Relative Resistance of Positional Isomers of Polychlorinated

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Environ. Sci. Technol. 2000, 34, 2792-2798

Relative Resistance of Positional Isomers of Polychlorinated Biphenyls toward Reductive Dechlorination by Zerovalent Iron in Subcritical Water HWA K. YAK, QINGYONG LANG, AND CHIEN M. WAI* Department of Chemistry, University of Idaho, Moscow, Idaho 83844

Relative resistance of positional isomers within the same homologue of polychlorinated biphenyls (PCBs) was studied by comparing the reduction efficiencies (REs) of these isomers by 100-mesh zerovalent iron in subcritical water at 250 °C and 10 MPa. The REs for meta and para isomers were found to be significantly higher than that of the ortho’s. These results revealed that variation in relative resistance of the positional isomers to reductive dechlorination does exist, and the order increases from para to meta to ortho substituents. This variation in relative resistance to reduction is correlated with the lowest unoccupied molecular orbital (LUMO) energy of individual PCB congeners. A model based on the empirical relative resistance of the PCBs to reductive dechlorination is developed, and an equation is established to predict the efficiency of a reductive dechlorination system that employs zerovalent iron and pressurized hot water.

Introduction Polychlorinated biphenyls (PCBs) are a class of toxic semivolatile organic compounds of great adverse environmental impacts that is widely and massively distributed in our environment (1-4). Many scientists had endeavored to devise remediation techniques to effectively treat PCB-contaminated matrices including water, oils, sediments, and soils (5-8). Although PCBs are considered toxic substances by toxicologists, toxicities of individual congeners may vary a great deal (9). PCBs can be classified into ortho or nonortho congeners based on the presence or absence of orthosubstituted chlorine(s). Because the electrons on the ortho chlorine repel the electron density on the aromatic phenyl ring, the conformation of the PCB molecule is determined by the presence of ortho chlorines. Contrarily, the meta and para chlorines are too far from the aromatic nucleus to cause this effect. The angle between the two phenyl rings is known as the twist angle of the molecule. The effect of the number of ortho chlorines on the twist angle of the molecule has been calculated by Greaves et al. using the semiempirical general molecular orbital program MOPAC (version 5.0) (10). For zero-, one-, two-, three-, and four-chlorine substitution, the twist angles were found to be approximately 41, 56, 76, 86, and 87 degrees, respectively. At three- and four-chlorine substitution, the phenyl rings are almost orthogonal to each * Corresponding author phone: (208)885-6552; fax: (208)885-6173; e-mail:[email protected]. 2792

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other. In general, the PCBs with ortho chlorine(s) are loosely referred to as nonplanar and those without are referred to as coplanar. The coplanar PCBs are of much more toxicological interests for they resemble, both in structure and activities, the most potent manmade toxic organic molecules2,3,7,8tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD)swhich has been used as a yardstick to measure the toxic equivalence quotient (TEQ) of other toxic organic substances (9). Based on structure-activity relationship, the most toxic PCBs are substituted at the para and at least one meta position of both phenyl rings and do not contain any ortho-chloro substituents (9). This principle defines a subset of four compounds, namely 3,3′,4,4′-TCB (BZ# 77), 3,4,4′,5-TCB (BZ# 81), 3,3′,4,4′,5-PCB (BZ# 126), and 3,3′,4,4′,5,5′-HCB (BZ# 169), as most toxic in the PCB family. Of these four compounds, all but BZ# 81 elicit comparable biologic and toxic effects (similar TEQs) to those of 2,3,7,8-TCDD (9). Toxicologists classify PCBs substituted at one para and multiple meta positions that do not contain any ortho chlorines as having intermediate potency. Ortho-substituted PCBs without any meta- and para-chloro substituents are considered least potent (9). We reported recently that the higher-substituted homologues indigenous in Aroclor 1260 were completely reduced to their lower-substituted counterparts by reductive dechlorination with zerovalent iron in subcritical water at 250 °C and 10 MPa (11). Aroclor 1260 is a mixture of over 40 PCB congeners, with over 90% of its content comprised of the penta-, hexa-, and heptachlorobiphenyl homologues (12). The chromatograms of Aroclor 1260 are thus fairly congested, with many peaks partially or completely unresolved. It is conceivable that specific and quantitative studies using Aroclor 1260 or any complex PCB mixture may not be the best choice. For the purpose of investigating the response of various positional isomers toward reductive dechlorination, single PCB congeners should provide more exact and definitive answers. Understanding of the reaction of the planar and nonplanar PCBs to reducing agents such as zerovalent iron may provide further insight into the mechanisms as well as the feasibility of reductive dechlorination by zerovalent metals under subcritical water conditions.

