Abstract
In this paper, the mechanism of the redox cycling of arsenic under dark conditions was studied to help explain the high prevalence of As(III) in groundwater where no photochemical redox cycling is expected to occur. Most research has focused on the photochemical oxidation and reduction of metals and metal ions with dark redox cycling not given as much attention. A full understanding of the geochemical cycle and speciation of arsenic makes it equally important to study reduction of As(V) to As(III) under dark conditions.The reduction of As(V) by FA in the absence of light is hypothesised to occur via complexation, which can be greatly enhanced by the presence of iron. Fe(III) is envisaged to play two roles:• It facilitates binding of arsenate by FA through intermetallic bridging which results in the reduction of As(V) to the intermediate As(IV).• It can be reduced by FA to Fe(II) which then can reduce As(IV) to As(III). The reduction of As(V) is felt to occur in two one-electron steps, where the As(IV) is reduced by Fe(II).In solutions with no added Fe(III), binding of the negative arsenate by the negative FA occurs through inter-metallic bridging by cationic metals inherent in the FA. Competition from H+ ions for the binding sites on FA at lower pH results in the diminished reduction.Keywords: abiotic reduction, mechanism, As(V), fulvic acid, Fe(III)
Highlights
Our previous study has demonstrated that As(V) can be reduced to As(III) by fulvic acid (FA) under both light and dark conditions in the presence and absence of Fe(III) (Tongesayi and Smart, 2006)
Substantial reduction of As(V) by FA occurs in the dark at pH 6. This suggests that photochemical reduction is not the only abiotic mechanism responsible for the production of As(III)
The amount of reduction has been shown to increase with an increase in FA concentration
Summary
Our previous study has demonstrated that As(V) can be reduced to As(III) by fulvic acid (FA) under both light and dark conditions in the presence and absence of Fe(III) (Tongesayi and Smart, 2006). This means that photochemical reduction is not the only mechanism by which As(V) can be reduced to As(III) by FA. Most previous research (Wittbrodt et al, 1996; Hug et al, 1997; Fukushima et al, 1999; Gaberell et al, 2003; Dutta et al, 2005) has focused on photochemical Fe(III)-mediated oxidation or reduction of heavy metals in natural waters; redox cycling under dark conditions has not been given as much attention. This work is important in groundwater systems where the presence of DOM and iron may result in As(III) contamination
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