Practical considerations to optimize aquatic testing of particulate material, with focus on nanomaterials

Aquatic testing of particulate materials (PMs), e.g., nanomaterials (NMs) and microplastics (MPs), poses inherent challenges potentially hindering the application of existing test guidelines (TGs). Those TGs are primarily designed for hazard assessment of the dissolvable form of a material, whereas the guidance document on aquatic and sediment toxicological testing of NM (OECD Guidance Document 317) encourages the inclusion of potential colloidal fractions in the assessment. A prerequisite for the testing of PMs is the preparation of stable dispersions. However, testing difficulties may result from the fact that nano-scale PMs are inherently unstable when dispersed in test media, leading to the need for differentiation of potential chemical vs. physical effects caused by the tested material. Aquatic testing of unstable PMs will likely result in inconsistent and non-uniform uptake and exposure scenarios and thus effects observed in the respective test systems. Maintaining stable exposure conditions is often very challenging given the constantly changing size of the PM and its agglomerates, requiring observed endpoints to be based on measured concentrations and particle size distributions present in the water phase, while neglecting agglomerated and settled particulates. In this paper we describe the current state of PM-testing, demonstrate PM-specific challenges in aquatic testing (e.g., test duration, physical effects, instability, biodegradation, bioaccumulation) with a focus on NMs, considering a set of most relevant TGs, and provide proposed testing considerations to optimize aquatic testing of PMs.