Because of As toxicity, the complexity of its chemistry, and the recent lowering of the maximum contaminant level of As in municipal drinking water, there has been considerable interest for improved methods to remove As from water. Although Al and Fe hydroxides have been extensively studied as adsorbents for As removal during water treatment, coprecipitated Al:Fe hydroxides have received only minimal attention. The theoretical and experimental feasibility of coprecipitated Al:Fe hydroxide systems were evaluated by studying their mineralogy, stability, and As adsorption and desorption behavior. The broad XRD peaks revealed that Al was substituted into the ferrihydrite structure and that this was the only major product up to about a 2:8 Al:Fe molar ratio. Gibbsite and bayerite were identified when Al content was higher. The rate of recrystallization of ferrihydrite into goethite and hematite was significantly reduced as Al substitution was increased. In general, adsorption capacity of both AsV and AsIII decreased with increase in Al:Fe molar ratio; however, similar AsV adsorption capacities were observed with Fe and Al:Fe hydroxides with Al:(Al+Fe) molar ratios < 0.20. Both AsIII and AsV adsorption isotherms were effectively described by Langmuir and Freundlich equations. Adsorption maxima of AsV on Fe and Al:Fe hydroxides were observed at pH 3 to 7, and that of AsV on Al hydroxide was observed at pH 5.2, with significant decreases in adsorption with increase and decrease in pH. Adsorption maxima of AsIII decreased by approximately 4 % for each 10 % increase in Al substitution up to 5:5 Al:Fe molar ratio. Adsorption maxima of AsIII on Fe and Al:Fe hydroxides were observed at pH 8 to 9. AsIII adsorption on Al hydroxide was negligible. Counterion Ca2+, compared to Na+, enhanced the retention of AsV, especially at pH > 7. Counterion concentration did not significantly affect AsV adsorption. Though phosphate desorbed both AsV and AsIII from all Al:Fe hydroxides, quantitative desorption was never observed. The results of this study indicate the possible utility of coprecipitated Al:Fe hydroxide in wastewater treatment. Based on adsorption/desorption behavior and stability of the Al:Fe hydroxide product, the preferred Al:Fe molar ratio was 2:8.

The adsorption behavior of silicic acid and arsenite alone and competitively on goethite over a broad pH range (3-11) at environmentally relevant concentrations was investigated utilizing pH adsorption data and zeta potential measurements. Both addition scenarios (Si before As(III) and As(III) before Si) were examined. The results of the adsorption experiments and zeta potential measurements were then used to model the single ion and competitive ion adsorption on goethite with the CD-MUSIC model implemented in the FITEQL 4.0 computer program. Silicic acid adsorption was reduced by the presence of arsenite for all but one of the adsorption scenarios examined, while in contrast silicic acid had little effect upon arsenite adsorption. However, the presence of silicic acid, regardless of the addition scenario, dramatically increased the arsenite equilibrium solution concentration over the entire pH range investigated. The CD-MUSIC model was able to predict the single ion adsorption behavior of silicic acid and arsenite on goethite. The modeled zeta potential data provided further evidence of the CD-MUSIC model's ability to describe the single anion adsorption on goethite. Our model was also able to collectively describe adsorption and zeta potential data for the low Si-arsenite adsorption scenario quite well however, our model under-predicted silicic acid adsorption for the high Si-arsenite competitive scenario. / Master of Science