Photoinduced Oxidation of Arsenite to Arsenate in the Presence of Goethite

Abstract

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 × 10(-5) M s(-1) m(-2)) was significantly faster than in the presence of ferrihydrite (6.73 × 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 × 10(-6) L s(-1) m(-2)) was experimentally determined to be considerably higher than if ferrihydrite was present (2.0 × 10(-7) L s(-1) m(-2)) at a solution pH of 5.

The fate of arsenic adsorbed on iron oxides in the presence of arsenite-oxidizing bacteria

“…The concentration of As(III) and goethite were chosen to ensure that the ratio of As to Fe (Fe/As z 100)after incubation was appropriate for the determination of the ratio of As(III) and As(V) by XANES analysis. In other studies on As(III) oxidation in the presence of iron oxides, the Fe/As ratio ranged from 5 to 580 (Ona-Nguema et al, 2005Bhandari et al, 2011Bhandari et al, , 2012. The flasks were shaken at 25 C at 150 rpm for 10 h. The adsorption reached equilibrium in 10 h. Abiotic control experiments of goethite only were also conducted.…”

“…It is reported that goethite interacting with dissolved Fe(II) produced the highest redox activity compared with other Fe(III) (oxy)hydroxides (Elsner et al, 2004). Photoinduced oxidation of As(III) by goethite has also been reported and can be significantly restricted by humic acid (Bhandari et al, 2012;Wang et al, 2013).…”

Removal of As(III) and As(V) from Water Using Magnetic Core-Shell Nanomaterial Fe3O4@Polyaniline

“…In general, As(V) is stable in aerobic environment and As(III) often exists in anaerobic environment. The toxicity of arsenic species is different, generally the toxicity of inorganic arsenic compounds is about 100 times higher than organic arsenic compounds, and the toxicity of inorganic As(III) compounds are approximately 60-80 times higher to humans than As(V) compounds [13][14][15]. So the world faces major threat to community health from the water supplies with contamination, which has been a clear common view, and stimulated many studies to develop new technologies for water purification.…”

The Effect of Iron Oxide Nanoparticles on the Fate and Transformation of Arsenic in Aquatic Environments

“…Therefore, microbial and photochemical processes are important in the environment because they can cause the reduction and/or oxidation of arsenic and iron Langner et al, 2000;Emett et al, 2001;Bhandari et al, 2011Bhandari et al, , 2012. Once iron oxide minerals are present, arsenic will adsorb onto these minerals and might undergo surface mediated reduction or oxidation reactions (Yan et al, 2008).…”

“…Abiotic oxidation of As(III) by dissolved oxygen is a possible mechanism of species transformation although the process is relatively slow (Zhao et al, 2011). Recent studies have shown that photoinduced oxidation of As(III) to As(V) can occur in the presence of nanoferrihydrite and goethite (Bhandari et al, 2011(Bhandari et al, , 2012. Bhandari et al (2011) assumed that in As(III)-ferrihydrite system, the oxidation of As(III) occurred on the ferrihydrite surface until the surface becomes saturated with As(V).…”