Experimental Section Chemicals and Instruments. Electrolytic iron powder (100 Mesh), HPLC grade acetone, hexanes, water, and sea sand were purchased from Fisher Scientific (Fair Lawn, NJ). All single congener standards were purchased from Ultra Scientific (North Kingstown, RI) in solid form with purity over 99% and were used without further purification. An Isco 260D Syringe Pump was used to pressurize the water, and a GC oven was used for maintaining the temperature to (2 °C. A HPLC solvent pump was connected to the system to deliver the hexanes. High temperature and pressure extraction cells were purchased from Keystone Scientific, Inc. (Bellefonte, PA). A Hewlett-Packard 5890 GC-FID was used for analyses of the PCB standard and samples. The 30-m DB-5 capillary column purchased from Alltech (Deerfield, IL) had a 0.32 mm i.d. and 0.25 µm film thickness. The carrier gas was chromatographic grade N2 (Oxarc, Spokane, WA). Sample injection volume was 1 µL splitless with the injector temperature set at 280 °C and the detector set at 300 °C. Separation was conducted with a temperature program that held column temperature at 70 °C for 1 min and then ramped at 15 °C/min until it reached 300 °C, where it was held for 5 min. 10.1021/es990689b CCC: $19.00

 2000 American Chemical Society Published on Web 05/17/2000

FIGURE 1. Possible reduction pathways of BZ# 52.

FIGURE 2. Possible reduction pathways of BZ# 77. Procedures. A detailed description of the experimental procedures, including a schematic of the subcritical water extraction (SWE) apparatus, has been reported previously (11) and hence will be abbreviated here. The SWE conditions for this work were set at 250 °C and 10 MPa, with reaction times ranging from 0.5 to 8 h. The amount of 100-mesh iron powder used for each experiment was between 0.50 and 3.0 g, depending on the protocol. The amount of single congeners used ranges from 180 to 220 µg for each congener (100 µL of ca. 2000 ppm solution). Due to the small amount of iron used, PCBs were spiked onto 1.0 g of sea sand instead of directly onto the iron powder. Fluoranthene, a polyaromatic hydrocarbon, was used as the internal standard at 20.4 ppm. Peak areas of the standard solution and the sample solution were normalized to the peak area of the internal standard after GC analyses. Reduction efficiency of a particular PCB congener was calculated by dividing the difference in peak area between the corresponding peak in the standard and the sample by the peak area of this peak in the standard.

Results and Discussion 1. Resistance of Positional Isomers toward Reductive Dechlorination. In the previous paper (11), we discussed the observation that the process of reductive dechlorination may have occurred stepwise, with a chlorine atom being replaced by a hydrogen atom in a single reduction step. Based

on this postulation, a reduction pathway scheme can be deduced for every PCB congener. For the purpose of this study, some symmetrical congeners had been chosen to enhance simplicity of the argument. BZ# 52 (2,2′,5,5′-TCB) contains one ortho chlorine and one meta chlorine on each of the phenyl ring and is a good candidate to compare the resistive effect of ortho and meta position chlorines. On the other hand, BZ# 77 (3,3′,4,4′-TCB) contains one meta chlorine and one para chlorine on each of the phenyl rings and will elucidate the effect of meta and para position chlorines. Figures 1 and 2 summarize the possible reduction pathway schemes for these two congeners. Should there exist no preference for reductive dechlorination on the various positions, one would expect the products in the first generation to be equal in quantity, which should also be true for the other generations. Our observations, however, do not support this rationale. The result of reductive dechlorination of 2,2′,5,5′-TCB (BZ# 52) by 0.5 g of 100-mesh zerovalent iron is shown in Figure 3. In this and the following cases, the reduction products are identified with single congener standards. It is obvious that there is a favored reduction pathway for BZ# 52. Displacement of either one of the meta position chlorines to form 2,2′,5TCB (BZ# 18) was much more favored than that of either one of the ortho position chlorines to form 2,3′,5-TCB (BZ# 26) with a ratio of BZ# 18/BZ# 26 being approximately 20/1. VOL. 34, NO. 13, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1: Descriptions of PCB Congeners with Their Respective Reduction Efficiencies (RE) and Lowest Unoccupied Molecular Orbital (LUMO) Energiesa BZ# homologue 4 11 15 52 54 77 153 155 207

di di di tetra tetra tetra hexa hexa nona

Cl position 22 33 44 2255 2266 3344 224455 224466 223344566

ortho meta para 2 0 0 2 4 0 2 4 4

0 2 0 2 0 2 2 0 3

0 0 2 0 0 2 2 2 2

RE (%)

