Spatiotemporally ordered topological transformation in layered double hydroxides enables synergistic mineralization of As^III/Cd²⁺

Abstract

The co-contamination of arsenite (As^III) and cadmium (Cd²⁺) poses a significant challenge in environmental remediation due to their divergent chemical speciation and transformation pathways. Here, a redox-active layered double oxide (MgMn-LDO) is designed, and achieves adsorption capacities of 821.7 mg g^‒1 for As^III and 1895.6 mg g^‒1 for Cd²⁺ in their coexisting system, presenting a state-of-the-art performance. Unexpectedly, the co-adsorption system not only enhances individual adsorption capacities but also accelerates the adsorption rate by 181‒fold compared to single-component systems. The LDO undergoes a four-stage spatiotemporally ordered topological transformation, which effectively decouples the oxidation of As^III from the adsorption of Cd²⁺ and reverses the conventional competitive sequence. This stepwise mechanism ensures preferential oxidation of As^III to As^V, followed by the alteration of Cd²⁺ adsorption pathway to isomorphous substitution, expanding diffusion pathways and accelerating As immobilization. Large-scale experiments demonstrate this material’s potential in synergistic remediation As/Cd in mining wastewater and contaminated soils.

Here authors prepare a defect-rich Mg–Mn layered double hydroxide, which via a spatiotemporally ordered topotactic transformation, achieving synergistic comineralisation of As^III and Cd²⁺.

Introduction

Arsenite and cadmium co-contamination (As^III/Cd²⁺) poses a significant global environmental challenge. However, remediation efforts are hindered primarily by the distinct chemical forms and transformation mechanisms of these two pollutants^1–6. To tackle this issue, nano-composite materials have emerged as a promising solution, leveraging their multi-factor adjustability to synchronous As^III/Cd²⁺ immobilization^7,8. Among these, nano-composite metal oxides with redox activity stand out for their unique advantage in the remediation process, as they can efficiently pre-oxidize the uncharged and highly mobile As^III into As^V anions⁹. This transformation not only enhances the immobilization of As^III but also significantly reduces its toxicity^10–12. However, in As^III/Cd²⁺ co-contaminated environments, the oxidation capacity of such materials often fails to function fully. The primary reason is that the strong coordination ability of Cd²⁺ preferentially occupies all of adsorption sites formed by surface hydroxyl (‒OH) groups generated through oxide hydration, including oxidation sites^13–15. Moreover, while highly oxidative components (such as MnO₂) rapidly convert As^III to As^V, they may also cause extensive reduction of Mn⁴⁺ to Mn²⁺, leading to structural collapse and secondary pollution from Mn²⁺ release. Hence, overcoming the competitive inhibition effects of As^III and Cd²⁺, while maintaining a balance between oxidative activity and structural stability, remains a significant challenge^16,17. The ideal immobilization material should possess spatiotemporally ordered oxidation-adsorption sites and self-stabilizing properties. Through functional decoupling and spatiotemporal matching, they address multiple pollutants in sequence while maintaining stability under complex conditions, thus directing pollutant transformation pathways for efficient and synergistic As^III/Cd²⁺ remediation.

Despite decades of research, nano-composite metal oxides still face significant limitations in heavy metal immobilization, with adsorption capacities typically capped at 200 mg g^‒1,18–20. These shortcomings arise from structural deficiencies, including insufficient active sites and their poor accessibility and utilization. To overcome these challenges, a structural redesign must prioritize two critical strategies: (1) introducing defects at the atomic scale can drive water dissociation and generate bulk-phase ‒OH groups. This approach leverages the entire nanoparticle volume, rather than just the 20–30% confined to surface regions, leading to a three to fivefold enhancement in adsorption performance; (2) developing channel structures with precisely controlled sizes of 2–3 nm is essential. These dimensions are carefully balanced: they are large enough to minimize diffusion resistance and improve active site accessibility, yet compact enough to facilitate the formation of tightly coordinated inner structures upon heavy metal binding. This dual functionality significantly boosts immobilization efficiency. Among various materials, layered double oxides (LDOs) offer promising structural advantages and have demonstrated exceptional performance in immobilizing single heavy metals, such as As, Pb, and Cd^21–24. For instance, MgMn-LDO (Mg:Mn = 2:1) has previously been reported to exhibit excellent Cd removal efficiency due to the formation of CdCO₃ and Cd(OH)₂²⁵. However, the challenge lies in the synchronous immobilization of As^III/Cd²⁺, where competitive inhibition during interfacial adsorption significantly diminishes their effectiveness. Fortunately, LDOs possess a unique property: topological transformation, allowing them to revert to the LDH structure^26–30. By harnessing this transformation, it becomes possible to spatially and temporally decouple the oxidation process of As^III from the adsorption process of Cd²⁺, effectively mitigating the competitive inhibition effect. This innovative strategy can pave the way for overcoming current limitations and achieving efficient synchronous immobilization of As^III/Cd²⁺.

