Aggregation of positively charged polystyrene nanoplastics in soil–root systems

The occurrence of nanoplastics in agricultural land is a potential threat to human health and food security, yet the fate of nanoplastics is soil–root systems is poorly known. Here we investigated the aggregation behavior of polystyrene nanoplastics bearing different charges in cucumber root exudates and soil leachate, and in the presence of common environmental Ca 2+ ions. Results show an increase in the aggregation size of positively charged amino-modified polystyrene nanoplastics, whereas negatively charged nanoplastics did not aggregate and showed no significant changes in particle size. This is possibly due to the adsorption of organic acids from soil leachates and root exudates onto the surfaces of nanoparticles. We observed that the critical condensation concentrations of Ca 2+ for negatively charged nanoplastics increased by more than 1.4 times in root exudates and soil leachates compared with that in deionized water. Our findings suggest that root exudates and soil leachates can reduce the aggregation of negatively charged nanoplastics in Ca 2+ solutions.