When does the aggregation matter? The CLUMP scheme for classifying nanoparticles based on the influence of aggregation on environmental fate

Abstract

Engineered nanoparticles (ENPs) released into aquatic environments can undergo multiple removal processes, including dissolution, sedimentation, advection, and aggregation with natural particles. This study quantitatively assesses the probability that heteroaggregation is the dominant removal pathway among these mechanisms. The aggregation propensity of 36 ENP types was evaluated by estimating attachment efficiencies (α) derived from DLVO theory across a wide range of environmental conditions. Results show that α decreases with particle size at low Hamaker constants but increases with size at higher values. The calculated α values were incorporated into an environmental fate model to quantify the relative importance of aggregation versus other removal processes. Aggregation dominance probabilities varied widely among ENPs - from a few percent to 100% - depending on material composition, particle size, and zeta potential. Based on these outcomes, a five-tier CLUMP classification was developed to categorize ENPs according to the frequency of heteroaggregation dominance. This classification framework provides a comparative measure of nanomaterial mobility and environmental stability, offering a practical tool to support environmental fate modeling and risk assessment.