Effect of water matrix on the removal of antibiotics and estrogens by nanofiltration membranes

This study provides novel insights into the interactions among humic acid (HA), calcium ion (Ca²⁺), and nanofiltration membranes, revealing their critical impact on rejecting nine antibiotics and six estrogens, common environmental pollutants. In the presence of HA, the normalized water flux of NF270 and NF90 was decreased to 92% and 89%, respectively, and was further reduced to 71%-83% and 74%-79% with the addition of Ca²⁺, resulting from the reduced hydrophilicity by HA-Ca-induced deposition. HA reduced antibiotic rejection by 27% for NF270 and 3% for NF90, attributed to weakened charge repulsion and size exclusion (reflected by decreased membrane surface charge and boron rejection). Adding Ca²⁺ further decreased antibiotic rejection by up to 48% for NF270 and 6% for NF90, as charge repulsion and size exclusion were further diminished. HA also decreased estrogen rejection by 8%-16% for NF270 and 5%-14% for NF90, due to weakened size exclusion. After adding Ca²⁺, estrogen rejection showed inconsistent trends due to competing effects of size exclusion and hydrophobic interaction. Therefore, size exclusion and charge repulsion are the main rejection mechanisms for the negatively charged antibiotics, however, size exclusion and hydrophobic interaction are for the neutral and hydrophobic estrogens. The reduced membrane permeability and selectivity highlight the importance of effectively managing organic matter and divalent cations in feedwater to ensure both treatment efficiency and long-term membrane stability.