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Introduction

This page provides an overview of research on contaminant remediation with zero-valent iron led by Drs. Paul Tratnyek and Rick Johnson at the Center for Groundwater Research (CGR). The page contains archival information that should be useful to newcomers, and features recent results for those interested in keeping up on what we are currently doing in the area. Comments, links, etc. are welcome.

[Venn Diagram]Our general objectives are two-fold: (i) to further develop the mechanistic understanding of chemical processes involved in groundwater remediation using reduction by zero-valent iron, and (ii) to use this understanding to develop strategies for managing and improving remediation performance. Please note that we do not provide remediation services or do feasibility studies for particular sites. For those things, see EnviroMetal Technologies and others links provided below.

The primary focus of our work is on "iron walls", which are permeable reactive barriers (PRBs) containing the zero-valent metal (ZVM) iron. However, not all PRBs are made with ZVMs, and not all environmental applications of ZVMs involve PRBs (as illustrated on the right). Some of our work is relevant to PRBs, in general, and to the environmental applications of ZVMs, in general.

Environmental applications of ZVMs also overlap with the burgeoning field of nanotechnology. Recently, our interests have grown to include the mobility and reactivity of iron nanoparticles, mainly to remediation of contaminated groundwater but also to medical diagnosis and therapy. Some details from our first major publication in this area are given below under featured recent results. Further information on the significance of this work can be found in a recent news story and press release.


Context

[Iron Wall Cartoon]The first field-scale application of iron metal in groundwater remediation was done at Base Borden, Ontario, by Bob Gillham and Stephanie O'Hannesin of the University of Waterloo. The design, which has come to be regarded as a "conventional iron wall", consisted of a treatment zone formed by excavating an area isolated by sheet pile, refilling with a mixture of granular iron and sand, and removing the sheet pile to leave an in situ, permeable, iron-bearing, treatment zone. Contaminated groundwater then flowed through the treatment zone (as illustrated on the right) resulting in a plume of treated water containing just dechlorination products and dissolved iron. For chlorinated ethenes (PCE and TCE) the products are mostly fully dechlorinated (i.e., little vinyl chloride was observed), although some chlorinated alkanes produce partial dechlorination products that can be problematic.

The success of the field test at Base Borden eventually led to commercialization of the technology. Since then, a great deal of interest has developed in the groundwater remediation community over the prospects of new treatment strategies (especially PRBs) based on contaminant reduction by granular iron (and other ZVMs). There have now been many feasibility studies, pilot tests, small to medium scale demonstration projects, and full-scale applications performed by numerous groups.


Background

Clearly, the zero-valent metal (usually granular iron) is the bulk reducing agent in these systems. However, corrosion of iron metal yields ferrous iron and hydrogen, both of which are possible reducing agents relative to contaminants such as chlorinated solvents. Thus, around 1992, we formulated a heuristic model (shown below/right) consisting of three possible "mechanisms".

[Matheson & Tratnyek (1994) Fig. 2]Pathway A represents direct electron transfer (ET) for Fe(0) to the adsorbed halocarbon (RX) at the metal-water interface, resulting in dechlorination and production of Fe(II). Pathway B shows that Fe(II) resulting from corrosion of Fe(0) may also dechlorinate RX, thereby producing Fe(III). Pathway C shows that H2 from the anaerobic corrosion of Fe(II) might react with RX if an effective catalyst is present.

The figure is a simplified representation, but the original discussion recognized subtleties such as the difference between dissolved and adsorbed Fe(II) and the possible role of intermediates in the formation of H2, such as atomic H or hydride. This discussion is contained in Matheson and Tratnyek (1994) Environ. Sci. Technol. 28(12): 2045-2053.

Click on the image to the left to view a full page version of the figure that is suitable for use in presentations. This figure is similar to the version that was originally published in 1994 by Matheson and Tratnyek and since has been reproduced in numerous publications by others. Remember that copyright for the original figure resides with the publisher (ACS).

Since then, it has become apparent that hydrogenation plays a minor role in most systems and that iron surfaces will be covered with precipitates of oxides (or carbonates and sulfides) under most environmental conditions. Therefore, we are now most concerned with how the oxide layer mediates transfer of electrons from Fe(0) to adsorbed RX. Consideration of this problem has lead us to formulate another heuristic model (shown below/right), again consisting of three "mechanisms".

[Scherer et al. (1998) Fig. 1] Pathway I shows essentially direct ET from Fe(0) to RX in a corrosion pit, or similar defect in the oxide film. Pathway II shows the oxide film mediating ET from Fe(0) to RX by acting as a semiconductor. Pathway III shows the oxide film as a coordinating surface containing sites of Fe(II) that complex and reduce RX.

