Arsenite removal in a goethite/oxalate system under UVA irradiation: Roles of different reactive species in acidic and neutral conditions

Abstract

In this work, arsenite (As(III)) removal in a goethite/oxalate suspension under UVA-irradiation (λmax = 365 nm) was explored under different conditions (initial pH, oxalate concentration, goethite dosage, and phosphate). SEM, EDS, FTIR, XRD and XPS were used to investigate the morphology, composition, functional groups, structure, and surface properites of goethite. The greatest efficiency of As(III) removal was 95.52% at pH 3.0, 100.0 µg·L–1 As(III), 0.05 g·L–1goethite, and 0.5 mmol·L–1oxalate. The removal of As(III) in the goethite/oxalate system was strongly pH-dependent. An increase in pH was accompanied by a reduction in photo-oxidation efficiency and an increase in adsorption efficiency. Addition of appropriate amounts of oxalate can enhance As(III) removal at pH 3.0 by promoting the photo-dissolution of iron oxide. However, the addition of oxalate was detrimental to As(III) removal at pH 7.0 due to its competition for photo-generated hole (hvb+) as well as adsorption sites. Increasing dosage from 0.0 to 0.5 g·L–1 accelerated As(III) removal at both pH 3.0 and pH 7.0. ESR, radical-scavenging, and N2-purging experiments showed that As(III) removal was mainly achieved by photo-oxidation at pH 3.0 and HO· appeared to be the primary oxidant for the As(III) oxidation, which was strongly dependent on HO2·/O2·–. However, adsorption served as a major pathway and photo-oxidation played a minor role at pH 7.0. As(III) photo-oxidation was predominantly attributed to the direct reaction between As(III) and hvb+ under neutral conditions. In addition, O2 played an important role in As(III) photo-oxidation under acidic and neutral conditions. Moreover, goethite remained morphologically intact and demonstrated good reusability up to five regeneration cycles. These findings show that As(III) can be effectively removed from water containing iron oxides and carboxylic acid, which is important for the remediation of arsenic-contaminated waterbodies and wastewaters.