AWWA ACE58105

Adsorption technologies are one of the preferred methods for removing arsenic in small-scale drinking water systems. Because of its low cost and high surface area, activated alumina has been the preferred adsorbent. However, adsorption processes employing activated alumina are sensitive to pH, with the optimum pH for arsenate removal ranging from 5.5 to 6.0. Because the pH of most groundwaters lies above this range, water treatment plants must often adjust pH prior to the adsorption process. This effect is most likely due to a decrease in adsorption of As (V) or other protolyzable anions as the pH increases and the surface charge of the adsorbent becomes less positive. Even though the isoelectric point (pHiep) of pure gamma alumina (the crystal form of many activated alumina) is 8.0, its pHiep shifts to 6.0 when the ratio of mass of adsorbate to surface area of adsorbent is 164 ug/m2, resulting in an unfavorable pH range for arsenate removal in most water treatment scenarios. Although this effect will occur with most adsorption media, materials having higher pHiep than activated alumina have higher arsenate adsorption capacities at pH values typical of drinking water (pH = 6.0 ~ 8.0). Magnesium aluminate spinel (pHiep = 10.2) was prepared by a sol-gel process and investigated as an arsenate adsorbent to overcome this pH effect. This spinel provides greater arsenate adsorption (g of arsenate adsorbed per m2 of active surface) over the pH range of typical drinking water than does *-alumina (pHiep = 8.0) prepared by a similar process. Includes 34 references, table, figures.