Arsenic retention and mobility in freshwater aquatic environments is a great concern because of its toxic effects on plants, animals and human health. In the past, arsenic has been referred to as one of the "Big Four" metals of environmental concern (the others being lead, cadmium and mercury). Arsenic is retained in aquatic sediments by adsorption at mineral surfaces, by precipitation as metal arsenate's at high oxidation levels, and as arsenic sulfides under reduced conditions. Because arsenate, As(V), is less mobile than arsenite, As(III), the reduction of arsenate to arsenite is a primary mechanism for increasing arsenic mobility.
Predictions of arsenic mobility rely on assumptions of thermodynamic equilibria. This is a large problem because the distribution of arsenate and arsenite are often observed to be out of equilibrium. Slow reaction kinetics are a logical reason for the disequilibria, yet reaction rates and mechanisms are poorly understood.
This proposed laboratory study will investigate arsenic reduction in anoxic sediments with a high arsenic content. It will test the following hypotheses: 1) As(V) reduction occurs through microbial respiration; 2) adsorbed As(V) is released from ferric oxyhydroxides during reductive dissolution of the iron phase, yet may be reabsorbed to other substrates; 3) the sequential use of electron acceptors for microbial respiation follows thermodynamic predictions; and 4) the appearance of As(III) in solution is closely tied to sulfide, and the fate of As(III) (precipitated or complexed in solution) is determined by the total concentration of sulfate, and the relative rates of arsenate and sulfate reduction.
A multistage chemostat will be employed to investigate reduction rates of Fe(III), As(V), and S(VI) in sediment cores. This technique is often used in studies on the effects of nutrient limitation on microbial growth and competition between populations; however, in this proposal, the chemostat will be used to simulate the continuous flux of water, nutrients, and solutes through a sediment package. The advantage to this technique is that consortium of bacteria, indigenous to the sediment core, can spatially arrange itself among the linked vessels of the chemostat, according to its sequential use of various electron acceptors along a decreasing PE gradient. Thus this setup will closely mimic changes in the microbial population with depth, as found in an anaerobic sediment. Also, it provides an opportunity to sample and observe chemical processes occurring within each linked microcosm, at pe conditions determined by the microbial population.
Similar flow-through reactors have been used to study reaction kinetics in inorganic systems where the chemistry of the inlet solution is often manipulated to study the reactor response to changing and non-steady-state conditions. Similar techniques will be applied to the chemostat system to observe the effect of a change in solution chemistry (pH, nutrient, and total concentrations of Fe, As, and S) on microbially-mediated kinetics of Fe(III), As(V), and S(VI) reduction.
