Peroxi-Electrocoagulation for PFAS Mitigation: The Impact of Water Quality and Dissolved Organic Matter on Removal Pathways

The recent addition of per- and polyfluoroalkyl substances (PFAS) to the National Primary Drinking Water Regulationfor per- has increased the need for research on PFAS treatment technologies for water and wastewater. Electrochemical treatment processes have been widely investigated for PFAS removal. Peroxi-electrocoagulation (electrocoagulation paired with hydrogen peroxide (EC:H2O2)) was evaluated as a novel water treatment process for PFAS mitigation due to the multimechanistic removal pathways that can proceed during treatment, including chemical degradation via oxidation, and physical separation pathways such as sorption to flocs, flotation layer accumulation, and foam fractionation. This work investigated the impacts of varying water quality conditions and dissolved organic matter (DOM) composition on PFAS mitigation efficacy and the corresponding removal pathways. Sources of DOM were an additional point of focus to provide insight into the role of DOM characteristics (i.e., aromaticity, molecular weight) on the fate of PFAS in EC:H2O2. This aim was studied by conducting EC:H2O2 with five different types of DOM (including humic acid, fulvic acid, oxalic acid, salicylic acid, and one natural river DOM). EC:H2O2 was effective as a PFAS mitigation technology using a bicarbonate electrolyte matrix and different types of DOM (including reference DOM and natural DOM). Generally, PFAS removal was higher at pH 3 compared to pH 6.3, ostensibly due to enhanced oxidant yield, interactions between iron and PFAS, and foam formation. At pH 3, oxidation was a key route of removal for the carboxylic acids including perfluorooctanoic acid (PFOA) and 5:3 fluorotelomer carboxylic acid (5:3 FTCA). A combination of chemical degradation and physical separation processes contributed to the removal of sulfonic acids including 6:2 fluorotelomer sulfonic acid (6:2 FTS) and perfluorooctanesulfonic acid (PFOS). However, in the presence of DOM, especially the <1 kDa low molecular weight and low aromatic autochthonous components, PFAS were more readily removed via physical sorption to the flotation layer, potentially due to the formation of DOM-iron-PFAS complexes. Regarding engineering applications, EC:H2O2 may have limited feasibility for PFAS mitigation in drinking water due to the highly acidic pH conditions needed and the release of metals during treatment. Accordingly, EC:H2O2 may better serve as a pretreatment and foam fractionation technology for higher strength wastewaters (such as membrane concentrates and industrial wastewaters) prior to more dedicated liquid-stream destructive technologies such as electrooxidation or supercritical water oxidation.