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 reductant and is capable of transforming (degrading) or sequestering a variety of contaminants found in groundwater and soil.

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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, metals and metalloids, nitroaromatic compounds (including energetic compounds), nitrate, and certain dyes[1][4][2][3]. ZVI may be applied in both ex situ and in situ applications. When applied in situ, it is one of many 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:

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[1]:

ZVI can also be oxidized in water in the absence of DO, although this reaction is much slower[1]:

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 Zn2+)[1].

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[5].
• Nano-scale ZVI (nZVI). Nano-scale ZVI is typically defined as ZVI particles with a diameter of 1 to 100 nanometers[6]. 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[7].
• 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[2][3].
• 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[8]. ZVI has also been combined with granular activated carbon with the intention of promoting reduction of sorbed contaminants[9]. 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)[1][5]
• In situ soil mixing[8]
• Injection-based treatments, including establishment of reactive treatment zones and source zone targeted injection[7][10][8]

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[1]:

1. Transformation. Transformation processes involve chemical reactions that convert a contaminant to distinct products.
2. 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:

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[11][12][3][2]. 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[12][11] [13][1]. 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[13] as shown in the following redox reaction:

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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[13] as shown here:

Heavy metals and metalloids. ZVI has been applied to treatment of arsenic and heavy metal species including chromium, lead, copper, zinc, and uranium species[3][2][6][1]. The process by which these contaminants are treated can generally be categorized as a sequestration process[1]. Sequestration of these contaminants typically involves an often complex set of processes, which may include adsorption, redox reactions, co-precipitation, and precipitation[2]. 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[2]. The sequestration process involves reduction of Cr(VI) to Cr(III) followed by Cr(III) immobilization through precipitation as Cr(OH)3 or incorporation into an iron (hydr)oxide shell on the ZVI surface[2][1] [3].

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[3]. 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[1][14]:

An example is the reduction of nitrobenzene to aniline[14]:

Context and Outlook

There are interconnected relationships between environmental applications of ZVMs such as ZVI and other established and emerging environmental technologies (Fig. 1)[15]. 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.

Figure 1. Overlap between areas of environmental technology related to ZVMs such as ZVI (reprinted with permission from Tratnyek et al.

References

See Also

Overview of Permeable Reactive Barriers, Permeable Treatment Zones, and Application of Zero-Valent Iron Overview of In Situ Chemical Reduction

The nZVI particles are, like other nanoparticles, usually smaller than 100&#;nm in diameter. They are highly reactive in oxic and aqueous environments, such as soils, forming an outer shell formed by Fe oxyhydroxides around the Fe(0) core. In order to produce nZVI material that would be easily manageable and applicable, nZVI producers provide forms stabilized by various coatings, e.g., carboxymethyl cellulose, polyacrylic acid, guar gum, etc., or Fe oxyhydroxides in suspension13,14,15. Fast nZVI oxidation can result in reduced reactivity, and surface passivation is thus essential for a successful application of nZVI in certain conditions16. This can be achieved by synthesizing various composites, e.g., with biochar17, zeolites18, etc.

Additionally, the interactions of nZVI with soil organic matter significantly influence its behavior and efficiency, as the evolution of the iron oxyhydroxide shell can be altered by dissolved organic matter in soils19. The actual composition of the Fe oxyhydroxides in the nZVI shell after it is equilibrated with the soil is mainly dependent on (i) the original nZVI coating; (ii) soil physico-chemical parameters, e.g., Eh, pH; (iii) composition of soil microorganisms communities, (iv) nZVI aging in soil; and include mainly the formation of ferrihydrite, magnetite, goethite and possibly lepidocrocite13,14,20, which can have different geochemical and adsorption properties. It is not possible to generalize which Fe oxyhydroxides will be formed in soils after nZVI application, and this highlights the need for further long-term studies involving soils with contrasting properties and microorganism communities.

The physical-chemical properties of engineered nanoparticles are one of the most critical factors controlling their environmental behavior. In this context, the processes at the soil-root interface play a vital role in the behavior of engineered nanoparticles and contaminants21,22. Different parameters such as soil type, organic matter content, pH, Eh, ionic strength, and aqueous chemistry can change aggregation kinetics, transformation, and subsequent behavior of engineered nanoparticles and their efficiency23. Natural organic matter alters its stability through electrostatic and steric interactions. The transformation process of engineered nanoparticles is controlled by a combination of factors, depending on the particles&#; characteristics and the environmental receptors10,24,25. Additionally, the changes in the hydraulic conditions in soils after nZVI application need to be considered26. However, studies describing the mobility of engineered nanoparticles in general, including nZVI, in the soil environment are often contradictory, showing that this area is still open to discussion.

Metal(loid) stabilization in soils treated with nZVI

The processes responsible for immobilizing metal(loid)s on the nZVI surface include mainly adsorption and, to some extent, reduction. The nZVI particles can remove various metal(loid)s simultaneously; however, the type of adsorption reaction involved in the process depends on the redox potential of metal species. While metals with a standard redox potential more negative than Fe(0) are rather adsorbed on the Fe oxyhydroxides formed in the nZVI shell, e.g., Cd, Zn, other metal(loid)s with a standard redox potential much more positive than nZVI can be reduced and precipitated when in contact with the zero-valent core, e.g., Cr, As, U, etc., or the combination of the two mechanisms, e.g., Pb, Ni10. The specific retention mechanisms, including the forms of the Fe oxyhydroxides in the shell and metal(loid) speciation after interactions with nZVI can be identified by various solid state analyses, i.e., X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS) techniques, such as X-ray absorption near edge structure spectroscopy (XANES) and extended X-ray absorption spectroscopy (EXAFS), X-ray diffraction (XRD), scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDS), and transmission electron microscopy (e.g., HR-TEM)10,14,20. However, recovering and separating the nZVI particles from the soil after application for further investigations is problematic, and specialized separation techniques, e.g., magnetic separation, must be used27. Sulfidation of nZVI particles can lead to promising results as it can sequester metals as sulfides, which could be resistant to subsequent reoxidation28,29.

