Microbes Enhanced Mobility of Arsenic, without Iron Reduction in Deep Bangladesh Aquifer

The role of microbial arsenate and iron reduction in As mobilization can be difficult to distinguish. A set of incubation experiments was conducted anaerobically over 3 months using deep Pleistocene (orange) aquifer sands from Bangladesh containing low HCl-leachable Fe(II)/Fe of 0.05 and phosphate-extractable As of 0.14 mg/kg, all as arsenate. The experiments assess the role of microbial reduction in a setting of great practical importance because the deep Pleistocene aquifer is a viable source of drinking water for Bangladesh where shallow Holocene aquifers often exceed 50 ìg/L of As. Sands, resuspended in artificial groundwater(Agw) were inoculated with wild-type Shewanella sp. ANA-3, capable of Fe(III), Mn(IV), As(V) reduction. The equivalent of 0.035 mg/kg sedimentary As was mobilized on day 23 by this strain. The controls with or without lactate and without strain of Shewanella released less than the equivalent of 0.003 mg/kg As from the solid phase. We observed that As release was limited by microbes with or without lactate whereas Fe and Mn release was limited by lactate. The reduction of Fe do not enhance the As release. Furthermore, Shewanella sp. ANA-3 strain significantly converted the solid phase As to a mobilizable form by 92 days. Microbial processes responsible for this conversion may therefore be the critical process to evaluate to assess the vulnerability of Pleistocene aquifers of Bangladesh.