ARSENIC SPECIES IN NEAR-SURFACE IRON OXYHYDROXIDE BANDS IN BANGLADESH: MICROCHARACTERIZATION BY ELECTRON PROBE, XAFS SPECTROSCOPY, AND X-RAY DIFFRACTION

Millions of Bangladeshis suffer adverse health effects due to arsenic (As) contamination of potable ground water. Near the ground surface, sub-lateral enrichments of secondary iron(III) oxyhydroxide (“iron bands”) containing 200-800 hundred ppm As have been observed. The bands are common in eastern and central Bangladesh, and in some locations they have been traced for at least a kilometer.

The iron bands occur in silty sediments, via the upward capillary flow of Fe(II) and As(III)-rich ground water in response to hot, dry conditions at the ground surface. Formation of a thin (mm-thick) layer of Mn oxide just above the Fe band confirms the direction of water movement. Measured bulk Fe contents of the layers approach 20 wt.% (5x background) with a maximum As enrichment of 800 ppm (80x background). Sediment above and below the bands contains approximately 4 ppm As. Bulk x-ray absorption fine structure (XAFS) spectroscopy indicates that 90-100% of the total As in the iron bands is present as As(V), and that > 80% of the total Fe is present as Fe(III ). SEM analysis shows several generations and morphologies of iron oxyhydroxide, ranging from star-shaped and acicular forms of goethite to boytroidal (presumably x-ray amorphous) Fe(III) oxyhydroxide.

Some of the iron bands contain millimeter-thick veinlets even more enriched in As (1-5 wt%) and have strong correlations with Fe, Ca, and P. The amount of P and Fe (20 wt% as P2O5 and 65-70 wt% as Fe2O3, respectively) and the veinlet morphology indicate the formation of a secondary Fe-phosphate phase. However, microbeam XAFS does not reveal significant differences between the As and Fe species in the veinlets and in the bulk Fe oxyhydroxide band material, despite obvious morphological differences.

Although the available data do not yet allow selection of the most probable mechanism of veinlet formation, they clearly formed after the iron bands. Evidence suggests that the iron bands formed prior to modern intensive cultivation practices through a cyclic process of creation during the dry season and incomplete destruction in the rainy season. The recent advent of pond aquaculture and widespread irrigation during the dry season could alter this balance in favor of net destruction, thereby releasing a large load of previously stored arsenic into the shallow ground water system.