A brief history of contaminant remediation using Fe metal
It has long been known that metals can react with certain chemicals and change their form. A well-known example is the use of Mg in the Grignard reagent for organic synthesis. This technique won Victor Grignard the Nobel prize in 1912, which he shared with Paul Sabatier who had made advances in nickel-catalyzed hydrogenation. Numerous applications of metal/organic reactions followed. By 1977, Archer had noted corrosion of various metals by chlorinated solvents such as 1,1,1-trichloroethane and carbon tetrachloride, as well as the degradation of the solvents themselves. This research was done to find methods for reducing the corrosion of metal vessels containing the solvents and maintain the solvent stability. Evidently the environmental remediation potential of these observations was not fully realized. In 1980 Sweeney patented the use of iron metal to reduce certain hydrocarbons in aqueous waste streams. Although this was an environmental usage, the idea of using these reactions on contaminants that had already been released to the subsurface environment had not yet evolved. In 1982 Gould investigated metallic iron as a reductant for Cr(VI) at pH<5 in wastewaters and proposed reaction mechanisms. In 1987, Hagenmaier and colleagues observed the catalytic effects of copper on the decomposition of polychlorinated dibenzo-p-dioxins (PCDD) and polychlorinated dibenzofurans (PCDF) and on other polychlorinated aromatic compounds. However, they used an experimental temperature of 120 °C for treatment of liquid laboratory wastes from dioxin analysis.
The relatively recent idea that iron metal can be used for in situ remediation of subsurface contaminants has resulted primarily from work done at the University of Waterloo, Ontario, Canada. Glenn Reynolds, a Masters student at the university, was carrying out research to investigate the sorption of organic contaminants by different well casing materials. It was noted in Reynold's work that the concentration of the halogenated hydrocarbon bromoform (CHBr3) declined when in contact with steel and aluminum casing materials. This result was available in 1984, and was hypothesized to be reductive dehalogenation, but its full significance was not realized until a few years later when the data were re-evaluated. It was at this time that the environmental possibilities for these reactions were realized. A peer-reviewed publication of Reynold's research occurred in 1990. An abundance of research activity ensued, particularly at the University of Waterloo, and primarily involving the use of granular iron due to its low cost and reactivity. By 1992, a number of conference proceedings on the use of zero-valent iron for contaminant remediation were appearing.
The Subsurface Restoration Conference-The Third International Conference on Ground Water Quality Research, was held in Dallas Texas, June 1992. It was jointly sponsored by the Robert S. Kerr Environmental Research Laboratory, Ada, OK, The University of Waterloo, Ontario, Canada, and Rice University, Houston, TX, under the auspices of the National Center for Ground Water Research. The Waterloo researchers presented two posters at this conference. One was by Robert Gillham and David Burris on dehalogenation, denitrification, and bioaugmentation. The other was by David Blowes and Carol Ptacek, showing that the granular iron could also reduce chromium from its Cr(VI) valence state to the Cr(III) valence state, resulting in a less toxic and immobile Cr(OH)3 precipitate. These posters also promoted the concept of using the iron in permeable reactive subsurface barriers. Many scientists first became aware of this new technology because of these posters.
Proceedings continued to appear in 1993 as some additional groups began to give presentations on their early findings. By 1994 a deluge of proceedings and, finally, additional peer-reviewed research publications began to appear. Results supporting the hypothesis of reductive dechlorination were published in the journal Ground Water by Robert Gillham and Stephanie O'Hannesin in 1994. At the 209th American Chemical Society National Meeting, April 2-7, 1995, the Symposium on Contaminant Remediation with Zero-Valent Metals was held. Forty abstracts were presented from thirty different research groups. As of this revision in February 2010 (original writing was September 1997), laboratory research continues and both pilot and full-scale field implementations have increased in number. However this is a remediation technique that has still not reached its full potential.
Numerous research organizations have tested metallic iron and other zero-valence-state metals, bimetallics, and both micro- and nanoscale particles of various materials, in combination with a variety of suspension media, for their ability to remediate a wide variety of contaminants and in an attempt to understand the reaction mechanisms. Much research has gone into increasing reaction rates while sustaining the reactions over the time periods needed to remediate substantial subsurface plumes. A significant body of research has developed. In addition, a great deal of thought and effort has gone into all aspects of designing and emplacing reactive subsurface barriers of these materials.
The contaminant remediation mechanisms and the effects of geochemical reactions in these iron-bearing subsurface systems are complex and remain incompletely understood except in specific circumstances. Understanding these processes in the natural environment requires a basic comprehension of subsurface hydrology as well as of the solution and surface chemical reactions that participate in the processes. Knowledge of these factors is important for understanding reactive barriers; their design, emplacement, and monitoring. Although the bulk phase chemistry is complicated, it is primarily the chemistry of corroding metal systems when using iron as the reactive material. These have been studied extensively and are reasonably well understood. It is the application of these corrosion systems to environmental contamination problems that is unique and melds previously disparate branches of chemistry.