Tuesday, 11 July 2006

Mineralogy and Arsenic Bonding in Arsenic Contaminated Rice-paddy Soils of Bangladesh.

G. Norman White1, Richard H. Loeppert2, L. R. Drees3, B. Biswas1, and G.M. Panaullah4. (1) Texas A&M University, Dept. of Soil & Crop Sciences, Texas A&M Univ., College Station, TX 77843-2474, (2) Texas A & M University, Dept. of Soil & Crop Sciences, College Station, TX 77843-2474, (3) Department of Soil and Crop Sciences, Texas A&M University, College Station, TX 77843, (4) CIMMYT Office in Bangladesh, House no.18, Road no. 4, Sector no. 4, Uttara, Dhaka, Bangladesh

The natural arsenic contamination of the Ganges-Meghna-Brahmanputra groundwater aquifer in Bangladesh and the extensive use of this water for irrigation of dry season rice are resulting in increased concentrations of arsenic in paddy soils. The arsenic in these soils was found to be associated with sand and silt particle-size fractions even though most of the arsenic was extractable by ammonium oxalate in the dark, a characteristic more often associated with small, poorly crystalline materials in clay fractions. The apparent incongruent nature of these observations led a detailed examination of the mineralogy and arsenic localization of several paddy soils. The alluvial paddy soils of Bangladesh contain high concentrations of Fe-rich di- and trioctahedral micas and trioctahedral chlorites in all size fractions. The trioctahedral micas and chlorites are highly susceptible to weathering in the alternating oxidizing and reducing environment present in rice paddies. It was shown that most of the arsenic is associated with a poorly crystalline Fe-oxide component formed from the Fe released during the weathering of trioctahedral micas and chlorites. Examination of particle-size fractions using X-ray diffraction and scanning electron microscopy/energy dispersive X-ray analysis (SEM/EDS) showed that most of the trioctahedral micas and chlorites were partially weathered to vermiculite and smectite. Examination of the sand fraction using synchrotron X-ray microprobe analysis indicated that most Fe-oxide grains and mica aggregates contained adsorbed arsenate. Appreciable arsenic was not observed at the edges of most individual mica or vermiculite flakes. Especially high arsenate concentrations were associated with small micaceous rock fragments and Fe-oxide coatings on root surfaces. It is hypothesized that Fe oxides at the internal surfaces of weathering mica schist aggregates provide sites favorable for oxidation of dissolved Fe2+ and continued accumulation of Fe oxide, even under reduced soil conditions. These reactions have important implications to retention, solubility, movement, and bioavailability of arsenic in the seasonally reduced paddy soils of Bangladesh.

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