Zerovalent iron (ZVI) is the most commonly used zerovalent metal (ZVM) for contaminated site remediation. Recent industrial and urban activities have led to elevated concentrations of a wide range of contaminants in groundwater. The use of ZVI for the treatment of toxic contaminants in groundwater and wastewater has received wide attention and encouraging treatment efficiencies have been documented. ZVI is typically applied as a reductant and is capable of degrading or sequestering a variety of contaminants found in groundwater and soil: chlorinated organic compounds, nitroaromatic compounds, arsenic, heavy metals, nitrate, dyes, and phenol.

ZVI has the ability to dehalogenate chlorinated compounds and oxidize soluble metals by chemical reduction and has been used many years as a granular material in permeable reactive barriers. Due to the high specific surface area, nano- and micro- sized ZVI-particles (nZVI & mZVi) are more reactive than granular materials. Moreover, nZVI- and mZVI-particles can be readily injected into the soil matrix, thus allowing for active treatment of source and plume areas, even at greater depth.

The particle size should be designed to easily be incorporated into the soil matrix during injection. Barrier application require a larger particle size that can be mixed with backfill material and remain suspended, not separating, during mixing and application or in the subsurface. Typically this would require a particle size greater than 60 mesh (250 micron). Injection requires a smaller particle size than barrier construction. ZVI for injection will typically be less than 100 mesh (150 micron). This size allows easier pre-injection mixing and suspension and better flow in the soil matrix to achieve a larger radius of influence (ROI). The distribution of particle sizes should be very narrow. Large particles will have less reactivity than smaller particles and could make up a disproportionate amount of the mass. Fine particles will have a higher reactivity and may lead to premature exhaustion of the iron. Barriers will typically have 90% of particles greater than 100 mesh (150 micron). Particles larger than 20 mesh will be less reactive and not be needed to suspend within the soil matrix. Injection particle ranges are typically 100 to 400 mesh (37 – 150 micron) to have a good balance of reactivity and longevity. Source area treatment particle sizes of 400 mesh and less (less than 37 micron) are typical to take advantage of higher reactivities

There are several factors that affect reactivity (particle size, impurities) which can reduce the effectiveness for remediation. Engineered ZVI products for environmental remediation can insure a consistent reactivity and a maximum longevity.

ZVI manufactured for other purposes (Steel Shot, Steel Grit, Steel Aggregate, Cast Iron Aggregate, Steel Ballast. etc.) and re-purposed for remediation may include impurities, metals and materials that will not aid in the reactivity or longevity. Re-purposed ZVI products are typically spherical in shape due to previous usage, this limits the available surface area (reactivity) for the same quantity compared to ZVI manufactured specifically for remediation projects.

Since the remediation with injectable ZVI-particles is based on direct contact between the ZVI- particle and the contaminant, the mobility and stability of the ZVI-particle in the soil is of crucial importance for the effectiveness of the remediation. Improved distribution of ZVI can be achieved by high injection velocities; mechanical modifications of the subsurface via fracturing (pneumatic or hydraulic) or dilatation (pressure pulse technology). A thorough preliminary study is necessary to check the feasibility of ZVI-particle injection (ISCR) for the treatment of chlorinated solvents. The preliminary study consists of the following phases:

1. Conceptual Site Model (CSM)
The contaminated soil volume and depth is essential to determine the required injection depths, distances and volumes. Information about the amounts of electron donors, contaminants and other electron acceptors (nitrate, sulphate, oxygen, Fe(II)/Fe(III) ) is essential to determine the needed amount of ZVI. Hydrogeological parameters such as hydraulic permeability, the average groundwater flow velocity and groundwater flow direction are needed to determine the radius of influence of the injections, the volumes that can be injected, the time period of injection, reflux of the injected solution and the number of injections (distance between injection points).

2. Lab Tests
Lab tests allow for the investigation of degradation kinetics (batch tests), stoichiometry (batch test with aquifer samples), potential to inject a ZVI-solution, mobility (column tests), stability of the ZVI-particle suspension (sedimentation tests) and overall feasibility of a ZVI application for a particular site.

3. Field Test
Since it is difficult to exactly simulate the conditions in the aquifer, it is recommended to conduct a field test. A field test can provide information about the injection method and the maximum injection pressure, flow rate and radius of influence. It also allows for the observation of possible rebound effects and the establishment of a reasonable remediation target. ZVI-particles can be injected via several injection methods. The chosen injection method is of great influence on the radius of influence of the injection and particle distribution. Each injection method has its own specific advantages and limitations and the choice is, amongst others, determined by the site specific conditions and available remediation budget. During injection it is important to avoid contact between the ZVI and oxidizing agents since these diminish the reactivity and, in addition, can cause safety hazards due to strong exothermic reactions. ZVI-particle injection is an expensive remediation technique since ZVI-particles (especially the nano-sized and/or modified ZVI-particles) are expensive and the radius of influence (because of limited mobility) is low.

3.1 Direct Push Injection Technology
Direct Push Injection (DPI) methods rely on the hydraulic downward advancement of small diameter (1.25-3.25 inch) hollow steel rods into the target zone. Each DPI point consists of a series of threaded 3-5 foot long steel drive rods that are advanced via series of connected rod joints to the desired application depth prior to injection of the remedial reagent. Direct push techniques generally rely on the displacement of soil around the diameter o rod tip. Soil displacement via the DPI rods does create localized areas of compaction immediately around the injection rods. The user and applier should be aware that these areas of compaction may alter the application of reagent into the desired target zone. DPI is very effective in shallow application (<50 feet bgs.) but may become cost prohibitive due to time to drive the rods in deeper application and rocky soils.

