# Carica papaya seed- derived functionalized biochar: an environmentally friendly and efficient alternative for dye adsorption

Francisco J Cano, Rocío Sánchez-Albores, A. Ashok, J. Escorcia-García, A.

Cruz-Salomón, Odín Reyes-Vallejo, P J Sebastian, S. Velumani

► To cite this version:

Francisco J Cano, Rocío Sánchez-Albores, A. Ashok, J. Escorcia-García, A. Cruz-Salomón, et al. Carica papaya seed-derived functionalized biochar: an environmentally friendly and efficient alternative for dye adsorption. Journal of Materials Science: Materials in Electronics, 2025, 36 (11), pp.663. 10.1007/s10854-025-14725-y. hal-05439929

HAL Id: hal-05439929

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Submitted on 3 Jan 2026

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<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons CC BY 4.0 - Attribution - International License
*Carica papaya* seed-derived functionalized biochar:
an environmentally friendly and efficient alternative
for dye adsorption

Francisco J. Cano<fnref n="1" />, Rocío Sánchez-Albores<fnref n="2" />, A. Ashok<fnref n="3" />,<fnref n="4" />, J. Escorcia-García<fnref n="5" />, A. Cruz-Salomón<fnref n="2" />,
Odín Reyes-Vallejo<fnref n="6" />,\* P. J. Sebastian<fnref n="7" />, and S. Velumani<fnref n="8" />

Received: 1 December 2024

Accepted: 2 April 2025

© The Author(s), 2025

ABSTRACT

Biochar is emerging as a sustainable material for addressing critical environmental challenges, including water pollution and carbon sequestration. In this study, biochar derived from *papaya seed* biomass via slow pyrolysis was investigated for its efficacy in removing organic dyes from aqueous solutions. Both biochar and chemically functionalized biochar were comprehensively characterized using XRD, FTIR, XPS, BET surface area analysis, and SEM/EDS to elucidate their structural, morphological, and surface properties. The adsorption performance was assessed using methylene blue (MB), rhodamine B (RhB), malachite green (MG), and methyl orange (MO) as model contaminants, with particular attention to adsorption kinetics and equilibrium behavior. The unmodified biochar exhibited moderate adsorption capacities, primarily driven by π-π interactions and a limited number of functional groups, achieving removal efficiencies of 55.5, 97.8, and 12.9% for MB, MG, and RhB, respectively, while MO showed negligible adsorption. Remarkably, chemical functionalization with KMnO₄ significantly enhanced biochar performance, increasing surface area and functional group diversity. This modification enabled the complete removal (100%) of MB and MG within six minutes, alongside substantial improvements for RhB and MO, achieving 100 and 88% removal, respectively. Adsorption kinetics followed a pseudo-second-order model (R<fnref n="2" /> > 0.99), indicating chemisorption as the dominant mechanism, while Langmuir isotherms (R<fnref n="2" /> = 0.993 – 0.999) suggested

<footnote n="1">¹Institute of Molecules & Materials of Le Mans (IMMM) – UMR CNRS 6283, Le Mans Université, 72000 Le Mans, France</footnote>

<footnote n="2">²Escuela de Ciencias Químicas, Universidad Autónoma de Chiapas (UNACH), 29140 Ocozocoautla de Espinosa, Chiapas, Mexico</footnote>

<footnote n="3">³Instituto de Física, Universidad Nacional Autónoma de México, 04510 Mexico City, Mexico</footnote>

<footnote n="4">⁴Postdoctoral Scholarship Program at DGAPA-UNAM, Coyoacán, C.P. 04510 Mexico City, Mexico</footnote>

<footnote n="5">⁵Conacyt-Cinvestav-Unidad Salillo, Ave. Industria Metalúrgica 1062, Parque Industrial, 25900 Ramos Arizpe, Coahuila, Mexico</footnote>

<footnote n="6">⁶Sección de Electrónica de Estado Sólido-Ingeniería Eléctrica (SEES), CINVESTAV - IPN, San Pedro Zacatenco, 07360 Mexico City, Mexico</footnote>

<footnote n="7">⁷Instituto de Energías Renovables-UNAM (IER-UNAM), 62580 Temixco, Morelos, Mexico</footnote>

<footnote n="8">⁸J. Mike Walker '66 Department of Mechanical Engineering, Texas A&M University, College Station, TX 77843, USA</footnote>
monolayer adsorption. Additionally, the intraparticle diffusion model highlighted the synergistic contributions of surface interactions and pore diffusion to overall adsorption efficiency. These findings establish chemically functionalized papaya seed biochar as a highly efficient, low-cost adsorbent for dye removal, underscoring its potential for environmental remediation. The innovative utilization of papaya seed biomass not only adds value to agricultural waste but also advances sustainable water treatment strategies, reinforcing its significance in environmental science and engineering.

# 1 Introduction

Biochar, a carbonaceous material produced via the controlled pyrolysis of biomass [1], has gained increasing prominence as a multifunctional resource for addressing critical environmental challenges. Its high porosity, structural robustness, and diverse surface functionalities make it a highly versatile and efficient adsorbent, with broad applications in environmental remediation and sustainable energy solutions [2]. Unlike conventional adsorbents such as functionalized carbon and zeolites, biochar is derived from renewable feedstocks, including agricultural and organic waste—such as pineapple peels, moringa leaves, and coffee grounds—thus promoting a sustainable waste valorization approach [3]. Its applicability extends across multiple disciplines, including soil amendment [4], carbon sequestration [5], and wastewater treatment [6], where it plays a pivotal role in the removal of organic and inorganic contaminants. These distinctive physicochemical properties, coupled with its low-cost production and scalability, position biochar as a promising alternative for advancing circular economy principles and enhancing sustainable environmental management strategies.

Among the most persistent environmental pollutants, synthetic dyes pose a significant ecological threat due to their toxicity, chemical stability, and bioaccumulation potential in aquatic ecosystems [6]. The discharge of dye-laden industrial effluents—originating primarily from the textile, leather, paper, and plastics industries—represents a major environmental challenge, as these pollutants exhibit high resistance to biodegradation and can persist in water bodies for prolonged periods [7]. Even at

low concentrations, synthetic dyes significantly reduce light penetration, thereby disrupting aquatic photosynthesis and altering entire ecosystems [8]. Moreover, many dyes and their degradation byproducts exhibit mutagenic, carcinogenic, and cytotoxic properties, posing serious risks to human health and aquatic biota [9]. Conventional wastewater treatment technologies, including coagulation, flocculation, and advanced oxidation processes, often face limitations such as high operational costs, secondary sludge generation, and incomplete dye mineralization [10]. Consequently, there is an urgent need for cost-effective, sustainable, and highly efficient adsorbents capable of removing these pollutants from aqueous media.

Although extensive research has been conducted on biochar for dye removal, the utilization of *Carica papaya* seeds—a highly abundant yet underutilized agro-industrial byproduct—remains largely unexplored. Their unique biochemical composition, including lignocellulosic content and mineral constituents, enhances the potential for producing biochar with favorable adsorption properties [11, 12]. Rich in carbon and bioactive compounds, *Carica papaya* seeds offer a dual advantage: they present a sustainable route for waste valorization while potentially yielding biochar with enhanced adsorption capacity. However, the current literature lacks a comprehensive assessment of the synthesis, chemical functionalization, and adsorption performance of biochar derived from these seeds, highlighting a critical research gap that this study seeks to address.