LUMO (eV)

9.3 28 31 30 20 62 76 57 98

-0.1151 -0.4553 -0.5449 -0.5066 -0.2098 -0.844 -0.857 -0.6129 -0.9899

a 0.35 g of 100-mesh iron powder was adopted for each congener under subcritical water conditions of 10 MPa and 250 °C. Reaction time is 2 h.

FIGURE 3. Reductive dechlorination of BZ# 52 after 2 h of reaction time at 10 MPa and 250 °C with 0.5 g of 100-mesh zerovalent iron in subcritical water. IS stands for internal standard.

FIGURE 4. Reductive dechlorination of BZ# 77 after 2 h of reaction time at 10 MPa and 250 °C with 0.5 g of 100-mesh zerovalent iron in subcritical water. IS stands for internal standard. Examination of the second-generation products again proves this preference to be working. 2,2′-DCB (BZ# 4) is present as the major product among the second-generation products, while 2,3′-DCB (BZ# 6) and 2,5-DCB (BZ# 9) are relatively minute in comparison. This result shows conclusively that ortho position chlorines are much more resistant to reductive dechlorination than the meta ones. The result of reductive dechlorination of 3,3′,4,4′-TCB (BZ# 77) under the same conditions is shown in Figure 4. In this system all the possible dechlorination products in the scheme are present in measurable quantities. The ratio of the firstgeneration products, 3,3′,4-TCB (BZ# 35)/3,4,4′-TCB (BZ# 37), is about 3/2. The initial inference from this observation is that there is a preference to remove the para position chlorines over the meta ones. The second-generation products show three peaks on the chromatogram because 3,4DCB (BZ# 12) and 3,4′-DCB (BZ# 13) are unresolved under the chromatographic conditions. In these second-generation products, the amount of 3,3′-DCB (BZ#11) is about twice as high as the 4,4′-DCB (BZ# 15). Further examination of the third-generation products, or the monochloro homologues, shows that 3-MCB (BZ# 2) is again about 1.5 times more than 4-MCB (BZ# 3). These repeated confirmations lead us to conclude that meta position chlorines are somewhat more resistant to reductive dechlorination than the para ones. A comparison of the results in Figures 3 and 4 also reveals that the amount of third-generation products present in the 2794

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BZ# 77 system is significantly more than the BZ# 52 system and that biphenyl is present in the BZ# 77 system while nondetectable in the BZ# 52 system. These qualitative observations provide further evidence that the ortho position chlorines are much more unyielding to reductive dechlorination than the meta and para ones, causing the reaction to stall at BZ# 4 (2,2′-DCB). To further confirm these observations and to investigate if more concrete quantitative data can be obtained, experiments were done with the PCB congeners of the same homologue placed together in the reaction vessel to undergo identical treatment simultaneously. In this manner discrepancies such as undetectable leaks in the cell, variation from addition of internal standard, or even some unknown factors can be eliminated. Three homologues were examined in this manner in which 0.35 g of 100-mesh iron powder was adopted for each congener present in the system. The dichlorobiphenyl homologue included BZ# 4, 11, and 15; the tetrachlorobiphenyl homologue consisted of BZ# 52, 54, and 77; and the hexachlorobiphenyl homologue comprised of BZ# 153 and 155. The descriptions of the congeners and the results are shown in Table 1. The relative standard deviations (RSDs) of this set of result range from 3.7 to 9.5%, with a general trend that lower homologues have lower RSDs. The dichlorobiphenyl homologue showed reduction efficiencies well in the range that fits in to the previous discussion, with the diortho BZ# 4 exhibiting reduction efficiency (RE) of 9.3%, the dimeta BZ#11 28%, and the dipara BZ# 15 30%. The efficiencies of reduction of meta and para substituents were approximately three times faster than that of the ortho’s. The tetrachlorobiphenyl homologue best illustrates our model. BZ# 54, which contains four ortho chlorines, showed the most resistance to dechlorination (RE ) 20%). BZ# 52, which contains 2 ortho and 2 meta chlorines, is less tenacious to dechlorination (RE ) 30%). While BZ# 77, which contains 2 meta and 2 para and no ortho chlorines, showed a reduction efficiency twice as high as BZ# 52 and three times as high as BZ# 54 (RE ) 62%). Even when both isomers contain two para chlorines and differ only in the meta chlorines, as in the case of BZ# 153 and 155 in the hexachlorobiphenyl homolog, the variation in resistance toward reduction still stood out as quite significant (RE ) 76% and 57%, respectively). Thus, this second set of experiment further confirms that the relative resistance of positional isomers increases from para to meta to ortho positions. 2. Correlation of Reduction Efficiency with LUMO. A question that naturally arises at this point is why do certain lower congeners show higher reduction efficiencies than the higher ones in this report? For instance, BZ# 15 has a RE of 31% while BZ# 54 only shows 20%, and BZ# 77 shows 62% while BZ# 155 only shows 57% (Table 1). Although this

