MICROCHEMICAL JOURNAL ARTICLE NO.
60, 290 –295 (1998)
MJ981668
Quantification of Total Polychlorinated Biphenyl by Dechlorination to Biphenyl by Pd/Fe and Pd/Mg Bimetallic Particles John G. Doyle, TeriAnn Miles, Erik Parker, and I. Francis Cheng1 Department of Chemistry, University of Idaho, Moscow, Idaho 83844 –2343 Received September 4, 1998; accepted September 25, 1998
INTRODUCTION More than 20 years after their use was discontinued, polychlorinated biphenyls (PCBs) remain a persistent environmental problem. This is attributable to the chemical stability and hydrophobic nature of PCBs (1). There is therefore much interest in their analysis in environmental and biological samples. However, PCB analysis can be problematic. This arises because of the possibility of PCB mixtures containing up to 209 congeners. Most commercial mixtures contain about 100 components. Errors in the analysis of PCBs are further compounded by wide variations in the response of each congener to the predominate method of analysis, GC-ECD (2, 3). Environmental aging, varying degrees of volatility for each congener, and coelution of matrix species also add to the uncertainty of PCB analysis (1, 2). These features may give rise to congener populations that are different from the original commercial mixtures and thus skew total PCB estimates (1, 2). Reaction of the chlorinated biphenyls to one species would be of great benefit, especially to techniques employing chromatographic separations of complex matrices. Over the years several papers have been published that allow for the estimation of total PCB based on either the complete hydrodechlorination of PCBs to biphenyl or the perchlorination to decachlorobiphenyl (4 – 8). Analysis of the former may then be conducted by GC-FID, potentiometric detection of Cl2, or HPLC, and analysis of the latter by GC-ECD. The difficulties in these methods lie in the procedures involved in the chemical derivatization of the PCB to a single species. For hydrodechlorination methods this requires either the use of LiAlH4 at elevated temperatures under inert atmosphere, specialized catalyst preparation, or the handling of Na metal (1, 4, 5, 6, 7). Perchlorination also involves extensive procedures (1, 8). A new method of PCB analysis based on hydrodechlorination is presented. A recent method for the room temperature and atmospheric pressure dechlorination of PCB was recently demonstrated (9). The process is based on an aqueous reduction by the Pd/Fe bimetallic. The driving force for the dechlorination of PCBs to biphenyl is based on the dissolution of iron metal: Fe 3 Fe21 1 2e 2,
E 0 5 20.44 V vs standard hydrogen electrode.
(1)
Palladium serves as a hydrodehalogenation catalyst as with previous schemes that involve other chemical reducing agents (4, 5, 6, 10). The complete dechlorination process is 1
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Other metals may also provide the driving force for the palladium-catalyzed reaction in Eq. two. Magnesium provides higher reducing force (E 0 5 22.022 V) and was also used in this study. Relative to sodium metal (E 0 5 22.713 V), the reactivity of Mg(0) is moderated by the hydrolysis of its ions (pKsp[Mg(OH)2] 5 11.15) on the metal surface. The demonstrated method does not require any specialized instrumentation or any lengthy chemical workup. Complete dechlorination to biphenyl is rapid and reproducible. The resulting biphenyl is analyzed by HPLC with UV detection at 200 nm. MATERIALS AND METHODS Chemicals. Magnesium (98%, 20 mesh) was obtained from Aldrich Chemicals. Iron (99%, 40 –70 mesh) and K2PdCl6 were purchased from Alfa-Aesar. Biphenyl (2000 mg/ml) and Arochlor 1221 (1000 mg/ml) standards were supplied by Supelco of Bellefonte, Pennsylvania. Both 2-propanol and acetonitrile were of HPLC quality and obtained from Fisher Scientific. All water was of 18 MV cm purity and from a Millipore-Q reagent water system. Preparation of Pd/Fe and Pd/Mg bimetallics. Palladized iron was prepared by weighing 4 g of iron and washing the particles in 1 M HCl under streaming Ar gas for 40 min. The iron was then twice rinsed with deoxygenated water under Ar. Approximately 5 ml of deoxygenated water was added to the acid-etched Fe particles and enough K2PdCl6 was added to produce 0.05% (w/w) Pd/Fe. The mixture was vigorously shaken under Ar with allowances for the occasional venting of H2 gas for 5 min or until the characteristic orange color of dissolved PdCl22 disappeared. The Pd metal spontaneously deposits on the Fe 6 particle surface via the following reaction: 2 21 PdCl22 6 1 2Fe 3 Pd 1 6Cl 1 2Fe .
(3)
Palladized magnesium was produced by much simpler means. About 0.7 (6 0.05) g of Mg was weighed out and emptied in the reaction vessel (20-ml bottle). To this, dry K2PdCl6 (0.004 6 0.0005 g) was added. Palladization ensued after 3 ml of the solution containing Arochlor 1221 solution was added. PCB dechlorination simultaneously occurred as the Pd/Mg bimetallic formed. No precautions were taken to prevent oxygen from entering the system. PCB dechlorination. To either Pd/Fe or the K2PdCl6/Mg, 3 ml of Arochlor solution was added. The dechlorination solvent system consisted of 50/50 (v/v) 2-propanol/water. Reaction vessels were loosely capped to allow the escape of H2 and the dechlorination reactions were allowed to run for 10 (6 2) min before they were sampled.
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FIG. 1. Calibration curve for biphenyl standards using HPLC analysis (see Materials and Methods). Detection was UV absorbance at 200 nm.
