Per- and polyfluoroalkyl substances (PFAS) are recalcitrant in the environment and require extreme conditions to initiate chemical transformation reactions. In general, the techniques capable of destroying PFAS are not amenable to in situ remediation of impacted groundwater. Therefore, the main objective of this project was to develop a cost-effective, in situ method using commercially available drinking water coagulants as sorption enhancers to sequester the six PFAS in the USEPA’s Unregulated Contaminant Rule 3 (UCMR3) list (PFOS; PFOA; perfluorononanoic acid [PFNA]; perfluorohexane sulfonate [PFHxS]; perfluoroheptanoic acid [PFHpA]; and perfluorobutane sulfonate [PFBS]) in groundwater systems to prevent their migration to drinking water supplies. The central hypothesis of this work was that the addition of chemical coagulants used in the drinking water industry and in other water treatment applications would enhance PFAS sorption to the soil, reducing PFAS mobility in the subsurface over long periods of time.
The overall objective of the project was accomplished by completing four Tasks:
- Identify the optimum coagulant and its dosage for the sequestration of PFAS in a groundwater system. Batch experiments containing PFAS, one of the four selected coagulants, and soil excavated from Tinker AFB or sand were carried out to determine PFAS-coagulant adsorption isotherms. Controls without coagulant were also performed. These experiments evaluated PFAS removal as a function of coagulant identity, dose, time, and solution conditions. It further determined the weathering characteristics of the coagulant and coagulant-PFAS complex under environmental conditions.
- Determine the optimum method of delivering coagulants into the groundwater system. Three methods for delivering coagulants to the groundwater system, including alginate beads, osmotic pumps, and commercially available floc logs, were evaluated to provide time-release of the coagulant into the dissolved phase of a simulated groundwater system. A fourth method specifically tailored to low organic carbon soils was developed, in which coagulant and powdered activated carbon (PAC) was combined to create a nonsettleable slurry that could be injected in to a simulated aquifer. This addition increased the organic carbon content of the soil, and increased the retention of PFAS by as much as 4000 times.
- Evaluate the effectiveness of selected coagulants to sequester PFAS using one-dimensional columns. One-dimensional column studies were performed to determine PFAS sequestration following the addition of coagulant and simultaneously with the addition of coagulant. This task evaluated the sequestration of PFAS, and long-term release of PFAS if coagulant treatment is halted. The mechanism of complexation was also investigated.
- Investigate distribution of sorption enhancer in a heterogeneous system in two-dimensional aquifer cell studies. Two-dimensional aquifer cell studies were performed to determine PFAS sequestration following the addition of coagulant, to evaluate the maximum removal capacity in representative aquifer materials, and to determine the potential for pore space blockage.
Two coagulants worked well at increasing the retention of PFAS in saturated soil. One in particular, polydimethylamine diallyldimethyl ammonium chloride (polyDADMAC), increased the sorption coefficient of PFAS by as much as a full order of magnitude. The mechanism was determined to be a complexation of PFAS with the coagulant, which in turn had a higher affinity for the sorbed phase than the free PFAS. The sorption of the complex appears to be irreversible, and the complex was shown to be abiotically and biotically resistant to degradation. Further development of the method showed that by combining polyDADMAC with PAC, the project team would be able to effectively increase the organic carbon content of the soil. This increase resulted in a soil that would require as much as 4,100 pore volumes of PFAS at 100 parts per billion before breakthrough would occur.
The methods developed in this project could be implemented in the field in several ways. Options include creation of a permeable absorptive barrier (PAB) of either polyDADMAC or polyDADMAC+PAC. In the case of a polyDADMAC+PAC PAB, one could also inject polyDADMAC upgradient to create complexes with PFAS to increase the effectiveness of the PAB (Figure A.1). Another approach would be to use an existing drinking water well to push the polyDADMAC or polyDADMAC+PAC into the aquifer around the well to act as a treatment zone such that the resulting drinking water pulled back up the well would be PFAS free (Figure A.2).
Aly, Y.H., C. Liu, D.P. McInnis, B.A. Lyon, J. Hatton, M. McCarty, W.A. Arnold, K.D. Pennell, and M.F. Simcik. 2018. In Situ Remediation Method for Enhanced Sorption of Perfluoro-Alkyl Substances onto Ottawa Sand. Journal of Environmental Engineering, 144(9).
Anderson, E.L., M.P.S. Mousavi, Y. Aly, X.V. Chen, M.F. Simcik, and P. Buhlmann. 2019. Remediation of Perflurooctylsulfonate Contamination by In Situ Sequestration: Direct Monitoring of PFOS Binding to Polyquaternium Polymers. ACS Omega, 4(1):1068-1076.
Aly, Y. 2016. Enhanced Sorption of Perfluoroalkyl Substances (PFAS) onto Ottawa Sand (Master’s Thesis). University of Minnesota, School of Public Health, Division of Environmental Health Sciences.
Aly, Y. 2019. Enhanced Adsorption of Perfluoroalkyl Substances in Groundwater; Development of a novel in situ Groundwater Remediation Method (PhD Thesis). University of Minnesota, Water Resource Sciences Program.
McCarty, M. 2016. Development of a Novel Perfluoroalkyl Substance Sequestration Scheme Using Alginate Macrobeads and Common Water Treatment Polymers (Master’s Thesis). University of Minnesota, Civil Environmental and Geoengineering.