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.

Automated summary

In this study, the chromate adsorption efficacy of MgAl-layered double hydroxide (LDH) was investigated. The results showed that Cl-intercalated LDH had a higher adsorption capacity compared to CO3-LDH, and fast chromate adsorption occurred within a short period of time. The crystal structure and morphology of LDH were well preserved after the adsorption, with most of the chromate ions being adsorbed on the surface. Additionally, LDH exhibited a high removal efficiency in practical applications.