FIGURE 5. Plot of reduction efficiencies of PCB congeners versus calculated LUMO energies. R 2 of the trend line is 0.895, giving r ) 0.946. SWE conditions are described in Table 1 caption. variation can be conceptually and qualitatively explained by the presence of ortho chlorines on the more tenacious congeners, it cannot be quantitatively represented. Greaves et al. had calculated lowest unoccupied molecular orbital (LUMO) energies of the entire PCB family using the semiempirical general molecular orbital program MOPAC (version 5.0). A partial listing of this calculated LUMO energies had been published (10), and the complete listing was provided through personal communication. Arbon et al. were the first group of researchers to study the correlation of absorbed dose of γ-radiation (reported as dose constants in their publication) and the LUMO energies of various PCB congeners (13). These authors found a strong correlation between the LUMO energies and the dose constants of 25 PCB congeners ranging from mono- to decachlorobiphenyl. They also found that the highest dose constants occur only for the para/meta-substituted congeners within a homolog, which have the lowest LUMO energies. The presence of orthosubstituent(s) was observed to lower the dose constant of that particular congener (13). Figure 5 plots the reduction efficiencies (REs) versus the LUMO energies of the congeners studied in this report. Careful observation of this figure reveals that the congeners with the lowest LUMO energies exhibit highest REs, while those with high LUMO energies exhibit low REs. An R 2 value of 0.895 (or r-value of 0.946) suggests that reduction efficiencies of PCBs by zerovalent iron can be strongly correlated to their individual calculated LUMO energies. To test the practicality of this correlation, two congeners of the trichlorobiphenyl homolog, BZ# 18 (2,2′,5-TCB) and BZ# 35 (3,3′,4-TCB), were employed. The results, which are shown in Table 2, indicate that the estimated and empirical values agree with each other within experimental errors and deviation of the regression trend line. Similar to the results of Arbon et al.’s, our results show that the reduction efficiencies of individual PCB congeners can be predicted based on the calculated PCB LUMO energies. As mentioned earlier, the presence of one or more ortho chlorines significantly changes the conformation of the PCB molecule, resulting in the nonplanarity of the phenyl rings and restricting their free spinning along the biphenyl-bridge. This conformation forces the electron cloud of the ortho chlorine on one phenyl ring to be hanging on top of the other phenyl ring, which is likely to hinder the reduction of the molecule.

TABLE 2: Descriptions of Test PCB Congeners and Their Lowest Unoccupied Molecular Orbital (LUMO) Energies, Reduction Efficiencies (RE), and Their Respective Estimated RE by LUMO and by Equation 4a estimated RE (%) BZ#

Cl position

ortho

meta

para

LUMO (eV)

RE (%)

by LUMO

by eq 4

18 35

225 334

2 0

1 2

0 1

-0.3286 -0.6737

20 47

23 55

26 47

a 0.35 g of 100-mesh iron powder was adopted for each congener under subcritical water conditions of 10 MPa and 250 °C. Reaction time is 2 h.

