Confinement-induced acceleration of CO_2 mineralization through interfacial water thinning

Domain: physics
Published: 2025-12-29
Editorial outcome: major_revision

Abstract
Current projections demand atmospheric CO2 removal at the gigatonne-per-year scales through mineral carbonation, yet conventional aqueous routes remain energy-intensive and kinetically limited. Here we apply a water thinning strategy to engineer the interfacial water-film (IWF) with thicknesses below 8.5 nm. We achieve complete carbonation of calcium hydroxide (Ca(OH)2) within 1 h under ambient conditions, realizing a 16-fold acceleration compared to bulk solution systems. The IWF exhibits confinement-induced saturation kinetics, with the reaction rate increasing with CO2 partial pressure and then approaching a plateau around 2.02 kPa. Within this film, pocket-like sites in the water confined between particles use oriented interfacial hydroxyls to organize CO2 and ions, facilitating carbonation even at pCO2 as low as 0.04 kPa. Crucially, this IWF enables rapid carbonation of industrial alkaline wastes such as carbide slag under ambient conditions, obviating the requirement for pretreatment or other energy-intensive operations. Its demonstrated efficacy across different Ca-rich residues highlights its wide applicability and scalability for CO2 sequestration. Our findings establish water nanostructuring as a universal platform to unlock Earth-abundant minerals for scalable CO2 removal applications.

Introduction
Mitigating rising atmospheric CO2 levels is critical for addressing climate change, and carbon capture and storage (CCS) technologies are central to this endeavor1. Among the various strategies, mineral carbonation, converting CO2 into stable carbonate minerals, offers a promising route for long-term sequestration at gigatonne scales2–5. In particular, direct aqueous carbonation is considered as the most promising approaches due to it avoids the need to extract reactive components from minerals prior to carbonation, resulting in simpler operation and lower energy consumption4,6. However, practical implementation is hampered by slow reaction kinetics and limited selectivity, largely due to challenges in efficiently delivering CO2 to reactive mineral surfaces, which can be blocked by reaction products or impurities7–10.

An effective strategy to enhance reaction kinetics is to tailor specific surface environments that provide abundant, selective active sites for CO2. Various methods exist for controlling the surface environment, including grinding, thermal pretreatment of reactants, and surface modification of the reactant3,5. By modulating the surface environment, customized reactive sites can alter reaction pathways or influence the formation of product layers, thereby enhancing the kinetics of carbonation. Among the numerous surface control techniques, the formation of ultrathin water films on mineral surfaces has emerged as a compelling approach because of its environmental and economic advantages11,12. These water films with thicknesses ranging from angstroms to nanometers separate the CO2 from the reactive surface and serve as the primary medium for the desired dissolution-precipitation processes13–17. These quasi-two-dimensional thin water film environments potentially lower the activation energy for certain carbonation reaction systems18,19. The current factors contributing to the reduction of activation energy or the acceleration of carbonation reactions are argued to be attributed to the control of the dielectric constant and viscosity of the water film20. This, in turn, influences surface morphology, charge, ionic species, solubility, pH, ionic diffusion rates, and infiltration pathways21. Such mechanistic benefits have been demonstrated in silicate systems under elevated or supercritical CO2, where films thinner than 2 monolayers can activate dissolution and transport22,23, while intermediate films of several tens of monolayers induce passivation24. However, these reaction conditions remain too stringent for practical deployment.

At ambient temperature and pressure, Gao et al.25 and Tao et al.16 generated water films via water vapor adsorption. It was demonstrated that maximum CO2 uptake occurs at a critical water content where the onset of capillary condensation intensifies liquid–gas interactions, leading to adsorption-driven accelerated carbonation at an optimal humidity. These studies provide a useful framework for moisture-dependent carbonation, yet they remain constrained by the control of environmental humidity and are insufficiently robust under low CO2 partial pressures relevant to air.

In this study, we prepared an nanometer-scale water film from the liquid phase using a bulk-phase thinning strategy. In situ observations of water thinning and carbonation reaction rates enabled us to determine that the critical thickness of the interfacial water-film (IWF) that accelerates carbonation is ≈ 8 nm. Interestingly, the carbonation rate exhibited a sharp transition rather than a gradual change across the boundary. Furthermore, once within the interfacial region, the reaction mechanism followed the typical Michaelis–Menten kinetics. This phenomenon allows for the complete carbonation of calcium hydroxide (Ca(OH)2) within a time frame of just 1 h, which is 16 times faster than in bulk solution. Density functional theory (DFT) and ab initio molecular dynamics simulations reveal that this kinetic behavior originates from a structurally organized interface, where surface hydroxyl groups align adjacent water molecules into a hydrogen-bonded network that forms electrostatically stabilized “pocket-like” reactive sites. These sites promote CO2 enrichment, ion desolvation, and calcium–carbonate ion (Ca2+–CO32-) pairing, offering a mechanistic basis beyond water content alone. Importantly, we also observed sustained carbonation activity at atmospheric CO2 (≈ 400 ppm). These findings offer fundamental guidance toward developing scalable and energy-efficient routes for solid waste mineralization, with implications for direct air capture (DAC).

Results
Accelerated carbonation via IWF
We utilized an evaporation-deposition (ED) method to coat Ca(OH)2 precursors with IWF of varying thicknesses (see details in the Methods). Subsequently, we employed in situ Raman spectroscopy to monitor the carbonation process, as illustrated in Fig. 1a. Figure 1b presents the in situ Raman spectra of the carbonation process for 0.395 mmol Ca(OH)2 coated with a 24 nm IWF (film thickness calculation see Supplementary Fig. 1, Supplementary Tables 1, 2) under 101.32 kPa CO2 and 20 °C over 0–9 h. A broad peak corresponding to water is observed in the range of 3000–3600 cm−1, which decreases markedly within 0–7 h as bulk water is removed by the CO2 gas flow, but the residual signal remains detectable even at 9 h and did not completely vanish until 11 h (Supplementary Fig. 2). The peak intensity at 1087 cm-1, corresponding to the symmetric C–O stretching vibration mode (ν1) of calcium carbonate (CaCO3)26, only slightly increased until the 7 h, indicating a slow, minor carbonation process. After 30 min, the peak intensity at 1087 cm−1 increased, and vibrational bands at 712 cm−1 and 285 cm−1, corresponding to the doubly degenerate in-plane O–C–O deformation bending mode (ν4) of calcite (L), became evident simultaneously, indicating that the CaCO3 crystal phase formed during the carbonation process was calcite27,28. At the 8 h, the Raman peaks at 357 cm−1 and 3621 cm−1 associated with Ca(OH)2 had completely disappeared, with the 3621 cm−1 peak corresponding to the O–H stretching vibration29, leaving only the characteristic peaks of calcite. So, the carbonation reaction was completed rapidly within 1 h. As a comparison, the carbonation degree reached only 12% (see Supplementary Fig. 3) after 8 h in bulk water (244 nm, see Supplementary Fig. 4). It indicates that the IWF markedly accelerates carbonation at ambient temperature and pressure.

We conducted two-components multivariate curve resolution–alternating least squares (MCR–ALS)30,31 chemometrics analysis on the Raman spectroscopy data to determine the contents of Ca(OH)2 and CaCO3 (see details in the Methods), and the quantitative results show high consistency with those derived from thermogravimetric analysis (TGA) (see Supplementary Fig. 5). As depicted in Fig. 1c, the carbonation process begins with an induction period of 0 to 7 h, during which only weak characteristic Raman signals of CaCO3 are observed. During this period, as CO2 flow removed water from the surface of the solid precursor, the thickness of the IWF decreased from the initial 24 nm, first rapidly dropping to ≈ 9 nm, indicating the onset of capillary bridges formation32–34, and then continuing to decrease until reaching ≈ 8 nm (Fig. 1d). Subsequently, beyond the 7 h mark, the carbonation reaction immediately begins and exhibits a sharp increase of the carbonate content. The carbonation is quickly completed within 1 h and yields 0.395 mmol of CaCO3 corresponding to a carbonation degree of ≈ 100%. Reproducibility is confirmed by three independent runs, which show consistent trends (Supplementary Fig. 6). Accordingly, the time-dependent X-ray diffraction (XRD) results reveal a rapid formation of calcite (PDF card no. 83-0577) from 7 h onwards, accompanied by nearly complete disappearance of characteristic peaks associated with portlandite (PDF card no. 84-1263) for 8 h (Supplementary Fig. 7). By fitting a phase transition model (PT model) controlled by nucleation and mass transfer in liquid nanofilms (see details in the Methods)35, we ascertain that when the thickness of IWF decreases to ≈ 8 nm, the nucleation rate of CaCO3 is ≈ 8.4 Å−3 h−1 (the grey thick solid line in Fig. 1c). This rate exceeds 0.54 Å−3 h−1 and is about 16 times of that in bulk water (Supplementary Fig. 3). This indicates that the formation of CaCO3 is accelerated as water transitions from bulk form to the film thickness below 8.5 nm.

When the initial water content is adjusted by, for instance, setting the initial IWF thickness at 12 nm (Supplementary Fig. 8a), the induction period for the onset of carbonation reaction is shorten to 2 h. However, the onset of the rapid carbonation still occurs at an IWF thickness of ≈ 8 nm (Supplementary Fig. 8b). Different initial IWF thicknesses reach carbonation completion in almost the same time, indicating that once a thickness of below 8.5 nm is reached, carbonation will proceed at the similar nucleation rate at around 8.4 Å−3 h−1.

Confinement-induced reaction kinetics
We measured the carbonation process at CO2 flow rates of 2, 5, 15 and 20 mL min−1 (Supplementary Fig. 9). It is evident that as the flow rate increased from 2 mL min−1 to 20 mL min−1, the induction period before carbonation began decreased from 19 h to 4 h. This reduction may be attributed to the rapid airflow accelerating the thinning of IWF, thereby shortening the induction period. However, the time required to accelerated stage of carbonation remains constant at 1 h. This indicates that the acceleration of the carbonation reaction by the IWF is independent of the CO2 flow rate. We further assessed the impact of different CO2 partial pressures (0, 0.04, 0.25, 10.13, 50.66, and 101.32 kPa) on the accelerated stage of carbonation (Fig. 2a), in which the nucleation rates were fitted to 0, 2.4, 6, 8.4, 8.4, and 8.4 Å−3 h−1, respectively. Moreover, we compared the performance of the PT model with the Avrami–Erofeev (AE)36 and pseudo-second-order (PSO)37 models in describing the carbonation process. The PT model consistently exhibited higher R2 values than the other two models, demonstrating its superior effectiveness in describing the carbonation kinetics (see Supplementary Table 3). Even under ambient air CO2 pressure (0.04 kPa), the IWF still accelerated the carbonation reaction, indicating high selectivity for CO2. As the CO2 partial pressure increased, the nucleation rate progressively rose. However, the saturation was reached at 10.13 kPa, where the nucleation rate plateaued. We performed the kinetic fitting of the nucleation rates at different CO2 partial pressures (Fig. 2b) and confirmed that they follow Michaelis–Menten kinetics38–40, adhering to the corresponding equation:
τ = τmax pCO2 / (K + pCO2) (1)
where τ is the carbonation nucleation rate (Å−3 h−1), τmax represents the maximum nucleation rate of the carbonation reaction (Å-3 h-1), pCO2 is the partial pressure of CO2 (kPa), and K denotes the CO2 partial pressure at which the nucleation rate τ reaches 0.5 τmax (kPa). This is a typical saturation kinetics, implying that the reaction site can selectively accommodate the substrate in a pocket-like manner and become rapidly saturated by these substrates41. As illustrated in Fig. 2b, in the IWF, a fixed number of pocket-like reaction sites restrict and control the spatial configuration of the substrate, selectively capturing CO2 (see Supplementary Fig. 10 for a schematic illustration of the structural features of these “pocket” sites). Form the fitting with Eq. (1), we obtain the critical CO2 concentration at the pCO2 of 2.02 kPa, after which the nucleation rate no longer increases with further increments in partial pressure. It suggests that these “pocket-like sites” can be fully saturated at this sub-atmospheric pressure, thus enabling nucleation rates even at low CO2 partial pressures42. In line with these findings, we further carried out continuous air carbonation at room temperature, during which nearly complete carbonation of Ca(OH)2 was achieved after nine successive IWF renewals (Supplementary Fig. 11), thus experimentally validating the capability of IWF to drive full carbonation under ambient air conditions.

