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• Water pollution from heavy metals poses a significant threat to ecosystems and human health. Heavy metals like Cr(Ⅵ) are particularly concerning due to their high solubility and mobility in water [1]. In aqueous solutions, Cr(Ⅵ) primarily exists as HCrO₄^-, Cr₂O₇^2-, or CrO₄^2-, with its form varying based on pH level and concentration [2]. The WHO sets the permissible Cr(Ⅵ) levels in industrial wastewater at 0.1 mg/L and in drinking water at 0.05 mg/L [3]. Due to its toxicity and carcinogenic nature, there is an urgent need to develop methods to reduce Cr(Ⅵ) levels below these thresholds [4,5]. Current Cr(Ⅵ) treatment methods, including ion exchange, chemical precipitation, adsorption, membrane filtration, photocatalytic reduction, and electrochemical reduction, often face economic and technical challenges.

  Adsorption is a widely used strategy for Cr(Ⅵ) removal due to its efficiency and stability [4]. Various adsorbents, including biomass, activated carbon, graphene oxide, clay minerals, metal oxides, and polymers, have been employed. However, many struggle with purifying low-concentration Cr(Ⅵ) wastewater, limiting their application. Conductive polymer-based composites (CPs) have gained interest for their straightforward synthesis, non-toxicity, environmental stability, and affordability [3,6].

  Polypyrrole (PPy), a leading conductive polymer, shows promise in heavy metal removal due to its electronic and redox properties and affinity for Cr(Ⅵ) [7]. However, PPy particles tend to aggregate in water, reducing adsorption performance [8]. To enhance Cr(Ⅵ) removal, hybrid composites of PPy and layered double hydroxides (LDHs) have been synthesized [9]. Layered double hydroxides (LDHs) are a typical type of inorganic layered material with abundant surface hydroxyl groups, easy surface functionalization, and high water dispersibility, making them promising heavy metal removal materials. However, due to their weak affinity for Cr(Ⅵ), pure LDHs may not have the anticipated high adsorption capacity for Cr(Ⅵ). Therefore, introducing PPy, which has a high affinity for Cr(Ⅵ), into highly water-dispersible LDHs is a good modification strategy. This study designed an all-in-one PPy/LDH filter and examine its feasibility for treating Cr(Ⅵ)-contaminated water, which support a novel approach to efficiently eliminate the trace amounts of Cr(Ⅵ) in water.

  The process of creating PPy/LDH composite materials is illustrated in Fig. 1a. Numerous hydroxyl groups (−OH) on the surface of LDH form hydrogen bonds with the benzenoid amine groups (−NH−) in pyrrole monomers. Initially, these pyrrole monomers are adsorbed onto the LDH surface. The introduction of ammonium persulfate then triggers the in situ polymerization of the attached pyrrole monomers, forming PPy coatings [10]. Scanning electron microscopy (SEM) showed that NiFe-LDH has a smooth, block-like structure without noticeable rough particles, while pure polypyrrole consists of clusters of spherical particles (Fig. S1 in Supporting information). SEM images of PPy/LDH composites with varying pyrrole monomer concentrations (Fig. S2 in Supporting information) reveals a consistently rough texture. Transmission electron microscopy (TEM) further elucidates the features of the PPy/LDH composites (Figs. 1b and c), showing bulk LDH and polypyrrole particles coated on LDH. Elemental distribution within the PPy/LDH composites, including Ni, Fe, C, N, and O, was verified through energy dispersive spectroscopy (EDS) mapping (Fig. 1d), indicating a uniform spread across the material. EDS line scanning spectrograms highlight a distinct carbon layer on the PPy/LDH surface, confirming the presence of the PPy coating (Fig. S3 in Supporting information).

  Figure 1

  

  Figure 1.  (a) Schematic illustration of the synthesis of PPy/LDH. (b, c) TEM images of PPy/LDH, and (d) EDS mapping of PPy/LDH.

