Dual-metal-doped perovskite adsorbents for efficient removal of humic acid

Domain: biology
Published: 2026-03-12
Editorial outcome: major_revision

Abstract
Humic acid (HA), a complex organic compound of natural matter, is harmless on its own but can form carcinogenic disinfection byproducts, such as trihalomethanes and haloacetic acids, during water chlorination and disinfection processes, posing serious risks to aquatic organisms and human health. We present a B-site dual-metal-doping strategy to fabricate ferromagnetic perovskite-type adsorbents for rapid and efficient HA degradation. Perovskite oxides, specifically LaFeO3, were selected for their unique crystal structure, tunability, and inherent ferromagnetic properties. By doping Ti and Co at the B-site, we created a bifunctional perovskite absorbent that exhibits a rapid HA adsorption rate with a high adsorption capacity of 381 mg g-1, while also enabling regeneration in situ or magnetic recovery for Fenton regeneration. Computational simulations provide molecular-level insights into the interactions between the perovskite adsorbent and HA. Our findings reveal the potential of perovskite materials as highly effective, regenerable catalytic adsorbents for organic compounds, paving the way for their development in sustainable water remediation applications.

Introduction
With the expanding global population and diminishing freshwater resources, developing advanced wastewater treatment and reuse technologies has become critically important for sustaining water availability1-3. Among the water contaminants, HA—formed from the decay of plant and animal matter—significantly contributes to natural organic matter (NOM) in freshwater4-8. HA is a high molecular weight organic substance with a complex and stable structure that is refractory and resists biodegradation under natural conditions9,10. If left untreated, HA in water supplies can form stable complexes with heavy metal ions, enhancing their persistence and mobility, which leads to the accumulation of pollutants in sediments and organisms11-16. Furthermore, during chlorination, HA can form carcinogenic, teratogenic, and mutagenic disinfection byproducts such as trihalomethanes and haloacetic acids, severely compromising water quality and human health17-22. Hence, the effective removal of HA during wastewater treatment processes is crucial for ensuring safe and clean water. Currently, three main approaches exist for HA removal: biological methods (e.g., biological treatment23-25), chemical methods (e.g., electrolysis26, flocculation/coagulation27, and catalytic oxidation28-30), and physical methods (e.g., membrane filtration31,32 and adsorption33,34). Among these, adsorption is widely employed due to its cost-effective, simple design, operation ease, environmental friendliness, and high efficiency35. However, traditional adsorbents such as resins and activated carbon are limited by their low adsorption capacity, low specific surface area, and poor regeneration properties36,37.

Perovskite oxides, represented by the formula ABO3, are a diverse family of mixed metal oxides with highly tunable compositions and physicochemical properties, enabling applications in solar cells38,39, fuel cells40,41, electromagnetic materials42-44, chemical sensors45, and catalysts46,47. In the perovskite lattice, A-site elements, typically rare earth metals or alkaline earth metals (e.g., La, Pr, Gd) with larger ionic radii, occupy the central position of the cubic lattice and stabilize the framework, while B-site elements, usually transition metals (e.g., Ti, Mn, Fe, Co, Cr) with smaller ionic radii, occupy corner-sharing octahedra and predominantly govern the electronic structure, redox behavior, and surface chemistry48. This structural flexibility offers a powerful platform for tailoring properties relevant to water treatment, including surface charge, hydrophilicity, oxygen vacancy concentration, and magnetic separability49. For instance, Ti doping in LaFeO3 perovskites prepared by a sol-gel method enhances catalytic activity, improving the removal and mineralization of chlorophenols50, and can also modulate magnetic properties by influencing Fe spins through the unpaired electrons of Ti3+ ions51. Similarly, Co doping introduces abundant active sites that accelerate methane activation, whereas Sr doping increases oxygen vacancies, facilitating oxygen-ion migration and improving reactivity and resistance to coking52. However, despite extensive work on such doping strategies, the use of perovskite oxides as integrated adsorbent–regenerable materials for pollutant removal, including HA, remains underdeveloped. Conventional approaches to perovskite-based adsorbent enhancement still focus primarily on maximizing adsorption capacity, with comparatively less consideration to regeneration efficiency, energy input, or compatibility with continuous operation. This leads to a persistent gap between material-level optimization and process-level requirements. Moreover, the rational design and mechanistic understanding of dual B-site doping remain limited, even though exploiting complementary effects between metal ions of differing ionic radii and electronic structures could, in principle, simultaneously enhance multiple functionalities and improve material stability during extended use53,54.

Here we implement a dual B-site doping strategy in LaFeO3, simultaneously introducing Ti and Co to obtain a perovskite-type adsorbent (LFCTO) for efficient remediation of HA-contaminated water. LFCTO exhibits rapid HA uptake and a high maximum adsorption capacity of 381 mg g-1 in batch adsorption tests, which is attributed to its increased specific surface area and improved surface hydrophilicity. Concurrently, the Co/Ti centers act as Fenton-active sites for H2O2 activation, allowing in situ oxidative degradation of adsorbed HA and efficient regeneration of the adsorbent under mild conditions. The dual-doping strategy also enhances the ferromagnetism of LFCTO, enabling efficient magnetic separation and reuse. By coupling systematic experimental characterization with Density Functional Theory (DFT)and related quantum-chemical calculations, we elucidate the atomic-level synergistic effects of dual B-site doping that underpin both adsorption and regeneration. We demonstrate that LFCTO adsorbent can be regenerated in situ within a continuous adsorption-regeneration cycle, or magnetically recovered for ex situ chemical regeneration. LFCTO maintains its crystalline structure and stable HA removal performance for over 280 hr of continuous operation, enabling an energy-efficient, one-step regeneration route that avoids the high-temperature calcination or solvent-intensive elution required by conventional adsorbents.

Results
Fabrication of LFCTO absorbents

As a typical ABO3-type perovskite oxide, LaFeO3 has an atomic ratio of approximately 1:1:3 for La, Fe, and O55,56. In the case of dual-doped LFCTO, La occupies the A-site, while Fe, Co, and Ti occupy the B-site (Fig. 1a). The LF0.55Co0.45-xTixO3 (x = 0.1, 0.2, 0.3, and 0.4) oxides were successfully synthesized using a facile sol-gel method. The resulting perovskite oxide-type adsorbents exhibited uniform block-like porous structure (Fig. 1b, Supplementary Fig. 1a-d) and the high-resolution TEM (HRTEM) image (Fig. 1c) depicted lattice fringes of 0.28 nm, corresponding to the (110) plane of LaFeO3. Elemental composition analysis using energy dispersive X-ray spectroscopy (EDX) (Supplementary Fig. 1e) confirmed an atomic ratio of La: (Fe + Ti + Co): O was approximately 1:1:3, verifying the successful synthesis of LFCTO via the sol-gel method. X-ray diffraction spectroscopy (XRD) (Fig. 1e) revealed that the diffraction peaks of LFCTO closely matched the characteristic peaks of LaFeO3 (JCPDS no. 75-0541), consistent with its crystal structure, which was also confirmed by the selected area electron diffraction (SAED) pattern (Fig. 1d)57,58. Analysis of the (110) crystal plane (Fig. 1f) showed that as the Ti doping ratio increased, there was an overall left shift in the diffraction peaks, indicating an increase in crystal plane spacing59. Interestingly, at a doping ratio of x = 0.3, the diffraction peak shifted to its original position, maintaining the original configuration of LaFeO3. This shifting phenomenon, attributed to changes in the metal-oxygen bond lengths induced by doping, suggest that Ti/Co double doping can stabilize the perovskite crystal configuration. The Fourier transform infrared spectroscopy (FTIR) (Fig. 1g) revealed peaks at approximately 576 cm-1, indicating the presence of metal oxide bonds, specifically attributed to the Fe-O stretching vibration characteristic of the octahedral FeO6 group in LaFeO3 60.

Most adsorbents are difficult to separate from wastewater streams, which poses challenges for their reuse. Consequently, the magnetic separation of suspended adsorbents has been extensively researched61. The M- H loop behavior of LFCTO is studied from vibrating sample magnetometer at room temperature varying the applied magnetic field (H) during the range of 0 to ± 30 kOe exhibit, as shown in Fig. 1h and Supplementary Table 1 with the saturation magnetization measured as the primary indicator of their recovery capability62. As the Ti doping ratio increased, the magnetization initially increased and then decreased. In LFCTO, Ti can exchange-couple with Fe, shifting the Fe spin moment, which facilitates small-angle rotation tilt, thereby enhancing overall ferromagnetism due to uncompensated Fe spins. As the Fe spin moment increases, the presence of uncompensated spins allows free motion of the domain walls, further enhancing magnetization. However, high concentrations of nonmagnetic Ti can increase the antiferromagnetic spin alignment of Fe3+-O-Fe3+ and Fe4+-O2--Fe4+ through super exchange interactions, inhibit domain wall motion, resulting in decreased magnetization63. The results indicated that LFCTO-0.3 and LFCTO-0.2 exhibited clear ferromagnetic behavior (M = 7.36 emu g-1 and 5.24 emu g-1, respectively), which is advantageous for recycling and utilization in water treatment processes64.

To further elucidate the effect of Ti/Co doping on the magnetic properties of the materials, we performed theoretical validation using Density Functional Theory (DFT) calculations. LaFeO3 is a G-type antiferromagnetic crystal, and its weak magnetism observed experimentally has been attributed to Oxygen vacancies introduced during synthesis (Fig. 1i, l). Pristine LaFeO3 crystals lack unpaired electrons, resulting in zero spin density throughout the crystal lattice, and are thus non-ferromagnetic. Introducing Co dopants significantly alters the electronic structure of the closed-shell LaFeO3 crystal. This is due to the electronic configuration of Co (III), which is [Ar]4s03d6. According to Hund's rule, the 3d orbitals of Co(III) have four unpaired electrons, leading to a net magnetic moment. The difference in the number of unpaired electrons compared to Fe(III) disrupts the originally stable G-type antiferromagnetic structure when Co is incorporated into the lattice. Fig. 1j, m show that Co-doped LaFeO3 crystals exhibit prevalent spin density, indicating the potential to become paramagnetic or ferromagnetic materials. Conversely, higher doping levels of Ti reduces the magnetism of the system because the electronic configuration of Ti (III) is [Ar]4s03d1, with only one unpaired electron in the 3d orbital. This results in a system with fewer unpaired electrons, leading to non-magnetism or weak ferromagnetism. Fig. 1k, n further confirm this, showing only one unpaired electron present in the system.

Microstructure and metal ion state analysis of LFCTO absorbents

The specific surface areas and pore size distributions of LFCTO, which influence its adsorption capacity and rate, were determined by N2 sorption porosimetry. As shown in Fig. 2a, the N2 adsorption/desorption isotherms of LFCTO exhibited typical type IV curves with hysteresis between P/P0 = 0.5 and 1.0, indicating a mesoporous structure65. The pore size distribution (Fig. 2b) revealed that LFCTO had pore size ranging from 2 to 30 nm, with LFCTO-0.3 possessing the smallest pore size among them. These results indicate that LFCTO-0.3 had the largest specific surface area of 28.01 m2 g-1 compared with LFCTO-0.1, LFCTO-0.2 and LFCTO-0.4. Further analysis of pore volume showed that LFCTO-0.3 had the high total pore volume of 0.121 cm3·g-1 (Supplementary Table 1). This suggests that LFCTO-0.3 could exhibit enhanced adsorption compared to other LFCTO samples.

We performed X-ray photoelectron spectrometry (XPS) to determine the surface chemical composition and chemical states of LFO and LFCTO-0.3 (Fig. 2c-f). The XPS survey spectra (Fig. 2c) confirm that Ti and Co are successfully doped into the LFCTO-0.3 lattice. The A-site La element, which stabilizes the crystal structure, is present as La3+ (Supplementary Fig. 2a). The characteristic peaks of Ti 2p at 458.1 eV and 463.8 eV correspond to Ti 2p3/2 and Ti 2p1/2 of Ti4+, respectively (Supplementary Fig. 2b). Compared to LFO, the Fe2+/Fe3+ ratio increases (Fig. 2d), and the redox dynamics between Fe2+/Fe3+ and Co2+/Co3+ promote the generation of surface oxygen vacancies to maintain charge neutrality. This is further confirmed by the O 1s XPS spectrum (Fig. 2f), where peaks below 530 eV correspond to lattice oxygen and peaks above 530 eV correspond to chemisorbed oxygen66. Compared to LFO, the increased ratio of lattice oxygen to chemisorbed oxygen in LFCTO-0.3 indicates that Ti/Co doping promotes the creation of more oxygen vacancies in the LFO lattice, thus improving the adsorption performance of the material67.

Electron microscopy revealed a uniform, block-like porous structure in the adsorbents. The crystalline structure was confirmed by XRD and electron diffraction, showing a well-defined perovskite structure and successful incorporation of Ti and Co through doping. The N2 sorption measurements quantitatively demonstrated a mesoporous structure, where LFCTO-0.3 exhibited the highest specific surface area (28.01 m2 g-1) and pore volume (0.121 cm3 g-1). XPS analysis provided crucial insights into the surface chemistry, revealing that Ti/Co doping not only modified the surface composition but also promoted oxygen vacancy formation through Fe2+/Fe3+ and Co2+/Co3+ redox couples. FTIR spectroscopy confirmed the presence of metal-oxygen bonds, which are essential for electron transfer during adsorption. Additionally, magnetic measurements showed optimal magnetization (7.36 emu g-1) at Ti doping ratio of 0.3, facilitating practical separation and reuse. These structural features, including high surface area, abundant oxygen vacancies as active sites, and efficient mass transfer channels through mesopores, work synergistically to enhance the material's adsorption capacity.

Static HA adsorption on LFCTO adsorbent

Using HA as the target contaminant, Supplementary Fig. 3 illustrates the differences in adsorption performance among various adsorbents: unmodified (LFO), single-doped (LFTO and LFCO), and dual-doped (LFTCTO). The removal rates of HA in water revealed that adsorption efficiency improved with the doping of Ti and Co metals, with the dual-doped LFCTO demonstrating the most significant effect. Among these, LFCTO-0.3 exhibited the highest adsorption performance. This improvement is attributed to dual doping at the B sites, which effectively enhances the specific surface area, increases the number of adsorption sites for pollutant molecules, and further boosts adsorption. The simultaneous doping Ti and Co at the B-site promotes the formation of oxygen vacancies on surface of lanthanum ferrite (Fig. 2f, Supplementary Fig. 4). These oxygen vacancies facilitate the redox cycle at the B-site, generate electron holes, promote charge transfer, and enable subsequent regeneration through hydrogen peroxide. Nevertheless, it should be noted that all HA adsorption experiments were conducted in the absence of H₂O₂, so that HA removal in this stage proceeds exclusively via adsorption onto LFCTO; H₂O₂ is introduced only in the subsequent regeneration step, where it participates in a Fenton-like process to oxidatively degrade the pre-adsorbed HA and restore the adsorption sites.

