Comparison of arsenate and arsenite removal behaviours and mechanisms from water by Fe La binary composite (hydr)oxides

The present study compares the performances and mechanisms of As(V) adsorption with As(III) removal by a series of FeLa binary composite (hydr)oxides. The As(V) and As(III) adsorption properties were studied by batch adsorption experiments, and their adsorption mechanisms were explored by SEM-EDS, FTIR, XRD and XPS. The FeLa binary composite (hydr)oxides exhibited excellent As(V) removal performances, and their adsorption properties of As(V) were obviously enhanced with increasing La contents. However, their As(III) removal performances were extremely poor, and the increase of Fe contents could not significantly affect As(III) adsorption performances. The adsorption processes of As(V) were rapid, but the As(III) adsorption were slower. The maximum adsorption capacities of As(V) on Fe/La 3:1, Fe/La 1:1, and Fe/La 1:3 were 166.69, 271.65 and 360.81 mg/g, respectively, and those of As(III) were 30.71, 26.44 and 28.64 mg/g, respectively. Solution pH and presence of PO43−, and citrate can clearly influence the removal efficiencies of As(V) and As(III). The predominant mechanisms of As(V) were the formation of LaAsO4 through ligand exchange reactions under all experimental conditions and coprecipitation of La(III) and As(V) under acidic and neutral conditions, while the formation of inner-sphere surface complexes through ligand exchange reactions between Fe–OH groups and As(III) contributed to the As(III) removal. This work emphasizes the different adsorption properties between As(V) and As(III) by FeLa composite (hydr)oxides, and indicates great application potential in As(V) water treatment and the need for pretreatment for As(III) removal, such as pre-oxidation of As(III) to As(V).