The Sorption of Antidepressant Pharmaceuticals on Virgin and Aged Microplastics Is Lower than Bioconcentration in Protozoa

The simultaneous occurrence of various pollutants in the aquatic environment raises questions about their mutual interactions. There is a gap in research on the sorption of polar substances on microplastics. This study aimed to assess the adsorption of the antidepressants sertraline, fluoxetine and duloxetine on microplastic polystyrene, polyethylene terephthalate and polyvinyl chloride, each in two versions: virgin and aged. To assess the affinity of the tested drugs for plastic and planktonic organisms, the experiment was conducted in microplastic suspensions and in a mixture of microplastics with the protozoan Spirostomum ambiguum. The Fourier transform infrared technique assessed the identity of microplastics and changes during ageing. No significant differences were found between the sorption of the tested drugs on virgin and aged microplastics. The sorption of sertraline onto microplastics was 1.5–3 times lower in the presence of the protozoa than in samples with microplastics alone. Moreover, its concentration in the protozoan cells was 10–30 times higher than in the microplastics. Considering that the amount of plankton in freshwaters is much greater than that of microplastics, it should be concluded that microplastics have a negligible share in the transport of antidepressants in surface waters.