Polystyrene nanoplastics and wastewater displayed antagonistic toxic effects due to the sorption of wastewater micropollutants

The knowledge about the interaction of nanoplastics with other aquatic pollutants and their combined effects on biota is very scarce. In this work, we studied the interaction between polystyrene nanoplastics (PS NPs) (30 nm) and the micropollutants in a biologically treated wastewater effluent (WW). The capacity of PS NPs to sorb micropollutants was studied as well as their single and combined toxicity towards three freshwater organisms: the recombinant bioluminescent cyanobacterium, Anabaena sp. PCC 7120 CPB4337; the duckweed, Spirodela polyrhiza and the cladoceran, Daphnia magna. The endpoints were the inhibition of bioluminescence, the growth inhibition of the aquatic plant and the immobilization of D. magna after 24, 72 and 48 h of exposure, respectively. Combination Index (CI)-isobologram method was used to quantify mixture toxicity and the nature of interactions. PS NPs sorbed a variety of chemicals present in WW as micropollutants in a range of tens of ng/L to μg/L. It was found that those pollutants with positive charge were the main ones retained onto PS NPs, which was attributed to the electrostatic interaction with the negatively charged PS NPs. Regarding the toxicological effects, single exposure to PS NPs affected the three tested organisms. However, single exposure to WW only had a negative impact on the cyanobacterium and S. polyrhiza with no observed toxicity to D. magna. Regarding PS NPs-WW combined exposure, a reduction of toxicity in comparison with single exposure was observed probably due to the sorption of micropollutants onto PS NPs, which resulted in lower bioavailability of the micropollutants. In addition, the formation of PS NPs-WW heteroaggregates was observed which could result in lower bioavailability of PS NPs and sorbed micropollutants, thus lowering toxicity. This study represents a near-realistic scenario approach to the potential sorption of wastewater pollutants onto nanoplastics that could alter the toxicological effect on the biota.