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Nanoremediation: Information for Decision Makers from NanoRem

Thematic Page 4: Factors Affecting Potential Deployment Risks from nZVI Release into the Environment

Contents


1.  Aim
2.  Introduction
3.  Context of Natural Iron NPs within the Environment
4.  Transport of nanoparticles in the Environment
5.  Toxicity of Nanoparticles
6.  Persistence, Bioaccumulation and Biomagnification of Nanoparticles
7.  Human Exposure to Nanoparticles due to Occupational Exposure
8.  NanoRem Activities
9.  Additional Resources on the NanoRem Web Site
10.  References
11.  Feedback and Opinion


1 Aim



nZVI is the dominant nanoparticle used in remediation to date. The aim of this page is to summarise the state of current knowledge on the fate and transport of nZVI particles within the subsurface, and on the consequent risks posed by these particles to human health and the environment. 
 

This thematic page summarises information provided by a chapter of the NanoRem report: 'A Risk/Benefit Appraisal for the Application of Nano-Scale Zero Valent Iron (nZVI) for the Remediation of Contaminated Sites'. The full report including additional information, detail and referencing can be downloaded from: 
www.nanorem.eu/Displaynews.aspx?ID=525


2 Introduction



Nanoparticles within the subsurface can only present a risk to a receptor, if the particles travel from the point of injection through the groundwater and make contact with this remote receptor. The risk to the receptor is a function of:
 
  • Fate – will the nanoparticles survive in the environment?
  • Transport – will the nanoparticles reach the receptor?
  • Toxicity – how will the nanoparticles harm the receptor? 
 

The fate, transport and toxicity of the nZVI nanoparticles will depend on the composition of the particles, including any coatings, the particle size; and the target organism.
 

Additional information can be found in other FAQs and thematic pages: 
Thematic Page 1: Application of nZVI in Remediation
Thematic Page on: Outline Risk Assessment Protocol for Field Nanoremediation deployment within the NanoRem Project,
FAQ: Nanoremediation: Information for Decision Makers from NanoRem


3 Context of Natural Iron NPs within the Environment 



Iron is a major component of many soil-forming minerals and naturally occurring nanoparticles occur in the form of colloids including clay, oxides and organics, which represent the vast majority of nanoparticulate matter experienced within the natural environment. These naturally occurring nanoparticles exhibit cluster forming behaviour and it has been suggested (Gilbert et al. 2007) that nZVI particles will behave similarly to natural nanoparticles (containing iron) and thus will be transformed into various iron oxyhydroxides. However, this proposition has not been universally accepted and the UK Government (HM Government 2010) has taken the position that “methodologies to determine trace levels and the state of manufactured nanoparticles in complex media such as soils, sediments and waters are required.” This task forms a major research area for the NanoRem project.
 

Laboratory studies have shown that nZVI particles (especially unmodified particles) (see: Thematic Page 1 : Application of nZVI in Remediation) tend to agglomerate very rapidly and produce potentially stable micrometre-sized clusters (Tratnyek & Johnson 2006, Elliott 2010). If this occurs, nZVI will adopt the behaviour of larger sized environmental colloids resulting in the loss of nano-specific characteristics, and a reduction in the exposed reactive area of the iron. Passivation may also occur through the reaction of the nZVI with non-target contaminants, groundwater constituents, (such as nitrate and dissolved organic matter), the subsurface matrix, and dissolved oxygen. The longevity of nZVI is thus very highly specific to geological conditions.
 

O’Carroll et al. (2013) stated that all nZVI particles form an outer (hydr)oxide layer in aqueous solutions as a result of reactions with water, which dramatically reduces the reactivity of the particle, as even with the zero valent iron core the particle would still be expected to essentially behave chemically as iron hydroxides.  However, nZVI with an iron hydroxide shell may retain the capacity for contaminant reduction (see Figure 1) as the iron hydroxide layer may still allow electron transfer from the inner metal core, through defects in the shell surface or via the oxide conduction band. Alternately, electron transfer may occur via Fe2+ sorbed on the particle’s surface and the hydroxide shell may act as a contaminant adsorbent in its own right (O’Carroll et al. 2013).
 