Accelerated nucleation behaviors
Figure 3a shows a scanning electron microscopy (SEM) image that visually demonstrates the typical structure of Ca(OH)2 composed of plate-like polycrystalline aggregates, with the pore structure consisting of slit-like pores formed by the stacking of these plate-like crystal grains, consistent with slit-like mesopores observed in the BJH pore size distribution (combine with Supplementary Fig. 12)43. As the carbonation reaction progresses at 20 °C, the surface of the Ca(OH)2 becomes covered by nanoparticles (Fig. 3b), with sizes ranging from several tens to hundreds of nanometers. No micron-sized crystals were observed. After the completion of the carbonation process (Fig. 3c), the product remains at the nanoscale, and these nanoscale particles no longer exhibit any features of the precursor’s morphology. Instead, they display the characteristic rhombohedral morphology of calcite (see the inset of Fig. 3c)44. Under thin water film conditions, the carbonation products are dispersed, isolated calcite microcrystals rather than a dense layer45 covering the surface of portlandite nanoparticles. Such a morphology allows the reactive surface to remain continuously exposed to CO2 and water, effectively preventing product-layer passivation. The high mobility and dynamic turnover of the water film likely facilitate this mechanism by promoting ion transport and continuously removing products from the surface. As a result, carbonation proceeds rapidly throughout the system. In contrast to the carbonation in bulk water, only a small number of CaCO3 nanoparticles can be observed at the corresponding time, with Ca(OH)2 partially covering the surface, leaving a substantial portion of unreacted areas exposed (Supplementary Fig. 13). These results further confirm the high nucleation rate of calcite within the IWF. To further evaluate the effect of particle size, a large-particle Ca(OH)2 sample with a broad micrometer-scale distribution and blocky morphology was tested in IWF (Supplementary Figs. 14b, 15, 16b). Despite its difference from the nanoscale precursor in both particle size and morphology (Supplementary Figs. 14a, 15, 16a), this sample exhibited nearly identical carbonation behavior, achieving complete conversion within 1 h (Supplementary Fig. 17). These results indicate that carbonation reactivity in our system is minimally affected by particle size but instead governed by IWF-mediated processes.

We collected high-resolution transmission electron microscopy (HRTEM) data at the 7.5 h of the reaction (Fig. 3d), during which the precursor had undergone partial carbonation. We can observe well-defined Ca(OH)2 particles along with freshly formed CaCO3 adhering to them. The CaCO3 appears in the form of small particle aggregates. The selected area electron diffraction (SAED) results in the inset further confirm the coexistence of the two phases. By magnifying the joint region between the Ca(OH)2 precursor and CaCO3 in Fig. 3d, we can clearly observe an amorphous area at the junction, as shown in Fig. 3e. This interfacial amorphous layer is characterized by a lack of long-range order and reduced structural coherence, and likely functions as a transitional zone that facilitates non-classical nucleation pathways. The structural disorder and chemical heterogeneity within this region can lower the interfacial energy barrier for nucleation46,47, thereby promoting the heterogeneous crystallization of calcite at the substrate surface. The formation of abundant rhombohedral calcite single crystals observed at the end of the reaction (Supplementary Fig. 18) provides further support for the role of this amorphous interphase in enhancing interfacial nucleation and subsequent crystal growth.

Mechanism of confinement-induced carbonation
We utilized atomic force microscopy (AFM) to observe the surfaces of clean samples (Fig. 4a) and samples with IWF (Fig. 4c). Force curve mapping of the marked points is displayed in the top and bottom sections of Fig. 4b, respectively. The clean surface of Ca(OH)2 and the surface with IWF exhibit average surface roughness (Ra), measuring 5.61 nm and 3.05 nm, respectively. The addition of water results in a smoother surface of Ca(OH)2, likely due to the partial dissolution of some exposed protrusions on high-energy surfaces in the aqueous environment, which can provide more active sites for the carbonation reaction48. It is noteworthy that the force curves of Ca(OH)2 with IWF exhibit a distinct resistance effect during the retraction of the probe; the retraction was affected by the adhesion force of the IWF, resulting in a wider retraction distance (Fig. 4b). This confirms the presence of strongly adsorptive water layers on the sample surface49.

We further investigated the surface of the calcium hydroxide precursor using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) (Fig. 4d), constructing the IWF with deuterated water with the aim to simplify the DRIFTS fitting and analysis by obtaining an O–D stretching mode through Gaussian peak fitting (see details in the Methods). Based on the results of vibrational analysis using DFT methods, we calculated the theoretical infrared spectra at the interface of Ca(OD)2 and water (Fig. 4e). By fitting the DRIFTS results with these theoretical vibrational peaks, our findings indicate that the absorption spectrum of the initial precursor can be deconvoluted into five peaks (the upper spectrum in Fig. 4d). Specifically, the peak I at 2680 cm−1 corresponds to the O–D stretching vibration in Ca(OD)250; the peaks II at 2706 cm−1 and 2620 cm−1 correspond to the symmetric and asymmetric O–D stretching vibrations in bulk water51, respectively; the peak III at 2514 cm-1 represents the O–D stretching vibration of weakly deuterium-bonded52 interfacial water as well as interfacial O–D vibrations, the peak IV at 2290 cm−1 reflects the O–D stretching vibration of strongly deuterium-bonded interfacial water53. As the IWF becomes thinner (the middle spectrum in Fig. 4d), the intensity of the symmetric and asymmetric O–D stretching vibrations of bulk water diminishes, while the signal attributed to the O–D stretching of interfacial water increases, indicating a higher proportion of interfacial water in the thinner films. When the IWF thickness decreases below 8.5 nm (the lower spectrum in Fig. 4d), the O–D stretching vibrations of bulk water disappear. Additionally, the O–D stretching vibrations of strongly deuterium-bonded interfacial water become invisible, leaving only interfacial O–D vibrations and the weakly deuterium-bonded O–D vibrations of interfacial water. It suggests that only interfacial water exists at this stage. The disappearance of strongly deuterium-bonded interfacial water is probably due to the fact that when the concentration of dissolved ions (e.g. Ca2+) in the system rises, the ions progressively disrupt the hydrogen-bond network54.

Further, we employed DFT calculations to investigate the solvent structure near the Ca(OH)2–water interface and the carbonation reaction under its constraints. Firstly, through ab initio molecular dynamics (AIMD) simulations, we examined the equilibrium water structure near the interface. As shown in Fig. 5a, the OH groups in water spontaneously adsorb onto the highly reactive Ca(OH)2 surface (see the crystal structure of Ca(OH)2 in Supplementary Fig. 19), forming a chemically adsorbed layer. In this layer, oxygen atoms from the OH groups are ionically bonded to specific Ca atoms on the solid surface, resulting in a highly oriented OH layer. By analyzing the distribution of O and H atoms along the Z-axis over 15 ps of equilibration (Fig. 5b), we can clearly observe that this oriented OH remains stably adsorbed on the substrate surface terminated with Ca atoms. Due to the directional nature of hydrogen bonding20, an oriented layer of water molecules forms above the OH layer, located ≈ 4 Å from the solid surface, corresponding to the Z-axis of the second peak in Fig. 5b. These oriented interfacial water molecules capture Ca2+ and CO32- species within pocket-like sites through electrostatic interactions and hydrogen bonding21. Our metadynamics analysis55 reveals that both Ca–O and C–O coordination numbers increase in the presence of surface-bound OH- groups (Supplementary Fig. 20) than H2O, suggesting a more favorable environment for the formation of Ca–O–C linkages, which in turn facilitates the nucleation of CaCO3. The rapid saturation of the captured Ca2+ and CO32- results in a high degree of supersaturation relative to CaCO3, thereby accelerating the carbonation reaction.

Furthermore, through simulations of the water film using Clay force filed (ClayFF) (Supplementary Figs. 21, 22), as shown in Fig. 5c, the radial distribution function (RDF) between the carbon atoms of carbonate ions and the oxygen atoms of water molecules reveal that, in regions close to the interface (Z=0–4 nm), the RDF values are markedly lower than those at greater distances. This indicates that CO32- near the interface is coordinated with fewer water molecules, i.e., their coordination environment is less saturated. This may originate from the interactions between lattice Ca2+ and CO32- with the solid surface of the “pocket-like sites” occupy the coordination spaces otherwise available to water, resulting in the coordination unsaturation at the interface56. The coordination unsaturation could influence the nucleation barrier by affecting the desolvation energy of ions57.

Therefore, we further investigated the reaction barrier for the formation of CaCO3 through the combination of Ca2+ and CO32- under different water coordination numbers (as illustrated by the reaction equation in Fig. 5d). As shown in Fig. 5d, the carbonation barrier reaches its highest point when only a single water molecule coordinates with the respective ion. Under such conditions, where the number of water molecules is low, resembling those during gas-solid reactions, the barrier is highest, and the carbonation reaction proceeds very slowly (see Supplementary Fig. 23). Increasing the coordination to two water molecules will greatly lower the carbonation barrier. The barrier reaches its lowest value (224.93 kcal mol−1) when the ion is coordinated by three water molecules, matching levels observed with critical IWF (< 8.5 nm). Beyond eight water molecules, resembling bulk water, the barrier increases again. This trend aligns closely with experimental observations, where carbonation rates peak at IWF thicknesses below 8.5 nm, reaching 16 times the nucleation rate in bulk water. The non-covalent interaction (NCI) analysis results illustrate the interactions between CO32- and Ca2+ with varying numbers of coordinated water molecules (Fig. 5e). It can be observed that the interaction between CO32- and water molecules largely relies on hydrogen bonding and strengthens as the coordination number increases. In contrast, the interaction between water molecules and Ca2+ is mainly ionic and weakens with increasing coordination number. A critical point emerges when ions are coordinated with three water molecules, minimizing the energy required for Ca2+ and CO32- to desolvate and form water-coordinated calcium carbonate ion clusters, and the electrostatic interaction between Ca2+ and CO32- is maximized. This suggests that the oriented chemical bonds near the interface can influence the coordination environment of ions to reduce the ionic desolvation energy. As a result, the coordination unsaturation of Ca2+ and CO32- at the interface confined in the “pocket-like sites” will obviously lower the carbonation barrier to accelerate the precipitation of CaCO3 under ambient conditions.