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  X-ray diffraction patterns (XRD) in Fig. 2a displayed that both PPy and PPy/LDH show a broad peak around 2θ = 24°, signifying the amorphous nature of the PPy homopolymer due to π-π interactions within the polymer chains [3]. Notably, in the PPy/LDH composites, the characteristic peak of NiFe LDH is absent, likely obscured by the polymer matrix enveloping the LDH and the low crystallinity of LDH in the composites. When different amounts of pyrrole monomer are added to the PPy/LDH composites, the XRD patterns display a wide diffraction peak like that of PPy (Fig. S4 in Supporting information). The surface elemental composition of NiFe-LDH and PPy/LDH was analyzed through X-ray photoelectron spectroscopy (XPS). The XPS survey spectrum reveals the presence of C, N, and O on the PPy/LDH surface, corroborating the existence of PPy on the composite (Fig. 2b). High-resolution C 1s, O 1s, Fe 2p, and Ni 2p XPS spectra (Fig. S5 in Supporting information) of both NiFe-LDH and PPy/LDH hybrids show notable differences. The presence of C 1s in PPy/LDH indicates the incorporation of PPy, while diminished peaks of Ni and Fe suggest effective coating of LDH by PPy. Additionally, the shift in O 1s peaks between LDHs (531.45 eV) and PPy/LDH (531.04 eV) supports the successful synthesis of the hybrid materials [10].

  Figure 2

  

  Figure 2.  (a) XRD patterns of PPy/LDH with different composite proportions. (b) XPS spectra, (c) N₂ adsorption/desorption isotherms and (d) pore-size distribution of NiFe LDH and PPy/LDH.

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  The Brunner-Emmet-Teller (BET) surface areas for NiFe-LDH, PPy, and PPy/LDH are 151.23, 58.67, and 72.14 m²/g, respectively (Fig. 2c). The intermediate surface area of PPy/LDH, between NiFe-LDH and PPy, suggests a successful combination of the two materials. According to IUPAC classification, all three materials exhibit Type Ⅳ isotherms with H3 hysteresis loops, indicating slit-like pores in mesoporous structures (Fig. 2d) [6]. The pore sizes, ranging from 2 nm to 50 nm, confirm the mesoporous nature of these adsorbents. The calculated BET surface area, pore diameter, and pore volume are also displayed (Table S1 in Supporting information). PPy/LDH, with its superior surface area, pore diameter, and volume compared to PPy, will possess enhanced adsorption capacity for Cr(Ⅵ) [11].

  The removal performance of NiFe LDH and PPy/LDH for Cr(Ⅵ) was compared in Fig. 3. The efficiency of Cr(Ⅵ) removal by PPy/LDH is consistently higher than by NiFe LDH across a pH range of 1-11 (Fig. 3a). For LDH, removal efficiency decreases from 79.3% to 10.1% as pH increased. In contrast, PPy/LDH maintains a removal efficiency above 95% in the pH range of 1-7, but efficiency sharply declines at pH 9 and 11. Cr(Ⅵ) species at different pH levels (Fig. S6 in Supporting information) show that Cr(Ⅵ) exists as negatively charged anions across a wide pH range [12,13], with HCrO₄^- dominant from pH 1.0 to 6.5 and CrO₄^2- dominant above pH 6.5. The isoelectric point of PPy/LDH is at pH 8.13 (Fig. S7 in Supporting information), indicating that the material's positively charged surface is more favorable for Cr(Ⅵ) adsorption under acidic conditions. The decline in removal efficiency under alkaline conditions is due to strong electrostatic repulsion between the negatively charged adsorbent and pollutant. Under low pH, the protonated PPy/LDH surface becomes positively charged, enhancing electrostatic attraction to the negatively charged Cr anions [14]. As solution pH increases, protonation of PPy/LDH decreases, reducing adsorption effectiveness. Post-adsorption pH changes (Fig. S8 in Supporting information) show that pH tends to rise after Cr(Ⅵ) adsorption under acidic and neutral conditions, indicating H⁺ consumption, while pH decreases under alkaline conditions due to OH^- ion competition [15]. At high pH, HCrO₄^- converts to CrO₄^2-, increasing the negative charge on the adsorbent surface and inhibiting further anion adsorption [16]. The results demonstrate that PPy/LDH is highly effective in removing Cr(Ⅵ) across a broad pH range, particularly under acidic conditions.

