POTENTIAL ROLE OF FE(III)-REDUCING BACTERIAL IN MEDIATING MOBILITY OF ARSENIC FROM SEDIMENT: IMPLICATION FROM BATCH INCUBATIONS

Severe arsenic (As) contamination in groundwater has been reported in the littoral area of the southwestern Taiwan for decades. Although microbial-mediated reductive dissolution of iron (Fe) oxyhydroxides was postulated to account for high levels of As in this area, detailed description concerning with functions of reducing bacterial, particularly iron-reducing bacterial (FeRB), was still instinct. In this study, the amounts of As release were measured directly via incubations inoculated with various sources of FeRB (enriched from cores and groundwater), using specific depths of in-situ sediment amended with artificial groundwater under anaerobic condition lasting 3 months. For various enrichments, reducing forms of As and Fe (i.e. As(III) and Fe(II)) were predominantly in comparison with the control set in the end of experiment, indicating the reducing ability of FeRBs. Although the simultaneous increase of As(III) and Fe(II) was observed at the beginning, two enrichments exhibited separate reducing ability on As and Fe, leading to a significant difference on the distribution of these reducing species. Our result showed that the single Fe reducing activity only affects the moderate level of the As concentration in the presence of the reducing condition. The high level of the As concentration only accumulates with the aid or combination of the As-reducing activity. After the initial equilibrium (ca. 15 days), few amounts of As(III) decrease with time (from 30 μg/L to 10 μg/L), suggesting that As(III) re-adsorbed onto the surface Fe minerals or transformed into new minerals, such as arsenopyrite (FeAsS), under the defined experimental condition. Similar tendency of concentrations of As(III) and Fe(II) was observed between different sediment sets, but the levels of As(III) were different, revealing the bioavailability of As affects its release into aqueous phase. Hence, the high level of As in groundwater was attributed to not only the activity of reducing bacterial, specifically for As-reducing, but also the bioavailable amounts of As.