TURMOIL REGARDING THE REDOX STATE OF ARSENIC IN SULFIDIC WATERS

With the onset of sulfate reduction in highly reducing environments, arsenite is converted to thiolated species. These thioanions until recently were believed to be As(III) species such as AsS[SH][OH]^- and As₃S₄[SH]₂^-. Now, novel ion-chromatographic evidence alleges that these thiolated species are instead As(V) species: H_nAsO_xS_4-x^n-3. Contrary to most prior evidence, it is claimed that sulfide drives As(III) to As(V) with coupled reduction of As(III) to As(0). Until this issue is clarified, predictions of arsenic mobility in highly reducing environments will be uncertain. New experimental results allow a comparison of arsenic solubilities in sulfidic solutions in contact with As₂S₃ alone and with As₂S₃+S⁰ (the latter solutions containing polysulfides as well as sulfide). A series of experiments at pH 7.0±0.5 indicates that polysulfides fail to enhance orpiment solubilities, contrary to expectation if As(V) thioarsenates were the predominant dissolved species. On the other hand, at pH ~8.5, orpiment quantitatively consumes polysulfides in a manner consistent with formation of As(V) thioanions. The redox state of As in sulfide solutions thus appears to be very pH-sensitive. An effort to reinterpret the data of Wilkin et al. (2003) in terms of As(V) species requires implausible assumptions about protonation equilibria. Solubilities of As(V) thioanions would have a greater pH dependence than observed by these previous authors. The cumulative evidence is best reconciled by assuming that As(III) thioanions predominate at near-neutral pH in contact with orpiment, but that these thioanions undergo redox disproportionation in the pH 12 mobile phase used in ion chromatography. It is proposed here that OH^- combines with thioarsenites to form reactive intermediates which are potentially strong electron-pair donors (reducing agents). In mildly alkaline solutions (pH ~8.5), these intermediates can quantitatively reduce polysulfides. In strongly alkaline solutions (pH ~12), they can reduce other As(III) thioanions to As(0).