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• <br />1.4 <br />1.2 <br />1.0 <br />0.8 <br />0.6 <br />0.4 <br />°> 0.2 <br />W 0.0 <br />-0.2 <br />-0.4 <br />-0.6 <br />-0.8 <br />-1.0 <br />HSe04 (aq) <br />H2Se03o ' <br />(aq) Se04 2 (aq) <br />HSe03 (aq) <br />H2Se° (aq) <br />- SeOa 2 (aq) <br />HSe" (aq) <br />0 2 4 6 pH 8 10 12 14 <br />Figure 4 - Eh-pH diagram for the system Se-O-H at 25° C and 1 atm. <br />Selenium Pure Phase Minerals <br />Several pure phase minerals such as Se metal and iron selenide (the selenium analog of <br />pyrite) have been proposed to explain the relatively low mobility of selenium in soils <br />(EPRI,1984). However, theses phases have not been directly observed in soils and are <br />speculative. <br />Selenium Solid-Solution Phases <br />Selenium in the form of selenite is believed to co-precipitate with iron hydroxide, as iron <br />and selenium tend to correlate in many soils, and selenium occurs with the reducible <br />iron leachates in sequential extraction tests. <br />The substitution of selenium for hydroxide into the iron hydroxide phase is given by the <br />following reaction: <br />(1+x)Fe+3 + RSe03 2 + (3-x) OH- --> [Fe2(SeO3)3 2H2O]X[Fe(OH)3]1-X <br />• <br />is