Remediation of Arsenic-Containing Ferrihydrite in Soil Using Iron- and Sulfate-Reducing Bacteria: Implications for Microbially-Assisted Clean Technology

The iron-reducing bacterium (IRB) Shewanella oneidensis MR-1 and a sulfate-reducing bacterium (SRB) Desulfovibrio vlugaris miyazaki, were used in varying sequences for batch and column arsenic extraction experiments to assess their use in alleviating arsenic pollution from arsenic-containing ferrihydrite in soil. Both batch (four conditions) and column experiments show variations in biological sequence affect the arsenic release and species. The results of arsenic release from batch conditions are: IRB-SRB (2.28 mg/g) > SRB-IRB (1.72 mg/g) > SRB+IRB (0.55 mg/g) > Control group (pure water, 0.04 mg/g). Further cycle operations from batch experiments showed that IRB-SRB operation always obtained the best performance of releasing arsenic. The column experiments achieved greater arsenic release: SRB+IRB (10.87 mg/g) > IRB-SRB (7.54 mg/g) > SRB-IRB (6.48 mg/g) relative to arsenic in the pristine samples (36.6 mg/g). Flow methods could obtain more releasing performance than batch experiment. The hydraulic conductivity in the column experiments was best in the SRB-IRB operation (average water difference: 0.39 psi for SRB-IRB, 0.86 psi for IRB-SRB and 1.07 psi for SRB+IRB). After the biological operation, chemical treatment (0.5 M, (NH4)2HPO4) only desorbed 11.14-39.81% of the total arsenic quantity of each operation. Reductive dissolution is the main driving reason for arsenic release, though the formation of FeS was found to hinder arsenic release. Analysis of the solids showed that the SRB+IRB operation results in 58.07% arsenic released. These results suggest that microbially-assisted reduction using ironand sulfate-reducing bacteria may be an effective technology for remediating arsenic-bearing soil e.g. paddy soil using systems such as an in-situ injection and pump.