| Adsorption technologies are the most preferred method for removing arsenic in small-scale drinking water systems. However, adsorption processes employing activated alumina have shown to be very sensitive to pH, with optimum pH for arsenic removal ranging from 5.5 to 6.0. This is outside the pH range of most groundwaters, and thus forces water treatment plants to adjust the pH prior to the adsorption process. This pH effect is likely due to the adsorption of As(V) or other protolyzable anions on the isoelectric pH (pHiep) of the activated alumina. Even though the pHiep of activated alumina is approximately 8.2, the pHiep shifts to lower pH values when the adsorption media adsorbs protolyzable anions. This causes a significant drop in pHiep to a value around 5.5∼6.0. This, in turn, results in an unfavorable range for arsenic removal in most water treatment scenarios. Although this effect will occur with most adsorption media, materials having higher pHiep than activated alumina have higher arsenic adsorption capacities at pH values typical of drinking water (pH = 6.0∼9.0). Magnesium aluminate spinel (pHiep = 10.2) was prepared by a sol-gel process and investigated as an arsenate adsorbent to overcome this pH effect.; The second most pressing problem associated with arsenic removal is As(III), arsenite. Since this trivalent arsenic species remains uncharged at the pH of drinking water, it is difficult to remove by adsorption or ion exchange. During this study, a TiO2/Al2O3 mixed oxide was developed which has been shown to effectively remove As(III) without requiring a separate oxidation process. The effect of dissolved oxygen on the rate of oxidation for this newly developed process was also monitored. Finally, As(III) photooxidation rates were compared for a synthetic test solution containing arsenic and a ground water containing arsenic as well as iron, phosphate, silica, organic matter, and bicarbonate. |
