We identify key the data gaps that need to be filled in order to proceed with meaningful ecological risk assessments, whether they are more global/ regional in nature, or for site specific assessments. Finally, we attempt to draw conclusions from the literature about the relative sensitivity of different model organisms, as well as the importance of particle properties on fate, transport and effects.Nanoceria is used in electronic and optical devices, polishing agents for glass and of silicon wafers, exterior paints, metallurgy, and diesel fuel additives.Additionally, nanoceria is used in automotive catalytic converters.It is also used in catalysts in petroleum refining, in the fluid catalytic cracking process . Based on the amount of total global CeO2 annual production and global nanoceria production rates,roughly 15–25% of total CeO2 production is nano . Cerium oxide is used in these applications in both nano and non-nano form and quantitative estimates of cerium oxide use within specific applications do not distinguish between nanoceria and its bulk counterpart.There are few studies that quantify the release of engineered nanomaterials during use, and even less nanoceria specific studies. One of the few studies by Park et al., indicates that 6–100% of CeO2 will be released during the use phase of diesel fuel additives.This has not yet been validated by other researchers. In laboratory conditions, particles filters from diesel cars removed 99.9% of Ce present in fuel additives.However the manuscript does not specify whether the Ce additive was in the nanoscale. Considering the applications and the likelihood that the nanomaterials are released to the environment, the following assumptions were made. For example,vertical growers nanoceria in batteries is enclosed within a protective casing, which is likely to minimize release during use.
If the batteries are disposed of improperly, the most likely environmental compartment would be soil, with negligible release to air, water or wastewater treatment plants . Similar assumptions were made for metallurgical products, catalysts in FCC, polishing powders used in industry , and other applications. Experimental studies have been conducted to measure the release of various manufactured nanoparticles from surface paints on exterior facades. Kaegi et al. measured concentrations as high as 600 μg L−1 of nano-TiO2 in runoff from newly-painted building facades,and estimated that as much as 30% of nano-Ag is released from surface paints within a year of paint application.However, no data exist on nanoceria released from paints. Based on similar information, estimated nanoceria concentrations in treated WWTP effluent discharged to waterbodies are expected to be in the range of 0.003–1 μg L−1 . In biosolids, nanoceria concentrations are expected to be around 0.53–9.10 mg kg−1 .These estimated concentrations are expected to increase as nanoceria is used more widely, and there will likely be accumulation of CeO2 in soils and sediments, further increasing exposure concentrations in these media.The detection and characterization of nanoceria under conditions relevant to environmental, toxicological and biological systems remains a challenging, and frequently impossible, task. However, there is little or nothing that is ceria-specific, but applies to all nanomaterials. However, aspects of characterization are included here since it is fundamental to understanding of all nanomaterials, including nanoceria. Essential general aspects are listed below: i) In environmental systems, the specific and accurate detection and characterization of manufactured nanoceria remains essentially impossible,due to the gap between metrology and analysis and the complexity of the system . Total Ce detection is useful as it acts an upper limit of nano-ceria concentrations for risk assessment, but is not synonymous with manufactured nanoceria.
The discussion below applies primarily to spiked materials, mainly in the laboratory or mesocosm. ii) As with other nanomaterials, nanoceria should be fully characterized using suitable preparation methods and a multi-method metrological approach. In a multi-method approach, independent techniques operating on independent measuring principles provide cross-validation of measured properties. The source of the nanomaterial also needs to be fully reported, given the likely effects on properties. Fuller discussion is given elsewhere.iii) A number of properties require characterization which can be grouped as size, shape, morphology, aggregation/ agglomeration, surface charge and dissolution . These groups, or classes, contain several individual properties. For instance, for size, an average size should be reported, along with some measure of spread .iv) Given the changes that are well known to occur upon storage or changing media,it is essential to perform appropriate measurement over temporal and spatial scales which adequately capture the dynamics of the nanomaterial system. Although, none of the points above are ceria specific, nanoceria is capable of oxygen storage, which is size and shape dependent.Nanoceria is generally thought to have low solubility in water,although this is size and oxidation state dependent. Where dissolution and solubility are low, study is rendered simpler because dissolved ions should have little impact on toxicity. However, recent work has shown potential effects of even low level dissolution.Nano-ceria has two stable oxidation states IJCeIJIII) and Ce) under environmental conditions and cerium has the ability to transition readily between these two states.This redox activity gives nanoceria some of its key properties.However, oxidation state and morphology are usually poorly controlled or defined and spatially variable within an individual particle,giving rise to poorly reproducible data and uncertainties in understanding toxicity or exposure. These uncertainties, along with dynamic changes that occur in complex media, could explain the variable environmental and toxicological results that are seen in the literature for nanoceria.Table 1 shows a non-definitive selection of studies of nanoceria in a variety of different environmental, toxicological and standard complex media. These studies are examples of some of the most complete characterization in the literature, although there is still little consistency between studies and it is often not clear which nanomaterial properties require analysis because it is not well understood how each affects biological or environmental processes.
