Enhanced adsorption of arsenate and antimonate by calcined Mg/Al layered double hydroxide: Investigation of comparative adsorption mechanism by surface characterization.

This study aims to investigate adsorption mechanisms of As(V) and Sb(V) on calcined Mg/Al layered double hydroxide (LDH). The calcination process of Mg/Al LDH (CLDH) considerably enhanced the adsorption capacity of As(V) and Sb(V) via reconstruction and new formation of brandholzite-like compound, respectively. The maximum adsorption capacity for As(V) and Sb(V) by CLDH was 102.9 mg/g and 303.3 mg/g, respectively. The regeneration efficiency of As(V) by 0.5 M NaOH with 5 M NaCl mixed solution reached 72.3% during five regeneration cycles, while Sb(V) regeneration gradually decreased to 32.0% at the fifth regeneration cycles due to the irreversible surface complexation mechanism. Extended X-ray absorption fine structure (EXAFS) data displayed that Al plays a dominant role in the adsorption of As(V) on CLDH through bidentate-binuclear inner-sphere complex. On the other hand, XRD and EXAFS data revealed that Sb(V) formed a brandholzite-like structure Mg[Sb(OH)6]2·6H2O which forms outer-sphere complex by the hydrogen bonding between hexagonal plates of magnesium and antimony hydroxide. Although the formation of brandholzite-like structure causes the partial transformation of original structure of Mg/Al LDH during reconstruction process with decreasing regeneration efficiency, it could be attributed to high binding affinity between Sb(V) and Mg/Al LDH.