View source for Zerovalent Iron (ZVI) (Chemical Reduction - ISCR)

Zerovalent iron (ZVI) is the most commonly used zerovalent metal (ZVM) for environmental remediation. ZVI is typically applied as a [https://en.wikipedia.org/wiki/Reducing_agent reductant] and is capable of transforming (degrading) or sequestering a variety of contaminants found in groundwater and soil.
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'''Related Article(s):'''
*[[Remediation Technologies]]
*[[Metal and Metalloid Contaminants]]


'''CONTRIBUTOR(S):''' [[Dr. Alexandra Salter-Blanc]]


'''Key Resource(s):'''
*[http://dx.doi.org/10.1201/9780203912553.ch9 Permeable Reactive Barriers of Iron and Other Zero-Valent Metals]<ref name = "Tratnyek2003">Tratnyek, P.G., Scherer, M.M., Johnson, T.L. and Matheson, L.J., 2003. Permeable reactive barriers of iron and other zero-valent metals. In:Tarr MA, ed. Chemical Degradation Methods for Wastes and Pollutants: Environmental and Industrial Applications. New York, NY, Marcel Dekker, Chapter 9, pp. 371-422. [http://dx.doi.org/10.1201/9780203912553.ch9 doi: 10.1201/9780203912553.ch9]</ref>
*[http://dx.doi.org/10.1016/j.advwatres.2012.02.005 Nanoscale Zerovalent Iron and Bimetallic Particles For Contaminated Site Remediation]<ref name="O'Carrol2013">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, pp. 104-122. [http://dx.doi.org/10.1016/j.advwatres.2012.02.005 doi:10.1016/j.advwatres.2012.02.005]</ref>
*[http://dx.doi.org/10.1016/j.jhazmat.2013.12.062 The Use of Zero-Valent Iron For Groundwater Remediation and Watewater Treatment: A Review]<ref name="Fu2014">Fu, F., Dionysiou, D.D. and Liu, H., 2014. The use of zero-valent iron for groundwater remediation and wastewater treatment: a review. Journal of Hazardous Materials, 267, pp. 194-205. [http://dx.doi.org/10.1016/j.jhazmat.2013.12.062 doi:10.1016/j.jhazmat.2013.12.062]</ref> 

==Introduction==
ZVI is a reductant used in the treatment of organic and inorganic contaminants found in various environmental media, including groundwater and soil. ZVI has been shown to treat a wide variety of contaminants including [[Chlorinated Solvents| chlorinated solvents]], [[Metal and Metalloid Contaminants| metals and metalloids]], nitroaromatic compounds (including [[Munitions Constituents - Deposition| energetic compounds]]), nitrate, and certain dyes<ref name = "Tratnyek2003"/><ref>Zhang, W.X., 2003. Nanoscale iron particles for environmental remediation: an overview. Journal of Nanoparticle Research, 5(3-4), pp. 323-332. [http://dx.doi.org/10.1023/a:1025520116015 doi:10.1023/A:1025520116015]</ref><ref name="O'Carrol2013"/><ref name="Fu2014"/>. ZVI may be applied in both ex situ and in situ applications. When applied in situ, it is one of many [[Chemical Reduction (In Situ - ISCR)| in situ chemical reduction (ISCR)]] approaches. Below, we present an overview of basic ZVI redox chemistry, types of ZVI, applications, and contaminant treatment examples.

==Chemistry==
ZVI behaves as a reductant in many environmentally-relevant oxidation-reduction (redox) reactions. ZVI, which has a negative reduction potential, has the tendency to donate electrons (i.e., to act as a reductant and in turn be oxidized) in the presence of species with more positive reduction potentials. The redox half reaction that describes the oxidation of ZVI is:
[[File:Salter-Blanc-Chemistry-Equation 1.PNG|center|180 px]]

ZVI can participate in redox reactions with a number of oxidants. For example, ZVI can be oxidized by oxygen in the presence of water to form various iron (II) species. A common example is the rusting of iron over time when oxygen and water are present. The overall redox reaction for the oxidation of ZVI in water in the presence of dissolved oxygen (DO) is given below<ref name = "Tratnyek2003"/>:<br />
[[File:Salter-Blanc-Chemistry-Equation 2.PNG|center|300 px]]

ZVI can also be oxidized in water in the absence of DO, although this reaction is much slower<ref name = "Tratnyek2003"/>:
[[File:Salter-Blanc-Chemistry-Equation 3.PNG|center|300 px]]

In addition to oxygen and water, other oxidants, including certain contaminants, may also participate in redox reactions with ZVI. These reactions may result in the transformation (degradation) or immobilization of contaminants (see Contaminants Treated section below). The oxidation of ZVI also leads to the formation of various iron oxide species on the surface of the material, which can potentially adsorb contaminants (e.g., metal cations like Zn<sup>2+</sup>)<ref name = "Tratnyek2003"/>.

