Zerovalent Iron Permeable Reactive Barriers

Permeable reactive barriers (PRBs) are in situ treatment zones created below ground to clean up contaminated groundwater. PRBs take advantage of natural groundwater migration to transport contaminants to a defined treatment zone. Contaminants are removed from groundwater in the PRB and treated groundwater passes through the permeable zone; eventually a “clean front” is created on the down-gradient side of the PRB. Zerovalent Iron (ZVI) was the first reactive material used in PRBs for groundwater remediation and it continues to be the primary material used in the construction of these treatment systems. ZVI PRBs can treat groundwater contaminated with chlorinated solvents and their breakdown products such as tetrachloroethene (PCE), trichloroethene (TCE), cis-1,2-dichloroethene (cis-DCE), vinyl chloride (VC), 1,1,1-trichloroethane (1,1,1-TCA), 1,1-dichloroethane (1,1-DCA), 1,2-dichloroethane (1,2-DCA), chloroform, and carbon tetrachloride; explosives such as TNT and RDX; cations of Pb, Cd, Ni, Zn, and Hg; and anions of Cr, As, Sb, Se, U, and Tc.

Related Article(s):

• Zerovalent Iron (ZVI) (Chemical Reduction - ISCR)
• Chemical Reduction (In Situ - ISCR)
• Metal and Metalloids - Remediation
• Chlorinated Solvents

CONTRIBUTOR(S): Dr. Richard Wilkin

Key Resource(s):

• Permeable Reactive Barriers: Lessons Learned/New Directions. Interstate Technology Regulatory Council (2005).
• Permeable Reactive Barrier: Technology Update. Interstate Technology Regulatory Council (2011)

Introduction

A PRB is commonly constructed using granular iron, limestone, organic-carbon, zeolites, apatite, or mixtures of these materials with sand or gravel. PRBs are installed hydraulically down-gradient from contaminant source zones to prevent off-site plume migration or to protect sensitive receptors. Design considerations for PRBs include:

1. understanding contaminant flux to determine the residence time requirement for effective treatment;
2. understanding geochemical conditions to identify compatibility issues between the selected reactive medium and site-specific groundwater; and
3. understanding groundwater flow to ensure the contaminant plume is intercepted for treatment (See Fig. 1) so contaminant flow beneath, around, or above the treatment system does not occur. PRBs are generally keyed into impermeable hydrostratigraphic units, such as clay layers or bedrock, to prevent underflow of contaminants beneath the treatment zone.

Figure 1. Conceptual Model of PRB. Adapted from Wilkin, et al., 2002

Zerovalent Iron

The first field-scale test of a PRB constructed with ZVI was conducted in 1991 at the Canadian Forces Base, Borden, Ontario by researchers from the University of Waterloo. The first full-scale ZVI-based PRB was constructed in Sunnyvale, California in 1994 to treat groundwater contaminated with TCE and cis-DCE. Long-term performance studies have documented sustained removal of trichloroethene and hexavalent chromium by ZVI PRBs and their continued hydraulic performance for up to 15 years.

ZVI degrades chlorinated ethenes, ethanes, and methanes via reductive abiotic reactions that largely circumvent the production of toxic chlorinated daughter compounds, such as vinyl chloride, which can be produced during biological reduction. For example, Arnold and Roberts reported that >85% of PCE and >90% of TCE treatment by zerovalent iron is accounted for by β-elimination reactions that avoid production of cis-DCE and VC.

Zerovalent Iron has also been used in pilot or full-scale tests to treat groundwater contaminated with metals (e.g., chromium), metalloids (e.g., arsenic), and radionuclides (e.g., uranium). Contaminant removal processes are complex and involve a combination of sorption to the surface of the ZVI or secondary minerals formed in the PRB, reductive precipitation, and co-precipitation. Other possible uses of ZVI for groundwater include treatment of explosives such as nitro aromatic compounds. Treatment applications of ZVI for organic and inorganic contaminants encountered in groundwater at hazardous waste sites are summarized in Table 1.

Table 1. Groundwater Contaminants Treated by Granular ZVI

Contaminant Class	Contaminant	Removal Mechanism
Chlorinated Ethenes	Tetrachloroethene, PCE Trichloroethene, TCE cis-1,2-Dichloroethene, cis-DCE trans-1,2-Dichloroethene, trans-DCE 1,1- Dichloroethene, 1,1-DCE Vinyl Chloride, VC	α-elimination β-elimination Hydrogenolysis
Chlorinated Ethanes	Hexachloroethane, HCA 1,1,1,2-Tetrachloroethane, 1,1,1,2-TeCA 1,1,2,2-Tetrachloroethane, 1,1,2,2-TeCA 1,1,1-Trichloroethane, 1,1,1-TCA 1,1-Dichloroethane, 1,1-DCA	α-elimination β-elimination Hydrogenolysis
Chlorinated Methanes	Tetrachloromethane, CT Trichloromethane, TCM	α-elimination β-elimination Hydrogenolysis
Explosives	Trinitrotoluene (TNT) Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX)	Adsorption Reduction to 2,4,6-triaminotoluene (TAT)
Anionic Metals	Chromium Arsenic Antimony Selenium Uranium Technetium	Reductive precipitation Co-precipitation Adsorption
Metals	Lead Cadmium Nickel Zinc Mercury	Precipitation Co-precipitation Adsorption