3. Reaction Mechanism and Kinetic Studies. The mechanism of reduction of alkyl halides by zerovalent iron in an aqueous environment at ambient temperature has been examined by Matheson et al. (14). Three possible pathways were identified in this publication, which included direct electron transfer from iron metal at the iron surface, reduction by Fe2+ from corrosion of iron, and catalyzed hydrogenolysis by the hydrogen formed from reduction of water. The first pathway was thought to be most probable. Inspired by their proposition, Weber used 4-aminoazobenzene (4-AAB) to conclusively show that the reduction of 4-AAB to form aniline at 25 °C required surface contact between the iron and the substrate (15). The voltammetric reduction of organohalogen compounds including PCBs in dimethyl sulfoxide (DMSO) was studied extensively by Farwell et al. (16, 17). The PCB reduction pathways were found complex in comparison to the reduction pathways for chlorinated benzenes. The reaction mechanism of PCBs and zerovalent iron in water at high temperature and pressure, on the other hand, has not been published in the literature. Electron transfer at the iron surface is certainly a probable process for PCB dechlorination in this system and is assumed to be the reaction mechanism in this article. For the surface mediated mechanism, the reaction between PCBs and zerovalent iron can be written as

C12HnCl(10-n) + Fe + H+ f C12H(n+1)Cl(9-n) + Fe2+ + Cl- (1) VOL. 34, NO. 13, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 3: Kinetics of Dechlorination of BZ# 77 by Initial Rate Study at 250 °C and 10 MPa for 1 h expt #

BZ# 77 (µmol)

Fe(0) (g)

initial rate (µmol BZ# 77/h)

1 2 3 4 5

0.644 0.644 0.644 0.644 1.29

0.5 1 2 3 0.5

0.29 0.37 0.41 0.48 0.61

FIGURE 7. Kinetics study of reductive dechlorination of BZ# 77 by 0.5 g of 100-mesh zerovalent iron after 0.5, 1, 2, 4, and 8 h of reaction times at SWE conditions of 10 MPa and 250 °C. Concentration of PCB congener is from dissolving products in 10 mL of hexanes.

FIGURE 6. Plot of reduction efficiency of BZ# 77 versus the initial amount of Fe present in the system at SWE conditions of 10 MPa and 250 °C for 1 h. The proton in this equation may come from the ionization of water or from an added source. The ferrous ion formed as a product typically associates itself with the hydroxide ion from ionization of water as a short-lived intermediate. At temperatures above 60 °C, ferrous hydroxide is metastable and quickly undergoes a disproportionation reaction to form magnetite, which is a highly stable compound (18). This is indeed what we observed empirically after the experiments. Once an iron particle is completely coated with magnetite, the particle is passivated and ceases to participate in the reduction of water or PCBs. Although this passivation process is well-known, the rate of passivation at various temperatures and for various sizes and shapes of iron particles are not well understood at all. For this reason the kinetics study results shown herein should be viewed as a rough approximation only until better understanding of the rate of passivation is attained from future studies. From eq 1, the rate equation of PCB reduction can be expressed as

rate ) k[PCB]x[Fe]y[H+]z

(2)

The method of initial rate study at 250 °C and 10 MPa for both BZ# 77 and Fe(0) was performed, and the results are shown in Table 3. Doubling the initial amount of iron from 0.5 to 1.0 g and from 1.0 to 2.0 g gave fractional orders of 0.4 and 0.2, respectively, thus indicating that a complex process is involved with the participation of iron in this equation. A plot of reduction efficiency of BZ# 77 with respect to the initial amount of iron present is depicted in Figure 6 to show that the reduction efficiency does not improve significantly beyond approximately 1.5 g of iron. Table 3 also shows that doubling the initial amount of BZ# 77 from 0.644 to 1.29 µmol approximately doubles the rate of reaction, thus suggesting that the order of BZ# 77 in this equation is one. Hinz et al. recently studied the dechlorination of nonachlorobiphenyl (BZ# 207) with zerovalent iron in subcritical water and showed that the reaction followed a first-order kinetics with respect to the concentration of the PCB (19). 2796

