Arsenic speciation and removal in adsorption systems

In this research arsenic removal from potable water by adsorption was investigated in batch and continuous flow packed bed adsorption reactors. Different water sources were tested, along with the effects of pH, and the concentrations of natural organic matter (NOM) and sulfate. The regenerability of the sorbents after exposure to arsenic was also investigated.; Arsenate was effectively removed by all three adsorbents. Specifically, arsenic concentrations <5 mug/L were achieved in columns packed with activated alumina (AA), iron-oxide-coated sand (IOCS), and a strong base ion exchange resin with 15-min EBCT for more than 2000, 2000, and 700 hrs, respectively. Only the oxide adsorbents could effectively remove arsenite, with adsorption densities for arsenite much lower than for arsenate.; NOM and sulfate levels affected the performance of the resin but not that of activated alumina or IOCS. Regeneration did not significantly modify the adsorption behavior of the media; the arsenic recovery from the spent oxides, however, was usually less than 50% of the sorbed arsenic. NaCl solutions fully recovered adsorbed arsenic from the spent resin.; In many of the column tests, arsenic removal improved after an initial, almost complete breakthrough. The cause of the unusual breakthrough profiles was identified as unexpected redox reactions which altered arsenic speciation. Specifically, direct measurements of arsenic speciation demonstrated that either arsenite oxidation or arsenate reduction could occur in the feed reservoir and, in the packed columns. The amount of NOM adsorbed on the media had a minimal effect on arsenic speciation.; In batch speciation tests, arsenite was oxidized to arsenate even in autoclaved samples. When bacterial colonies were inoculated into autoclaved solutions, arsenite oxidation was less pronounced. More complex chemically and microbially mediated redox reactions occurred in water samples that were not autoclaved. Specifically, arsenate was initially reduced to arsenite, and arsenite was later oxidized to arsenate.; Exposure to light significantly altered the redox reactions in unfiltered samples containing blue-green algae. The presence of phosphate inhibited the biotic redox reactions.; This research provides an insight into factors affecting arsenic treatability by adsorption. It also demonstrates the ease with which arsenic speciation can change and affect its treatability.