3. Types of nZVI Particles Used in Remediation Projects
4. Application of nZVI in the Field
5. NanoRem Activities
6. Additional Resources on the NanoRem Web Site
8. Feedback and Opinions
The aim of this page is to outline the use of nano-scale zero valent iron (nZVI) for in situ remediation projects, how the particles are amended to enhance their longevity, and to describe how the particles are introduced into the ground. This thematic page focusses on nZVI as it has been the most widely used nanoparticle type in practice.
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.
For an environmental risk to exist, all of the following must be present - a source of contamination (S), a receptor (R) and a pathway(s) (P) linking the two - i.e. a contaminant linkage (S-P-R). If any of the above links are absent then there is no environmental risk. In most cases where a contaminant linkage exists, risk management is achieved using a combination of measures, such as contaminant mass removal (source); plume control (pathway management); and institutional controls (restricting use of a resource) to break this pollution linkage as shown in Figure 1 (Vegter et al. 2002, Nathanail and Bardos 2004).
A range of technologies have been developed to manage contaminant linkages utilising civil engineering approaches, and physical, chemical and biological processes. A number of technologies are now available for in situ treatment of soil and groundwater. However, source reduction for the removal of dense non-aqueous phase liquid (DNAPL) sources is fundamentally limited by the accessibility and availability of the DNAPLs (Teutsch 2001). Complete mass removal is rarely possible and residual DNAPL is frequently trapped within the fine pore structure of the aquifer. As groundwater passes through the aquifer the DNAPL will slowly dissolve into the groundwater, which can give rise to continual low concentrations of contaminants within the groundwater, which may be in excess of groundwater threshold values at compliance points.
Figure 1 Possible Risk Management Actions
An emerging in situ technology is nanoremediation, to reduce the source and/or to manage the contaminant plume along the pathway. Since the first documented case of nZVI being used to address groundwater contamination in 2000, at Trenton, New Jersey, USA, up to 2013, over 70 nZVI nanotechnology remediation projects have been documented at either pilot or full scale worldwide.
The most common usage of nanoremediation documented in the field has been in pathway management where an in situ treatment zone is created by the direct injection of nanoparticles in order to treat a defined volume of aquifer. This technology will usually be targeted at parts of the plume with high dissolved contaminant concentrations. NanoRem is also investigating whether nanoparticle based remediation technologies can provide more effective source management approaches in comparison with other technologies.
3 Types of nZVI Particles Used in Remediation Projects.
A range of nanoparticles are being used or are in development for nanoremediation. Nanoparticles, such as nZVI, are by their nature highly reactive. Their active lifespan in situ is limited by a number of processes, namely:
- Agglomeration - where nZVI particles are attracted to each other and aggregate into larger particles. In almost all cases this reduces their effective surface area and their mobility in water.
- Passivation - where nanoparticle surfaces are chemically inactivated (although activity may remain within particles).
- Immobilisation in the aquifer solid matrix (e.g. through the processes of sorption).
These processes place limitations on treatment effectiveness by restricting the ability of nanoparticles to reach and treat contaminants in situ.
To overcome these limitations, a number of modifications have been developed to improve the effectiveness of nZVI by reducing the scale of agglomeration and the immediacy of passivation. These include:
- Stabilisation – using a range of coatings, including biopolymers such as starch, chitosan, and carboxymethyl cellulose.
- Emulsification - where aqueous nZVI is surrounded by an oil-liquid.
- Anchoring nZVI onto carbon, cellulose acetate, polymeric resin or silica to prevent agglomeration and aid dispersion of the nZVI.
Other modifications have included the development of bimetallic nanoparticles (BNP) which typically utilize small quantities (i.e. <1% by weight) of other metals (e.g. nickel, palladium, platinum, etc.) to serve as catalysts to increase reaction rates and the types / ranges of compounds that are amenable to reduction by the iron.
4 Application of nZVI in the Field.
There are two main methods by which nZVI particles could be applied in the field: direct injection into the aquifer, and potentially contained in situ treatment applications such as the matrix of an engineered permeable reactive barrier. Published field studies show the application of all nZVI, including modified forms, has been by direct injection.
4.1 Direct Injection
Direct injection is used in both source and pathway management. In pathway management it is used to create an in situ treatment zone for a defined volume of aquifer.
The performance of directly injected remediation projects is constrained by the ability of the nZVI particles to disperse throughout the aquifer to ensure the entire contaminated plume is addressed. This accessibility is constrained by the permeability of the subsurface, subsurface heterogeneities and their potential to limit flow and/or create preferential pathways of flow and discontinuities such as the phase difference between the groundwater and the non-aqueous phase liquid (NAPL).
The aim of the injection process is to introduce a known amount of nZVI particles into the aquifer at a known depth, (see Figure 2). The injection may be gravity fed or introduced under pressure (either pneumatic or hydraulic). The main injection processes commonly used are:
- Direct push techniques involving a direct push rig or stationary injection point to introduce nZVI slurry into the treatment zone.
- Pneumatic or hydraulic fracturing uses air or water to create a fracture network of preferential flow paths around the injection point and enhance nZVI distribution.
- Pressure pulse technology uses regular pulses of pressure while injecting the nZVI slurry.
- Liquid atomization injection combines an nZVI-fluid mixture with a carrier gas (for example nitrogen) to create an aerosol that can be dispersed into the treatment zone.
