Unraveling the synergistic impact of calcium ions and pH on the rejection of per- and perfluoroalkyl substances by nanofiltration in complex water matrices

The influence of coexisting calcium ions (Ca 2 + ) and fluctuating pH on the removal of diverse per- and polyfluoroalkyl substances (PFAS) by nanofiltration (NF) in complex natural waters remains unclear. This study systematically investigated the rejection of 14 representative PFAS as a function of Ca 2+ concentration (0, 0.1, 1.0 mM) and pH (4.5, 6.5, 8.0) in the presence of humic acid (HA). Results show that increasing Ca 2+ to 1.0 mM significantly enhanced the rejection of most PFAS by strengthening size exclusion, albeit at the cost of aggravated flux decline. More compellingly, raising the pH from 4.5 to 8.0 boosted the rejection of short-chain PFAS (e.g., PFBA and PFBS) by 15–20 % due to enhanced electrostatic repulsion. Mechanistic analysis revealed that Ca 2+ promoted the formation of a denser cake layer via “ion bridging” with HA and PFAS, whereas high pH intensified electrostatic repulsion between membrane and anionic PFAS by increasing the negative charge of the membrane surface. This work elucidates the intrinsic trade-off between rejection efficiency and membrane fouling governed by water chemistry, providing critical insights for optimizing NF processes to treat complex waters contaminated with emerging PFAS.