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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,

S. Hoque
Department of Soil, Water & Environment, University of Dhaka, Dhaka-1000, Bangladesh.

S. M. I. Huq
Department of Soil, Water & Environment, University of Dhaka, Dhaka-1000, Bangladesh.

K. Q. Kibria
Department of Soil, Water and Environment, University of Dhaka, Dhaka-1000, Bangladesh.

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

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 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.

Keywords: Soil, Water, Arsenic, Contamination

Location: Nowabganj, Dhaka, Bangladesh

Start Date:

End Date:

Program Area: Crop-Soil-Water Management

Commodity: Contamination of soil

Objectives:

To identify the evidence of bacterial activity in the release of arsenic

Methodology:

An experiment has been conducted with fresh and scaled sediment samples of arsenic contaminated aquifers collected from drill-holes at Nawabganj (24.7"N 88.2"E) at a depth of 100-102 ft to observe sulfur oxidation potential Ten gram of sediment sample was taken in a conical flask and elemental sulfur (S") was applied at a rate of 0 and 0.2gS/50gm soil. Treatment was replicated thrice. Water level was maintained at 40% of the sediment sample to ensure the activity of the organism, if any, responsible for the oxidation of S0 The sample was incubated at room temperature 30° ± 3°C for 21 days. The sample was extracted with 500µg Pml^-1 from Ca (H₂P0₄) 2H₂o solution for sulphate (sediment extractant ratio of I : 5). Sulphate was determined turbi dimetrically. Hydrogen ion concentration, pH, (sediment to water ratio of 1: 2.5), was determined. 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. lt was noted that pH was decreased from 7.2 to 7. Sulphate concentration of 79.83 μgSg^-1 sediment was found in S-treated sediment compared to 17.3 μgSg^-1 in 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. An incubation experiment was conducted with arsenic contaminated aquifer sediments (fresh and scaled) collected from drill-hole at Laxmipur (22.6"N 90.5°E) by using a sterilized Thiobacillus media. The nutrient broth was incubated at 35°C for 21 days and after filtration the samples were analyzed for arsenic content.

Source of Information: JOUR. GEOL. SOC. INDlA,VOL.6l, 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 AI 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