Chromium mobility in freshwater wetlands

A wetland at a chromium-contaminated electroplating site was studied to determine its ability to immobilize subsurface chromium contamination. First, a site characterization was conducted to determine the lateral and vertical extent of chromium contamination in the sediment and pore water. The wetland was found to be highly contaminated, with sediment concentrations of up to 50,000 μg g−1. Chromium was partitioned largely on the sediment, with Kdʹs up to 317,000 mL g−1. No Cr(VI) was detected in the pore water. Sequential chemical extractions performed on the sediment found 60–90% of the chromium bound in the Fe/Mn-oxide and residual fractions of the soil, with very little exchangeable or organic-bound chromium present. These results indicate that the chromium is very tightly bound to the sediment. XPS determined a very low Cr/Si ratio on the solid surface. Batch leaching experiments using the contaminated sediment were conducted at pH 3, 4, and 5. Leaching of chromium from the sediment increased with lower pH, ranging from 0.02% to 0.34% of the total, and appeared to be solubility controlled. Results indicate that the wetland has been highly effective in immobilizing Cr, by reducing the Cr(VI) and precipitating it as a relatively insoluble Cr(III)-hydroxide.