Evaluation of nanoparticle (NP) toxicity in respect to NP physicochemistry and reactivity in the aquatic environment
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The increasing application of nanoparticles (NPs) has led to increased occurrence of engineered NPs in the aquatic environment. Understanding the toxicity of NPs in the aquatic environment is dependent on linking NP physicochemistry with toxicological responses and while research has been moving towards this direction, the link has not been fully understood yet. The present study critically reviewed adsorption and desorption processes of organic environmental contaminants on TiO2 NPs and evaluated interactions of NPs with compounds of different physicochemistry already existing in the aquatic environment as contaminants. Aquatic toxicity of the new generation lead-halide perovskite NPs was evaluated relative to lead ion dissolution. Finally, the sedimentation of NP agglomerates during a traditional fish early-life stage toxicity test, a major limitation of assessing NP toxicity in the aqueous phase, was addressed by development of an exposure chamber designed to keep NP agglomerates in homogeneous dispersion. The model organisms used in the present study to evaluate NP toxicity were larvae and adult zebrafish Danio rerio and the unicellular green fresh water alga Chlorella vulgaris. The main findings were: 1) sorption of environmental contaminants on NPs can change the bioavailability of the contaminant in the aqueous phase. Specifically, sorption of copper and benzo(a)pyrene (under fluorescent light) on NPs reduced the adsorbent bioavailability. On the contrary, benzo(a)pyrene and anthracene, when adsorbed on TiO2 or Si NPs, were photo-catalysed under UVA and in the case of benzo(a)pyrene, highly toxic photoby-products showed increased bioavailability in larval zebrafish; 2) lead-halide perovskite acute toxicity was attributed to lead ion dissolution based on induction of metallothionein 2 gene expression through aqueous and dietary exposure, and 3) the perovskite-spiked diets did not disrupt zebrafish gut microbiome after a 14-d exposure while disruption of gut microbiota by equivalent Pb(NO3)2 diets was observed; finally, 4) higher toxicity was found when NPs were tested using an exposure chamber that allowed continuous NP dispersion, indicating toxicity is depended on the dispersion state of NPs. This study has expanded our knowledge on NP surface physicochemistry and interactions with surrounding compounds in the aqueous phase; has confirmed metal ion dissolution out of metallic NPs and linked perovskite NP toxicity to lead ion dissolution as well as linked NP toxicity to NP dispersion in the aqueous phase contributing to a better understanding of NP properties and reactivity relation to toxicity in the aquatic environment.