Again, this is a simplified representation, but we hope it will helpful in directing future research. The original version of the figure, and an extended discussion of its implications, were published in 1998 by Scherer, Balko, and Tratnyek (in "Kinetics and Mechanisms of Reactions at the Mineral-Water Interface", D. Sparks and T. Grundl, Eds., ACS Symp. Ser. No. 715; pp. 301-322).

And as with the figure above, you can click on this image to view a full page version of the figure that is suitable for use in presentations. Remember that copyright for the original figure resides with the publisher (ACS).

Further discussion of these two figures can be found in the abstract to the plenary/opening talk we gave at the 2000 Theis Conference on "Iron in Ground Water". The abstract is available here as a PDF file (78 KB).


Featured Recent Results

[Tratnyek & Johnson, 2006, Fig. 1]Nanotechnology for Groundwater Remediation: Among the many applications of nanotechnology that have environmental implications, remediation of contaminated groundwater using nanoparticles containing zero-valent iron (nZVI) is one of the most prominent examples of a rapidly emerging nanotechnology with considerable potential benefits. There are, however, many uncertainties regarding the fundamental features of this technology, which have made it difficult to engineer applications for optimal performance or to assess the risk to human or ecological health. Here we address three of the fundamental features that commonly contribute to misunderstanding of this technology: showing (i) that the nZVI used in groundwater remediation are larger than particles that exhibit "true" nano-size effects, (ii) that the higher reactivity of this nZVI is mainly due to its high specific surface area, and (iii) that the mobility of nZVI will be less than a few meters under almost all relevant conditions. One implication of its limited mobility is that human exposure due to remediation applications of nZVI is likely to be minimal. There are, however, many characteristics of this technology about which very little is known: e.g., how quickly nZVI will be transformed and to what products, whether this residue will ever be detectable in the environment, and how surface modifications of nZVI will alter its long-term environmental fate and effectiveness for remediation.

We have had many requests for the concept diagram (Figure 1) in this article, a version of which is shown to the right (click on it for a closer view). You can also download high resolution versions of each of the three parts of this figure from the links below.

The figure shows three approaches to application of iron particles for groundwater remediation: (Fig. 1A (988 KB)) a conventional "permeable reactive barrier" made with millimeter-sized construction-grade granular iron; (Fig. 1B (444 KB)) a "reactive treatment zone" formed by sequential injection of nano-sized iron to form overlapping zones of particles adsorbed to the grains of native aquifer material; and (Fig. 1C (380 KB)) treatment of non-aqueous phase liquid (DNAPL) contamination by injection of mobile nanoparticles. In B and C, nanoparticles of iron are represented by black dots and zones that are affected by nanoparticles are represented as pink plumes. In B, the nanoparticles are assumed to have little mobility in the porous medium; whereas in C, nanoparticles modified to impart significant mobility are necessary. Note that reaction will only occur when contaminant—either dissolved in the groundwater or as DNAPL—comes into contact with the iron surfaces.

The full article is Tratnyek and Johnson (2006) NanoToday 1(2): 44-48. The publisher's version is available at [http://dx.doi.org/10.1016/S1748-0132(06)70048-2]. You can also download a PDF of the pubished version from here.

[Nurmi et al. 2005, Fig. 6]Effects of Nano-sized Particles: In this 2005 paper, we provide a thorough characterization of the structure and reactivity of two prototypical nano-sized iron materials. One material was obtained from Wei-Xian Zhang to represent the borohydride reduction method that is widely used for laboratory-scale synthesis of nano-sized iron. The other material was the nano-sized iron produced by the Toda Kogyo Corp. for environmental applications (RNIP). Using carbon tetrachloride as a model contaminant, we found that while the mass normalized rate contants are greater for nano-sized iron than micron-sized iron (Fisher Electrolytic), the surface-area normalized rate constants are all about the same. With respect to the products formed from reduction of carbon tetrachloride, the greatest practical concern is minimizing the yield of chloroform (which is a persistant and hazardous byproduct). The figure (right) shows that RNIP nano-sized iron generally gave lower yeilds of chloroform than borohydride-produced nano iron or Fisher electrolytic micro iron. Details on this work can be found in Nurmi et al. (2005) Environ. Sci. Technol. 39(5): 1221-1230 [http://dx.doi.org/10.1021/es049190u].

