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The amount of substitution of arsenic into ferric hydroxide is determined by the pH of • <br />the solution (more arsenic substitution occurs at lower pH values) and the concentration <br />of arsenic in solution (higher arsenic concentrations result in more substitution). <br />Arsenic Adsorption { <br />Arsenic. adsorbs to solid surfaces due partly to interactions between the negatively <br />charged ions and a positively charged surface. Therefore, arsenic adsorption tends to be r <br />favored for solid materials which are positively charged. The surface charge of the <br />material depends on the type of solid, the pH of the water, and the concentration of <br />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 arsenate ions become less prevalent. The <br />pH of the ZPC is different for different materials, as shown in Table 1. <br />Table 1 DH of the Zero-Point-of-Charae (aHzoc) for Various Minerals' <br />Material Formula Hzvc <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 y-A100H 8.2 <br />Aluminum Hydroxide A-AI OH 3 5.0 <br />Amorphous Silica Si02 2.0 <br />Mangan se Dioxide 6-Mn02 2.8 <br />Montmorillonite Clay Na0.2Cao.vA12Si401o OH 2.10 H2O 2.5 <br />Kaolinite Clay A12Si2O5(OH)4 4.6 <br />1. Data from Stumm and Morgan (1981) <br />The materials with a higher pHzPc are able to maintain a positive charge at a higher pH <br />than for materials with a lower pHzpc. Of the materials listed in Table 3-1, amorphous <br />ferric hydroxide is the best anion adsorbent at higher pH values (below 8.5). <br />Under typical Eh/pH conditions, As(+3) is a neutral ion and does not adsorb well to <br />negatively or positively charged surfaces. Therefore, As(+3) is roughly 4-10 times more <br />mobile than As(+5) (Duel and Swoboda,1972). In addition, As(+3) is about 60 times <br />more toxic to humans than arsenate (Houslow,1980). <br />• <br />Arsenic has a strong affinity for iron phases and minerals. Strong correlations between <br />arsenic and iron have been found in soils (Woolson et al., 1971; Duel and Swoboda 1972); <br />in ores (Shnyukov,1963); within ferrihydrite impurities in phosphate pebbles (Stow, <br />• <br />1969); and in sediments impacted by arsenic-containing groundwaters (Whiting, 1992).