Biphenyl analysis. Products of the dechlorination reaction were analyzed by isocratic HPLC. The system consisted of a Rheodyne 7120 valve with a 50-ml sampling loop, Acuflow Series I single piston pump with a coiled 3-m dampening loop, and a Hewlett– Packard Model 79853C UV detector tuned to 200 nm. The separation column was a 10-cm C-8 Zorbax cartridge column. The mobile phase was a degassed solution of 65/35 acetonitrile/water. Further analyses of selected dechlorination runs were conducted by GC-FID and GC-ECD. RESULTS Studies of catalyzed and uncatalyzed systems. The importance of Pd on each metal system is evident by the biphenyl yield from the dechlorination of the Arochlor 1221 solutions. In the absence of Pd, PCB dechlorination was essentially nil after 10 min of reaction time on either Fe(0) or Mg(0) as observed by GC or HPLC (see below for procedure). Subsequent studies always employed the use of Pd dechlorination catalysts. Pd/Fe versus Pd/Mg dechlorinations. The preparation of Pd/Mg was found to be much simpler and faster than the Pd/Fe system. The dechlorination yields as measured by HPLC (see below) for either system were identical. Unless otherwise noted, the studies below employed the use of the Pd/Mg bimetallic system. Gas chromatographic studies. A 1-ml sample of dechlorination solution was obtained after 10-min reaction time using 5 3 1025 M Arochlor 1221 with either bimetallic system.
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FIG. 2. HPLC chromatograms for 1.11 3 1025 M Arochlor 1221 (bottom) and for 3.0 ml of this solution treated with 0.77 g Mg and 4.5 mg K2PdCl6 for 10 min (top). The large peak in the top chromatogram is biphenyl.
GC-FID analysis indicated that biphenyl was the predominate species. Biphenyl concentration in the original Arochlor mixture was found to be 3.3% (w/w) as measured by GC-FID. The other minor peaks that appeared were attributed to solvent components in 2-propanol. Analysis of chlorinated products by GC-ECD indicated that PCBs were completely dechlorinated to the detection limit of the method ('1 ng), in agreement with previous investigations (9). HPLC analysis of dechlorination mixtures. To determine quantitatively the biphenyl yields from the dechlorination mixtures a biphenyl calibration curve was developed by diluting the standard solutions in 50/50 (v/v) 2-propanol/water. The solutions were injected onto the HPLC with detection at 200 nm. The results of this study are presented in Fig. 1. The estimated limit of detection (LOD) for biphenyl in these circumstances is 5 3 1027 M. Figure 2 shows the HPLC results for an Arochlor 1221 mixture before and after dechlorination by Pd/Mg. The large peak in Fig. 2 (top) is attributable to biphenyl. A calibration curve for biphenyl from the Arochlor 1221 mixture is presented in Fig. 3. Biphenyl yields from the various dechlorination runs were found to average 98%. These yields are reflected in the ratio of slopes from Figs. 1 and 3. The estimated LOD for total PCB in Fig. 3 is 5 3 1027 M. DISCUSSION Advantages of the presented method are the speed and ease of the analytical procedure ('10 min) requiring no specialized laboratory equipment or precautions. Dechlorination
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FIG. 3. Calibration curve of biphenyl from the dechlorination of Arochlor 1221.
of PCBs by Pd/Mg involves only the mixing of dry Mg powder and K2PdCl6 with 3 ml of analyte solution. The detection limit of the described dechlorination method is about 100 ppb (5 3 1027 M) or 5 ng without any sample preconcentration. This compares very well with present methods for total PCB analysis and with those involving hydrodechlorination to biphenyl with detection by GC-FID and HPLC-UV 248 nm (5 and 100 ng, respectively) (1, 4, 6). The primary disadvantage of this method is the need for a biphenyl background prior to PCB dechlorination. This feature is shared by all dechlorination methods. Another disadvantage is the volatility of the biphenyl, which could skew total PCB measurement. However, this sort of systematic error was not observed on the time scale of 10 min. In principle, the described PCB dechlorination method can be adapted to schemes involving extraction and preconcentration steps of complicated matrices such as soils and biological tissues. The Pd/Mg dechlorination system may also be employed in the analysis of other halocarbon mixtures such as lindane and DDT/DDE. Because of the ease of this method, hydrodechlorinations may find wider use in analytical procedures. ACKNOWLEDGMENT The authors express their gratitude to Professor Quintus Fernando for his helpful advice.
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3. Mullin, M. D.; Pochini, C. M.; McCrindle, S.; Romkes, M.; Safe, S. H.; Safe, L. M. Environ. Sci. Technol., 1984, 18, 468. 4. Zimmerli, B. J. Chromatogr., 1974, 88, 65–75. 5. Cooke, M.; Nickless, G.; Prescott, A. M.; Roberts, D. J. J. Chromatogr., 1979, 156, 293–299. 6. Seidl, G.; Ballschmiter, K. Fresenius Z. Anal. Chem., 1979, 296, 281–284. 7. Ware, M. L.; Argentine, M. D.; Rice, G. W. Anal. Chem., 1988, 60, 383–384. 8. Steinwandter, H.; Bru¨ne, H. Fresensius Z. Anal. Chem., 1983, 314, 160. 9. Grittini, C.; Malcomson, M.; Fernando, Q.; Korte, N. Environ. Sci. Technol., 1995, 29, 2898 –2900. 10. Cheng, I. F.; Fernando, Q.; Korte, N. Environ. Sci. Technol., 1997, 31, 1074 –1078.