Methods
Construction of IWF
IWF was deposited on the surface of Ca(OH)2 particles using the ED method. Specifically, 0.15 g CaO (analytical reagent, 98%, Macklin Co., Ltd.) was placed in an alumina crucible within a stainless steel reactor 0.1 or 0.5 mL deionized water was added near the crucible to control the thickness of the IWF. The inlet and outlet valves of the reactor are then sealed, and the temperature is raised to 100 °C to generate water vapor, maintained for 1 h. The reactor was subsequently cooled to room temperature, resulting in wetted Ca(OH)2 precursors with 12 or 24 nm IWF. For the bulk solution method, the amount of water added near the crucible in the ED method is increased to 1.5 mL. The samples obtained after the addition of water were respectively named ED/P/0.1, ED/P/0.5, ED/P/1.5.

Carbonation experiments
We employed in-situ Raman spectroscopy to monitor the carbonation process. A sample of 0.0392 g ED/P/0.5 or 0.0357 g ED/P/0.1 was placed in an in-situ reaction cell. To investigate the effect of CO2 flow rate, a mass flow controller (MKS, 179A) regulated the overall integral gas flow rates to 2, 5, 15, and 20 mL min−1, while maintaining the CO2 partial pressure at 101.32 kPa. As for the overall integral gas flow rate was kept constant at 15 mL min−1, the different proportions of N2 were mixed according to the desired CO2 partial pressures (0, 0.04, 0.25, 10.13, 50.66, and 101.32 kPa). The reaction temperature was consistently maintained at 20 °C. Raman spectra were collected using an HR800 Raman spectrometer (Jobin Yvon, France) equipped with an Olympus BX40 microscope (focal length 1 µm) and a 532 nm laser. To minimize background interference, fluorescence detrending was performed using baseline correction and cosmic ray removal (LabSpec, Horiba Scientific, USA). For bulk water conditions, the carbonation process caused changes in the surface water layer thickness of Ca(OH)2 due to gas flow, leading to challenges in maintaining clear focus during in situ Raman monitoring. To address this, the precursor ED/P/1.5 was placed in a stainless-steel reactor, where carbonation experiments were conducted by introducing CO2 at 101.32 kPa at a flow rate of 15 mL min−1.

Characterization
The crystallinity and chemical composition of the samples before and after carbonation were analyzed by XRD (D8 Advance, Bruker, Germany) with Cu-K α radiation (λ=1.5418 Å) at 40 mA and 40 kV. Scans were performed over a 2θ range of 10° to 50° with a step size of 0.05° s−1. A field emission scanning electron microscope (FE-SEM, JSM-7900F, JEOL, Japan) was employed for morphological analyses. Prior to imaging, the samples were coated with a platinum (Pt) layer using a JEC-3000FC sputter coater (JEOL, Japan) to enhance conductivity. The carbonation precursor and products were dispersed in methanol and deposited onto porous carbon grids (Agar Scientific) for imaging analysis using TEM (JEM-F200, JEOL, Japan) operating at an acceleration voltage of 200 kV. The specific surface area of the dry precursor was measured and calculated using a physical adsorption analyzer (ASAP 2020 PLUS HD88). Particle size distributions of nanoscale and micron-sized Ca(OH)2 samples were measured using laser diffraction instruments, the Zetasizer Nano ZS90 and Mastersizer 2000, respectively. The chemical composition of fresh solid wastes (CS, SS and FGDR) was analyzed using X-ray fluorescence spectroscopy (XRF, Malvern Panalytical Zetium, Netherlands).

Verification of IWF presence using AFM
To verify the presence of the IWF, 0.0292 g of anhydrous Ca(OH)2 and 0.0392 g of ED/P/0.5 were selected as samples, with ED/P/0.5 transferred to AFM (Bruker MultiMode 8) for testing once the accelerated carbonation reaction began. Mica sheets were used as substrates. AFM force–volume contact mode was conducted at room temperature using silicon tips mounted on nitride cantilevers (SCA-NASYST-AIR, Bruker; length 115 µm, width 25 µm, thickness 650 nm, resonant frequency 70 kHz, spring constant 0.4 Nm−1, tip radius ≤ 2 nm, reflective front coating). Samples were sealed with Parafilm immediately after preparation and measured promptly under ≈ 50% RH to preserve IWF. Low-force settings and hydrophobic tips were applied to enhance sensitivity to capillary/adhesive interactions, enabling reliable detection of IWF via adhesion features in the force–distance curves. Ra was also determined from the AFM topography data.

MCR–ALS analysis
Two-components MCR–ALS chemometric analysis was performed on the datasets under different partial pressures (0, 0.04, 0.25, 10.13, and 50.66, 101.32 kPa) and initial IWF thicknesses (12 nm and 24 nm). The MCR–ALS results calculate the relative concentration of Ca(OH)2 and CaCO3 as functions of time. The initial mass of Ca(OH)2 involved in the reaction, 0.0292 g, was converted into the corresponding molar amount 0.395 mmol.

Raman–TGA validation of CaCO3 quantification
To validate the quantification of carbonation using Raman spectroscopy, a series of reference samples were prepared by physically mixing known ratios of Ca(OH)2 and CaCO3 to represent different levels of CaCO3 content. These samples were characterized by both TGA and Raman spectroscopy (Supplementary Fig. 5). In the TGA analysis, the weight loss attributed to CaCO3 decomposition was evaluated in the temperature range of 500–850 °C58, which effectively excludes interference from surface-bound water and hydroxide dehydration. The CaCO3 content wCaCO3 (wt.%) was obtained using the empirical relationship:
wCaCO3 = Δm / 0.44 * 5.32 wt.% (R2=0.9948) (2)
where Δm is the weight loss between 500 °C and 850 °C (wt.%), 0.44 is the dimensionless mass fraction of CO2 in CaCO3, and the subtraction of 5.32 (wt.%) accounts for the apparent mass loss observed in the 0 wt.% CaCO3 reference sample (pure Ca(OH)2).

Raman spectra were collected from the same reference samples, and the Raman-derived CaCO3 contents were compared with those obtained from TGA. A strong linear correlation (R2=0.9970) was obtained:
y = 1.01x + 2.13 wt.% (3)
where x is the TGA-derived CaCO3 content (wt.%) and y is the Raman-derived CaCO3 content (wt.%). The coefficient 1.01 is dimensionless because both variables are expressed in wt%. The slope being close to unity and the small intercept (≈ 2.13 wt.%) confirm the high consistency and reliability of Raman-based quantification in comparison with TGA. Ref. 62 has also reported quantitative relationships between the carbonate content and Raman peak intensity (≈ 1084 cm-1) during cement paste carbonation, further validated by TGA measurements.

Calculation of real-time IWF thickness
The IWF thickness d (nm) was calculated as:
d = (mw / ρ * Vpore) * 10^3 / (ρ * Sext * mp) (4)
where mw is the mass of water covering Ca(OH)2 (g), and ρ is the density of water, typically 1 g cm−3, Vpore is the cumulative pore volume of 0.0757 cm3 g−1, Sext is the external specific surface area (m2 g−1), and mp = 0.0292 g is the mass of fully dried Ca(OH)2 precursor. The factor 10^3 is a unit-conversion factor that converts the calculated thickness from μm to nm.

To obtain Sext, we applied a surface-area balance:
Sext = SLangmuir - SBJH - Smicropore + Sbase (5)
where SLangmuir = 16.9881 m2 g-1 is the Langmuir surface area, SBJH = 14.8933 m2 g−1 denotes the BJH mesopore wall contribution, Smicropore = 0.6414 m2 g−1 is the micropore area, and Sbase accounts for pore end-faces that are not included in the side-wall area. Following an equivalent-cylinder approximation for each pore-size bin i, the two-end base area is obtained from the incremental pore area Fi (Supplementary Table 2) as
Sbase = 1/λ * Σ Fi (6)
with a pore-depth factor λ = 0.8 (dimensionless) to parameterize the average pore segment height hi = λDi relative to the bin diameter Di. As emphasized by the IUPAC technical report, pore shape and aspect ratio are model-dependent assumptions rather than universal constants; λ is a pragmatic geometric simplification63.

To estimate the mass of water covering Ca(OH)2, the samples were weighed before and after vacuum drying. The weighing was performed using an analytical balance (Mettler Toledo ME204E, readability = 0.1 mg, linearity ±0.2 mg), which corresponds to an uncertainty of ±0.0001 g in mw. Propagation of this error into the thickness calculation yields an uncertainty of ≈ ±0.32 nm for the IWF thickness, which does not alter the main conclusion regarding the critical thickness regime. To calculate the real-time variation in IWF thickness, Ca(OH)2 precursor prepared using the ED method (with an added water volume of 1 mL) was dried at 60 °C to obtain vary water contents (mw) at time intervals (see Supplementary Table 1). Using Eq. (4), the corresponding thickness of the IWF was calculated. Raman spectra of Ca(OH)2 at different water contents were then collected (Supplementary Fig. 1a) to determine the integrated area A in arb. u. of the O–H stretching vibration peak of H2O (3000–3600 cm−1) (see Supplementary Table 1). A polynomial fit between A and the IWF thickness yielded (Supplementary Fig. 1b):
d(t) = 4.75 * 10^-10 nm * A^2 - 8.80 * 10^-5 nm * A + 12.27 nm (7)
where d(t) is the IWF thickness (nm) at time t. The coefficients (4.75 * 10^-10 nm·(arb. u.)−2, 8.80 * 10^-5 nm·(arb. u.)−1, and 12.27 nm) are empirical fitting parameters. The integrated area of the O–H stretching vibration peak of H2O (3000–3600 cm−1) during carbonation can be substituted into Eq. (7) to obtain the real-time thickness of the IWF. Due to limitations in Raman detection, the O–H stretching vibration peak of H2O (3000–3600 cm−1) is undetectable when the IWF thickness falls below 8 nm.

Distinction of interfacial and bulk water by DRIFTS
DRIFTS were recorded using a Bruker Tensor II spectrometer equipped with a CaF2-window reaction cell and an MCT detector. The calcium hydroxide precursor was prepared via the ED method by adding 50% (v/v) D2O (99.90 at.% 2H, Macklin, China) to 0.5 mL of water. Subsequently, 0.0392 g of the sample was placed in the reaction cell, with CO2 introduced at 15 mL min−1 and a pressure of 101.32 kPa. The resolution was set to 4 cm−1, and spectra were collected over time (0, 0.2, 7 h). Isotope-diluted HDO was employed instead of pure H2O to analyze the O–D stretching frequency shifts, simplifying the fitting and analysis of the interfacial water structure.