  Figure 3

  

  Figure 3.  (a) Adsorption removal of Cr(Ⅵ) by NiFe-LDH and PPy/LDH under pH 2.0 to 11.0. (b) Ni and (c) Fe leaching from NiFe-LDH and PPy/LDH under pH 2.0 to 11.0. (d) Adsorption isothermal kinetics of NiFe-LDH and PPy/LDH for Cr(Ⅵ) at pH 5.0. (e) Column adsorption operation of Cr(Ⅵ) at pH 5.0. Experimental conditions: [Cr(Ⅵ)] = 10 mg/L, [PPy/LDH] = 0.25 g/L, T = 25 ℃.

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  The stability of adsorbents is vital for evaluating the performance of adsorbents, as metal ions leaching will increase the complexity of the subsequent removal process. Partial ion leaching often occurs in acidic media for most LDH-based adsorbents, but PPy coating reduces LDH laminate destruction in acidic reactions. As shown in Fig. 3b, NiFe-LDH exhibits higher nickel ion leaching across pH 2 to 7, risking secondary pollution and reducing stability and reusability. PPy/LDH, however, shows minimal nickel leaching (0.147 mg/L at pH 3) and none at pH 5 and above, indicating higher security of PPy/LDH. No Fe leaching was observed for either adsorbent (Fig. 3c).

  Increased PPy coating improved removal efficiency, which plateaued at higher concentrations (Fig. S9 in Supporting information). Adsorption isotherms (Fig. 3d) showed that PPy/LDH has a Cr(Ⅵ) adsorption capacity of 440.4 mg_Cr/g, much higher than that of NiFe-LDH (19.4 mg_Cr/g). The Langmuir model fits better (R² > 0.99) than the Freundlich model (Table S2 in Supporting information), indicating monolayer homogeneous adsorption behaviour for Cr(Ⅵ) on PPy/LDH [17]. The adsorption capacity of various polypyrrole and LDH-based materials to Cr(Ⅵ) was compared with that of PPy/LDH (Table S3 in Supporting information). Notably, unlike the lower acidic pH condition reported in other literature, PPy/LDH still showed excellent adsorption performance at pH 5.0, with better adsorption performance than most of the other PPy-based materials mentioned above. A column adsorption experiment at pH 5.0 (Fig. 3e) shows PPy/LDH's effective treatment volume is 300 BV, with no measurable Cr(Ⅲ) or Cr(Ⅵ) detected in the effluent, suggesting that PPy/LDH was qualified to effectively remove Cr from water. Furthermore, during the column adsorption experiment, Ni and Fe concentrations in the effluent were monitored (Fig. S10 in Supporting information), with Ni levels remaining below 20 µg/L and Fe levels below 200 µg/L, demonstrating the long-term stability of the PPy/LDH filter. We also found that the PPy/LDH composite exhibits strong application potential for real-world scenarios, e.g., 10 mg/L of Cr(Ⅵ) can be efficiently removed by 0.25 g/L PPy/LDH to safety levels in three natural water matrices, including Pearl River water, tap water, and wastewater treatment plant (WWTP) effluent water (Fig. S11 in Supporting information). Therefore, the PPy/LDH composite could serve as a reliable and efficient filter for Cr(Ⅵ) remediation across a broad range of environmental and industrial contexts.

  The adsorption kinetics of PPy/LDH was investigated using the pseudo-first-order and pseudo-second-order kinetic equations. PPy/LDH shows a rapid initial adsorption rate for Cr(Ⅵ) (Fig. S12a in Supporting information), which gradually decreases until equilibrium. In addition, the pseudo-second-order model is more suitable for describing the adsorption process of Cr(Ⅵ) on PPy/LDH (R² > 0.99) (Figs. S12b and c in Supporting information), indicating that the adsorption of Cr(Ⅵ) ions on PPy/LDH is likely controlled by chemisorption [18]. The Weber and Morris intraparticle diffusion model (Fig. S12d and Table S4 in Supporting information) showed three-stage adsorption for Cr(Ⅵ) ions (Text S6 in Supporting information). The adsorption process is influenced by both boundary layer and intraparticle diffusion [19,20], implying early electrostatic adsorption and later chemical reduction.

