Platinum Metals Rev., 1997, 41, (1), 2
The Dechlorination of Hydrocarbons
Palladised Iron Utdlised for Ground Water Purification
The pollution of groundwater and soils by industrial chemicals is a cause of significant environmental concern. Treatment with palladium is suggested as a means of decontaminating certain occurrences of polluted soil and groundwater. Palladium can be chemically deposited onto zero-valence iron (Fe0) and the resulting bimetal is very effective in dechlorinating many of the toxic chemicals, produced as industrial effluents, that are found in soils and groundwater. With a palladium content of 0.05 per cent (w/w), the rate at which trichloroethene is dechlorinated is one to two orders of magnitude faster than with iron alone. Similarly, a bimetal made of palladium/iron readily dechlorinates dichloromethane and polychlorinated biphenyls which are not easily reduced by Fe0.
This report describes the treatment of contaminated water with a palladium/iron bimetal. Such contaminated water is typically the legacy of past practices in which chlorinated degreasing solvents were discharged to soils following use. These solvents have been used in dry-cleaning, all aspects of metals manufacturing, metal plating, engine maintenance, and in many other cleaning and fabrication applications. Consequently, thousands of sites have been affected. Other common chlorinated hydrocarbon pollutants include pesticides and herbicides, chlorophenols used in the wood-treatment industry, and polychlorinated biphenyls (PCBs) which were used in electrical capacitors and as heat transfer fluids.
Our recent studies have demonstrated that palladium/iron rapidly dechlorinates toxic hydrocarbons, such as chlorinated ethenes, phenols and biphenyls (1–3). This research began after the publication of a report suggesting the use of zero-valence iron as a means of dechlorinating contaminated groundwater in situ (4). Iron alone, however, suffers from relatively slow reaction rates and does not dechlorinate important compounds, such as dichloromethane and PCBs. By chemically plating iron filings with palladium to approximately 0.05 per cent (w/w), the dechlorination rate for trichloroethene is increased by one to two orders of magnitude (5), and the dechlorination reaction is extended to include recalcitrant compounds, such as PCBs.
The majority of the experiments conducted to date have employed 0.05 per cent (w/w) palladium. Zero-valence iron, nominally 40-mesh size, is palladised by reacting it with a solution of 99 per cent hexachloropalladate (K2PdCl6), according to the following reaction:
The solution at first exhibits the red-orange colour of the hexachloropalladate(IV) ion, which changes to pale yellow or clear as the reaction proceeds. X-ray photoelectron spectroscopy, see Figure 1, has shown that the Pd(IV) ion sorbs onto the iron surface, followed by rapid reduction to Pd(0) (6). The reduction potentials of iron and palladium:
are such that the reaction is rapid and results in a stable surface which is relatively impervious to most chemical agents.
When produced in bulk, it is important to manage the production of hydrogen caused by the water-induced corrosion of iron:
Similarly, following palladisation, the material should be dried under an inert atmosphere to minimise corrosion by atmospheric oxygen. Dechlorination kinetics for volatile compounds were measured with batch studies using zero-headspace extractors (ZHEs) which prevent losses due to the leakage of volatile compounds (7). The essential components of a ZHE are a stainless steel cylinder and piston. The upward movement of the piston is controlled by applying a positive pressure at the bottom. At a given time during the reaction, a sample can be withdrawn or displaced by slowly raising the piston, such that there is no headspace, thus assuring the gaseous by-products are collected; they are then analysed by gas chromatography.
The dechlorinations of 1,1,2-trichloroethylene by iron and palladium/iron are compared in Figure 2. The rate constant for the reaction was approximately 0.0931/hour with iron, but improved to 1.17/hour with 0.05 per cent (w/w) palladium/iron (5). Figure 2 also shows that there is a difference in the by-products observed from the two reaction conditions. Intermediates 1,1-dichloroethene and 1,2-dichloroethene are observed when iron is the reductant. However, neither of these two compounds is observed when 1,1,2-trichloroethylene is reduced by palladium/iron. Similarly, although vinyl chloride is observed in both instances, the persistent concentration is reduced by approximately an order of magnitude when palladium/iron is used instead of iron. These results demonstrate that palladium/iron has a potential applicability to a wide variety of domestic and waste waters.
