ANALYZING ENGINEERED REMOVAL OF CHROMIUM FROM DRAINAGE WATERS SOURCED FROM LATERITIC SOILS IN INDONESIA

Intense tropical weathering leads to the formation of lateritic soils. When developed over mafic and ultramafic rocks, laterite soils become highly enriched in the immobile elements of these rocks including Fe, Mn, and Cr. Interaction of Mn with Cr leads to the oxidation of Cr(III) to Cr(VI); unlike Cr(III), Cr(VI) is highly mobile and is readily leached from soils by runoff. In addition to greater mobility, Cr(VI) is also toxic, and is a known carcinogen. Weathering of mafic and ultramafic rocks indeed leads to natural environmental and human health risks over large tracts of the Indian subcontinent. Here we investigate the geochemical behavior of Cr in waters draining lateritic soils mined for Ni on Sulawesi Island, Indonesia. This drainage undergoes engineered treatment to remove dissolved Cr(VI). Reduction to Cr(III) is one way to remove Cr as Cr(III) is much less soluble than Cr(VI) and is more particle reactive. Conversion of Cr(VI) to Cr(III), takes place either naturally or artificially through reaction with strong reductants such as Fe(II). In June 2012, we sampled an engineered Cr removal system that uses the aerial addition of FeSO₄ to surface streamwaters through suspended faucets to reduce Cr(VI) and effectively remove Cr from downstream water. Prior to the faucets, the drainage system contains elevated concentrations of dissolved Cr, with total concentrations as high as 20μM in pooled surface waters; much of this dissolved Cr is present in the hexavalent redox state. The FeSO₄ faucets dramatically decrease dissolved Cr concentrations—after treatment Cr concentrations are reduced by two to three orders of magnitude. However, the reducing capacity of the treatment appears to weaken with distance from the faucet; this is suggested by the changes in concentrations of redox sensitive elements between faucets. To further assess Cr removal efficiency, and possible redox recycling we are conducting measurements of Cr isotopes. The large isotopic fractionation induced by reduction at the faucets enriches the downstream Cr(VI) in the ⁵³Cr isotope, and the extent of this enrichment can be used to calculate the fraction of reduction at a given location.