  The ionic strength effect shows negligible impact on Cr(Ⅵ) removal by PPy/LDH, indicating inner-sphere complexation dominates (Fig. S13a in supporting information) [21]. Competitive anions Cl^-, NO₃^-, and SO₄^2- have negligible effects on Cr(Ⅵ) removal (Fig. S13b in Supporting information), confirming strong inner-sphere interaction of Cr(Ⅵ) removal by PPy/LDH. Adsorption thermodynamics were evaluated to assess internal energy changes during adsorption. Cr(Ⅵ) adsorption by PPy/LDH increases with temperature (Fig. S13c in Supporting information), suggesting an endothermic and entropy-driven process [22]. Negative ∆G⁰ values indicate a spontaneous process, with higher temperatures favouring adsorption. Positive ΔH⁰ (+21.54 kJ/mol) and ΔS⁰ (+ 86.68 kJ/mol) values (Fig. S13d and Table S5 in supporting information) reveal that the adsorption process is endothermic and increases randomness at the solid-liquid interface [4]. Recycle measurements also indicate that Cr(Ⅵ) removal efficiency of PPy/LDH decreases gradually with the increase of the number of cycles, and could still reach > 95% removal efficiency in the second cycle (Fig. S14 in Supporting information). The decrease in adsorption capacity may be attributed to polymer chain breaking due to repeated oxidation reactions with highly oxidizing Cr(Ⅵ) species and the occupation of adsorption sites by chromium species in the previous cycles [23].

  The interaction mechanism between Cr(Ⅵ) and PPy/LDH was explored through X-ray photoelectron spectroscopy (XPS) analysis. As shown in Fig. 4a, the presence of Cr 2p peaks on PPy/LDH samples after Cr(Ⅵ) adsorption confirms chromium loading. Deconvoluted Cr 2p XPS spectrum (Fig. 4b) shows that Cr(Ⅵ) (579.1 and 588.5 eV) and Cr(Ⅲ) (577.1 and 586.7 eV) simultaneously existed on the surface of PPy/LDH, indicating both adsorption and chemical reduction of Cr(Ⅵ) to Cr(Ⅲ) by the electron-rich polypyridine groups in PPy/LDH composites [14,24]. In acidic conditions, PPy can switch between a neutral (PPy⁰) and oxidized (PPy⁺) state [25], allowing electrons to transfer from PPy⁰ to Cr(Ⅵ), reducing it to Cr(Ⅲ) [26]:

  $ \mathrm{PPy}^{+}+\mathrm{e}^{-} \rightarrow \mathrm{PPy}^0 $

  (1)

  $ 3 \mathrm{PPy}^0+\mathrm{HCrO}_4^{-}+7 \mathrm{H}^{+} \rightarrow \mathrm{Cr}^{3+}+4 \mathrm{H}_2 \mathrm{O}+3 \mathrm{PPy}^{+} $

  (2)

  $ 6 \mathrm{PPy}^0+\mathrm{Cr}_2 \mathrm{O}_7^{2-}+14 \mathrm{H}^{+} \rightarrow 2 \mathrm{Cr}^{3+}+7 \mathrm{H}_2 \mathrm{O}+6 \mathrm{PPy}^{+} $

  (3)

  $ 3 \mathrm{PPy}^0+\mathrm{CrO}_4^{2-}+4 \mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{Cr}^{3+}+8 \mathrm{OH}^{-}+3 \mathrm{PPy}^{+} $

  (4)

  Figure 4

  

  Figure 4.  XPS characterization of PPy/LDH before and after Cr(Ⅵ) adsorption. (a) Survey spectra, (b) Cr 2p spectra, N 1s spectra of PPy/LDH (c) before and (d) after Cr(Ⅵ) adsorption.

