As is well known, hydrotalcite-like compounds, such as layered-double-hydroxide (LDH) materials, have shown great potential applications in many fields owing to their unique characteristics, including a higher anion exchange capacity, a structure memory effect, low costs, and remarkable recyclability. While the lower surface area and leaching of metal ions from LDH composites reduce the process efficiency of the catalyst, combining LDH materials with other materials can improve the surface properties of the composites and enhance the catalytic performance. Among organic compounds, carbon materials can be used as synergistic materials to overcome the defects of LDHs and provide better performance for environmental functional materials, including adsorption materials, electrode materials, photocatalytic materials, and separation materials. Therefore, this article comprehensively reviews recent works on the preparation and application of layered double-hydroxide-based carbon (LDH–C) composites as synergistic materials in the field of environmental remediation. In addition, their corresponding mechanisms are discussed in depth. Finally, some perspectives are proposed for further research directions on exploring efficient and low-cost clay composite materials.
Pb(ii) adsorption by MnO2/MgFe-layered double hydroxide (MnO2/MgFe-LDH) and MnO2/MgFe-layered metal oxide (MnO2/MgFe-LDO) materials was experimentally studied in lab-scale batches for remediation property and mechanism analysis.
A novel pomelo peel biochar/MgFe-layered double hydroxide composite (PPBC/MgFe-LDH) was synthesised using a facile coprecipitation approach and applied to remove cadmium ions (Cd (II)). The adsorption isotherm demonstrated that the Cd (II) adsorption by the PPBC/MgFe-LDH composite fit the Langmuir model well, and the adsorption behaviour was a monolayer chemisorption. The maximum adsorption capacity of Cd (II) was determined to be 448.961 (±12.3) mg·g−1 from the Langmuir model, which was close to the actual experimental adsorption capacity 448.302 (±1.41) mg·g−1. The results also demonstrated that the chemical adsorption controlled the rate of reaction in the Cd (II) adsorption process of PPBC/MgFe-LDH. Piecewise fitting of the intra-particle diffusion model revealed multi-linearity during the adsorption process. Through associative characterization analysis, the adsorption mechanism of Cd (II) of PPBC/MgFe-LDH involved (i) hydroxide formation or carbonate precipitation; (ii) an isomorphic substitution of Fe (III) by Cd (II); (iii) surface complexation of Cd (II) by functional groups (-OH); and (iv) electrostatic attraction. The PPBC/MgFe-LDH composite demonstrated great potential for removing Cd (II) from wastewater, with the advantages of facile synthesis and excellent adsorption capacity.
Pb (II) adsorption by MnO2/ MgFe-layered double hydroxide (MnO2/ MgFe-LDH) and MnO2/ MgFe- Layered metal oxides (MnO2/ MgFe-LDO) materials were experimentally studied in lab-scale batch for remediation property and mechanism analysis. Based on our results, the optimum adsorption capacity for Pb (II) was achieved at the calcination temperature of 400 ℃ for MnO2/ MgFe-LDH. Langmuir and Freundlich adsorption isotherm models, quasi-primary and quasi-secondary kinetics, Elovich model, and thermodynamic studies were used for exploring Pb (II) adsorption mechanism by the two composites. In contrast to MnO2/ MgFe-LDH, MnO2/ MgFe-LDO400℃ have a stronger adsorption capacity, the results show that the Freundlich adsorption isotherm model (R2 > 0.948), the quasi-secondary kinetic model (R2 > 0.998), and the Elovich model (R2 > 0.950) provide great fits to the experimental data, indicating the adsorption of a predominantly chemisorption. The maximum adsorption capacity of MnO2/ MgFe-LDO400°C for Pb (Ⅱ) was 531.86 mg·g− 1 at the dosage of 1.0 g·L− 1, pH of 5.0, and temperature of 25°C. The thermodynamic model suggests that MnO2/ MgFe-LDO400℃ is spontaneously heat-absorbing during the adsorption process. Through characterization analysis, the main mechanisms involved in the adsorption process were include precipitation action, complexation with functional groups, electrostatic attraction, cation exchange and isomorphic replacement, and memory effect. Besides, MnO2/ MgFe-LDO400°C has excellent regeneration ability in five adsorption/desorption experiments. The above results highlight the powerful adsorption capacity of MnO2/ MgFe-LDO400°C and may inspire the development of new types of nanostructured adsorbents for wastewater remediation.
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