Layered double hydroxide/eggshell membrane: An inorganic biocomposite membrane as an efficient adsorbent for Cr(VI) removal

Hexavalent chromium, Cr(VI), is carcinogenic and mutagenic to the living organisms, and hence more hazardous than other heavy metals. The choice of Cr(VI) removal adsorbents has always been a hot issue in environmental protection. Here we report a novel inorganic biocomposite membrane, layered double hydroxide/eggshell membrane (LDH/ESM) as adsorbent for Cr(VI) removal, in which ESM serves as the substrate and template to immobilize the LDH formed by an in situ hydrothermal crystallization method. X-ray diffraction (XRD) and scanning electron microscope (SEM) images showed that MgAl–CO32−–LDH grown on the fibers of ESM, and the final LDH/ESM composite membrane both maintained the network structure of the ESM substrate. After being treated with an NaCl/HCl mixed solution, the interlayer CO32− anions in the LDH portion of the composite membrane were exchanged for Cl−. The ESM, MgAl–CO32−–LDH/ESM and MgAl–Cl−–LDH/ESM composite membrane were evaluated as adsorbent to remove Cr(VI), respectively, and the effect of the pH values on the adsorption was mainly investigated. It showed that the MgAl–Cl−–LDH/ESM composite membrane had a higher adsorption capacity than ESM and MgAl–CO32−–LDH/ESM—even when the pH value of the Cr(VI) solution was not further adjusted by addition of acid, which is of practical significance in a wastewater treatment plant. The adsorption isotherm of the MgAl–Cl−–LDH/ESM composite membrane showed a good fit with the Langmuir isotherm model.