Removal difference of Cr(VI) by modified zeolites coated with MgAl and ZnAl-layered double hydroxides: Efficiency, factors and mechanism

Abstract As the 7th most hazardous heavy metal contaminant, chromium has attracted extensive attention due to its ecotoxicity and persistence in the environment. Layered double hydroxides (LDHs) are recognized as high-efficiency adsorbent for capturing Cr(VI). In this study, LDHs-modified zeolites (zeolites/ZnAl-LDHs and zeolites/MgAl-LDHs) were synthesized by a co-precipitation method of layered double hydroxides (LDHs) using zeolites as a template, and the morphology and microstructure of LDHs-modified zeolites were investigated through SEM, XRD, EDS and FTIR. Subsequently, the adsorption process was systematically studied. The isothermal sorption results showed that zeolites/ZnAl-LDHs exhibited a higher removal capacity Cr(VI),which was 2.3 times that of zeolites/MgAl-LDHs and 4 times that of original zeolites; the adsorption kinetic process of LDHs-modified zeolites were in line with the pseudo-second-order kinetic model, and zeolites/ZnAl-LDHs had higher adsorption rate than zeolites/MgAl-LDHs; the adsorption of Cr(VI) by LDHs-modified zeolites was a spontaneous endothermic process. Additionally, the high-efficiency pH range for Cr(VI) removal was 4.0-7.0. The existence of the coexisting cation promoted the adsorption of Cr(VI),while the anions showed the inhibiting effect. The results demonstrated that electrostatic adsorption and ion exchange were the dominant Cr(VI) removal mechanisms. Furthermore, LDHs-modified zeolites showed a significantly higher removal efficiency and superior reusability. The results of the present study showed that LDHs could be applied in practical engineering, and zeolites/ZnAl-LDHs was the more effective adsorbent for purifying chromium-containing waste water through the research of adsorption performance and mechanism.