In this work, the framework structure of MgMn-LDO (LDO) is successfully constructed through calcination of MgMn-LDH (LDH) precursor. As a result, the adsorption capacity of LDO for As^III and Cd²⁺ can reach 821.7 and 1895.6 mg g⁻¹ in co-contamination, presenting a state-of-the-art result (Supplementary Table 1)^31,32. Conventional materials such as iron-manganese oxides and graphene nanotubes exhibit adsorption capacities for As^III rarely exceeding 100 mg g⁻¹ due to their limited active sites^33,34. Unexpectedly, compare the adsorption of As^III or Cd²⁺ alone, not only was adsorption capacity improved, but rate also increased by 181 times. Kinetics and mechanistic analysis indicate that the quadruple topological transformation of the adsorbent from MgMn-LDH →MgMn-LDO →MgMn-LDO-H₂O (LDO(H)) → MgMn-LDH(As^III) (LDH(As^III)) → CdMgMn-LDH(As^III/Cd²⁺) (Cd-LDH(As^III/Cd²⁺)), triggered sequentially by heat, H₂O, As^III, and Cd²⁺, is the key factor (Fig. 1a). The distinct As^III oxidation and Cd²⁺ adsorption energies successfully reversed the usual sequence in As^III/Cd²⁺ co-adsorption, causing As^III oxidation to precede Cd²⁺ adsorption (Fig. 1b). Through electron transfer between As^III and Mn⁴⁺, the LDO undergoes a topological transformation into an LDH, enabling As^III/As^V to be deeply fixed within the LDH interlayers. Meanwhile, the original adsorption pathway for Cd²⁺ is altered, causing Cd²⁺ to enter the layer via isomorphic substitution, which induces lattice expansion and significantly enlarges the bulk diffusion channels. This multi-level topological transformation plays a central role by not only safeguarding the high reactivity of the adsorbent during the process but also conferring a stable mineralized encapsulation in the final adsorption structure.

Fig. 1. The adsorption capacity of LDO and four-level topological transformation.

a Schematic presentation for the four-level topological transformation. b The Gibbs free energy diagram of four-level topological transformation. As represents As^III and Cd represents Cd²⁺ in figures.

Results and discussion

As^III and Cd²⁺ shifted from remediation targets to mutual adsorption enhancers

Typically, when metal oxides are used to treat multiple heavy metals (such as Cd²⁺/Pb²⁺ and Cu²⁺/Zn²⁺), their adsorption efficiency tends to be lower compared to treating a single heavy metal^35–38. This is especially true for As^III/Cd²⁺ co-contaminants, as the primary adsorption sites on metal oxides are the surface hydroxyl groups (‒OH). The strong coordination ability of Cd²⁺ with these ‒OH groups often significantly hinder the adsorption of As^III,39. Unexpectedly, although the solution pH value increased with adding LDO calcined at 350 °C (Supplementary Figs. 1‒4), the maximum adsorption capacity for As^III can reach 821.7 mg g⁻¹, which is five times the current highest value (Starch‒FeMnO_x, 161.29 mg g^‒1)³¹; and for Cd²⁺ reach 1895.6 mg g^‒1, which is three times the current highest value (graphene/δ‒MnO₂, 644 mg g^‒1) (Fig. 2a, b, Supplementary Figs. 5‒8, and Supplementary Tables 2‒4)⁴⁰. More importantly, these values are higher than the adsorption capacities of LDO for treating As^III (691.6 mg g^‒1) and Cd²⁺ (1723.1 mg g^‒1) individually (Fig. 2c). This anomaly indicates that As^III and Cd²⁺ become triggers for mutually enhancing adsorption performance. In addition, the structure of LDO is stable (Supplementary Figs. 9‒13), and its removal efficiency for As^III/Cd²⁺ is independent of pH value, coexisting salts, and metal ions (Supplementary Figs. 14–18).

Fig. 2. As^III and Cd²⁺ promote each other’s adsorption performance.

A comparison chart of the adsorption capacities of reported adsorbents in the literature for a As and b Cd. c The removal capacity of LDO for As and Cd in single and co-contaminated system. d The sorption kinetics of As and Cd in different As/Cd initial concentration. e Weber‒Morris intraparticle diffusion plots of LDO for As/Cd co-contaminated system. f The residual concentrations of As and Cd at varying adsorbent dosages across different initial concentrations. The concentration of before and after remediation of g natural mining wastewater and h mining site soil in solution leached. The test results are the averages of three tests; As represents As^III and Cd represents Cd²⁺ in figures.

The adsorption of both As^III and Cd²⁺ are chemical, with intraparticle diffusion controlling rate, supported by kinetics and fittings. A series of concentrations were used to fit the adsorption process. Specifically, the initial concentration of As^III remain unchanged and the initial concentration of Cd²⁺ is = 50, 100, 200, 300, 500, and 600 mg L^‒1. All adsorption process follows pseudo-second-order kinetics, with an excellent fit indicated by an R² value (> 0.99) (Fig. 2d and Supplementary Figs. 19‒23 and Supplementary Table 5). The fit results of Weber‒Morris model have been showed in Fig. 2e and Supplementary Table 6. The kinetic adsorption of As^III and Cd²⁺ is divided into three stages, including external diffusion, intraparticle diffusion, and equilibrium stages. Among them, intraparticle diffusion is the main rate-controlling step⁴¹. As confirmed by the rate constant of the second stage is significantly lower than that of the first stage and R₂² > R₁².

The deep-remove of As^III/ Cd²⁺ co-contaminates remains an unresolved challenge in practical applications^42,43. LDO demonstrates deep-remove capabilities in As^III/Cd²⁺ co-contaminated systems. As evidenced in Fig. 2f, when the initial concentrations of As^III/Cd²⁺ are 0.1/0.1 and 1.5/1.5 mg L^‒1, 0.5 g L^‒1 of the adsorbent LDO achieves low residual concentrations of As^III/Cd²⁺ (As^III ≤ 4 μg L^‒1; Cd²⁺ ≤ 2 μg L^‒1), which meet WHO drinking water standards^44,45. Especially, even when the initial concentrations of As^III/Cd²⁺ rise to 12.0 mg L^‒1, 1.0 g L^‒1 of the adsorbent still achieves low residual concentrations with As^III = 5 μg L^‒1 and Cd²⁺ = 1.8 μg L^‒1, which is currently unattainable by other materials.

