Particle size and interlayer anion effect on chromate adsorption by MgAl-layered double hydroxide

The chromate adsorption efficacy of MgAl-layered double hydroxide (LDH) was investigated along with the two aging conditions (room temperature and post hydrothermal treatment) and two intercalated anions (Cl⁻ and CO₃²⁻). Based on the adsorption isotherm, Cl^-intercalated LDH (Cl-LDH) followed the Langmuir isotherm model, and its adsorption capacity (60–70 mg/g) was 4–15 times higher than that of CO₃-LDH to the affinity of anions to LDH. Moreover, adsorption kinetics results indicated that fast chromate adsorption occurred within 30 min, and reached a plateau within 1 h due to chemical interactions. After the chromate adsorption, the characteristic crystal structure and morphology were well preserved, and based on the powder X-ray diffraction (PXRD), zeta-potential, and scanning electron microscopy (SEM) results, most of the chromate ions were possibly adsorbed on the surface of LDH with partial intercalation interlayer spaces. The X-ray photoelectron spectroscopy (XPS) Cr 2p spectra of the chromate-adsorbed LDH exhibited characteristic peaks attributed to trivalent chromium and chromate ions, indicating the reduction of hexavalent chromium by adsorption onto the hydroxyl groups (-OH) in LDH. The Cl-LDH aged at room temperature (Cl-RT) exhibited 100% adsorption efficacy during the fifth cycle and an average desorption efficacy of 80% in the recyclability test. Moreover, the practical applicability of the LDH was further confirmed by conducting a simulated box test, where the box was filled with silicate sand, in which it exhibited a high chromate removal efficacy (around 70%) during 180 min. Therefore, it could be confirmed that the LDH prepared in this study is an effective material for in-situ soil and groundwater remediation.