 

Figure 1 Core–shell structure of nZVI depicting various mechanisms for the removal of metals and chlorinated compounds (O’Carroll et al. 2013, adapted from Li et al. 2006).

 

Over time, the shell tends to expand while the core shrinks, but core reactivity can remain for a period of several months or longer (Liu & Lowry 2006).


4 Transport of Nanoparticles in the Environment



Until recently the lack of any effective analytical technique to determine nanoparticle concentration in environmental media has significantly hampered reliable reporting on the transportation and fate of nanoparticles. Recent experiments have used indirect methods such as measurement of total Fe and tracer injection in column experiments, and the use of multiple methods at field-scale, including indirect measurements such as pH and ORP as well as measuring specific conductance as a conservative tracer. One of the aims of the NanoRem project is the development and utilisation of a methodology to determine the distribution and concentration of nanoparticles within the subsurface.
 

The active lifespan of nanoparticles and the distance they can travel from the point of injection is highly dependent on both the nanoparticle and the subsurface conditions. For example the presence of calcium ions within the groundwater has been shown to increase aggregation of particles. It has even been suggested that the injection of calcium ions could be used as a barrier to transport.  There is still considerable debate within the literature but the broad consensus is that under ideal conditions nZVI particles may remain active for up to 1 – 2 years, but are unlikely to travel more than 3 – 4 metres from the point of injection in sand and gravel aquifers.


5 Toxicity of Nanoparticles
 

Many of the concerns regarding nZVI applications in remediation are related to whether or not it is toxic and likely to negatively affect ecosystem and human health. Concerns regarding the toxicity of nZVI can be broadly split into those related to the toxicological effect of the actual iron and those related to the particles’ nano-scale size. 
 

Iron is an essential element for growth and a deficiency of iron can cause ill health effects, though it should be noted that iron can be toxic in excess.  In terms of toxicity related to the nano-scale size of nZVI, several papers have suggested that nanoparticles below 30nm in size may behave differently to naturally occurring forms and may be cytotoxic.  Auffan et al. (2009) proposed a sub division of NPs into those smaller and those larger than 30 nm, as the larger NPs essentially behaved as their bulk counterparts.  As most nanoremediation projects use particles in the 10 – 100 nm size range (O’Carroll et al. 2013, Karn et al. 2009), it is possible that many injected nanoparticles would not pose toxicity issues over and above conventional ZVI applications.
 

Many of the toxicity studies reported have used concentrations of iron in the 100mg/l range which is greatly in excess of any concentrations likely to be experienced in the sub-surface environment beyond a nanoremediation treatment zone.
 

Studies have produced somewhat contradictory results with some researchers finding toxic effects, while others reported no significant toxicity issues for nZVI. It is likely that that nZVI toxicity is highly dose and species dependent, and Saccá et al. (2013) concluded that it is unrealistic to thoroughly establish the toxicity of nZVI in soil based on currently available evidence. A full review of arguments for and against toxicity can be found in Bardos et al. (2014).
 

It has been suggested that other constituents of nanoparticles such as coatings used to increase mobility, or the addition of trace amounts of rare metals to form bimetallic nanoparticles (BNP) (see: Thematic Page 1: Application of nZVI in Remediation) could additionally affect the toxicity of the nanoparticle. The amount of rare metals added to the iron nanoparticle is typically below 0.1 %, so when it is considered that the concentration of nanoparticles injected into the aquifer is typically in the region of several kg/m3, the concentration of rare metal within the ground is likely to be insignificant and below any groundwater standards at compliance points.  The coatings used to modify nanoparticles are generally non-toxic materials such as bi-polymers. However some coatings such as surfactants may have an adverse effect in the environment, though this is no different from alternative in situ technologies which utilise surfactants.
 