3.1.1 Expendable Tip Method
The DPI rod string is fitted with an expendable point. Upon achieving the desired depth the expendable tip is “dropped” or knocked out of the end of the lead rod. A reagent is then injected via the open rod. This method is simple and is generally only appropriate for target zones that are reasonably homogenous. This method is potentially limiting because the remedial reagent must be applied in a “bottom up” fashion, meaning that the reagent is being pumped out the end of the lead rod at a known rate while slowly raising the rod set. This method provides a lower level of application flexibility (bottom up only) and may tend to focus the injected remedial substrate downward rather than outward. In most cases, this method provides limited ISRR distribution and is therefore not recommend.

3.1.2 Horizontal Injection Tool
Horizontal injection tooling is typically composed of a modified section of the lead rod. This section of the lead rod is typically equipped with a sleeve that covers a set of injection ports or the lead rod may be pressure activated injection ports. Upon reaching the desired depth the operator begins injection of the remedial reagent through the injection ports in the rod. The horizontal injection method allows the operator to apply the reagent in a “top-down” as well as a “bottom up” operation. This method provides greater flexibility and enhances the outward injection of the reagent.

3.2. Hydraulic Fracturing and Injection
Hydraulic fracturing involves injecting a reagent into the subsurface at a pressure that initially exceeds the combined lithostatic pressure, hydrostatic pressure, cohesive strength of the formation, and other sources of resistance such as pressure loss through the injection tooling. The lithostatic and hydrostatic pressures are essentially equivalent to the weight of the soil and water columns, respectively, above the depth of injection. The cohesive strength is a measure of how well the soil particles are adhered to one another. Clays generally have significantly greater cohesive strength than sands. Other pressure losses, such as friction from the sidewalls of the injection rods, will create additional resistance that must be overcome. Once this pressure has been overcome and a fracture has been created, the pressure required to continue the injection will be lower.

Hydraulic fracturing is particularly suited to fracturing of consolidated soils, bedrock and media with low permeability but can be applied to all soil types. The potential benefit of employing a fracturing method is increased lateral distribution from a given injection location. Because the reagent is emplaced in a fracture that occupies a very small fraction of the subsurface, there is no longer a need to fill up the entire effective pore space to achieve a certain placement radius. The radius of influence will depend on several factors such as soil type, application method, injection rates, injection depth, and reagent viscosity, where more viscous reagent slurries enhance fracture propagation.

3.3 Pneumatic Fracturing and Injection
As the name suggests, pneumatic fracturing uses a gas to fracture the media and inject the reagent, with or without the use of packers to isolate the injection depth. The injection method is completed in two steps, pneumatic fracturing and pneumatic injection, which are completed sequentially. As with hydraulic fracturing, pneumatic fracturing is used to create and /or enhance subsurface fractures with controlled bursts of high-pressure gas at pressures exceeding the natural in situ geostatic pressures and at flow volumes exceeding the natural permeability of the subsurface. Fracturing allows greater volumes of reagents to be distributed in the subsurface and provides better access to hydraulically isolated zones in the plume.

The type of gas used depends on the reagent. For oxidative reagents, compressed air can be used. For reducing reagents, nitrogen gas is used to avoid injection of oxygen into the aquifer. Pneumatic fracturing and injection has been applied in many types of geologic media including sands, silts, silty clays, and highly weathered fractured bedrock, and up to depths of 160 feet.

3.4 Injection Wells
Injection wells are appropriate for injection of solutions containing very fine ZVI particles (nZVI and mZVI < 400 mesh). Injection wells are typically screened in the uppermost portion of the water table, usually from 10 feet to 25 feet below the water table. Nested injection wells are designed for injections into aquifers of greater thickness and for injection into DNAPL zones located at the bottom of aquifers. Injection wells are commonly constructed of polyvinylchloride (PVC) or stainless steel pipe with the screen interval placed in the vertical section intended for treatment. Usually these wells are constructed with the intention of being temporary or semi- permanent. Occasionally more permanent type wells such as monitoring wells or pumping wells are used for injection purposes. Monitoring well should only be used for injection if they are not part of the compliance network, are screened in the right interval, and are tested to ensure their seals can contain the injection pressure.

The most significant difference between common monitoring wells and injection wells is that the injection wells are screened in a deliberate way to intersect only the selected zones identified for treatment. Unlike groundwater monitoring wells injection wells should not be screened across multiple zones or above the water table unless there is forethought and an intention to treat the capillary fringe area or vadose zone.

References

  • In Situ Chemical Reduction using Zero Valent Iron injection – A technique for the remediation of source zones, Veerle Labeeuw, April 12 2013
  • The Use of Zero-Valent Iron For Groundwater Remediation and Wastewater Treatment: A Review, Journal of Hazardous Materials, Volume 267, 28 February 2014, Pages 194-205 Permeable Reactive Barrier Technologies for Contaminant Remediation, EPA/600/R-98/125 September 1998
  • Technical Report: Subsurface Injection of In Situ Remedial Reagents (ISRRs), Los Angeles Regional Water Quality Control, September 16, 2009