Tracking the degree of carbonation via XRD
We take the time evolution of the normalized reflection intensity of calcite (104) collected by XRD at d = 3.0 Å as a measure of portlandite carbonation conversion fraction α(t):
α(t) = [I(t) - I(t=0)] / max[I(t) - I(t=0)] (8)
where I is the XRD diffraction intensity and t is the time. This method has been proven in the study of quantifying the dehydration process of gypsum35.

Determination of CO2 sequestration capacity and carbonation degree of solid wastes
The thermal characteristics of fresh and carbonated solid waste samples were analyzed using a PerkinElmer STA 8000 instrument to determine the CO2 sequestration capacity. Samples were heated from 30 to 1000 °C at a constant heating rate of 10 °C min−1 under a flowing N2 atmosphere. The weight loss observed in the temperature ranges of 30–105 °C, 105–500 °C, and 500–850 °C was attributed to physically adsorbed water evaporation, hydroxide (e.g., Ca(OH)2) and MgCO3 decomposition, and CaCO3 decomposition, respectively58,59. The CO2 mass fraction ωCO2 was calculated from the dry mass at 105 °C and the weight loss between 500 and 850 °C:
ωCO2 = Δm500–850°C / m105°C (9)
where Δm500–850°C and m105°C are expressed in g. The CO2 sequestration capacity K (g CO2 per g dry solid) was calculated as:
K = ωCO2 / (1 - ωCO2) (10)
and reported as g CO2 per kg dry solid by converting g to kg. “1” denotes the total mass fraction of the sample (dimensionless).

To obtain the net CO2 sequestration capacity, the intrinsic CO2 content of the fresh solid waste (arising from pre-existing CaCO3) was subtracted from the measured values. For CS, the carbonation degree was determined by normalizing the net CO2 sequestration capacity obtained under each method to the CO2 sequestration capacity of fully carbonated CS:
carbonation degree = (Kmethod - Kfresh) / (Kfull - Kfresh) (11)
where Kmethod is the sequestration capacity under a given carbonation method, Kfresh is that of fresh CS, and Kfull is that of fully carbonated CS.

Kinetic model of calcite nucleation and growth
Our kinetic model posits that the conversion of portlandite to calcite takes place exclusively during the nucleation and growth phases, and it characterizes the changes in the conversion rate (α(t)) through time integrals of nucleation and growth volumes. Based on this model, we obtained the nucleation rate and growth flux by fitting the time series of the conversion rate (α(t)) obtained from in situ Raman spectroscopy. The result presented in this paper is as follows:
α(t) = [Jσ * ετ * (Jσ * Vs * ετ)^-1 * Vs * ετ * et * (Jσ * Vs * ετ)] / [Jσ * Vs * ετ] * e^(-t * Jσ * Vs * ετ) (12)
Let Ve denote the volume of calcite after complete carbonation. At time t the total volume can be expressed as α(t)Ve = Vn(t) + Vg(t), where Vn(t) represent the volume of accumulated nuclei up to that time, and Vg(t) corresponds to the volume gained from the growth of these nuclei. The nucleation rate τ indicates the number of nuclei generated per unit volume of calcite per unit time, allowing us to describe Vn(t) as the integral over time of Vs * τ * V(t), where Vs is the volume of an individual nucleus, τ is nucleation rate, and V(t) is the remaining volume of portlandite at time t. Given that the molar quantity of CaCO3 remains constant during the phase transition, V(t) can be expressed as the variation in ε(1-α(t))Ve, where ε is the molar volume ratio portlandite and calcite, yielding:
Vn(t) = ∫0t Vs * τ * ε(1-α(t))Ve dt (13)
Given the observation of nuclei with varying sizes, which suggests that particle growth due to Ostwald ripening is not significant, we explicitly assume that the specific surface area (σ) of the formed portlandite remains constant. Here σ was determined to be 12.3811 m2 g−1 using the BET method. Consequently, Vg(t) represents the accumulated growth volume up to time t and can be expressed as:
Vg(t) = ∫0t Jσ * α(t)Ve dt (14)
At this point, the volume added to the calcite due to growth is equal to the accumulation of the volume flux J over time and surface area. Combining this with Eq. (13) yields:
α(t)Ve = ∫0t Vs * τ * ε(1-α(t))Ve dt + ∫0t Jσ * α(t)Ve dt (15)
Considering the boundary condition that α(t)=1 at t = tmax, solving the differential Eq. (15) gives:
J = - W * (Vn * tm * ετe^(-Vn * tm * ετ)) / σ (16)
Here, W represents the Lambert W function. Thus, the flux is determined by the nucleation rate τ and the time tmax when the phase transformation is expected to be completed.

Additionally, the constant ki describes the induction period of the transformation (α(t)<0.01) and is modeled using zero-order kinetics. During the induction period, the concentration of the reactant, specifically the solubility of portlandite, remains constant as it is in a pure phase:
dα / dt = ki (α≤0.01) (17)

Computational details
We performed all calculations of the first principle employing the CP2K64 program package. All the atoms were described by the DZVP-MOLOPT-SR-GTH basis sets65. In the mixed Gaussian-plane-wave scheme (GPW), the atomic core electrons are described by Goedecker-Teter-Hutter pseudopotentials66. The Perdew-Burke-Ernzerhof (PBE) functional with D367 correction was adopted for describing the electronic structure68,69, and a cutoff of 450 Ry for the auxiliary plane-wave basis set was used. Geometry optimization was performed until all the forces in the atoms were smaller than 0.25 eV nm–1. The Born-Oppenheimer molecular dynamics (BOMD) simulation is to simulate the water molecules on Ca(OH)2. Production runs of 20 ps and the gamma-only k-mesh were used in all AIMD calculations and analysis. The successive simulations with micro-canonical (NVE) ensemble were carried out with a timestep of 0.5 fs, where the initial temperature was set at 300 K by a Nose–Hoover chain thermostat. The models are drawn by the VMD 1.9.3 software70.

Metadynamics calculations were performed using CP2K to investigate the free-energy landscape associated with the coordination environment of interfacial species. The collective variables (COLVARs) were defined as the coordination numbers of the interfacial Ca and C atoms with surrounding oxygen atoms. Prior to the metadynamics runs, the system was equilibrated in the canonical (NVT) ensemble to ensure structural stability. A 5-ps trajectory from the equilibrated system was then collected for the evaluation of the COLVARs. No bias potential was applied during these simulations, such that the free-energy profiles reflect the intrinsic configurational sampling of the system.

