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• <br />E <br />0 <br />1.4 <br />1.2 <br />1.0 <br />0.8 <br />0.6 <br />0.4 <br />>> 0.2 <br />v <br />U 0.0 <br />-0.2 <br />-0.4 <br />-0.6 <br />-0.8 <br />-1.0 <br />H3ASO4 <br />_ (aq) <br />H2AsO4 (aq) <br />HAs04 2 (aq) Cr <br />. <br />cc <br />HAS0° (a q) 3 q) `?V <br />0 <br />Q <br />H2AS03 (aq) - - ?- . <br />HAS03 2 (aq) <br />4S0 3 <br />3 <br />aq) <br />0 2 4 6 pH 8 10 12 14 <br />Figure 2 - Eh-pH diagram for the system As-O-H at 25° C and 1 atm. <br />As(+3) is predominantly a neutral species (H3AsO3°) below a pH of about 9. HZAsO3- and <br />HAsO3-2 do not become important until the pH exceeds 9 su, which is higher than <br />observed in the vast majority of natural waters. <br />Arsenic Pure Phase Minerals <br />Pure phase arsenic minerals such as orpiment (As2%), realgar (AsS), and arsenopyrite <br />(FeAsS) occur mainly in ore deposits formed from hydrothermal fluids within the <br />Earth's crust. A few pure phase arsenic minerals occur under low temperature and low <br />pressure conditions at the Earth's surface, such as scorodite (FeAsO4.2H20 at low pH), <br />and arsenic sulfides (under reducing conditions). However, the vast majority of pure <br />phase arsenic minerals are too soluble to be present in soils that are in contact with <br />water. <br />Arsenic Solid-Solution Phases <br />Arsenic forms solid-solution phases with ferric hydroxide and iron hydroxysulfates such <br />as jarosite (HFe3(OH)6(SO4)2) and schwertmannite (Fe8O8(OH)6SO4) and with amorphous <br />silica. Arsenate, like silicate, has a tetrahedral form (a central atom coordinated with four <br />oxygen atoms) which may facilitate the incorporation of arsenate into amorphous silica. <br />Amorphous phases such as ferric hydroxide or schwertmanite tend to substitute <br />hydroxide or sulfate for arsenate. A reaction to form an iron-arsenic solid-solution is as <br />follows: <br />Fe+3 + xAsO4-3 + (3-3x) OH- -? [FeASO4 2H20JJFe(OH)3]1-X