Although biochars derived from various biomass sources—including microalgae [13], pistachio shells [14], rice husks [15, 16], corn straw [17], and pineapple crown biomass [18]—have demonstrated high efficacy
in dye adsorption from aqueous media, previous studies have also validated the potential of biochars obtained from papaya peels [19], stems, and leaves [20], as well as papaya seed-chitosan hydrogel beads [21], for the removal of these pollutants. However, the adsorptive performance of chemically functionalized biochar derived from *Carica papaya* seeds remains largely underexplored, particularly in terms of its efficacy across a broad spectrum of organic dyes and the fundamental interaction mechanisms governing adsorption. This study aims to bridge this knowledge gap by systematically assessing the adsorption capacity, kinetic behavior, and molecular interaction mechanisms of *Carica papaya* seed-derived biochar for multiple dye contaminants. By doing so, it seeks to provide novel insights into optimizing biochar properties for enhanced wastewater treatment efficiency.

There exist different physical, chemical, and biological strategies that can modify the properties of biochar for higher pollutant removal efficiency [22]. A comparative analysis of different functionalization methods, highlighting their respective advantages and limitations, is presented in Table S1 [23]. In this context, chemical functionalization has emerged as a key strategy for enhancing the intrinsic adsorption capabilities of biochar [24]. Agents such as potassium permanganate (KMnO₄), phosphoric acid (H₃PO₄), and potassium hydroxide (KOH) induce significant structural and chemical modifications, including increased porosity and the incorporation of oxygenated functional groups [25, 26], thereby optimizing interactions such as electrostatic attractions, hydrogen bonding, and π-π stacking [17]. In this study, KMnO₄ is employed as the functionalization agent due to its proven efficacy in controlled pore development and selective surface oxidation, ultimately broadening the adsorptive interactions available for both cationic and anionic contaminants.

While advanced materials such as nanocomposites and photocatalysts have been investigated for wastewater treatment [27–30], biochar retains distinct advantages, including cost-effectiveness, facile synthesis, and operational efficiency under ambient conditions. Furthermore, its renewable origin and scalability reinforce its potential for deployment in both industrial and rural settings, aligning with sustainability objectives and environmental stewardship principles.

This research pioneers the valorization of *Carica papaya* seeds as a precursor for the synthesis of chemically functionalized biochar. Through a comprehensive characterization process—including X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), Brunauer–Emmett–Teller (BET) surface area analysis, and scanning electron microscopy (SEM)—the structural and chemical transformations induced by KMnO₄ functionalization are elucidated. The specific objectives of this study are: (i) to demonstrate the feasibility of converting an underutilized agro-industrial residue into a high-performance adsorbent; (ii) to optimize the functionalization process by tailoring the porous architecture and surface functionalities of biochar; and (iii) to systematically evaluate its adsorption capacity, kinetics, and molecular interaction mechanisms using a suite of model organic dyes (methylene blue, malachite green, rhodamine B, and methyl orange). By addressing these research gaps, this study not only establishes the novel contribution of *Carica papaya* seed-derived biochar to wastewater remediation but also provides a benchmark for the development of sustainable, cost-effective, and high-performance materials for environmental applications.

## 2 Experimental details

### 2.1 Preparation of biochar and functionalized biochar

The biochar was synthesized by a controlled pyrolysis process. The papaya seeds were first thoroughly washed with water to remove extraneous matter, followed by air drying for one week to reduce the surface moisture content. The biomass was then subjected to a second washing process and placed in an oven for drying at 65 °C for 48 h to achieve complete desiccation. The sieved biomass was then placed in a ceramic crucible and introduced into a pyrolysis reactor under a constant flow of nitrogen gas to create an inert atmosphere. The reactor was heated to a target temperature of 450 °C at a controlled heating rate of 10 °C/min and maintained at this temperature for 1 h to facilitate thermal decomposition of the organic matter. Following the pyrolysis process, the reactor was allowed to cool to ambient temperature by natural convection. Finally, the resulting biochar was ground
using a mortar to achieve a homogeneous particulate form.

Subsequently, the functionalized version was performed using a method adapted from established protocols for the synthesis of graphene oxides [31]. The functionalization procedure commenced with the preparation of a mixed acid solution, consisting of 43.2 mL of concentrated sulfuric acid (H₂SO₄, ACS reagent, 95.0–98.0%) and 4.8 mL of concentrated orthophosphoric acid (H₃PO₄, 85%). This acid mixture was then gradually added to 0.1 g of biochar powder under continuous stirring for 3 h while maintaining the system in a cold bath at 4 °C to ensure temperature control and prevent exothermic reactions. Following this, 6 g of potassium permanganate (KMnO₄) was introduced into the reaction mixture, and the system was allowed to react for an additional 2 h. After this period, a solution containing 12 mL of hydrogen peroxide (H₂O₂, 30 wt%) and 13 mL of hydrochloric acid (HCl, 36.5–38.0%) was added dropwise to the mixture under vigorous stirring for 30 min. The reaction was subsequently quenched by dilution with distilled water to neutralize the residual acidity. The exfoliation of the treated material was then performed through sonication for 30 min. Following exfoliation, the resulting dispersion was subjected to multiple washing cycles, until the pH of the supernatant reached a neutral value of 7. Finally, the material was collected and dried in an oven at 65 °C for 12 h to yield the functionalized biochar.

## 2.2 Characterization of materials

The crystalline structure of biochars was investigated using X-ray diffraction (XRD) analysis, conducted on a Bruker D2 PHASER diffractometer. The system utilized Cu-Kα radiation (λ = 1.5405 Å) under operational conditions of 40 kV and 30 mA. Diffraction data were recorded over a 2θ range spanning 7° to 80°, with a scanning rate of 0.02°/min. Fourier transform infrared (FTIR) spectroscopy was employed to identify functional groups within the samples, using a PerkinElmer Frontier spectrometer equipped with a diamond attenuated total reflectance (ATR) accessory. The spectra were acquired within the wavenumber range of 600–2000 cm⁻¹ at a resolution of 4 cm⁻¹. X-ray photoelectron spectroscopy (XPS) was performed using a PHI VersaProbe II system under a vacuum pressure of 2 × 10⁻⁸ mTorr. Monochromatic X-ray radiation from an aluminum

anode (1486.6 eV) was employed to analyze the surface chemical composition of the materials. Surface morphology via scanning electron microscopy (SEM) and Energy-Dispersive Spectroscopy (EDS) analysis were examined on a Hitachi S5500 field-emission microscope operating at an accelerating voltage of 5 kV, providing high-resolution imaging. Nitrogen adsorption-desorption measurements were carried out to assess the specific surface area and pore characteristics of the materials using the Brunauer–Emmett–Teller (BET) method on a Micromeritics Gemini 3240 analyzer. Before measurement, the samples underwent degassing at 150 °C for 12 h under a nitrogen atmosphere to remove residual moisture and volatile compounds.

## 2.3 Adsorption procedure

Adsorption experiments were conducted in batch mode using 100 mL solutions with an initial dye concentration of 10 ppm (MB, MG, RhB, and MO), employing 0.1 g of biochar and functionalized biochar powders as adsorbents. The experiments were performed at an optimal pH of 7, ensuring neutral conditions to enhance comparability among the dyes. To facilitate effective dye-adsorbent interactions, the mixtures were vigorously stirred under dark conditions for an optimized contact time of 40 min. Aliquots were extracted at predefined intervals, immediately centrifuged, and analyzed using a UV-Vis spectrophotometer (JASCO model V-670) to monitor dye concentration variations over time. The maximum absorbance wavelengths were determined as 664 nm for MB, 617 nm for MG, 555 nm for RhB, and 464 nm for MO, with H₂O serving as the baseline reference. The effect of biochar functionalization on dye removal efficiency was systematically examined, and key adsorption parameters, including removal efficiency (R%) and equilibrium adsorption capacity (qₑ, mg/g), were quantified using their respective equations.

$$R(\%) = \left( \frac{C_0 - C_e}{C_0} \right) \times 100 \qquad (1)$$

$$q_e = \left( \frac{C_0 - C_e}{W} \right) \times V \qquad (2)$$

In this context, C₀ and Cₑ (mg/L) represent the initial and equilibrium dye concentrations, respectively. W(g)
denotes the mass of the adsorbent used, while V(L) corresponds to the volume of the solution.