Molecular dynamics simulations of nanoscale water films were conducted with the LAMMPS simulation package71. The system consisted of over 108,270 atoms in total. Simulations were conducted in the NVT ensemble at a fixed temperature, with a total production time of 5 ns. The simulation setup included a slab layer of 20 Å thickness and an effective water film of 140 Å thickness. Interactions were described using the ClayFF, which has been widely applied for modelling hydrated mineral surfaces and water–solid interfaces.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Report on the manuscript intitled “Ultrafast CO2 Mineralization via Water Thinning” by\nMin Su, Chunli Wang, Yufang Wang, Zhanxin Geng, Jing Zhang and Zhang Lin\nThis paper reports on mineral carbonation using the conventional aqueous route that remains energy-intensive and\nkinetically slow (at the scale of millimeters per year). More specifically, this work focuses on the use of a water thinning\nstrategy aiming at engineering the nanometric-thick interfacial water-film (IWF) to achieve calcium hydroxide ([Ca(OH)2]\ncomplete carbonation within 1 h under ambient conditions. This IWF-based strategy is shown to be 16 times faster\ncompared to bulk solution systems. Carbonation efficiency was experimenatally measured using Raman spectroscopy, time-\ndependent X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), Atomic Force Microscopy\n(AFM) and reflectance infrared Fourier transform spectroscopy (DRIFTS).\nThe authors went on into providing explanations of the observed phenomenon that involve the concept of an enzyme-\nmimetic confinement effect with “biomimetic reaction pockets” that enhance CO2 capture efficiency even at pCO2 = 0.04\nkPa” that is the content of CO2 in ambient air. This is the key achievement of the present work that puts the nanoconfined\nIWF as the enabler for a direct utilization of industrial alkaline wastes such as slags, achieving near-complete carbonation\nopening the route to unlock Earth-abundant minerals for scalable CO2 sequestration applications.\nThis is timely and well-written paper that merits publication in Nature Communication once the following comments and\nindications are fully addressed.\n1- First, the authors have to clarify what they actual mean by “enzyme-mimetic confinement effect. Is the “pockets” conclusion\nsolely based on the fact that the acceleration of the carbonation reaction is independent of the CO2 flow rate and the\nresemblance with the Michaelis-Menten kinetics equation? What are the attributes of these “pockets” is terms of pore size,\nsurface chemistry and extended defects…To me, this is a too-far stretch. Can the author defend their position or consider\nremoving the enzyme resemblance section?\n2- The authors have to position the novelty of their work with respect to recent papers that precisely address the role of\nnanometric IWF for an efficient carbonation of minerals such as Portlandite. In particular, I urge the authors of the present\nwork to cite and compare their approach/results with that recently proposed by Y. Gao et al, Moisture-driven carbonation\nkinetics for ultrafast CO2 mineralization, Proceedings of the National Academy of Sciences, 122, 1, e2418239121, 2025 and\nY. Tao et al, Optimal CO2 intake in metastable water film in mesoporous materials, Nature Communications, 15, 10790,\n2024. In these papers, the authors utilized experimental/atomistic simulations approach to unravel the moisture-driven\nmineral carbonation mechanisms. They show that maximum CO2 uptake occurs at a critical water content where the onset\nof capillary condensation intensifies liquid–gas interactions, leading to adsorption-driven ultrafast carbonation at an optimal\nmoisture content. Conversely, they show that high moisture shifts carbonation kinetics to a surface dissolution–driven\nprocess hence they provide a very unified view for the very-debated optimal CO2 mineralization conditions.\nSo, the main finding of the present paper on the role a nanometric IWF, is not exactly new but has the merit of comforting\nvery recent results on a very important subject such as the permanent docking of atmospheric CO2 as a solid form\n(limestone). To me the novelty of the present paper lies in the fact that it addresses the concentration of CO2 in the\natmosphere. But this point although claimed as an achievement, needs to be strengthened and further demonstrated. This is\nabsolute priority.\n3- The authors of the present work complemented their experimental findings with some Density functional theory (DFT)\ncalculations and unraveled the role of oriented hydroxyl (OH) groups present in the IWF, which provide low-energy barrier\n“pocket-like” reactive sites. This seems to me a very limited view that does not compare with advanced molecular methods\nsuch as metadynamics for reaction free energy barrier calculations using either ab initio energy calculations or reactive\ninteratomic potentials (see for instance and refer to Xinping Zhu et al, Collective molecular-scale carbonation path in\naqueous solutions with sufficient structural sampling: From CO2 to CaCO3, J. Chem. Phys., 14;161(18), 2024). I urge the\nauthors of the present work to position their work with respect of these more modern and reliable numerical techniques to\nevaluate free energy barrier. They also need to compare their values to experimental literature.\nI do agree with the fact that the electrostatic interaction between Ca2+ and CO3-2- ions dominates in the formation of\ncarbonates but is mediated through the water in the IWF. A dissolution of Ca2+ step (going from the solid Portlandite phase\ninto solution, so that it can eventually react with the carbonate ions and form carbonates phases) is necessary and not\nshown in the modelling part of the present paper.\nCa2 dissolution has been shown to be related to the ionic strength of the IWF (pH and other ions, see and refer to Y. Yong et\nal, CSH decalcification in seawater: The view from the nanoscale, Cement and Concrete, 175, 107385, 2024). I see no\nmention of ionic strength or pH in the modelling part of the present paper that to me, considers an oversimplistic desolvation\nand water-coordinated calcium carbonate ion clusters formation mechanism. Furthermore, the “pockets” explanation from the\nDFT part of this manuscript is extremely local (OH groups) and by the way not at all at the spatial scale of an enzymatic\nreaction.", "reviewer_2": "This work explores the use of ultrathin water films on mineral surfaces as a promising strategy to significantly enhance the\nperformance of mineral carbonation processes for Ca-rich wastes. Specifically, by applying a water-thinning approach to\nengineer interfacial water films (IWFs) with thicknesses below 13.5 nm, the authors report complete carbonation of\nPortlandite (Ca(OH)₂) within 1 hour under ambient conditions, which resulted in a 16-fold acceleration compared to\nconventional bulk solution systems. The authors further state that this nanoconfined water architecture is applicable to\nindustrial alkaline wastes, such as carbide slag, achieving near-complete carbonation without any pretreatment—a process\nthat typically takes several days in standard slurry reactors. According to the manuscript, the IWF exhibits enzyme-mimetic\nconfinement effects, forming biomimetic reaction pockets that lower desolvation energy barriers and improve CO₂ capture\nefficiency. The manuscript has a degree of novelty but require major revision. Specific comments:\n1- However, the main bottleneck in mineral carbonation technologies remains the extraction of reactive oxides (e.g., CaO,\nMgO) from bulk waste or rock. The carbonation step itself is not the primary limitation to broader deployment and this must\nbe clarified in the manuscript.\n2- Ca(OH)₂ (portlandite) is significantly more reactive than CaO commonly present in Ca-rich wastes. An important aspect\nthat is insufficiently addressed is the particle size of the synthesized portlandite powder. This parameter is neither clearly\nreported nor easily assessed from the data provided, yet it can strongly affect reactivity. Therefore, the particle size\ndistribution of the tested materials should be explicitly analysed and stated.\n3- The comparative performance results for carbide slag (e.g., Fig. S12) are misleading. Several existing studies\ndemonstrate similar carbonation efficiency under ambient conditions using conventional reactors and the same material,\ncalling into question the claimed exceptional performance. Examples from the literature include:\n• Bonfante et al., Materials and Structures, 2024, 57", "reviewer_181": "Cement bypass dust carbonated at atmospheric pressure and low\ntemperature (20–60°C) within 20–60 minutes achieved a carbonation degree of 82%, outperforming earlier studies.\n• Li et al., Separation and Purification Technology, 317, 2023", "reviewer_123932": "Direct aqueous carbonation of carbide slag (72.8% CaO content vs. 89% in the present work) under ambient conditions showed a CO₂ capture capacity of 214.05 g CO₂/kg slag within 10 minutes.\nThese studies (and other available) need to be discussed and clearly compared with this work.\n4- Page 3 – Lines 79–83: It is unclear why the water signal disappears during carbonation, especially considering water is\ngenerated in the reaction. This needs clarification.\n5- Carbonation efficiency on page 3 is evaluated using Raman spectroscopy for quantifying carbonation extent. The authors\nshould validate this measurement using another technique such as thermogravimetric analysis (TGA) to strengthen the\nreliability of the results.\n6- The discussion on enzyme-like catalysis is somewhat ambiguous. As shown in the references above, similar carbonation\nrates have been achieved without employing nanometric interfacial water films.\n7- Page 5 – Lines 157–160: If the portlandite nanoparticles are completely coated under the thin-film process, does the\nformation of a passivating product layer limit further carbonation as many previous works have shown? This important issue\n(when scale-up is applied) is not discussed (in relation to the many available previous works) and the kinetics should\nconsider it.\n8- While the mechanistic explanation is reasonable, the broader discussion (particularly on Page 8) is weak and misleading.\nThe study does not address the fundamental challenge in mineral carbonation: transporting reactive cations (Ca²⁺, Mg²⁺) to\nreact with CO₂. Moreover, claims about the effectiveness of this technique for Ca-rich waste carbonation are overstated, as\nother studies have already demonstrated similar efficiencies and reaction times under ambient conditions. The single\nreference cited does not adequately represent the current state of the art. It is required that all these statements are\nmoderated or removed from the abstract, conclusion and discussion sections. Moreover, recent works where the effect of\ncarbonation using different thin layer of water have been published and should be discussed. For example, Thompson et al,\nEnviron. Sci.: Nano, 2024, 11, 1412 suggest the opposite behaviours as discussed in this paper (carbonation rates and\nextents declined with decreasing water coverage). Therefore, relevant literature must be discussed and differences noted in\nthis work well addressed.\nIn conclusion, while this work presents a novel thin-film approach that appears to reduce diffusion limitations and shows\nsome interesting features at the nano-scale, it does not substantially advance the broader field of mineral carbonation. The\npractical implications and scalability of the proposed method remain highly uncertain. Therefore, the reviewer believes that\nalthough the concept is innovative, its overall contribution to overcoming real-world challenges in mineral carbonation is\nlimited and very specific to Portlandite, which does not well represent the Ca-rich wastes.", "reviewer_3": "The manuscript titled \"Ultrafast CO2 Mineralization via Water Thinning\" reports an experimental study for determination of\nthe effective thickness of interfacial water films (IWF) for enhanced carbonation of calcium hydroxide (Ca(OH)2). The study\nclaimed significant advancements in carbonation rates, achieving complete carbonation within one hour, which was\nsubstantially faster than conventional methods under ambient conditions. These results were compared with other\nexperimental measurements and computational simulations such as in situ Raman spectroscopy and density functional\ntheory (DFT), respectively. This reviewer evaluate such a systematic study of IWF would be impactful in that it provides a\nmethod to overcome the limitations of conventional aqueous carbonation methods. While the manuscript presents promising\nresults, the demonstration of the quantitative analysis are ambiguous. For example, almost all numbers provided in this work\nare shown without estimation errors or uncertainties, which are mandatory for any scientific report. In addition,\nmodel analysis results are shown without the quality of fit, which is necessary to evaluate the validity of the model. Therefore, it is\nnot possible to assess the credibility of the findings, which would require substantial revision of the work including\nindependent experiments or replication studies.\nThis reviewer provides three main suggestions that can improve the quality of the work for its potential publication.