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F, r -I <br />L.A <br />Arsenic Geochemistry <br />Arsenic occurs in two oxidation states in natural waters, +3 (arsenite) and +5 (arsenate). <br />As(+5) exists predominantly as a negatively charged ion (anion) above a pH of about ?. <br />As(+5) is predominantly monovalent (charge of -1) over the pH range of 2 to 7 (f12As04), <br />divalent from pH 7 to 11.5 (HAs042-) and trivalent at pH values above 11.5 (As04;-), as <br />shown in Figure 1. <br />Arsenic <br />1 <br />0.9 <br />0.8 <br />0.7 <br />0.6 <br />A <br />c 0.5 <br />Q <br />0.4 <br />0.3 <br />0.2 <br />0.1 <br />0 <br />0 <br />pH <br />Figure 1 - Arsenate speciation as a function of pH (alpha is the fraction of the total <br />dissolved arsenate consisting of the given species). <br />The aqueous arsenate and arsenite species distribution with Eh and pH are shown in <br />Figure 2. <br />C] <br />0 <br />2 4 6 8 10 12 14