Introduction

The use of engineered nanomaterials (ENMs) is growing continuously due to the increasing number of applications of nanotechnology, promoting the development of a new generation of smart and innovative products and processes that have created tremendous growth potential for a large number of industrial sectors (Savolainen et al, 2013)

Due to its potential to develop new added value products, a staggering number of ENMs is already available on the market, however, along with the benefits, there is an on-going debate about their potential effects on the human health or the environment (Boverhof, D. et al, 2015). The uncertainties are extensive since it is now well-established

that ENMs exhibit unique physical and chemical properties different from those of the same material in bulk form (Kumar et al, 2011.), affecting their physicochemical and biological behaviour, which can lead to adverse effects to both humans and the environment (Ray PC, et al, 2009).

At the same time, the use, production and disposal of ENMs raise concerns about their environmental impact at all stages of the value chain, considering that nanostructured materials can be released to the air, soil or water in common industrial processes and/or accidental events (Gottschalk and Nowack, 2011), and ultimately accumulate in the soil, water or biota (Köhler et al, 2008.), endangering the health of living organisms and ecosystems.

Besides the above, given that the production volumes of some of these materials are already exceeding thousands of tonnes and that the global demand for ENMs is expected to exceed $3.1 trillion by 2020 (Roco et al, 2011.), the likelihood of unintended release will tend to increase in the near term, and therefore, the environmental release of ENMs during production, use, and end-of-life becomes unavoidable (N.Wu et al, 2013).

Despite this situation, it is currently not possible to precisely asses the ecological impacts of the release of ENMs into the environment, which is mainly due to the lack of understanding of the inherent physicochemical properties of ENMs and mechanisms driving exposure and release (N.Wu et al, 2013), as well as the scarce existing knowledge on the transport, transformation, degradation and possibly accumulation of ENMs in the environment (N.Wu et al, 2013) upon release, all of them of paramount importance for characterizing and evaluating whether a potential risk for humans and the environment is present.

Moreover, despite large research efforts, to date, still little is known about the concentration, fate and toxicity of ENMs when released into the environment (Giese,B. et al, 2018). These knowledge gaps are partly due to the lack of techniques suitable for collecting, preserving, and storing samples containing ENMs (Lin et al, 2010), as well as to a poor understanding of nanomaterial properties and behaviour in the environment. So far, environmental scientists have not been able to fill these gaps due to a number of reasons including the fact that ENMs consist of several highly diverse class of substances, and their exposure concentrations are frequently very low (Gottschalk, F. et al, 2012).

In this situation, it is important to start estimating possible environmental concentrations. Several studies have demonstrated the presence of ENMs in relevant environmental compartments, including air, soil, and water. A recent study published by the author (C.Fito-López et al. 2016) predicted levels of release using modelling approaches of

350.000 and 123.000 tons of ENMs to the water and soil compartment respectively, which means concentrations up to 200 μg/L in water and 900 μg/kg in soil. Considering published data, recent studies reported significant concentrations in the environment, with values ranging from 0.003 ng/l (fullerenes) to 21 ng/l (TiO2) for surface waters, and values around 1 ng/kg (fullerenes) to 89 g/kg (TiO2) in sludgetreated soils. Similarly, modelling studies conducted by Gottschalk et al. in 2013 reported concentrations TiO2 in rivers ranging from 3 ng/l to 1.6 mg/l, and Hendren et al. in 2013 reported concentrations of nano-silver in sludge ranging from 4.3 μg/kg to 13.03 μg/kg.

Within REACH regulation, the predicted environmental concentration values (PEC) are compared with the predicted no effect concentration levels (PNEC) of ENMs for risk assessment purposes, and the ratio between PEC and PNEC is eventually used as an indicator or risk. If the PEC is greater than the PNEC (i.e. ratio > 1), means that there is a risk of effects to the environment. However, neither the PECs nor the PNECs are known. As a result, although environmental concentrations of ENMs are growing, there is still a lack of knowledge base to undepin emission control measures and/or a legislative framework.

In this context, a fundamental step towards a quantitative assessment of the risks of new pollutants (i.e. ENMs) to the environment is to support the current estimated levels with measured data on their environmental concentrations.

