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• <br />Selenium Geochemistry <br />Selenium occurs in solution in oxidation states of -2 (selenide), +4 (selenite), and +6 <br />(selenate). Selenium can exist in the zero-valent oxidation state as a solid, but not in <br />solution. Selenide (Se-'-) is the most reduced form of selenium and tends to form <br />insoluble compounds with metals and replace sulfide in metal-sulfide minerals such as <br />pyrite- (FeS2). Under low pH conditions (<4), hydrogen selenide (H2Se) is formed, which <br />can volatilize from solution. As shown in Figure 1, HSe- is the dominant selenide <br />species above a pH of about 1, while H2Se1' predominantes below a pH of 1. The Se-'- <br />species is not present except at very high pH values and is not an important species. In <br />fact, selenide species in general are present in solution in very low concentrations, due to <br />the formation of selenide solid phases as will be discussed further below. <br />Selenide <br />0.9 <br />0.8 <br />0.7 <br />is 0.6 <br />a 0.5 <br />Q <br />0.4 <br />0.3 <br />0.2 <br />0.1 <br />0 <br />pH <br />Figure 1 - Selenide speciation as a function of pH (alpha is the fraction of the total <br />dissolved selenide consisting of the given species). <br />Selenite is a negatively charged ion (anion) in solution at pJA values greater than about <br />2.5. The selenite species flSeOx- is predominant within the pH range 2.5 to 8.5, while <br />W01-2 is stable above a pH of 8.5 (see Figure 2). <br />E <br />0 2 4 6 8 10 12 14