The dyes were selected based on their structural diversity, charge characteristics, and environmental relevance as representative organic pollutants. MB and MG are cationic dyes widely used in the textile, pharmaceutical, and paper industries, whereas RhB is a zwitterionic xanthene dye with applications in biological staining and laser technologies. In contrast, MO is an anionic azo dye commonly employed in textile dyeing and printing. This selection enables a comprehensive assessment of the adsorption efficiency of biochar under different electrostatic interaction scenarios and molecular frameworks, providing insights into the adsorption mechanism.

The adsorption kinetic experimental data were fitted and analyzed using pseudo-second-order kinetic and intra-particle diffusion equations.

$$ \text{Pseudo-second-order kinetic: } \frac{t}{q_t} = \frac{1}{k_2 q_e^2} + \frac{1}{q_e} t \quad (3) $$

$$ \text{Intra-particle diffusion model } q_t = k_{id} t^{1/2} + C, \quad (4) $$

where $q_t$ (mg/g) stands for the adsorption capacity at time t; $k_2$ (g/mg/h) represents the adsorption rate constants of pseudo-second-order kinetics; $k_{id}$ (mg/[g·min$^{1/2}$]) is the intraparticle diffusion rate constant; C is a constant related to the thickness of the boundary layer.

The isothermal adsorption experiment data were fitted and analyzed using the Langmuir model. The fitting equations are as follows:

Langmuir model:

$$ \frac{C_e}{q_e} = \frac{1}{q_m}C_e + \frac{1}{K_L q_m}, \quad (5) $$

where $q_m$ (mg/g) represents the maximum adsorption capacity when adsorption equilibrium is reached; $K_L$ (L/g) is the adsorption constant of the Langmuir equation; Langmuir isotherm can be discussed in terms of the dimensionless constant, $R_L$ which is calculated using the following equation:

$$ R_L = \frac{1}{(1 + q_m K_L)} \quad (6) $$

# 3 Results and discussion

## 3.1 Microstructural analysis

X-ray diffraction patterns (Fig. 1) reveal that the biochar has a predominantly amorphous structure, characteristic of pyrolysis at moderate temperatures, as evidenced by the absence of distinct signals and the presence of diffuse and broad signals [32, 33]. However, analysis by Lorentzian deconvolution (Fig. S1) allowed the identification of minor crystalline phases, such as graphite (planes (002) and (100) at 24.02° and 40.66°, respectively) [34] and calcite (28.58° and 29.55°) [35, 36]. Additionally, the signals at 29.96° and 31.13° are attributed to the presence of crystalline potassium chloride [37]. The detection of a signal at 9.82° and 43.30° suggests the possible formation of oxygenated lamellar structures, similar to graphene oxide-like regions [25, 38]. These findings highlight the heterogeneous microstructural nature of the material, with localized graphitic domains embedded in a mostly amorphous matrix.

On the other hand, chemical functionalization with KMnO$_4$ resulted in significant structural transformations. Crystalline phases such as graphite and CaCO$_3$ were eliminated and replaced by reflections corresponding to MnO$_2$ and manganese phosphates [39, 40]. This change aligns with the oxidizing conditions imposed by the combined action of KMnO$_4$, H$_2$SO$_4$, and H$_3$PO$_4$, which promote oxidative dissolution and restructuring of the carbonaceous matrix.

## 3.2 Chemical composition analysis

### 3.2.1 Elemental composition by XPS

XPS was employed to analyze the elemental composition of the biochar surface before and after chemical functionalization. Figure 2 presents the survey spectra of both biochar and functionalized biochar, while Table 1 summarizes the atomic ratios of the quantified elements.

In the biochar sample, XPS analysis revealed characteristic signals for C 1s, O 1s, K 2p, and Ca 2p, indicating the presence of carbonaceous structures along with residual inorganic species derived from the biomass precursor. Following chemical functionalization, a significant alteration in surface composition was
<figure><img src="image_2.png" /><figcaption>**Fig. 2** XPS spectra of biochar and functionalized biochar</figcaption></figure>

<figure><img src="image_3.png" /></figure>
<table><caption>**Table 1** Atomic ratios of elements determined from high-resolution XPS spectra</caption><thead><tr><th>Atomic ratio (%)</th><th>C</th><th>O</th><th>K</th><th>Ca</th><th>Mn</th><th>P</th><th>S</th><th>O/C by XPS</th></tr></thead><tbody><tr><td>Biochar</td><td>76.74%</td><td>13.80%</td><td>8.17%</td><td>1.29%</td><td>—</td><td>—</td><td>—</td><td>0.18</td></tr><tr><td>Functionalized biochar</td><td>35.14%</td><td>43.81%</td><td>—</td><td>—</td><td>5.76%</td><td>11.58%</td><td>3.72%</td><td>1.25</td></tr></tbody></table>

observed, marked by the emergence of new spectral features corresponding to P 2p, S 2p, and Mn 2p. This transformation is attributed to interactions with the functionalization reagents (HCl, H₂SO₄, and KMnO₄), which not only modified the surface chemistry but also facilitated the incorporation of oxygenated functional groups. The substantial increase in the O/C atomic ratio from 0.18 to 1.25 post-functionalization strongly suggests an enrichment of oxygen-containing functionalities. Notably, while XRD failed to detect distinctive carbon-related signals after functionalization, XPS provided clear evidence of its presence, highlighting the superior surface sensitivity of this

technique in characterizing chemically modified carbonaceous materials.

### 3.2.2 Functional composition by FTIR

The FTIR spectrum of biochar within the 4000–400 cm⁻¹ wavelength range reflects a complex chemical composition, influenced by its lignocellulosic origin and pyrolysis process (Fig. 3a). A peak at 3103 cm⁻¹ is indicative of OH-stretching vibration in the hydroxyl groups of phenols in aliphatic compounds, which are characteristic of organic residues present in biochar [41]. In the 600–900 cm⁻¹ region, the

<figure><img src="image_4.png" /><figcaption>**Fig. 3** FTIR spectra of a–b biochar, and c–d functionalized biochar</figcaption></figure>
band at 872 cm⁻¹ confirms the presence of carbonates (calcite) [42], consistent with XRD results. The signals at 761 and 709 cm⁻¹ correspond to out-of-plane deformations of C-H bonds in aromatic rings, indicating the retention of aromatic structures. The band at 615 cm⁻¹ is attributed to out-of-plane vibrations of inorganic compounds, such as metal oxides, present as residual minor fractions [43].

In the 900-1800 cm⁻¹ range, characteristic bands were identified by Lorentzian deconvolution (Fig. 3b), which allowed to assignment of specific molecular vibrations to key functional groups, based on previous investigations on carbonaceous materials [44, 45]. The band at 1586 cm⁻¹, associated with C=C vibrations in aromatic rings, suggests the presence of graphitic domains that facilitate π-π interactions with aromatic molecules. The signals at 1471 cm⁻¹ and 1270 cm⁻¹, attributed to vibrations of phenolic groups, highlight the ability of biochar to establish hydrogen bonds, a crucial mechanism in the interaction with polar compounds. These functional groups, in combination with the band at 1389 cm⁻¹ associated with C-O-C vibrations in ethers, predominate at the edges of carbonaceous structures, which significantly increases the affinity of biochar towards polar and aromatic molecules in aqueous media. Whereas the signals at 1114 cm⁻¹ and 1035 cm⁻¹, related to epoxide groups and skeletal C-C and C-O vibrations, confirm a degree of partial oxidation in aromatic domains (corroborated by XPS). These features, combined with a partially ordered carbonaceous structure, provide evidence of a functionally versatile matrix.