This work presents a thorough review of the current knowledge on the concentration of engineering nanomaterials in freshwater ecosystems and soil, both data on especial interest to support risk assessors on the evaluation of the potential risks posed by the manufacturing and use of ENMs to the environment.

Material and methods

The platform Web of ScienceTM, from Thomson Routers, was selected in order to gather data on the concentration of engineered nanomaterials in the environment. WoS is one of the largest, only true collection of research data, books, journals, proceedings, publications and patents. For the identification of data, following “key words” were stablished:

1. Nano* and Exposure* and occup*

2. Nano* and environment* and concentration*

3. Nano* and Fate*

For each source, parameters as nanoparticle type, location, method used to calculate concentration values, and contextual information on the environmental conditions were stored in a database for data comparison and analysis purposes.

Results

Environmental concentrations at natural compartments

Table 1 shows current predicted concentration (PEC) values compiled from peer reviewed publications to elucidate the current levels of ENMs in air, water and soil compartments. The data retrieved showed that ENMs may be present in different environmental matrices (air, water and soils) due, for example, direct release through open windows when powdery material is used incautiously or from accidental spills, handling activities, falling powders or transferring activities. Furthermore, this release may also be produced into rivers and soils via untreated or treated wastewater. As can be seen in table 1, most of the data compiled is referred to the concentration of ENMs in water, while the presence of these compound in soils is less studied.

Higher concentrations of ENMs are expected in freshwater ecosystems, where quantities above 76 µg/L were retrieved for ZnO nanoparticles. Moreover, current data exists only for four countries in Europe. From the perspective of nanomaterials, titanium dioxide (TiO2) and silver (Ag) are the most measured compounds. Table 1 and 2 shows data on the concentration levels estimated for water and soil compartment respectively.

Environmental concentrations at urban compartments

Several publications have been found in the literature which measures the concentration of airborne NMs in urban and rural environments. In Figure 1 are exposed the locations where the studies compiled were carried out

Figure 2 shows data on the concentration of ultrafine particles (1 – 100 nm) for urban atmospheres. In this compartment, there are more available studies, giving information on air quality levels both in cities and the countryside. The data cover most of the countries of the European Union and some of them are summarized in the table above, which shows nanoparticle concentrations in urban areas, roadside and background levels for different cities of Europe. According to these data, Birmingham, Milan, Zurich, Barcelona and Leicester are the cities with the higher concentration of nanoparticles.

The data showed that relevant concentration of ENMs can be expected in urban compartments, however, an evident lack of information on the chemical nature of particles below 100 nm is such compartment shall be noticed. Current studies show compositions ranging from metals and metal oxides to phosphates, in particular SiO2, CeO2 Zn, Mn, Fe, Co, Ni, Cd, and Pb (Kumar et al., 2014).

Figure 3 shows the analysis of the environmental concentration in those countries where a higher number of studies have been retrieved. The data show metal oxide nanoparticles are expected in the freshwater ecosystems, in particular, TiO2 and ZnO, which can be attributed to the use of ZnO and TiO2 in cosmetics and paints, two product categories that dominate the market.

Conclusions

The potential concentration of ENMs in relevant compartment have been studied. Results obtained shows that still little is known about the concentration of ENMs, partly due to the lack of techniques suitable for collecting, preserving, and storing samples containing ENMs, as well as to the scarce number of studies available so far.

Few studies have been conducted in real natural systems, and possible existing forms and the proportion of ENMs in the environment remain unclear. In the case of water compartments, several studies are available mainly related with laboratory studies. Nevertheless, few studies have been designed to study soil concentrations.

Even though the knowledge is growing and data are gradually becoming available, there are still substantial gaps that need to be bridged before achieving robust and comprehensive environmental assessments of ENMs. Nowadays, environmental exposure assessment for

ENMs is mainly based on probabilistic approaches or makes use of different scenarios to provide estimates.

Robust measurement methods for determination of low ENM concentrations in water, sediments and soil are needed, with only a few on-line measurement methods available.

Acknowledgments

This research was conducted as part of the European projects LIFE NanoRISK (LIFE12 ENV/ES/000178), and NanoMONITOR (LIFE14/ENV/ES/000662).

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Notas de autor

cfito@itene.com