Adsorption experiments were carried out with different concentrations of HA (C = 10, 30, and 50 mg L-1) at a fixed adsorbent dosage (C = 0.2 g L-1) and pH = 7 to compare the removal efficiency and adsorption capacity, aiming to identify the dual-doped perovskite oxide adsorbent with optimal performance (Supplementary Text 2). The results showed that all dual-doped LFCTO samples exhibited prominent adsorption for HA. Notably, LFCTO-0.3 demonstrated higher removal efficiency across different concentrations compared to other doping ratios (Fig. 3a), likely due to its largest specific surface area and smallest pore size among all dual-doped modified adsorbents (Fig. 2b and Supplementary Table 1). As the HA concentration increased, the adsorption sites were more fully utilized, further enhancing the adsorption effect. When the concentration of HA increased from 10 to 30 mg L-1, the adsorption capacity of LFCTO-0.3 increased significantly. However, as the HA concentration was increased to 50 mg L-1, the increase in adsorption capacity became less pronounced (Supplementary Fig. 5). Therefore, for subsequent experiments exploring varying conditions, an HA concentration of 30 mg L-1 was selected.

The amount of adsorbent is a critical factor affecting adsorption performance. As shown in Fig. 3b, the HA removal efficiency by LFCTO-0.3 increased with an increase in adsorbent dosage. When the adsorbent dosage was raised from 0.1 g L-1 to 0.2 g L-1, the removal efficiency of HA significantly increased from 82% to 97%. However, further increasing the dosage to 0.4 g L-1 only marginally increased the removal efficiency to 99%. This plateau is attributed to adsorbent aggregation at higher dosages, which reduces the accessible surface area and decreases the total number of active adsorption sites. Additionally, the limited number of HA molecules relative to the increased adsorbent restricts further improvements in removal efficiency. Meanwhile, the adsorption capacity gradually decreased as the adsorbent dosage increased from 0.1 to 0.4 g L-1. This reduction is due to the presence of unsaturated adsorption sites at higher dosages, leading to a decrease in per-unit adsorption capacity. Considering both the removal efficiency and adsorption capacity, 0.2 g L-1 was selected as the optimal adsorbent dosage. Additionally, TOC analysis was performed on the aqueous solutions before and after the adsorption process to quantify the organic matter removal efficiency (Supplementary Table 2). TOC decreased from 7.85 mg L-1 (pre-adsorption) to 1.83 mg L-1 (post-adsorption) while IC only slightly changed (0.91→0.77 mg L-1), indicating that HA is removed from the aqueous phase mainly via adsorption onto LFCTO-0.3.

To optimize the adsorption process for a wide range of real-world wastewater conditions, the effect of pH on adsorption was investigated (Fig. 3c). The adsorption performance of LFCTO-0.3 under various acidic and alkaline conditions is influenced by three primary factors: the surface charge of the adsorbent, the solubility of HA, and changes in HA molecular configuration68,69. The results indicated that the removal performance of HA by LFCTO-0.3 gradually increased as the solution pH ranged from 3 to 7. Under acidic conditions, HA molecules tend to polymerize and often exist as undissociated molecules or weakly ionic species, resulting in lower solubility. This enhances the interaction between HA and the adsorption sites on LFCTO-0.3, thereby improving the adsorption performance. The zero point charge (pHpzc) of LFCTO-0.3 was determined to be 7.5 through Zeta potential measurements (Fig. 3d). This critical parameter governs the surface charge characteristics of the adsorbent: when solution pH is below 7.5, the LFCTO-0.3 surface becomes positively charged, enabling favorable electrostatic attraction with negatively charged species (formed through the dissociation of HA: HA+H2O→H3O++A-). This electrostatic interaction mechanism explains the enhanced adsorption performance observed under acidic conditions70,71. As the pH increased from 7 to 11, the removal efficiency of HA by LFCTO-0.3 gradually decreased. This reduction is primarily due to electrostatic repulsion between the negatively charged adsorbent surface and HA molecules under alkaline conditions. Additionally, the increased solubility of HA in alkaline solutions further diminishes the adsorption capacity of LFCTO-0.372. Hence, LFCTO-0.3 demonstrated notably effective adsorption performance for HA under both acidic and neutral conditions. Furthermore, under high pH conditions, LFCTO samples show robust structural stability of LFCTO under elevated pH, which is attributed to its robust crystal structure and strong metal-oxygen bonds in the framework (Supplementary Fig. 6). Considering that the pH of HA solutions is generally around 7, the highest adsorption performance of LFCTO-0.3 at pH 7 indicates that highly efficient removal of HA can be achieved without the need for pH adjustment.

Electrostatic and thermodynamic mechanisms of HA adsorption

To investigate the enhanced adsorption and reaction mechanisms between HA and LFCTO-0.3, quantum chemical calculations were performed on HA molecules in different protonated states under water environment (Fig. 3g). The results revealed significant differences in the electrostatic potential properties of HA molecules under various protonation states, corresponding to different pH conditions. At lower pH levels, both carboxyl groups of the HA molecule are protonated and electrically neutral. The electrostatic potential projection on its van der Waals surface shows a maximum surface electrostatic potential is 61.9 kcal mol-1 and a minimum of -39.6 kcal mol-1, indicating significant polarization. The negatively charged regions are located near the carboxyl and nitro groups, while the positively charged regions are near the saturated cycloalkane structures. As the ambient pH increases, the HA molecule undergoes further ionization. At least one of the carboxyl groups becomes deprotonated, imparting a negative charge to the molecule. The van der Waals surface potentials becomes entirely negative, with the highest value decreasing to -5.3 kcal mol-1 and the lowest to -148.1 kcal mol-1. With a further decrease in ambient proton concentration, the maximum electrostatic potential shifts to -39.6 kcal mol-1 If the ambient proton concentration is reduced even further, all active hydrogens on the HA molecules dissociate, significantly enhancing the molecule's negative charge. The van der Waals surface then exhibits strong negative potential, with the highest being -80.3 kcal mol-1 and the lowest -242.5 kcal mol-1. These calculations indicate that HA molecules tend to adsorb onto positively charged sites of the adsorbents due to electrostatic interactions. Consistent with the zeta potential test results, the increase in pH leads to HA molecules becoming more electronegative. Thus, as pH increases from 3 to 7, the enhanced negative charge of HA strengthens the electrostatic attraction with the positively charged surface of LFCTO-0.3, enhancing their removal from water. However, as the pH continues to increase beyond 7, the removal efficiency significantly decreases. This reduction is primary due to the adsorbent surface becoming less positively charged or even negatively charged at higher pH levels, leading to electrostatic repulsion between the adsorbent and the negatively charged HA molecules. Furthermore, it was demonstrated that LFCTO-0.3 not only possesses a wide range of adsorption adaptability but also exhibits appreciable removal performance for various types of contaminants in water, especially for anionic contaminants (Supplementary Fig. 7).

The adsorption kinetics depend on the physical and chemical properties of the adsorbent, which influence the adsorption mechanisms. To model the experimental data of HA adsorption onto LFCTO-0.3, pseudo-first order kinetic (PFO), pseudo-second order kinetic (PSO), and intra-particle diffusion models were employed (Supplementary Text 2)73. The fitting curves are shown in Fig. 3e and Supplementary Fig. 8, and the corresponding kinetic parameters are presented in Supplementary Table 3. The ID model fitting yield correlation coefficients (R2) ranging from 0.341 ~ 0.945, indicating that intra-particle diffusion was not the rate-determining step in the adsorption process74. In comparison, the PFO model showed lower correlation with the experimental data (R2= 0.834 ~ 0.990) than the PSO model, which exhibited a prominent fit with high correlation coefficients (R2 > 0.996). Additionally, the equilibrium adsorption capacities calculated using the PSO kinetic equation (Supplementary Equation 4) were consistent with the experimental data. Therefore, the adsorption of HA onto LFCTO-0.3 is better described by the PSO model, suggesting that the adsorption process is predominantly governed by chemisorption, with physisorption playing a secondary role. To investigate the relationship between adsorption capacity and equilibrium concentration, both Langmuir and Freundlich isotherm models were applied75. The correlation coefficients (R2) and corresponding parameters for both models are listed in Fig. 3f and Supplementary Table 4. The Langmuir model, with a correlation coefficient of R2 = 0.983, provided a better fit to the experimental data better than the Freundlich model (R2 = 0.979). This indicates that the adsorption process follows monolayer adsorption, primarily characterized by chemisorption76.

Thermodynamic analysis further elucidated the significant role of temperature in the adsorption process from an energy perspective. For practical applications, thermodynamic changes were studied at 298 K, 308 K, and 318 K. As the adsorption temperature increased, the adsorption performance of LFCTO-0.3 for HA improved, and the thermodynamic equilibrium constant (Kd) also increased. The results (Fig. 3h and Supplementary Table 5) showed that ∆G < 0, indicating that the adsorption process is spontaneous and fall within the range of physical and chemical adsorption (-20 ~ -40 kJ·mol-1)77. Furthermore, the absolute value of ∆G increased with increasing temperature, confirming the spontaneity of the adsorption process at higher temperatures. The positive value of ∆S suggests an increase in the enthalpy at the solid-liquid interface during adsorption, reflecting greater dispersal and interaction of HA molecules on the LFCTO-0.3 interface. This enhanced the solid-liquid interfacial area, promoting further adsorption. Additionally, ∆H > 0 indicates that the adsorption process is endothermic, implying that higher temperatures favor adsorption. These results demonstrated that the adsorption of HA onto LFCTO-0.3 is a spontaneous, endothermic chemical process with improved performance at elevated temperatures.

Hydrophilicity and structural defects in LFCTO adsorbent

Low-Field Nuclear Magnetic Resonance (LF-NMR) was used to quantify the surface hydrophilicity of water trapped in the absorbents. Compared to pure water, the presence of the adsorbent causes the water to enter nanoconfined regions. The schematic in Fig. 3i illustrates the water-binding capacity and surface hydrophilicity of the perovskite oxide, representing three distinct temporal domains: sub-nanometer confinement space I, nanoconfinement II, and free space III. The LF-NMR characterization revealed that the T2 relaxation times of LFO and LFCTO were 932.4 and 142.8 ms, respectively (Fig. 3i and inset). Given equal amounts of adsorbent and solvent in both samples, the transverse relaxation time (T2) exhibited a negative correlation with the bound water content78. From LFO to LFCTO-0.3, protons adsorbed on the particle surfaces become more tightly bound, significantly shortening the NMR relaxation time and increasing the bound water content. This increase is primarily due to the improved hydrophilicity of the LFCTO-0.3 surface achieved through doping modification. The presence of more bound water on and within the adsorbent indirectly enhances its adsorption properties. This finding aligns with the type and content of surface oxygen in perovskite oxides determined by XPS O 1s spectroscopy (Fig. 2f). The ratio of oxygen vacancies to lattice oxygen (OV/OL), which indicates the abundance of oxygen vacancies in the material, increased from 0.61 in LFO to 1.05 in LFCTO-0.3 (Supplementary Table 6). The dual doping of Ti and Co in LFCTO-0.3 promotes the formation of structural defects, generating more oxygen vacancies and active sites, thereby improving adsorption performance. Compared with LFO, LFCTO-0.3 possesses a larger number of oxygen vacancies and active sites, significantly enhancing its adsorption capacity.

The Independent Gradient Model based on Hirshfeld partition (IGMH) is a computational interaction method that utilizes the electron density gradient and has gained widespread popularity due to its universality and robustness79. We applied IGMH to calculate the adsorption behavior of HA molecules on the surface of LaFeO3 crystals. In the diagrams, the red isosurfaces represent regions of attraction, while blue denotes repulsion areas, with their size and color intensity being directly proportional. The results indicate that pristine LaFeO3 crystals possess notable adsorption capacity for HA molecules (Fig. 3j), mainly driven by interactions between Fe and O atoms, with HA molecules being adsorbed onto the crystal surface through two or more Fe-O interactions. As discussed previously, Co doping leads to a transition of the crystal material from an antiferromagnetic to a ferromagnetic state. Given that closed-shell organic molecules like HA are diamagnetic, the enhanced adsorption performance of HA on the Co-doped LaFeO3 surface is relatively modest (Fig. 3k). In contrast, Ti doping reverts the material back to an antiferromagnetic state, restoring these interactions. Due to the B-site dual-doping of Co and Ti during synthesis, the number of adsorption sites and anisotropy of the crystal increase, leading to a stronger adsorption capacity compared to the pristine LaFeO3 crystals (Fig. 3l). The dual-doping effectively enhances the crystal’s ability to adsorb HA molecules, showcasing a notable improvement in adsorption performance.

Building on the findings from the aforementioned studies, we describe the adsorption mechanism of HA onto LFCTO as follows. The adsorption capacity of LFCTO is attributed to the well-structured surface, rich oxygen-containing functional groups, and enhanced specific surface area and pore volume through dual metal doping with Ti and Co. The surface of the adsorbent undergoes protonation or deprotonation processes, generating pH-dependent surface charge distributions. These charge variations create electrostatic interactions between the adsorbent and negatively charged HA molecules, which facilitate the efficient capture of HA through selective surface adsorption mechanisms. Furthermore, doping modification induces charge compensation mechanisms and crystal structure strain, both of which contribute to the formation of oxygen vacancies. The presence of these oxygen vacancies greatly enhances electron migration within LFCTO, significantly increasing its chemical reactivity. This not only accelerates the adsorption kinetics but also provides favorable conditions for the regeneration of the adsorbent, enhancing its overall performance and economic viability.

