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Nanoremediation: Information for Decision Makers from NanoRem

Thematic Page 8 Managing Deployment Risks

Contents

1.  Aim
2.  Considering Introduced nZVI as a Contaminant Source Term
3.  Considering Pathways for Introduced nZVI
4.  Considering Receptors for Introduced nZVI

5.  Developing a Conceptual Site Model
6.  NanoRem Activities
7.  Additional Resources on the NanoRem Web Site
8.  References
9.  Feedback and Opinion

1 Aim


nZVI is the dominant nanoparticle used in remediation to date.
In common with many other technologies utilised for the in situ treatment of contaminated soils and groundwaters, nZVI can pose health and safety risks if improperly handled. The aim of this page is to review how potential deployment risks for nZVI (if they exist) might be managed.  

A sound understanding of the fate and transport of nZVI is needed to ensure remediation objectives are achieved while negative impacts are avoided (Karn et al. 2009). These considerations will be site specific. The use of a conceptual site model and the source – pathway – receptor paradigm provide a framework for determining likely deployment risks and how they might be managed. For the purposes of clarity this discussion refers to risks from the use of introduced nZVI as “deployment risks” to distinguish these risks from the risks from contamination being managed as a remediation process. 


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

A more detailed treatment of how NanoRem assesses and manages deployment risks for its case study sites is provided in TP9 Outline risk assessment protocol for field nanoremediation deployments within the NanoRem project.

2 Considering Introduced nZVI as a Contaminant Source Term

 

Key considerations are:
  • The type of nZVI, mass/rate and mode of deployment
  • Interaction of nZVI with aquifer materials in the treatment zone
  • Interaction of nZVI with groundwater in the treatment zone
  • Interaction of nZVI with target contaminants in the treatment zone 


nZVI is most likely to be introduced into the subsurface via an injection process (although PRB applications are considered possible) (see Thematic Page 1: Application of nZVI in Remediation). The type of the nZVI and its mode of deployment may impact on the speed of reaction, the proportion of unreacted particles that can potentially leave the treatment zone, and the release of other substances, for example metal ions from BNP, or surfactant coatings. Potential reactivities and selectivities are different for different types of nZVI and also dependent on their surface modifications. The reactivity realised in the subsurface is also a function of subsurface conditions (US EPA 2008). Hence, the mass deployed is likely to be well in excess of that stoichiometrically required to treat the target contaminant/s to compensate for the interactions between the nZVI and the subsurface environment. Ideally all nZVI is consumed in the treatment zone, in which case there would be no downstream pathway and hence no deployment risk. However, this may be difficult to achieve. It will usually be assumed that some proportion of the particles may escape, either in pristine or reacted form and monitoring for the release of “renegade” particles may be required.

3 Considering Pathways for Introduced nZVI

Currently practical nZVI applications are limited to remediation in the saturated zone. Hence assuming that nZVI has been properly handled and administered then pathways to human receptors via direct contact or via air borne dusts will not exist. Pathways of most concern will be via movement of water in the subsurface, away from the zone of nZVI introduction. Physical characteristics of the receiving aquifer largely determine transport in the saturated zone. Important characteristics include (Kim et al. 2012, O’Carroll et al. 2013, US EPA 2008):
 

  • Hydraulic gradient and conductivity (permeability), as these determine the quantity of water travelling through the aquifer.
  • Aquifer chemistry, in particular, ionic strength, redox and pH.
  • Aquifer surface characteristics.
  • Degree of sorting of porous media. This strongly dictates the (non-retarding) residence time of particles being transmitted.
  • Groundwater average linear velocity.
  • Effective porosity of aquifer (the porosity available which is interconnected and can usefully transmit water flow.

nZVI will be transported as a particle rather than as a solute. Conventional solute transport models are therefore not valid to use, and risks based on nZVI migration from the treatment zone will be based on colloidal transport theory.


nZVI application may affect aquifer porosity, at least in a localised way, depending on mode of deployment, e.g. by causing clogging near injection points. As a result of this altered porosity, the likelihood of nZVI migration to surface water receptors may also be affected.


Distance to the receptor clearly makes a significant difference to the likelihood of any negative outcomes of risks being realised. By increasing the ‘pathway’ length, greater distance to the receptor provides the equivalent of a safety coefficient to attenuation processes such as passivation, agglomeration, filtration and sorption of the original form of nZVI.  


