Mobilization of arsenic on nano-TiO₂ in soil columns with sulfate reducing bacteria

Arsenic (As) remediation in contaminated water using nanoparticles is promising. However, the fate and transport of As associated with nano-adsorbents in natural environment is poorly understood. To investigate the fate of adsorbed As on nano-TiO₂ in changed redox condition from oxic to anoxic, we added the As(V)-TiO₂ suspension in groundwater to an autoclaved soil column which inoculated a sulfate-reducing bacterium, Desulfovibrio vulgaris DP4. The dissolved As(V) in effluent increased to 798 μg/L for the biotic column and to 1510 μg/L for the abiotic control, and dissolved As(III) was observed only in biotic column. The total As (dissolved plus particulate) in the biotic column effluent (high to 2.5 mg/L) was substantially higher than the abiotic control (1.5 mg/L). Therefore SRB restrained the release of dissolved As, and facilitated the transport of particulate As. Micro-XRF analysis suggested that the nano-TiO₂ with As was mainly retained in the influent front and that its transport was negligible. Our pe-pH calculation and XANES analysis demonstrated that generated secondary iron minerals containing magnetite and mackinawite mainly were responsible for dissolved As retention, and then transported with As as particulate As. The results shed light on the mobilization of adsorbed As on a nano-adsorbent in an anoxic environment.