Cyclic performance and regeneration of LFCTO adsorbent

We evaluated the reusability of LFCTO-0.3 for the adsorption of HA to assess its potential for industrial applications. Fig. 4a illustrates a schematic diagram of the adsorption-desorption cycle. Under static condition at pH 7.0, 0.2 g L-1 of absorbent was added to 200 ml of HA solution (C = 30 mg L-1). After the adsorption process was complete, LFCTO-0.3 was recovered using a strong magnet. The adsorbent was then immersed in a diluted hydrogen peroxide (H2O2, 0.15 M, 200 mL) solution to facilitate chemical desorption for 60 min, leveraging the Fenton properties inherent in ferrite to generate hydroxyl radicals that break down the adsorbed HA (Supplementary Fig. 9). Following three washes with deionized water and drying at 60℃, the adsorbent was reused in the next adsorption experiment. After five adsorption-desorption cycles, the removal efficiency of LFCTO-0.3 to HA remained at 89.87% (Fig. 4b), demonstrating that the magnetic adsorbent retained its adsorption performance after chemical desorption. This also suggests that LFCTO-0.3 maintained its Fenton activity and magnetism throughout the cycles. Free radical trapping experiments reveal that significant OH was observed, confirming effective H₂O₂ activation by the Fe²⁺/Fe³⁺ and Co²⁺/Co³⁺ redox pairs in LFCTO. No hydroxyl radical (OH) signals were detected when either LFCTO adsorbent or H2O2 existed independently (Supplementary Fig. 10). During the adsorption stage, the active sites on the LFCTO surface are partially occupied by target pollutants, reducing the number of sites available for OH generation. In the subsequent degradation stage, the generated OH is consumed in the H₂O₂-driven oxidative degradation of the adsorbed HA (Supplementary Fig. 11). To clarify the dominant HA removal mechanism, additional O2⁻ trapping and oxygen vacancies experiments were conducted (Supplementary Fig. 12-13). The O2⁻ signal did not disappear during either adsorption and degradation. In fresh LFCTO, the oxygen vacancy (OV) fraction is 24.90%, increasing to 30.02% after HA adsorption and further to 32.69% following H₂O₂-mediated HA degradation, consistent with the Electron Paramagnetic Resonance (EPR) characterization trend. The slight increase in O2⁻ is attributed to the exposure of previously hidden vacancies via H₂O₂-induced mild oxidative reconstruction (removing surface-adsorbed HA residues or hydroxyl groups) and to the structural flexibility imparted by Ti/Co dual doping, which prevents excessive oxidation-induced lattice collapse and enables stable accumulation of oxygen vacancies. To further understand the molecular interactions, DFT calculations were performed to determine the molecular orbital structure (Supplementary Fig. 14), which confirmed the feasibility of using H2O2 for the oxidative removal of HA molecules. The calculations indicated favorable interactions between H₂O₂, LFCTO-0.3, and the adsorbed HA, facilitating the oxidative degradation of HA during the desorption process.

Moreover, the concentrations of dissolved metal ion gradually decreased with each cycle. After the fifth cycle, the concentrations of La3+, Co3+, Fe3+, and Ti4+ were 0.017 mg L-1, 0.048 mg L-1, 0.026 mg L-1, and 0.012 mg L-1, respectively (Supplementary Fig. 15). These levels are below the national wastewater discharge standards, indicating minimal leaching of metal ions during regeneration. The crystal structure and surface elemental composition of LFCTO in the fresh, post-adsorption, and post-degradation states were characterized by XRD and XPS (Supplementary Figs. 16-17). Comparison of the patterns and spectra across these states shows only minor variations in peak intensity and surface element ratios, with no evidence of phase transformation or lattice collapse after degradation, demonstrating the structural stability of the perovskite under H₂O₂ regeneration. Furthermore, the crystal structure of the perovskite adsorbent remained unchanged after five cycles, as confirmed by XRD analysis, indicating good structural stability (Supplementary Fig. 18).

To investigate the regeneration and stability performance of LFCTO-0.3 during the dynamic adsorption process, we conducted five cycles of adsorption-desorption experiments using a fixed bed column packed with LFCTO-0.3, as illustrated in Fig. 4c. The breakthrough curves (Fig. 4d) indicate that LFCTO-0.3 maintained high adsorption performance after five cycles. As the cyclic experiment progressed, both the time to reach the breakthrough point and the saturation time decreased, suggesting a reduced adsorption capacity of LFCTO-0.3 for HA. This reduction is likely due to incomplete desorption within a fixed recovery period of 20 min. Despite this, LFCTO-0.3 exhibits good stability and regenerative capability, as confirmed by XRD analysis.

Notably, LFCTO-0.3 ranks among the highest in adsorption capacities for HA when compared to other adsorbents reported in the literature (Fig. 4e; Supplementary Data 1). It rapidly achieves HA removal with a maximum adsorption capacity of 381 mg g-1 under 2 hr (Supplementary Data 1; Detailed comparison and methodology are provided in the Supporting Information). In addition, traditional regeneration methods for reported adsorbents typically involve high-temperature calcination or solvent elution using agents like NaOH, resulting in elevated energy consumption and potential secondary pollution80-82. In contrast, our adsorbent can be regenerated in under 20 min (Fig. 4d) through the Fenton reaction, facilitated by LFCTO’s ability to generate radicals. The impressive adsorption and regeneration properties of LFCTO-0.3 make it particularly suitable for use in adsorption columns. Its easy dispersion and straightforward magnetic separation also allow its application in existing Advanced Oxidation Processes (AOP) tanks without requiring structural modifications to sewage treatment facilities. The versatility of LFCTO-0.3 supports flexible application scenarios. These findings suggest that LFCTO-0.3 could serve as an environmentally friendly and magnetic adsorbent with marked HA removal performance, offering significant advantages in ease of regeneration, stability, and reduced environmental impact compared to conventional adsorbents.

Methods
Chemical reagents

HA was purchased from Alfa Aesar (China) Chemical Co., Ltd. Methylene blue, Congo red, Rhodamin B, sodium hydroxide (NaOH), and anhydrous ethanol were purchased from Sinopharm Chemical Reagent Co., Ltd. Oxytetracycline hydrochloride, sodium citrate, ammonia, hydrochloric acid, and hydrogen peroxide were purchased from Aladdin Reagent (Shanghai) Co., Ltd. Tetrabutyl titanate and polyethylene glycol 600 were purchased from Shanghai Lingfeng Reagent Co., Ltd. La(NO3)3·6H2O, Fe(NO3)3·9H2O, and Co(NO3)3·6H2O were purchased from Shanghai Macklin Biochemical Co., Ltd. The chemicals used in this investigation were of analytical grade and no additional purification steps were conducted on any of the reagents. All solutions were prepared using deionized water (DI).

Synthesis of LFCTO adsorbents

LaFeO3 (LFO) and LaFe0.55Co0.45O3 (LFCO) perovskite oxides were synthesized using a sol-gel method followed by calcination. First, the metal compounds of La(NO3)3·6H2O, Fe(NO3)3·9H2O, and Co(NO3)3·6H2O were mixed, stirred, and dissolved in a beaker. The molar ratio was La: Fe = 1:1 for preparing LFO and La: Fe: Co = 1:0.55:0.45 for preparing LFCO. Sodium citrate was added to the mixed metal solution to achieve a molar ratio of 1:5 between metal cations and citrate anions. The pH of the metal solution was adjusted to 4 using 20% ammonia solution. The mixed metal solution was stirred continuously at 80°C until it transformed into a sol-gel state. The resulting wet gel was placed in a preheated oven and dried at 120°C overnight. The dried gel was then placed in a muffle furnace and heated from 20°C to 300°C at a rate of 5°C min-1. It was then held at 300°C for 3 h before being heated further to the final temperature of 700°C, where it was held for 5 h. After natural cooling of the muffle furnace, the desired product (LFO and LFCO) was obtained.

LaFe0.55Ti0.45O3 (LFTO) and LaFe0.55Co0.45-xTixO3 (LFCTO) were synthesized by adding titanium dioxide sol to the above mixed metal solution. Titanium dioxide sol was prepared by adding tetrabutyl titanate, anhydrous ethanol, PEG600, and deionized water in a molar ratio of 3:12:3:1 to form a transparent sol. The prepared Ti dioxide sol was slowly added into the mixed metal solution and stirred sufficiently for 1 hour at 50°C. The molar ratio was La:Fe:Ti = 1:0.55:0.45 for preparing LFTO. For the preparation of dual-doped LaFe0.55Co0.45-xTixO3 (LFCTO, x=0.1, 0.2, 0.3, and 0.4), the molar ratio is La:Fe:Co:Ti =1:0.55:0.45-x:x. For example, the preparation of dual-doped LFCTO-0.3, the corresponding molar ratio is La:Fe:Co:Ti =1:0.55:0.15:0.3. Sodium citrate and ammonia solution were then added sequentially to the mixed metal solution in the same steps as above. The mixed metal solution was stirred continuously at 80°C until it transformed into a sol-gel state. The perovskite oxide-type adsorbent, LFTO and LFCTO were subsequently obtained using the same dry and annealing conditions.

Material characterizations

The surface morphology of metal oxide was characterized by scanning electron microscopy (SEM, HITACHI S-4800, Japan). The elemental composition of metal oxide was obtained by energy-dispersive X-ray spectroscopy (EDX, Horiba, Japan). The morphological structure was further analyzed by transmission electron microscopy (TEM, FEI Tecnai F20, USA). The crystal morphology of the perovskite oxide-type adsorbents was analyzed by X-ray diffraction spectroscopy (XRD, RIGAKU MiniFlex600, Japan). The functional groups of perovskite oxide-type adsorbents were identified by Fourier transform infrared spectroscopy (FTIR, Thermo Nicolet8700, USA). The textural properties of the perovskite oxide-type adsorbents were assessed by N2 adsorption/desorption analysis (BET, Micromeritics ASAP 2460, USA). Metal cation states and oxygen forms were investigated by X-ray photoelectron spectrometry (XPS, ESCALAB 250XI, USA). Ferromagnetism was measured using a vibrating sample magnetometer (VSM, Quantum Design Dyna Cool, USA) at room temperature. The zeta potential of perovskite oxide-type adsorbents in aqueous solution was determined by laser scattering method (Malvern Nano ZS90, UK). Additionally, the absorbance of HA at 254 nm was determined by UV-visible spectrophotometry (Perkin Elmer Lambda950, USA). The dissolution concentrations of leached metal ions were measured by inductively coupled plasma spectrometry (ICP, Optima 7000DV, USA). The hydrophilicity and water distribution characteristics of the adsorbents were characterized by low-field nuclear magnetic resonance spectroscopy (LF-NMR, NM42-40H-I, Suzhou Niumag). The oxygen vacancies and free radicals were characterized by Electron Paramagnetic Resonance (EPR, Bruker EMXplus-6/1, Germany).

Computational simulation

DFT calculations were performed on the small molecules, lanthanum ferrate crystals, and their doped variants involved in the experiments. The wavefunction analysis, molecular visualization, and structural modeling were carried out with Multiwfn 3.8, VMD 1.9.3, and ChimeraX 1.90 software, respectively. The details are described in the Supplementary Text.

