Campbell, and 4 co-authors, reported in 2006 that bacterial reduction of arsenic(V) and iron(III) oxides influences the redox cycling and partitioning of arsenic (As) between solid and aqueous phases in sediment-groundwater systems. Two types of anaerobic bacterial incubations were designed to probe the relative order of As-(V) and Fe(III) oxide reduction and to measure the effect of adsorbed As species on the rate of iron reduction, using hydrous ferric oxide (HFO) as the iron substrate. In one set of experiments, HFO was pre-equilibrated with As(V) and inoculated with fresh sediment from Haiwee Reservoir (Olancha, CA), an As-impacted field site.T
The second set of incubations consisted of HFO (without As) and As(III)- and As(V)- equilibrated HFO incubated with Shewanella sp. ANA-3 wild-type (WT) and ANA-3ΔarrA, a mutant unable to produce the respiratory As(V) reductase. Of the two pathways for microbial As(V) reduction (respiration and detoxification), the respiratory pathway was dominant under these experimental conditions. In addition, As(III) adsorbed onto the surface of HFO enhanced the rate of microbial Fe-(III) reduction. In the sediment and ANA-3 incubations, As-(V) was reduced simultaneously or prior to Fe(III), consistent with thermodynamic calculations based on the chemical conditions of the ANA-3 WT incubations.
The authors concluded that the effect of sorbed As on Fe(III) reduction was significant. The presence of adsorbed As(III) increased the rate of Fe reduction in the ANA-3 WT microcosm experiments. In the incubations with HFO/As(V), sorbed As(V) was reduced to As(III) within approximately 40 hours. Once most of the As(V) was reduced (>40 h) and As(III) was the dominant species adsorbed onto the HFO, the rate of Fe reduction was comparable to the HFO/As(III) case (0.024 mM Fe/h for HFO/As(V) microcosm and 0.028 mM Fe/h for the HFO/As(III) microcosm) from 40 to 130 h. The HFO only and HFO/As(V)/mutant incubations show substantially less Fe reduction (0.009 and 0.003 mM Fe/h, respectively) than observed in the HFO/As(V) and HFO/As(III) microcosms, which indicates that sorbed As(III) enhances the bacterial reduction of HFO.
The rate of microbial reduction of Fe(III) oxyhydroxides depends on the surface area and crystallinity of the oxide mineral. Changes in these properties could be related to the observed differences in rates of Fe reduction. The time scale of this experiment (˘4 days) is considerably less than the time scale of ˘15 days over which substantial recrystallization of HFO, with or without adsorbed As, was observed in abiotic batch experiments. In flow-through experiments, however, a transition from HFO to lepidocrocite has been observed within 2 days of incubation with bacteria. The absence of dissolved As in our study suggests that surface area did not decrease substantially over the course of the experiment.
The enhancement of microbial Fe reduction observed when As(III) was adsorbed on the HFO substrate was unexpected and may have significant environmental implications. Bacterial reduction of As(V) to As(III) may increase the rate of reductive dissolution of poorly crystalline Fe hydroxides, possibly leading to an increase in As mobility into groundwater in sediment systems.
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