==Types==
Many forms of ZVI are currently used in remediation applications. These include:
*'''Granular ZVI.''' Granular ZVI is used in remediation applications and is typically construction- or industrial-grade material. Sources of granular ZVI include scrap iron (e.g., from the manufacture of automotive parts), which is milled down and sorted to specific grain sizes, as well as direct production of granular material from molten iron<ref name = "ITRC2011PRB">ITRC, 2011. Permeable Reactive Barrier: Technology Update. Interstate Technology and Regulatory Council. [[Media:ITRC-2011-PRB_Tech_Update.pdf|Report pdf]]</ref>.  
*'''Nano-scale ZVI (nZVI).''' Nano-scale ZVI is typically defined as ZVI particles with a diameter of 1 to 100 nanometers<ref name="Li2006">Li, X.Q., Elliott, D.W. and Zhang, W.X., 2006. Zero-valent iron nanoparticles for abatement of environmental pollutants: materials and engineering aspects. Critical Reviews in Solid State and Materials Sciences, 31(4), pp. 111-122. [http://dx.doi.org/10.1080/10408430601057611 doi:10.1080/10408430601057611]</ref>. nZVI typically has a greater reactivity than granular ZVI per mass of material. This is attributed to larger overall surface area of the nZVI particles<ref name="Tratnyek2006">Tratnyek, P.G. and Johnson, R.L., 2006. Nanotechnologies for environmental cleanup. Nano today, 1(2), pp. 44-48. [http://dx.doi.org/10.1016/S1748-0132(06)70048-2 doi:10.1016/s1748-0132(06)70048-2]</ref>. 
*'''Bimetallic nZVI.''' ZVI-based bimetallic particles are composed of ZVI with a second zerovalent metal deposited on the particle surface. Bimetallic particles of ZVI doped with palladium, platinum, nickel, silver, or copper have increased reactivity toward a number of contaminants when compared to ZVI alone<ref name="O'Carrol2013"/><ref name="Fu2014"/>. 
*'''Combination products.''' ZVI and nZVI are often combined with other amendments to capitalize on other remediation processes such as in situ bioremediation and sorption. For combined ZVI treatment and in situ bioremediation, biostimulants (carbon sources) are often combined with ZVI<ref name="Tratnyek2014">Tratnyek, P.G., Johnson, R.L., Lowry, G.V. and Brown, R.A., 2014. In situ chemical reduction for source remediation. In Chlorinated Solvent Source Zone Remediation, pp. 307-351. Springer New York. [http://dx.doi.org/10.1007/978-1-4614-6922-3_10 doi: 10.1007/978-1-4614-6922-3_10]</ref>. ZVI has also been combined with granular activated carbon with the intention of promoting reduction of sorbed contaminants<ref>Battelle, 2007. Demonstration of the AquaBlok Sediment Capping Technology: Innovative Technology Evaluation Report. U.S. Environmental Protection Agency, Washington, DC, EPA/540/R-07/008. [[Media:Battelle-2007-AquaBlok.pdf|Report pdf]]</ref>. Several commercial formulations are available for both combined approaches.  
Properties and reactivity of these various forms of ZVI can vary due to the presence of “impurities” (inherent or intentionally-added) in the materials; because of this, ZVI products that fall into the same category described above may behave differently in remediation applications. Systematic research into the effects of impurities is limited.