Unfortunately, most studies on metal(loid) stabilization in soils using nZVI are only laboratory-based. However, the first obtained results were promising for subsequent field applications at a larger scale30. Arsenic is well known for its high affinity to Fe oxyhydroxides, and nZVI could thus present an attractive and efficient amendment. In one of the first studies, Gil-Díaz et al. evaluated As stabilization in a soil treated with nZVI, and the application significantly decreased As availability determined by various extraction procedures; however, the studied soil was spiked with As, which does not represent natural conditions31, which is a common issue in several similar laboratory investigations. The dominant mechanism seemed to be, in this case, As adsorption followed by possible reduction and diffusion through the thin oxide nZVI layer, resulting in an intermetallic phase with the Fe(0) core after breaking of As&#;O bonds at the particle surface32. Chromium(VI) is another good target metal for remediation by nZVI. Its reducing properties result in the transformation of Cr(VI) into Cr(III), which lowers its mobility and availability in the environment, which has also been shown in contaminated soils33,34,35. The efficiency of the stabilization process depends on the (i) pH of the treated soil. As expected, anionic metal(loid)s (e.g., As, Cr) are preferentially retained in acidic conditions, cationic metals (e.g., Cd, Pb, and Zn) are better immobilized in near-neutral-alkaline soils; and (ii) the presence of several metal(loid)s and their competition for nZVI-sorption sites36. However, when the stabilization efficiency of nZVI is compared with common iron-based materials (micro- or macro-sized)7, it is not possible to justify the costs associated with nZVI when used extensively on large, contaminated sites.

Toxicity and interactions with plants and microorganisms

It has been suggested that the generation of Fe2+ and reactive oxygen species by nZVI results in cell membrane disruption, and together with oxidative stress, these are the main mechanisms contributing to nZVI cytotoxicity and altering the taxonomic and functional composition of indigenous microbial communities14,37,15. These unwanted effects can result in severe toxicity for soil biota, especially in soils with low organic matter contents38. These drawbacks can be alleviated by the co-addition of inorganic and especially organic materials, e.g., compost, biosolids, bentonite, etc., which are beneficial for microbial communities and earthworms in nZVI-treated soils39,40,41,42,43.

In general, iron-based nanomaterials have not shown significant toxicity toward bacteria and plants at concentrations lower than 50&#;mg/L and at 5&#;mg/L for earthworms, except for ball-milled nZVI44. Higher nZVI concentrations (500 and &#;mg/kg) inhibited growth and respiration and increased avoidance of earthworms and oxidative stress in E. fetida as a result of nZVI application45. It is evident that the potential nZVI toxicity depends on environmental conditions, physico-chemical characteristics of the soil, concentration, aggregation, and reactivity of nZVI, and contamination types and levels37,46. Generally, it is possible to assume that plants can take up Fe from nZVI into their aboveground parts13,47. While Wojcieszek et al. () indicate that the Fe in the aerial parts is mainly in the form of particles and not originating from dissolved species, this question remains open, and the exact mechanisms need to be clarified using advanced techniques, e.g., synchrotron Mössbauer and isotope techniques, which is currently under investigations. Counteractive effects of arbuscular mycorrhiza and nZVI on plant physiology and metal(loid) uptake in the arbuscular mycorrhiza fungal&#;root&#;nZVI system need to be taken into account, and its presence is usually beneficial for alleviating the potential stress caused by nZVI to plants13.

The nZVI dose, as for any other material, is one of the crucial factors influencing its potential toxicity. For example, nZVI additions higher than 1&#;2% (w/w), which have been commonly studied under laboratory conditions (e.g., refs. 20, 31, 36, 38, 43, 45, 48), would not be economical and could potentially aggravate the associated toxicological risks. As for other engineered nanoparticles, another important point that needs to be taken into account is the potential toxicity and health-related risks of nZVI to humans, e.g., nZVI has the potential to induce cardiovascular disease through oxidative and inflammatory mediators produced from the damaged lung epithelium in chronic lung diseases49 or directly damage DNA50. Nevertheless, risks associated with ingesting the nanoparticles from the soil seem to be minimal.

nZVI composites

Few recent studies suggest that nZVI composites with biochar can be beneficial for selective sorption as well as for the stability of the amendments in soils17,51,52. Biochar-supported nZVI improved the stability, mobility, and stabilization efficiency of nZVI in a Cr(VI)-contaminated soil33. Using nZVI-biochar composites also helps limit the nanoparticles&#; aggregation in the soil and thus possibly increases the efficiency and reduces nZVI leaching through the soil profile. Additionally, using such composites could improve the selectivity of the amendments and possibly reduce application costs as more nZVI surfaces will be available for sorption due to limited aggregation and the influence of the biochar as another sorbent. The properties and preparation conditions of the biochar, e.g., pore structure, functional groups, feedstock composition, and pyrogenic temperature, are crucial for the resulting properties of the composite53. Other possible materials improving the efficiency through limiting nZVI particle aggregation include e.g., zeolites18, bentonite42, vermiculite54, etc.

The materials used for synthesizing the composites should be chosen in accordance with the target contaminants and specific soil conditions. When biochar is used, biowaste materials used as feedstock should be cost-efficient and improve the LCA of the final product. For example, the use of pyrolyzed biosolids, e.g., sewage sludge, could be promising as it would somehow reduce the footprint of this waste43,55; however, the presence of various contaminants in the sludge must be carefully considered.

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