Chloroethenes, such as trichloroethene (TCE) and tetrachloroethene (PCE), are commonly encountered groundwater contaminants at industrial facilities, dry cleaners, and military installations. Many chloroethene plumes emanate from aquifer formations contaminated by dense non-aqueous phase liquids (DNAPLs). While significant effort has been directed toward improving methods for recovering NAPL from the subsurface, it is now generally accepted that source remediation technologies will not achieve complete mass removal. Technologies capable of in situ mass destruction provide opportunities to treat persistent NAPL contamination.
The objectives of this project were to develop and evaluate nanoscale zero valent iron (nZVI) technologies for treatment of DNAPL source zones and to assess transport and reaction behavior for nZVI treatment systems, leading to the development of application guidelines.
The integrated research program combined multi-scale laboratory experiments with mathematical modeling. Batch experiments with different nZVI and combinations of oil-in-water emulsions were conducted to develop, characterize, and refine nanoscale iron delivery systems. Column and two dimensional aquifer cells were used to evaluate transport and reaction behavior of nanoscale iron in representative and heterogeneous porous media. A mathematical model was developed for simulating nZVI delivery and reactivity in NAPL source zones.
A number of processes that tend to limit the effectiveness of aqueous slurry nZVI injection for in situ DNAPL mass transformation, even under the most favorable conditions, were identified. These processes include pore clogging (and associated injection pressure increases), groundwater flow bypassing of the treated zone, DNAPL mobilization, unfavorable iron to DNAPL mass ratios, and reaction limitations due to dissolution mass transfer. Column transport experiments demonstrated the superior injection and mobility performance of biodegradable micro-emulsion nZVI formulations. However, reactivity studies suggest that emulsification will tend to slow aqueous phase reactions and promote contaminant solubilization. Controlled emulsion partitioning to the NAPL may offer promise for sustained in situ reaction, but further research is needed to address reaction limitations due to low water solubility in the DNAPL. The trapping number concept and a model based on modified clean bed filtration theory were successfully implemented to reproduce experimental observations of nZVI injection, transport, and reaction.
The experimental and simulation results together provide a fundamental understanding of effective options for targeted treatment of NAPLs using nZVI. Processes controlling the transport and delivery of nZVI nanoparticles in porous media containing DNAPL were quantified. Improvement in nZVI delivery was achieved by novel ZVI encapsulation formulations that directly target NAPLs for effective in situ destruction. Chemical reaction pathways and rates for nZVI particles in contact with chloroethene liquid phases were assessed. A multiphase, multicomponent numerical simulation module capable of predicting ZVI delivery, reactivity, and effects on NAPL mobilization was developed. This project also provided operational guidelines for treatment of NAPL source zones with nZVI.