Abstract:
Remediation efforts and contaminant transport predictions generally neglect the complicated dissolution and transport behavior associated with multi-component nonaqueous phase liquid (NAPL) sources. Therefore, it is important to understand the diffusion and dissolution processes occurring in these multicomponent systems as a function of mole fraction, molecular similarity/dissimilarity, and nonideal hydraulic factors. A series of laboratory scale NAPL-aqueous phase dissolution experiments were conducted to assess dissolution and intra- NAPL diffusion as a function of multicomponent NAPL composition (mole fraction) for both trichloroethene (TCE) and toluene (TOL). Predetermined volumes of target NAPL compounds were mixed with an insoluble n-hexadecane (HEX) NAPL to create mixtures that vary by NAPL composition. The ideality of resulting target compound dissolution was evaluated by quantifying NAPL-phase activity coefficient through Raoult's Law analysis. The results show that dissolution from the NAPL mixtures behave ideally for mole fractions above 0.2. As the target compound fraction of the NAPL mixture gets smaller, the dissolution behavior becomes increasingly nonideal (larger NAPL-phase activity coefficients). The TOL:HEX mole fraction mixtures show greater nonideality at equilibrium and initial elution concentrations for batch and column experiments when compared to TCE:HEX systems. Mass flux reduction analysis shows that the 0.5:0.5:, 0.2:0.8, and 0.1:0.9 mole fractions of both TCE and TOL behave similarly while the 0.05:0.95 mole fractions of TCE and TOL behave the most nonideally and exhibit mass flux reduction before any other mole fractions. Overall, the dissolution rates were constant and not controlled by NAPL composition-dependent factors. The results of this work may be used to improve transport predictions, remediation design, and risk assessments especially for sites contaminated by complex NAPL mixtures.
