Simultaneous Removal of Arsenite As (III) and Arsenate As (V) From Drinking Water Using a Novel Photoactive Adsorbent

With the maximum contaminant level for arsenic in drinking water decreasing from 50 ppb to 10 ppb in 2006, a heavy financial burden will be placed on small water-treatment facilities serving less than 10,000 people. At present, adsorption on activated alumina is preferred for removing arsenic in these small facilities because this process has relatively low capital costs and low operating expenses. However, adsorption processes employing activated alumina require a pH between 5.5 and 6.0 for optimum arsenic removal. Because these pH values are outside the pH range at which most water treatment plants operate (pH between 6 and 8.5), most plants install a separate pH adjustment unit before the adsorption process. This pH effect is likely due to the adsorption of arsenate or other protolyzable anions on the activated alumina, which lowers its isoelectric pH (IEP) from 8.2 to approximately 5.5 to 6. Although this effect will occur with most adsorption media, materials having higher IEPs than activated alumina have higher arsenic adsorption capacities at pH values typical of drinking water. Another concern with arsenic removal is treating arsenite, which is uncharged at the pH of drinking water and so is difficult to remove by adsorption or ion exchange. Processes for removing arsenite typically oxidize arsenite to arsenate and then adsorb the arsenate. Use of a TiO2 photocatalyst has been shown to significantly accelerate the photooxidation of arsenite. Recent studies in the laboratory of Professor Anderson at the University of Wisconsin have shown that both arsenic species can be removed in a photocatalytic adsorption process using a novel thin-film material. In addition, because this material has a higher IEP than alumina, it is not necessary to adjust the pH of the removal process. These new materials not only adsorb both species equally well but have significantly higher capacities for removing arsenic than activated aluminas. In this research project, Microporous Oxides Science and Technology will select an appropriate substrate for these thin-film adsorbents and obtain data to demonstrate that this process is not only superior to present arsenic removal systems, but competes favorably with other systems in cost while treating other types of contaminants (e.g., the photocatalyst mineralizes most organics to CO2, water, and mineral salts, and may photoplate heavy metals on the photocatalyst). By placing these thin films on optical wave-guide delivery systems, and by using a unique pulsed UV light source furnished by Novatron, Inc., that will likely destroy pathogens such as Cryptosporidium, it should be possible to develop a commercially viable, cost-effective drinking water treatment system for small and mid-sized communities.