Lastly, because of logistical or other constraints, characterization is often not performed as fully as necessary to interpret such processes. Some of the most powerful techniques for the visualization of nanoparticles are transmission electron microscopy , atomic force microscopy and scanning electron microscopy . These techniques not only provide direct visual images but can be used to quantify other properties such aggregation, dispersion, sorption, size, structure and shape of the nanoparticles,although the sample preparation may alter considerably the sample. These techniques have been extensively applied to nanoceria, occasionally in complex media. Van Hoecke et al.and Rodea-Palomares et al. used TEM to visualize the interaction between the nanoceria and algal cells in order to test whether the nanoparticles are taken up or adsorbed by the algal cell wall. Zhang et al.used TEM to further investigate the needle like clusters on the epidermis and in the intercellular spaces of cucumber roots after treatment with nanoceria over 21 days. In some cases, TEM has been coupled with spectroscopy, for instance TEM coupled with EDS was used to determine the elemental composition of ceria clusters on both the root epidermis and in the intercellular regions of the cucumber plant.Merrifield et al.used AFM to image and quantify the size of PVP-coated nanoceria while compared them using TEM and DLS in toxicology exposure media. TEM confirmed that the larger particles are aggregates composed of smaller individual particles , but that nanoceria properties did not measurably change in the exposure media tested. In the same study, EELS was used to quantify the oxidation states showing that the smallest and the largest samples were composed of entirely CeIJIII),vertical grow with only small amounts of Ce present in the largest sample. Such spectroscopy is essential to microscopy imaging in complex media and is required to unambiguously identify the nanoparticles of interest in the presence of materials with similar sizes, shapes and electron densities/tip interactions. Microscopy, although a powerful single particle method, remains challenging when attempting to provide statistically meaningful measurements. Much data reported in the literature is pictorial and non-quantitative; careful design and timeconsuming analysis are required to be able to determine representative parameters with confidence. Nanoparticle tracking analysis is another widely used characterization technique which utilises microscopy to determine size distributions and number concentration of nanoparticles in liquid samples. NTA has been infrequently used for nanoceria, for instance to determine the mean size of nanoceria in green alga and crustaceans and to better understand the effect of natural organic matter on the particle-size distribution of nanoceria settling in model fresh water as a function of time.However, the methodology has some limitations in complex and realistic media.X-ray photoelectron spectroscopy has been used in only one relevant study, to our knowledge, in this case to understand the antioxidant capacity of nanoceria to DNA. The calculation of CeIJIII) : CeIJIV) ratios was performed,in an analogous manner to EELS, within a multi-method approach. Similarly, synchrotron-based X-ray spectroscopy has been used in several studies to assess Ce speciation. Studies using micro X-ray fluorescence coupled with X-ray absorption near edge structure in natural matrices have been conducted concluding that nanoceria can undergo biotransformations within a matrix, so the modifications, the mechanism and extent of these transformations should be fully addressed.Scanning transmission X-ray microscopy is an analytical microscopy which, with extended X-ray absorption fine structure spectroscopy, provided 2D quantitative maps of chemical species at concentrations which are environmentally relevant.
X-ray microscopy can in principle provide a spatial resolution down to ~30 nm while imaging the specimen in the aqueous state without the need for sample preparation.Synchrotron-based techniques provide direct structural information regarding the nanoparticles and their interaction with the environment.It is clear that X-ray spectroscopy, XPS and EELS are complementary methods for oxidation state analysis and combination may prove fruitful. Field flow fractionation has also been used on nanoceria to measure the size distribution of nanoceria in synthesized samples as well as to understand the aggregation behavior of other nanoparticles in the presence and absence of humic substances.ICP-MS can be used as a detector for FFF, but has not been for environmental or toxicological studies of nano-ceria, to our knowledge. Preliminary studies have shown the feasibility of ICP-MS for nanoceria analysis in single particle mode, although its further application in real systems has yet to be demonstrated. Infrared spectroscopy has also been used to study biotransformations in plants by comparing the molecular environment of the sample before and after exposure hence concluding that cerium speciation changes after incubation of nanoceria in different exposure media over 21 days. Ultraviolet-visible spectroscopy has been used to monitor the dynamic aggregation process of nanoceria in various waters with time along with DLS and TEM.Nanoparticles properties are altered by the water chemistry such as pH, ionic strength, nature of electrolytes or presence of NOM. One of the most important changes may be aggregation of nanoparticles: between the same nanoparticle, homoaggregation, or between nanoparticles and an environmental particle, heteroaggregation. The increase in size of the aggregates affects their transport, behavior, reactivity, uptake by organisms, and toxicity. In pure water, the stability of non-coated nanoparticles in solution depends on their surface charge. Nanoparticles brought into close contact via Brownian diffusion processes will repel each other if the charge is strong enough to overcome attractive forces. Nanoceria surface charge is dependent of the pH; nanoceria are positively charged at low pH, negatively charged at high pH and have an isoelectric point at approximately pH 8.The methods of synthesis and the cleanup of nanoceria have been shown to play a role in affecting the experimental point of zero net charge for nanoceria suspensions, which range from 6.5 to 8.1.Differences in the reported PZC may also come from differences in the method applied to determine the PZC, the order of titration process, and sorption of anions used in the titration.These authors measured the aggregation kinetic of nanoceria and compared to the theoretical prediction of Derjaguin–Landau–Verwey– Overbeek . At pH 11, the experimental critical coagulation concentration was higher for the monovalent , than the divalent salts, 80 mM and 16 mM respectively. They showed that DLVO theory could predict quite well the stability of nanoceria at this pH. However, this model fails to explain aggregation behavior as solution conditions become more environmentally relevant and non- DLVO forces may also play important roles between particles.In a water-saturated column packed with sand, water composition has also been shown to control the transport and deposition of nanoceria.