Preparation of ZnAl layered double hydroxides supported by silica for the treatment of Cr(VI) and Cu(II) in aqueous solution

Abstract A novel adsorbent ZnAl–LDHs/SiO2 (ZA/SiO2) was prepared by blending urea mixture of ZnSO4 and Al2(SO4)3 while using SiO2 as a support form. The adsorption properties of ZA/SiO2 for the removal of toxic metal ions (Cu(II) and Cr(VI)) from water were evaluated. By batch experiment method to investigate the ZA/SiO2 adsorption of Cu(II) and Cr(VI) solution treatment effect. The sorption kinetics curves of Cu(II) and Cr(VI) on ZA/SiO2 were L-shaped. What’s more, the solid concentration effect was found in the process of sorption kinetics. Langmuir and Freundlich sorption isotherm models were used to analyze the adsorption data. The results showed that the adsorption conforms to Langmuir and Freundlich adsorption isotherm models. However, the adsorption capacity of ZA/SiO2 compounds for Cu(II) and Cr(VI) is greatly improved. The adsorption capacity of Cu(II) is 158 mg·g−1 and of Cr(VI) is 176 mg·g−1, which were 3.6 and 1.8 times of ZnAl-LDHs (ZA), respectively. Density functional theory (DFT) was utilized for the analysis of intrinsic mechanism and specific pathways. The primary mechanism for removing Cr(VI) from water mainly included the intercalation of Cr2O7 2− and exchange between Cr2O7 2− and OH−, excluding Cr(OH)3 precipitation. Regarding the primary mechanism for eliminating Cu(II) from water, it involves isomorphic substitution as the predominant process, except for the formation of Cu(OH)2 precipitates.