Chemical functionalization of biochar (Fig. 3c) induced a significant reconfiguration, evidenced by the appearance of new functional groups and structural defects. The band at 645 cm⁻¹, associated with Mn-O vibrations, confirms the incorporation of manganese oxides, known for their catalytic activity and affinity towards polar and anionic species. Likewise, the peak at 853 cm⁻¹, corresponding to P-O stretches in phosphates, points to the incorporation of phosphorus as an additional adsorbent site.

Table 3 summarizes the molecular changes observed in the functionalized biochar in the range 900-1800 cm⁻¹ (Fig. 3d). The significant increase of epoxide groups (from 8.6% to 76.5%) stands out, resulting from a hierarchical restructuring that enhances the density functional and generates defects favoring pore opening, as confirmed by BET analysis. In parallel, a decrease in phenolic groups (from 10.2%

to 1.2%) and ethers (from 26.5% to 8.3%) was observed, reflecting their selective conversion to epoxides. The reduction in aromatic conjugated domains (from 30.2 to 2.4%) and C-C and C-O skeletal structures (from 24.5% to 9.9%) suggests a loss of graphitic order (supported by XRD), compensated by the introduction of R-C=O...H-O-C-R type bonds (1.6% in the functionalized biochar), which enhance the interaction with polar and anionic contaminants.

Comparative analysis with chemically functionalized biochars reported in the literature reveals that the observed transformations are consistent with the strong oxidative environment created by KMnO₄, further supporting its effectiveness as a functionalization agent [46]. The following section provides a detailed overview of the biochar functionalization mechanism, outlining the key stages and transformations that occur during the functionalization process (Table 2).

## 3.3 Morphological analysis

The SEM analysis (Fig. 4a) reveals that the biochar exhibits a disordered and porous surface morphology, with stacked lamellar structures and irregularly shaped blocks. These features are consistent with the amorphous nature observed in the XRD analysis, indicating a heterogeneously distributed carbon matrix. Notably, rhombohedral particles embedded in the porous matrix and basal plane (Fig. 4b) were identified as calcium carbonate (CaCO₃) crystals. This identification is based on their characteristic morphology, which aligns with previous descriptions of such crystals formed during the carbonization process [47] and, confirmed by EDS analysis showing a calcium content of 12.78%. The formation of these particles can be attributed to the reaction

<table><caption>**Table 2** Functional composition according to FTIR spectra deconvolutions</caption><thead><tr><td>Bonds</td><td>Biochar (%)</td><td>Functionalized biochar (%)</td></tr></thead><tbody><tr><td>C-C and C-O skeletal</td><td>24.5</td><td>9.9</td></tr><tr><td>Epoxy</td><td>8.6</td><td>76.5</td></tr><tr><td>Phenol</td><td>10.2</td><td>1.2</td></tr><tr><td>Ether</td><td>26.5</td><td>8.3</td></tr><tr><td>C=C aromatic</td><td>30.2</td><td>2.4</td></tr><tr><td>R-C=O...H-O-C-R</td><td>-</td><td>1.6</td></tr></tbody></table>
<figure><img src="image_5.png" /><figcaption>**Table 3** BET-Specific surface area and pore volume of biochar and functionalized biochar</figcaption></figure>

<table><thead><tr><th>Sample</th><th>Specific surface area (m²/g)</th><th>Pore volume (cm³/g)</th><th>O/C ratio by EDS</th></tr></thead><tbody><tr><td>Biochar</td><td>1.673</td><td>0.0017</td><td>0.20</td></tr><tr><td>Functionalized biochar</td><td>4.688</td><td>0.0116</td><td>1.20</td></tr></tbody></table>

between calcium in the biomass and carbon dioxide during pyrolysis [48]. These results highlight the coexistence of mineral phases and carbonaceous

components in biochar, which could influence initial adsorption properties.

Upon chemical functionalization (Fig. 4c), significant morphological transformations were observed, indicative of oxidation-induced reconfiguration. The lamellar structure became more fragmented and exfoliated, suggesting an increase in specific surface area (SSA) and pore volume compared to the untreated biochar (Table 3). This morphology has been reported as characteristic of materials with high degrees of oxidation, such as graphene oxides [49]. The observed structural modifications are consistent with the removal of CaCO₃ and the expansion of graphitic sheets facilitated by oxidative treatment. Similar enhancements have been reported
in biochars derived from various sources, including *corn straw* [50], *oak woodchips char* [51], and *moringa oleifera leaves* [52], among others. However, the increase in SSA and pore volume observed in this study is less pronounced than those reported in these references and commercially functionalized carbons. This may be attributed to the high concentration of oxygenated functional groups introduced during functionalization, potentially leading to an agglomeration effect that hinders the development of more extensive porosity [53].

Despite the moderate SSA values, surface functionalization—evidenced by an increase in the O/C ratio from 0.20 to 1.20—plays a crucial role in enhancing adsorptive performance by introducing oxygenated functional groups that improve surface reactivity and chemical affinity with adsorbates. Although the absolute O/C values obtained by XPS and EDS differ slightly due to the inherent surface and bulk sensitivities of these techniques, both exhibit a consistent trend, reinforcing the robustness of the compositional analysis. While strategies based on multistage functionalization or hybrid approaches combining physical and chemical functionalization could further increase SSA and porosity, the induced surface functionalization partially compensates for this limitation. In this context, the resulting material exhibits an optimized balance between surface chemistry and textural properties, which is particularly relevant for specific adsorption applications, where the presence of functional groups can be as decisive as the extent of accessible surface area. EDS analysis further confirmed the presence of manganese (Mn), phosphorus (P), chlorine (Cl), and sulfur (S), formed through the interaction of KMnO₄ and auxiliary reagents.

## 3.4 Biochar functionalization mechanisms

The biochar functionalization mechanism involves a series of chemical and structural transformations demonstrated by XRD, SEM, FTIR, and XPS. These transformations are induced by controlled interactions with oxidizing agents and specific environmental conditions, which promote the formation of highly porous and functionalized structures. This process, divided into interconnected steps, allows precise tuning of the physicochemical characteristics of the functionalized biochar, such as its surface reactivity, its lamellar structure, and its density of functional groups. The

stages that constitute the chemical functionalization mechanism of biochar are described in detail below (Fig. 5).

**Initial stage (I) – Intercalation and chemical modification:** The first stage of the biochar functionalization mechanism is characterized by the rapid interaction between the carbonaceous material and the oxidizing acidic medium composed of H₂SO₄ and H₃PO₄. During this process, acid molecules intercalate between the carbon layers, causing a significant expansion of the lamellar structure of the biochar. This phenomenon, which occurs in a matter of minutes, increases the accessibility of the interlaminar spaces, facilitating the penetration of chemical agents and preparing the material for further chemical functionalization steps. The initial intercalation not only modifies the structure of the biochar but also optimizes its surface reactivity, which is essential for subsequent transformations.

