Abstract
The effect of simulated solar radiation on the oxidation of arsenite [As(III)] to arsenate [As(V)] on the layered manganese oxide, birnessite, was investigated. Experiments were conducted where birnessite suspensions, under both anoxic and oxic conditions, were irradiated with simulated solar radiation in the presence of As(III) at pH 5, 7, and 9. X-ray absorption spectroscopy (XAS) was used to determine the nature of the adsorbed product on the surface of the birnessite. The oxidation of As(III) in the presence of birnessite under simulated solar light irradiation occurred at a rate that was faster than in the absence of light at pH 5. At pH 7 and 9, As(V) production was significantly less than at pH 5 and the amount of As(V) production for a given reaction time was the same under dark and light conditions. The first order rate constant (kobs) for As(III) oxidation in the presence of light and in the dark at pH 5 were determined to be 0.07 and 0.04 h−1, respectively. The As(V) product was released into solution along with Mn(II), with the latter product resulting from the reduction of Mn(IV) and/or Mn(III) during the As(III) oxidation process. Post-reaction XAS analysis of As(III) exposed birnessite showed that arsenic was present on the surface as As(V). Experimental results also showed no evidence that reactive oxygen species played a role in the As(III) oxidation process.Electronic supplementary materialThe online version of this article (doi:10.1186/s12932-016-0037-5) contains supplementary material, which is available to authorized users.
Highlights
The metalloid arsenic is one of the most common contaminants in ground water, where it is derived mainly from oxidative weathering of rocks [1] and from industrial effluents [2, 3]
The As(V) concentrations after 8 h are similar for the oxic and anoxic experiments in the presence of light (197 and 196 μM, respectively) and similar in the dark experiments (136 and 147 μM, respectively). These results show that light irradiation has a greater influence on the oxidation As(III) and oxygen does not appear to play a major role in the oxidation mechanism
We have shown that the oxidation of As(III) by Na-birnessite can be enhanced via irradiation with simulated solar light
Summary
The metalloid arsenic is one of the most common contaminants in ground water, where it is derived mainly from oxidative weathering of rocks [1] and from industrial effluents [2, 3]. Arsenic in aquatic systems exists mainly in the 3+ and 5+ oxidation states, occurring primarily as arsenite (H3AsO3) and the oxyanion arsenate (AsO43−), respectively. Both forms pose health risks, the trivalent species is considered more toxic and more mobile than the pentavalent form [4]. Photochemical processes that facilitate or enhance the oxidation of trivalent arsenic have received considerable interest in recent years. Semiconductors such as TiO2 are already in use in arsenic remediation schemes, the activation of this particular material requires ultraviolet (UV) radiation for the oxidation of As(III) [7,8,9,10,11]. Neither TiO2 nor the iron oxyhydroxides show significant activity toward As(III) oxidation in the absence of light
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