Data availability

The data supporting the findings of the study are included in the main text and supplementary information files. Raw data can be obtained from the corresponding author upon request.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The paper in overall is good, the synthesis, characterization and discussion are in depth and the literature review detailed.\nMy main concern is that the models used (PFO, PSO, BDST, Thomas, Yoon-Nelson) are, at best, rough approximations. I\nam wondering why the authors do not use more accurate models that are available in the literature and for some software\ncan be used as well. These models cannot offer any significant insight as they are phenomenological. The author must\nexplain why the use such models and discuss the weakness and limitations of these models. See here on the merits and\nfallacies of models doi.org/10.1016/j.jhazmat.2018.12.023.\nPage 8 - BET results are provided but not the most important, that is the specific surface area.\nPage 9 - Do the used HA concentrations represent those found in natural waters or wastewater? The study on the effect of\nadsorbent is trivial; we do not expect anything else but the removal to increase when you are using more adsorbent. The\nplateau is, in my view, unexpected as at some point the mass of adsorbent is enough to remove almost all HA.\nFigure 3, Figure 4 and in other instances - the label is too long please shorten and explain the graph in the text.\nLines 275-276, 282, 284 and in other instances - too many figures digits are used implying unattainable accuracy\nPage 15 - How the Kd was calculated? It seems as a distribution coefficient rather than equilibrium constant. If so the\nthermodynamic analysis is wrong. Anyways, if the process is chemical in natire then the application of adsorption models is\nproblematic.", "reviewer_2": "The manuscript presents an innovative approach using B-site dual-metal-doping (Ti and Co) to fabricate ferromagnetic\nperovskite-type adsorbents for the rapid and efficient degradation of humic acid (HA). The abstract highlights the key\nfindings, including the high adsorption capacity of 381 mg/g and the potential for in-situ regeneration or magnetic recovery\nfor Fenton regeneration. The application of various adsorption models (BDST, Thomas, and Yoon-Nelson) to describe the\ndynamic adsorption behavior is a solid methodological approach, and the computational simulations provide valuable\ninsights into the interactions between the perovskite adsorbent and HA molecules.\nHowever, there are several areas that require further clarification and improvement:\n1.The figure in the abstract describes degradation, while the title is about adsorption, which does not reflect the main theme\nof the paper.\n2.The dynamic adsorption section lacks parallel experimental data\n3.The abstract does not highlight the novelty and significance of this research. Why was this study conducted? Why was\nperovskite selected for HA removal? What is special about the perovskite used in this study, and what distinguishes this\nwork from previous research? Please clarify why this particular type of perovskite was chosen.\n4.Lines 134-135: How did the author arrive at the conclusion that “Surface oxygen vacancies disturb the antiparallel spin\nordering in Fe³⁺-O-Fe³⁺ and Fe⁴⁺-O²⁻-Fe⁴⁺ through super-exchange interactions, leading to increased magnetization”?\nPlease explain which characterization technique(s) led to this conclusion.\n5.Line 141: \"Ms = 7.36 emu/g and 5.24 emu/g, respectively.\" How did the author calculate the ferromagnetic behavior?\nPlease provide the calculation method and include this information in the manuscript.\n6.Table S1: Please provide the volumes of microporosity and mesoporosity. Without these values, it is difficult to confirm the\nstatement in Line 165 regarding the mesoporous structure. Also, the total pore volume and pore width appear similar across\nall four samples in Table S1 and Figure 2b. Please clarify.\n7.The authors claim that oxygen vacancies are present in LFTCO. However, there is a lack of sufficient experimental\ncharacterization to confirm the existence of oxygen vacancies (O-v). Additionally, the statement in lines 199-200 suggesting\nthat electron mobility can boost adsorption is not adequately explained. Further clarification and supporting evidence are\nneeded to substantiate this claim.\n8.Lines 231-251: The pH_pzc of the adsorbents in Figure 3d should be discussed when analyzing the effect of solution pH.\n9.Lines 493-496: H₂O₂ was used for the desorption of the used adsorbents. The concentration and volume of H₂O₂, as well\nas the reaction time, should be specified. Why was hot water used for desorption? Additionally, how can the author be\ncertain that the adsorbed HA was fully broken down?\n10.The structure of the adsorbent is crucial to its adsorption capacity, but it was not evaluated throughout the manuscript.\nPlease address this issue.\n11.The XRD in Figure 1e does not appear to fit well with FaFeO₃. Please check. The XRD of the spent adsorbents in Figure\nS10 shows an increase in the intensity of LaFeO₃ peaks after recycling. Please explain this observation.\n12.The mechanism of HA adsorption removal, including the relationship between the adsorbent structure and the role of\noxygen vacancies, has not been shown or discussed. Please elaborate on this mechanism."}, "author_response": "Dear Reviewer 1,\nWe are grateful for the opportunity to revise our manuscript submitted to Nature\nCommunications. The authors sincerely thank the reviewers for their insightful\ncomments, which have been very helpful our revisions and improvements to the\nmanuscript. All comments have been carefully considered, and we have addressed them\npoint by point below.\nThe paper in overall is good, the synthesis, characterization and discussion are in depth\nand the literature review detailed. My main concerns are as follows:\n1. My main concern is that the models used (PFO, PSO, BDST, Thomas, Yoon-Nelson)\nare, at best, rough approximations. I am wondering why the authors do not use more\naccurate models that are available in the literature and for some software can be used\nas well. These models cannot offer any significant insight as they are phenomenological.\nThe author must explain why the use such models and discuss the weakness and\nlimitations of these models. See here on the merits and fallacies of models\ndoi.org/10.1016/j.jhazmat.2018.12.023.\nWe appreciate the reviewer's comments regarding the kinetic models employed in our\nresearch. We fully acknowledge the concerns raised about the use of the pseudo-first\norder (PFO), pseudo-second order (PSO), Bed Depth Time of Service (BDST), Thomas,\nand Yoon-Nelson models.\nOur decision to utilize these models was a well-considered choice based on the aspects\nbelow. We are well aware that these models are phenomenological approximations.\nHowever, their simplicity in calculation is a significant advantage. This simplicity\nmakes them particularly suitable for the preliminary screening of adsorption properties\nunder diverse conditions. For instance, when we are exploring the potential of LFCTO\nmaterials for adsorption, these models allow us to quickly compare different\nexperimental setups and materials. We can rapidly identify promising directions for\nfurther investigation. In addition, they serve as a standardized approach for comparative\nanalysis. In the field of adsorption studies, having a common set of models for\ncomparison is highly convenient. It allows us to directly compare the adsorption\nperformance of LFCTO materials with those of other materials in the literature.\nThis not only helps in understanding the relative adsorption capabilities but also\nhighlights the unique adsorption properties of LFCTO materials.\nIt's important to note that we do not regard these models as definitive predictive tools.\nInstead, they act as a starting point for gaining initial insights into the adsorption\nmechanisms of LFCTO materials. The adsorption mechanism cannot be determined by\nfitting the kinetic equation alone.\nWe have reviewed the reference recommended by the reviewer and discussed the\nstrengths and weaknesses of each model. The discussion has been added to the\nsupplementary information, under the model descriptions section, as follows:\nLines 173-180: “These aforementioned models are phenomenological and widely\napplied in adsorption studies due to their computational simplicity, making them ideal\nfor preliminarily screening and comparative analysis of adsorption properties across\ndiverse conditions. The models offer valuable information into the fundamental\nunderstanding of adsorption kinetics. Note that the adsorption mechanism cannot be\ndetermined solely by fitting the kinetic equations. Instead, it requires integration with\nanalytical techniques for a comprehensive and accurate determination. A detailed\ndiscussion of the model selection and limitations is provided in Table S11.”\nTable S11. Advantages and disadvantages of the PFO, PSO, BDST, Thomas and Yoon-\nNelson model.\nModel Purpose Key Advantages Limitations\nAn empirical model that\nSimple mathematical form lacks physical\nsignificance and cannot\nDescribes initial Easy to calculate\nPFO represent a specific\nadsorption rate\nQuick preliminary\nadsorption mechanism\nassessment\nCannot accurately\nreflect the nature of the\nadsorption process\nBetter describes Assumes uniform\nchemisorption adsorption sites\nEstimates\nMore accurate for many May overestimate\nPSO adsorption\nsystems adsorption capacity\nequilibrium\nConsiders interaction between Less accurate for\nadsorbate and adsorbent complex systems\nAssumes constant driving\nRelates bed depth to service\nforce\ntime\nPredicts column Limited by linear\nBDST Useful for fixed-bed design\nperformance\nisotherm assumptions\nEstimates breakthrough\nDoes not account for\ncharacteristics\ncomplex interactions\nConsiders both adsorption\nEstimates\nNeglects axial dispersion\nand mass transfer\nmaximum\nThomas\nadsorption Widely used in column studies Linear driving force\ncapacity approximation\nProvides kinetic parameters\nRequires fewer parameters\nAssumes linear driving\nSimplifies Quick estimation of\nYoon- force\nbreakthrough breakthrough time\nNelson\ncurve analysis Less accurate for\nNo need for detailed system complex systems\ncharacteristics\n2. Page 8 - BET results are provided but not the most important, that is the specific\nsurface area.\nThe specific surface area of the material has been provided in Supplementary Table S1.\nTo clarify the results further, we have included the BET surface area in Fig. 2a and\nupdated the revised manuscript accordingly as follows:\nLines 169-171: “These results indicate that LFCTO-0.3 had the largest specific surface\narea of 28.01 m2/g compared with LFCTO-0.1, LFCTO-0.2, and LFCTO-0.4.”\nLFCTO-0.3 28.01 m2/g\nLFCTO-0.4 24.37 m2/g\n60\n40\n20\n0\n0.0 0.2 0.4 0.6 0.8 1.0\nFig. 2 | a, BET analysis of LFCTO.\n3. Page 9 - Do the used HA concentrations represent those found in natural waters or\nwastewater? The study on the effect of adsorbent is trivial; we do not expect anything\nelse but the removal to increase when you are using more adsorbent. The\nplateau is, in my view, unexpected as at some point the mass of adsorbent is enough to remove almost\nall HA.\nThe humic acid (HA) concentration selected in this study ranges from 10 to 50 mg/L,\nbased on environmental relevance. Natural water bodies typically contain HA\nconcentrations of 5-30 mg/L (Appl. Catal. B-Environ. 194, 32-41 (2016), Chemosphere.\n312, 137193 (2023)), while wastewater systems (including municipal and industrial\nwastewater) have total humic substance concentrations of 118-228 mg/g, with about 42%\nbeing HA (J. Water Process Eng. 53, 103679 (2023), Desalin. Water Treat. 144, 57-64\n(2019)). The selected range not only encompasses typical natural water and wastewater\nenvironments but also simulates variations in HA concentrations during actual water\ntreatment processes (Eng. Proc. 57, 22 (2023)). This range allows for a comprehensive\nevaluation of adsorbent removal performance and provides a reliable theoretical and\nexperimental basis for water treatment technologies.\nFig. 3b analyzes the impact of adsorbent dosage on HA removal and adsorption\n)1-g·3mc(\ndebrosda\nytitnauQ\nRelative pressure (p/p )\n0\nperformance. By evaluating both the HA removal rate and adsorption capacity, we aim\nto determine an optimal adsorbent dosage for subsequent experiments. The plateau\nregion occurs due to the constant HA concentration. As the adsorbent dosage increases,\nmore active adsorption sites are exposed, ultimately achieving over 99% removal\nefficiency. This plateau validates the expected adsorption performance and suggests an\neffective dosage range for HA treatment.\n4. Figure 3, Figure 4 and in other instances - the label is too long please shorten and\nexplain the graph in the text.\nWe appreciate the reviewer's suggestion regarding figure labeling. We have simplified\nthe labels accordingly to improve readability, and incorporated part of the explanations\ninto the text. The updated simplified labels are as follows:\nLines 277-283: “Fig. 3 | HA adsorption performance and mechanistic analysis.\nAdsorption performance of LFCTO-0.3: the effect of a, HA concentration, b, the amount\nof adsorbent and c, pH. d, The zeta potential of LFCTO-0.3 and HA. e, PSO model\nfitting to static adsorption capacity (qt). f, Langmuir isotherm and Freundlich isotherm\nfitting. g, The electrostatic potential of HA. h, Thermodynamic for HA adsorption on\nLFCTO-0.3. i, LF-NMR spectra of LFO and LFCTO-0.3 using water as probe molecule.\nThe weak interactions within HA-LaFeO and its doped systems. j, HA-LaFeO system;\n3 3\nk, HA-Co doped LaFeO system; l, HA-Ti doped LaFeO system.”\n3 3\nLines 453-456: “Fig. 4 | Dynamic adsorption and corresponding models under\nvarious conditions. a, Flow chart of dynamic adsorption experiment. b, BDST analysis.\nThe effect of c, influent flow rate, d, bed height, e, feed concentration and f, feed pH on\ndynamic adsorption of HA. The analysis of g-j, Thomas model for c-f and k-n, Yoon-\nNelson model for c-f, respectively.”\nAfter simplifying the graphic labels, we ensured that all necessary information remains\naccessible to readers by moving the detailed descriptions to the revised manuscript.\nThis includes the calculation conditions for different protonated states of HA molecules\nin water environment, as used in the quantum chemistry analysis in Fig. 3g. In Fig. 3j-\nl, the adsorption behavior of HA molecules by IGMH on LaFeO crystal surface is\n3\nillustrated using different colors, sizes and colors of isosurfaces. The following sentence\nwas added to the revised manuscript:\nLines 286-288: “To investigate the enhanced adsorption and reaction mechanisms\nbetween HA and LFCTO-0.3, quantum chemical calculations were performed on HA\nmolecules in different protonated states under water environment (Fig. 3g).”\nLines 382-383: “In the diagrams, the red isosurfaces represent regions of attraction,\nwhile blue denotes repulsion areas, with their size and color intensity being directly\nproportional.”\n5. Lines 275-276, 282, 284 and in other instances - too many figures digits are used\nimplying unattainable accuracy.\nWe appreciate the reviewer's suggestions on the significant figures. The values in Lines\n275-276, 282, and 284 represent surface electrostatic potentials from quantum chemical\ncalculations of HA molecules, where three decimal places were originally retained to\nreflect the computational results. However, we agree that these levels of precision are\nnot necessary to convey the same meaning. Therefore, we have adjusted the significant\nfigures throughout the manuscript to align with the actual experimental accuracy. The\nspecific changes in the manuscript are as follows:\nLines 291-294: “The electrostatic potential projection on its van der Waals surface\nshows a maximum surface electrostatic potential is 61.9 kcal/mol and a minimum of -\n39.6 kcal/mol, indicating significant polarization.”\nLines 298-301: “The van der Waals surface potentials become entirely negative, with\nthe highest value decreasing to -5.3 kcal/mol and the lowest to -148.1 kcal/mol. With a\nfurther decrease in ambient proton concentration, the maximum electrostatic potential\nshifts to -39.6 kcal/mol.”\nLines 303-304: “The van der Waals surface then exhibits strong negative potential, with\nthe highest being -80.3 kcal/mol and the lowest -242.5 kcal/mol.”\nFig. 3g. The electrostatic potential of HA.\n6. Page 15 - How the Kd was calculated? It seems as a distribution coefficient rather\nthan equilibrium constant. If so the thermodynamic analysis is wrong. Anyways, if the\nprocess is chemical in natire then the application of adsorption models is\nproblematic.