Zerovalent Iron (ZVI) (Chemical Reduction - ISCR)

Zerovalent iron (ZVI) serves as the most prevalent zerovalent metal (ZVM) utilized for environmental remediation efforts. Generally, ZVI acts as a reducing agent, effectively degrading or sequestering numerous contaminants present in soil and groundwater.

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Overview of ZVI in Environmental Remediation

ZVI functions as an effective reductant for the remediation of a multitude of organic and inorganic pollutants found in various environmental matrices like groundwater and soil. The use of ZVI is demonstrated in treating an extensive range of contaminants, including chlorinated solvents, heavy metals, nitro-aromatic compounds, nitrates, and some synthetic dyes. ZVI can be applied both in situ and ex situ, reinforcing its flexibility in in situ chemical reduction (ISCR) methods. This section elucidates the fundamental redox chemistry of ZVI, various ZVI types, applications, and examples of contaminant management.

Redox Chemistry of ZVI

ZVI operates as a reductant in numerous oxidation-reduction (redox) reactions pertinent to environmental science. Its negative reduction potential favors electron donation, making it reduce other species with higher reduction potentials. The oxiation of ZVI can be articulated through the redox half-reaction:

ZVI can react with a variety of oxidants. For instance, in the presence of water and oxygen, ZVI oxidizes producing several iron (II) species, mirroring the common phenomenon of iron corrosion in moist environments. The overall redox reaction showcasing ZVI's oxidation in water with dissolved oxygen (DO) can be summarized below:

ZVI also reacts in anaerobic conditions, albeit at a significantly slower rate. Beyond oxygen and water, ZVI can engage in redox reactions with a host of other oxidants, including various contaminants, leading to their transformation or immobilization (detailed in the Contaminants Treated segment). The oxidation of ZVI not only engages surface contamination but also drives the formation of diverse iron oxide species that potentially absorb contaminants like metallic cations.

Varieties of ZVI Utilized in Remediation

Numerous forms of ZVI have found applications in remediation strategies including:

• Granular ZVI. Frequently employed in remediation, this variant typically constitutes industrial-grade materials sourced from scrap iron, subsequently milled to specific grain sizes.
• Nano-scale ZVI (nZVI). This consists of ZVI particles measuring between 1 to 100 nanometers, exhibiting heightened reactivity relative to their granular counterparts due to their expansive surface area.
• Bimetallic nZVI. Comprising ZVI with a secondary zerovalent metal, bimetallic nZVI showcases enhanced reactivity toward various contaminants compared to standard ZVI.
• Combination products. It is common practice to blend ZVI or nZVI with supplemental remedial agents to unlock added benefits from bioremediation solutions or sorption processes. Such combinations often include biostimulants or activated carbon.

The properties and reactivity of these various ZVI forms can differ, influenced by impurities (either inherent or deliberately added) present in the materials. Consequently, various ZVI products categorized under similar classifications may exhibit differing behaviors in remediation contexts.

Real-World Applications of ZVI

ZVI finds usage across multiple remediation applications such as:

• Permeable reactive barriers (PRBs).
• In situ soil mixing.
• Injection treatments like reactive treatment zones and target-specific injections.

Effective Treatment of Contaminants

ZVI has been explored for remediation of numerous contaminants, with the removal processes generally categorized into:

1. Transformation. Chemical reactions that convert contaminants into differing products.
2. Sequestration. Primarily concerned with adsorption and reduction without actual degradation of the contaminant.

Highlighted below are reaction processes for common contaminants:

Chlorinated solvents. ZVI facilitates the degradation of chlorinated solvents like trichloroethene (TCE) through dechlorination processes defined by redox reactions, wherein ZVI plays the role of a reductant.

Heavy metals and metalloids. For elements like arsenic, chromium, and lead, ZVI aids in the sequestration by reducing their mobility in the environment through adsorption and precipitation processes.

Nitroaromatic compounds. ZVI has shown efficacy in transforming nitroaromatic compounds, resulting in the formation of amines.

Future Prospects of ZVI Applications

Significant interrelationships exist between ZVI environmental applications and other established as well as emerging technologies. While ZVI is closely associated with permeable reactive barriers, it also overlaps with the expanding domain of nanotechnology through the adoption of nZVI. Current trends exhibit research focusing on nZVI, exploring its synthesis, enhancement, and environmental transport while venturing into studies about how sources and impurities impact reactivity and the development of multipurpose products targeting emerging contaminants.

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