[Miehr et al. 2004, Fig. 1]Role of the "Reductate": In a 2004 paper, we take a uniquely-broad approach to characterizing the reactivity of iron metal with contaminants. Eight contaminants (called "reductates" because they are being reduced) were reacted with nine types of granular iron, and the kinetics were compared from a variety of perspectives. One perspective is shown in the figure (right) where the reaction rate is represented by the circle size (on a log scale). Note that the relative reactivity among reductates is not always the same for each reductant (type of iron). Also, the relative reactivity among reductants is not always the same for each reductate. The whole data set was subjected to correlation analysis, and surprising good correlations were obtained considering the diversity of the types of reductates included in the analysis. Details on this work can be found in Miehr et al. (2004) Environ. Sci. Technol. 38(1): 139-147 [http://dx.doi.org/10.1021/es034237h].

[Agrawal et al. 2002, composite fig]Effects of carbonate: In a 2002 paper, we describe a range of interrelated effects of dissolved carbonate species on the dehalogenation of a chloroalkane (1,1,1-trichloroethane, TCA). Of particular interest is the increased rate of dechlorination by iron that has been exposed to carbonate for only a short period (a few hours), and then the gradual decrease in dechlorination rates due to passivation of the iron by accumulation of precipitates, mainly siderite (ferrous carbonate) but also probably green rust. These effects are illustrated in the figure on the right, which shows how the first order rate constant for TCA dechlorination (represented by the ratio of Vmax/Km obtained with a mixed-order kinetic model) vs. the time of exposure to carbonate solution before initiating the dechlorination experiment). This work can be found in Agrawal et al. (2002) Environ. Sci. Technol. 36(20): 4326-4333 [http://dx.doi.org/10.1021/es025562s].


Selected Early Publications

A complete list of papers and extended abstracts from Tratnyek's group (including all those on zero-valent metals) is available at http://www.ebs.ogi.edu/tratnyek/pubs/.

[Chemistry & Industry Cover]An early overview of the technology and its development for chemists and chemical engineers: "Putting corrosion to use: remediating contaminated groundwater with zero-valent metals" by P. G. Tratnyek. Chemistry & Industry, 1 July 1996. No. 13, Cover (see left) and pp. 499-503.

A summary of published kinetic data for reduction of chlorinated solvents by iron metal: "Kinetics of halogenated organic compound degradation by iron metal" T. L. Johnson, M. M. Scherer, and P. G. Tratnyek, Environ. Sci. Technol. 1996, 30(8): 2634-2640 [http://dx.doi.org/10.1021/es9600901].

Quantitative structure activity relationships (QSARs) are derived that allow prediction and systematic validation of kinetic data: "Correlation analysis of rate constants for dechlorination by zero-valent iron" M. M. Scherer, B. A. Balko, D. A. Gallagher, and P. G. Tratnyek, Environ. Sci. Technol. 1998, 32(19): 3026-3033 [http://dx.doi.org/10.1021/es9802551].

The first Symposium on Contaminant Remediation with Zero-Valent Metals took place at the ACS National Meeting in Anaheim, CA (2-7 April 1995). Even back then, the call for papers attracted 40 papers, representing 30 different research groups. The Preprint Extended Abstracts to this symposium contain a large amount of literature on the subject. They filled pages 689-835 of the "Blue Book" from the Division of Environmental Chemistry. The organizers (Paul Tratnyek and Martin Reinhard) have assembled a subject index to these abstracts. It can be downloaded from here as a PDF file (18K).


Links to Other Resources

  • ETI: EnviroMetal Technologies Inc., Guelph, Ontario
  • "zerovalentiron.com": ARS Technologies
  • PRB Site by Powell & Associates Science Services
  • RTDF: Remediation Technologies Development Forum
  • FRTR: Federal Remediation Technologies Round Table
  • CLU-IN: Technology Innovation Office, U.S. EPA
  • RUBIN: Rubin Network Online, Germany
  • PEREBAR : EU Project on Long-term Performance of PRBs

Boolean string that includes variations on iron, zero-valent, etc. Google Web Search, Google Scholar Search.


Acknowledgments

Current Funding: The Strategic Environmental Research and Development Program (SERDP); Environmental Security Technology Certification Program (ESTCP); and the Environmental Management Science Program (EMSP).

Past Funding: The Petroleum Research Fund (PRF); the National Science Foundation (NSF), Bioengineering and Environmental Systems Program; U.S. Environmental Protection Agency (NCERQA), Office of Exploratory Research; Camille and Henry Dreyfus Foundation, Environmental Chemistry Postdoctoral and Special Grants Programs; University Consortium Solvents-In-Groundwater Research program (UCSGRP) and its corporate sponsors.

Group: Leah Matheson, MSE Technologies, Inc.; Abinash Agrawal (Wright State University); Timothy Johnson (AMEC), and Michelle Scherer (University of Iowa); and Barb Balko (Lewis & Clark College).


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