**Intermediate stage (II) – Controlled oxidation and by-product formation:** This subsequent step begins when KMnO₄ is added to the acid mixture to induce gradual oxidation of the biochar. Although KMnO₄ is the oxidant used, the actual active species is Mn₂O₇ [54], which can selectively oxidize aliphatic unsaturated double bonds over aromatic double bonds. When KMnO₄ reacts with H₂SO₄, it generates dimanganese–heptaoxide (Mn₂O₇) and, combined with the strongly acidic medium, facilitates the oxidation process (reactions 7 and 8). The higher the amount of KMnO₄, the more oxidation is promoted in the material [55].

$$KMnO_4 + 3H_2SO_4 \rightarrow K^+ + MnO_3^+ + H_3O^+ + 3HSO_4^- \quad (7)$$

$$MnO_3^+ + MnO_4^+ \rightarrow Mn_2O_7 \quad (8)$$

When the combination is added to deionized water, the dilution heat of H₂SO₄ causes a rapid increase in the temperature of the mixture, resulting in a secondary expansion of the volume of the intercalated carbon [56]. Under these conditions, the Mn(VII) compound can produce ozone and oxidizing radicals, such as hydroxyl radicals or atomic oxygen, which contribute to the oxidation of carbon, forming oxygenated functional groups [57]. At this stage, functional groups are initially formed that are mainly present at the edges as phenols and ether and, as oxidation continues, more functional groups are formed and occupy the basal planes of the
<figure><img src="image_6.png" /><figcaption>**Fig. 5** Chemical functionalization mechanism of biochar</figcaption></figure>

lamellae such as epoxide groups [58]. This destructive oxidation leads to the formation of permanent defects. However, a certain amount of insoluble manganese compounds, such as $MnO_2$, are generated and grow on the surface of the functionalized biochar [59], as demonstrated by XRD. Subsequently, the addition of $H_2O_2$ reduces $Mn^{7+}$ and $Mn^{4+}$ to $Mn^{2+}$.

Contrary to XRD analysis, which did not detect significant carbon-related peaks after functionalization, XPS analysis confirmed the presence of carbon while highlighting surface functionality. It also revealed the formation of $MnO_2$ and manganese phosphates as dominant compounds on the biochar surface. The presence of these manganese species is not a by-product of over-oxidation but an intentional aspect of the functionalization strategy, designed to create a hybrid material with enhanced adsorptive properties. The incorporation of manganese phases within the carbon matrix improves the performance of the material in applications requiring synergistic interactions between carbon and manganese-based species.

**Final stage (III)—Exfoliation and structural improvement:** Finally, the third step is the exfoliation of the functionalized biochar. This process involves the separation of the individual layers, especially

under sonication and/or agitation conditions in aqueous solution, achieving highly porous and functionalized structures, confirmed by SEM images and BET analysis. When the prepared biochar is sufficiently oxidized, it is spontaneously exfoliated into mono-lamellar layers by simple agitation in water.

This comprehensive functionalization mechanism, incorporating both chemical modification and the formation of manganese-based species, highlights the creation of a novel material that is not merely a "pure" activated biochar, but rather a functionalized hybrid material with tailored properties for advanced applications.

## 3.5 Adsorption removal of dyes

### 3.5.1 Adsorption on biochar

As depicted in Fig. 6a, MB exhibited a moderate adsorption efficiency, achieving a removal rate of 55.47% ($q_e$ = 5.21 mg/g). This behavior can be attributed to the limited presence of oxygenated functional groups on the surface of the biochar, which are critical for establishing effective electrostatic interactions with the cationic species of the dye. In contrast,
<figure><img src="image_7.png" /><figcaption>**Fig. 6** Effect of contact time of biochar on the adsorption of dyes: a MG, b MB, c RhB, and d MO</figcaption></figure>

Fig. 6b shows that MG exhibited a significantly higher removal efficiency, attaining 97.84% ($q_e$ = 9.78 mg/g). This enhanced performance is likely due to the planar structure of the aromatic rings in MG, which facilitates the formation of stable π-π stacking interactions between the dye and the adsorbent [60]. These interactions, which are more robust and stable than those of MB, are further reinforced by the molecular complexity and the nitrogen atom in MG, which disrupts the electronic conjugation in MB, thereby decreasing the stability of their respective interactions [61]. Additionally, MG, being a larger and structurally more complex molecule, possesses a greater ability to establish multiple π-π interactions compared to MB, which is

smaller and less conjugated [62, 63]. This expanded capacity allows the dye to maximize its adsorption by more efficiently exploiting the graphitized regions of the biochar surface, favoring more intense and stable interactions.

Furthermore, Fig. 6c reveals that RhB exhibited a notably low adsorption rate, with only 12.88% removal ($q_e$ = 1.26 mg/g), attributed to the sensitivity of the dye to pH. In neutral media, the cationic interaction is weakened, limiting its effective removal [64]. This observation is consistent with previous studies, which reported nearly complete inhibition of RhB removal at higher pH levels ($pH > 6.0$) [65]. In contrast, Fig. 6d demonstrates that MO, an
anionic dye, exhibited a negligible removal rate of 2.08% ($q_e$ = 0.20 mg/g), due to the inability of the non-functionalized biochar to establish favorable interactions with negatively charged species, given the scarcity of specific functional groups [66]. These results underline the importance of the chemical and structural properties of the adsorbent, as well as the intrinsic characteristics of the dyes, on the adsorption performance. Additionally, the UV-Vis absorption spectra illustrating dye adsorption over time are provided in Fig. S2 of the Supporting Information, offering a visual representation of the progressive reduction in dye concentration across different time intervals.

### 3.5.2 Adsorption on chemically functionalized biochar

Chemical functionalization substantially improved the adsorption of all the dyes evaluated (Fig. 7). The UV-Vis absorption spectra of dye adsorption at different time intervals are presented in Fig. S3. Of the Supporting Information. The structural and functional changes of the biochar, confirmed by XRD and FTIR, played a crucial role in this improvement. Figure 7a shows that the functionalized biochar achieved complete MB removal (100%; $q_e$ = 10 mg/g) in only 3 min, attributed to the increase of oxygenated functional groups that increased the chemical affinity and active sites available for adsorption [49]. In addition, the higher specific

<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /><figcaption>**Fig. 7** Effect of contact time of functionalized biochar on the adsorption of dyes: a MG, b MB, c RhB, and d MO</figcaption></figure>
surface area and pore volume favored faster diffusion of the molecules toward these active sites [55].

The MG adsorption efficiency remained high (98.5%; $q_e$ = 9.94 mg/g), despite the partial removal of graphitic regions during functionalization (Fig. 7b). Surface functionalization allowed new interactions, such as hydrogen bonds and dipole-dipole bonds, which compensated for the loss of $π-π$ interactions [67]. In the case of RhB (Fig. 7c), functionalization overcame pH limitations in the non-functionalized biochar, achieving 100% removals for RhB ($q_e$ = 10 mg/g) and 88% for MO ($q_e$ = 8.95 mg/g). This behavior reflects the role of oxygenated groups and transition elements (Mn, P, S) detected by EDS, which contribute to specific interactions with anionic dyes [68].

### 3.5.3 Kinetics

To ensure a more rigorous evaluation, the adsorption kinetics of the dyes on biochar and functionalized biochar were analyzed using both the pseudo-first-order and pseudo-second-order models. However, the pseudo-second-order model exhibited a superior fit to the experimental data, with correlation coefficients ($R^2$ > 0.99) across all analyzed systems. Nevertheless, the parameters obtained from the pseudo-first-order model are presented in Table S2 and Fig. S3 for reference. This model suggests that the adsorption process is predominantly governed by chemical mechanisms,

such as electrostatic interactions or covalent bonding between the adsorbate and the adsorbent [69]. The corresponding model fits are illustrated in Fig. 8, where the kinetic curves of t/q~t~ versus time (t) are displayed.