\nThe parameter K represents the equilibrium adsorption coefficient at a specific\nd\ntemperature T, characterized as a dimensionless constant derived from equation (8). The\ndetailed methodology is provided in the supplementary text. This thermodynamic\nanalysis is primarily based on equations (9), (10) and (11), which are used to calculate\nthermodynamic parameters such as ΔG°, ΔH° and ΔS°. Accordingly, the adsorption\nof humic acid by the adsorbents was evaluated from a thermodynamic perspective.\nWith reference to other studies (Sep. Purif. Technol. 339, 126637 (2024), Sep. Purif.\nTechnol. 278, 119630 (2021), Water Res. 44, 1489-1497 (2010)), we believe it is\nreasonable and feasible to conduct thermodynamic analysis using K . However, we note\nd\nthat equation (10) mistakenly mixed the equilibrium adsorption coefficient K with the\nd\nthermodynamic equilibrium constant K . Accordingly, we have revised and recalculated\nc\nΔG°. The results show that the values of ∆G at different temperatures are -24.43, -26.43\nand -28.43 kJ/mol, respectively. Based on the ΔG°, physical adsorption and chemical\nadsorption are the two major adsorption mechanisms. Chemical adsorption typically\nranges from -80 to -400 kJ/mol, physical adsorption ranges from 0 to -20 kJ/mol, and\nphysical-chemical adsorption ranges from -20 to -40 kJ/mol. Therefore, the adsorption\nprocess likely involves both physical and chemical adsorption behaviors, and the\nrevised formula does not alter the initial judgment of adsorption type. Additionally, we\nused the thermodynamic equilibrium constant K to calculate the ΔG at 298K using the\nc\nformula ΔG=-RTlnK (J. Hazard. Mater. 381, 120974(2020), J. Hazard. Mater. 174,\nc\n137-143 (2010)). The calculated ΔG is -22.74 kJ/mol, which is close to the values\nobtained using K , confirming that the adsorption type remains unchanged. The\nd\ncomparison of the two methods verifies the feasibility of using K for thermodynamic\nd\nanalysis.\nThe detailed calculation in this section has been updated accordingly and included in\nthe supplementary information, under Table S4 as follows:\n∆𝐺(cid:3087) = ∆𝐻(cid:3087) −𝑇∆𝑆(cid:3087) (10)\nTable S4. Thermodynamic parameters for humic acid adsorption on LFCTO-0.3.\nT ∆G ∆S ∆H\nContaminant\n(K) (kJ.mol-1) (J.mol-1K-1) (kJ.mol-1)\n298 -24.43\nHumic acid 308 -26.43 199.11 34.89\n318 -28.43\nLines 306-319: “The ΔG° is calculated by K , and the results show that the values of\nd\n∆G at different temperatures are -24.43, -26.43 and -28.43 kJ/mol, respectively. Based\non the ΔG°, physical adsorption and chemical adsorption are the two major adsorption\nmechanisms. Chemical adsorption typically ranges from -80 to -400 kJ/mol, physical\nadsorption ranges from 0 to -20 kJ/mol, and physical-chemical adsorption ranges from\n-20 to -40 kJ/mol. Therefore, the adsorption process likely involves both physical and\nchemical adsorption behaviors. Additionally, we used the thermodynamic equilibrium\nconstant K to calculate the ΔG, as follows equations (16):\nc\nΔG=-RTlnK (16)\nc\nwhere K is the thermodynamic equilibrium constant, L/mol; ∆G is the change in Gibbs\nc\nfree energy, kJ/mol; R is the standard gas constant, 8.314 J/(mol·K); and T is the\nabsolute temperature, K.\nThe calculated ΔG is -22.74 kJ/mol, which is close to the values obtained using K ,\nd\nconfirming that the adsorption type remains unchanged. The comparison of the two\nmethods verifies the feasibility of using K for thermodynamic analysis.”\nd\nDear Reviewer 2,\nWe are grateful for the opportunity to revise our manuscript submitted to Nature\nCommunications. The authors sincerely thank the reviewers for their insightful\ncomments, which have been very helpful our revisions and improvements to the\nmanuscript. All comments have been carefully considered, and we have addressed them\npoint by point below.\nThe manuscript presents an innovative approach using B-site dual-metal-doping (Ti\nand Co) to fabricate ferromagnetic perovskite-type adsorbents for the rapid and\nefficient degradation of humic acid (HA). The abstract highlights the key findings,\nincluding the high adsorption capacity of 381 mg/g and the potential for in-situ\nregeneration or magnetic recovery for Fenton regeneration. The application of various\nadsorption models (BDST, Thomas, and Yoon-Nelson) to describe the dynamic\nadsorption behavior is a solid methodological approach, and the computational\nsimulations provide valuable insights into the interactions between the perovskite\nadsorbent and HA molecules. However, there are several areas that require further\nclarification and improvement:\n1. The figure in the abstract describes degradation, while the title is about adsorption,\nwhich does not reflect the main theme of the paper.\nThe graphical abstract illustrates the dual-functional performance of LFCTO adsorbent,\nwhere adsorption is the primary mechanism for humic acid removal. Alongside its\nexcellent adsorption capacity, the material exhibits an innovative self-cleaning feature:\ntransition metal redox cycling in the Ti/Co-doped perovskite activates H O to generate\n2 2\nfree radicals, which degrade saturated humic acid, facilitating desorption. This self-\ncleaning capability enhances the material's practical applicability by enabling efficient\nregeneration while complementing the main adsorption function. We would like to\nretain the original design as the graphical abstract highlights both the adsorption and\nthe synergistic self-cleaning feature, aligning with the paper's focus on adsorption-\nbased humic acid removal.\n2. The dynamic adsorption section lacks parallel experimental data.\nWe appreciate the reviewers' suggestions to include parallel experimental data. The\ndynamic adsorption experiments using LFCTO-0.3 were primarily designed to\ninvestigate the adsorption behavior under practical conditions. The obtained\nbreakthrough curves exhibited characteristic S-shaped patterns across different\nexperimental parameters, which are typical for dynamic adsorption processes. The\nexcellent agreement between the experimental data and theoretical predictions from\nBDST, Thomas, and Yoon-Nelson models further validates the reliability of our\nfindings.\nSingle breakthrough curve experiments are widely accepted and commonly reported in\nfixed-bed column studies for the removal of organic pollutants. Using the Web of\nScience, we performed a search with the terms [\"breakthrough\" (Topic) AND \"dynamic\nadsorption\" NOT \"gas\" (Topic) and \"Last 5 years\" (Publication) and Review Article\n(Exclude-Document Types)] to search for the last five years of work on the dynamic\nadsorption of adsorbents in solution environments containing breakthrough curves\nwhile also excluding review articles.\nThe results indicated that among the top 50 publications of the search results, 5 studies\ndid not include breakthrough curves at all, as their focus was on gas adsorption or metal\nion adsorption. Of the remaining studies, 4 reported parallel experimental data for\nbreakthrough curves in dynamic adsorption, while the remaining 41 publications\n(including 1 publication in Nature Communication) reported single-breakthrough curve\nstudies without parallel experiments. The search results align with our approach in this\nstudy, but we acknowledge the limitation and will incorporate parallel experiments in\nfuture studies for more rigorous validation. We are also happy to perform and\nsupplement such data if the reviewers consider the experiment indispensable for this\nstudy.\n3. The abstract does not highlight the novelty and significance of this research. Why\nwas this study conducted? Why was perovskite selected for HA removal? What is\nspecial about the perovskite used in this study, and what distinguishes this work from\nprevious research? Please clarify why this particular type of perovskite was chosen.\nWe thank the reviewer for their insightful comments. Humic acid (HA) poses a\nsignificant challenge in water treatment due to its stable, recalcitrant structure, which\nnot only enriches heavy metals but also generates carcinogenic disinfection byproducts,\nthreatening water quality and human health. Existing adsorption strategies suffer from\nslow adsorption rates, low efficiency, poor reproducibility, and limited mechanistic\nunderstanding. We selected perovskite oxides, specifically LaFeO₃, for their unique\ncrystal structure, exceptional tunability, and inherent ferromagnetic properties. By co-\ndoping Ti and Co, we developed an innovative adsorbent (LFCTO) with a high\nadsorption capacity of 381 mg/g, rapid adsorption rates, and the ability to regenerate\nefficiently via magnetic field-assisted Fenton processes. This distinguishes our work\nfrom previous studies by offering a highly efficient, economical, and environmentally\nfriendly solution for organic pollutant removal.\nWe have carefully revised the abstract to better highlight the innovative aspects of our\nresearch approach while ensuring it does not overlap too much with the key information\ncovered in the Introduction. The new abstract is as follows:\nLines 17-32: “Humic acid (HA), a complex organic compound of natural matter, is\nharmless on its own but can form carcinogenic disinfection byproducts, such as\ntrihalomethanes and haloacetic acids, during water chlorination and disinfection\nprocesses, posing serious risks to aquatic organisms and human health. We present a\nB-site dual-metal-doping strategy to fabricate ferromagnetic perovskite-type\nadsorbents for rapid and efficient HA degradation. Perovskite oxides, specifically\nLaFeO , were selected for their unique crystal structure, exceptional tunability, and\n3\ninherent ferromagnetic properties. By doping Ti and Co at the B-site, we created a\nbifunctional perovskite absorbent that exhibits a rapid HA adsorption rate with a high\nadsorption capacity of 381 mg/g, while also enabling regeneration in situ or magnetic\nrecovery for Fenton regeneration. The dynamic adsorption behavior of HA is\naccurately described and predicted by the Bed Depth Service Time (BDST), Thomas,\nand Yoon-Nelson models, while computational simulations provide molecular-level\ninsights into the interactions between the perovskite adsorbent and HA. Our findings\nreveal the potential of perovskite materials as highly effective, regenerable catalytic\nadsorbents for organic compounds, paving the way for their development in sustainable\nwater remediation applications.”\n4. Lines 134-135: How did the author arrive at the conclusion that “Surface oxygen\nvacancies disturb the antiparallel spin ordering in Fe3+-O-Fe3+ and Fe4+-O2--Fe4+\nthrough super-exchange interactions, leading to increased magnetization”? Please\nexplain which characterization technique(s) led to this conclusion.\nWe thank the reviewer for their insightful comment. Upon review, we acknowledge that\noxygen vacancies are not directly related to the antiparallel spin ordering. We apologize\nfor this oversight. The magnetic measurements presented in Fig. 1h demonstrate that\nthe magnetic properties of LFCTO increase with the Ti doping ratio. However,\nexcessive doping leads to a decline in magnetization, which is consistent with\npreviously reported findings (J. Alloys Compd. 712, 870-877 (2017)). The possible\nexplanation for this observation is that an increase in Ti4+ enhances the\nantiferromagnetic (AFM) spin arrangement of Fe3+-O-Fe3+ and Fe4+-O2--Fe4+ through\nsuper-exchange interactions, ultimately leading to a decrease in magnetization. We have\nrevised this part of the manuscript accordingly as follows:\nLines 138-141: “However, high concentrations of nonmagnetic Ti4+ can increase the\nantiferromagnetic (AFM) spin alignments of Fe3+-O-Fe3+ and Fe4+-O2--Fe4+ through\nsuper exchange interaction and inhibited domain wall motion, resulting in decreased\nmagnetization63.”\n5. Line 141: \"Ms = 7.36 emu/g and 5.24 emu/g, respectively.\" How did the author\ncalculate the ferromagnetic behavior? Please provide the calculation method and\ninclude this information in the manuscript.\nIn this study, a vibrating sample magnetometer (VSM) was used to characterize the\nmagnetic properties of LFCTO materials at room temperature within a magnetic field\nrange of ±30 kOe. Saturation magnetization (Ms) was determined by identifying the\nmaximum magnetization value in the high-field plateau region of the M-H curve, where\nfurther increases in magnetic field do not significantly change magnetization. The\ncalculation follows the principle: 𝑀𝑠 = lim 𝑀 (𝐻).\n(cid:3009)→(cid:2998)\nFor the LFCTO samples, the saturation magnetization values were: LFCTO-0.1: Ms =\n0.95 emu/g, LFCTO-0.2: Ms = 5.24 emu/g, LFCTO-0.3: Ms = 7.36 emu/g, LFCTO-0.4:\nMs = 0.58 emu/g.\nWe have revised the manuscript to include a detailed description of the magnetic\ncharacterization method and the precise calculation procedure for determining the\nsaturation magnetization value.\nLines 129-133: “The M- H loop behavior of LFCTO is studied from vibrating sample\nmagnetometer at room temperature varying the applied magnetic field (H) during the\nrange of 0 to ± 30 kOe exhibit, as shown in Fig. 1h and Supplementary Table S1 with\nthe saturation magnetization measured as the primary indicator of their recovery\ncapability62.”\n6. Table S1: Please provide the volumes of microporosity and mesoporosity. Without\nthese values, it is difficult to confirm the statement in Line 165 regarding the\nmesoporous structure. Also, the total pore volume and pore width appear similar across\nall four samples in Table S1 and Figure 2b. Please clarify.\nWe appreciate the reviewer for their insightful comments. The previous characterization\nfocused primarily on the mesoporous region and showed incomplete pore size\ndistribution curves that did not converge to baseline. We have expanded our analysis to\ncover the complete pore size range, and the micropore and mesopore volumes have\nbeen provided in Supplementary Table S1.\nThe N adsorption/desorption isotherm of LFCTO exhibits a typical type IV curve with\n2\nhysteresis between P/P = 0.5-1.0, which is characteristic of mesoporous materials due\n0\nto capillary condensation. Additionally, the pore size distribution (Fig. 2b) confirms that\nthe pores are mainly in the range of 2-50 nm, with a significant concentration around\n10-20 nm, supporting that LFCTO is a mesoporous material.\nThe adsorption and desorption isotherm and pore size distribution of the LFCTO across\nthe full pore range, have now been provided as follows:\nFig. 2 | a, BET analysis and b, pore size distribution of LFCTO.\nThe micropore and mesoporous volumes have been included in Supplementary Table\nS1, as follows:\nTable S1. Physiochemical property parameters of LFCTO.\nSaturate Micropore Mesoporous Total pore\nS\nBET\nAdsorbent magnetization volume volume volume\n(m2/g)\nMs (emu/g) (cm3/g) (cm3/g) (cm3/g)\nLFCTO-0.1 0.95 15.55 0.0011 0.0565 0.0784\nLFCTO-0.2 5.24 26.90 0.0014 0.1210 0.1404\nLFCTO-0.3 7.36 28.01 0.0012 0.1065 0.1209\nLFCTO-0.4 0.58 24.37 0.0013 0.1017 0.1189\nWe have also revised the text on total pore volume data in the manuscript as follows:\nLines 171-173: “Further analysis of pore volume showed that LFCTO-0.3 had the high\ntotal pore volume of 0.121 cm3·g-1 (Supplementary Table S1).”\nRegarding the comment on the total pore volume and pore width appearing similar\nacross all four samples in Table S1 and Figure 2b, we can confirm that the total pore\nvolumes and pore widths of the four samples were indeed similar, demonstrating that\nCo/Ti doping does not significantly alter the total pore volume and pore width of\nLFCTO. It is also important to note that pore diameter is not the primary factor\ndetermining adsorption characteristics. Instead, the high adsorption performance of\nLFTCO is largely attributed to a combination of factors, such as high specific surface\narea, which provides abundant active adsorption sites, oxygen vacancies that alter the\nelectronic structure, and electrostatic interactions between the material surface and\npollutants. These microscopic structural features collectively contribute to LFCTO's\nadsorption efficiency.\n7. The authors claim that oxygen vacancies are present in LFTCO. However, there is a\nlack of sufficient experimental characterization to confirm the existence of oxygen\nvacancies (O-v). Additionally, the statement in lines 199-200 suggesting that electron\nmobility can boost adsorption is not adequately explained. Further clarification and\nsupporting evidence are needed to substantiate this claim.\nWe thank the reviewer for pointing out the lack of oxygen vacancy analysis. To address\nthis, we have supplemented the manuscript with Raman spectral characterization of the\nLFO and LFCTO-0.3. In the Raman spectra of the original LFO, only weak peak\nintensity and peak position near 700 and 430 cm-1 were observed, suggesting a low\nconcentration of oxygen vacancies on the surface of the LFO. In contrast, the Raman\nspectrum of LFCTO-0.3 exhibits a more pronounced peak around 700 cm-1. This\npronounced increase in peak intensity indicates a higher concentration of oxygen\nvacancies, confirming that Co/Ti doping effectively increases the oxygen vacancy on\nthe surface of LFCTO.