Removal of hexavalent chromium ions by core-shell sand/Mg-layer double hydroxides (LDHs) in constructed rapid infiltration system

This study aimed to determine the efficiencies and mechanisms of hexavalent chromium (Cr(VI)) removal with modified sand coated by Mg-LDHs in constructed rapid infiltration system (CRIS). Mg-LDHs (MgAl-LDHs, MgFe-LDHs) were prepared by co-precipitation method and in situ coated onto the surface of original sand. Scanning Electron Microscope (SEM) and X-Ray Fluorescence Spectrometer (XRFS) were used to analyze physicochemical properties of sand/Mg-LDHs composites. Results obtained confirmed the successful LDHs-coating modification. During the purification experiments, the average removal rates of Cr(VI) were 47.62% for sand/MgAl-LDHs, 34.15% for sand/MgFe-LDHs and 11.61% for original sand, respectively. The sand/Mg-LDHs had a higher Langmuir adsorption capacity and desorption capability for Cr(VI) compared to original sand. The adsorption kinetic data of different adsorbents were better described by pseudo-second-order model. Intra-particle diffusion model was also used to elucidate the adsorption mechanism. Moreover, extracellular polymeric substances, biomass and enzymatic activity of microbes on the modified and original sand were testified and analyzed to study microbial effect on Cr(VI) removal in test columns. Through a rough economic estimation, the reagent cost of sand/MgAI-LDHs synthesis was only RMB (sic) 0.24/Kg. It could be concluded that MgAl-LDHs modified sand is an efficient and economical substrate of CRIS for Cr(VI) removal.