There has been little study of the toxicity of spent NPs. However, Phenrat et al. (2009b) demonstrated that the oxidized form of one type of nZVI was far less toxic than the fresh nZVI with Fe0 in its core, and Auffan et al. (2009b) showed that the toxicity of the Fe series decreased as the materials became more oxidized, and stated that “analysis of published data suggests that
chemically stable metallic nanoparticles have no significant cellular toxicity whereas nanoparticles able to be oxidised, reduced or dissolved are cytotoxic and even genotoxic for cellular organisms”.
 

Toxicity may also arise from other constituents of the injected nanoparticles such as coatings (to improve stability) or the metals used to form BNP, though few toxicological studies have been carried out. Hildebrand et al. (2010) found limited toxicological effect of palladium doped nano-scale iron oxides, though it should be considered that the low percentage of noble metal used in the BNP (0.1%) results in very low concentrations within groundwater. Coatings are generally non-toxic materials such as biodegradable polymers; however some surfactants are potentially toxic within the environment. Additional work is required to quantify any risk.
 

The use of nanoremediation may introduce indirect effects into the aquifer, such as reduced hydraulic conductivity, changes in pH or redox. However, these issues are similar to those associated with other in situ technologies utilising chemical reagents.


6 Persistence, Bioaccumulation and Biomagnification of Nanoparticles

 

While the US EPA has raised the possibility of biomagnification of nanoparticles there are few data currently existing to either prove or disprove this hypothesis and the data tend to be contradictory. Furthermore, it has been suggested that overall, bioaccumulation and biomagnification of nanoparticles are unlikely to occur due to rapid passivation limiting the bulk of added nZVI to the injection zone, with few nZVI particles being transported to areas where exposure of external organisms to nZVI could take place. It is therefore probable that exposure is likely to be restricted to microorganisms within the treatment zone (Keane 2002). 


7 Human Exposure to Nanoparticles due to Occupational Exposure



The most likely human exposure route to nanoparticles is occupational exposure during site operations, and this should be significantly restricted by the use of appropriate PPE. In the case of ingestion, it has been postulated that nanoparticles below 20 nm can pass through the intestinal barrier and are likely to end up in the bloodstream (Gatti & Rivasi 2002). However, most nanoremediation projects use particles in the 10 – 100 nm size range so this should not be an issue. In any case, once such fine particulate iron enters the low pH stomach environment, it will end up in an ionic form. Ionic iron, at low levels, can then be metabolised as a nutrient.
 

Inhalation is a potentially more significant exposure route, especially if the nanoparticles are delivered to site as a powder, though the use of appropriate PPE should mitigate this route. However, it is possible that spillage during injection may leave residual particles coating equipment. Once dry, these fine powders could be a source of exposure, but due to oxidation in air these particles would be expected to be largely Fe-oxide aggregates at this point and thus no longer likely to pose a significant threat as a redox-active material.


8 NanoRem Activities



The uptake of nano technology for the remediation of contaminated groundwater has not been as rapid as was originally anticipated. Amongst the reasons cited for the limited uptake, is uncertainty in the toxicology of nanoparticles within the environment, and the lack of analytical techniques capable of tracing the distribution and the active life span of nanoparticles (or lack of reliable delivery means).
 

The NanoRem project aims to address these issues by:
 
  • Developing techniques to accurately determine the concentration of NPs.
  • Utilising these techniques to measure the distribution and concentration of NPs in the sub-surface after injection both at a number of test sites, and in reaction chambers at Stuttgart University.
  • Using this data to calibrate fate and transport models for nanoparticles within the natural environment.
  • Undertaking toxicity testing on both raw and weathered nanoparticles.
  • Holding a workshop with acknowledged experts in toxicology and risk assessments to identify the actual risks presented by nanoparticles to human health and the environment.

9 Additional Resources on the NanoRem Web Site
 

This thematic page summarises information provided by a chapter of the NanoRem report: 'A Risk/Benefit Appraisal for the Application of Nano-Scale Zero Valent Iron (nZVI) for the Remediation of Contaminated Sites'. The full report including additional information, detail and referencing can be downloaded from: 
www.nanorem.eu/Displaynews.aspx?ID=525.
 

