The Role of Zero Valent Iron in Permeable Reactive Barriers

Environmental remediation refers to the removal, treatment, or containment of pollutants and hazardous substances in a contaminated area. Its ultimate goal is to eliminate all possible contaminants of concern (COCs) from soil, water, or air and restore the environment’s original or acceptable condition.

Environmental remediation services play a crucial role in minimizing the detrimental effects of pollution on ecosystems and human populations. Among the vast array of remediation technologies being used, zero valent iron (ZVI) has become an invaluable tool in today’s environmental conservation efforts.

Introduction to Zero Valent Iron

Zero valent iron is an engineered metal used as a reductant in environmental remediation. As a “zero-valent” metal, ZVI is highly reactive. In effect, it becomes useful in treating organic and inorganic COCs in environmental media, such as groundwater and soil.

ZVI’s reactivity stems from its negative reduction potential. It participates in a series of reduction-oxidation (redox) reactions to facilitate the removal of a wide range of pollutants. In these reactions, it acts as a reducing agent undergoing an oxidation reaction while interacting with COCs with a positive reduction potential.

Depending on factors such as particle size, surface area, and the presence of other substances or coatings, ZVI’s reactivity can be enhanced further. This can make it even more effective in breaking down COCs, such as chlorinated solvents, heavy metals, and organic compounds, and render them non-toxic or less toxic.

One of the most common applications of ZVI is their integration as the primary reactive material in permeable reactive barriers (PRBs). In this application, ZVI inactivates passing pollutants by reducing their ionic state, degrading them into less harmful or non-harmful compounds. This makes permeable reactive barriers an effective in situ treatment method for contaminated groundwater to aid soil and groundwater remediation efforts.

Understanding Permeable Reactive Barriers

Permeable reactive barriers are subsurface emplacements used in the in situ treatment of groundwater. They are typically constructed using reactive media, such as limestone, organic carbon, apatite, zeolites, granular iron, or their mixtures with sand and gravel. These reactive materials are carefully selected during PRB design as it affects the type of contaminant removed.

There are several ways to install PRBs within a contaminated area. They can be in the form of trenches, overlapping borings installed in series, or as grouped injection points. Regardless of the type, they are installed beneath the soil and are positioned perpendicular to the direction of groundwater flow. This allows the barrier to gradually remove COCs as groundwater permeates through the barrier.

PRBs apply to a wide range of COCs. However, as mentioned above, the design must consider the appropriate reactive material for a specific contaminant. This involves understanding the nature of the COC and its mechanism of removal, which can be any of the following:

● Adsorption: COCs adhere to the reactive material’s surface as contaminated groundwater passes through the PRB.
● Precipitation: Dissolved COCs in groundwater react with the material to form solid particles trapped in the barrier wall.
● Reaction: The COC interacts with the reactive material through various chemical reactions to form less harmful versions.
● Biodegradation: Microbes consume and digest the COC to make them less soluble in groundwater to form precipitates or to alter their form into less harmful compounds.

The Role of Zero Valent Iron in Permeable Reactive Barriers

ZVI was one of the earliest materials used in PRBs. It is still extensively used in sustainable remediation due to its high reactivity to various pollutants and sustained long-term performance in groundwater remediation.

ZVI has been proven effective in removing COCs, as shown in various pilot and full-scale applications in PRBs. The following examples illustrate ZVI’s COC-removal capabilities in groundwater and soil remediation:

● ZVI facilitates the removal of heavy metals during wastewater treatment and groundwater remediation. The removal process takes place in the form of an abiotic reduction reaction where ZVI acts as a reducing agent. It then reacts with positively-charged metal ions like chromium and arsenic to convert them into less toxic precipitates.
● As the primary reactive material in PRBs, ZVI assists in the degradation of organic pollutants like pesticides, petroleum, and phenolic compounds. ZVI acts as a reducing agent in reducing the organic compound to reduce them into less harmful substances.
● In a process called reductive dechlorination, ZVI reacts with chlorinated solvents like trichloroethylene (TCE) or perchloroethylene (PCE) to break them down into harmless products. This process is used for sustainable groundwater remediation to remove chlorine-containing pollutants and reduce their environmental impact.

Reductive dechlorination is particularly beneficial to ecosystems as it circumvents the production of toxic chlorinated daughter compounds like vinyl chloride. In other pathways, such as biological reduction, breaking down chlorinated compounds can form substances like vinyl chloride, a known carcinogen.

In all of these examples, the actual removal of COCs from groundwater involves a combination of different removal pathways. These include the formation of secondary products in the PRB, sorption on the surface of the ZVI, and reductive precipitation and co-precipitation.

Importance in Environmental Remediation Services

Without any sustainable means to fuel progress, pollution will continue to threaten ecosystems and human health. Environmental remediation services then serve a crucial role in reducing the impact of pollution and restoring these sites to their acceptable, if not originally untouched, condition. Zero-valent iron, and its integration into permeable reactive barriers, have become an invaluable tool in this quest to protect the environment.

As a powerful reducing agent, it is an effective device for removing pollutants from contaminated sources. It is also a sustainable remediation option, as it eliminates COCs without creating new issues in the process. Above all, it is a dynamic option with a wide range of applications, from treating contaminated soils to removing pollutants in groundwater and wastewater.

Effective Environmental Remediation Solutions at CAP Remediation

CAP Remediation is a leading provider of environmental remediation services dedicated to providing the amendments you need to make your project successful. Our team has extensive experience in in situ remediation solutions for treating chlorinated and hydrocarbon contaminants and is dedicated to making your project successful. Contact us today to learn more about our environmental remediation services.