Separation of trace pharmaceuticals individually and in combination via forward osmosis

With a high rejection coefficient for trace pharmaceuticals and personal care products (PPCPs), forward osmosis (FO) membrane separation has become a cutting-edge technology in water treatment owing to its low energy consumption and low membrane fouling. Wastewater contains many types of PPCPs, and one pharmaceutical molecule affects the separation behaviors of other pharmaceuticals in FO. Therefore, simultaneous FO of multiple PPCPs needs to be investigated. In this study, the separation behaviors of four trace pharmaceuticals (ciprofloxacin (CIP), sulfamethoxazole (SMX), acetaminophen (ACP), carbamazepine (CBZ)), individually (termed "single pharmaceuticals") and in combination (termed "binary pharmaceuticals" as two pharmaceuticals were studied simultaneously), during FO were investigated at trace concentrations using ultra-high performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS). The results showed that for single pharmaceuticals, the molecular sieve dominates their retention rate-the retention rate increases with increasing Stokes radius of the molecules (29.1 → 94.8% for 0.35 → 0.47 nm). For binary pharmaceuticals, the retention rates of both pharmaceuticals without charge decrease with increasing total molecule number (for ACP + CBZ, 31.4 → 52.1% (ACP), 75.1 → 83.0% (CBZ)). Negatively charged pharmaceuticals are mutually exclusive with the negatively charged FO membrane, resulting in the increase of the retention rate of pharmaceuticals (83.1 → 90.1% (CIP) when CIP + ACP → CIP + SMX). In the presence of a positively charged pharmaceutical, the retention rate of negatively charged pharmaceuticals decreases (85.7 → 80.4% (SMX) when SMX + ACP → SMX + CIP) because the positively charged pharmaceutical neutralizes the negative charge on the FO membrane surface, resulting in the weakening of electrostatic repulsion between the negatively charged pharmaceutical and FO membrane surface. The positively charged molecule attracts the negatively charged molecule, forming a couple of molecules with larger molecule weight and increasing the retention rate of the pharmaceuticals (80.4 → 88.2% (SMX) when pH = 7 → 5 for SMX + CIP). The results suggest that the interactions between pharmaceuticals cannot be ignored in the process of removing PPCPs by FO.