Additional summary information is also available on the following online pages:


FAQs
 

Currently we have the following FAQ pages:
 

 
 


THEMATIC PAGES
 



10 References


AUFFAN, M., ROSE, J., BOTTERO, J. Y., LOWRY, G. V., JOLIVET, J. P., AND WIESNER, M. R. 2009a. ‘Towards a definition of inorganic nanoparticles from an environmental, health and safety perspective’, Nature, 4, 10, 634-641.
http://wiesner.cee.duke.edu/node/847
 

GATTI, A.M. AND RIVASI, F. 2002 ’Biocompatability of micro- and nanoparticles. Part I: in liver and kidney’, Biomaterials, 23, 11, 2381-2387.

http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0CCAQFjAA&url=http%3A%2F%2Fwww.researchgate.net%2Fpublication%2F11358796_Biocompatibility_of_micro-_and_nanoparticles._Part_I_in_liver_and_kidney&ei=ICMTVKu9JsPy7Aa4lYGYBQ&us
 

ELLIOTT, D.W. 2010 CLU-IN Webinar Presentation. [Online] Available at:
http://www.clu-in.org/conf/tio/nano-iron/prez/1280x1024/ppframe.cfm?date=502
 

GILBERT, B., LU, G. AND KIM, C.S. 2007 'Stable cluster formation in aqueous suspensions of iron oxyhydroxide nanoparticles', Journal of Colloid and Interface Science, 313, 1, 152-159.

http://www.sciencedirect.com/science/article/pii/S0021979707004791
 

HM GOVERNMENT 2010.  ‘UK Nanotechnologies Strategy- Small Technologies, Great Opportunities’. First published March 2010 © Crown Copyright URN 10/825

http://www.bis.gov.uk/assets/BISPartners/GoScience/Docs/U/10-825-uk-nanotechnologies-strategy
 

KARN, B., KUIKEN, T. AND OTTO, M.  2009 ‘Nanotechnology and in situ remediation: A review of the benefits and potential risks’, Environmental Health Perspectives, 117, 12, 1823-1831. DOI:10.1289/ehp.0900793.
http://ehp.niehs.nih.gov/0900793/
 

KEANE, E. 2009 ‘Fate, Transport, and Toxicity of Nanoscale Zero-Valent Iron (nZVI) Used During Superfund Remediation’, United States Environmental Protection Agency Report. [Online] Available at:
http://83.cebed1.client.atlantech.net/download/studentpapers/Zero-Valen-Iron-Keane.pdf
 

LIU, Y.Q. AND LOWRY, G.V. 2006 ‘Effect of particle age (Fe-o content) and solution pH on NZVI reactivity: H-2 evolution and TCE dechlorination’. Environmental Science and Technology, 40, 19, 6085-6090.
 

O’CARROLL, D., SLEEP, B., KROL, M., BOPARAI, H., AND KOCUR, C. 2013 ‘Nanoscale zero valent iron and bimetallic particles for contaminated site remediation’ Advances in Water Resources, 51, 104-122.

http://www.sciencedirect.com/science/article/pii/S0309170812000358
 

TRATNYEK, P.G. AND JOHNSON, R.L. 2006 ‘Nanotechnologies for environmental cleanup’, Nano Today, 1, 2, 44-48.

http://www.sciencedirect.com/science/article/pii/S1748013206700482
 

SACCÀ, M. L., FAJARDO, C., COSTA, G., LOBO, C., NANDE, M., AND MARTIN, M. 2014 ‘Integrating classical and molecular approaches to evaluate the impact of nanosized zero-valent iron (nZVI) on soil organisms’, Chemosphere  107 104 184-9.

http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0CB0QFjAA&url=http%3A%2F%2Fwww.ncbi.nlm.nih.gov%2Fpubmed%2F24287264&ei=BR8TVLqdB9Pn7AalDg&usg=AFQjCNFaRZdPKC_kD8IwkQiPTXeor4CTmA

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