A Multi-Scale Investigation of Mass Transfer Limitations in Surfactant-Enhanced Aquifer RemediationEPA Grant Number: R825405
Title: A Multi-Scale Investigation of Mass Transfer Limitations in Surfactant-Enhanced Aquifer Remediation
Investigators: Mayer, Alex , Pope, Gary A.
Institution: Michigan Technological University , The University of Texas at Austin
Current Institution: Michigan Technological University , The University of Texas at Houston
EPA Project Officer: Chung, Serena
Project Period: October 1, 1996 through October 31, 1999 (Extended to October 31, 2000)
Project Amount: $299,792
RFA: Environmental Fate and Treatment of Toxics and Hazardous Wastes (1996) RFA Text | Recipients Lists
Research Category: Hazardous Waste/Remediation , Land and Waste Management , Safer Chemicals
Description:Significant progress has been made in the application of surfactants to enhance remediation of aquifers contaminated by nonaqueous phase liquids (NAPLs). The addition of surfactant lowers the interfacial tension between the NAPL and water and also increases the solubility of the organic contaminants in the water. Solubility enhancement is the primary mechanism for removing dense NAPLs (DNAPLs), such as chlorinated solvents. However, almost all of the work on surfactant enhanced remediation of NAPL contamination has assumed that local equilibrium exists between the NAPL and the aqueous/surfactant solution. Although a few studies have shown that equilibrium may not always occur, very little work has been conducted to determine the conditions where mass transfer limitations are important for these systems. Mass transfer limitations have the effect of slowing the rate of NAPL removal by the surfactant solution.
The goal of the proposed work is to investigate mass transfer between residual NAPLs and aqueous phases containing surfactants. The investigation will yield mass transfer relationships for surfactant-enhanced NAPL dissolution. The dependence of mass transfer rates on remediation design variables surfactant constituents, types and concentrations and aqueous phase flow rates will be determined. The proposed work also will attempt to identify the phenomena responsible for mass transfer limitations in surfactant-enhanced NAPL dissolution, such as the viscosity of aqueous-surfactant solutions, diffusivities in aqueous-surfactant solutions, mass exchange from NAPL to aqueous solutions, and mass exchange from aqueous solution to micelles.
The development of the mass transfer relationships will occur at three scales: the pore scale, macroscopic scale (1-10 cm), and meso scale (10-100 m). First, laboratory column tests will be conducted to determine macroscopic mass transfer rate coefficients. The column tests will yield mass transfer rates as a function of aqueous phase velocities and surfactant types, dosages, and constituents. Next, residual NAPL dissolution will be simulated at the pore scale using a modified pore scale simulator. The averaged mass transfer rates produced in the pore scale model will be calibrated to the laboratory results. The calibrated parameters will include effective diffusivities and NAPL/aqueous phase/micellar mass exchange rates. Lastly, the macroscopic mass transfer rate coefficients will be incorporated into a continuum-based model for simulating NAPL dissolution at the meso scale.
This project will result in a greater understanding of the chemical processes involved in enhanced remediation processes. This improvement will occur by developing mass transfer rates that have been derived from pore-scale phenomena and are described as a function of variables that are relevant to remediation design aqueous phase flow rates and surfactant properties. A greater understanding of mass transfer limitations will result in more intelligent design of surfactant enhanced remediation, e.g., surfactant dose, type, constituents, and application rates, which presumably will result in more cost-effective remediation efforts. In addition, the improved understanding expected from the proposed work could result in the development of site selection criteria for surfactant-enhanced aquifer remediation.