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Evidence of Bacterial Activity in the Release of Arsenic - a Case Study from the Bengal Delta of Bangladesh

A. A. Khan
Department of Geology, University of Dhaka, Dhaka-1000, Bangladesh.

M. A. Hoque
Department of Geology, University of Dhaka, Dhaka-1000, Bangladesh.

S. Hoque
Department of Geology, University of Dhaka, Dhaka-1000, Bangladesh.

S. M. I. Huq
Department of Soil, Water & Environment, University of Dhaka, Dhaka-lOOO, Bangladesh.

K. Q. Kibria
Department of Soil, Water & Environment, University of Dhaka, Dhaka-lOOO, Bangladesh.

Abstract/ Executive Summary:

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 o.f thiosulphate oxidizers. In a preliminary in vitro study the presence of thiobocillus activity that was instrumental in lowering the pH by 0.2 unit was noted. However, a detail 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 thiobacillus treated samples show some correlation, All the samples, except one, show much greater concentration of arsenic in thiobacillus treated samples, thus indicating the release of arsenic from sediment by the acid formation.

Keywords: Arsenic, Groundwater, Contamination, Sediments,

Location: Chapai Nawabganj district, Bangladesh

Start Date:

End Date:

Program Area: Risk Management in Agriculture

Commodity: Arsenic

Objectives:

To determine the evidence of bacterial activity in the release of Arsenic - a case study from the Bengal delta of Bangladesh

Methodology:

An experiment has been conducted with fresh and sealed sediment samples of arsenic contaminated aquifers collected from drill-holes at Nawabganj (24.7"N 88.2"E) at a depth of 100-102ft to observe sulfur oxidation potential Ten gram of sediment sample was taken in a conical flask and elemental sulfur (S^O) was applied at a rate of 0 and 0.2gS/S0gm soil. Treatment was replicated thrice. Water level was maintained at 40'>10 of the sediment sample to ensure the activity of the organism, if any, responsible for the oxidation of So. The sample was incubated at room temperature 30° ± 3°C for 21 days. The sample was extracted with 500μgPm1^-1 from Ca (H₂P0₄).2H₂0 solution for sulphate (sediment extractant ratio of 1 : 5). Sulphate was determined turbidimetrically. Hydrogen ion concentration, pH, (sediment to water ratio of 1: 2.5), was determined by Jenway glass electrode pH meter. All the glassware used in this experiment was washed with freshly prepared distilled water. The same water was also used for preparing solutions and maintaining water level of the sediment. The conical flask was covered with parafilm to allow air passage and to prevent water loss through evaporation. Results obtained by this experiment indicated that pH of the sediment was slightly decreased due to addition of elemental sulfur. It was noted that pH was decreased from 7.2 to 7. Sulphate concentration of 79.83μgSg^-1 sediment was found in S-treated sediment compared to 17.3μgSg^-1in control treatment. The increase in S-concentration due to S-addition indicated the oxidation of elemental S to sulphate. However, the exact reaction can not be obtained, as the specific organism was not identified. The above findings led to conduct further detailed investigation on the possible role of microorganism in arsenic release from sediments. An incubation experiment was conducted with arsenic contaminated aquifer sediments (fresh and sealed) collected from drill-hole at Laxmipur (22.6"N 9O.5"E) by using a sterilized ThiobacilIus media. The nutrient broth was incubated at 35°C for 21 days and after filtration the samples were analyzed for arsenic content. A control, using only the nutrient broth was included in the experiment. A total of nine fresh and sealed sediment samples collected from different depths of arsenic contaminated aquifer with three replications were tested. For the determination of total arsenic content, sediment samples were wet digested with aqua regia. Water-soluble arsenic was also determined by using sediment: water ration of 1:3. Arsenic concentration in the samples (both sediments and water) was determined.

Source of Information: Jour.Geol.Soc.India, Vol.61, Feb. 2003

Article Link (Online Journal):

Funding Source:

Total Budget (Tk.) :

Achievement/Findings:

Arsenic in the aquifer sediments in the Bengal delta mostly occurs by adsorption on grain coatings of FeOOH and MnOOH. The release of arsenic and its mobility in the groundwater from these sediments is due to the acid formation principally role played by chemolithotrophic bacteria. The presence of higher percentage of organic matter, Fe and Al oxides enhances the oxidation rate that in turn reduces the aquifers environment for arsenic release and solubility. However, the study shows no relationship of the release of arsenic with the total arsenic content of the sediments. The preliminary experiment indicated the presence of Thiobacillus activity in the sediments. The study warrants for further detailed study on the characterization of bacteria in the release of arsenic.

Knowledge Management: Journal