\nAdditionally, the XPS spectrum of O 1s in our previous analysis (Fig. 2f) also supports\nthe existence of oxygen vacancies. Compared with LFO, the ratio of lattice oxygen to\nchemisorbed oxygen in LFCTO-0.3 increased, indicating that Ti/Co doping promoted\nthe generation of additional oxygen vacancies within the LFO lattice.\nWe have included the Raman spectra of LFO and LFCTO-0.3 and their analysis in the\nsupplementary information, as follows:\n400 600 800 1000 1200\n0 1000 2000 3000\nFig. S5. Raman spectra of LFO and LFCTO-0.3.\nLines 199-204: “In the Raman spectra of the original LFO, only weak peak intensity\nand peak position near to 700 and 430 cm-1 were observed, suggesting few of oxygen\nvacancies on the surface of the LFO. In contrast, the Raman spectrum of the LFCTO-\n0.3 exhibits a more pronounced peak around 700 cm-1. This enhancement in peak\nintensity indicates a higher concentration of oxygen vacancies, confirming that Co/Ti\ndoping effectively increases the oxygen vacancy on the surface of LFCTO.”\nWith regard to the comment that “electron mobility can promote adsorption”,\nwe\nintended to convey that the presence of oxygen vacancies facilitates the redox cycle at\nthe B-site, enhances charge transfer, and enables subsequent regeneration through\nhydrogen peroxide. We have revised the discussion on this part as follows:\nLines 214-221: “This improvement is attributed to dual doping at the B sites, which\neffectively enhances the specific surface area, increases the number of adsorption sites\nfor pollutant molecules, and further boosts adsorption. The simultaneous doping of Ti\nand Co at the B-site promotes the formation of oxygen vacancies on the surface of\nlanthanum ferrite (Fig. 2f, Supplementary Fig. S5). These oxygen vacancies facilitate the\nredox cycle at the B-site, generate electron holes, promote charge transfer, and enable\nsubsequent regeneration through hydrogen peroxide.”\n).u.a(\nytisnetnI\nLFCTO-0.3\nLFO\nRaman shift (cm-1)\n).u.a(\nytisnetnI\nLFCTO-0.3\nLFO\nRaman shift (cm-1)\n8. Lines 231-251: The pH_pzc of the adsorbents in Figure 3d should be discussed when\nanalyzing the effect of solution pH.\nWe appreciate the valuable suggestion regarding the importance of pH in\npzc\nunderstanding pH-dependent adsorption behavior. In our revised manuscript, we have\nthoroughly addressed this point by incorporating the pH analysis from the Zeta\npzc\npotential measurements (Fig. 3d). Specifically, we identified that LFCTO-0.3 has a\npH of 7.5, which serves as a critical parameter in explaining the adsorption\npzc\nmechanism. By including this pH analysis, we have strengthened the mechanistic\npzc\nunderstanding of the pH-dependent adsorption process and provided a more\ncomprehensive explanation of the adsorbent's behavior under different pH conditions.\nThe effect of pH on the surface charge characteristics of the adsorbent and its direct\npzc\neffect on the adsorption properties at different pH values have been added to the\nmanuscript:\nLines 260-266: “The zero point charge (pH ) of LFCTO-0.3 was determined to be 7.5\npzc\nthrough Zeta potential measurements (Fig. 3d). This critical parameter governs the\nsurface charge characteristics of the adsorbent: when solution pH is below 7.5, the\nLFCTO-0.3 surface becomes positively charged, enabling favorable electrostatic\nattraction with negatively charged species (formed through the dissociation of HA:\nHA+H O→H O++A-). This electrostatic interaction mechanism explains the enhanced\n2 3\nadsorption performance observed under acidic conditions70,71.”\nWe have improved Fig. 3d to illustrate the pH determination and its implications for\npzc\nsurface charge characteristics, as follows:\n20\n10\n0\npH =7.5\npzc\n-10\n-20\n-30\n-40\n3 5 7 9 11\nFig. 3 | d, The zeta potential of LFCTO-0.3 and HA.\n9. Lines 493-496: H O was used for the desorption of the used adsorbents. The\n2 2\nconcentration and volume of H O , as well as the reaction time, should be specified.\n2 2\nWhy was hot water used for desorption? Additionally, how can the author be certain\nthat the adsorbed HA was fully broken down?\nWe thank the reviewer for their valuable comment. We have provided the desorption\nconditions in the manuscript, as follows:\nLines 523-526: “The adsorbent was then immersed in a diluted hydrogen peroxide\n(H O , 0.15 M, 200 mL) solution to facilitate chemical desorption for 60 min,\n2 2\nleveraging the Fenton properties inherent in ferrite to generate hydroxyl radicals that\nbreak down the adsorbed HA (Supplementary Fig. S12).”\nWe would like to clarify that hot water was not employed in the desorption process. The\nregeneration procedure involved desorption using hydrogen peroxide (H O , 0.15 M,\n2 2\n200 mL), followed by thorough rinsing with deionized water and controlled drying at\n60℃ to prepare the adsorbent for subsequent adsorption cycles.\nTo address the question of whether the adsorbed humic acid was fully broken down,\nwe\nconducted several analyses. A notable color change was observed between the original\nHA solution and the H O -regenerated solution, providing initial evidence of molecular\n2 2\nstructural disruption. We then performed permeation gel chromatography to compare\nthe molecular weight distribution of the original HA solution with the H O -regenerated\n2 2\n)vm(\nlaitnetop\nateZ\nLFCTO-0.3\nHA\npH\nsolution. The results revealed a significant transformation, with the number-average\nmolecular weight decreased from 5557 to 3035 Da, accompanied by a notable\nnarrowing of the molecular weight distribution spectrum. FTIR analysis of the LFCTO-\n0.3 adsorbent before and after adsorption-regeneration cycles demonstrated remarkable\nspectral consistency with the pristine material. No intermediate molecular fragments or\nresidual functional groups were detected on the adsorbent surface, confirming the\nabsence of residual HA or its degradation byproducts. These findings collectively\nconfirm that the adsorbed HA was fully decomposed and effectively removed during\nthe regeneration process.\nRelevant characterizations and detailed descriptions have been added to the\nsupplementary information:\nFig. S12. a, A physical diagram of the original HA solution and the H O -regenerated solution.\n2 2\nb, Molecular weight distribution diagram of the original HA solution and the H O -regenerated solution.\n2 2\nc, FTIR spectra of the LFCTO-0.3, regenerated LFCTO, used LFCTO and HA.\nTable S12. Statistical results of molecular weight of the original HA solution and the\nH O -regenerated solution.\n2 2\nSample Mn Mw Mp Mz Mz+1 polydispersity\noriginal HA solution 5557 6632 7472 7661 8535 1.19\nH O -regenerated\n2 2\n3035 3601 2508 4388 5251 1.19\nsolution\nLines 279-296: “The molecular structure transformation of humic acid during H O\n2 2\nregeneration was systematically investigated through multiple analytical approaches.\nInitial visual evidence emerged from the notable chromatic variation between the\noriginal HA solution and the H O -regenerated solution, suggesting potential\n2 2\nmolecular structural modifications (Fig. S12a). To quantitatively assess the structural\nchanges, permeation gel chromatography (GPC) was employed to compare the\nmolecular weight distribution of the original and regenerated HA solutions\n(Supplementary Fig. S12b and Table S12). The analysis revealed a significant\nmolecular transformation, characterized by a dramatic reduction in number-average\nmolecular weight from 5557 Da to 3035 Da. Concurrently, a pronounced narrowing of\nthe molecular weight distribution spectrum was observed, indicating substantial\nmolecular fragmentation. FTIR analysis of the LFCTO-0.3 adsorbent further\ncorroborated these findings (Fig. S12c). Comparative spectral analysis before and after\nadsorption-regeneration cycles demonstrated remarkable spectral consistency with the\npristine material. Critically, no intermediate molecular fragments or residual\nfunctional groups were detected on the adsorbent surface. These comprehensive\nanalytical results conclusively demonstrate that the humic acid adsorbed on the\nadsorbent surface was fully decomposed and effectively removed during the hydrogen\nperoxide regeneration process.”\n10. The structure of the adsorbent is crucial to its adsorption capacity, but it was not\nevaluated throughout the manuscript. Please address this issue.\nWe thank the reviewer for their valuable comment. To address the evaluation of the\nadsorbent structure and its relationship to adsorption properties, we would like to\nhighlight that structural characterizations had been previously conducted and included\nin the manuscript. However, we acknowledge that the corresponding discussions could\nbe presented more clearly to emphasize the connection between structure and\nadsorption performance. As part of our revisions to improve readability and coherence,\nwe have made the following adjustments:\nLines 193-207: “Electron microscopy revealed a uniform, block-like porous structure\nin the adsorbents. The crystalline structure was confirmed by XRD and electron\ndiffraction, showing a well-defined perovskite structure and successful incorporation\nof Ti and Co through doping. The N sorption measurements quantitatively\n2\ndemonstrated a mesoporous structure, where LFCTO-0.3 exhibited the highest specific\nsurface area (28.01 m2/g) and pore volume (0.121 cm3/g). XPS analysis provided\ncrucial insights into the surface chemistry, revealing that Ti/Co doping not only\nmodified the surface composition but also promoted oxygen vacancy formation through\nFe2+/Fe3+ and Co2+/Co3+ redox couples. FTIR spectroscopy confirmed the presence of\nmetal-oxygen bonds, which are essential for electron transfer during adsorption.\nAdditionally, magnetic measurements showed optimal magnetization (7.36 emu/g) at Ti\ndoping ratio of 0.3, facilitating practical separation and reuse. These structural\nfeatures, including high surface area, abundant oxygen vacancies as active sites, and\nefficient mass transfer channels through mesopores, work synergistically to enhance\nthe material's adsorption capacity.”\n11. The XRD in Figure 1e does not appear to fit well with FaFeO .\n3\npeaks after recycling. Please explain this observation.\nWe thank the reviewer for their valuable comment. We have verified that the XRD\npatterns of both pristine LaFeO and its dual-doped derivatives match well with the\n3\nstandard LaFeO pattern (PDF #75-0541), showing well-crystallized planes. The slight\n3\nvariations in peak positions can be attributed to the successful incorporation of dopants\ninto the perovskite structure, which is consistent with our design strategy.\nRegarding the intensity variation in Fig. S11, the XRD measurements were not\nconducted under strictly quantitative conditions, which may lead to apparent intensity\ndifferences. The recycled absorbents were collected from multiple parallel experiments\nfor subsequent cycles and characterization. We have carefully compared the XRD\npatterns after the 1st, 3rd, and 5th cycles, and found that the peak intensity variations\nwere within acceptable ranges, indicating no significant structural changes. The\ndetailed experimental procedures for XRD measurements have been included as\nsupporting text under Fig. S11. The consistent peak positions and relative intensities\nbetween fresh and recycled samples demonstrate that our perovskite adsorbents\nmaintain excellent crystallinity and structural stability throughout multiple adsorption-\nregeneration cycles, which is crucial for their practical applications.\nA description of the XRD measurements has been added under Fig. S11, as follows:\nLines 264-273: “The XRD measurements were performed to evaluate the structural\nstability of the adsorbents. The recycled adsorbents were collected from parallel\nexperiments for subsequent cycles and characterization. While the measurements were\nnot conducted under strictly quantitative conditions, the peak positions and relative\nintensities remained consistent throughout multiple adsorption-regeneration cycles.\nCareful comparison of the XRD patterns after the 1st, 3rd, and 5th cycles revealed that\npeak intensity variations were within acceptable ranges, confirming no significant\nstructural changes during the recycling process. The consistent crystallographic\nfeatures between fresh and recycled samples demonstrate the excellent structural\nstability of our perovskite adsorbents during multiple adsorption-regeneration cycles.”\n12. The mechanism of HA adsorption removal, including the relationship between the\nadsorbent structure and the role of oxygen vacancies, has not been shown or discussed.\nPlease elaborate on this mechanism.\nWe thank the reviewer for your suggestion. We have included a detailed discussion on\nthe removal mechanism of humic acid by LFCTO, addressing the physical properties\nof LFTCO, electrostatic interactions between LFCTO and humic acid, and the role of\noxygen vacancies, as follows:\nLines 397-411: “Building on the findings from the aforementioned studies, we describe\nthe adsorption mechanism of humic acid onto LFCTO as follows. The excellent\nadsorption capacity of LFCTO is attributed to the well-structured surface, rich oxygen-\ncontaining functional groups, and enhanced specific surface area and pore volume\nthrough dual metal doping with Ti and Co. The surface of the adsorbent undergoes\nprotonation or deprotonation processes, generating pH-dependent surface charge\ndistributions. These charge variations create electrostatic interactions between the\nadsorbent and negatively charged humic acid molecules, which facilitate the efficient\ncapture of HA through selective surface adsorption mechanisms. Furthermore, doping\nmodification induces charge compensation mechanisms and crystal structure strain,\nboth of which contribute to the formation of oxygen vacancies. The presence of these\noxygen vacancies greatly enhances electron migration within LFCTO, significantly\nincreasing its chemical reactivity. This not only accelerates the adsorption kinetics but\nalso provides favorable conditions for the regeneration of the adsorbent, enhancing its\noverall performance and economic viability.”\nIn addition to the revisions made based on the comments from the reviewers, the\nmanuscript has been thoroughly proofread to ensure consistency and accuracy.\nAdditionally, the acknowledgments section has been updated, as noted in the\nmanuscript."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors addressed all comments in a satisfactory way. I retain my opinion that both equilibrium and kinetics analysis, at\nbest, is weak. I recommend publication of this paper subject to the editor's view on my concerns.", "reviewer_2": "The authors have revised their manuscript according to the reviewers’ comments. However, several critical issues remain\nunaddressed, and the explanations provided are insufficient:\nOriginal Comment: The figure in the abstract illustrates degradation, whereas the title focuses on adsorption, which does not\nalign with the main theme of the paper.\nThis issue persists. The revised abstract states, “We present a B-site dual-metal-doping strategy to fabricate ferromagnetic\nperovskite-type adsorbents for rapid and efficient HA degradation.” However, both the model and experimental data indicate\nthat adsorption is the primary mechanism of HA removal. Adsorption and degradation are fundamentally different processes.\nAdsorption is predominantly governed by the porous structure and surface functional groups, while degradation involves the\nformation of intermediates and modifications to surface chemical properties. However, the paper does not discuss surface\nfunctional groups or surface properties.\nThe authors should provide a scientifically sound explanation rather than a superficial revision.\nOriginal Comment: Table S1: Please provide the volumes of microporosity and mesoporosity. Without these values, it is\ndifficult to verify the statement in Line 165 regarding the mesoporous structure. Additionally, the total pore volume and pore\nwidth appear similar across all four samples in Table S1 and Figure 2b. Please clarify.\nIn the revised manuscript, Table S1 still shows that the total pore volume and pore width are similar across all four samples,\nas also depicted in Figure 2b. There is insufficient evidence to support the claim that such differences would result in\nsignificant variations in adsorptive removal efficiency.\nMany conclusions in the manuscript seem overly speculative and lack adequate supporting evidence.\nOriginal Comment: The authors claim that oxygen vacancies exist in LFTCO. However, there is insufficient experimental\ncharacterization to confirm the presence of oxygen vacancies (O-v).