Sorption of psychoactive compounds on passive sampling sorbents in the presence of wastewater suspended particulate matter

Understanding interfacial processes that govern contaminant uptake is essential for interpreting data from passive sampling (PS) systems in aquatic environments. This study investigates the sorption mechanisms of eight psychoactive compounds (PCs) on two commonly used polymeric sorbents - (hydrophilic-lipophilic Balanced (HLB) and mixed-mode cation exchanger (MCX)) - under environmentally relevant water conditions. Laboratory exposure experiments were performed to derive partition coefficients (K_SW) and sorption rate constants (k_X) and investigate the sorption dynamics in the presence and absence of suspended particulate matter (SPM). At pH 7.8, sorption was largely governed by lipophilic interactions, with MCX exhibiting slightly higher K_SW for cationic compounds but relatively slower accumulation rates due to cation exchange. Compound-specific properties influenced sorption behavior at varying pH, while decreased conductivity lowered sorption efficiency. Sorbed concentrations on both sorbents reduced with increasing temperature, and thermodynamic analysis confirmed that the sorption processes are exothermic and spontaneous. SPM was found to enhance sorbent uptake through two mechanisms: partitioning of particulate-bound species and facilitated transport of compounds to the sorbent surface. These findings highlight the importance of considering SPM-mediated transport into the design of passive sampling strategies for monitoring emerging contaminants in dynamic aquatic systems.