Abstract:
The presence of nonaqueous phase liquids (NAPLs) in soil and groundwater is difficult and expensive to remediate. Complications exist for remediation of multicomponent NAPL sources due to differences in dissolution behavior at the molecular level. The dissolution behavior of two contaminants of concern (COC), trichloroethene (TCE) and toluene (TOL), was compared as binary mixtures within hexane, decane, and hexadecane. The relative ideality of mass transfer processes for TCE and TOL from the binary NAPL mixtures was evaluated by comparing aqueous-phase COC concentrations calculated using Raoult’s Law to the observed equilibrium aqueous-phase COC concentrations for a series of batch dissolution experiments. As mole fraction ratios of the COCs (i.e., TCE and TOL) within the NAPL source decrease, dissolution nonideality generally increases for such multicomponent NAPL mixtures. A series of comprehensive equilibrium batch experiments was conducted to understand and quantify the systematic influence of bulk NAPL carbon-chain length on the dissolution behavior of TCE and TOL. The differences between the observed COC equilibrium and Raoult’s Law-predicted concentrations are likely due to specific intra-NAPL component interactions that occur and thereby affect mass transfer dynamics from the multicomponent NAPL mixture. However, no particular correlation between the observed COC aqueous-phase equilibrium concentrations (via dissolution) and the COC-NAPL mixture’s bulk NAPL carbon chain length was determined. A static equilibrium-solubility model was used to estimate activity coefficients for TCE and TOL within various carbon-length aliphatic bulk NAPL mixtures (i.e., hexane, decane, hexadecane). The xlUNIFAC Model was used to simulate the mixtures for comparison to the batch experimental systems, following the UNIFAC group contribution methods for estimating phase equilibrium. TOL (aromatic structure) showed greater nonideal dissolution behavior than TCE (aliphatic structure) in the presence of the different bulk-NAPL components used for this study. The results of this work suggest that the prediction of aqueous phase concentrations in groundwater of COCs from complex multicomponent NAPL sources is highly dependent upon both compositional and molecular structural variations. Such impacts should be taken into account when designing and evaluating a particular remediation strategy and/or predicting COC concentrations from a NAPL source zone region.
