ZVI products, such as AquaZVI and MicroZVI, 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.
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 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 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.
AquaZVI™ is an aqueous suspension of sulfidated, colloidal zero-valent iron (ZVI) that promotes the destruction of many organic pollutants, including chlorinated solvents, pesticides, haloalkanes and energetics. It is engineered to promote multiple pathways for contaminant degradation which leads to faster cleanup while minimizing daughter product formation. In addition, AquaZVI is easy to handle and simple to inject leading to significantly better reagent distribution.
In many cases this improved formulation of ZVI can also destroy contaminants through a direct chemical reaction as shown in Figure 3.
Figure 3. Example of chlorinated ethene degradation pathways and products. The top line with single-line arrows represents the reductive dechlorination (hydrogenolysis) pathway. The downward facing double-arrows represent the abiotic beta-elimination pathway.
AquaZVI is composed of colloidal, sulfidated zero-valent iron (ZVI) particles suspended in an aqueous medium with environmentally-acceptable, proprietary dispersants. The passivation technique of sulfidation, completed through proprietary processing methods, provides unparalleled reactivity with chlorinated hydrocarbons like PCE and TCE, and increases its stability and longevity in situ by minimizing undesirable side-reactions.
In addition to superior reactivity, AquaZVI is designed for easy handling that is unmatched by any ZVI material on the market. Shipped as an aqueous suspension, AquaZVI requires no powder feeders, no thickening with guar, and pneumatic or hydraulic fracturing is not mandatory. When diluted with water in the field, the resulting suspension is easy to mix and inject using direct push or injections wells.
AquaZVI is diluted with water on site and easily applied into the subsurface through low-pressure injections. AquaZVI can also be mixed with products like 3-D Microemulsion or PlumeStop® prior to injection.
MicroZVI™ is an In Situ Chemical Reduction (ISCR) reagent that is comprised of colloidal ZVI particles suspended in glycerol and environmentally acceptable, proprietary dispersants. MicroZVI is especially effective for promoting the rapid biological degradation of chlorinated ethenes and other volatile organic compounds (CVOCs). Zero-valent iron (ZVI) stimulates anaerobic biological degradation by creating a strongly reducing and hydrogen rich environment for reductive dechlorination. MicroZVI also reacts with and destroys CVOCs through a direct abiotic chemical reaction.
The small ZVI particle size is a key feature of MicroZVI. With an average diameter of a few micrometers, MicroZVI particles are small enough to be suspended in water and injected directly into contaminated aquifers at low pressures. This particle size, however, is still large enough to provide long-acting reactivity without requiring the mechanical mixing, thickening, high injection pressures, and fracking needed to inject larger ZVI products. This includes the ability of MicroZVI to be injected using direct push technology and through horizontal and vertical screened wells. MicroZVI can also be mixed with and co-injected along with other organic remediation amendments (ex. 3-D Microemulsion®), pH modifiers, and anaerobic microbes (ex. Bio-Dechlor Inoculum® Plus).
MicroZVI is diluted with water on-site and easily applied into the sub-surface using low pressure injections. MicroZVI is compatible with and can be co-applied with products like 3-D Microemulsion (3DME) and Bio-Dechlor Inoculum Plus® (BDI+).