EFFECTIVE STERILIZATION TECHNIQUES FOR LONG TERM INCUBATIONS OF BANGLADESH AQUIFER SANDS

The involvement of micro-organisms in the release of As from Bangladesh aquifer sands to groundwater is a topic of active research because it affects millions of people who rely on shallow tube wells as their source of drinking water. One of the difficulties in conducting incubation studies to study the mechanisms of As mobilization is setting up proper abiotic controls for comparison. For this reason, four different sediment sterilization methods - autoclaving, gamma irradiation, addition of antibiotics, and addition of sodium azide - were investigated to determine both their efficacy in sterilizing sediment and to what extent they preserve the original sediment geochemistry. While autoclaving has been successful as a sterilization method it is known to alter amorphous iron (oxy)hydroxides, which dominate the surface coating of these sediments. Studies have shown that gamma irradiation, antibiotics, and sodium azide have little or no effect on the surface chemistry of the sediment. However, these methods have not been shown to be effective in sediment over long durations.

The sediments used in this study were collected from a deep Pleistocene aquifer in Araihazar, Bangladesh, with groundwater containing <5 ug/L As. Approximately 3 g of sediment, 6 mL of artificial groundwater, and 10 mM of acetate were added to glass bottles. Autoclaved samples were heated for one hour once or on three consecutive days. Gamma-ray irradiated samples received a dose of 25kGy. Guillard's reagent (the antibiotic) or 10mM sodium azide were added to the other samples before sealing the bottles and purging with N2 or a 5% H2/95% N2 mixture. Iron reduction is used as a proxy for changes in overall microbial activity and monitored on the basis of both diffuse spectral reflectance (reduced Fe oxides turn grey) and Fe speciation in different leachates of the solid phase. In addition, the Fe, Mn, S, P, and As content of artificial ground water is being monitored by High Resolution Inductively Coupled Plasma- Mass Spectrometry (HR ICP-MS). The first results of this on-going experiment indicate that all the amendments were successful in sterilizing the sediments after 30 days, whereas the non-sterilized control turned grey after only 14 days.