\n(1) Reconsideration of the use of the term “enzyme-mimetic confinement effects”\nMichaelis–Menten kinetics is a mathematical model traditionally associated with enzyme-catalyzed reactions, describing\nhow reaction rates depend on substrate concentration. However, it is important to recognize that the presence of Michaelis–\nMenten kinetics in a set of measurements does not unequivocally indicate an enzymatic process. This model can apply to\nvarious systems beyond enzymes due to its general applicability to processes involving saturation kinetics. For example,\ncertain chemical reactions involving catalysts or intermediates can show saturation kinetics, resembling the Michaelis–\nMenten model. When I focus on the data in this specific work, the relationship shown in Figure 2b is the very case where the\nauthors found such relationship, which in my opinion, is extremely generic. Therefore, connecting this observation with\nbiological processes clearly looks overinterpretation, which the authors should avoid.\n(2) Recapturing questionable data analysis\nIt is uncertain whether some of the model analysis properly explains the experimental data. For example, this reviewer is\nunsure whether the fitting lines for 101.32 kPa data in Figures 2a and S5 are representative of the data. The authors should\nshow the quality of fit and, if possible, comparison with other models.\nThis review also thinks that equation 2 in the supporting information, which is the most critical equation of this work because\nit shows how to calculate the thickness of IWF, is too simple to explain the physics. This relation assumes that all solid\nparticles are isolated with all surfaces accessible for chemical reactions. This assumption is known to be unapplicable in\nporous media where various reactions will be controlled by the confinement. In such environment, the distribution of liquid\nbecomes extremely nonhomogeneous due to the interplay of various physical forces and the structural\ncharacteristics of the porous material. Based on this consideration, it is hard to accept that the quantification approach that\nthe authors utilized can be validated.\n(3) Comparison with computational study\nWhile DFT calculations can provide a mechanistic insight into the molecular scale process near the solid-water interface, it\nmay not be the most suitable method to quantify the specific IWF determined by the experiments. For example, the\nexperimentally determined IWF, 13.5 nm thick, is approximately one order of magnitude thicker than what are shown, 3-7\nwater monolayers, by DFT.\nAdditional comments\nL. 167: Define the identity of the transition zone and how it can structurally promote calcite growth on the solid substrate\ncompared to its growth in solution or on a bare substrate surface.\nL. 172-173: It is unclear how AFM can visualize IWF. It is also unclear how IWF was preserved prior to and during AFM\nmeasurements. The authors should consider describing how the AFM measurements can perturb the IWF structure and/or\nhow to minimize such alternation.\nL. 202: Interfacial water: what is the theoretical basis by which one can explain the spatial extension of “thick” interfacial\nwater over IWF, which was determined to be 13.5 nm, which is equivalent to ~40 molecular layers of water.\nSupporting information\nL. 44: Provide information on AFM tips.\nSection 1.3: Explain how to measure the mass of water covering Ca(OH)2."}, "author_response": "Responses to the comments of the reviewers\nWe would like to thank the editor and the referees for their very useful and constructive remarks,\nwhich gave us the opportunity to revise and to improve our manuscript. We first quote a comment\nfrom the referees’ reports, then provide our specific reply (in blue), followed by details of the relevant\nrevisions made to the manuscript (blue slanted font with quotation marks). The implemented changes\nhave been highlighted in red in the revised manuscript.\nReviewers' comments:\nReviewer #1 (Comments to the Author):\nThis paper reports on mineral carbonation using the conventional aqueous route that remains energy-\nintensive and kinetically slow (at the scale of millimeters per year). More specifically, this work\nfocuses on the use of a water thinning strategy aiming at engineering the nanometric-thick interfacial\nwater-film (IWF) to achieve calcium hydroxide ([Ca(OH) ] complete carbonation within 1 h under\n2\nambient conditions. This IWF-based strategy is shown to be 16 times faster compared to bulk solution\nsystems. Carbonation efficiency was experimentally measured using Raman spectroscopy, time-\ndependent X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM),\nAtomic Force Microscopy (AFM) and reflectance infrared Fourier transform spectroscopy (DRIFTS).\nThe authors went on into providing explanations of the observed phenomenon that involve the\nconcept of an enzyme-mimetic confinement effect with “biomimetic reaction pockets” that enhance\nCO capture efficiency even at pCO = 0.04 kPa that is the content of CO in ambient air. This is the\n2 2 2\nkey achievement of the present work that puts the nanoconfined IWF as the enabler for a direct\nutilization of industrial alkaline wastes such as slags, achieving near-complete carbonation opening\nthe route to unlock Earth-abundant minerals for scalable CO sequestration applications.\nfollowing comments and indications are fully addressed.\nResponse:\nWe sincerely thank the reviewer for the positive and encouraging overall assessment of our work, and\nwe greatly appreciate the recognition of the novelty, timeliness, and potential impact of our IWF-based\ncarbonation strategy. This feedback is highly valuable to us, and detailed responses to the specific\nP\nA\nG\nE\ncomments are provided below.\n1. First, the authors have to clarify what they actual mean by “enzyme-mimetic confinement effect. Is\nthe “pockets” conclusion solely based on the fact that the acceleration of the carbonation reaction is\nindependent of the CO flow rate and the resemblance with the Michaelis–Menten kinetics equation?\ndefects…To me, this is a too-far stretch. Can the author defend their position or consider removing the\nenzyme resemblance section?\nResponse:\nWe thank the reviewer for pointing out the need to clarify the terms “enzyme-mimetic confinement\neffect” and “pockets”. As presented in the previous version, the carbonation nucleation kinetics in our\nsystem as a function of CO partial pressures can be well fitted by the Michaelis–Menten model for\n2\nenzyme-mimetic catalysis on the substrate. However, as the reviewer has pointed out that the\ncarbonation behavior is irrelevant to enzyme and might cause overstatement of the terminology. Based\non this consideration, we are willing to revise “enzyme-mimetic confinement effect” to “confinement-\ninduced saturation kinetics”, while retaining the term “pockets”. We emphasize that under the\nconfinement of an 8-nm-thick water film (revised from the previously reported 13.5 nm according to\nthe recalculated method suggested by Reviewer 3’s second comment; all subsequent discussions adopt\n8 nm in place of 13.5 nm), when the inflowing CO is below the amount required for the saturation\n2\nrate, the carbonation reaction rate increases with the net CO flux. Once the saturation rate is reached,\n2\nhowever, further increases in CO flux no longer affect the reaction rate. The following revisions have\n2\nbeen made in the revised Manuscript:\n(Page 1, Lines 19–21) “The IWF exhibits confinement-induced saturation kinetics, forming\nreaction pockets that simultaneously enhance CO capture efficiency and reduce the\n2\ndesolvation energy barriers of hydrated ion, even at pCO as low as 0.04 kPa.”\n2\n(Page 1, Lines 27) We have replaced “enzyme-like activity” with “confinement-induced.”\n(Page 3, Lines 70) We have replaced “enzyme-catalyzed Michaelis–Menten kinetics” with\n“Michaelis–Menten kinetics.”\n(Page 4, Lines 133) We have replaced “Enzyme-like reaction kinetics” with “Confinement-induced\nreaction kinetics.”\n(Page 5, Lines 155–156) We have replaced “enzyme-catalyzed reaction kinetics” with “saturation\nkinetics.”\nP\nA\nG\nE\n(Page 7, Lines 208) We have replaced “enzyme-Like carbonation” with “confinement-induced\ncarbonation.”\n(Page 9, Lines 298–299) “The carbonation process follows typical Michaelis–Menten kinetics,\nexhibiting confinement-induced catalytic activity,”\n(Page 17, Lines 514) We have replaced “Enzyme-like reaction kinetics” with “Confinement-induced\nreaction kinetics.”\nWe retain the term “pockets” to describe localized reaction microenvironments formed in the\ninterparticle regions. As confirmed in Supplementary Fig. 12, the N adsorption–desorption isotherm\n2\nof the precursor shows a typical H3-type hysteresis loop, consistent with slit-shaped mesopores\nbetween crystal grains, and the corresponding pore size distribution lies mainly in the mesoporous\nrange of 2–20 nm. Under bulk water coverage, the supplied water exceeds the cumulative pore volume\n(0.0757 cm3/g), leading to complete saturation of the pores prior to evaporation as a result of capillary\nforces1-3. This is expected for hydrophilic mesopores, where surface hydroxyl groups adsorb the first\nwater layer4,5, thereby reducing the interfacial energy, while capillary forces drive spontaneous\nsaturation even in pores approaching ~100 nm6,7. As the continuous water film fully covering the\nparticle surface gradually thins, surface tension drives the redistribution of liquid, leading to the\nformation of capillary bridges (pendular menisci) (Supplementary Fig. 10). At this stage, the\nevaporation rate of water decreases significantly (see revised Fig. 1d)8-10.\nThe formation of capillary bridges prevents the interparticle regions from being completely filled\nwith water, leaving localized voids that act as pockets for gas storage, thereby enabling the uptake of\nCO and its subsequent reaction. This phenomenon resembles what has been reported in the\n2\nliterature11,12, where hydrophilic mesopores undergo capillary condensation at optimal relative\nhumidity, remaining incompletely saturated with residual voids, thereby allowing CO to be enriched\n2\nand the formation of mixed H O/ CO metastable phases, thereby creating structural bases for localized\n2 2\nreactant enrichment. It is noteworthy that, once the water film thins from the bulk phase, the oriented\nhydroxyl groups provide a unique solid–liquid interfacial environment (as illustrated in\nSupplementary Fig. 10). This interfacial configuration further enhances CaCO nucleation, resulting\n3\nin significantly accelerated reaction rates compared to those reported in previous studies, increasing\nfrom 18.33% h-1 to 100% h-1 12. The above discussion was added in the revised Manuscript:\n(Page 4, Lines 110–113) “During this period, as CO flow removed water from the surface of the solid\n2\nprecursor, the thickness of the IWF decreased from the initial 24 nm, first rapidly dropping to\nP\nA\nG\nE\napproximately 9 nm, indicating the onset of capillary bridges formation32-34, and then continuing to\ndecrease until reaching 8 nm (Fig. 1d).”\n(Page 5, Lines 159–160) “(see Supplementary Fig. 10 for a schematic illustration of the structural\nfeatures of these “pocket” sites).”\n(Page 6, Lines 169–172) “Figure 3a visually demonstrates the typical structure of Ca(OH) composed\n2\nof plate-like polycrystalline aggregates, with the pore structure consisting of slit-like pores formed by\nthe stacking of these plate-like crystal grains, consistent with slit-like mesopores observed in the BJH\npore size distribution (combine with Supplementary Fig. 12)43.”\n“\nSupplementary Fig. 12. (a) N adsorption–desorption isotherms of the nanoscale Ca(OH) precursor,\n2 2\nexhibiting a Type IV curve with an H3 hysteresis loop in the relative pressure range of 0.5–0.9,\nindicative of slit-shaped mesopores formed by aggregated plate-like crystal grains. (b) BJH pore size\ndistribution (based on desorption branch), showing a dominant mesopore population in the range of\n2–20 nm.”\n“\nFig. 1d The thickness of the adsorbed IWF on Ca(OH) during carbonation. The inset of (d) illustrates\n2\nthe variation in the thickness of IWF. The thickness of IWF were obtained by fitting the relationship\nbetween the Raman spectra and the volume of water (see details in Supplementary Method 1.4). Initial\nIWF thickness = 24 nm, pCO = 101.32 kPa.”\nA\nG\nE\n“\nSupplementary Fig. 10. Schematic illustration of the structural features of “pocket” sites. These\npockets originate within the interparticle regions, resulting from capillary bridges formed through the\nthinning of water films. Within these pockets, oriented hydroxyl groups formed through solid–water\ninteractions constitute the solid–liquid interface of the pockets, some of which are partially filled with\nCO . Oriented interfacial hydroxyls within these pocket-like sites alter the local coordination\n2\nenvironment, lowering ion desolvation energy. This reduces the carbonation barrier of confined Ca2+\nand CO 2-, thereby promoting CaCO precipitation under ambient conditions.”