Interactions between pharmaceutically active compounds and organic foulants in ultrafiltration processes

In this study, we systematically investigated the fundamental interactions between pharmaceutically active compounds (PhACs) and organic fouling layers during ultrafiltration (UF) processes. Two representative PhACs, propranolol (PRP) and naproxen (NPX), were filtered through polyethersulfone membranes fouled with natural organic matter surrogates, including humic acid, bovine serum albumin, and sodium alginate. The results suggested that electrostatic interactions contributed substantially to the association of ionic PhACs with organic fouling layers under the investigated UF conditions. Electrostatic attraction likely promoted the association of cationic PRP with negatively charged fouling layers, whereas electrostatic repulsion likely limited the association of anionic NPX with these layers under neutral and alkaline conditions. Experiments conducted at high ionic strength (1 M NaCl) further supported these mechanisms by suppressing electrostatic interactions, resulting in decreased PRP and improved NPX association with fouling layers. When electrostatic contributions were reduced, the observed PhAC retention was consistent with hydrophobic interaction, with possible additional contributions from π-π interactions and hydrogen bonding. These findings highlight the influence of foulant chemistry on pharmaceutical association.