Redox reactions of arsenic in As-spiked lake water and their effects on Asadsorption

Removal of inorganic arsenic from AS-Spiked Lake Washington water by sorpti on onto activated alumina, iron-oxide-coated sand (IOCS), and an ion exchan ge resin was studied in laboratory systems. The sorption results could be r ationalized only by invoking unanticipated changes in arsenic speciation be tween As(Ill) and ASM in the feed reservoir. The changes in AS speciation w ere subsequently confirmed in independent experiments, and the factors cont rolling the changes were investigated. oxidation and reduction reactions so metimes occurred in the same system sequentially over a period of days to w eeks, and in other systems both oxidation and reduction seemed to be procee ding simultaneously. Because As(Ill) has a much lower affinity for the adsorbents than does AS(V ), arsenic breakthrough was much more rapid when the influent arsenic was i n the reduced form. in some cases, the combination of speciation changes in the influent and differential sorption of AS(III) and ASM generated breakt hrough curves for total arsenic in which the breakthrough increased and the n declined for a period before increasing again. Reduction of ASM to As(III) was facilitated by filtration of the sample and by incubating the sample in the light, and it was impeded by the addition of PO4 to solution. Neither filtration nor exposure to light had a discerni ble effect on the rate of As(III) oxidation. As(III) oxidation was facilita ted by autoclaving the sample either before or after the AS(III) was added. Although arsenic speciation changes are reproducible and explainable when i nvestigating the behaviour of a given water sample under well controlled la boratory conditions, they are not easily extrapolated to other laboratory s ystems or field conditions, because they result from chemical and microbial ly mediated reactions that are strongly interrelated and evolve with time.