QUANTITATIVE ARSENIC SPECIATION IN MINE TAILINGS USING X-RAY-ABSORPTION SPECTROSCOPY

X-ray absorption fine structure spectroscopy (XAFS) was used to determ ine arsenic (As) oxidation state, local coordination (to a radius of a pproximate to 7 Angstrom around As), and the relative proportion of di fferent As species in model compounds and three California mine wastes : fully oxidized tailings (Ruth Mine), partially oxidized tailings (Ar gonaut Mine), and roasted sulfide ore (Spenceville Mine). Mineralogy w as characterized by Rietveld refinement of X-ray powder diffraction pa tterns. The spatial distribution of As in the mine wastes (at several micrometers spatial resolution) was determined by electron microprobe analyses. X-ray absorption near edge structure (XANES) analysis indica tes that As5+ is the dominant oxidation state in the mine samples, but mixed oxidation states (nominally As-0 and As5+) were observed in the Argonaut Mine waste. Non-linear, least-squares fits of mine waste EXA FS (Extended XAFS) spectra indicate variable As speciation in each of the three mine wastes: As5+, the Ruth Mine sample is sorbed on ferric oxyhydroxides and aluminosilicates (probably clay) in roughly equal po rtions. Tailings from the Argonaut Mine contain approximate to 20% As bound in arsenopyrite (FeAsS) and arsenical pyrite (FeS(2-x)Asx) and a pproximate to 80% As5+ in a precipitate such as scorodite (FeAsO4.2H(2 )O); however, no precipitate was detected by X-ray diffraction or micr oprobe analysis, suggesting that the phase is poorly crystalline or ha s low abundance (total As in sample = 262 ppm). Roasted sulfide ore of the Spenceville Mine contains As5+ substituted for sulfate in jarosit e [KFe3(SO4)(2)(OH)(6)] or incorporated in the structure of an unident ified Ca- or K-bearing phase, and As5+ sorbed to the surfaces of hemat ite or ferric oxyhydroxide grains. Determination of solid-phase As spe ciation in mine wastes by XAFS spectroscopy is a valuable first step i n the evaluation of its bioavailability, because the mobility and toxi city of As compounds vary with oxidation state. As bound in precipitat es, as in the Argonaut mine sample, is considered to be less available for uptake by organisms than when sorbed on mineral surfaces or copre cipitated with poorly crystalline phases, as found for the Ruth and Sp enceville mine wastes.