The Effect of Organic Carbon on the Adsorption of Fluorene by Aquifer Materials

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Abstract
The movement of nonpolar organic contaminants through aquifers depends partially on the adsorption of these contaminants on the aquifer materials. Since adsorption can be influenced by the fraction of organic carbon (foc) and its distribution among the aquifer materials, we investigated the relationship between aquifer foc and the adsorption of organic contaminants by the aquifer materials. Three aquifer materials, fractionated into 21 subsamples comprising sand, silt, and clay-sized particles and characterized with respect to grain size and foc, were used in batch equilibrium experiments to determine adsorption isotherms of fluorene. Fluorene was used as a representative nonpolar organic contaminant. The results indicate that the foc decreased as median grain-size (ϕ) increased. The decrease was rapid as ϕ increased to about 85 μm, but was gradual with further increases in ϕ. However, the bulk of each aquifer material (about 90%) was larger than 53 μm and, therefore, most of the aquifer organic carbon was associated with material above this grain size. The adsorption isotherms of fluorene by subsamples of the three aquifer materials and by whole aquifer samples were linear for aqueous concentrations below 38% of the water solubility of fluorene. In addition, the measured values of Kd (the slope of the linear isotherm) from whole aquifer samples were within a factor of 1.5 of the calculated mean values weighted by the fraction of different grain-size ranges. A linear relationship was found between Kd and foc, with Kd increasing as foc increased. Further, values of Kd/foc, which represent the partition coefficient of the contaminant between 100% organic carbon and water (Koc), for subsamples and whole samples differed by a factor of 2.5. No consistent correlation between Koc and grain size was found among the samples from the three aquifers. Despite the spread in the values of Koc, reasonable values of Kd can be estimated from the product of foc and Koc.