\n3 3\n2. The authors have to position the novelty of their work with respect to recent papers that precisely\naddress the role of nanometric IWF for an efficient carbonation of minerals such as Portlandite. In\nparticular, I urge the authors of the present work to cite and compare their approach/results with that\nrecently proposed by Y. Gao et al, Moisture-driven carbonation kinetics for ultrafast CO\n2\nmineralization, Proceedings of the National Academy of Sciences, 122, 1, e2418239121, 2025 and Y.\nTao et al, Optimal CO intake in metastable water film in mesoporous materials, Nature\nsimulations approach to unravel the moisture-driven mineral carbonation mechanisms. They show that\nmaximum CO uptake occurs at a critical water content where the onset of capillary condensation\n2\nintensifies liquid–gas interactions, leading to adsorption-driven ultrafast carbonation at an optimal\nmoisture content. Conversely, they show that high moisture shifts carbonation kinetics to a surface\ndissolution–driven process hence they provide a very unified view for the very-debated optimal CO\n2\nmineralization conditions.\nSo, the main finding of the present paper on the role a nanometric IWF, is not exactly new but\nhas the merit of comforting very recent results on a very important subject such as the permanent\ndocking of atmospheric CO as a solid form (limestone). To me the novelty of the present paper lies\n2\nin the fact that it addresses the concentration of CO in the atmosphere. But this point although claimed\nA\nG\nE\nas an achievement, needs to be strengthened and further demonstrated. This is absolute priority.\nResponse:\nWe sincerely thank the reviewer for this insightful and constructive comment. We fully agree that it\nis essential to position our findings in the context of the recent advances made by Gao et al.12 and Tao\net al.11, which have significantly deepened the understanding of moisture-driven carbonation\nmechanisms using both experimental and atomistic approaches. These studies elegantly demonstrate\nthat an optimal water content—associated with the onset of capillary condensation—maximizes CO\n2\nuptake by intensifying gas–liquid interactions, thereby shifting the rate-determining step from surface\ndissolution to adsorption-limited processes. These remarkable works offer a coherent framework for\nunderstanding the accelerated mineralization of carbon dioxide under appropriate humidity conditions.\nCompared with the studies by Gao and Tao, our water film is produced via bulk phase thinning\nrather than vapor adsorption. Consequently, our system achieves nearly complete carbonation of\nCa(OH) within only 1 h under ambient conditions, which is significantly faster than the best result\n2\nreported in a previous study11, where ~55% conversion was obtained after 3 h at 98% relative humidity,\nand Gao et al.12 reported ~23% carbonation degree after 60 min under pure CO flow. We attribute\n2\nthis additional acceleration of water-film facilitated carbonation, relative to prior reports, to the solid–\nwater interface acting as an active contributor to the reaction dynamics.\nMoreover, we demonstrate that nanometric IWF can still promote carbonation of Ca(OH) under\n2\natmospheric CO levels (~400 ppm). This observation suggests that IWF-coated mineral systems\n2\npossess a degree of selective CO adsorption and activation, which could be beneficial direct air\n2\ncapture (DAC) applications. Consistent with this view, we further conducted continuous air-\ncarbonation experiments at room temperature in which nanometric water films were re-established by\nsequential re-wetting; after nine cycles, the Ca(OH) precursor achieved nearly complete carbonation\n2\n(Supplementary Fig. 11).\nThis water thinning strategy is operationally simpler than humidity-controlled protocols, offers a\nmore convenient pathway toward DAC-relevant operation while enabling near-complete conversion,\nand further supports the notion that IWF-coated minerals can sustain reactivity in nitrogen-dominated\nair. These points are now clarified with additional explanations in the revised manuscript, and a\nsystem-level mechanistic investigation of the air-carbonation regime is currently under active\nexploration.\nIn summary, compared with the studies by Gao and Tao et al., we obtained an optimized water\nP\nA\nG\nE\nstate from bulk-phase water rather than from vapor adsorption. This allowed us to establish a solid–\nliquid interface that favors carbonation, resulting not only in an accelerated reaction rate but also in a\nmore practical approach for air-based carbonation. These advances have been explicitly integrated\ninto the revised manuscript, where we now cite and compare our results with Gao et al. and Tao et al.\nThe following revisions have been made in the revised Manuscript:\n(Page 2, Lines 59–64) “At ambient temperature and pressure, Gao et al.25 and Tao et al.16 generated\nwater films via water vapor adsorption. It was demonstrated that maximum CO uptake occurs at a\n2\ncritical water content where the onset of capillary condensation intensifies liquid–gas interactions,\nleading to adsorption-driven ultrafast carbonation at an optimal humidity. These studies provide a\nuseful framework for moisture-dependent carbonation, yet they remain constrained by the control of\nenvironmental humidity and are insufficiently robust under low CO partial pressures relevant to air.”\n2\n(Page 3, Lines 77–80) “Importantly, we also observed sustained carbonation activity at\natmospheric CO (~400 ppm). These findings offer fundamental guidance toward developing\n2\nscalable and energy-efficient routes for solid waste mineralization, with significant\nimplications for direct air capture (DAC).”\n(Page 5, Lines 164–167) “In line with these findings, we further carried out continuous air\ncarbonation at room temperature, during which complete carbonation of Ca(OH) was\n2\nachieved after nine successive IWF renewals (Supplementary Fig. 11), thus experimentally\nvalidating the capability of IWF to drive full carbonation under ambient air conditions.”\n“\nSupplementary Fig. 11. Raman spectra of Ca(OH) before and after air carbonation with 9\n2\nsuccessive IWF renewals. The initial spectrum shows a pronounced OH stretching band at\n~3620 cm-1 characteristic of portlandite. After nine cycles of water addition under ambient air,\nthe OH band almost disappears, while strong carbonate peaks emerge at ~1086 and ~712 cm-\n1, confirming the nearly complete transformation of Ca(OH) into calcite.”\nA\nG\nE\n3-1 The authors of the present work complemented their experimental findings with some Density\nfunctional theory (DFT) calculations and unraveled the role of oriented hydroxyl (OH) groups present\nin the IWF, which provide low-energy barrier “pocket-like” reactive sites. This seems to me a very\nlimited view that does not compare with advanced molecular methods such as metadynamics for\nreaction free energy barrier calculations using either ab initio energy calculations or reactive\ninteratomic potentials (see for instance and refer to Xinping Zhu et al, Collective molecular-scale\ncarbonation path in aqueous solutions with sufficient structural sampling: From CO to CaCO , J.\n2 3\nChem. Phys., 14;161(18), 2024). I urge the authors of the present work to position their work with\nrespect of these more modern and reliable numerical techniques to evaluate free energy barrier. They\nalso need to compare their values to experimental literature.\nResponse:\nWe sincerely thank the reviewer for the thoughtful and constructive comments. We further\ncomplemented the calculation of free energy barriers using first-principles-based metadynamics\nsimulations to explore the intrinsic free-energy landscape of interfacial Ca and C coordination\nenvironments by evaluating their coordination numbers after NVT equilibration without applying bias\npotentials. The results highlight the promoting effect of interfacial hydroxyl groups on the association\nbetween calcium ions and carbonate ions. The corresponding data are presented in Supplementary Fig.\n20, and the following discussion has been added to the revised Manuscript:\n(Page 8, Lines 254–258) “Our metadynamics analysis55 reveals that both Ca–O and C–O coordination\nnumbers increase in the presence of surface-bound OH⁻ groups (Supplementary Fig. 20) than H\nsuggesting a more favorable environment for the formation of Ca–O–C linkages, which in turn\nfacilitates the nucleation of CaCO .”\nP\nA\nG\nE\n“\nSupplementary Fig. 20. Two-dimensional contour plot of the free energy as a function of Ca–O and\nC–O coordination numbers. (a) The lowest free energy corresponds to a C–O coordination number\nof approximately 1.9 and a Ca–O coordination number of 3.2 under the surface-bound OH⁻\nenvironment. (b) Under the interfacial water environment, these values are around 1.84 and 2.4,\nrespectively. The increased coordination numbers of both Ca–O and C–O in the presence of surface-\nbound OH⁻ suggest a higher propensity for the formation of Ca–O–C linkages, thereby facilitating\nCaCO nucleation.”\n3\n3-2 I do agree with the fact that the electrostatic interaction between Ca2+ and CO 2- ions dominates in\n3\nthe formation of carbonates but is mediated through the water in the IWF. A dissolution of Ca2+ step\n(going from the solid Portlandite phase into solution, so that it can eventually react with the carbonate\nions and form carbonates phases) is necessary and not shown in the modelling part of the present paper.\nCa2+ dissolution has been shown to be related to the ionic strength of the IWF (pH and other ions, see\nand refer to Y. Yong et al, CSH decalcification in seawater: The view from the nanoscale, Cement and\nConcrete, 175, 107385, 2024). I see no mention of ionic strength or pH in the modelling part of the\npresent paper that to me, considers an oversimplistic desolvation and water-coordinated calcium\ncarbonate ion clusters formation mechanism.\nP\nA\nG\nE\nResponse:\nWe agree with the reviewers’ comments regarding the influence of ionic strength or pH on the\nsolubility of Ca2+. In our experimental system, aside from protons, the predominant cation present is\nCa2+, and the introduction of other extraneous ions has not been considered. The pH of the IWF is\napproximately 12.6, which is close to that of a saturated Ca(OH) solution, where Ca2+ maintains the\n2\nsolubility equilibrium at a concentration of ~0.0234 M13. During the operation of the IWF, the ionic\nstrength of the solution varies with the distance from the surface. As illustrated in the revised\nSupplementary Information (Supplementary Fig. 21), we further simulated the ion distribution with\nClay force filed. From Supplementary Fig. 22, we observed that the number density of Ca2+ increases\nwith distance from the surface and becomes approximately constant at around 8 nm. This is attributed\nto the high point of zero charge of Ca(OH) (~12.7), which renders the pristine Ca(OH) surface\n2 2\nsignificantly positively charged. Consequently, negatively charged ions such as OH- and CO 2- are\n3\nstrongly adsorbed, while Ca2+ is repelled. Since the dissolution rate of Ca(OH) is much higher than\n2\nthe nucleation rate of CaCO 14, the Ca2+ concentration in the IWF is expected to remain close to its\n3\nsolubility equilibrium value until the complete consumption of Ca(OH) . Therefore, when the bulk\n2\nwater is progressively thinned, the dissolution of Ca2+ is not the rate-limiting step for carbonation, and\nthus it was not the primary focus of our consideration.\nMoreover, considering that carbonate ions near the solid–liquid interface are the key determinants\nof the carbonation rate, supplementary simulations were conducted to examine the coordination of\nthese ions with water molecules within the IWF as a function of distance from the interface. As shown\nin Fig. 5c, the radial distribution function (RDF) between the carbon atoms of carbonate ions and the\noxygen atoms of water molecules indicates that, at regions close to the interface (Z = 0–4 nm), the\nRDF values are significantly lower than those at farther distances. This suggests that carbonate ions\nnear the interface are coordinated with fewer water molecules, i.e., their coordination is less saturated.\nThis observation supports the conclusion presented in Fig. 5d and 5e of the manuscript, namely that\nthe ions in the IWF exhibit under-coordination, which leads to a reduced nucleation barrier and thereby\naccelerates the carbonation process. The following discussion has been added to the revised\nManuscript:\n(Page 8, Lines 261–269) “Furthermore, through simulations of the water film using (Clay force filed)\nClayFF (Supplementary Fig. 21 and 22), as shown in Fig. 5c, the radial distribution function (RDF)\nbetween the carbon atoms of carbonate ions and the oxygen atoms of water molecules reveals that, in\nregions close to the interface (Z = 0–4 nm), the RDF values are markedly lower than those at greater\ndistances. This indicates that CO 2- near the interface are coordinated with fewer water molecules,\n3\ni.e., their coordination environment is less saturated. This may originate from the interactions between\nlattice Ca2+ and CO 2- with the solid surface of the \"pocket-like sites\" occupy the coordination spaces\n3\notherwise available to water, resulting in the coordination unsaturation at the interface56.”\n“\nSupplementary Fig. 21. Schematic representation of the simulated structure of IWF.”