Dr. Schaefer is a chemical engineer with a wide array of experience in fate and transport of organic contaminants in soil and bedrock. In particular, his studies have focused on pore-scale phenomena, non-aqueous phases in porous media, coupled diffusion and reaction processes, and treatment of emerging contaminants. Dr. Schaefer has carried out bench and field-scale experiments and developed physically based numerical models to describe processes such as diffusion, sorption, interfacial mass transfer, mineral-based reactions, and microbial growth and transport. Much of Dr. Schaefer’s work has included work with NAPL systems, where he has worked on issues ranging from rate limited dissolution of chlorinated solvents, to co-solvent/surfactant flooding, to thermodynamic models that predict activity coefficients of hydrocarbon mixtures. Dr. Schaefer’s more recent research efforts have focused on electrochemical disinfection and treatment of perfluorinated compounds.
PLATFORM PRESENTER - Emerging Contaminants: Tick Tock
Electrochemical Treatment of Poly- and Perfluoroalkyl Substances
The presence of poly- and perfluoroalkyl substances (PFASs) is a rapidly growing environmental concern. Cost-effective and sustained in situ destruction of PFASs under relevant environmental conditions has yet to be demonstrated. As part of this SERDP-funded research project, both short and long-term bench scale studies were performed to assess the effectiveness of boron-doped diamond electrodes for treatment of PFASs under relevant field conditions, and using AFFF-impacted groundwater. Reaction mechanisms were assessed, and defluorination pathways were observed.
Initial experiments focused on assessing the impacts of chloride and hydroxyl radical scavengers on both the rates of PFOA and PFOS removal, and on the rate of defluorination. A range of current densities was examined. Treatment in natural groundwater and at environmentally relevant PFOA and PFOS concentrations also was evaluated. Results showed that the presence of chloride, hydroxyl radical scavengers, and components in natural groundwater has no measureable impacts on the observed rates of PFOA and PFOS removal and defluorination in the batch studies for the range of current densities tested. This finding suggests that PFOA and PFOS treatment is controlled by a surface reactions on the BDD anode, and that the presence of any organic co-contaminants that might serve as hydroxyl radical scavengers would likely not adversely impact anodic treatment of PFOA and PFOS. Finally, PFOA removal was greater than that of PFOS, especially at lower current densities.
The second phase of testing focused on long-term treatment. Results of this long-term treatment showed that the removal of PFOA and PFOS did not diminish over time (approximately 5 weeks). However, perchlorate generation (from naturally present chloride) decreased by nearly an order of magnitude. These results are encouraging, and suggest that PFAS treatment can be sustained, and the generation of high levels of perchlorate may only be a transient phenomenon. Finally, AFFF-impacted groundwater was evaluated in multiple systems. Removal of PFOS, PFOA, and a large suite of precursor compounds was evaluated as a function of groundwater geochemistry and current density. Results indicate that groundwater geochemistry may have a significant impact on overall treatment effectiveness. Assessment of AFFF-impacted groundwater is ongoing.
Abiotic Dechlorination by Natural Ferrous Minerals
The importance of natural attenuation mechanisms is receiving increased attention. Of particular interest are PCE and TCE dechlorination reactions that occur in low permeability (e.g., rock matrices, clays) materials, as even very slow dechlorination reactions can have a beneficial impact on dissolved plume longevity and mass discharge. Abiotic dechlorination reactions facilitated by the presence of ferrous minerals has been shown to be an important process, and is the focus of this presentation.
Abiotic dechlorination of PCE and TCE in several natural rock and clay systems was evaluated in a series of laboratory experiments. Reaction rates were evaluated as a function of ferrous mineral content, as determined by both the 1,10-phenanthroline method and magnetic susceptibility. In addition, dechlorination was determined under both oxic and anoxic conditions. Tests were repeated after partial mineral extraction to determine the specific ferrous minerals responsible for the observed dechlorination reactions. Finally, the relevance of the dechlorination reactions was assessed by performing a series of diffusion simulations.
Initial results suggest that slow abiotic dechlorination reactions occur in both natural clays and rock in the presence of ferrous minerals. Ferrous mineral content appears to be an important parameter for predicting abiotic dechlorination rates. Correlation of magnetic susceptibility to abiotic reactivity, or to ferrous iron content, was poor. Initial testing to compare dechlorination under oxic versus anoxic conditions suggest that dechlorination under these conditions occurs under very different mechanisms. Testing using multiple clay and rock types currently is underway. We anticipate that the results of this study will be used to determine how useful magnetic susceptibility or other ferrous mineral testing is with respect to predicting abiotic dechlorination rates.