\nIn the revised manuscript, the authors provide Raman spectra comparing LFO and LFCTO-0.3 in Figure S5. However, this\nevidence is not sufficiently convincing. EPR characterization would be more appropriate for confirming oxygen vacancies.\nAdditionally, the paper does not evaluate changes in surface chemistry after adsorption.\nGiven the presence of oxygen functional groups and the electron transfer between Fe²⁺/Fe³⁺ and Co²⁺/Co³⁺ redox couples,\nit is questionable whether adsorption is indeed the dominant removal mechanism. Are the adsorption models used in the\npaper appropriately justified?\nFigure 3c: At higher pH values, the LFCTO surface could undergo oxidation. The authors should use XRD to characterize\npotential phase changes and further explore the underlying mechanisms."}, "author_response": "Response to second round comments:\nReviewer #1 Comments:\nThe authors addressed all comments in a satisfactory way. I retain my opinion that both equilibrium and kinetics\nanalysis, at best, is weak. I recommend publication of this paper subject to the editor's view on my concerns.\nWe thank the reviewer for the careful assessment. We acknowledge that the models used are phenomenological\napproximations and previously provided a detailed explanation in the revised manuscript and supplementary\ninformation (Table S11) discussing the strengths, limitations, and rationale for their use (based on the original\ncomment from Reviewer #1 to discuss the above). Please note that these models were employed as standardized\ntools for comparative analysis and preliminary screening, which is common practice in adsorption studies. In light of\nthese limitations, and to avoid overinterpretation, we have omitted the dynamic analysis because it is not essential to\ninterpreting the results. In addition, we note that Reviewer #2 commented positively, stating “The application of\nvarious adsorption models (BDST, Thomas, and Yoon-Nelson) to describe the dynamic\nadsorption behavior is a solid methodological approach”\nReviewer #2 Comments:\nOriginal Comment: The figure in the abstract illustrates degradation, whereas the title focuses on adsorption, which\ndoes not align with the main theme of the paper.\nReviewer raised concerns about the graphical abstract, suggesting that it focuses on degradation while the title\nemphasizes adsorption. Our work includes both adsorption and degradation mechanisms, and both are depicted in\nthe graphical abstract. Adsorption is the primary mechanism for HA removal, while degradation (via Fenton reactions)\nplays a crucial role in decomposing HA and regenerating the perovskite, thereby enabling the perovskite to adsorb\nmore HA in subsequent cycles. We believe the graphical abstract accurately represents the dual functionality of the\nmaterial, highlighting both adsorption and the synergistic self-cleaning feature.\nThis issue persists. The revised abstract states, “We present a B-site dual-metal-doping strategy to fabricate\nferromagnetic perovskite-type adsorbents for rapid and efficient HA degradation.” However, both the model and\nexperimental data indicate that adsorption is the primary mechanism of HA removal. Adsorption and degradation are\nfundamentally different processes. Adsorption is predominantly governed by the porous structure and surface\nfunctional groups, while degradation involves the formation of intermediates and modifications to surface chemical\nproperties. However, the paper does not discuss surface functional groups or surface properties.\nThe authors should provide a scientifically sound explanation rather than a superficial revision.\nWe agree with Reviewer that adsorption is the primary mechanism of HA removal, as shown by both our model and\nexperimental data. Degradation is the 2nd step of the process (Fig. 4), which is to regenerate the perovskite for the\nnext adsorption cycle of HA, with the use of diluted hydrogen peroxide to generate hydroxyl radicals (Fig. S13).\nOriginal Comment: Table S1: Please provide the volumes of microporosity and mesoporosity. Without these values,\nit is difficult to verify the statement in Line 165 regarding the mesoporous structure. Additionally, the total pore volume\nand pore width appear similar across all four samples in Table S1 and Figure 2b. Please clarify. In the revised\nmanuscript, Table S1 still shows that the total pore volume and pore width are similar across all four samples, as also\ndepicted in Figure 2b. There is insufficient evidence to support the claim that such differences would result in\nsignificant variations in adsorptive removal efficiency. Many conclusions in the manuscript seem overly speculative\nand lack adequate supporting evidence.\nReviewer #2 commented on the similarity of total pore volumes across the samples and requested micropore and\nmesopore volumes. We would like to clarify that we have re-performed N₂ adsorption and desorption analyses on all\nfour samples and included the micropore and mesopore volumes in Table S1. Contrary to the reviewer’s comment,\nthe total pore volume is not the same across the samples. As shown in Table S1, the pore volume of LFCTO-0.2 is\nalmost double that of LFCTO-0.1.\nOriginal Comment: The authors claim that oxygen vacancies exist in LFTCO. However, there is insufficient\nexperimental characterization to confirm the presence of oxygen vacancies (O-v).\nIn the revised manuscript, the authors provide Raman spectra comparing LFO and LFCTO-0.3 in Figure S5. However,\nthis evidence is not sufficiently convincing. EPR characterization would be more appropriate for confirming oxygen\nvacancies. Additionally, the paper does not evaluate changes in surface chemistry after adsorption.\nWe thank the reviewer for the careful assessment. While we agree that EPR is a powerful tool, it is not suitable for\nour samples due to their magnetic properties. We previously performed EPR analysis but encountered limitations due\nto the ferromagnetic nature of the materials. Therefore, we used Raman spectroscopy and XPS analysis to confirm\nthe presence of oxygen vacancies. Both techniques provide robust evidence for oxygen vacancies. We believe these\nmethods are appropriate and sufficient for characterizing oxygen vacancies in our materials.\nGiven the presence of oxygen functional groups and the electron transfer between Fe²⁺/Fe³⁺ and Co²⁺/Co³⁺ redox\ncouples, it is questionable whether adsorption is indeed the dominant removal mechanism. Are the adsorption models\nused in the paper appropriately justified?\nThank you for your insightful comments regarding the removal mechanism and the role of Fe2+/Fe3+ and Co2+/Co3+\nredox pairs in our study. To address your concerns about the dominant removal mechanism, we conducted additional\nfree radical trapping experiments to provide direct evidence for our mechanistic interpretation. The experimental\nresults (shown in Figure 1) demonstrate that no hydroxyl radicals (•OH) were detected when either LFCTO adsorbent\nor H O existed independently, confirming the absence of oxidative degradation under these conditions. However,\n2 2\nwhen LFCTO and H O coexisted, significant •OH signals were observed, indicating the effective activation of H O\n2 2 2 2\nby Fe2+/Fe3+ and Co2+/Co3+ redox pairs. These findings substantiate our proposed mechanism that adsorption\ndominates the HA removal process in the absence of H O , while the redox pairs primarily contribute to adsorbent\n2 2\nregeneration through H O activation rather than direct oxidative removal. This mechanistic understanding not only\n2 2\nvalidates our initial conclusions but also provides valuable insights for the practical application and regeneration of\nthe LFCTO adsorbent.\nDMPO- ·OH\n© ©\n© ©\n)\n.u\n.a LFCTO+H O\n( 2 2\ny\ntis\nn\ne\ntn LFCTO\nI\nH O\n2 2\n3460 3480 3500 3520 3540\nMagnetic Field (G)\nFigure S9. Hydroxyl radical capture spectra under H O , LFCTO and LFCTO+ H O conditions.\n2 2 2 2\nFigure 3c: At higher pH values, the LFCTO surface could undergo oxidation. The authors should use XRD to\ncharacterize potential phase changes and further explore the underlying mechanisms.\nThank you for your valuable suggestion regarding the potential oxidation of LFCTO under high pH conditions.\nFollowing your recommendation, we conducted XRD characterization of LFCTO samples after exposure to various\npH conditions (pH values ranging from 7 to 11). The XRD patterns (shown in Figure 2) reveal that all characteristic\ndiffraction peaks of LFCTO remained unchanged after high pH treatment, with no emergence of new peaks or phase\ntransitions. The peaks at 2θ = 32.2, 46.2, 57.4 and 76.7 maintained their positions and intensities, indicating the\npreservation of the original crystal structure. These results collectively demonstrate the remarkable structural stability\nof LFCTO even under elevated pH conditions, which can be attributed to the robust crystal structure and strong metal-\noxygen bonds in the material framework.\nFigure S6. XRD spectra of LFCTO samples treated under high pH conditions.\n).u.a(\nytisnetnI\n2 0 3 0 4 0\n5\ne\n0\nta ( ° )\n6 0\np\np\np\n7 0\nHHH === 791\n1\n8 0"}, {"round": 3, "reviewer_comments": {"reviewer_2": "The manuscript reports a B-site dual-doped LaFeO₃ material for rapid adsorptive removal of humic acid (HA) and facile\nregeneration. After revision, the manuscript has been improved to some extent.\nHowever, several key issues remain:\nNovelty and Introduction: The current innovation mainly lies in the relatively high adsorption capacity and the use of H₂O₂ for\nregeneration. These features alone are not sufficient to demonstrate strong originality, as similar perovskite-based\nadsorbents and Fenton-regenerable systems have been widely reported. The Introduction should be strengthened to clearly\ndefine the research gap and highlight what is new in this work—such as the specific synergistic effect of dual-metal doping,\nmechanism insight, or structure–function correlation.\nRemoval mechanism: Please clarify whether HA removal occurs mainly via adsorption or involves Fenton-like oxidative\ndegradation. Provide supporting evidence such as TOC analysis, desorption experiments, or radical identification to\ndistinguish between adsorption and degradation processes.\nOxygen vacancies: To support the claim of oxygen-vacancy-induced activity enhancement, solid-state EPR characterization\n(g ≈ 2.003) is strongly recommended.\nH₂O₂ regeneration: H₂O₂ may alter the adsorbent’s structure and oxygen-vacancy concentration. Comparative\ncharacterization before and after regeneration (XRD, XPS, and EPR) is suggested to explain the observed performance\nchanges.\nOverall evaluation:\nThe manuscript shows potential, but the current novelty—mainly large adsorption capacity and H₂O₂ regeneration—is\nlimited. The authors should further clarify the mechanism, substantiate the oxygen-vacancy contribution, and better articulate\nthe scientific innovation before the paper can be considered for external review."}, "author_response": "Reviewer #2 Comments:\nComments 1: Novelty and Introduction: The current innovation mainly lies in the\nrelatively high adsorption capacity and the use of H O for regeneration. These\n2 2\nfeatures alone are not sufficient to demonstrate strong originality, as similar\nperovskite-based adsorbents and Fenton-regenerable systems have been widely\nreported. The introduction should be strengthened to clearly define the research gap\nand highlight what is new in this work-such as the specific synergistic effect of dual-\nmetal doping, mechanism insight, or structure-function correlation.\nWe sincerely thank the reviewer for their insightful and constructive comments,\nwhich have helped us clarify and better emphasize the novelty and mechanistic\ncontributions of our work. In the revised manuscript, we have substantially\nstrengthened the introduction to more clearly define the research gap and to highlight\nthe specific innovations of this study, including the synergistic effects of dual-metal B-\nsite doping, mechanistic insight into adsorption and regeneration, and the establishment\nof structure-function correlations.\nIt is well-established that the B-site of perovskites can be doped with various metal\nions (e.g., Cu, Fe, Co, Mn) or their combinations to modulate the coordination\nenvironment for oxygen vacancy formation. In general, such doping strategies increase\nthe surface oxygen vacancy content, thereby providing more active sites for adsorption\nand altering the electronic structure. However, most prior studies have focused on the\nmacroscopic improvement of catalytic or adsorptive performance, while the deeper\nmechanistic understanding of the underlying synergistic effects introduced by specific\ndopant combinations has often remained limited. In this work, we go beyond simply\nreporting a high adsorption capacity or the use of H₂O₂ for regeneration by expanding\nthe understanding of B-site bimetallic engineering. Specifically, we elucidate how\nrational B-site bimetallic doping regulates intrinsic material properties to achieve a\nstep-change in performance, particularly in adsorption capacity, magnetic\nrecoverability, and facile regeneration. By carefully engineering B-site of perovskite\nLaFeO₃ with a synergistic Co/Ti dual-doping strategy—rationally selected based on\ntheir complementary roles (Co endows redox activity to promote oxygen vacancy\nformation and charge transfer, while Ti modulates lattice stability and spin\nconfiguration to fine-tune magnetic performance)—and combining systematic\nexperimental characterizations and density functional theory (DFT) calculations, we\ndemonstrate that this strategy not only effectively increases oxygen vacancy\nconcentration, and improves surface hydrophilicity that provide additional active sites,\nbut also synergistically enhances saturation magnetization for the recovery of the\nmaterial for practical applications in water treatment.\nTaking the enhanced HA adsorption as an example, we demonstrated that the\ncoordinated bimetallic doping optimizes the atomic and electronic structure, leading to\nkey enhancements in adsorption performance: elevated oxygen vacancy density\n(providing abundant binding sites for HA molecules) and improved surface\nhydrophilicity (strengthening interfacial interactions between the adsorbent and\naqueous HA). These are core targets in conventional adsorbent design for boosting\nadsorption capacity. Beyond these traditional optimizations, our dual-doping strategy\nfurther achieves deliberate magnetic functionalization by regulating the spin\nconfiguration of the LaFeO₃ lattice: the synergistic incorporation of Co (with four\nunpaired 3d electrons) and Ti (with one unpaired 3d electron) disrupts the intrinsic G-\ntype antiferromagnetic order of pristine LaFeO₃, generating a significant net saturation\nmagnetization (Ms = 7.36 emu/g for LFCTO-0.3) that enables efficient and rapid\nadsorbent recovery via external magnetic separation. Specifically, the proposed\nmechanism and the underlying structure-activity relationship were established through\nour DFT calculations. Pristine LaFeO exhibits a G-type antiferromagnetic order with\n3\nzero net magnetic moment. Co-doping introduces multiple unpaired electrons, which\ndisrupts the antiferromagnetic structure and enhances the overall magnetism. In\ncontrast, Ti single-doping introduces only one unpaired electron, thereby reducing the\ntotal magnetic moment of the system (Fig 1i-n, Manuscript). This theoretical validation\ndemonstrates that by rationally selecting and balancing the type and proportion of B-\nsite metal dopants, we can predictably and deliberately tailor the magnetic properties.\nMeanwhile, our mechanistic investigation was further strengthened by employing\nLF-NMR to address a key question: how doping alters the surface hydrophilicity and\nwater interaction (Fig 3i, Manuscript). The results unambiguously show that our doping\nmodification creates a more hydrophilic surface (shorter transverse relaxation time for\nLFCTO-0.3), leading to a higher bound water content that indirectly enhances the\nadsorption process. This unique characterization not only solidifies our mechanism but\nalso highlights the innovative approach of our work in fundamentally understanding\nproperty-performance relationships.\nFurthermore, we elucidate the origin of the enhanced adsorption performance at\nthe atomic level by employing IGMH analysis to visualize the interactions between HA\nand the material surface. The results indicate that pristine LaFeO adsorbs HA\n3\nmolecules primarily through well-defined Fe–O interactions. In contrast, Co single-\ndoping induces a magnetic transition while only moderately enhancing HA adsorption,\nwhich suggests that magnetism alone is not the dominant driver. Most importantly, the\nCo/Ti dual-doping strategy achieves markedly enhanced adsorption by synergistically\nincreasing both adsorption site density and surface anisotropy. This is vividly\ndemonstrated by the pronounced and widespread attractive regions observed in the\nIGMH plot (Fig 3j-l, Manuscript), confirming the superior adsorption performance\nachieved through rational dual-doping design.