Summer 2019 Currents

Physico-chemical Destructive Technologies for Treatment of PFAS Brian J. Yates, P.E., Civil and Environmental Engineer, Burgess & Niple, Inc. Blossom Nwedo Nzeribe, Ph.D., Environmental Engineer, GSI Environmental, Inc. Per- and polyfluoroalkyl substances (PFAS) are a class of thousands of exclusively anthropogenic chemicals that have been in use since the 1940s. Their chemical and thermal stability as well as their oil- and water-repellency characteristics, make them useful in many consumer products and across many different industries. These same properties, however, also make them environmentally recalcitrant, and PFAS contamination has been reported worldwide. Health studies over the past 20+ years have indicated that PFAS are linked to adverse health outcomes including high cholesterol, ulcerative colitis, thyroid disease, testicular cancer, kidney cancer and pregnancy-induced hypertension (eclampsia). Other negative health outcomes have also been proposed as being caused by elevated levels of PFAS in blood serum. In response, the United States Environmental Protection Agency (US EPA) has set a drinking water lifetime health advisory level of 70 ng/L for a combination of two PFAS (PFOS and PFOA) and individual states have set even lower values for drinking water, groundwater, soil, and other environmental matrices.

The best available technologies for removal of PFAS in water are granular activated carbon, ion exchange resins, and reverse osmosis. These treatment technologies, however, do not destroy, but rather concentrate PFAS in another matrix, which remains a challenge for remediation efforts. There are no full-scale best available technologies for the destruction of PFAS owing to the strength of the C-F bond (the strongest in organic chemistry); however, some emerging technologies are showing promise.

This article aims to introduce the reader to the current state of the science with respect to emerging destructive technologies for PFAS in water. These include chemical oxidation processes, chemical reduction processes, ultrasonication, and plasma technologies. This article is a summary of a more extensive review published in Critical Reviews in Environmental Science and Technology earlier this year (Crit. Rev. in Environ. Sci. Technol. 2019, doi.org/10.1080/10643389.2018.1542916).

Chemical Oxidation Processes Traditional chemical oxidation processes using chlorine, permanganate, and ozone are not effective in destroying PFAS in water due to the strength of the C-F bond; however, PFAS have been shown to be susceptible to oxidation by high-energy free radicals or by direct electrochemical oxidation. Three technologies which have shown promise for the destruction of PFAS in water by chemical oxidation include activated persulfate, electrochemical oxidation, and photochemical oxidation.

Persulfate is a strong oxidizing agent that can be activated by several methods, including heat, pH, transition metals, and microwaves. Upon activation, persulfate forms the sulfate and hydroxyl radicals. The sulfate radical is effective in the sequential removal of the perfluoroalkyl moiety from PFAS. This mode of removal is known as the “unzipping” mechanism and has been shown for PFOA. Complete mineralization of PFOA is possible however, only one study has demonstrated that PFOS can be destroyed by heat-activated persulfate. Temperature and pH affect the efficiency with which PFOA is removed as higher temperature and lower pH promote radical-radical reactions, which lowers the efficiency of their reaction with target PFAS. Also, degradation of PFAS by activated persulfate is suppressed by other solutes (e.g., chloride, bicarbonate, and organic matter). www.asce.org/ewri • EWRI Currents • Volume 21, Number 3 • Summer 2019