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Figure 7 shows the decrease in BZ# 77 concentration (in logarithmic scale) with time for a fixed amount of zerovalent iron (0.5 g) in subcritical water at 250 °C and 10 MPa. The dechlorination reaction appears to follow a pseudo-firstorder process below treatment times of 2 h or less (marked with a broken line). The rate of dechlorination between 2 and 8 h slows down significantly, and the reaction order is no longer well defined. The fact that there exists a nonlinearity in overall reaction order is likely due to the competitive reaction of water with zerovalent iron discussed in the previous paragraph. With the huge excess of water present in the reaction system, the availability of iron to the PCBs decreases fairly quickly. Observations of the generation of hydrogen from the reduction of water at these SWE conditions show that hydrogen production increases exponentially for the first hour and plateaus by about 2 h. These observations suggest that most of the surface of the iron particles had already been coated with magnetite at the end of 2 h, thus causing the overall kinetics of dechlorination to slow significantly beyond 2 h of treatment time. It was mentioned earlier that the reduction of PCBs in SWE might have occurred stepwise based on our previous observation. In this study we employed BZ# 77 to further investigate this point. Experiments at 0.5, 1, 2, and 4 h of reaction times were performed, and the results are shown in Figure 8. Examining the first generation products (BZ# 35 and 37) shows that they are present in all time scales. On the other hand, the second-generation products (BZ#11, 12, 13, and 15) are not present at the 0.5 h time scale but are present after the 1-h experiment. These two observations indicate that the first reduction step happens with the first chlorine atom being replaced by a hydrogen atom. Similarly, the thirdand fourth-generation products show that the reduction process progresses sequentially. Hence it is concluded that the reductive dechlorination of PCBs in SWE does follow a stepwise pathway. The mass balance in this series of experiments averaged to 90 ( 8%. Based on our previous experiments with Aroclor 1260 and present observations, the rest of the carbon mass was present as several aliphatic hydrocarbon products existing as normal and branched chain hydrocarbons between eight and 12 carbons in length. The presence of these clusters of hydrocarbons and the lack of appropriate calibration standards complicate exact quantitation of these reduction products. An estimation of the concentrations of these products was done by using decane and dodecane, and the amount was found to be about 5% of the initial mass of BZ# 77. The presence of this cluster of hydrocarbon also poses the question if they are the reduction products of biphenyl by zerovalent iron. Experiments with

FIGURE 8. Product distribution of reductive dechlorination of BZ# 77 as the parent compound by 0.5 g of 100-mesh zerovalent iron after 0.5, 1, 2, and 4 h of reaction times. SWE conditions were 10 MPa and 250 °C. Concentration of PCB congener is from dissolving products in 10 mL of hexanes. biphenyl and zerovalent iron at the same SWE conditions yield average recovery of 95 ( 4.3%, indicating that very little, if any, of the biphenyl is reducible by zerovalent iron. Hence the production of aliphatic hydrocarbon happens before the PCB is reduced to biphenyl. 4. Prediction of Reduction Efficiency. The reduction efficiency (RE) of various congeners may be calculated based on the total number of Cl and their distribution in the PCBs. Assuming that Cl in each position (ortho, meta, and para) has fixed contribution and is independent to each other in dechlorination, we can express RE by a linear combination of the contribution from each position according to the following equation

RE ) A(Clo) + B(Clm) + C(Clp)

(3)

where Clo ) number of ortho chlorines and 0 e Clo e 4, Clm ) number of meta chlorines and 0 e Clm e 4, Clp ) number of para chlorines and 0 e Clp e 2, and A, B, and C are empirically determined reduction coefficients (RECs) relating to Clo, Clm, and Clp, respectively. By substituting the chlorine numbers at various positions and RE results in Table 1 for the eight congeners mentioned, we can find the best-fit values for A, B, and C. The average and standard deviation of the RECs were computed as follows:

A ) 0.057 ( 0.011 B ) 0.14 ( 0.028

Acknowledgments

C ) 0.19 ( 0.026 Hence a general equation for the specified experimental conditions can be written as

RE ) 0.057(Clo) + 0.14(Clm) + 0.19(Clp)

