A versatile test system to determine nanomaterial heteroagglomeration attachment efficiency

Engineered and incidental nanomaterials are emerging contaminants of environmental concern. In aquatic systems, their transport, fate, and bioavailability strongly depend on heteroagglomeration with natural suspended particulate matter (SPM). Since particulate contaminants are governed by different mechanisms than dissolved contaminants, harmonized, particle-specific test systems and protocols are needed for environmental risk assessment and for the comparability of environmental fate studies. The heteroagglomeration attachment efficiency (α _het) can parametrize heteroagglomeration in fate models which inform exposure assessment. It describes the attachment probability upon nanomaterial-SPM collision and reflects the physicochemical affinity between their surfaces. This work introduces a new versatile test system to determine α _het under environmentally relevant conditions. The test matrix combines model SPM analogs and an adjustable model hydrochemistry, both designed to represent the process-relevant characteristics of natural freshwater systems, while being standardizable and reproducible. We developed a stirred-batch method that addresses shortcomings of existing strategies for α _het determination and conducted heteroagglomeration experiments with CeO ₂ (<25 nm) as a model nanomaterial. Single-particle ICP-MS allowed working at environmentally relevant concentrations and determination of α _het values by following the decrease of non-reacted nanomaterial over time. The α _het values received for the model freshwater test matrix were evaluated against a natural river-water sample. Almost identical α _het values show that the model test system adequately reflects the natural system, and the experimental setup proved to be robust and in line with the theoretical concept for α _het determination. Combinations of natural SPM in model water and model SPM in natural water allowed further insight into their respective impacts. The α _het values determined for nano-CeO ₂ in the natural river water sample (0.0044-0.0051) translate to a travel distance of 143-373 km downstream until 50% is heteroagglomerated, assuming an average flow velocity of 5 km h ⁻¹, an SPM concentration of 20-45 mg L ⁻¹, and experimental mixing conditions (i.e., G ∼ 97 s ⁻¹). These half-lives illustrate the importance of heteroagglomeration kinetics.