A laboratory evaluation of the sorption of oil sands naphthenic acids on soils

Abstract

The adsorption characteristics of an oil sands tailings water (OSTW) mixture of naphthenic acids were determined using a batch partitioning method for two soils from the Alberta oil sands region. The soils were mineral peat mixtures produced during salvage operations, with Soil 1 having a higher organic carbon fraction (foc) than Soil 2. Naphthenic acids are a significant toxic byproduct of bitumen extraction, and are acutely toxic to aquatic organisms. The sorption of naphthenic acids as affected by a high ionic strength solution was examined using a synthetic groundwater (SGW) mixture.The adsorption isotherms were found to be linear in all cases. All tests were conducted at 4oC, and at a pH of 8.0 ± 0.4, reflective of the conditions in a tailings settling facility near Fort McMurray, AB. The adsorption characteristics of the naphthenic acids in the SGW solution were compared to that of the mixture in Milli-Q water for the two soils. In the presence of SGW, the adsorption coefficient (Kd) for the mixture of naphthenic acids on Soil 1 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 for the mixture of naphthenic acids on Soil 2 was also observably higher in the SGW mixture, increasing from 1.3 ± 0.15 mL/g to 3.7 ± 0.2 mL/g. In order to determine whether preferential sorption is exhibited by a particular species within the mixture, the relative fractional abundance of the individual naphthenic acids was plotted as a 3-dimensional histogram for carbon numbers 5 to 37. It was found that for all Z families (where Z is a measure of the number of carbon ring structures), naphthenic acids within the middle range of carbon numbers showed preferential sorption. A two sample t-test confirmed that the naphthenic acids in the carbon number groupings 15 to 24 and 25 to 37 sorbed significantly in the SGW mixture when compared to those in the carbon number grouping of 5 to 14. It was concluded that select constituents of oil sands naphthenic acids mixtures sorb strongly to soil under conditions of elevated salinity and therefore adsorption could be an important attenuating mechanism in groundwater transport. Furthermore, preferential sorption of the individual naphthenic acids is important with respect to toxicity since lower molecular weight naphthenic acids are believed to have a more pronounced toxic effect. Overall, the measured adsorption coefficients indicate that there can be significant sorption of OSTW derived naphthenic acids to soils.