The values in Table 4 revealed that chemical functionalization significantly increased the adsorption rate ($k_2$) for MB and MG, indicating a faster occupancy of the active sites due to increased accessibility and functional density. In contrast, for RhB, a reduction in $k_2$ was observed, suggesting that intraparticle diffusion became the limiting factor. The structural complexity of RhB created transport barriers to the internal pores, slowing down the kinetics despite the increase in adsorption capacity. The negligible adsorption of MO on biochar led to its exclusion from further analysis, though a significant improvement in adsorption was observed for functionalized biochar.

### 3.5.4 A mechanistic model for MB dye diffusion

The Weber and Morris model applied to biochar reveals a multi-stage-controlled adsorption process, combining rapid surface interactions and significant transport into the internal pores of the material [70]. This mechanism, illustrated in Fig. 9, consists of three main stages: *surface diffusion*, *intraparticle diffusion*, and *equilibrium*.

<figure><img src="image_12.png" /><figcaption>**Fig. 8** Pseudo-second-order kinetic study of dye adsorption by a biochar and b functionalized biochar</figcaption></figure>
<table><caption>**Table 4** Summarizes the kinetic model results for both biochar and functionalized biochar</caption><thead><tr><th rowspan="2">Material</th><th rowspan="2">Dye</th><th rowspan="2">q<sub>e</sub> (mg/g) (experimental)</th><th colspan="3">Pseudo-second-order</th></tr><tr><th>k<sub>2</sub> (min<sup>-1</sup>)</th><th>q<sub>e</sub> (mg/g) (calculated)</th><th>R<sup>2</sup></th></tr></thead><tbody><tr><td rowspan="4">Biochar</td><td>MB</td><td>5.547±0.080</td><td>0.159</td><td>5.215</td><td>0.990</td></tr><tr><td>MG</td><td>9.785±0.407</td><td>0.207</td><td>9.939</td><td>0.999</td></tr><tr><td>RhB</td><td>1.356±0.346</td><td>3.468</td><td>1.267</td><td>0.992</td></tr><tr><td>MO</td><td>0.208±0.036</td><td>—</td><td>—</td><td>—</td></tr><tr><td rowspan="4">Functionalized Biochar</td><td>MB</td><td>10±0.704</td><td>15.388</td><td>10.002</td><td>0.999</td></tr><tr><td>MG</td><td>9.944±0.010</td><td>2.006</td><td>9.845</td><td>0.999</td></tr><tr><td>RhB</td><td>10±0.061</td><td>0.057</td><td>10.461</td><td>0.998</td></tr><tr><td>MO</td><td>8.956±0.130</td><td>0.327</td><td>9.007</td><td>0.999</td></tr></tbody></table>

<figure><img src="image_13.png" /><figcaption>**Fig. 9** Intraparticle diffusion model for dye adsorption in biochar a MB, b MG, and c RhB</figcaption></figure>
In stage I (surface diffusion), adsorption occurs rapidly on the most accessible active sites on the biochar surface. This initial behavior, characterized by a steep slope in the diffusion curves, reflects a high affinity between the dye molecules and the surface functional sites, resulting in rapid occupancy during the first few minutes. For MB, this stage is short, reaching a maximum of 38.25% adsorption in approximately 3 min. In comparison, MG and RhB show more gradual kinetics, extending up to 6 min, suggesting differences in the ease of interaction with surface sites. The high affinity of MG in this phase is attributed to its planar structure, which facilitates stable π-π interactions with graphitic regions of the biochar.

Stage II (Intraparticle diffusion) involves a transition to a slower process, dominated by the diffusion of the dye molecules into the internal pores. For MB, adsorption increases from 38.25 to 55.63% in 12 min, indicating saturation of the surface sites and increasing reliance on internal transport processes. RhB, on the other hand, shows a more modest increase (from 8.98 to 12.93%), reflecting lower affinity and higher barriers to intraparticle diffusion. On the other hand, MG undergoes a minimal change (from 94.7 to 97.8% in 18 min), indicating that the initial occupation of surface sites was almost complete, followed by efficient diffusion into the pores. The reduction of the velocity constants ($k_p_2$) observed at this stage (Table 5) is evidence of a higher diffusion resistance, determined by the structure of the dye and the heterogeneity of the biochar.

The final stage III (equilibrium) corresponds to the equilibrium of the adsorption process, with variations in the stability of the interactions depending on the dye. MB shows fluctuations between 45 and

52%, attributable to relatively weak π-π interactions, possibly affected by the nitrogen atom in its aromatic structure [61]. RhB shows smaller fluctuations (12-13%), indicating more stable interactions but with some reversibility. MG, on the other hand, shows no desorption, confirming the formation of strong and specific chemical interactions with biochar.

### 3.5.4.1 Impact of chemical functionalization
Chemical functionalization of the biochar substantially modifies the adsorption processes at each stage, improving the initial efficiency and altering the internal transport mechanisms (Fig. 10). In stage I (0-3 min), the increased density of oxygenated functional groups and the increased accessibility to the active sites facilitate fast and effective interactions at the surface. For MB, adsorption occurs almost exclusively at this stage, reaching equilibrium rapidly and eliminating the need for intraparticle transport.

The most significant differences emerge during stage II. MG reaches equilibrium in only 6 min, thanks to its high surface affinity and efficient diffusion. In contrast, RhB undergoes a slower process, reaching equilibrium in 24 min, due to the diffusive resistance generated by its complex structure. MO shows a gradual increase, with equilibrium at 18 min, reflecting a higher resistance to intraparticle transport, possibly linked to its anionic nature and specific interactions with residual elements such as Mn.

Finally, in stage III, common to all systems, no significant fluctuations associated with desorption or readsorption processes are observed, evidencing stable and specific chemical interactions

<table><caption>**Table 5** The intraparticle model calculated parameters for biochar and functionalized biochar</caption><thead><tr><th rowspan="2">Material</th><th rowspan="2">Dye</th><th colspan="2">1st step</th><th colspan="2">2nd step</th><th colspan="2">3rd step</th></tr><tr><th>k<sub>p1</sub> (mg/g min<sup>1/2</sup>)</th><th>R<sup>2</sup></th><th>k<sub>p2</sub> (mg/g min<sup>1/2</sup>)</th><th>R<sup>2</sup></th><th>k<sub>p3</sub> (mg/g min<sup>1/2</sup>)</th><th>R<sup>2</sup></th></tr></thead><tbody><tr><td rowspan="4">Biochar</td><td>MB</td><td>2.195</td><td>0.999</td><td>0.858</td><td>0.734</td><td>—</td><td>—</td></tr><tr><td>MG</td><td>3.969</td><td>0.990</td><td>0.1251</td><td>0.984</td><td>—</td><td>—</td></tr><tr><td>RhB</td><td>0.469</td><td>0.990</td><td>0.165</td><td>0.998</td><td>—</td><td>—</td></tr><tr><td>MO</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td rowspan="4">Functionalized Biochar</td><td>MB</td><td>5.678</td><td>0.999</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>MG</td><td>5.516</td><td>0.999</td><td>0.379</td><td>0.997</td><td>—</td><td>—</td></tr><tr><td>RhB</td><td>4.309</td><td>0.999</td><td>0.808</td><td>0.967</td><td>—</td><td>—</td></tr><tr><td>MO</td><td>4.710</td><td>0.999</td><td>0.211</td><td>0.825</td><td>—</td><td>—</td></tr></tbody></table>
<figure><img src="image_14.png" /><figcaption>**Fig. 10** Intraparticle diffusion model for dye adsorption in functionalized biochar a MB, b RhB, c MG, and d MO</figcaption></figure>

promoted by the functional groups introduced during
functionalization.