PRB Design and Site Conditions

Like other in-situ technologies used for groundwater cleanup, PRB designs rely on accurate site characterization data for geology, hydrology, and groundwater geochemistry. Site hydrologic aspects critical for PRB design and performance include seasonal consistency in groundwater flow direction to maintain migration of contaminants to the PRB at an appropriate flow velocity. High flow velocities (≳2 m/d) can result in short residence time in the treatment zone and early breakthrough of contaminants; low flow velocities (≲0.1 m/d) result in little change in contaminant concentrations over time, so wells in down-gradient regions respond slowly. Geochemical considerations for the design of ZVI PRBs include redox conditions and the concentrations and make up of major ions dissolved in local groundwater. Geochemical conditions that are favorable for the sustained performance of ZVI PRBs include:

Dissolved O₂ concentrations ≲2 mg/L: Excess dissolved oxygen leads to rapid iron corrosion and formation of low density iron oxide precipitates that may lead to hydraulic failure.

Nitrate-N concentrations ≲10 mg/L: Like dissolved oxygen, nitrate enhanced iron corrosion leads to rapid iron corrosion and formation of low density iron oxide precipitates that may lead to hydraulic failure.

Bicarbonate alkalinity ≲250 mg CaCO₃/L: Carbonate minerals (aragonite, siderite, chukanovite, carbonate-green rust) precipitate in the moderately alkaline pH environment (pH^~10) produced by ZVI PRBs and most of the porosity and reactivity reduction in PRBs can be accounted for by these minerals (see Figure 2).

Sulfate concentrations ≳100 mg/L: High dissolved hydrogen generated by iron corrosion creates a favorable environment for sulfate-reducing bacteria and methane-producing archaea. Metal sulfide precipitates like mackinawite (FeS) provide a source of secondary reactivity for chlorinated solvents and metals. Thus, levels of sulfate in influent groundwater promote natural sulfidation of ZVI PRBs and potentially enhanced treatment capacity.

Total dissolved solids below ≲1500 mg/L: The amount of secondary mineralization and clogging potential increases with increasing solute levels.

Figure 2. False-color scanning electron micrograph of a zero-valent iron grain (pink) coated with secondary precipitates (iron oxide and aragonite; green) surrounded by native aquifer quartz grains (blue). The iron particle is approximately 1.5 mm in length.

Performance Monitoring

Performance monitoring of PRBs typically includes: 1) sampling groundwater monitoring wells located up-gradient of the PRB to establish influent contaminant concentrations and groundwater geochemistry; 2) sampling groundwater monitoring wells located within the PRB to establish in-wall behavior; and, 3) sampling groundwater monitoring wells located down-gradient of the PRB to establish contaminant concentrations and groundwater geochemistry in the effluent. Key monitoring parameters include: contaminant(s) and daughter product(s), pH, oxidation-reduction potential, specific conductance, alkalinity, calcium, iron, and sulfate. These monitoring parameters provide an indication of whether or not the expected geochemical environment is present within the treatment zone, and also that contaminant degradation is occurring.

For chlorinated solvents, analysis of the parent compounds and daughter products (e.g., ethene and ethane) aids in the performance analysis. Compound-specific isotope analysis can be helpful to understand contaminant degradation processes in complicated systems. Hydrologic monitoring typically consists of gradient analysis to determine flow directions and whether water level changes are abrupt in the vicinity of the PRB, indicating the potential for impeded groundwater flow. Core collection with solid-phase analysis and/or geophysical surveys (e.g., conductivity probing) are not common or routine components of performance monitoring, but can be considered in situations where anomalous behavior is encountered such as a failure to meet performance criteria.

Problems noted with the application of this, and other, in situtechnologies include:

• contaminants are present down-gradient of the PRB prior to installation;
• influent contaminant concentrations may increase with time such that necessary residence time requirements are not met;
• incomplete capture of the plume, especially along peripheries of the plume;
• unaccounted for high contaminant flux zones and early breakthrough; and,
• incomplete coverage of the reactive medium in the subsurface.

Many of these issues can be addressed with rigorous site characterization, which may incur extra cost and time prior to remediation, but can potentially avoid implementation of secondary measures.

Summary & Outlook

ZVI is perhaps the most widely studied and best understood reactive material used for groundwater remediation. It will continue to serve as a benchmark for other novel treatment media and for studying treatment options for emerging contaminants. The environmental industry is becoming accustomed to screening sites for potential ZVI applications and is actively taking advantage of improving ZVI performance, i.e., through natural sulfidation and treating up-gradient sources of contamination to extend the lifetime of the PRB. Advantages of using ZVI in PRBs for remediating contaminated groundwater include: wide range of treatable contaminants (organics and inorganics); expectation of complete subsurface coverage; and, flexible adaptation to variable contaminant flux and plume geometry. Disadvantages include: construction challenges and uncertainty in predicting long-term treatment and hydraulic performance. These uncertainties can often be addressed with rigorous site characterization.