A Laboratory Evaluation of the Sorption of Oil Sands Naphthenic Acids on Organic Rich Soils

Abstract

The adsorption characteristics of oil sands tailings pond water (OSTPW)-derived naphthenic acids on soils was determined using a batch partitioning method. The adsorption isotherms were found to be linear in all cases. All tests were conducted at 4°C, and at a pH of 8.0 ± 0.4, which reflects the pH of a tailings settling facility near Fort McMurray, AB. The adsorption characteristics of the naphthenic acids in a synthetic groundwater (SGW) solution was compared to that of the mixture in Milli-Q water. In the presence of SGW, the adsorption coefficient (K d) of the mixture of naphthenic acids on soil 1 with a higher organic carbon fraction (f oc) was an order of magnitude higher than that observed with the same soil and the Milli-Q water mixture, increasing from 1.9 ± 0.2 (mL/g) to 17.8 ± 1.5 (mL/g). The adsorption coefficient of the mixture of naphthenic acids on soil 2, with a lower f oc, was also observably higher in the SGW mixture, increasing from 1.3 ± 0.15 (mL/g) to 3.7 ± 0.2 (mL/g). The relative fractional abundance of the individual naphthenic acids was plotted in order to determine the presence of preferential sorption between individual species within the mixture. It was found that for all Z families (where Z is a measure of the number of rings), naphthenic acids within the carbon number range of 13 to 17 showed preferential sorption. The mixture in SGW showed more pronounced sorption relative to naphthenic acid mixture in Milli-Q water. The results indicate that mixtures of naphthenic acids sorb strongly to soils and that adsorption would be an important attenuating mechanism in groundwater transport. Furthermore, preferential sorption of the individual naphthenic acids is important from a toxicity stand point since different naphthenic acid species have varying degrees of toxicity.