Characterizing equilibrium dissolution of trichloroethene and toluene from binary NAPL mixtures as a function of molecular structure

Significant uncertainties exist, due to dissolution behavior variability, for the remediation of multicomponent nonaqueous-phase liquid (NAPL) contaminants impacting groundwater. Equilibrium batch experiments were conducted to quantify the systematic influence of bulk NAPL carbon-chain length on the dissolution behavior of trichloroethene and toluene within binary NAPL-sources of n-hexane, n-decane, and n-hexadecane. Different mole-fractions of the targeted contaminants within the bulk-NAPL sources were tested. Raoult's Law was used to assess the relative ideality of the mass-transfer processes for each dissolution experiment. Results show as the mole-fraction ratio of trichloroethene and toluene decrease, dissolution nonideality generally increases for the multicomponent-NAPL mixtures. The differences between the observed and Raoult's-Law-predicted concentrations are likely due to specific intra-NAPL component interactions that affect mass-transfer from the multicomponent-NAPL mixture. A semi-empirical thermodynamic model, xlUNIFAC, was used to estimate activity-coefficients for trichloroethene and toluene within various carbon-length aliphatic bulk NAPL mixtures. This group contribution model was used to simulate phase equilibrium for the mixtures compared to the batch systems. Toluene (aromatic-structure) showed greater nonideal dissolution behavior than trichloroethene (aliphatic-structure) in the presence of the different bulk-NAPL components. Results suggest that aqueous-phase concentration in groundwater, released from complex multicomponent-NAPL, is highly dependent upon both NAPL-mixture composition and molecular structure variability.