Arsenic leach fractions

Figure 12.1 Map showing locations of sites (squares) used for the soil profiles and boreholes (C and B), and contents of arsenic in the leach fractions of the top layer of the soil profiles. Squares with stars denote samples used for the Pb isotope study other squares are locations of profiles used in a geochemical study (Ayuso, unpublished data). Dashed lines enclose areas containing wells characterized by Lipfert and Reeve (2004) and Lipfert et al. (2007) as containing high-arsenic groundwater (As >1.3 pmol L ), medium arsenic groundwater, and low arsenic groundwater in the Mount Percival recharge area (most wells have <0.13 jimol L-1). Solid line encloses the drainage basin in this study.

Soil samples were air dried and prepared for lead isotope analysis using 50-100 mg of the <0.2-mm size fraction. Sulfides, Fe-hydroxides, and other secondary minerals in the Penobscot Formation can be used to monitor the composition of labile Pb and provide the means to discriminate labile (anthropogenic) lead from lead inherited from the parent rocks and sulfides (e.g., Ayuso and Foley, 2008). A cool mild acid leach (1,5N HC1 + 3N HNO3) was used to attack the secondary minerals. This solution likely reflects the labile Pb (e.g., Erel et al., 1997) captured in the Fe-hydroxide, carbonate, or organic materials, or other secondary minerals (clays). These minerals can contain lead, arsenic, and other elements derived from outside of the watershed. Mixed solutions of HF—HN03 were used for final dissolution of the residual fractions. Table 12.2 summarizes the Pb-isotopic data for the leach fractions of the soil horizons (together with Pb and As contents) Table 12.3 shows equivalent data for the residues. 

Leach fractions from deeper in the soil profiles ( >10 cm) are somewhat shifted from the field of aerosols and gasoline, and the field of the Fe-hydroxides toward higher values of 208Pb/207Pb and toward the field of the arsenical pesticides (Fig. 12.6). Higher values of 208Pb/207Pb than the Fe-hydroxides and pesticides in deeper portions of the soil profiles may indicate a more prominent role for imported lead sources (Peruvian and Mexican Pb ores), or a natural source that has not yet been identified (Fig. 12.6). 

Cobalt sulfide and arsenide ores are often found mixed with those of nickel and copper. The mixed ore is roasted with Na2C03 and KNO3, which removes part of the sulfur and arsenic as volatile species. The residue contains the metal oxides, as well as sulfate and arsenate, and the latter are removed by leaching with water. The metal oxide mixture is then dissolved in hot HCl or H2SO4, and the individual metal oxides are fractionally precipitated using Ca(OH)2 and NaOCl. This process gives the mixed valent oxide C03O4, which is then reduced to the metal by treatment with charcoal. 

Only limited work has been done on the bioaccessibility of metals in windborne mine waste and tailings material, and so much must be inferred. Mullins and Norman (1994) analyzed the size distribution, metal content, and metal extraction by simulated biofluids (lung, gastric, intestinal) of surface materials (soils) collected from several mine waste piles in the Butte, Montana, district. They found that the concentrations of arsenic, cadmium, copper, manganese, and lead were commonly greatest in the smallest size fractions (<4.7 pm) of the waste dump material. The percentage of metals leached from the hne fraction was quite variable, but not in any consistent way, between different metals, different dumps, and different extraction fluids. 

Chemical leach tests of the <50 p.m size fraction of dust samples collected around Owens Lake, using water (Reheis etal, 2001, and our unpublished data) and SLFs (our unpubhshed data), show that the dusts are sufficiently aUcahne and reactive to shift the pH of water and SLF to values near 10.5 and 9.5, respectively. Arsenic, chromium, vanadium, molybdenum, hthium, zinc, and other trace metals or metalloids are readily solubilized from the dusts. The trace metals or metalloids leached in the greatest quantities are those that form oxyanion species or abundant carbonate complexes in solution, and that are therefore mobilized most effectively under the alkaline conditions generated by the alkaline dusts. 

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