3.5.5 Adsorption isotherms

The adsorption isotherms were examined in this study
by linearly fitting the experimental data to two mod-
els: the Langmuir and Freundlich isotherms. Fig. S4
presents the linear curve fitting of the experimental
adsorption data using the Freundlich isotherm model.
Furthermore, the parameters associated with each
model investigated are listed in Table S3. The most
suitable isotherm model was determined by com-
paring their correlation coefficients (R²). Given that

the Langmuir model exhibited the highest R² value
(R² > 0.98), it was identified as the best fit among the
two, confirming monolayer adsorption on homoge-
neous surfaces [71]. This model also provided key
insights into the favorability of the adsorption process,
expressed through the R_L coefficient. The graphical
representations of the model fit are shown in Fig. 11,
while the corresponding calculated parameters are
listed in Table 6.

The biochar showed favorable adsorption for MB (R<sub>L</sub> = 0.014) and RhB dyes (R<sub>L</sub> = 0.015) and highly favorable for MG (R<sub>L</sub> = 0.001), which is related to the previously described interactions between the dye and the biochar surface. While chemical functionalization increased its affinity towards all the dyes evaluated. For
<figure><img src="image_15.png" /><figcaption>**Fig. 11** Linear fitting of experimental adsorption data to Langmuir adsorption isotherm for the removal of dyes by **a** biochar and **b** functionalized biochar</figcaption></figure>

<table><caption>**Table 6** Fitting parameters for Langmuir isothermal on biochar and functionalized biochar</caption><thead><tr><th rowspan="2">Material</th><th rowspan="2">Dye</th><th colspan="4">Langmuir model</th></tr><tr><th>q<sub>max</sub> (mg/g)</th><th>K<sub>L</sub> (L/mg)</th><th>R<sub>L</sub></th><th>R<sup>2</sup></th></tr></thead><tbody><tr><td rowspan="4">Biochar</td><td>MB</td><td>8.067</td><td>6.705</td><td>0.014</td><td>0.985</td></tr><tr><td>MG</td><td>84.643</td><td>78.993</td><td>0.001</td><td>0.999</td></tr><tr><td>RhB</td><td>1.721</td><td>6.500</td><td>0.015</td><td>0.998</td></tr><tr><td>MO</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td rowspan="4">Functionalized biochar</td><td>MB</td><td>470.234</td><td>463.862</td><td>0.000</td><td>0.999</td></tr><tr><td>MG</td><td>85.309</td><td>79.587</td><td>0.001</td><td>0.998</td></tr><tr><td>RhB</td><td>80.374</td><td>74.351</td><td>0.001</td><td>0.993</td></tr><tr><td>MO</td><td>34.556</td><td>30.036</td><td>0.003</td><td>0.998</td></tr></tbody></table>

MB, the value of $R_L = 0$ indicated irreversible adsorption, while for RhB ($R_L = 0.001$), MG ($R_L = 0.001$), and MO ($R_L = 0.003$), the $R_L$ values suggest highly favorable adsorption, close to irreversibility. This behavior is consistent with the results obtained in stage 3 of the intraparticle diffusion model, where, upon reaching equilibrium, no significant desorption of the dyes was observed.

## 4 Comparison of recent studies

Table 7 presents a comparison of recent studies highlighting the use of biochar as an adsorbent for the removal of dyes and other complex pollutants. The biochar from this research showed outstanding performance under optimal conditions. This efficiency significantly exceeds that of other biochars reported in the literature. For example, biochar derived from moringa leaves functionalized at 450 °C achieved efficiencies of 88% for MB and 93% for MG [52], while biochar obtained from pineapple waste processed at 105 °C showed much lower efficiency (~54% for MB) [72]. These results highlight the importance of using higher pyrolysis temperatures and appropriate functionalization treatments to optimize adsorption capacity.

Biochar, a highly versatile carbonaceous matrix, can be produced from a wide range of biomass sources, including agricultural wastes (moringa leaves, pineapple peels), lignocellulosic residues (corn straw, sugarcane bagasse) and unconventional organic materials (papaya seeds, pomegranate peels). This diversity of raw materials not only promotes the use of waste but also makes it possible to adjust the final properties of biochar according to the type of biomass used and the production conditions, such as pyrolysis temperature and activating agents. Beyond dye removal, biochar has demonstrated its
<table><caption>**Table 7** Recent studies on pollutant adsorption by various biomass – derived biochar</caption><thead><tr><th rowspan="2">Biomass</th><th rowspan="2">Pyrolysis conditions (Temperature)</th><th rowspan="2">Functionalization type</th><th rowspan="2">Pollutant</th><th rowspan="2">Initial concentration</th><th colspan="2">Max. Removal efficiency</th><th rowspan="2">Contact time evaluated</th><th rowspan="2">References</th></tr><tr><th>Before functionalization</th><th>After functionalization</th></tr></thead><tbody><tr><td>Oil palm fiber</td><td>450 °C</td><td>Functionalization temp. 753 °C</td><td>SO<sub>2</sub></td><td>300 ppm</td><td>8.62 mg/g</td><td>33.09 mg/g</td><td>73 min</td><td>[76]</td></tr><tr><td rowspan="2">Corn straw</td><td rowspan="2">500 °C</td><td rowspan="2">KOH</td><td>Cr(VI)</td><td>10 mg/L</td><td>—</td><td>116.97 mg/g</td><td>120 min</td><td rowspan="2">[50]</td></tr><tr><td>NAP</td><td>15 mg/L</td><td>—</td><td>450.43 mg/g</td><td>180 min</td></tr><tr><td>Sugarcane bagasse</td><td>200 °C</td><td>NaOH</td><td>Sulfamethoxazole (SMX)</td><td>25 mg/L</td><td>—</td><td>99.41%</td><td>150 min</td><td>[74]</td></tr><tr><td rowspan="3">Moringa leaves</td><td rowspan="3">450 °C</td><td rowspan="3">Chemical</td><td>MB dye</td><td>10 ppm</td><td>33.1%</td><td>88.0%</td><td>40 min</td><td rowspan="3">[52]</td></tr><tr><td>RhB dye</td><td>10 ppm</td><td>12.6%</td><td>71.5%</td><td>40 min</td></tr><tr><td>MG dye</td><td>10 ppm</td><td>88.1%</td><td>93.0%</td><td>40 min</td></tr><tr><td>Pineapple waste biomass</td><td>105 °C</td><td>Chemical</td><td>MB dye</td><td>1000 ppm</td><td>≈52%</td><td>≈54%</td><td>12 h</td><td>[72]</td></tr><tr><td>Water lily through</td><td>110 °C</td><td>Physical and chemical</td><td>MG dye</td><td>100 ppm</td><td>—</td><td>100%</td><td>75 min</td><td>[77]</td></tr><tr><td rowspan="2">Pomegranate peels</td><td rowspan="2">800 °C</td><td rowspan="2">Phosphoric acid</td><td>Antibiotic ciprofloxacin</td><td>25 mg/L</td><td>—</td><td>142.86 mg/g</td><td>420 min</td><td rowspan="2">[73]</td></tr><tr><td></td><td>25 mg/L</td><td>—</td><td>5.92 mg/g</td><td>360 min</td></tr><tr><td>Jack fruit peel</td><td>550 °C</td><td>Chemical</td><td>RBB dye</td><td>25 ppm</td><td>—</td><td>90%</td><td>420 min</td><td>[75]</td></tr><tr><td>Reed straw</td><td>300 °C</td><td>Steam</td><td>MV dye</td><td>50 mg/L</td><td>—</td><td>99.9%</td><td>90 min</td><td>[78]</td></tr><tr><td>Algae biochar</td><td>400 °C</td><td>—</td><td>DNA</td><td>—</td><td>6.9%</td><td>—</td><td>50 min</td><td>[79]</td></tr><tr><td>Sediment biochar</td><td>400 °C</td><td>—</td><td>DNA</td><td>—</td><td>66.47%</td><td>—</td><td>50 min</td><td></td></tr><tr><td>Peanut shell</td><td>900 °C</td><td>—</td><td>RBB dye</td><td>25 ppm</td><td>100%</td><td>—</td><td>30 min</td><td>[80]</td></tr><tr><td rowspan="4">Carica papaya seed</td><td rowspan="4">400 °C</td><td rowspan="4">KMnO<sub>4</sub></td><td>MB dye</td><td>10 ppm</td><td>55.47%</td><td>100%</td><td>3 min</td><td rowspan="4">This work</td></tr><tr><td>MG dye</td><td>10 ppm</td><td>97.84%</td><td>98.5%</td><td>6 min</td></tr><tr><td>RhB dye</td><td>10 ppm</td><td>12.88%</td><td>100%</td><td>30 min</td></tr><tr><td>MO dye</td><td>10 ppm</td><td>2.08%</td><td>88%</td><td>18 min</td></tr></tbody></table>

