Review articleSaltwater ecotoxicology of Ag, Au, CuO, TiO2, ZnO and C60 engineered nanoparticles: An overview
Introduction
The manipulation of the matter at the nanoscale (≤ 100 nm) generates engineered nanomaterials (ENMs) with peculiar physico-chemical characteristics (Liu et al., 2014). Recently, the application of nanotechnology in many fields of industrial production has progressively grown, making engineered nanoparticles (ENPs) a new emerging potential category of environmental contaminants (Manzo et al., 2013a). Their innovative and economic potential is threatened by a limited understanding of the related environmental health and safety (EHS) issues. It is commonly accepted that ENPs can be dispersed into the environment during their productive process, use and end-life (Fan et al., 2014), potentially entering the marine compartment and thus posing potential risks for the biota (Hanna et al., 2013).
In the last decade, aquatic ecotoxicity studies about ENP effects grew rapidly, stressing more on freshwater rather than saltwater or terrestrial species (Corsi et al., 2014, Libralato, 2014, Minetto et al., 2014, Libralato et al., 2016, Lofrano et al., 2016, Vale et al., 2016). Saltwater is a complex matrix pushing ahead ENP instability and promoting the rapid formation of agglomerated/precipitated forms (Callegaro et al., 2015). Despite the huge number of papers, their nanoecosafety is still fragmentary, and the comparison of multiple studies can be difficult, since experimental designs and testing conditions are rarely consistent across studies (Salieri et al., 2015). The shrinking time to market of new ENMs drives the need for pressing actions by policymakers and stakeholders that are still slow to arrive. Starting from highly scattered information and considering a special focus on saltwater, this paper will: (i) provide an overview of ENMs ecotoxicological effects from existing data; (ii) refine papers on cross-cutting selection criteria; (iii) support a “mind the gap” approach stressing on missing data supporting hazard and risk assessment.
This review examined 529 papers including original nanoecotoxicological researches embracing freshwater and saltwater environments up to the end of December 2015. Bibliographic search engines were Google Scholar, PubMed, Scopus and Web of Science. Review papers were not considered. Papers investigated aquatic ecotoxicology on 78 ENPs including metals and metalloids (n = 10), metal oxides (n = 33), organics (n = 3), quantum dots (QD) (n = 5) and “others” (n = 27) (Table S1). The “others” category comprised: ALEX (aluminium NPs), C60HxC70Hx, C60OH24, C70, carbon-iron, CD-Se, cotton nanofibers, fluorescent NPs (FNP), fluorescent silica (FS), graphene oxide, hydroxy apatite (HA), L-ALEX (aluminium NPs), Mg(OH)2, Mn-ZnS, Mo/NaO, N-isopropilacrilamide (NIPAM), N-isopropilacrilamide/N-tertbutylacrylamide (NIPAM/BAM), polyethyleneglycol(PEG)-Fe3O4, PEG-QD, polymethylmethacrylate (PMMA), polystyrene, sodium alginate-polyvinylalcohol-ZnO (SA-PVA-ZnO), sodium dodecyl-sulphate/didodecyl-dimethyl ammonium bromide (SDS/DDAB), tricalcium phosphate (TCP), tobramycin polymeric, Zn–Se, zero valent iron (ZVI). An overview of ENP-biological model pair and the list of recorded species are available in Table S1 and Table S2, respectively.
The relative abundance of studies for ENP was reported in Fig. 1. The attention was mainly focused on metal oxides (65%) and then on metals and metalloids (20%), organics (8%), QD (2%) and others (5%). The most investigated ENPs were: TiO2 (31%), Ag (12%), ZnO (11%), CuO (6%), C60 (5%), Au (3%), CeO2 (3%) and carbon nanotubes (3%).
Focusing on saltwater species, the statistics drastically changed. Only 126 papers (24%) accounted for saltwater nanoecotoxicology including 38 ENPs. They were grouped as metal oxides (n = 20), metals and metalloids (n = 5), organics (n = 3), QD (n = 2) and others (n = 6). A detailed overview of ENP-biological model pair and the list of ENPs relative abundance are available in Table S3, and Fig. 2 (considering saltwater species).
The consistency of data showed to vary according to the exposure scenario making their interpretation very case specific. Various ENPs were tested on several species and endpoints increasing the responses within and between each ENP category and group of organisms (Kahru and Dubourguier, 2010). Nanotoxicity/nanosafety data are increasing day-by-day, but the importance to regulators is often unclear or unproven mainly because their extreme case specific outlook (Minetto et al., 2014). Selected ENPs were considered after a skimming process including (i) the ENPs studied with a frequency ≥ 5%; (ii) the presence of analytical concentrations; (iii) the presence of detailed experimental design including organism exposure conditions, ENP characterization and/or dispersion methods; and (iv) the compliance with quality assurance and quality control procedures. Six ENPs met these criteria (nAg, nAu, nCuO, nTiO2, nZnO and C60) and were investigated within this review.
Section snippets
Ecotoxicity of ENPs
An overview concerning the ecotoxicity effects on saltwater species of nAg, nAu, nCuO, nTiO2, nZnO and C60 was proposed in Table S4. For each ENP, toxicity data were classified according to the taxonomy of biological models. The ENP administration/exposure conditions and uptake were also specified. The reviewed papers not cited in this review, thus not meeting the above-mentioned criteria, were listed in Supplementary materials (Table S5) with the relative references.
Discussion
Data revision highlighted how it is still difficult determining a clear framework about nanoecosafety to saltwater organisms. Information appeared fragmentary and incomplete, and sometimes with a profile of limited ecological significance. From Table S6, it can be observed that i) nevertheless non-ecologically relevant exposure concentrations were taken into consideration (sometimes up to several hundreds of mg/L or mg/kg depending on ENP and its way of exposure - water or sediment), frequently
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2022, Aquatic ToxicologyCitation Excerpt :A wide range of such materials are currently being considered and investigated (e.g., carbon nanotubes, titanium dioxide (TiO2)) as potential photocatalysts in water treatment (Lu et al., 2016). Amongst these NMs, the metal-oxide-semiconductor, TiO2, appears to be the most promising and most abundant NP in water (Minetto et al., 2016). The production of nanohybrids (NHs) consisting of metal-oxide NPs and carbon-based nanomaterials (C-NMs) has increased and attracted defined research interest because NHs can be applied to improve the surface area, multifunctional properties and catalytic efficiency of photocatalytic NPs (Baek et al., 2020).