== Applications ==
ZVI is applied in a number of remediation applications such as:
*Permeable reactive barriers (PRBs)<ref name = "Tratnyek2003"/><ref name = "ITRC2011PRB"/> 
*In situ soil mixing<ref name="Tratnyek2014"/> 
*Injection-based treatments, including establishment of reactive treatment zones and source zone targeted injection<ref name="Tratnyek2006"/><ref>CityChlor, 2013. In Situ Chemical Reduction using Zero Valent Iron Injection, April 12, 2013. [[Media:CityChlor-2013-ZVI_Injection.pdf|Report pdf]]</ref><ref name="Tratnyek2014"/>

==Contaminants Treated==
ZVI has been studied and applied for the treatment of a number of contaminants. The processes responsible for contaminant removal can be grouped into two main categories<ref name = "Tratnyek2003"/>:
#'''Transformation'''. Transformation processes involve chemical reactions that convert a contaminant to distinct products.
#'''Sequestration'''. Sequestration processes do not involve contaminant degradation, but instead primarily involve adsorption, reduction, and co-precipitation. 
Here, we introduce reaction processes for some common families of contaminants: <br>

'''Chlorinated solvents'''. Many chlorinated compounds, including the chlorinated solvents tetrachloroethene (PCE), trichloroethene (TCE), and carbon tetrachloride, are degraded by ZVI in transformation reactions known as dechlorination reactions<ref name="Roberts1996">Roberts, A.L., Totten, L.A., Arnold, W.A., Burris, D.R. and Campbell, T.J., 1996. Reductive elimination of chlorinated ethylenes by zero-valent metals. Environmental Science & Technology, 30(8), pp. 2654-2659. [http://dx.doi.org/10.1021/es9509644 doi:10.1021/es9509644]</ref><ref name="Matheson1994">Matheson, L.J. and Tratnyek, P.G., 1994. Reductive dehalogenation of chlorinated methanes by iron metal. Environmental Science & Technology, 28(12), pp. 2045-2053. [http://dx.doi.org/10.1021/es00061a012 doi:10.1021/es00061a012]</ref><ref name="Fu2014"/><ref name="O'Carrol2013"/>. The dechlorination process is a redox reaction in which ZVI serves as the reductant and the chlorinated solvent is the oxidant. The two major relevant dechlorination pathways are hydrogenolysis and reductive β-elimination<ref name="Matheson1994"/><ref name="Roberts1996"/> <ref name="Arnold2000">Arnold, W.A. and Roberts, A.L., 2000. Pathways and kinetics of chlorinated ethylene and chlorinated acetylene reaction with Fe (0) particles. Environmental Science & Technology, 34(9), pp. 1794-1805. [http://dx.doi.org/10.1021/es990884q doi:10.1021/es990884q]</ref><ref name = "Tratnyek2003"/>. In hydrogenolysis (also known as reductive dehalogenation), a chlorine (or other halogen) atom in the molecule is replaced with a hydrogen atom. An example is the reduction of PCE to TCE<ref name="Arnold2000"/> as shown in the following redox reaction:<br />
[[File:Salter-Blanc-Contaminants Treated-Equation 4.PNG|center]]

In reductive β-elimination (also known as vicinal dehalogenation), two chlorine atoms (or other halogens) bonded to adjacent carbon atoms are removed, leaving behind two electrons. These electrons go on to form an additional bond between the carbons (i.e., a single bond becomes a double bond or a double bond becomes a triple bond). An example is the reduction of PCE to dichloroacetylene<ref name="Arnold2000"/> as shown here:
[[File:Salter-Blanc-Contaminants Treated-Equation 5.PNG|center]]

'''Heavy metals and metalloids'''. ZVI has been applied to treatment of arsenic and heavy metal species including chromium, lead, copper, zinc, and uranium species<ref name="Fu2014"/><ref name="O'Carrol2013"/><ref name="Li2006"/><ref name = "Tratnyek2003"/>. The process by which these contaminants are treated can generally be categorized as a sequestration process<ref name = "Tratnyek2003"/>. Sequestration of these contaminants typically involves an often complex set of processes, which may include adsorption, redox reactions, co-precipitation, and precipitation<ref name="O'Carrol2013"/>. These processes result in reduced mobility of the contaminants.
One common heavy metal contaminant treated with ZVI is hexavalent chromium (Cr(VI)). Cr(VI) is highly toxic and mobile<ref name="O'Carrol2013"/>. The sequestration process involves reduction of Cr(VI) to Cr(III) followed by Cr(III) immobilization through precipitation as Cr(OH)<sub>3</sub> or incorporation into an iron (hydr)oxide shell on the ZVI surface<ref name="O'Carrol2013"/><ref name = "Tratnyek2003"/> <ref name="Fu2014"/>.

