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Treatment of PFAS Residuals Using a Membrane Distillation Crystallizer
Citation:
Huggett, E., M. Nadagouda, C. Patterson, T. Lee, T. Speth, AND H. Salih. Treatment of PFAS Residuals Using a Membrane Distillation Crystallizer. ACS Fall Meeting, Chicago, IL, August 21 - 26, 2022.
Impact/Purpose:
Over the last half century, per- and polyfluoroalkyl substances (PFAS) have been widely used in numerous industrial and commercial applications (e.g., firefighting foams, stain repellants, and nonstick coatings). PFAS are reproductive and developmental toxins, endocrine disrupters, potential carcinogens, and bio accumulative. In response, several provisional health-based guidelines for PFAS in drinking water have been established. The removal of PFAS from the environment to below provisional guidelines has been effective using current conventional treatment methods (e.g., advanced oxidation, ion exchange, and membrane filtration) but still require post-treatment. In this work, an innovative, scalable, and cost-effective remediation technology, membrane distillation (MD), is evaluated as a polishing step for membrane filtration and ion exchange concentrates. Reducing the volume of the brine (i.e., concentrate) is a sustainable strategy for meeting future stringent discharge regulations and reducing hazardous waste.
Description:
The extensive use of the nano-polymer per- and polyfluoroalkyl substances (PFAS) in consumer and industrial manufacturing has resulted in their release into the environment at concentrations of concern. High-pressure membrane processes (i.e., reversed osmosis and nanofiltration), activated carbon (AC), and anion exchange resins (AXR) have been shown to be very effective in removing PFAS when appropriately designed. All of these processes, however, generate problematic concentrated waste streams. The practice of AXR single-use mode followed by incineration of the spent resin often makes the AXR prohibitively expensive, and can be seen as environmentally unfriendly. When resin regeneration is used, it is typically conducted using a brine solution (100-150 g/L salts concentrations). Thus, the regeneration waste stream typically contains high salt concentrations in addition to the released PFAS which can be problematic. To deal with this waste, membrane distillation (MD) can be advantageous for treating regenerant solutions as the transport of vapor molecules across the membrane is not dramatically affected by the concentration of non-volatile contaminants. This research aims to treat AXR regeneration waste brines by MD. The MD process is used to remove water from the waste brine to further concentrate salts beyond their saturation limit. This treatment process will separate salts as crystals and produce highly concentrated PFAS solutions at smaller volumes that can then be treated by incineration or any other end-of-life destruction techniques. A model short-chain PFAS compound, perfluoropentanoic acid (PFPeA), at a concentration of 10 mg/L in the presence and absence of ion exchange resin regeneration brine (100 g/L NaCl) was tested using four commercially available membranes. Namely, unlaminated polytetrafluoroethylene (PTFE), polypropylene laminated PTFE, polyether ether ketone (PEEK), and polyvinylidene difluoride (PVDF) hydrophobic membranes. For each test, a new membrane with an area of 140 m2 was used with a constant permeate temperature of 20 °C (cold) and varying feed temperatures of 50 and 60 °C (hot). The conductivities of the feed and permeate, the PFAS, salts concentrations, and water flux were measured. The interaction of PFPeA with the different membranes, such as membrane fouling due to PFPeA adsorption, was also investigated. The unlaminated and laminated (with polyether) PTFE membranes showed the best performance in treating the PFPeA contaminated brine. At the same experimental conditions, the water flux through the unlaminated PTFE membrane was 50% higher than the flux through the PEEK membrane and 25% higher than the PVDF and laminated PTFE membranes. The laminated and unlaminated PTFE membranes achieved the highest rejection of NaCl and PFPeA (>99.7%) compared to 95% and 97% obtained by the PEEK and the PVDF membranes. The laminated PTFE membrane showed higher stability and mechanical strength than the other membranes. In contrast, the PEEK and PVDF membranes were proven to be very fragile.