- Injection via a gravity feed.
- Injection using foam surfactant carrying nZVI for delivery of nZVI into the vadose zone (developed to lab-scale: Ding et al. 2013, Shen et al. 2011).
To facilitate pumping the nZVI particles are generally introduced as a pumpable slurry, e.g. in water, vegetable oil or in gaseous nitrogen.
Figure 2 - Direct Injection of nZVI in the Field at the Trenton Facility, New Jersey (Photo courtesy of Geosyntec Consultants)
The operator may wish the natural groundwater process (advection and diffusion) to carry the reactive media away from the injection point, or may wish to actively control the process using pumping methods. This can include the use of pumps to direct the downstream groundwater flow or the use of recirculation pumps. Recirculation may be used via up-gradient injection wells and down-gradient extraction wells to improve delivery of the particles. In this case abstracted groundwater is reconditioned and mixed with additional nanoparticles and re-injected in the injection wells. (US EPA 2008).
4.2 Permeable Reactive Barriers (PRB)
A permeable reactive barrier or treatment wall treats dissolved phase contaminants in a fixed treatment zone within the groundwater flow path. The treatment zone can be considered conceptually in three parts: an active agent; a permeable matrix that supports and anchors the agent; and an amenable containment for this matrix. There are many possible PRB configurations ranging from highly engineered approaches like the Funnel and GateTM (Figure 3) where sheet piling or slurry walls direct groundwater flow in a funnel to an engineered treatment gate which contains the matrix and its active agents; to barriers where an active agent is simply injected or trenched in situ in a form that does not migrate. In this latter case the aquifer materials form the containment and the matrix.
Figure 3 - Funnel and GateTM and Continuous PRB Configurations (from Nathanail et al. 2007)
The use of micro sized zero valent iron within PRBs to treat plumes of chlorinated solvents has been well documented, and researchers have suggested that nanoparticles may also be suitable as the active component within the PRB matrix (Kanel 2007). The relatively short life span of active nZVI particles (typically 1 – 2 years) would limit their use unless the matrix can be recharged, though it has been postulated that iron BNPs stabilised in some kind of a matrix might have improved longevity. Furthermore, the matrix might render them more recoverable, and so facilitate the recycling of the precious metals used to dope the nZVI. Additional work needs to be undertaken to establish the use of nZVI as the active component within a PRB matrix.
Researchers have investigated the application of an applied electric field to enhance the mobility of nZVI in porous media (O’Carroll 2012). Their results suggested electrokinetic delivery enhanced migration of nZVI through the media studied and that utilization of an oxygen scavenger (Na2SO3) decreased oxidation of nZVI. In addition, stabilized nZVI transport was improved through electrophoresis compared to diffusion alone. Potentially similar techniques could be used to increase the zone of influence of wells in direct injection scenarios as well.
5 NanoRem Activities
The NanoRem project aims to overcome perceived problems associated with nanoremediation by undertaking a number of well-designed and fully documented field demonstrations in order to identify and overcome problems in the use of this technology, and thus encourage the uptake of nanoremediation for the treatment of contaminated groundwater. This information will then be widely disseminated to interested parties which will hopefully address any knowledge gaps and encourage the uptake of nanoremediation.
The anticipated information is expected to include, but not be limited to:
- The production of a number of different nanoparticles.
- The fate of nanoparticles within the saturated sub-surface.
- The effectiveness of these particles in treating different contaminants.
- Issues affecting scale up from bench to field using large tank experiments.
- The ease, or otherwise, of injecting the particles into the sub-surface.
- Methods for field scale observation of nanoparticle performance.
- Cost and performance information from field based case studies.
6 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:
Currently we have the following FAQ pages:
CHOWDHURY, A. I., O’CARROLL, D. M., XU, Y., AND SLEEP, B. E. 2012 ‘Electrophoresis enhanced transport of nano-scale zero valent iron’, Advances in Water Resources, 40, 71-82
DING, Y., LIU, B., SHEN, X., ZHONG, L., AND LI, X. 2013 ‘Foam-Assisted Delivery of Nanoscale Zero Valent Iron in Porous Media’, Journal of Environmental Engineering, 139, 9, 1206-1212.
KANEL, S.R., GRENACHE, J.M. AND CHOI, H. 2007 ‘Arsenic (V) removal from
groundwater using nano scale zero-valent iron as a colloidal reactive barrier material’,
Environmental Science and Technology, 40, 6, 2045-2050
SHEN, X., ZHAO, L., DING, Y., LIU, B., ZENG, H., ZHONG, L., AND LI, X. 2011 ‘Foam, a promising vehicle to deliver nanoparticles for vadose zone remediation’, Journal of Hazardous Materials 186, 2, 1773-1780.
Teutsch, G., Rügner, H., Zamfirescu, D., Finkel, M., AND Bittens, M. 2001 ’Source remediation vs. plume management: critical factors affecting cost-efficiency’, Land Contamination and Reclamation, 9, 1, 129-140
UNITED STATES ENVIRONMENTAL PROTECTION AGENCY (US EPA) 2008 ‘Selected Sites Using or Testing Nanoparticles for Remediation’, Office of Solid Waste and Emergency Response (OSWER Programme) Summary Sheet. [Online] Available at: