Influence of typical clay minerals on aggregation and settling of pristine and aged polyethylene microplastics

Microplastics (MPs) are emerging as a class of pollutants that are a potential threat to biological and human health. Aggregation and settling are crucial to controlling MPs transport and environmental fate. However, the influence of clay minerals in the aqueous environment on the aggregation-settling processes of larger size MPs and its mechanisms remain unclear. In this study, homoaggregation of pristine and aged polyethylene microplastics (PEs) and heteroaggregation-settling of PEs with typical clay minerals (chlorite, illite, kaolinite, montmorillonite) under different hydrochemical conditions (NaCl, CaCl₂, MgCl₂) were systematically investigated. The results showed that the cation type has a greater influence on the homoaggregation system. In detail, the aged PEs is more stable than pristine PEs in monovalent electrolyte solutions, but not in divalent electrolytes. In heteroaggregation systems, electrostatic repulsion dominates the interaction of PEs (pristine, aged) with clay minerals. However, the settling ratio of PEs (pristine, aged) contributed by clay minerals is not very dependent on the clay mineral type. Conversely, high NaCl concentrations are more conducive to the heteroaggregation-settling of PEs, which can be explained by the DLVO theory. The findings of this study provide new insights into the environmental fate and distribution of MPs in natural waters.