Towards a scalable continuous flow plasma water treatment architecture for PFAS destruction

Project SummaryPer and Poly fluorinated alkyl substances whose origins date back to the 1940s are a class ofcompounds possessing high thermal and chemical stability. The compounds have been used in a range ofconsumer products owing to these robust properties. Their unusual stability and thus long half-life have ledto the proliferation of these compounds in the environment. It is now well established that ingestion of thesechemicals can lead to the development of a range of health disorders from endocrine disruption to cancer.Regulations at the federal and state level are setting health advisories of maximum acceptable concentrationlevels. Unfortunately conventional water treatment systems cannot remove PFAS. The recalcitrant natureof these compounds owing to the strength and shielding effects of the carbon-fluorine bonds makesdestructive removal difficult (typical bond energies ~100 kcal/mol). Membrane and activated carbon arecapable of removing PFAS effectively but each creates a concentrated waste stream. The US EPA identifiesa research gap for low-cost methods to process concentrate streams from membrane processes or cheapermethods to regenerate activated carbon. The effort proposed here aims to use plasma to destroy PFAScompounds both in contaminated water systems and in concentrate streams derived frommembrane processes. While it is widely recognized that plasmas are effective at destroying PFASthe consensus is that work needs to be done on scaling up the process and quantifying overallimplementation and operational costs. To date plasma reactors—even the large ones—have beenpredominantly batch-mode reactors. It is desirable in industrial or municipal applications to treatwater in a flow reactor configuration. Our goal is to investigate the feasibility of a once-throughscalable flow plasma reactor capable of treating practical influent flows.PFAS removal via plasma treatment is predominantly a surface treatment process--essentially longchain hydrophobic PFAS compounds are degraded by energetic electrons ions and UV light exposure.Considerable destruction is initiated via reduction by solvated electrons derived from the plasma whereelectron addition leads to spontaneous bond cleavage and defluorination. To destroy the PFAS with plasmamultiple exposures of the liquid surface are required to reduce the contaminants to smaller chains andultimately to cause mineralization. In this regard one must circulate the volume of water to be treated pastthe plasma multiple times as in a batch reactor. This proposal aims to convert from batch-mode explored inprevious experiments to a flow reactor geometry while preserving batch mode kinetics. We hypothesizethat using a series of reactors with recirculation such that water exposure to plasma matches thecontact time in our batch reactor we can achieve similar degradation in a flow-through geometry.Removal is achieved through serial processing of the water as it passes from one microreactor to the nextso that the water receives the requisite number of passes through the plasma over the characteristic timeconstant as realized in a batch reactor. Furthermore we will use reverse osmosis concentrate as our inputfeedwater. This PFAS load is degraded to desired levels at the end of the final microreactor where thatwater is blended with RO permeate to achieve the desired MCL thus realizing a true flow-through reactor.The scalability of this system will be explored through the addition of parallel legs of microreactors. In allcases the kinetics through a given flow reactor will match the exposure realized in the batch reactor fromprevious work. The work proposed will investigate a scalable 10 gallons/min flow system with capacity forup to 50 gallons/min. Understanding the scalability of plasma-based water treatment systems can providea significant commercial and societal benefit. If the technology