Soil sorption of hydrophobic organic chemicals: Competitive effects and risk assessment

This dissertation focuses on competitive effects between co-occurring contaminants in soil, in particular, how chemical properties of competitors affect competition and on the effect of competition induced desorption on risk based soil clean-up standards.;Single solute sorption isotherm data were determined in three distinct sorbents for a set of organic compounds with varying physical-chemical properties. Additionally some of the most recent Linear Free Energy Relationship (LFER) models were tested for their ability to fit or predict sorption. Although the LFERs tested provided very good estimates of sorption they cannot currently be applied to different sorbents using the same fitting parameters. Additional effort is still required in the estimation of sorption to soil organic matter.;Binary systems consisting of 1,2-dichlorobenzene (12DCB) + competitor were investigated over a range of concentrations of competitor in three natural sorbents with distinct characteristics. Two models, the ideal adsorbed solution theory (IAST) and the Potential theory (Polanyi based multi-solute model), widely used in the prediction of multi-solute sorption equilibrium from single solute data were used to simulate competitive sorption. The effect of competitor structure on the degree of competition was also investigated to identify any relationships between competitive effect and competitor structure using molecular descriptors. On average the IAST model provided lower errors than the Potential model. Taking into consideration that both models rely solely on single solute data to predict multi-solute data both models can be considered satisfactory. Competitor volume seemed to have an effect on the degree of competition observed, structure using molecular descriptors. On average the IAST model provided lower errors than the Potential model. Taking into consideration that both models rely solely on single solute data to predict multi-solute data both models can be considered satisfactory. Competitor volume seemed to have an effect on the degree of competition observed, most likely due to the rigid nature of the competitive hard carbon domain which may prevent larger molecules from reaching the same pores in which the primary contaminant is located.;The effect of the presence of a competitor on the transport and release to groundwater of a low concentration of sorbed chemical (mimicking the residual amount of chemical left in soil after clean-up) was modeled combining water infiltration and chemical fate and transport components. The results confirm that the presence of a competitor can significantly increase the mobility and release of a primary contaminant in certain soil/solute combinations. All competitors simulated affected both the speed of movement and the maximum effluent concentration of the primary contaminant.