For many ENMs such as nanoceria,reactivity is highly size dependent. To accurately model material fates thus requires understanding how material surface properties affect integrity, how both change under varying environmental conditions such as pH, clay content,and organic matter content,and how surface properties and particle reactivity affect physicochemical processes that are parametrized in far-field models. This is especially true for ENMs used as pesticide delivery mechanisms, including carbon nanotube composites with specifically reactive surface monomers. Yet only recently has modeling attempted to address differing properties of a material’s structural variants .Evaluating computational model predictions is a challenge for ENMs, which presently are estimated to occur in the environment at low concentrations.Also, detection methods for ENMs in environmental media and distinguishing ENMs from natural chemical analogs are still under development,with more evaluation strategies needed including a framework for validating new ENM analytical detection methods.Fullerenes from incidental sources were quantified in river sediments collected from locations across the globe and quantified in the atmosphere over the Mediterranean Sea.Perhaps related to a viable exposure scenario,stacking pots fullerenes were quantified at relatively high concentrations in treated wastewater effluent and at ng/L to μg/L concentrations in river waters receiving effluent discharge.
While not necessarily nanoscale, similarly high concentrations of TiO2 were reported for sediments sampled near a WWTP outfall.The greatest uncertainty in ENM exposures is near-field , at the receptor where toxicant dose manifests as internal dose. Heteroaggregation is a dominant fate process for ENMs when they interact with natural colloids.Given sufficient residence time for ENMs in environmental matrices, heteroaggregation and to a lesser degree homoaggregation will affect localized compartmentalization, including stability in the water column and therefore, sedimentation.However, these processes do not preclude biological impacts under simulated environmental conditions, as has been shown for nanoceria in a complex aquatic mesocosm.Exposure can be confirmed by quantifying receptor body burdens, thereby allowing for quantitatively relating near-field exposure to biological effects.Thus, in the absence of detailed, biologically complex, near-field models for local exposures to environmental receptors, the ability to trace ENMs to biological receptors sampled directly from the environment becomes the best available approach to relate far-field exposures to biological impacts.Overall, material flow models and multimedia modeling of ENMs have advanced to inform ENM ecotoxicology. Available far-field modeling frameworks are adaptable to changing inputs despite uncertainties in production volumes. Major uncertainties remain at the nexus of ENM surface and core chemistries as related to nanomaterial transport, aggregation, and degradation characteristics. However, fundamental research is needed to discover and parametrize complex fate processes.
New approaches, such as assays that can be used to rapidly probe surface associations,demonstrate how to populate far-field models and how to determine near-field exposures associated with effects. Although existing models can simulate particle movement, deposition, and some transformations, the knowledge state regarding ENM environmental exposure conditions via measurements or modeling simulations cannot be assumed to accurately represent actual conditions at biological receptors.Many of the outstanding research issues and recommendations for evolving ENM ecotoxicology are echoed in the discourse for other chemicals of emerging concern .These include the need for systematically understanding ENM and decomposition product toxicity across various receptors within linked levels of biological organization,quantifying actual exposures and uptake into environmental receptors,gaining mechanistic insights into and biological markers for acute and chronic low level exposures,and understanding how environmental factors including cocontaminants affect ENM transformation and biological impacts. Still, how can the potential for exposure and impacts of ENMs be anticipated, prevented, managed, or mitigated? Further, what data and tools do decision makers need to inform their work? Innovation in nanotechnology hinges on having the science to evaluate ENM safety. While no formalized process for incorporating all exposure conditions and concepts of ENM transformation, dose, and body burden into risk assessments currently exists, a proposed framework approach to risk characterization over the life cycle of ENMs has been published and is available.This framework advocates an initial decision cutoff in regards to exposure; in the absence of exposure, the need for further assessment is diminished or negated.In this available framework, ENMs that are certain to rapidly dissolve into ionic components in a destined environmental compartment would be assessed for risk based on the released components rather than the original nanoparticles.
Persistent ENMs are expected to accumulate in matrices such as sediments.The consequences of ENMs to successive generations, biodiversity, and ecosystem services are not addressed by model organism-specific assays of discrete growth and mortality.Nonetheless, in this available framework, toxicity end points associated with standardized testing protocols for sediment, aquatic, and terrestrial standard population-level end points over short and long time frames are advocated for assessing hazards of simulated ENM concentrations in the environment.In this framework, sunlight is an environmental variable, bio-accumulation is measured, and ENM modifications during product and material life cycles that may change bio-availability are considered.While such a framework has broad organizational appeal, priority setting within the framework is required and thus could focus on tests that are relatively well aligned with likely exposure scenarios. Even with a risk assessment framework that considers ENMs across product life cycles and considers sediments, water, and soil in testing end points,major hurdles hinder regulatory agencies, and research scientists, in using concepts such as exposure conditions, ENM transformation, dose, and body burden in interpreting biological and computational findings for assessing risks. Toxicity tests developed for dissolved chemicals typically require significant modification for use with ENMs.Tests may not apply to ENMs if they are not appropriate for solids.Additional scientifically based hazard information from the peer-reviewed literature may or may not be available for consideration. ENMs used in ecotoxicity tests,strawberry gutter system which are sometimes laboratory-synthesized to overcome uncertainty regarding proprietary coating or other commercial formulations, may be insufficiently analogous to allow for extrapolating information or risk comparisons. Issues include the need to know test material characteristics and how they relate to testing results and the ENM life cycle. Even if an initial risk assessment considers ENM solubility,ENM dissolution is not instantaneous; therefore, at what stage of dissolution does the contaminant no longer pose a hazard as an ENM? Also, where biological impacts stem from ENM surface characteristics, how can mass concentration be used to judge hazards? Environmental ENM effects in bench top experiments can be indirect, stemming from physical nutrient depletion,or amplifying organism uptake of cocontaminants.Other indirect physical effects derive from ENMs adhering to the organism surface,light shading,or internal food displacement.Near-field exposures can result in biological hazards from specific ENMs based on their properties .By definition, ecological risk assessment is “the process for evaluating how likely the environment will be impacted as a result of exposure to one or more environmental stressors.” ERA involves predicting effects for individuals, populations, communities and ecosystems, and concerns itself with valuable ecosystem services such as nutrient cycling.Thus, conducting ERAs for ENMs could benefit from an ecological outlook. All levels of biological organization, and interactions between them, would be considered when assessing responses to ENM exposure . Release and exposure scenarios , use of functional assays for assessing environmental compartmentalization ,and combined life-cycle and multimedia modeling have important roles in focusing ENM ecotoxicology. Less recognized is that mechanistically based models of dynamic biological effects are informed by hazard assessment research. Different types of process-based, dynamic models allow for predicting effects from exposures stepwise, starting at sub-cellular levels, into individuals, through populations, and conceivably to communities and ecosystems. Developing process-based models requires researching key effects processes and ecological feed backs.Models are formalized to describe interactive processes culminating in toxicity such as reactive oxygen species generation and cellular damage. Process-based mathematical expressions evolve with empirically based discoveries or through model reconciliation with experimental data. Parameters are independent of toxicity testing protocols, although models could be informed by standard test results. Thus, ENM ecotoxicity research could support predictive toxicology by informing and populating process-based, dynamic ecological effects models.