\nTherefore, the innovation of our work extends beyond merely reporting a high\nadsorption capacity or the use of H O for regeneration. The core novelty lies in the\n2 2\nstrategic design of a B-site dual-doped perovskite that is conceptually envisioned and\nexperimentally validated to synergistically enhance adsorption, regeneration, and\nmagnetic recovery performance. This deliberate design is substantiated through\nprofound mechanistic insight and advanced characterizations, where we elucidate how\ndual-doping creates abundant oxygen vacancies, and ideal magnetic properties, and\nestablish the definitive structure-function relationship governing the high performance.\nUltimately, these fundamental findings converge in the demonstration of an integrated\nadsorption-regeneration platform, showcasing a sustainable and efficient strategy for\nwater treatment.\nTo reflect these points, we have substantially rewritten Paragraphs 2 and 3 of the\nIntroduction. The corresponding changes in the Introduction are provided in the revised\nmanuscript:\nManuscript Line 59-104: “Perovskite oxides, represented by the formula ABO ,\n3\nare a diverse family of mixed metal oxides with highly tunable compositions and\nphysicochemical properties, enabling applications in solar cells38,39, fuel cells40,41,\nelectromagnetic materials42-44, chemical sensors45, and catalysts46, 47. In the perovskite\nlattice, A-site elements, typically rare earth metals or alkaline earth metals (e.g., La, Pr,\nGd) with larger ionic radii, occupy the central position of the cubic lattice and stabilize\nthe framework, while B-site elements, usually transition metals (e.g., Ti, Mn, Fe, Co,\nCr) with smaller ionic radii, occupy corner-sharing octahedra and predominantly\ngovern the electronic structure, redox behavior, and surface chemistry48. This structural\nflexibility offers a powerful platform for tailoring properties relevant to water treatment,\nincluding surface charge, hydrophilicity, oxygen vacancy concentration, and magnetic\nseparability49. For instance, Ti doping in LaFeO perovskites prepared by a sol-gel\n3\nmethod enhances catalytic activity, improving the removal and mineralization of\nchlorophenols50, and can also modulate magnetic properties by influencing Fe spins\nthrough the unpaired electrons of Ti3+ ions51. Similarly, Co doping introduces abundant\nactive sites that accelerate methane activation, whereas Sr doping increases oxygen\nvacancies, facilitating oxygen-ion migration and improving reactivity and resistance to\ncoking52. However, despite extensive work on such doping strategies, the use of\nperovskite oxides as integrated adsorbent–regenerable materials for pollutant removal,\nincluding humic acid, remains underdeveloped. Conventional approaches to\nperovskite-based adsorbent enhancement still focus primarily on maximizing\nadsorption capacity, with comparatively less consideration to regeneration efficiency,\nenergy input, or compatibility with continuous operation. This leads to a persistent gap\nbetween material-level optimization and process-level requirements. Moreover, the\nrational design and mechanistic understanding of dual B-site doping remain limited,\neven though exploiting complementary effects between metal ions of differing ionic\nradii and electronic structures could, in principle, simultaneously enhance multiple\nfunctionalities and improve material stability during extended use53,54.\nHere we implement a dual B-site doping strategy in LaFeO ,\n3\nsimultaneously\nintroducing Ti and Co to obtain a perovskite-type adsorbent (LFCTO) for efficient\nremediation of HA-contaminated water. LFCTO exhibits rapid HA uptake and a high\nmaximum adsorption capacity of 381 mg/g in batch adsorption tests, which is attributed\nto its increased specific surface area and improved surface hydrophilicity. Concurrently,\nthe Co/Ti centers act as Fenton-active sites for H O activation, allowing in situ\n2 2\noxidative degradation of adsorbed HA and efficient regeneration of the adsorbent under\nmild conditions. The dual-doping strategy also enhances the ferromagnetism of LFCTO,\nenabling efficient magnetic separation and reuse. By coupling systematic experimental\ncharacterization with density functional theory and related quantum-chemical\ncalculations, we elucidate the atomic-level synergistic effects of dual B-site doping that\nunderpin both adsorption and regeneration. We demonstrate that LFCTO adsorbent can\nbe regenerated in situ within a continuous adsorption-regeneration cycle, or\nmagnetically recovered for ex situ chemical regeneration. LFCTO maintains its\ncrystalline structure and stable HA removal performance for over 280 hr of continuous\noperation, enabling an energy-efficient, one-step regeneration route that avoids the\nhigh-temperature calcination or solvent-intensive elution required by conventional\nadsorbents.”\nComments 2: Removal mechanism: Please clarify whether HA removal occurs\nmainly via adsorption or involves Fenton-like oxidative degradation. Provide\nsupporting evidence such as TOC analysis, desorption experiments, or radical\nidentification to distinguish between adsorption and degradation processes.\nWe thank the reviewer for the insightful comment. We confirmed that HA removal\noccurs firstly by adsorption followed by subsequent Fenton-like oxidative degradation.\nDuring the adsorption phase, HA molecules are captured and enriched on the external\nand internal surfaces of LFCTO-0.3 via surface interactions, enabling effective\nseparation from the aqueous phase as the first step. In the subsequent regeneration phase,\nthe introduction of H O into the system triggers a Fenton-like reaction catalyzed by\n2 2\nthe bimetallic sites (e.g., Fe, Co) on the material surface, which generates highly\noxidative radicals. These radicals rapidly degrade the adsorbed HA molecules into\nsmaller fragments or mineralize them completely, thereby clearing the adsorption sites\nin situ and achieving efficient regeneration and reuse of the material.\nIn response to the reviewer's request, we have performed further characterizations\nto support our findings. The TOC analysis of the aqueous solution revealed a significant\ndecrease from 7.85 mg/L (before adsorption) to 1.83 mg/L (after adsorption) (Table R1).\nThe significant decrease (76.69%) in TOC of the aqueous phase before and after\nadsorption confirms the effective removal of organic carbon from the water phase.\nMoreover, the adsorption kinetics, which fit a classic model and rapidly reach\nequilibrium, indicate that HA removal is achieved through occupancy of the surface\nsites, leading to temporary site blocking (Fig S3, S8, Supporting Information). EPR\nspectra confirmed that no hydroxyl radicals were detectable during the adsorption\nprocess (Fig S10, Supporting Information), ruling out the occurrence of a Fenton-like\noxidation process at this stage. Together, these results robustly support our findings that\nthe initial HA removal mechanism is adsorption, while the subsequently added H O\n2 2\nacts specifically in the regeneration stage via a Fenton-like reaction to degrade the pre-\nadsorbed HA molecules.\nTable R1. TOC test data of the HA solution before and after adsorption.\nSample TC (mg/L) IC (mg/L) TOC (mg/L)\nBefore adsorption 8.75 0.91 7.85\nAfter adsorption 2.59 0.77 1.83\nIn response to the reviewer’s comment, we have revised the manuscript to\nincorporate these new characterizations and clarifications, as follows:\n1. Manuscript Line 230-234: “Nevertheless, it should be noted that all HA adsorption\nexperiments were conducted in the absence of H₂O₂, so that HA removal in this\nstage proceeds exclusively via adsorption onto LFCTO; H₂O₂ is introduced only in\nthe subsequent regeneration step, where it participates in a Fenton-like process to\noxidatively degrade the pre-adsorbed HA and restore the adsorption sites.”\n2. Manuscript Line 263-268: “Additionally, TOC analysis was performed on the\naqueous solutions before and after the adsorption process to quantify the organic\nmatter removal efficiency (Supplementary Table S2). TOC decreased from 7.85\nmg/L (pre-adsorption) to 1.83 mg/L (post-adsorption) while IC only slightly\nchanged (0.91→0.77 mg/L), indicating that HA is removed from the aqueous phase\nmainly via adsorption onto LFCTO-0.3.”\n3. Supporting Information Line 288: We have added Table S2 to provide the detailed\nTOC/IC data for the HA solution before and after adsorption.\nComments 3: Oxygen vacancies: To support the claim of oxygen-vacancy-induced\nactivity enhancement, solid-state EPR characterization (g=2.003) is strongly\nrecommended.\nWe thank the reviewer for their valuable suggestions regarding the oxygen\nvacancies. In response, we have performed solid-state EPR measurements as well as\nXPS to confirm the presence of oxygen vacancies, and to provide direct experimental\nevidence to support that oxygen vacancies contribute to the activity enhancement in our\nstudy. The characteristic signal of the LFCTO sample at g≈2.003 in Fig R1a confirms\nthe presence of oxygen vacancies in the material, which directly supports the finding\nthat oxygen vacancies induce activity enhancement. In addition, XPS analysis of the O\n1s region (Fig R1b) reveals a systematic evolution of the oxygen-vacancy (OV) fraction\nacross different stages. In fresh LFCTO, the OV fraction is 24.90%; it increases to 30.02%\nafter HA adsorption and further to 32.69% after H₂O₂-mediated HA degradation. This\ntrend is consistent with the EPR results and indicates that oxygen vacancies are not only\npresent but also dynamically involved in the adsorption–regeneration process. The\nslight increase in oxygen-vacancy content after adsorption and degradation suggests\nthat mild oxidative reconstruction (e.g., removal of surface-adsorbed HA residues or\nhydroxyl groups) and the structural flexibility imparted by Ti/Co dual doping expose\nor stabilize additional vacancies, thereby contributing to sustained activity.\nFig R1 | Oxygen vacancies-related characterization of LFCTO across different\nstates (fresh, post-adsorption, and post-degradation). a, EPR spectra of oxygen\nvacancies. b, XPS spectra of the O 1s.\nTo solidify the link between oxygen vacancies and activity enhancement in the\nmain manuscript, we have revised the text as follows:\n1. Manuscript Line 454-464: “To clarify the dominant humic acid (HA) removal\nmechanism, additional O 2⁻ trapping and oxygen vacancies experiments were\nconducted (Supplementary Fig S12-13). The O 2⁻ signal did not disappear during\neither adsorption and degradation. In fresh LFCTO, the oxygen vacancy (O )\nV\nfraction is 24.90%, increasing to 30.02% after HA adsorption and further to 32.69%\nfollowing H₂O₂-mediated HA degradation, consistent with the EPR\ncharacterization trend. The slight increase in O 2⁻ is attributed to the exposure of\npreviously hidden vacancies via H₂O₂-induced mild oxidative reconstruction\n(removing surface-adsorbed HA residues or hydroxyl groups) and to the structural\nflexibility imparted by Ti/Co dual doping, which prevents excessive oxidation-\ninduced lattice collapse and enables stable accumulation of oxygen vacancies.”\n2. Supporting Information, Line 238: Fig S12 (DMPO–O₂⁻ radical-trapping EPR\nspectra of LFCTO across different states (fresh, post-adsorption, and post-\ndegradation)) was added.\n3. Supporting Information, Line 241: Fig S13 (Oxygen vacancies-related\ncharacterization of LFCTO across different states (fresh, post-adsorption, and post-\ndegradation). a, EPR spectra of oxygen vacancies. b, XPS spectra of the O 1s) was\nadded.\nComments 4: H O regeneration: H O may alter the adsorbent's structure and\n2 2 2 2\noxygen-vacancy concentration. Comparative characterization before and after\nregeneration (XRD, XPS, and EPR) is suggested to explain the observed performance\nchanges.\nWe thank the reviewer for the valuable comment regarding the H₂O₂ regeneration\nand its potential impact on the adsorbent's structure. Following the reviewer’s\nrecommendation, we conducted additional comparative characterizations of LFCTO\nbefore and after H₂O₂-based regeneration, including XRD, XPS, and radical-trapping\nEPR measurements. For these tests, an adsorbent dosage of 0.2 g/L was used to remove\n30 mg/L HA. After adsorption, H\n2\nwas introduced to degrade the adsorbed HA and\nregenerate the adsorbent, and the regenerated adsorbent was collected and characterized.\nThe XRD patterns (Fig R2) reveal that all characteristic diffraction peaks of LFCTO\nremained essentially unchanged across the fresh, post-adsorption, and post-degradation\nstates. No new peaks or phase transitions appear, and the peak positions are well\npreserved, indicating that the original perovskite crystal structure is maintained during\nthe adsorption–regeneration cycle. XPS analysis (Fig R3) further confirms that H₂O₂\nregeneration does not cause significant changes to LFCTO’s element valence states or\noverall surface chemical framework, supporting the structural stability of the adsorbent\nunder the applied H₂O₂ treatment.\nFig R2. XRD spectra of LFCTO across different states (fresh, post-adsorption, and\npost-degradation).\nFig R3. XPS spectra of LFCTO across different states (fresh, post-adsorption, and\npost-degradation).\nIn addition, we supplemented EPR radical trapping experiments, as shown in Fig\nR4,5. Fresh LFCTO exhibits a clear DMPO–OH signal, indicating that it possesses\nabundant active sites capable of generating OH through a Fenton-like reaction. During\nthe adsorption stage, the active sites on the LFCTO surface are partially occupied by\ntarget pollutants, reducing the number of sites available for OH generation, leading to\na decreased DMPO–OH signal. In the subsequent degradation stage (with H₂O₂), OH\nis generated and then consumed in the oxidative degradation of the adsorbed HA, but\nthe persistence of the DMPO–OH signal confirms that hydroxyl radicals actively\nparticipate in pollutant removal. Furthermore, the DMPO–O₂⁻ signal observed in Fig\nR5 verifies the generation of O₂⁻ species. Together, these data differentiate the\nadsorption step from the Fenton-like degradation step and confirm that HA removal\ninvolves both adsorption and radical-driven oxidation.\nFig R4. DMPO–OH radical-trapping EPR spectra of LFCTO across different states\n(fresh, post-adsorption, and post-degradation).\nFig R5. DMPO–O₂⁻ radical-trapping EPR spectra of LFCTO across different states\n(fresh, post-adsorption, and post-degradation).\nThese results collectively demonstrate that H₂O₂ regeneration does not induce\nsignificant crystal-structure degradation or major changes in the overall surface\nchemical framework. The observed performance changes can therefore be attributed\nprimarily to the evolution of oxygen-vacancy concentration and related surface/defect\neffects on adsorption and regeneration, rather than to structural collapse, which supports\nthe practicality and robustness of H₂O₂-based regeneration for LFCTO.\nWe have made the following changes to the manuscript to clarify the effects of\nH₂O₂ regeneration on crystal structure, surface elemental composition, and radical\nspecies:\n1. Manuscript Line 451-454: “During the adsorption stage, the active sites on the\nLFCTO surface are partially occupied by target pollutants, reducing the number of\nsites available for OH generation. In the subsequent degradation stage, the\ngenerated OH is consumed in the H₂O₂-driven oxidative degradation of the\nadsorbed HA(Supplementary Fig S11).”\n2. Manuscript Line 475-481: “The crystal structure and surface elemental composition\nof LFCTO in the fresh, post-adsorption, and post-degradation states were\ncharacterized by X-ray diffraction and X-ray photoelectron spectroscopy\n(Supplementary Figs S16-17). Comparison of the patterns and spectra across these\nstates shows only minor variations in peak intensity and surface element ratios, with\nno evidence of phase transformation or lattice collapse after degradation,\ndemonstrating the structural stability of the perovskite under H₂O₂ regeneration.”\n3. Supporting Information, Line 235: Figure S11 (DMPO–OH radical-trapping EPR\nspectra of LFCTO across different states (fresh, post-adsorption, and post-\ndegradation)) was added.\n4. Supporting Information, Line 261-267: Figure S16 (XRD spectra of LFCTO across\ndifferent states (fresh, post-adsorption, and post-degradation)) and Figure S17 (XPS\nspectra of LFCTO across different states (fresh, post-adsorption, and post-\ndegradation)) were added."}, {"round": 4, "reviewer_comments": {"reviewer_2": "I have no other comment."}, "author_response": ""}]

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