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  XPS analysis of N 1s before and after Cr(Ⅵ) adsorption (Figs. 4c and d) shows a shift to higher binding energy, indicating the involvement of nitrogenous groups in Cr(Ⅵ) reduction [25,27]. The deconvoluted N 1s spectrum reveals three peaks at 397.9, 399.8, and 401.4 eV, corresponding to quinonoid imine (=N–), benzenoid amine (–NH–), and doped imine (–NH·⁺–) electronic states, respectively (Fig. 4c) [8,25]. Post-Cr(Ⅵ) adsorption, the relative content of –NH– and –NH·⁺– decreased, while the relative content of =N– increased significantly, suggesting N functional groups transformation during Cr(Ⅵ) adsorption. The significant decrease in –NH·⁺– indicates its role in Cr(Ⅵ) reduction [28]. These findings imply that surface pyrrole nitrogen species in PPy/LDH participate in Cr(Ⅵ) adsorption and reduction. Deprotonated pyrrole nitrogen can bind Cr(Ⅲ) via Cr(Ⅲ)-N covalent bonds, forming a complex on the adsorbent [29]. The Cr(Ⅵ) removal mechanism by PPy/LDH may involve: (i) Electrostatic adsorption of negatively charged Cr(Ⅵ) onto positively charged nitrogen (–NH·⁺–) in PPy/LDH, (ii) electron transfer from protonated PPy nitrogen reducing Cr(Ⅵ) to Cr(Ⅲ), and (iii) covalent bonding of Cr(Ⅲ) to deprotonated pyrrole N.

  In summary, an organic-inorganic composite material (PPy/LDH) with high efficiency in Cr(Ⅵ) removal has been successfully prepared by in-situ oxidation polymerization. The polypyrrole (PPy) coating enhances the stability of the bimetallic hydroxide layer in acidic conditions, reducing metal ion leaching during adsorption. The optimal PPy/LDH exhibits a maximum adsorption capacity for Cr(Ⅵ) of 440.4 mg/g at pH 5, surpassing most previously reported PPy-based adsorbents. This increased capacity is due to improved specific surface area and surface functional groups. Importantly, PPy/LDH can reduce Cr(Ⅵ) concentration from 10 mg/L and 1 mg/L to below the WHO limit at 0.25 g/L dosage. PPy/LDH shows strong pH dependence for Cr(Ⅵ) removal, with negligible influence from ionic strength and coexisting anions (Cl^-, NO₃^-, and SO₄^2-). The Cr(Ⅵ) removal mechanism involves electrostatic interactions, chemical reduction, and surface chelation. The high adsorption capacity and efficiency of PPy/LDH make it a promising candidate for environmental pollution control.

  Declaration of competing interest

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

  CRediT authorship contribution statement

  Zhu Wang: Writing – original draft, Resources, Methodology, Funding acquisition, Formal analysis, Conceptualization. Shuangqiu Huang: Validation, Methodology, Investigation, Data curation. Danni Guo: Validation, Methodology, Investigation, Data curation. Wenhao Lao: Visualization, Software, Methodology, Investigation, Data curation. Yiping Feng: Writing – review & editing, Writing – original draft, Supervision, Methodology, Conceptualization. Tong Li: Writing – review & editing, Supervision, Software, Methodology, Conceptualization. Zhao-Qing Liu: Writing – review & editing, Resources. Chun Hu: Writing – review & editing, Resources.

  Acknowledgment

  This study was supported by the National Natural Science Foundation of China (Nos. 52370070, and 52070047).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2025.111090.

  
  
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  Figure 1  (a) Schematic illustration of the synthesis of PPy/LDH. (b, c) TEM images of PPy/LDH, and (d) EDS mapping of PPy/LDH.

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  Figure 2  (a) XRD patterns of PPy/LDH with different composite proportions. (b) XPS spectra, (c) N₂ adsorption/desorption isotherms and (d) pore-size distribution of NiFe LDH and PPy/LDH.

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  Figure 3  (a) Adsorption removal of Cr(Ⅵ) by NiFe-LDH and PPy/LDH under pH 2.0 to 11.0. (b) Ni and (c) Fe leaching from NiFe-LDH and PPy/LDH under pH 2.0 to 11.0. (d) Adsorption isothermal kinetics of NiFe-LDH and PPy/LDH for Cr(Ⅵ) at pH 5.0. (e) Column adsorption operation of Cr(Ⅵ) at pH 5.0. Experimental conditions: [Cr(Ⅵ)] = 10 mg/L, [PPy/LDH] = 0.25 g/L, T = 25 ℃.

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  Figure 4  XPS characterization of PPy/LDH before and after Cr(Ⅵ) adsorption. (a) Survey spectra, (b) Cr 2p spectra, N 1s spectra of PPy/LDH (c) before and (d) after Cr(Ⅵ) adsorption.

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