4 Considering Receptors for Introduced nZVI


Given that the pathways of concern are via the migration of water in the subsurface, the receptors of principal concern will be the biology of surface water bodies which may be in continuity with and fed by the groundwater within the aquifer, groundwater source protection zones, or anywhere else understood to host aquatic life. Groundwater itself is a receptor as well as a pathway and regulators will set or negotiate a compliance point at a site-specific distance from the treatment zone. The iron itself is unlikely to be of concern to regulators as a source of dissolved iron ions, but coatings and other modifiers may be considered as hazardous substances


The microbiology of groundwater could also be considered a receptor in its own right. However, all in situ remediation techniques impact aquifer ecology (e.g. by changing one or more of redox potential, pH, temperature, hydraulic conductivity, contaminant accessibility and availability, substrate availability). Such perturbations are usually short-lived and indeed are needed to effect treatment of the target contaminant(s). It may be viewed as disproportionate to regulate nZVI use based on its impacts on subsurface biology or pH-redox changes; a requirement not usually made of established in situ remediation techniques.  



5 Developing a Conceptual Site Model


Table 1 summarises possible pollutant linkages (connected source terms, pathways and receptors). Assuming adequate control of the above ground handling and injection of NPs into the subsurface, the most important pollutant linkage where there may be a significant possibility of harm is the risk of negative impacts on surface water biology, should nZVI migrate from groundwater treatment to surface water.


Table 1:  Possible pollutant linkages for nZVI used in remediation



 Source  Pathway  Receptor  Comments
 

nZVI: handling and transportation
Air Human health Should be effectively managed by
adequate health and safety provisions
Dermal / Ingestion Human health
nZVI powder / slurry mixing and application
 

 
Air Human health
Dermal / Ingestion (direct) Human health
Dermal / Ingestion (direct) Ecology Possible but should be managed by good site practice and human health and safety provisions
Injected sub-surface active nZVI
 

 

 
Dermal / Ingestion via aquifer –
 controlled waters receptor
Human health Perhaps an unlikely scenario given use
 at depth and dilution through an aquifer
Dermal / Ingestion via aquifer –
controlled waters receptor
Ecology and natural soil and
surface water microbial processes
Limited duration and extent to an already
 severely impacted water body
Aquifer Groundwater Potential negative impact from surface modifications
(e.g. surfactants) and metal ions released from BNP.
 Short-term perturbation in aquifer conditions
(e.g. redox, pH, microbial community)
Soils – Plant uptake Ecology Seen as an unlikely scenario given use at depth
 in the saturated zone
Soils – Plant uptake –dermal / ingestion Human health Seen as an unlikely scenario given use at
depth in the saturated zone


A conceptual site model for this pollutant linkage is illustrated below. However, it should be noted that a conceptual site model should be developed on a project-by-project basis to take account of site and NP-specific conditions and properties. This figure differentiates between nZVI directly injected into the subsurface and nZVI that might be partially constrained (e.g. within a partially contained barrier). As nZVI in the subsurface is not volatile, the primary transport mechanism in the pathway will be via aqueous advective dispersion. The remainder of these sections reviews the components of this pollutant linkage and the consequent information requirements needed for the regulation of in situ use of nZVI.






6 NanoRem Activities

Given the “dread” associated with the use of nanotechnologies which appears to lead to a heightened perception of risk of this technology’s use (See Thematic Page 5 Risk Perception Issues), NanoRem will:
 
  • Undertake fully documented field trials to establish the fate and transport of nanoparticles within the subsurface, preceded by large scale contained tank experiments to investigate nanoparticle fate and transport.
  • Ensure all field deployments will be subject to deployment risk assessment (see Thematic Page 9: Summary of the renegade nanoparticle risk assessment protocol for NanoRem field deployments), and outcomes will be reported with detailed conceptual site models.
  • Undertake toxicity testing on both raw and weathered nanoparticles.
  • Widely disseminate the information to ensure that the research development and demonstration outputs meet end-user and regulatory requirements and that information and knowledge is shared widely across the EU.



7 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





8 References
 

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/
 

KIM, H. J., PHENRAT, T., TILTON, R. D., AND  LOWRY, G. V. 2012 ‘Effect of kaolinite, silica fines and pH on transport of polymer-modified zero valent iron nano-particles in heterogeneous porous media’ Journal of Colloid and Interface Science, 370, 1, 1-10.

http://www.sciencedirect.com/science/article/pii/S0021979711015487
 

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
 

US ENVIRONMENTAL PROTECTION AGENCY 2008a. Nanotechnology for site remediation Fact Sheet.  [Online] Available at:

http://epa.gov/ncer/publications/workshop/pdf/10_20_05_nanosummary.pdf

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