Dynamic equilibrium dissolution of complex nonaqueous phase liquid mixtures into the aqueous phase

Area of Science:

• Environmental Science
• Geochemistry
• Hydrogeology

Background:

• Human health risks from groundwater contamination by nonaqueous phase liquids (NAPLs) are time-dependent due to varying compound solubilities.
• More soluble compounds (e.g., benzene, toluene, ethylbenzene, xylene - BTEX) dissolve faster than less soluble ones (e.g., polycyclic aromatic hydrocarbons - PAH).

Purpose of the Study:

• To experimentally determine long-term dissolution rates of diesel fuel components into groundwater.
• To interpret dissolution data using a dynamic equilibrium model based on Raoult's law.
• To assess the model's accuracy in predicting temporal aqueous concentrations and inform remediation strategies.

Main Methods:

• Utilized a continuous flow-through system with the slow-stirring method to simulate long-term dissolution from diesel fuel.
• Employed a dynamic equilibrium dissolution model, grounded in Raoult's law, for data interpretation.
• Observed and analyzed tailing behavior during compound depletion, attributing it to nonequilibrium effects like mass transfer limitations.

Main Results:

• Experimental results showed good agreement with the dynamic equilibrium model's predictions for temporal aqueous concentrations.
• A tailing behavior was observed as NAPL compounds approached depletion, indicating nonequilibrium effects.
• The model predicted a 1.5% increase in the mean molar mass of diesel fuel over the experimental period.

Conclusions:

• The dynamic equilibrium model effectively predicts the temporal evolution of aqueous concentrations during NAPL dissolution.
• The model supports initial response actions and long-term remediation planning for contaminated sites.
• Potential inaccuracies of the simple model at advanced stages of dissolution, due to significant changes in molar mass, were noted.