Photoinduced Oxidation of Arsenite to Arsenate in the Presence of Goethite

The photochemistry of an aqueous suspension of goethite in the presence of arsenite (As(III)) was investigated with X-ray absorption near edge structure (XANES) spectroscopy and solution-phase analysis. Irradiation of the arsenite/goethite under conditions where dissolved oxygen was present in solution led to the presence of arsenate (As(V)) product adsorbed on goethite and in solution. Under anoxic conditions (absence of dissolved oxygen), As(III) oxidation occurred, but the As(V) product was largely restricted to the goethite surface. In this circumstance, however, there was a significant amount of ferrous iron release, in stark contrast to the As(III) oxidation reaction in the presence of dissolved oxygen. Results suggested that in the oxic environment ferrous iron, which formed via the photoinduced oxidation of As(III) in the presence of goethite, was heterogeneously oxidized to ferric iron by dissolved oxygen. It is likely that aqueous reactive oxygen species formed during this process led to the further oxidation of As(III) in solution. Results from the current study for As(III)/goethite also were compared to results from a prior study of the photochemistry of As(III) in the presence of another iron oxyhydroxide, ferrihydrite. The comparison showed that at pH 5 and 2 h of light exposure the instantaneous rate of aqueous-phase As(V) formation in the presence of goethite (12.4 x 10(-5) M s(-1) m(-2)) was significantly faster than in the presence of ferrihydrite (6.73 X 10(-6) M s(-1) m(-2)). It was proposed that this increased rate of ferrous iron oxidation in the presence of goethite and dissolved oxygen was the primary reason for the higher As(III) oxidation rate when compared to the As(III)/ferrihydrite system. The surface area-normalized pseudo-first-order rate constant, for example, associated with the heterogeneous oxidation of Fe(II) by dissolved oxygen in the presence of goethite (1.9 x 10(-6) L s(-1) m(-2)) was experimentally determined to be considerably higher than if ferrihydrite was present (2.0 X 10(-7) L s(-1) m(-2)) at a solution pH of 5.