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 aqui fer sands. Microscopic examination of three sands was used to quantify the coating thicknesses. With measures of the coatings' organic conte nts and porosities, we predicted the relative sorption rates for (a) m ultiple HOCs on one sand end (b) one HOC with three sands. The predict ed relative rates and equilibrium coefficients were assessed using obs ervations of HOC transport through short sand columns operated at vary ing flow rates. We found that the column K-d values were always much l ess than predicted from K(oc)f(oc) or observed in batch tests. This su ggests that diagenetically produced sorbents may include organic matte r that is completely inaccessible for HOC sorption; procedures that di saggregate these sands could expose organic matter that does not sorb HOCs in the environment. Second, by modifying our retarded diffusion e xpectations with the inferred fraction of available organic carbon, f( avail), all observed sorption rates were consistent (within a factor o f 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(s w)(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 fac tors affecting sorption rates (e.g., tortuosity) were set equal to 0.0 01. Since oxide coatings are ubiquitous in aquifer sands, the model de scribed here should have wide applicability.