Unfortunately, not all chlorinated compounds are dechlorinated as readily as 1,1,2-trichloroethylene. For example, PCBs are notoriously difficult to degrade (8). A recent study required equipment comprising sealed ampoules and a temperature of 400°C for dechlorination to occur (9). With palladium/iron, however, all of the PCB congeners in Aroclor 1260 and 1254 – PCBs were typically sold and used commercially in mixtures called Aroclors – were dechlorinated within 10 minutes by reacting a methanol: water:acetone solution (mixed in a 1:3:1 volume ratio) with the 0.05 per cent w/w palladium/iron bimetal (3). Biphenyl, the principal reaction product, was identified in a hexane extract of the reaction mixture using gas chromatography/mass spectrometry, GC/MS. PCBs were not detected by GC/MS from hexane or methanol extractions of the palladium/iron following use. Parallel experiments were performed with unpalladised iron particles of size 10 μm. PCBs were detected in the hexane and methanol extracts of the unpalladised iron, but not biphenyl.
Preliminary work has also been performed with chlorophenols. As shown in the Table, a number of chlorophenols are easily degraded with palladium/iron. Present work is concentrating on pentachlorophenol (PCP), which is a widespread water and soil contaminant in the United States of America. The dechlorination of PCP from a solvent solution is shown in Figure 3. A solvent or surfactant is required because of the low aqueous solubility of PCP. Detoxifying contaminated soils by extraction with a solvent or surfactant is considered very expensive because of the cost of reclaiming the surfactant. This experiment, therefore, suggests that palladium/iron may solve the problem by providing a convenient means of degrading the contaminants within the original extraction solution.
Work with zero valence iron has suggested that the reductive dechlorination reaction is a simple electron transfer, following the sorption of the contaminant on the iron surface (10). A rapid two-electron transfer is suspected, based on thermodynamic calculations (11) and failure to observe free radical formation by electron spin resonance – as was attempted by the authors of this communication.
The precise role of the palladium has not been determined, however, two approaches are being considered. First, palladium may serve as an electron shuttle and may act in a manner similar to certain hydroquinones, such as jugalone. Research has shown that an azo-dye which is easily reduced by Fe0 is not reduced when the compound is prevented from sorbing directly to the iron.
The addition of jugalone to the system, however, facilitates electron transfer and the reduction then proceeds rapidly (12). Palladium may act in a similar fashion while bound to the iron. However, such an electron shuttle effect would essentially be an enhancement for the electron transfer process. The reaction rate for compounds which are reduced by zero valence iron would be increased. The effectiveness of palladium/iron for circumstances where zero valent iron is ineffective, such as for surfactant/solvent solutions or for dichloromethane and PCBs, can not, however, be explained by an electron shuttle effect.
Thus, it is believed that the propensity of palladium to sorb hydrogen (at 80°C it can sorb 900 times its own volume at a pressure of 1 atmosphere (13)) explains the increased reactivity of the palladium/iron. Hydrogen, formed as shown in Equation (v), is sorbed by the palladium forming a powerful reducing species that dechlorinates the chlorinated organic compound once it adsorbs onto the bimetal surface.
Additional research is being conducted to determine whether the use of palladium/iron has commercial value. First, little work has yet been performed regarding the selection of the iron base metal and the optimum palladium coverage. It is known that the reductive dechlorination occurs on the palladium/iron surface. However, the inter-relationships among iron surface area, palladium coverage and reactivity have not been adequately evaluated. Moreover, work with zero valence iron (5) has demonstrated that carbon impurities enhance the dechlorination of 1,1,2-trichloroethylene. Thus, impregnating carbon with palladium/iron or simply using a less pure, carbon-contaminated iron source may offer advantages both in terms of reactivity and cost.
The second major area where research is needed is with respect to the regeneration of the surface following use. Laboratory studies do not adequately simulate long-term field use. For example, clogging occurs with long-term use of zero-valence iron (14). Thus, because palladium/iron is more costly, it is expected that for viable commercial use, the surface will require periodic regeneration. Research in this area is centring on mineral acids and complexing agents, such as citric acid. It is hoped that these agents will remove iron oxides and other iron precipitates (such as carbonates and sulfates) which may accumulate when groundwater is passed through the palladium/iron.
It should be noted that sulfide is known to poison palladium catalysts. Fortunately, groundwaters used for domestic purposes rarely contain sulfide. Some potable waters do, however, contain sulfate which might be reduced by the sulfur-reducing bacteria that thrive in the reducing conditions created by the palladium/iron. This possibility is currently being investigated.
