Anion Exchange Resin and Inorganic Anion Parameter Determination for Model Validation and Evaluation of Unintended Consequences during PFAS Treatment

When implementing anion exchange (AEX) for perand polyfluoroalkyl substances treatment, temporal drinking water quality changes from concurrent inorganic anion (IA) removal can create unintended consequences (e.g., corrosion control impacts). To understand potential effects, four drinking water-relevant IAs (bicarbonate, chloride, sulfate, and nitrate) and three gel-type, strong-base AEX resins were evaluated. Batch binary isotherm experiments provided estimates of IA selectivity with respect to chloride (Kx/C) for IA/resin combinations where bicarbonate < sulfate <= nitrate at studied conditions. A multi-IA batch experiment demonstrated that binary isotherm-determined Kx/C values predicted competitive behavior. Subsequent column experiments with and without natural organic matter (NOM) allowed for the validation of a new ion exchange column model (IEX-CM; https://github.com/USEPA/Water_Treatment_Models). IA breakthrough was well-simulated using binary isotherm-determined Kx/C values and was minimally impacted by NOM. Initial AEX effluent water quality changes with corrosion implications included increased chloride and decreased sulfate and bicarbonate concentrations, resulting in elevated chloride-to-sulfate mass ratios (CSMRs) and Larson ratios (LRs) and depressed pH until the complete breakthrough of the relevant IA(s). IEX-CM utility was further illustrated by simulating the treatment of low-IA source water and a change in the source water to understand the resulting duration of changes in IAs and water quality parameters.

Automated summary

When implementing anion exchange for perand polyfluoroalkyl substances treatment, temporal drinking water quality changes from concurrent inorganic anion removal can create unintended consequences. Evaluation of four drinking water-relevant inorganic anions and three strong-base anion exchange resins showed that the selectivity of anions with respect to chloride predicted competitive behavior. Column experiments validated a new ion exchange column model and demonstrated that the breakthrough of inorganic anions was minimally impacted by natural organic matter. Initial effluent water quality changes included increased chloride and decreased sulfate and bicarbonate concentrations, leading to elevated chloride-to-sulfate mass ratios and depressed pH. The ion exchange column model was further illustrated by simulating changes in source water to understand the duration of changes in inorganic anions and water quality parameters.