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• <br />Vanadium Geochemistry <br />Vanadium (V) occurs in three oxidation states in natural waters, +3, +4, and +5. V(+4) <br />exists exclusively as the positively charged ion (VO''), and is stable only at pH values <br />less than 5.87. <br />V (+3) exists mainly as positively charged ions below a pH of 5.19, and a neutral species <br />above pH 5.19 (see Figure 1). <br />Vanadium III <br />n <br />U <br />1 <br />0.9 <br />0.8 <br />0.7 <br />0.6 <br />M <br />La 0.5 <br />a 0.4 <br />0.3 <br />0.2 <br />0.1 <br />0 <br />V-? V(OH) + V(OH) <br />V(OH)' <br />V V V <br />A A A <br />A /X ix <br />0 2 4 6 8 10 12 14 <br />pH <br />Figure 1 - Vanadium (+3) speciation as a function of pH (alpha is the fraction of the <br />total dissolved vanadium (+3) consisting of the given species). <br />V(+5) exists mainly as a positively charged species below a pl-I of 3.30, a neutral species <br />from 3.30 to 3.79, and negatively charged species at pH values greater than 3.79 (see <br />Figure 2). <br />