Arsenic(III) oxidation in a municipal groundwater distribution system and a model laboratory system

The primary route of human exposure to inorganic arsenic, a carcinogen, is through consumption of drinking water. Arsenic mobility in natural systems and treatability in engineered systems are governed by its oxidation state, thus redox reactions are important. As(III) oxidation was examined in both field and laboratory systems. In field studies, arsenic concentration and redox speciation were determined for the Hanford (CA) municipal groundwater distribution system. Laboratory studies were performed to determine As(III) oxidation kinetics and mechanism with manganite as a model oxidant.;Possible treatment technologies were evaluated for removal of arsenic from the groundwater. In field studies, As(III) was found to be the predominant form of arsenic. Arsenic concentrations are higher in shallower wells and lower in deeper wells; chloride also shows a distinct contrast between shallow and deep wells. As(III) oxidation can be inferred from observation of As(V) in a storage tank and at a sampling site within the distribution system. As(V) was also found in three shallower wells. Since reaction of As(III) with oxygen is known to be slow, observation of As(V) suggests either biotic or abiotic catalysis or reaction with an alternate oxidant.;Freshly precipitated manganese oxides are a plausible oxidant for As(III) in many natural and engineered systems. A novel method for determining arsenic redox speciation in heterogeneous system was applied to examine the kinetics of As(III) oxidation by manganite. The following parameters were varied: initial As(III) concentration, pH and competitive adsorbates. Measurement over time of total As(III) rather than dissolved As(III) concentrations is needed to determine oxidation kinetics. The rate of As(III) oxidation decreases with time; this can be attributed to product inhibition due to competitive sorption of As(V). Kinetic models were applied to interpret the effect of initial As(III) concentration on reaction rates. A two-site model was invoked to explain the product inhibition effect because a single reaction site was unable to capture the effect at low ratios of [As]T/[Mn]T. The rate of As(III) oxidation decreases with an increase in pH and in the presence of phosphate. Non-stoichiometric release of Mn(II) was observed during As(III) oxidation.