Effect of calcium on micropollutant removal from water with nanofiltration: Theoretical and experimental study

Calcium is widely present in natural waters and can influence the removal of micropollutants (MPs) by nanofiltration (NF) membranes, yet the underlying mechanisms remain insufficiently understood. This study provides a combined experimental and theoretical investigation into the mechanisms by which calcium influences the removal of organic MPs. We evaluated three proposed mechanisms: (i) calcium-MP complexation, (ii) membrane charge screening or charge reversal, and (iii) pore narrowing. Isothermal titration calorimetry showed that calcium–MP complexation is unlikely at environmentally relevant MP concentrations. Filtration experiments demonstrated that pore narrowing due to calcium precipitation is negligible. In contrast, our results reveal that calcium strongly interacts with deprotonated carboxyl groups in the membrane, altering membrane charge density and thereby governing MP rejection. Charge modification by calcium decreases the removal of small, positively charged MPs ≈ 100 g/mol but enhances rejection for larger positively charged MPs > 200 g/mol. These findings clarify the dominant role of calcium-induced membrane charge modification in MP removal with NF.