The contribution of additives to microplastic aquatic toxicity - A testing approach with model additives on selected aquatic organisms

The challenge in assessing the overall toxicity of micro- and nanoplastics (MNP) lies in the complexity and variety of additives and their release, which is additionally affected by ageing processes of MNP in the environment. Our study aims to develop further the methodology to differentiate between the physical (particle-related) and chemical effects of MNP (contribution of additives) on the overall toxicity of MNPs. A testing methodology is presented that assesses the effects of well-defined, additive-loaded MNPs, pristine additive-free MNPs, and additives separately. This approach is complemented by a comprehensive characterization of additive release, while also considering ageing processes. The results indicated that low-density polyethylene (LDPE) alone or with additives, independent of ageing regime, exhibited no significant toxic effects on two aquatic test organisms, cladoceran Daphnia magna and protozoan Tetrahymena thermophila. The inorganic additive ZnO alone caused immobility in D. magna and demonstrated cytotoxicity in T. Thermophila. In contrast, the organic benzotriazole derivative (Lubio® AS17) and the hindered amine light stabiliser (Lubio® UV18) as additives showed no cytotoxic effect in T. Thermophila, and low toxicity was observed in daphnids. Artificial ageing showed an impact on the quantity of leached/released additives, but not on the toxicity of MNPs to the tested organisms. In conclusion, our study provides a systematic approach to MNP design with known amounts of additives, their characterisation and ecotoxicological testing, which could be a basis for designing safer plastic materials in the future.