Which natural organic matter components limit PFAS removal by activated carbon?

Per- and polyfluoroalkyl substances (PFAS) are persistent micropollutants subject to strict drinking water regulations. Although granular activated carbon treatment can effectively remove PFAS from water, early breakthrough, especially for short-chain PFAS, requires frequent regeneration and increases treatment costs. While many studies have related the presence of natural organic matter (NOM) to early breakthrough, the mechanisms affecting PFAS adsorption remain unclear. In this study, we used resin and membrane fractionation to isolate distinct NOM fractions based on chemical characteristics and molecular size. Batch isotherm, kinetic, and column experiments showed that the presence of the <3 kDa fraction significantly reduced short-chain PFAS adsorption. As low molecular weight compounds (<3 kDa) are consistently detected in source waters at concentrations comparable to or higher than those included in the present study, their removal is crucial to improving short-chain PFAS removal during drinking water treatment. Conversely, long-chain PFAS adsorption was not notably affected by molecular weight fractions but was significantly reduced in the presence of the hydrophobic neutral (HON) fraction. The HON fraction consists of compounds with similar molecular characteristics as long-chain PFAS, leading to direct competition for adsorption sites. The HON fraction is typically present at lower concentrations compared to other NOM fractions in natural waters. This low concentration of the HON fraction helps explain the delayed breakthrough observed for long-chain PFAS, in contrast to short-chain PFAS, whose breakthrough is rapid due to the high concentration of the <3 kDa fraction. We found that the presence of specific NOM fractions can indicate reduced PFAS removal, suggesting that NOM fractionation techniques could be integrated into monitoring procedures at drinking water facilities, as the presence of specific fractions can indicate reduced PFAS removal.