Adsorption of a diverse range of pharmaceuticals to polyethylene microplastics in wastewater and their desorption in environmental matrices

It is proposed that microplastics discharged from wastewater treatment plants act as a vector of pharmaceuticals. In this study, adsorption of pharmaceuticals to polyethylene microplastics was investigated in municipal wastewater. Pharmaceuticals for study were selected to represent different speciation (anionic, cationic, and neutral) and a range of pH dependant octanol-water distribution coefficients (log DOW). Findings revealed adsorption favoured those in cationic form with the greatest hydrophobicity (e.g., fluoxetine log DOW 2.0 at pH 7.8). Adsorption of anionic pharmaceuticals was restricted due to repulsion with the microplastic's negatively charged surface. Only atorvastatin had any appreciable adsorption due to its comparatively high log DOW value (2.9). Those pharmaceuticals predominantly in neutral form (carbamazepine and ketamine) with log DOW values >= 2.4 had similar adsorption. Freundlich KF values were 3400, 386, 284, 259 and 218 (mg kg-1)(mg L-1)1/n for fluoxetine, propranolol, atorvastatin, ketamine, and carbamazepine, respectively. All pharmaceuticals with log DOW values <1.0 (atenolol, gliclazide, bezafibrate, and ifosfamide) did not adsorb to microplastics, irrespective of their speciation. Changing composition of wastewater (pH, dilution with stormwater and NaCl addition) within the range expected for municipal wastewater had limited influence on adsorption. Pharmaceutical desorption from microplastics was assessed in river water and simulated gastric and intestinal fluids. Solution pH was considered the most important factor for pharmaceutical desorption, influencing both pharmaceutical speciation and microplastic surface charge. Greatest desorption was observed for the cationic pharmaceuticals in gastric fluids due to a reduced surface charge of the microplastics under low pH conditions. Up to 50% desorption of fluoxetine occurred in gastric fluid at 37 degrees C. These findings show that pharmaceuticals adsorbed to microplastics are 'bioavailable'. However, this is often overlooked as an exposure route to aquatic organisms because water samples are normally pre-filtered prior to chemical analysis.

Automated summary

The study revealed that pharmaceuticals in cationic form with greater hydrophobicity are more likely to adsorb to microplastics in municipal wastewater. Despite being bioavailable to aquatic organisms, pharmaceuticals adsorbed to microplastics are often overlooked as an exposure route due to pre-filtration of water samples before chemical analysis.