What is zero valent iron used for?

Term denoting metallic iron, Fe(0), used for permeable reactive barrier in site remediation

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Zerovalent iron (ZVI) is jargon[clarification needed] that describes forms of iron metal that are proposed for use in groundwater remediation.[1][2][3][4]

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Model B

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ZVI operates by electron transfer from Fe0 toward some organochlorine compounds, a common class of pollutants. The remediation process is proposed to generate Fe2+ and Cl&#; and halide-free organic products, all of which are relatively innocuous.[5] Nanoscale ZVIs (nZVIs) are commonly used in remediation of chlorinated compounds and other pollutants.[6]

Type of ZVI

• Bulk Fe. Cast iron, consisting of scrap iron of construction grade, has been used as a reactive material for permeable reactive barriers for groundwater remediation. Reactions are generally believed to occur on the Fe (oxide) surface; however, graphite inclusions have been shown can also serve as a reaction sites.

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• Nanoscale Fe. In addition to using macroscale iron in PRBs, nanoparticles (1-100 nm diameter) of zerovalent iron (nZVI) are effective.

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• Zn. Zinc has showed much higher reactivity toward pentachlorophenol than iron. This indicates that zinc may be used as a replacement for ZVI in dechlorinating chlorinated phenols. Chlorinated phenols are sequentially dechlorinated and thus less chlorinated phenols have been identified as a reduction product.

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Type of contaminants treated

Many kinds of pollutants have been proposed, but few have been demonstrated in solving environmental challenges.

Further reading

• Tratnyek, P. G.; M. M. Scherer; T. J. Johnson; Matheson, L.J. (). Permeable reactive barriers of iron and other zerovalent metals. In: Tarr M. A. (ed.), Chemical Degradation Methods for Wastes and Pollutants; Environmental and Industrial Applications. Environmental Science and Pollution Control, Marcel Dekker, New York, pp 371&#;421. doi:10./.ch9

Notes

About In Situ Chemical Reduction (ISCR)

S-MicroZVI and other ZVI products are used in soil and groundwater remediation as an in situ chemical reduction (ISCR) reagents. Chemical reduction is the process of adding or donating electrons to contaminants, while chemical oxidation is the process of removing or accepting electrons from contaminants. The ZVI acts as a reducing agent to provide electrons directly to the contaminant for degradation or to support processes that require electrons to degrade contaminants.

Abiotic Degradation
Zero-valent iron can provide an abiotic degradation pathway involving the direct reaction of ZVI with groundwater contaminants. The abiotic, &#;beta-elimination pathway&#; for chlorinated ethenes is shown in the bottom of Figure 1. The abiotic pathway involves short-lived dichloroacetylene and chloroacetylene intermediates, bypasses the formation cDCE and VC, and ultimately results in ethene and ethane.

Figure 1: ISCR or beta-elimination (double-line arrows). Beta-elimination avoids the formation of cDCE and VC.

Biological Degradation
Biological degradation involves the destruction of contaminants by anaerobic bacteria that are supported by the molecular hydrogen that is produced by the fermentation of organic hydrogen donors or by the reaction of ZVI with water. The biological degradation pathway for perchloroethene (PCE) and trichloroethene (TCE) is provided in Figure 2. This pathway, called reductive dechlorination (or hydrogenolysis), involves the sequential replacement of a chlorine atom with a hydrogen atom and is always accompanied by the formation cis-1,2-dichloroethene (cDCE) and vinyl chloride (VC). Many common anaerobic bacteria can transform PCE to TCE and then to cDCE, but only Dehalococcoides ethenogenes (DHC) can transform cDCE and VC to ethene.

Figure 2: Reductive dechlorination sequentially replaces chlorine atoms with hydrogen atoms. The intermediates cDCE and VC are more toxic than parent compounds PCE and TCE.

Supplementing dechlorinating bacteria with zero-valent iron and organic hydrogen donors can enable more rapid and complete biodegradation. Zero-valent iron quickly deoxygenates groundwater and provides an electrochemically reducing environment that is highly fertile for the microbes involved in anaerobic bioremediation. In many situations this favorable environment can be sustained for several years.

ISCR-Enhanced Bioremediation
ISCR-enhanced bioremediation is a term describing the remediation approach that combines zero-valent iron (ZVI), an organic hydrogen donor, and contaminant-degrading microbes (native or bioaugmented) in order to degrade contaminants in soil and groundwater. This approach is most commonly used for chlorinated contaminants, e.g. chlorinated ethenes. ISCR-enhanced bioremediation is particularly effective because it stimulates anaerobic biological degradation by rapidly creating a reducing environment favorable to reductive dichlorination. Furthermore, ISCR-enhanced bioremediation may limit the formation of undesirable daughter products such as cDCE and VC by degrading parent compounds via direct chemical reduction.

ISCR-enhanced bioremediation can be used to treat contaminants such as chlorinated solvents, haloalkanes, and chlorinated pesticides. Contaminants that are resistant to abiotic degradation (e.g. 1,2-dichloroethane or dichloromethane) and compounds that can inhibit bioremediation (e.g. 1,1,1-trichloroethane or chloroform) may be effectively treated by ISCR-enhanced bioremediation. ISCR-enhanced bioremediation can be used for source zones, plumes, and barrier applications.

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