| The fate, transport, and bioavailability of contaminants in the subsurface have been closely linked to the binding of the contaminant by natural organic matter (NOM) present in solution and in sediments. Some potential causes of the wide variations in the binding of pollutants by NOM are the origin of the humic substance (the major component of NOM), the solution chemistry in the groundwater, and the presence of other inorganic constituents in the soil.; Binding constants for phenanthrene interaction with aquatic and soil humic substances, and with a natural water sample from the Dismal Swamp area of Virginia were determined under conditions of different ionic strength and pH using the fluorescence quenching technique. Within the relative error of the measurements, the variation in K{dollar}rmsb{lcub}oc{rcub}{dollar} for the humic substances of aquatic origin was not significant. A significant trend of increasing K{dollar}rmsb{lcub}oc{rcub}{dollar} with increasing pH was observed at low ionic strength for the interaction between the four PAHs and the soil humic acid. At higher ionic strength values the effect of increasing pH was not as significant. At fixed pH, phenanthrene binding by the humic acid was highest in the presence of sodium nitrate, followed by calcium nitrate, and aluminum nitrate.; For the soil humic acid, these results are consistent with a conformational model of humic substances which allows for a more open conformation at high pH, caused by increased charge-charge repulsive forces between acidic groups, and at low ionic strength, providing less charge screening of these repulsive forces. The more open conformation may allow increased access of phenanthrene to areas of the humic acid having a higher PAH affinity.; Experimental binding coefficients (K{dollar}rmsb{lcub}oc{rcub}){dollar} for phenanthrene binding by on humic-coated clay surfaces were lower than the results calculated for the bulk humic acid in the aqueous phase. Fractionation of the humic acid, based on the results of fluorescence quenching experiments with the residual humic material left after adsorption experiments, would indicate higher K{dollar}rmsb{lcub}oc{rcub}{dollar} values for the material adsorbed to the surface. The surface configurations adopted by the humic substance may be affecting the level of PAH binding by the humic acid. |
