Removal mechanism of arsenate by bimetallic and trimetallic hydrocalumites depending on arsenate concentration

Abstract We investigated the influence of initial arsenate concentration (C0) in the 5th order of magnitude on removal of arsenate by hydrocalumite (bimetallic layered double hydroxide, LDH) and Mg-doped hydrocalumite (trimetallic LDH) from aqueous solution. These hydrocalumites were prepared by the microwave-assisted hydrothermal treatment. There is a trend that the larger adsorption density of arsenate (Qe) values is observed with bimetallic LDH under low C0 values and with trimetallic LDH under high C0 values. The transitional C0 values ranged at 2.10–2.96 mM. Comprehensively understanding characterization results for the solid residues after adsorption of arsenate by X-ray diffraction, 27Al-nuclear magnetic resonance, and scanning electron microscopy–energy dispersive X-ray, the mechanism to remove arsenate was dependent on arsenate concentrations. At low arsenate concentration, partial intercalation and dissolution–reprecipitation (DR) happened together. With increasing C0, full intercalation and DR happened to bring out one phase of arsenate-bearing hydrocalumite. Under the very high C0, DR mechanism happened at the edge sites of LDH sheets, leading that the newly formed massive precipitates block the further intercalation with nitrate. As a result, two phases of LDH were observed. The greater Qe with bimetallic LDH in low concentration comes from high crystallinity to enhance partial ion-exchange, and greater Qe with trimetallic LDH in high concentration is derived from more fragile properties to enhance DR mechanism.