Adsorptive removal of arsenic using a novel akhtenskite coated waste goethite

In recent years, the synthesis of manganese-based mixed oxides that can promote both the oxidation and the adsorption of arsenic has aroused great interest among those who are involved in management of arsenic wastewater. In this work, waste goethite, BT9 (α-FeOOH, 0.25–1 mm, 232 m2 g−1), which was recycled from industrial fluidized-bed Fenton equipment, was combined with akhtenskite (ε-MnO2), called MnBT9, for use in the adsorptive removal of arsenic (As(III) and As(V)) from solution. MnBT9 was prepared using a fluidized-bed reactor (FBR) in which ε-MnO2, formed by oxidizing Mn2+ with sodium hypochlorite (NaOCl), was uniformly deposited onto BT9 support particles. Langmuir isotherms suggested that BT9 effectively adsorbed As(V) (28.25 mg g−1) but was mostly ineffective in adsorbing As(III) (8.03 mg g−1). Nevertheless, the deposition of MnO2 on MnBT9 markedly increased its capacity to adsorb As(III) (34.36 mg g−1) without significantly influencing the retention of As(V) by the BT9 substrate. K-edge XANES (X-ray absorption near edge structure) analysis revealed that As(III) was totally oxidized to As(V) on MnBT9. A pseudo-second-order model was used to elucidate the kinetics of adsorption; at pH 3.5, MnBT9 removed As(III) at a higher rate than it removed As(V), proving that oxidation was a limiting step of arsenic removal when the BT9 substrate dominated the adsorption of As(V) in either its native form or formed by the oxidation of As(III).