Owing to the convenient and economical synthesis method (Supplementary Table 7), the LDO can be scaled up for kilogram-scale production (Supplementary Figs. 24 and 25), verifying the high feasibility for practical applications in remediation of wastewater and contaminated soil. Firstly, when LDO was used to treat the mining water from Yiyang in Hunan province (detail information see Supplementary Table 8), the total concentration of As and Cd²⁺ decreased from 0.257 and 1.153 mg L^‒1 to 0.012 and 0.008 mg L^‒1, respectively (Fig. 2g), meeting the advisory level of Standard for irrigation water quality of China (GB5084-2021, As ≤ 0.05 mg L^‒1, Cd ≤ 0.01 mg L^‒1). Additionally, LDO has demonstrated excellent adsorption capacity for As^III/Cd²⁺ in domestic wastewater environments (Supplementary Figs. 26‒28). Secondly, when LDO was added the highly contaminated industrial soils located at Changde City, Hunan Province, the concentration of As and Cd in solution leached decreases 89.9% and 91.5%, respectively after remediation (Fig. 2h). Based on the dose-effect relationship, an optimal LDO dosage of 2–3% was identified, offering the best balance between treatment efficacy and LDO consumption (Supplementary Table 9 and detailed discussion). Moreover, the remediation efficiency for heavily contaminated soils remained high across various parameters, including different contamination levels, pH values, and soil types, with removal efficiencies consistently exceeding 87% (Supplementary Tables 10‒12 and detailed discussion). When the dose-effect relationship is not considered, the removal rates of As and Cd still exceeded 85% with a 10% LDO addition for treating high contaminated industrial soils with initial As and Cd concentrations of 700–3100 mg kg⁻¹ (Supplementary Table 13 and detailed discussion). These findings demonstrate the substantial practical potential of in-situ immobilization technique using LDO for the remediation of environments contaminated with heavy metals.

Bulk hydroxylation topology (Topology I and II)

In the treatment of As^III/Cd²⁺ using metal oxides, whether at oxidation sites or adsorption sites, the initial chemical reaction involves ligand exchange between As^III/Cd²⁺ and the ‒OH groups on the surface of the adsorbent^46,47. Thus, the variation in the ‒OH groups of LDO was monitored by X-ray photoelectron spectroscopy (XPS, Fig. 3a). The O 1s spectra of the LDO(H) demonstrates that the O_OH content reach 71.8%, far exceeding that of LDH (50.4%) and LDO (36.8%). Meanwhile, the content of lattice oxygen (O_L) in LDO(H) significantly reduced to 4.8% compared with that of conventional LDH (18.7%) and LDO (47.6%)⁴⁸. The reason lies in the formation of metal defects and oxygen defects during the transformation of LDH to LDO triggered by heat (Topology I)^49–51. Firstly, metal defects sites promote hydroxylation. The presence of Mg²⁺ defects can be confirmed by extended X-ray absorption fine structure (EXAFS, Supplementary Figs. 29 and 30 and Fig. 3b). The peak intensity of Mn‒M bonds at 2.73 Å for LDO is weaker than that of LDH, revealing an increased unsaturated metal site. Secondly, the oxygen defects (oxygen vacancy and clusters) can also effectively promote the hydroxylation. The electron paramagnetic resonance (EPR) of LDO shows a stronger signal located at g = 2.003 than that in LDH, demonstrating the presence of much more oxygen vacancies (Fig. 3c)⁵². In addition, microscopic changes show oxygen vacancies/clusters (Fig. 3d–f and Supplementary Figs. 31–33). LDH has irregular hexagonal nanosheets (40‒80 nm radial, ~20 nm thick). Calcination preserved morphology; hydrolysis distorted LDO(H) into thinner flakes. LDO’s 0.15 nm smaller lattice spacing (vs. LDH) indicates topological rearrangement, exposing defects to affect hydrolysis⁵³. Diffraction ring confirms retained LDH polycrystal structure. These observations indicate high-density oxygen vacancies and vacancy clusters induce localized charge redistribution, enhancing electrostatic interactions with polar water⁵⁴. In addition, the hydroxylation can be finished in 1 min as confirmed by XRD, where a series of characteristic peaks appeared at 2θ = 18.5, 37.8, 50.8, 58.7, and 62.1° (Fig. 3g). These peaks do not belong to Mg(OH)₂ or MnOOH, thus, confirming the formation of LDO(H). The hydroxylation rate of LDO is faster than that of Nano-MgO and commercial MgO (Supplementary Figs. 34 and 35). Moreover, LDH layers uniformly disperse Mg, Mn (Supplementary Fig. 36). Post-calcination, no separate MnOₓ phases beyond vanished peaks, indicating atomic-dispersed Mg²⁺ and Mn³⁺/Mn⁴⁺ in oxide matrix, supporting oxidation/adsorption site partitioning.

Fig. 3. Bulk hydroxylation of LDO.

a O 1s XPS spectra of LDH, LDO, and LDO(H). b Magnitude of k⁴-weighted Fourier transforms of R-space EXAFS spectra of Mn. c EPR spectrograms of the LDH and LDO. TEM images, HR-TEM images, and indexation of electron patterns of d LDH; e LDO, and f LDO(H). g XRD patterns of LDH, LDO, and LDO(H) under different hydration time. h Binding energies of H₂O and OH⁻ on Mg and Mn sites of LDO. i ΔGs of hydrating LDO to LDO(H) and MgO to Mg(OH)₂. j Gibbs free energy diagram of hydrating LDO to LDO(H) with geometries of representative intermediates displayed (I represent each hydration). The color code is the same with that in Fig. 1.

