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The solid-solution reactions involoving vanadium are not well documented. However, • <br />several investigations have been performed that suggest that vanadium is associated <br />with iron oxyhydroxide phases (EPRI,1984). Kaur et al. (2009) found that V+3 has a <br />similar ionic radius and hydrolytic properties as Fe+3' and can substitute for iron within <br />the mineral goethite (Fe00H) at concentrations as high as 13% (on a molar basis). A <br />reasonable solid solution reaction would be as follows: <br />(1) <br />xFe+3 (act + (1-x)V+3 (aq) + 2H20 (1) -+ FeXVl_XOOH (s) + 3H+ (aq) <br />Substitutions of vanadium (+:5) oxyanions for hydroxide within an amorphous iron <br />oxyhydroxide phase is also theoretically possible, but has not been demonstrated in the <br />literature. <br />Vanadium Adsorption <br />V(+5) adsorbs to solid surfaces due partly to interactions between the negatively <br />charged ions (at pH values greater than about 3.30 su) and a positively charged surface. <br />Therefore, V(+5) adsorption tends to be favored for solid materials which are positively <br />charged. The surface charge of the material depends on the type of solid, the pH of the <br />water, and the concentration of other anions in solution. <br />At low pH values, the water and mineral surfaces have higher concentrations of <br />hydronium ion (H30+) which imparts a positive charge to the surface. As the pH <br />increases, the hydronium ion concentration decreases relative to the hydroxide ion (OH-) • <br />concentration in both the water and the solid materials within the water. At a specific <br />threshold pH value called the pH of the zero-point-of-charge (ZPC), the surface charge <br />transitions from positive to neutral to negative. Once the surface charge becomes <br />negative, adsorption of the negatively charged V(+5) ions become less prevalent. The <br />pH of the ZPC is different for different materials, as shown in Table 1. <br />Table 1 nH of the Zero_Poin4.ef-Charge (pH...I for Various Minerals' <br />Material Formula Hzac <br />Magnetite Fe304 6.5 <br />Goethite Fe00H 7.8 <br />Hematite Fe203 6.7 <br />Amorphous Ferric hydroxide Fe(OH)3 8.5 <br />Aluminum Hydroxide -AIOOH 8.2 <br />Aluminum Hydroxide A-AI OH 3 5.0 <br />Amorphous Silica Si02 2.0 <br />Manganese Dioxide 6-Mn02 2.8 <br />Montmorillonite Clay Nao.2Cao.iA12Si401o OH 2.10 H2O 2.5 <br />Kaolinite Clay A12Si2O5(OH)4 4.6 <br />1. Data from Stumm and Morgan (11981) <br />The materials with a higher pHzrc are able to maintain a positive charge at a higher pH <br />than for materials with a lower pHzrc. Of the materials listed in Table 1, amorphous <br />ferric hydroxide is the best anion adsorbent at higher pH values (below 8.5). •