versatility in the elimination of other contaminants. For example, KOH – functionalized biochar derived from corn straw achieved an adsorption capacity of 116.97 mg/g for Cr(VI) [50], while biochar obtained from pomegranate peels and functionalized with phosphoric acid removed up to 142.86 mg/g of the antibiotic ciprofloxacin [73]. These diversified applications position biochar as a comprehensive solution for the treatment of a wide range of aquatic contaminants, including heavy metals and persistent organic compounds.

Chemical functionalization emerges as a determining factor in biochar performance. In our case, the use of KMnO₄ significantly improved surface functionalization and accessibility of active sites, achieving results comparable to those obtained with other functionalization agents, such as NaOH applied to sugarcane bagasse [50] and KOH on corn straw [74]. In contrast, non-functionalized biochars, such as those derived from algae or sediments, show limited adsorption capacities, underlining the need for functionalization treatments for more demanding applications. A distinctive aspect of our material is its adsorption

rate. It achieved complete MB removal in only 3 min, a remarkably higher efficiency compared to biochars such as jackfruit husk derivative, which required up to 420 min to achieve similar results [75]. This accelerated performance is attributed to the higher density of oxygenated functional groups and better accessibility of active sites generated during functionalization.

## 5 Future scope

The development of biochar and chemically functionalized biochar as sustainable adsorbents for dye removal represents a promising strategy for environmental remediation. However, for these materials to be considered viable alternatives for large-scale applications, a comprehensive evaluation of their regeneration potential and long-term stability is essential. The adsorption capacity of biochar is inherently linked to its surface chemistry, porosity, and structural integrity, all of which may undergo modifications upon interaction with contaminants. The presence of oxygenated functional groups, as
observed in this study, can enhance adsorption efficiency but may also affect the resilience of the material to regeneration processes.

Current research on biochar regeneration primarily explores thermal, chemical, and solvent-based treatments, each with distinct advantages and limitations. Thermal regeneration can effectively desorb organic pollutants but often leads to partial structural degradation and mass loss, reducing material reusability. Chemical regeneration, depending on the choice of regenerating agent, may selectively restore adsorption sites but also raises concerns about secondary pollution. Solvent-based methods can efficiently extract adsorbed contaminants but are generally costly and less sustainable. Given these trade-offs, future investigations should focus on hybrid regeneration strategies that optimize efficiency, minimize environmental impact, and enhance cost-effectiveness.

Beyond regeneration, evaluating the stability of biochar over multiple adsorption-desorption cycles is equally important. The interaction of biochar with dyes and other organic pollutants may induce structural modifications, surface passivation, or irreversible fouling, potentially compromising long-term performance. Systematic studies involving multiple regeneration cycles, coupled with advanced characterization techniques (e.g., in situ spectroscopic and microscopic analysis), are needed to elucidate the mechanisms governing biochar degradation and longevity.

Additionally, future research should consider the broader implications of biochar reuse within circular economy models. Repurposing exhausted biochar for secondary applications—such as catalysis, soil amendment, or energy storage—could further enhance its sustainability and economic viability. Integrating machine learning and predictive modeling approaches could also facilitate the design of next-generation biochars with tailored properties optimized for both adsorption efficiency and regeneration potential.

While this study provides critical insights into the functionalization and adsorption behavior of biochar, its long-term applicability remains an open question. Addressing the challenges associated with regeneration, stability, and lifecycle analysis will be fundamental in advancing biochar from a laboratory-scale adsorbent to a scalable, sustainable solution for industrial wastewater treatment.

Future studies should adopt a holistic approach that not only enhances adsorption performance but also ensures the durability and environmental compatibility of material in real-world applications.

## 6 Conclusion

This study establishes chemically functionalized biochar derived from *Carica papaya* seeds as a highly efficient and sustainable adsorbent for organic dye removal, demonstrating exceptional adsorption performance across multiple model pollutants. The chemical functionalization process significantly enhanced the textural and chemical properties of the biochar, resulting in a substantial increase in oxygenated functional groups (from 13.8 to 43.81%) and a notable expansion in surface area. These modifications collectively improved adsorption kinetics and capacity, enabling the material to achieve ultrafast adsorption equilibrium (6–9 min) for methylene blue (MB) and malachite green (MG), surpassing many conventional and biochar-based adsorbents. This makes *Carica papaya* seed biochar a highly promising candidate for rapid water decontamination applications. Kinetic modeling confirmed that the adsorption mechanism follows a pseudo-second-order process ($R^2 > 0.99$), suggesting strong chemisorptive interactions, while isotherm analyses revealed a monolayer adsorption process governed by the Langmuir model ($R^2 = 0.993–0.999$), indicating highly selective and well-defined adsorption sites. The synergistic effects of chemical functionalization were further validated through SEM, BET, FTIR, and EDS analyses, confirming significant structural and chemical modifications that underpin the enhanced adsorption capabilities. Beyond its high adsorption efficiency, this study highlights the transformative potential of agricultural waste valorization, positioning *Carica papaya* seed biochar as an economically viable and environmentally responsible alternative for water treatment technologies. Its abundant availability, cost-effectiveness, and scalability make it a compelling candidate for industrial wastewater remediation, offering a practical and sustainable solution that aligns with circular economy principles and global sustainability goals.
## Acknowledgements

The authors would like to acknowledge SECIHTI-CONAHCYT (Mexico) for the postdoctoral scholarship awarded to Odín Reyes Vallejo (CVU: 487411). The authors thank Ing. Rogelio Morán Elvira and MSc María Luisa Ramón García for their technical support with SEM images and XRD research, respectively.

## Author contributions

FJC: Formal analysis, investigation, data curation, writing—original draft preparation. RS – A: Characterization. AA: Visualization. JE – G: Characterization. AC – S: Characterization OR – V: Conceptualization, formal analysis, investigation, supervision, project administration, writing—review and editing. SPJ Equipment, materials, reagents, and supervision. SV: Equipment, materials, reagents, and supervision. All authors have read and agreed to the published version of the manuscript.

## Funding

Odín Reyes Vallejo (CVU 487411) acknowledges CONAHCYT (SECIHTI-MEXICO) for the postdoctoral fellowships, respectively.

## Data availability

The datasets generated during and/or analyzed during the current study are available from the corresponding author upon reasonable request.

## Declarations

**Competing interests** The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

**Supplementary Information** The online version contains supplementary material available at https://doi.org/10.1007/s10854-025-14725-y.

**Open Access** This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.

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