The spontaneous hydroxylation of LDO is also confirmed by density functional theory (DFT) calculations. All the binding energies between metal sites (Mg and Mn) in LDO and H₂O or OH⁻ are negative (Mg−OH⁻: −2.86 eV, Mg−H₂O: −1.58 eV, Mn−OH⁻: −3.32 eV, Mn−H₂O: −0.60 eV) (Fig. 3h and Supplementary Figs. 37 and 38). Moreover, metal sites (Mg, Mn) favor OH⁻ over H₂O (more negative binding energies), aiding H₂O dissociation and LDO hydroxylation. Mn binds OH⁻ stronger than Mg, making it optimal for H₂O dissociation. After that, the Gibbs free energy (ΔG) for the hydroxylation of LDO (with a chemical formula of Mg₁₂Mn₃O₂₂H₈) to LDO(H) (with a chemical formula of Mg₁₂Mn₃O₃₀H₂₄) is calculated to be −43.41 eV, which is far lower than that of MgO (with a chemical formula of Mg₈O₈) to Mg(OH)₂ (with a chemical formula of Mg₈(OH)₁₆) (−16.10 eV) (Fig. 3i, j and Supplementary Fig. 39). These results reveal that LDO hydroxylates more strongly than MgO. MgO only surface-hydroxylates while LDO fully does so via rich sites/defects. Its 2D structure enables spontaneous bulk hydroxylation via H₂O–induced interlayer hydrolysis, activating sites globally. This is key for high adsorption.

Prior oxidation of As^III triggers Topology III transformation

This process converts LDO back to LDH, thereby altering the adsorption pathway of Cd²⁺. To explore the sequence of As^III oxidation and Cd²⁺ adsorption, the following experiments were conducted. The same mass of LDO was used to treat equal masses of As^III and Cd²⁺, but the order of addition of As^III and Cd²⁺ was different in condition 1–3 (See Supplementary Information for details). Adsorption solids at 1 and 2 h (Fig. 4a–c): Condition 1 showed only Cd(OH)₂ peaks at 2θ = 18.8, 29.7, 35.5, 38.1, 49.3, 56.2, 59.1° (Supplementary Fig. 40). Condition 2 showed LDH peaks 2θ = 11.3, 22.3, 34.7, 38.2, 59.3, 60.1°: As^III restores LDO to LDH (Supplementary Figs. 41 and 42), with Cd²⁺ incorporating into LDH lattice without altering its layered structure. Condition 3 solids matched 2, meaning simultaneous As^III/Cd²⁺ generates stable LDH. Cd²⁺ did not affect As^III adsorption (Supplementary Figs. 43 and 44). Results confirm co-adsorption: As^III oxidation precedes Cd²⁺ adsorption, triggering LDO-to-LDH transformation.

Fig. 4. Prior oxidation of As^III alter the adsorption pathway of Cd²⁺.

XRD patterns of different adsorption process: a adsorption of Cd first, followed by adsorption of As; b adsorption of As first, followed by adsorption of Cd; and c simultaneous adsorption (# represent Cd(OH)₂; * represent (003) characteristic peak of LDH). d, e TEM-EDS elemental mapping and HRTEM results of Mg/Cd ions mass fraction in different As/Cd adsorption time (10 and 30 min). f The Mg²⁺/Mn^3+/4+ ions leaching concentration of different adsorption time and initial Cd concentration in co-contamination system. g The interlayer spacing of Cd-LDH (As/Cd) in different adsorption time and initial As concentration in co-contamination system. h Gibbs free energy diagram for the isomorphous substitution of Mg by Cd in LDH(As^III) with geometries of representative intermediates displayed (I represent the number of Mg of the MgMn-LDH(As) laminates replaced by Cd). i Gibbs free energy diagram for the isomorphous substitution of Mg by Cd in LDO(H) with geometries of intermediates displayed (I represent the number of Cd replacing Mg in MgMn-LDO(H)). As represents As^III and Cd represents Cd²⁺ in figures.

This topological shift alters Cd²⁺ adsorption from surface ligand exchange to LDH lattice substitution, as confirmed by time-resolved transmission electron microscopy (TEM) mapping (Fig. 4d, e) and ICP (Supplementary Fig. 45). Mg²⁺ in Cd-LDH(As^III/Cd²⁺) decreased from 23.29% to 8.97%, whereas Cd²⁺ increased from 15.71% to 31.04%, indicating Cd²⁺ replaces the Mg²⁺ via isomorphous substitution with Mg²⁺ leaching. This is supported by Mg²⁺ dissolution: Fig. 4f shows Mg²⁺ increased from 0 to 118 mg L^‒1 over 0‒6 h, and from 14 to 34 mg L^‒1 as Cd²⁺ initial concentration rose 10‒200 mg L^‒1. Mn leaching was negligible (near zero). As locates in LDH interlayers, shown by increased interlayer spacing with higher adsorption capacity. Fig. 4g shows Cd-LDH(As^III/Cd²⁺) interlayer spacing increased from 0.74 to 0.78 nm as time extended 0‒12 h, with adsorption capacity rising from 0 to 99.7 mg g^‒1 (Supplementary Fig. 46); simultaneously, spacing increased from 0.73 to 0.77 nm as As^III concentration rose 10‒500 mg L^‒1, with capacity up to 389 mg g^‒1 (Supplementary Figs. 47 and 48).

The spatiotemporal order of the oxidation of As^III and the isomorphous substitution of Cd²⁺ is further verified through the thermodynamics energy diagrams obtained by DFT calculations. As shown in Fig. 4h, after oxidation of As^III on LDH (As^III), the ΔGs for the isomorphous substitution of each Mg²⁺ cation by each Cd²⁺ cation are all smaller than 0 eV, indicating this process is spontaneous. In contrast, the ΔGs of isomorphous substituting Mg²⁺ in LDO(H) by Cd²⁺ is larger than 0 eV, demonstrating is thermodynamically unfavorable (Fig. 4i). Above all, we prove that the oxidative adsorption of As^III occurs before the isomorphous substitution of Cd²⁺.