A comprehensive fate and effects research agenda is needed for addressing ENM quantification in complex media.Such an agenda has allowed for assessing experimental compartmentalization,and sensitively assessing environmental persistence, toxicity, bio-accumulation, trophic transfer,and indirect effects from the uptake of ENMs coated in other hazardous materials.Such research could substantially inform ENM risk assessment for a relevant environmental exposure scenario. However, most ENMs have not been studied comprehensively along the entire exposure and effects continuum . Further, the approach is not sustainable. Rather, the need is to develop efficient approaches applicable within an overall approach to rapidly evaluate the large number of ENMs under commercialization . A research agenda that focuses on distilling key determinants of exposure and hazard for ENM-environment systems that can be measured experimentally would be most compelling. Thus, while the science of ENM ecotoxicology and exposure characterization has advanced, there are disconnects between how regulators review ENM-based products for environmental safety and the research that is conducted to evaluate hazards. Except for results published in open source outlets or directly reported, research may be unknown to government bodies. Ongoing synthesis of published research results is challenging due to high variability across study conditions and ENMs tested, and due to effort needed to regularly update such comparisons. Moreover, there is a systematic resistance to publishing “no effect” studies in the peer-reviewed literature.As a result, relying only on published research to inform regulatory decisions can present challenges. A life cycle-based framework facilitates exposure modeling and hazard testing to support risk assessment. However, extrapolation of effects to untested concentrations, study, or environmental conditions, and across biological levels of organization, requires understanding dynamic biological process-based effects, which current standard tests neither deliver nor sufficiently inform. Ultimately, exposure scenarios are useful for framing and focusing ENM ecotoxicology, and some version of a tiered intelligent testing and risk assessment strategy is needed. Such a conceptual tiered strategy considering the impact of the ENMs’ varying properties on ecological risks at different life cycle stages was proposed in the EU FP7MARINA project and is being further developed in the EU NANoREG program. This strategy considers several domains represented by specific tools ranging from relatively simple in the lower tiers to more complex and specific in the higher tiers. The framework aim is to structure information collection and generation for cost-efficient risk assessment, compliant with 3R animal-use testing principles , which should also be pursued by means of grouping ENMs. A strategy for grouping ENMs based on releases, uses, physicochemical properties, bio-accumulation, bio-availability, and effects for both human and ecological risk assessment is currently in development across a number of EU research projects such as MARINA, NANoREG, SUN, and GUIDEnano. These efforts have been challenged by the complexity of ENM identity and interactions, but this approach is necessary, as the costs for safety assessment on a case-by-case basis would be exorbitant.Therefore, a vision on ENM grouping is needed, which should apply in a regulatory context.Applying grouping in regulatory risk assessments should enable read-across, that is, filling a data gap by using information on one ENM, or a nonENM, for another substance in the same group.The above-mentioned tools should be fitted into a risk assessment strategy for ENMs. This strategy should be flexible enough to address different assessment goals depending on the user’s needs, considering all data already available as a starting point, contingent upon data quality evaluation and selecting the most appropriate tools to fill existing data gaps. Such a strategy should ideally be exposure driven, starting with identifying the most relevant exposure scenarios in the ENM life cycle, and evaluating completeness and quality of the available data from a risk assessment perspective. This facilitates careful prioritization of ENMs to optimize testing efforts and can inform more realistic ecotoxicological investigations. Doing so can allow one to screen-out irrelevant exposure routes, eliminate unnecessary testing, and support prioritization of exposure scenarios. Exposure assessment should begin with an analysis of plausible exposure scenarios; where none is expected, further testing may be precluded for the applicable use patterns and volumes.Researchers and regulators need to understand actual exposures at biological receptors. This exposure-driven approach can also provide important information on realistic environmental conditions to affect test designs for improved interpretation of laboratory toxicology studies. Such practices can ensue in the interim, while research continues to discover best hazard assessment practices. Experimental ENM toxicity assessments, using ecologically relevant receptors and across linked biological levels of organization, should inform developing and parametrizing dynamic process-based models. Such models should respond to future scenarios and predict impacts.