Finally, research is needed to determine the best method for deploying the palladium/iron material during a field-scale trial. Present work is evaluating packed bed reactors (15) but the use of foam iron or slurry reactors may have significant advantages for cost and ease of regeneration.
- 1N. Korte,, R. Muftikian,, C. Grittini,, Q. Fernando,, J. Clausen and L. Liang, ORNL/MMES, “ Research into Remedial Applications of Zero-Valence Metals. 2: Bimetallic Enhancements ”, in Am. Chem. Soc., Division of Environmental Chemistry, presented at the 209th ACS National Meeting, April 2–7, 1995, 35, (1), 752 – 753, Anaheim, Calif., 1995
- 2R. Muftikian,, Q. Fernando and N. Korte, Water Res ., 1995, 29, (10), 2434
- 3C. Grittini,, M. Malcomson,, Q. Fernando and N. Korte, Environ. Sci. Technol, 1996, 29, (11), 2898
- 4R. W. Gillham and S. F. O’Hannesin, Ground Water, 1994, 32, 958 – 967
- 5L. Liang,, N. Korte,, J. D. Goodlaxson,, J. Clausen,, Q. Fernando and R. Muftikian, “ Byproduct Formation during the Reduction of TCE by Zero-valence Iron and Palladised Iron ”, submitted to Ground WaterMonit. Rem ., 1996
- 6R. Muftikian,, K. Nebesny,, Q. Fernando and N. Korte, Environ. Sci. Technol, 1996, 30, (17), 3593
- 7R. L. Siegrist,, O. R. West,, C. K. Bayne,, R. A. Jenkins and C. A. Muhr, “ An Integrated Study of Sampling and Measurement Error for Soil VOCs ”. In final preparation for Ground Water Monit. Rem ., 1997
- 8D. C. Ayres, Platinum Metals Rev ., 1981, 25, (4), 160 ; op. cit ., 1988, 32, (4), 186 ; op. cit ., 1990, 34, (4), 180
- 9F. W. Chuang and R. A. Larson, “ Zero-Valent Iron-Promoted Dechlorination of Polychlorinated Biphenyls (PCBs) ”, in Am. Chem. Soc ., Division of Environmental Chemistry, presented at the 209th ACS National Meeting, April 2–7, 1995, 35, (1), 771 – 774, Anaheim, Calif ., 1995
- 10L. J. Matheson and P. G. Tratnyek, Environ. Sci. Technol, 1994, 28, 2045
- 11A. L. Roberts,, L. A. Totten,, W. A. Arnold,, D. R. Burns and T. J. Campbell, Environ. Sci Technol, 1996, 30, (8), 2654
- 12E. J. Weber, “ Iron-Mediated Reductive Transformations: Investigations of Reaction Mechanism ”, in Am. Chem. Soc ., Division of Environmental Chemistry, presented at the 209th ACS National Meeting, April 2–7, 1995, 35, (1), 702 – 705, Anaheim, Calif ., 1995
- 13F. A. Cotton and G. Wilkinson, “ Advanced Inorganic Chemistry ”, 1966, Interscience Publishers, New York, NY
- 14P. D. MacKenzie,, S. S. Baghel,, G. R. Eykholt,, D. P. Horney,, J. J. Salvo and T. M. Sivavec, “ Pilot Scale Demonstration of Chlorinated Ethene Reduction by Iron Metal: Factors Affecting Iron Lifetime ”, presented at the I&EC Special Symposium, Am. Chem. Soc ., September 17–20, 1995, p. 59, Atlanta, Georgia, 1995
- 15N. Korte,, L. Liang and J. Clausen, “ The Use of Palladized Iron as a Means of Treating Chlorinated Contaminants ”, presented at the I&EC Special Symposium, Am. Chem. Soc ., September 17–20, 1995, pp. 51 – 53, Atlanta, Georgia, 1995
The work described in this communication was supported by funds received from the Environmental Restoration Programs at the Portsmouth and Paducah Gaseous Diffusion Plants, and from the Office of Technology Development within the Department of Energy Office of Environmental Management, under the In Situ Remediation Integrated Program. The support of Jay Clausen from Paducah and Frank Anderson and Tom Houk at Portsmouth is gratefully acknowledged.
This is publication No. 4617, Environmental Sciences Division, Oak Ridge National Laboratory (ORNL). ORNL is managed by Lockheed Martin Energy Research Corp. for the U.S. Department of Energy under contract No. DE-AC05-960R22464.