'''Nitroaromatic compounds'''. Nitroaromatic compounds (NACs), including nitrobenzene, 2,4,6-trinitrotoluene (TNT), 2,4-dinitrotoluene, and 2,6-dinitrotoluene, can be transformed through reduction by ZVI<ref name="Fu2014"/>. The net reduction reaction for NACs (which is a combination of several individual reduction reactions) results in the formation of an amine, as shown below, where “Ar” represents the aromatic backbone of the molecule<ref name = "Tratnyek2003"/><ref name="Agrawal1995">Agrawal, A. and Tratnyek, P.G., 1995. Reduction of nitro aromatic compounds by zero-valent iron metal. Environmental Science & Technology,30(1), pp. 153-160. [http://dx.doi.org/10.1021/es950211h doi:10.1021/es950211h]</ref>: 
[[File:Salter-Blanc-Contaminants Treated-Equation 6.PNG|center]]

An example is the reduction of nitrobenzene to aniline<ref name="Agrawal1995"/>:
[[File:Salter-Blanc-Contaminants Treated-Equation 7.PNG|center]]

==Context and Outlook==
There are interconnected relationships between environmental applications of ZVMs such as ZVI and other established and emerging environmental technologies (Fig. 1)<ref name= "Tratnyek2010">Tratnyek, P., Salter, A.J., Nurmi, J.T. and Sarathy, V., 2010. Environmental applications of zerovalent metals. Iron vs. Zinc. Nanoscale Materials in Chemistry: Environmental Applications. In American Chemical Society. Washington, DC, pp. 165-178. [https://doi.org/10.1021/bk-2010-1045.ch009 doi: 10.1021/bk-2010-1045.ch009]</ref>. For example, ZVI was first popularized for environmental remediation because of the overlap of ZVM with PRBs. ZVI-based PRBs are now a well-established technology and applications are numerous. However, not all applications of ZVMs are PRBs, just as not all PRBs are constructed with ZVMs (Fig. 1). There is also overlap of ZVM with the growing and diverse field of nanotechnology, including the use of nZVI. Current ZVI research focuses primarily on nZVI, particularly its synthesis, modification, enhancement, and migration in environmental media. Other areas of current and future ZVI research include examining the influence of sources and impurities on reactivity, the use of combination products, and prospects for treating emerging contaminants.
[[File:SalterBlanc Figure1 ZVI.png|thumbnail|center|500 px|Figure 1. Overlap between areas of environmental technology related to ZVMs such as ZVI (reprinted with permission from Tratnyek et al. 2010<ref name= "Tratnyek2010"/>.]]

==References==

<references/>

==See Also==
[https://clu-in.org/techfocus/default.focus/sec/Permeable_Reactive_Barriers,_Permeable_Treatment_Zones,_and_Application_of_Zero-Valent_Iron/cat/Overview/  Overview of Permeable Reactive Barriers, Permeable Treatment Zones, and Application of Zero-Valent Iron]
[https://clu-in.org/techfocus/default.focus/sec/In_Situ_Chemical_Reduction/cat/Overview/ Overview of In Situ Chemical Reduction]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-107 Permeable Reactive Barriers for In-Situ Treatment of Chlorinated Solvents]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-1125 Influence of Groundwater Constituents on Longevity of Iron-Based Permeable Barriers]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-1140  Evaluation of Performance and Longevity at DoD Permeable Reactive Barrier Sites]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminants-on-Ranges/Protecting-Groundwater-Resources/ER-1232  Remediation of Explosives Contaminated Groundwater with Zero-Valent Iron]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-1485  Fundamental Study of the Delivery of Nanoiron to DNAPL Source Zones in Naturally Heterogeneous Field Systems]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-1487 Development and Optimization of Targeted Nanoscale Iron Delivery Methods for Treatment of NAPL Source Zones]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-199907  Evaluation of Performance and Longevity at Permeable Reactive Barrier Sites]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-200017  Treatment of Chlorinated Hydrocarbon Contaminated Groundwater with Injectable Nanoscale Bimetallic Particles]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/ER-200223  Remediation of TNT and RDX in Groundwater Using Zero-Valent Iron Permeable Reactive Barriers and Zero-Valent Iron In Situ Treatment Wells]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-200431  Emulsified Zero-Valent Nanoscale Iron Treatment of Chlorinated Solvent DNAPL Source Areas]