The electron transfer from As^III to Mn⁴⁺ during adsorption, forming As^V and Mn³⁺, as confirmed by XPS and XANES. For As, post-adsorption Cd-LDH(As^III/Cd²⁺) shows As 3d signals at 44.9 eV (As^III) and 45.9 eV (As^V, Fig. 5a); LDO raised As^V to 53.8% (vs. LDH’s 18.0%), showing stronger oxidation⁵⁵, which also can be confirmed by HPLC-ICP-MS, semi-in-situ FT-IR, and in-situ Raman (Supplementary Figs. 49‒51). For Mn, XPS shows Mn 2p peaks at 643.1 and 653.8 eV (Mn³⁺) (Fig. 5b); its proportion in Cd-LDH(As^III/Cd²⁺) (84.0%) exceeds LDO’s 35.9%, confirming Mn⁴⁺ reduction with As^III oxidation, as confirmed by XANES. Cd-LDH(As^III/Cd²⁺) has a 1.02 eV lower Mn K-edge (lower oxidation state) and an As K-edge 11874 eV peak (As^V, Supplementary Figs. 52 and 53), verifying electron transfer⁵⁶.

Fig. 5. Electron transfer triggers the topological transformation from LDO to LDH.

a The As 3d XPS spectra of Cd-LDH(As/Cd) and LDH(As/Cd). b The Mn 2p XPS spectra of LDO and Cd-LDH(As/Cd). c XRD patterns of LDO after adsorbing different anions. Magnitude of k⁴-weighted Fourier transforms of R-space EXAFS spectra of d As and e Mn. f The binding energy of Mg/Mn/Cd sites with As^III/As^V in LDH(As^III) and Cd-LDO(As^III/Cd). g The bonding energy of Mg/Mn‒O in LDH(As) and Cd-LDO(As/Cd). h The schematic diagram of As migration path (Top View). i The relative energies during As diffusion along path I and path II. As represents As^III and Cd represents Cd²⁺ in figures.

To clarify Mn⁴⁺/As^III redox roles in structural reconstruction, we conducted controlled experiments (Fig. 5c): (1) With only As^V/Cd²⁺, LDO showed no significant topological change, where XRD revealed no LDH peaks, matching LDO. (2) With Na₂CO₃ alone, LDO formed composite hydroxides (Mg(OH)₂·MnOOH) with peaks at 18.5°‒62.1° (no layered order). (3) With electron-donating ascorbic acid, LDO formed LDH (peaks at 11.0°‒59.3°), same as As^III/Cd²⁺ co-adsorption: its vicinal diols transfer electrons, reducing Mn⁴⁺ to Mn³⁺ and restoring LDH⁵⁷. O 1s XPS confirmed LDO→ Cd-LDH(As^III/Cd²⁺) transformation (M‒OH: 54.2%, O_L: 23.4% in product vs. 50.4%/18.7% in LDH; Supplementary Fig. 54). The aforementioned comparative experiments fully demonstrate that As^III undergoes oxidation by Mn⁴⁺ centers and occurs electron transfer. The reduction of Mn⁴⁺ triggers LDO→Cd-LDH(As^III/Cd²⁺) topological transformation via electron transfer.

The topological transformation from LDO to Cd-LDH(As^III/Cd²⁺) is accompanied by the intercalation of As^III and/or As^V and multi-site coordination, which is key to enhancing the removal capacity of As^III. The adsorption capacities of 821.7 mg g^‒1 for As^III/Cd²⁺ and 240.0 mg g^‒1 for As^V/Cd²⁺, respectively (Supplementary Fig. 55). Note that As^V/Cd²⁺ solution need Cd²⁺ concentration adjustment due to poor solubility. EXAFS probed As coordination: Cd-LDH(As^III/Cd²⁺) had As−O (1.23 Å, first shell) and As−M (2.80 Å, second shell) bonds (Fig. 5d). Its As–O coordination number (4.2) and distance (1.68 Å) matched Na₂HAsO₄ (4.0, 1.68 Å) (Supplementary Table 14), indicating binuclear AsO₄ tetrahedra is thermodynamically favored. These tetrahedra bridge MO₆ octahedra via corner/edge-sharing, boosting stability and electron transfer between As^III and Mn^4+58,59. This allows As incorporation into layers (not just surface), increasing sorption capacity post-transformation. Though As^V forms similar coordination, it doesn’t transfer LDO to LDH, causing lower adsorption. The coordination of Mn showed Mn‒O (1.47 Å) and Mn‒M (2.64 Å) peaks; Cd-LDH(As^III/Cd²⁺) had shorter Mn‒O (1.91 Å vs. 1.92 Å in LDH) and Mn–M (2.91/3.20 Å vs. 3.23 Å in LDH), confirming strong As−Mn interaction (Fig. 5e and Supplementary Table 15). Due to the inherently low affinity between As and MnO₂ in binary oxides compared to magnesium oxides, a higher proportion of Mn in the LDO leads to a reduction in adsorption efficiency, as confirmed by that the adsorption capacity decreases with increasing Mn content from Mg:Mn = 5:1, 4:1, 3:1, 2:1 (Supplementary Figs. 56–58). Therefore, to balance sufficient oxidation sites (Mn‒OH) and maximize adsorption sites (Mg‒OH), the optimal Mg:Mn = 5:1 in the LDO was chosen.

DFT calculations characterized adsorption-oxidation processes and active sites. As shown in Fig. 5f, As^III binding energy on Mn sites in LDO(H) (−1.48 eV) was far more negative than on Mg sites (−0.36 eV), meaning As^III preferentially adsorbs on Mn sites to drive Mn⁴⁺/As^III redox (forming Mn³⁺/As^V), converting LDO(H) to LDH (As^III). With Cd²⁺, As^III binding energies on Mn (−1.87 eV) and Mg (−1.60 eV) sites strengthened, boosting As^III adsorption. As^V binding energies on Mg/Mn/Cd sites were more negative than that of As^III, confirming stable Cd-LDH (As^III/Cd²⁺). However, no electron transfer occurs between Mn⁴⁺ and As^V, blocking LDH-forming topological transformation and limiting to low-capacity surface adsorption. Figure 5g shows calculated M‒O bond energies: As^III (AsO₂⁻) oxidation to As^V (AsO₄³⁻) requires O transfer from LDO, necessitating M‒O cleavage. Mn‒O (2.49 eV) is weaker than Mg‒O (2.63 eV), so Mn sites favor O transfer and oxidation^60,61. With Cd²⁺ (partly substituting Mg²⁺), LDO(H) lattice parameters increase, lowering Mg‒O (2.50 eV) and Mn‒O (2.10 eV) energies. This weakens Mn‒O bonds, aiding As^III oxidative adsorption.

The oxidation of As^III happens with a chemical formula of AsO₂⁻ + 2Mn⁴⁺ → AsO₄³⁻ + 2Mn³⁺, indicating that the oxidation of one As^III requires the collaboration of two Mn⁴⁺. However, the Mn⁴⁺ may not be adjacent to another Mn⁴⁺ in one LDO(H) layer since Mn is uniformly dispersed in LDO(H) layers. Therefore, the oxidation of As^III is likely to occur between two LDO(H) layers and thus facilitating the topological transformation of LDO(H) to LDH. Then the generated As^V binds tightly with two reduced Mn³⁺, forming a stable structure (Supplementary Fig. 59). Therefore, the subsequent diffusion pathways of As^III can be divided into two types: (1) directly through the As^V-occupied site (path I) and (2) bypassing this site (path II, Fig. 5h). The diffusion energy barriers of As^III for paths I and II are calculated by DFT to be 0.60 and 0.41 eV, respectively, as displayed in Fig. 5i. The optimized geometries of the diffusion intermediates are displayed in Supplementary Fig. 60. Therefore, bypassing As^V-occupied site is the favorable pathway for As^III diffusion.

The presence of Cd²⁺ triggers Topology IV transformation

The presence of Cd²⁺ significantly enhances the adsorption rate of LDO for As^III (Fig. 6a). When LDO is used alone for As^III adsorption (C₀ = 100 mg L^–1), it takes 7 h to reach adsorption equilibrium. However, when the introduction of Cd²⁺ (200 mg L^‒1), the adsorption of As^III by LDO reaches equilibrium in just 10 min. The adsorption rate constant (k) increases from 0.0183 to 3.3200 g h^‒1 mg^‒1 (Supplementary Figs. 61 and 62 and Supplementary Table 16), representing an approximately 181-fold enhancement. This phenomenon can be attributed to increased oxidation rate and diffusion rate.

Fig. 6. The presence of Cd²⁺ changes the local tensile strain, promoting electron transfer.

a The adsorption kinetic of LDO in As and As/Cd = 100/200 mg L^‒1. b Raman results of various adsorption time. c Magnitude of k⁴-weighted Fourier transforms of R-space EXAFS spectra of Mn. d Mulliken charges of Mn−O−Mg structure in LDO(H) and Mn−O−Cd structure in Cd-LDO(H). e Lattice expansion of Cd-LDH(As/Cd) compared to LDH(As) and corresponding electron density isosurface, with Mg−Mg and Mg−Mn distances labeled. f ICOHP of Mn‒O bond in LDH(As) and Cd-LDH(As/Cd).

Cd²⁺ substitution for Mg²⁺ changes the local tensile strain within the layers, promoting electron transfer of As^III‒Mn⁴⁺ and As^III oxidation. The variation in tensile strain can be observed in Raman spectroscopy, where the Mn⁴⁺‒O peak progressively shifts to lower wavenumbers from 626.8 to 606.7 cm^‒1 as increasing Cd²⁺ (Fig. 6b), indicating a longer Mn‒O bond is easier to break. The same conclusion can also be drawn from EXAFS, where Mn‒M of Cd-LDH(As^III/Cd²⁺) located at 2.64 Å is longer than that of LDH(As^III) (2.61 Å) in Fig. 6c. The correlation between localized tensile strain and electron transfer was systematically investigated through DFT calculations. Firstly, the Mulliken charge analysis shows that the partial charges of Mn and O are 1.82 e and ‒1.00 e in Mg‒O‒Mn (Fig. 6d), the more electrons are transferred from Mn to O due to the local tensile strain in the structure by Cd substitution, resulting in a charge of Mn and O are 1.86 e and ‒1.02 e, respectively, which demonstrates that Cd substitution promotes the electron transfer of Mn. This charge redistribution is further corroborated by the electron density isosurface calculations (Fig. 6e), which demonstrate that the Mg–Cd distance elongation (3.54 Å vs. Mg–Mg: 3.21 Å) promotes electron delocalization under tensile strain. The integrated crystal orbital Hamilton population (ICOHP) analysis provides additional insight (Fig. 6f)⁶², showing a reduction in the covalent character of the Mn–O bond (ICOHP: −1.33 to −1.11), which indicates an increased ability of Mn to accept electrons. Together, these results establish a coherent mechanism: Cd²⁺ induced strain promotes charge delocalization, destabilizes the Mn–O bond, and ultimately facilitates electron transfer from As^III to transfer to Mn, thereby enhancing the oxidation of As^III.

The isomorphous substitution of Cd²⁺ in LDH layers resembles natural mineral isostructural substitution⁶³. Specifically, larger radius induces structural relaxation, lattice expansion, and interlayer spacing variation, as confirmed by XRD patterns (Fig. 7a)⁶⁴. In 100 mg L^–1 As^III solution, as Cd²⁺: Mg²⁺ rises from 1:28 to 1:7, (003) peak of Cd-LDH(As^III/Cd²⁺) shifts from 11.6° to 11.2°; HRTEM shows lattice expansion (from 0.21 to 0.27 nm, Fig. 7b) and time-dependent growth (from 0.205 to 0.211 nm, Supplementary Fig. 63)⁶⁵. Increased spacing reduces As^III diffusion resistance, boosting its rate. Cd²⁺ replacing Mg²⁺ expands Cd‒As‒Mn lattice, modulating As^III migration via two mechanisms: (i) As^III distal to Cd²⁺ has lower activation energy (enhanced diffusion) from relaxed strain; (ii) First-shell As^III near Cd²⁺ faces narrowed channels (higher barriers) via charge redistribution (Fig. 7c). Thus, low Cd²⁺ substitution favors expansion; high levels favor channel narrowing⁶⁶.