\n“\nSupplementary Fig. 22. Variation of Ca2+ concentration as a function of the distance from the surface\ninto IWF. Considering the effects of ionic strength and pH on the reaction, we emphasize that in our\nsystem, the pH of the IWF is approximately 12.6, close to that of a saturated Ca(OH) solution, with\n2\nthe Ca2+ concentration near its solubility equilibrium (~0.0234 M)16. Simulations show that the Ca2+\nconcentration increases with distance from the surface and becomes approximately constant at around\n8 nm, as obtained using a clay force field (Supplementary Fig. 21).”\n“\nP\nA\nG\nE\nFig. 5c Variation of CO 2- concentration as a function of the distance from the surface into IWF.”\n3\n3-3 Furthermore, the “pockets” explanation from the DFT part of this manuscript is extremely local\n(OH groups) and by the way not at all at the spatial scale of an enzymatic reaction.\nResponse:\nWe appreciate the reviewer’s observation. As clarified in our response to the first comment, “pockets”\nare defined as localized reaction microenvironments formed in the interparticle regions of aggregates\nthrough the formation of capillary bridges. Their solid–liquid interface comprise oriented hydroxyl\ngroups, which provide the minimal molecular framework for stabilizing interfacial water layers. With\nregard to the “enzymatic active sites” analogy, we have revised the text to avoid this description. Our\nintention was not to imply an enzymatic mechanism, but rather to highlight that the observed\nsaturation-type kinetics originates from nanoscale confinement effects. Thus, the “pockets” concept\nrefers to confined aqueous regions created by packed particles and capillary bridges, which facilitate\nCO enrichment and CaCO nucleation under ambient conditions.\n2 3\nIn the revised version of the Supplementary Information, we have also included additional details\nregarding the aforementioned calculations:\n(Page 6, Lines 208–221) “Metadynamics calculations were performed using CP2K to investigate the\nfree-energy landscape associated with the coordination environment of interfacial species. The\ncollective variables (COLVARs) were defined as the coordination numbers of the interfacial Ca and\nC atoms with surrounding oxygen atoms. Prior to the metadynamics runs, the system was equilibrated\nin the canonical (NVT) ensemble to ensure structural stability. A 5 ps trajectory from the equilibrated\nsystem was then collected for the evaluation of the COLVARs. No bias potential was applied during\nthese simulations, such that the free-energy profiles reflect the intrinsic configurational sampling of\nthe system.\nMolecular dynamics simulations of nanoscale water films were conducted with the LAMMPS\nP\nA\nG\nE\nsimulation package13. The system consisted of over 108,270 atoms in total. Simulations were\nconducted in the NVT ensemble at a fixed temperature, with a total production time of 5 ns. The\nsimulation setup included a slab layer of 20 Å thickness and an effective water film of 140 Å thickness.\nInteractions were described using the ClayFF, which has been widely applied for modelling hydrated\nmineral surfaces and water–solid interfaces.”\nReviewer #2 (Comments to the Author):\nThis work explores the use of ultrathin water films on mineral surfaces as a promising strategy to\nsignificantly enhance the performance of mineral carbonation processes for Ca-rich wastes.\nSpecifically, by applying a water-thinning approach to engineer interfacial water-films (IWFs) with\nthicknesses below 13.5 nm, the authors report complete carbonation of Portlandite (Ca(OH) ) within\n2\n1 hour under ambient conditions, which resulted in a 16-fold acceleration compared to conventional\nbulk solution systems. The authors further state that this nanoconfined water architecture is applicable\nto industrial alkaline wastes, such as carbide slag, achieving near-complete carbonation without any\npretreatment—a process that typically takes several days in standard slurry reactors. According to the\nmanuscript, the IWF exhibits enzyme-mimetic confinement effects, forming biomimetic reaction\npockets that lower desolvation energy barriers and improve CO capture efficiency. The manuscript\n2\nhas a degree of novelty but require major revision.\nResponse:\nWe sincerely thank the reviewer for their constructive evaluation of our manuscript. We appreciate the\nrecognition of the novelty and potential significance of our interfacial water-film (IWF) strategy for\naccelerating mineral carbonation. We also acknowledge the reviewer’s observation regarding the\nrelatively high carbonation efficiency achieved without any pretreatment. In our work, the process was\nconducted under ambient conditions, without the use of specialized reactors, mechanical stirring, or\nother external enhancement measures that are often employed in existing studies. This demonstrates\nthat the IWF-based approach can effectively enhance carbonation efficiency in a more simplified and\npractical manner.\nSpecific comments:\n1. However, the main bottleneck in mineral carbonation technologies remains the extraction of reactive\noxides (e.g., CaO, MgO) from bulk waste or rock. The carbonation step itself is not the primary\nP\nA\nG\nE\nlimitation to broader deployment and this must be clarified in the manuscript.\nResponse:\nWe thank the reviewer for emphasizing the critical role of reactive oxide extraction in mineral\ncarbonation. Current carbonation reactions are generally classified into two main types: direct and\nindirect carbonation. Indirect carbonation is typically employed to produce calcium carbonate of\nhigher purity, whereas direct carbonation serves as the important route for CO capture and\n2\nsequestration.\nAs highlighted in several reviews15-17, a widely recognized challenge in indirectly mineral\ncarbonation lies in the energy-intensive pretreatment required to extract reactive Ca/Mg species from\nnatural minerals such as olivine or serpentine. Minerals are first subjected to chemical or thermal\ntreatments to extract Ca2+/Mg2+ ions into solution, followed by precipitation of carbonates. This\nsignificantly increases the overall cost and limits the scalability of such approaches. While such\nprocesses can yield high-purity carbonate products, they are heavily dependent on the efficiency of\nCa/Mg extraction16, which remains the main cost and energy bottleneck.\nIn contrast, direct aqueous (slurry-based) carbonation of alkaline industrial solid wastes offers a\nmore practical pathway for CO sequestration. In this process, the solid wastes are dispersed in water\n2\nto form a slurry, and carbonation proceeds through gas–liquid–solid reactions. These materials\ntypically contain readily available reactive calcium phases such as free CaO or Ca(OH) , eliminating\n2\nthe need for energy-intensive extraction16,18. In such systems, the main bottleneck shifts from reactive\noxides recovery to the carbonation reaction itself. Several studies17,19 have also clearly identified slow\nreaction kinetics and low conversion rates as major obstacles to the commercialization of carbonation\ntechnologies, often limited by factors such as mineral dissolution, CO mass transfer, and surface\n2\npassivation effects.\nTherefore, our study specifically focuses on addressing these key challenges in direct aqueous\ncarbonation by accelerating the carbonation process. We have revised the Manuscript to more\nrigorously reflect this distinction and to better align the motivation of our work with the current\nlimitations in the field:\n(Page 2, Lines 34–37) “In particular, direct aqueous carbonation is considered as the most\npromising approaches due to it avoids the need to extract reactive components from minerals\nprior to carbonation, resulting in simpler operation and lower energy consumption4,6.”\n2. Ca(OH) (portlandite) is significantly more reactive than CaO commonly present in Ca-rich wastes.\nA\nG\nE\nAn important aspect that is insufficiently addressed is the particle size of the synthesized portlandite\npowder. This parameter is neither clearly reported nor easily assessed from the data provided, yet it\ncan strongly affect reactivity. Therefore, the particle size distribution of the tested materials should be\nexplicitly analysed and stated.\nResponse:\nWe thank the reviewer for highlighting the role of particle size in Ca(OH) reactivity. To address this\n2\nissue, a large-particle Ca(OH) sample was synthesized to conduct additional experiments\n2\ninvestigating the effect of particle size on carbonation. Hydrodynamic particle size analysis revealed\nthat the sample exhibited a broad distribution in the micrometer range (approximately 1–30 μm),\nwhereas the nanoscale precursor (the particles used in the carbonation reaction in the original\nmanuscript) displayed an average diameter of ~164 nm (Supplementary Fig. 14). Powder XRD\ncombined with Scherrer analysis20 indicated average crystallite sizes of ~98 nm for the large-particle\nsample and ~27 nm for the nanoscale precursor (Supplementary Fig. 15). SEM imaging further\nconfirmed their contrasting morphologies, revealing smooth blocky grains in the micrometer-sized\nsample versus aggregated plate-like crystallites in the nanoscale precursor (Supplementary Fig. 16).\nDespite these distinctions in size and morphology, both samples achieved complete conversion within\n1 h under identical carbonation conditions (Supplementary Fig. 17a). The fitted CaCO nucleation\n3\nrates were also nearly identical (~8.4 Å-3·h-1) (Supplementary Fig. 17b), indicating that carbonation\nkinetics in our system are not governed by particle geometry or surface area.\nThe comparable carbonation reactivity observed for both nanoscale and large-particle Ca(OH)\n2\nindicates that particle size is not the determining factor in our system. Instead, the key lies in the\npresence of IWF formed by capillary bridges between aggregated particles8-10, which provide confined\naqueous domains for CO absorption11,12. Regardless of bulk particle dimensions, the aggregation of\n2\ncrystallites inevitably generates interparticle voids where capillary bridges establish stable water films\nunder ambient conditions. These confined films not only enrich CO locally but also interact strongly\n2\nwith the Ca(OH) substrate. The oriented water layers at the solid–liquid interface can lower ionic\n2\ndesolvation barriers, facilitate the coordination of Ca2+ and CO 2-, and thereby promote CaCO\n3 3\nnucleation. Thus, carbonation kinetics in our system are governed primarily by capillary-bridge-\nmediated confinement and interfacial configuration, rather than by the external particle size.\nCorresponding explanations have been added in the revised Manuscript:\n(Page 6, Lines 187–193) “To further evaluate the effect of particle size, a large-particle Ca(OH)\nA\nG\nE\nsample with a broad micrometer-scale distribution and blocky morphology was tested in IWF\n(Supplementary Fig. 14b, Fig. 15 and Fig. 16b). Despite its significant difference from the nanoscale\nprecursor (Supplementary Fig. 14a, Fig. 15 and Fig. 16a), this sample exhibited nearly identical\ncarbonation behavior, achieving complete conversion within 1 h (Supplementary Fig. 17). These\nresults indicate that carbonation reactivity in our system is minimally affected by particle size but\ninstead governed by IWF-mediated processes.”\n“\nSupplementary Fig. 14. (a) Particle size distribution of the nanoscale Ca(OH) and (b) blocky\n2\ncomparison, the large-particle sample yields a size distribution of D = 0.881 μm, D = 5.62 μm,\n10 50\nand D = 30.9 μm.”\n90\n“\nSupplementary Fig. 15. XRD patterns of the synthesized Ca(OH) precursors with nanoscale plate-\n2\nlike and micron-sized blocky morphologies. Crystallite sizes were then obtained from the Scherrer\n𝐾𝜆\nequation14 𝐷 = with K is typically = 1.24 for FWHM of crystals with hexagonal symmetry15, λ\n𝛽𝐶𝑂𝑆𝜃\n= 1.5418 Å.”\n“\nP\nA\nG\nE\nSupplementary Fig. 16. (a) and (b) are SEM images of the synthesized Ca(OH) precursors with\n2\nnanoscale plate-like and micron-sized blocky morphologies, respectively.”\n“\nSupplementary Fig. 17. (a) Time-resolved Raman spectra of the blocky Ca(OH) precursor during\n2\naccelerated stage of carbonation at 20 °C. (b) Concentration profiles of CaCO over time, obtained\n3\nby MCR–ALS analysis. Raman spectroscopy monitoring of the large-particle Ca(OH) during\n2\ncarbonation under ambient conditions (room temperature, atmospheric pressure) revealed that\ncomplete conversion to CaCO was achieved within 1 h, with a fitted calcite nucleation rate of 8.4 ×\n3\n10-3 Å-3·h-1, comparable to that of the nanoscale plate-like sample.”\n3. The comparative performance results for carbide slag (e.g., Fig. S12) are misleading. Several\nexisting studies demonstrate similar carbonation efficiency under ambient conditions using\nconventional reactors and the same material, calling into question the claimed exceptional\nperformance. Examples from the literature include:\n• Bonfante et al., Materials and Structures, 2024, 57"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors did a good job in clarifying all points raised by reviewers. The manuscript has clearly improved.\nTherefore, I support the publication of this paper in Nature Communications.", "reviewer_2": "The authors have effectively addressed all the raised comments, and the paper is now of publishable quality.", "reviewer_3": "I confirm that the revised manuscript fully addresses all concerns and suggestions raised from the previous review."}, "author_response": ""}]

Decision letter