Heteroaggregation of nanoplastic particles in the presence of inorganic colloids and natural organic matter

The presence and accumulation of micro- and nanoplastics in marine and fresh waters represent a huge environmental concern. Due to the complexity of nanoplastic surface chemistry and impact of the surrounding aquatic environment, the fate of nanoplastics is still difficult to evaluate. Our study aims to explore the effect of different water components such as natural organic matter and inorganic colloids as well as water composition on the stability of polystyrene nanoplastics. Heteroaggregation experiments are performed under contrasting conditions by considering mixtures of three components: nanoplastics, Fe2O3 and alginate and at different concentration ratios. It is found that the charge neutralization mechanism in most cases is responsible for the formation of large heteroaggregates. A shift in the optimal heteroaggregation concentration is observed in the presence of alginate indicating competitive effects between alginate and Fe2O3. The formation of primary heteroaggregates is found to be a requisite before the formation of large structures. The behavior of polystyrene nanoplastics is also studied here in natural water from the Rhone river. Nanoplastic particles are found to rapidly change their surface charge from positive to negative and form small heteroaggregates at low concentration. Increasing the nanoplastic particle concentration is found to result in the formation of large heteroaggregates when the isoelectric point is achieved indicating the importance of nanoplastic surface charge neutralization.