Evidence of bacterial activity in the release of arsenic - A case study from the Bengal delta of Bangladesh

The Quaternary sediments and groundwater of the Bengal delta are contaminated by arsenic, a naturally occurring toxic element. The results presented here indicate that arsenic is released from sediments by the acid formation. The activity is indicated by the decrease in pH value from 7.2 to 7, due to the role played by bacteria that acted on the particulate material present in the sediments. The decrease in pH value in the preliminary experiment indicated the presence of the activity of thiosulphate oxidizers. In a preliminary in vitro study the presence of Thiobacillus activity that was instrumental in lowering the pH by 0.2 unit was noted. However, a detailed microbiological study is yet to be done. The aquifer sediments those contain arsenic, in turn by such release, can contaminate groundwater. It is observed that the values of the incubation study did not show any relationship with the total arsenic content of the sediments. Water extractable fraction of arsenic also did not show any trend with the total arsenic content of the sediments, probably due to solubility product constant of the arsenic compound in the sediments. However, the arsenic concentration in water treated samples and in thiosulphate treated samples show some correlation. All the samples, except one, show much greater concentration of arsenic in thiosulphate treated samples, thus indicating the release of arsenic from sediment by the acid formation.