RELEASE AND CO-REDUCTION OF AS(V) AS A FUNCTION OF MICROBIAL REDUCTIVE DISSOLUTION OF FERRIHYDRITE

The effect of microbial reduction of Fe(III) and As(V) on the release of As from synthetic, amorphous ferrihydrite (HFO) was investigated by studying the kinetics of reductive dissolution of HFO in two batch-reactor experiments. In both experiments, a modified Fe-reducing growth medium containing 20 mM of lactate was dispensed into septum sealed serum bottles. Several bottles were inoculated with an enrichment culture containing an anaerobic Fe-reducing bacterium obtained from sediments at Milltown Reservoir near Missoula, MT. Heat kills and blank controls were run with each experiment. The first experiment established the rate of microbial Fe(III) reduction in batches containing 10 mM HFO. Fe(II) production began 31 hrs after inoculation and was described by the rate function: R=K(Xe-x) where R is the overall rate of production (mM/hr), K is a rate constant (1/hr), x is the concentration (mM) of the reduced ion at time t (hr), and Xe is the concentration (mM) of x at equilibrium. After 1200 hrs, 6.5% of the initial Fe(III) available as HFO was reduced with: K = 0.0026/hr and Xe = 0.67 mM Fe(II). The second experiment investigated the effect of microbial HFO reduction on As(V) mobility and redox state in batches containing a combination of 0.85 mM Fe(III), as HFO, and 0.009 mM As(V). 77% of the initial As(V) was adsorbed onto HFO surface sites. Fe(II) production began after 54 hrs. After 1300 hrs, 100% of the HFO was reduced with: K= 0.036/hr and Xe = 0.85 mM Fe(II). As(III) production began after 54 hrs and was described by the rate function: R = [(K*Ao)/Xe](Xe-x) where Ao is the initial concentration of As(V) in solution. Production of As(III) ceased when all HFO had been reduced. After 1300 hrs, 61% of the available As(V) was reduced with: K = 0.0012/hr and Xe = 0.0056 mM As(III). No Fe(II) or As(III) production was observed in the heat kills or blank controls. A third experiment was conducted with batches containing 1 mM As(V) and no HFO. After 250 hrs, no As(III) production was measured. Based on the results, we hypothesize that microbial reductive dissolution of HFO led to the co-reduction of As(V) to As(III). We are continuing experimentation to determine if the mechanism for reduction requires microbial adhesion to the HFO surface. Our experiments provide no evidence for As(V) reduction in the absence of HFO.