ARSENIAN PYRITE: THE MOST IMPORTANT SOLID ARSENIC PHASE IN GROUNDWATER UNDER SULFATE-REDUCING CONDITIONS

Although orpiment (As₂S₃) and realgar (AsS) have been reported at some As-contaminated sites, their XRD confirmation is lacking. The abundance of iron in aquifer sediments and groundwater makes it unlikely that pure As-S phases will form in nature. Alternatively, As-bearing pyrite appears to be the thermodynamically most favored phase in groundwater where sulfate-reducing bacteria (SRB) are active. This conclusion is based on our research on the origin of biogenic As-bearing pyrite in alluvial aquifers and Gulf Coast salt dome caprocks in USA, published data on pyrite from anoxic marine sediments, as well as recent voluminous descriptions of the phase in reducing groundwater in Bangladesh, India, Nepal, and Cambodia. Reported As concentrations in biogenic pyrite formed under sulfate-reducing conditions range from 0.5 to 4.0 wt. %. Although not documented from natural groundwater conditions, arsenopyrite (FeAsS) was produced in a laboratory experiment where SRB metabolism occurred in contact with excess iron and arsenic. The most-widely used geochemical modeling programs contain thermodynamic data for crystalline As-S phases, and perhaps some amorphous As-S phases and thioarsenite aqueous complex. The lack of thermodynamic data for low-temperature As-Fe-S solid solutions limits the utility of geochemical modeling programs in predicting the behavior of arsenic under reducing conditions. By excluding arsenian pyrite from consideration in geochemical modeling effectively assumes that the phase is not stable, which runs counter to published data on its importance in reducing groundwater. What is needed is new (experimental) thermodynamic data for various compositions of arsenian pyrite for meaningful prediction. Short of that, a reasonable estimate of thermodynamic properties of a common composition arsenian pyrite (e.g, 1 wt. % As) should be used in the modeling programs. We are in the process of evaluating several techniques for estimating such interim value(s).