Fig. 7. The emergence of Cd²⁺ regulates the diffusion channels of As.

a XRD patterns in different Cd/Mg molar rate. b High-resolution TEM of Cd-LDH(As/Cd) in different Cd/Mg molar rate. c The schematic diagram of As migration path after Cd replacement. d Relationships between energy barriers of As migration along two pathways versus atomic ratios of Mg to Cd. As represents As^III, Cd represents Cd²⁺, and Mg represents Mg²⁺ in figures.

To verify this mechanism, the diffusion energy barriers of As^III in Cd-LDH(As^III/Cd²⁺) with Cd²⁺/Mg²⁺ molar ratio of approximately 1:28, 1:14, and 1:7 following path I (away from Cd²⁺) and path II (close to Cd²⁺) are calculated and displayed in Fig. 7d. The optimized geometries of diffusion intermediates are shown in Supplementary Fig. 64. For Cd²⁺/Mg²⁺ = 1:28, the diffusion energy barriers are calculated to be 0.27 eV in path I and 0.42 eV in path II. The diffusion of As^III along the path away from Cd²⁺ is significantly easier than that close to Cd²⁺. With the increasing amount of Cd²⁺, the diffusion energy barriers of As^III along path I decreases progressively, reaching 0.24 eV for Cd²⁺/Mg²⁺ = 1:14 and 0.20 eV for a ratio of 1:7. On the other hand, the diffusion energy barriers of As^III along path II is further increased to 0.45 eV for Cd²⁺/Mg²⁺ = 1:14 and 0.46 eV for a ratio of 1:7. Therefore, DFT calculations verify that Cd²⁺ substitution benefits As^III diffusion by local expansion effect.

In summary, redox-active LDO synthesized via LDH thermal topotactic transformation achieves simultaneous As^III/Cd²⁺ remediation, with capacities of 821.7 mg g^‒1 (As^III) and 1895.6 mg g^‒1 (Cd²⁺). More importantly, their synergistic co-adsorption (vs. conventional competitive occupation) enhances As^III kinetics 181-fold, addressing low capacity/sluggish kinetics in coexisting systems. The defect-rich structure of LDO promotes water dissociation for bulk hydroxylation (critical for immobilization). The spatiotemporal orderliness of the material enables the oxidation of As^III to As^V coupled with the reduction of Mn⁴⁺ to Mn³⁺. This electron transfer activates structural memory effect of LDO, reconstructing LDH-like layers while sequestering As within the interlayer. Concurrently, Cd²⁺ shifts to lattice anchoring via isomorphous substitution, expanding As^III diffusion channels to synergistically accelerate kinetics. The scalable synthesis allows kilogram-scale production of LDO, with excellent performance in pilot tests using real mining wastewater/soil. This work establishes a transformative co-remediation strategy, offering insights for complex matrices.

Methods

Chemicals

Mg(NO₃)₂·6H₂O, Mn(NO₃)₂·4H₂O, NaOH, Cd(NO₃)₂·4H₂O, H₂O₂ were purchased from Beijing Innochem Chemical Reagent Company (Beijing, China). Experimental As^Ⅲ and As^Ⅴ solution were prepared by dissolving NaAsO₂ and Na₂HAsO₄·7H₂O in deionized. As standard stock solutions in HNO₃ (National Institute of Metrology, China) were used for preparation and calibration standards. In this work, all reagents were used as received without further purification. All solutions were prepared with deionized water.

Synthesis of MgMn-LDH

MgMn-LDH was synthesized via a traditional co-precipitation method. The Mg(NO₃)₂·6H₂O (1.0 mol) and Mn(NO₃)₂·4H₂O (0.2 mol) were firstly dissolved in 100 mL deionized water. Meanwhile, NaOH (1.92 mol) and H₂O₂ (1 mL) were also dissolved in another 100 mL deionized water. Then, the two solutions were poured into the colloid mill at the same flow rates with rotating of 210 × g at room temperature. The slurry was aged at 60 °C for 4 h. The products were washed with deionized and ethanol for three times, respectively, and dried at 60 °C drying for 12 h to obtain LDH (Mg:Mn = 5:1). Similarly, LDH samples with different ratios (Mg:Mn = 2:1, 3:1, 4:1) were synthesized by the same method, except different feeding ratio of Mg/Mn.

Synthesis of MgMn-LDO

MgMn-LDO was obtained by calcined the MgMn-LDH at different temperatures (200, 250, 350, 450, 550, and 650 °C) for 3 h in air, with a heating rate of 5 °C·min^‒1.

Adsorption experiments of As^III/V/Cd²⁺

All adsorption experiments were conducted at room temperature using a batch sorption method. A certain amount of NaAsO₂, Na₂HAsO₄·7H₂O, and Cd(NO₃)₂·4H₂O were dissolved in deionized water to obtain corresponding As^III, As^V, and Cd²⁺ solution. Then the LDO were added to the heavy metal ions solution and stirred until the adsorption equilibrium was reached. The supernatant was separated with a 0.22 μm membrane filter. The concentrations of heavy metals in solution were determined by inductively coupled plasma optical emission spectrometry (ICP-OES). All experiments were carried out in duplicate, and the average data were used for analysis. The heavy metals adsorption capacity and removal rate were calculated by the following equations:

$$Q=\frac{\left(C_0-C_{\mathrm{e}}\right)\times V}{m}$$ 1

$$R=\frac{C_0-C_e}{C_0}\times 100\%$$ 2

where C₀ (mg L⁻¹) and C_e (mg L⁻¹) are the initial and final concentrations of As^III/V and Cd²⁺, respectively, V (L) is the volume of aqueous solution, m (g) is the adsorbent mass, and Q (mg g⁻¹) is the adsorption amount for As^III/V and Cd²⁺, R is the removal rate of As^III/V and Cd²⁺.

