Role of calcium ions on the removal of haloacetic acids from swimming pool water by nanofiltration: mechanisms and implications

We investigated the removal of haloacetic acids (HAAs) from swimming pool waters (SPWs) by two nanofiltration membranes NF270 and NF90. The strong matrix effect (particularly by Ca2+) on membrane rejection prompts us to systematically investigate the mechanistic role of Ca2+ in HAA rejection. At typical SPW pH of 7.5, NF90 maintained consistently high rejection of HAAs (>95%) with little influence by Ca2+, thanks to the dominance of size exclusion effect for this tight membrane (pore radius ∼ 0.31 nm). In contrast, the rejections of both inorganic ions (e.g., Na+ and Cl−) and HAA anions were decreased at higher Ca2+ concentration for NF270 (pore radius ∼ 0.40 nm). Further tests show that the rejection of neutral hydrophilic molecular probes and the membrane pore size were not affected by Ca2+. Although Ca2+ is unable to form strong complex with HAAs, we observed the binding of Ca2+ to NF270 together with a reduction in its surface charge. Therefore, the formation of membrane-Ca2+ complex, which weakens charge interaction effect, was responsible for the reduced HAA rejection. The current study reveals important mechanistic insights of the matrix effect on trace contaminant rejection, which is critical for a better understanding of their fate and removal in membrane-based treatment.