| Knowledge of the pathways and processes controlling the partitioning of Arsenic (As) and Selenium (Se) species between readily bioavailable and less bioavailable forms is critical for understanding the environmental problems caused by excess concentrations of As or Se (which can be toxic to many organisms), and Se deficiency (Se is an essential nutrient for many organisms).; In the first part of this dissertation (Chapters 2–4) the partitioning of As and Se onto mineral solids is examined in water-mineral slurries. These sorption reactions are studied as a function of solution variables such as pH, ionic strength, and concentration of adsorbate. X-ray absorption spectroscopy (XAS), an element-specific molecular structure probe is used as the primary tool to examine As and Se sorption complexes. In the second part of this dissertation (Chapters 5–6), information gained from these and other model system studies is used to determine the dominant forms of As in mine waste samples and in a contaminated soil.; The bidentate, binuclear As(III), As(V), and Se(IV) sorption complexes formed on aluminum oxides and hydroxides and Mn hydroxides [As(V) only] are similar to those first determined on iron hydroxides several years ago. However, Se(IV) surface complexes on Mn hydroxides are distinct from any reported previously, and indicate either the formation of a coprecipitate or an edge-sharing, bidentate surface complex with Mn(II, III).; In mine wastes from former gold mining sites in California, we find that As association with iron hydroxides is abundant in completely oxidized tailings, and that ferric arsenate precipitates may form in tailings with residual sulfide due to local supersaturation with respect to ferric arsenate during As-pyrite and/or arsenopyrite oxidation. In a contaminated soil treated with ferrous sulfate and then cemented to immobilize As, we do not find evidence that a Fe(III) arsenate precipitate is the dominant form of sequestered As. Instead we propose that fully hydrated AsO4 moieties are coprecipitated in cement phases (these resemble outer-sphere complexes in terms of their EXAFS spectra. Unlike a true outer sphere complex, however, the cement forms a physical barrier to As reaction with solutions (as determined by sequential partial dissolution). |