35 47 ( 6%. The experimental results show 20% and 47% for BZ# 18 and BZ# 35, respectively (Table 2), which agree with the range of the predicted values. In Table 1 the experimental value of BZ# 207 (2,2′,3,3′,4,4′,5,6,6′-NCB) is shown as 98%, while the estimated value of 104 ( 6% also agrees with the empirical result. It should also be noted that RE in eq 4 may be larger than unity at higher number of chlorines. A RE larger than one simply suggests that the conditions employed are more than enough to completely reduce that particular congener to one with one less chlorine. In conclusion, our findings indicate that the most and intermediately toxic coplanar PCB congeners can be very easily reduced by zerovalent iron due to the lack of orthochloro substituents. On the other hand, our findings also imply that complete dechlorination of nonplanar congeners, especially those containing three or four ortho chlorines and no meta and para substituents, may be an arduous task to achieve for this system. Our model shows that the efficiency of a reductive dechlorination system employing zerovalent iron and subcritical water can be understood by semiempirically correlating calculated LUMO energies to the reduction efficiencies. By empirically establishing reduction coefficients for the ortho, meta, and para positions of a few PCB congeners, reduction efficiencies of the entire PCB family can be estimated using a simple linear combination of the contribution from each position. Both these methods provide a simplified and swift way to evaluate a reductive dechlorination system.

(4)

Equation 4 can be used to predict the effect of adding one more chlorine to the biphenyl nucleus. RE will be improved by about 6% if the chlorine is added to the ortho position, by about 14% for the meta position, and by about 19% for the para position. The equation can also be used to estimate the RE of any PCB congener under the specified experimental conditions. To illustrate this point, we employ the same two congeners, BZ# 18 and BZ# 35, mentioned earlier. Accordingly, the estimated RE for BZ# 18 is 26 ( 6% and for BZ#

The authors wish to thank J. Greaves, E. Harvey, and G. MacIntyre for sharing the LUMO energy data and Mike Pearson of Anatek Labs, Inc., Moscow, ID for analyzing some of the PCB samples.

Literature Cited (1) Jones, G. R. N. The Lancet 1989, Sep 30, 791. (2) Environmental Science Research, Vol. 37-Hazards, Decontamination, and Replacement of PCB; Crine, J.-P., Ed.; Plenum Press: New York, 1988. (3) Physical Behavior of PCBs in The Great Lakes; Mackay, D., Peterson, S., Eisenreich, S. J., Simmons, M. S., Eds.; Ann Arbor Science Publishers: Ann Arbor, MI, 1983. (4) PCB Poisoning and Pollution; Higuchi, K., Ed.; Academic Press: New York, 1976. VOL. 34, NO. 13, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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(5) Hitchman, M. L.; Spackman, N. C.; Ross, N. C.; Agra, C. Chem. Soc. Rev. 1995, 24, 423-430. (6) Arbon, R. E.; Mincher, B. J.; Knighton, W. B. Environ. Sci. Technol. 1996, 30, 1866-1871. (7) Renner, R. Environ. Sci. Technol. 1998, 32, 360A-363A. (8) Marques, C. A.; Selva, M.; Tundo, P. J. Org. Chem. 1994, 59, 3830-3837. (9) Environmental Toxin Series 1-Polychlorinated Biphenyls (PCBs): Mammalian and Environmental Toxicology; Safe, S., Hutzinger, O., Ed.; Springer-Verlag: New York, 1987. (10) Greaves, J.; Harvey, E.; MacIntyre, G. J. Am. Soc. Mass. Spectrom. 1994, 5, 44-52. (11) Yak, H. K.; Wenclawiak, B. W.; Cheng, I. F.; Doyle, J. G.; Wai, C. M. Environ. Sci. Technol. 1999, 33, 1307-1310. (12) Erickson, M. D. Analytical Chemistry of PCBs, 2nd ed.; CRC/ Lewis Publ.: Boca Raton, FL, 1997. (13) Arbon, R. E.; Mincher, B. J.; Knighton, W. B. Environ. Sci. Technol. 1994, 28, 2191-2196.

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(14) Matheson, L. J.; Tratnyek, P. G. Environ. Sci. Technol. 1994, 28, 2045-2053. (15) Weber, E. J. Environ. Sci. Technol. 1994, 30, 716-719. (16) Farwell, S. O.; Beland, F. A.; Geer, R. D. Electroanal. Chem., Interfacial Electrochem. 1975, 61, 303-313. (17) Farwell, S. O.; Beland, F. A.; Geer, R. D. Electroanal. Chem., Interfacial Electrochem. 1975, 61, 315-324. (18) Fujita, N.; Matsuura, C.; Ishigure, K. Corrosion 1990, 46(10), 804-812. (19) Hinz, D. C.; Wai, C. M.; Wenchawiak, B. W. J. Environ. Monit. 2000, 0, 1-4.

Received for review June 21, 1999. Revised manuscript received March 21, 2000. Accepted April 4, 2000. ES990689B