Sorption of nonpolar neutral organic compounds to model aquifer sands: implications on blocking effect.

The use of a calibrated two-component model with a single blocking parameter for both adsorption onto mineral surfaces and partitioning into soil organic matter of two nonpolar sorbates (i.e., 1,2,4-trichlorobenzene and benzene) was evaluated by using several humic acid (HA)-coated sands with different fractions of organic carbon (f(oc)) ranging from 0.006 to 0.154%. Sorption of nonpolar sorbates to both untreated and heated sands was nearly linear due to the reduction of heterogeneous adsorption potential by strongly adsorbed vicinal water molecules over hydrophilic mineral surfaces. Sorption of nonpolar sorbates to the HA-coated sands was also essentially linear, and resulted from a combination of adsorption onto mineral surfaces and partitioning into soil organic matter, with the dominance of either contribution depending on the properties of the sorbents (e.g., f(oc)) and the sorbates (e.g., K(ow)). Contrary to the previous studies suggesting that surface area is the only variable affecting the magnitude of the adsorption coefficient onto modified high-surface-area clay minerals, the difference in adsorptive affinity of nonpolar sorbates onto modified mineral surfaces (i.e., K(S,c)) relative to that for pure mineral surfaces (i.e., K(S)) also may be important for low-surface-area, coarse-grained model aquifer sands. However, the improvement in predicted sorption behavior by incorporating the blocking effect is insignificant for low-surface-area, coarse-grained model aquifer sands.