As^III oxidation and Cd²⁺ adsorption sequential experimental details

To investigate the sequence of As^III oxidation and Cd²⁺ adsorption during their co-adsorption process, batch experiments were conducted with initial concentrations of 100 mg L⁻¹ As^III and 200 mg L⁻¹ Cd²⁺ in 100 mL solution with 0.10 × g LDO adsorbent. Three independent experiments (As^III-only, Cd²⁺-only, and As^III/Cd²⁺ co-contamination) were started simultaneously. After 1 h adsorption with continuous mixing, suspensions were centrifuged (2100 × g, 3 min) for solid-liquid separation. Parallel experiments involved sequential addition: the complementary heavy metal was introduced to respective monometallic systems after initial 1 h adsorption, followed by 1 h reaction prior to centrifugation. All obtained solids were characterized by XRD analysis.

In manuscript, condition 1: Cd²⁺ was added first at 0 min, followed by As^III 1 h later; condition 2: As^III was added first at 0 min, followed by Cd²⁺ 1 h later; condition 3: As^III and Cd²⁺ were added simultaneously at 0 min.

The practical remediation details of contaminated soil

The experimental field was subdivided into three plots, each 3 × 3 m in size. Remediation agents were surface-applied at a dosage of 0.67 kg per plot (750 kg per hectare) and subsequently tilled to a depth of 20 cm to ensure thorough integration with the soil. Soil sampling was conducted at 0, 15, 30, 60, and 120 days to assess the stabilization efficiency of As and Cd. Sampling procedures strictly followed China’s Technical Specification for Soil Environmental Monitoring (HJ/T 166-2004). Specifically, five subsites (0–20 cm depth) were arranged in a plum blossom pattern within each plot; samples from these points were homogenized, and a 1 kg composite sample was retained for analysis. Following removal of plant debris and gravel, soils were nature-dry and sieved through a 40‒mesh screen. As concentrations were determined by atomic fluorescence spectrometry, while Cd levels were quantified using graphite furnace atomic absorption spectrometry.

The practical remediation details of contaminated wastewater

First, insoluble solids were removed from nature wastewater through 0.45 μm membrane filtration prior to adsorption experiments. Then, 0.5 × g of LDO was added into 500 mL of actual wastewater containing heavy metal ions. The adsorption time was 2 h under stirring, the supernatant was separated with a 0.22 μm membrane filter. The concentrations of heavy metals in actual wastewater were determined by ICP-MS.

The regenerative experiment details of MgMn-LDO

The regeneration performance of the MgMn-LDO was systematically evaluated via 10 consecutive laboratory cycling experiments. The experimental setup fixed the initial concentrations of As and Cd at 10 and 20 mg L^‒1, respectively, while 50 mg L^‒1 Na₂CO₃ solution was selected as the regeneration agent. The 1 g L^‒1 as the dosage standard to proceed regeneration test. The adsorption time was 2 h under stirring, the supernatant was separated with a 0.22 μm membrane filter. The concentrations of heavy metals in actual wastewater were determined by ICP-MS.

Characterization of materials

The crystal structure of resulting sample was analyzed by a powder X-ray diffraction device (JDX-3530, Tokyo, Japan) using Cu Kα radiation (λ = 1.5418 Å) with a 2θ value ranging from 5° to 70° with a scanning speed of 10° min^‒1. The surface binding states and elemental speciation of the samples before and after As^III/Cd²⁺ sorption was analyzed by XPS on a spectrometer (Thermo Scientific ESCALAB 250) with a focused monochromatic Al Ka X-ray source, in which all of the binding energies were calibrated with reference to the C 1s peak (284.8 eV). The data analysis was conducted using XPS peak software (version 2.3.12.8). The morphologies and microstructures of the samples were observed using field-emission scanning electron microscopy (Merlin VP Compact, 15 kV), and TEM (JEOL-2010, 200 kV, equipped with an EDX spectrometer). The concentrations of As^III and Cd²⁺ in the solutions were measured by ICP-OES (PerkinElmer 8300) and ICP-MS (PerkinElmer NexION 300X). EPR spectra were recorded on a JEOL JES-FA200 EPR spectrometer to detect the presence of oxygen vacancies. The XAFS data were processed according to the standard procedures using the Athena module implemented in the IFEFFIT software packages. The EXAFS spectra were obtained by subtracting the post-edge background from the overall absorption and then normalizing with respect to the edge-jump step. Subsequently, the χ(k) data of were Fourier transformed to real (R) space using a Hanning windows (dk = 1.0 Å⁻¹) to separate the EXAFS contributions from different coordination shells. To obtain the quantitative structural parameters around central atoms, least-squares curve parameter fitting was performed using the ARTEMIS module of IFEFFIT software packages.

Author contributions

M.Z. performed all experiments, data analysis, created the figures, and wrote the original draft. H.D. contributed to the partial measurements. M.Z. and X.C. completed the schematic drawing. S.X. conducted theoretical calculations. W.C. and M.S. contributed to the discussion. W.S. and X.K. conceived the experiments and supervised the project. X.D. proposed the idea.

Peer review

Peer review information

Nature Communications thanks Hailong Wang and the other anonymous, reviewer(s) for their contribution to the peer review of this work. A peer review file is available.

Data availability

The data generated in this study are provided in the Supplementary Information files. Additionally, all raw data for each graph have been compiled into an Excel file. Source data are provided with this paper.

Competing interests

The authors declare no competing interests.

Supplementary information

The online version contains supplementary material available at 10.1038/s41467-026-68326-2.