Assessment of the interaction between aquatic colloids and pharmaceuticals facilitated by cross-flow ultrafiltration

Interactions between pharmaceuticals and aquatic colloids are a key process regulating their environmental fate, but poorly understood. A validated cross-flow ultrafiltration (CFUF) system was used to isolate river colloids and to determine the partition of selected pharmaceuticals between colloidal (>1 kDa but <0.7 μm) and dissolved phases (<1 kDa) by liquid chromatography-tandem mass spectrometry (LC-MS-MS). The kinetics of pharmaceutical binding to colloids was rapid, reaching equilibrium within 5 min. The mass balance of chosen pharmaceuticals through CFLUF system was satisfactory for propranolol, sulfamethoxazole, meberverine, carbamazepine, indomethacine, diclofenac, and meclofenamic acid. The partition coefficient normalized to colloidal organic carbon content (K_coc) varied from 5.45 × 10⁴ to 7.54 × 10⁵ mL/g for the chosen pharmaceuticals, which are greater than those for endocrine disrupting chemicals (EDCs), suggesting substantially stronger colloidal interactions with pharmaceuticals than with EDCs. Linear relationships were demonstrated between log-K_coc and pharmaceutical properties such as log K_ow (octanol-water partition coefficient), highlighting the importance of compound hydrophobicity in controlling their binding with colloids. Such a finding is in contrast to that for EDCs whose K_coc values were independent of their K_ow values. The CFUF-LC-MS technique has the potential to become a widely applicable tool for quantifying the distribution of emerging organic pollutants between nanoparticles and the dissolved phase.