Estimating sorption rates of hydrophobic organic compounds in iron oxide and aluminosilicate clay-coated aquifer sands

We hypothesized that retarded diffusion in coatings controls the rate of sorption of hydrophobic organic compounds (HOCs) on quartzitic aquifer sands. Microscopic examination of three sands was used to quantify the coating thicknesses. With measures of the coatings' organic contents and porosities, we predicted the relative sorption rates for (a) multiple HOCs on one sand end (b) one HOC with three sands. The predicted relative rates and equilibrium coefficients were assessed using observations of HOC transport through short sand columns operated at varying flow rates. We found that the column K-d values were always much less than predicted from K(oc)f(oc) or observed in batch tests. This suggests that diagenetically produced sorbents may include organic matter that is completely inaccessible for HOC sorption; procedures that disaggregate these sands could expose organic matter that does not sorb HOCs in the environment. Second, by modifying our retarded diffusion expectations with the inferred fraction of available organic carbon, f(avail), all observed sorption rates were consistent (within a factor of 4) with k(r) approximate to 0.001 D-aq/delta(2)(1 + r(sw)(coat)K(oc)f(oc)(coat)f(avail)), where k(r) is the desorption rate constant, D-aq is the HOC's aqueous diffusivity, delta is the coating thickness, r(sw)(coat) is the ratio of solids-to-water in the coatings, f(oc)(coat) is the organic carbon content of the coating solids, and all other factors affecting sorption rates (e.g., tortuosity) were set equal to 0.001. Since oxide coatings are ubiquitous in aquifer sands, the model described here should have wide applicability.