QUANTIFYING NI DISTRIBUTION IN A CONTAMINATED AQUIFER USING SEQUENTIAL EXTRACTION AND ADSORPTION EXPERIMENTS

Improved prediction of the transport and bioavailability of transition metals in contaminated soils and sediments demands a better quantitative understanding of metal partitioning between aqueous solutions and mineral phases. In this study, Ni partitioning in a contaminated groundwater aquifer near Grand Rapids, MI was examined using a combination of sequential extraction and adsorption experiments. An operationally-defined sequential extraction procedure developed by Tessier et al. (1979, Anal Chem) was used to assess Ni and Fe distribution among five solid fractions (exchangeables (EX), carbonates (CAR), Fe/Mn oxides (FMO), organics/sulfides (ORG) and residual). Nearly all of the extracted Fe is associated with the FMO fraction. In most samples, the majority of the Ni was also extracted from the FMO fraction, however, the percentage of Ni extracted in the exchangeable fraction was also significant, ranging from 12-60%. Previous investigators have suggested that Ni sorption in the aquifer is controlled primarily by adsorption on ferrihydrite, an assumption seemingly consistent with the sequential extraction results in this study. To test this notion, the pH and ionic strength dependence of Ni adsorption onto the fine (<0.005” sieve), high surface area (N2 BET = 20m2/g) portion of the aquifer sediments was measured and compared to predicted Ni adsorption on ferrihydrite using stability constants from Dzombak and Morel (1990, Hydrous Ferric Oxide). Poor agreement between model predictions and measured data suggests that ferrihydrite alone cannot account for Ni speciation in the sediments. Therefore, the pH and ionic strength dependence of Ni adsorption on pure kaolinite, synthetic silica and aquifer sediments after extraction of EX, EX+CAR, EX+CAR+FMO or EX+CAR+FMO+ORG was measured. Results suggest that silica, kaolinite, organic matter and Mn oxides, as well as Fe oxides, likely play a significant role in Ni speciation in the contaminated sediments.