Environ. Sci. Techno/. 1995, 29, 98-103
Formal Reduction Potentials and Redox Chemistty of ilted Biphenyls in a Q I N G D O N G HUANG A N D JAMES F. RUSLING* Deparment of Chemistry (U-60), Uniuersity of Connecticut, Storrs, Connecticut 06269-3060
Microemulsions are clear, stable fluids which contain water, oil, and monolayers of surfactant. Redox studies of pollutants in such microheterogeneous fluids containing water may be more relevant to environmental interactions than studies in organic solvents. In this paper, cyclic and square wave voltammetry on a mercury-film electrode were used to estimate the formal potentials of the first reduction peak and to investigate the rate-determining step in the reduction of polychlorinated and polybrominated biphenyls. Slow kinetics of the initial electron transfer to form a halobiphenyl anion radical was the ratedetermining step. Formal potentials in the microemulsion were shifted considerably positive relative to values in homogeneous organic solvent, with the more heavily halogenated congeners showing the largest shifts. Implications of the results with respect to PCB toxicity and anaerobic degradation are discussed.
98 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, NO. 1,1995
Polychlorinated biphenyls (PCBs) are among the most widespread pollutants in the global ecosystem ( 1 ,2). Brown and co-workers showed that PCBs are partly dehalogenated by anaerobic bacteria in Hudson River sediments (3). Aquatic sediments of varying activityhave been discovered recently ( 4 ) . Anaerobic sediments facilitate reductive dechlorination, but removal of the last few chlorines is difficult. Anaerobic dechlorinations of PCBs may occur by a radical pathway similar to their catalytic reductions (5). Toxic effects of PCBs are thought to result from the action of a PCB-receptor complex on DNA (6).Participation of PCB radicals is also possible. Formal reduction potentials of PCBs are controlled by the free energy required to produce PCB anion radicals. A reductant of the proper formal potential provides this energy. Electrochemical methods are capable of estimating formal potentials ofhalobiphenyls (7) as well as elucidating details of redox processes that may be important for understanding their chemistry in environmental contexts. Electrochemical dechlorination removes chlorines stepwise from PCBs to eventuallyyield biphenyl, which is much less toxic than PCBs. Electrochemical catalytic dehalogenation can destroy PCBs in water-based surfactant media ( 5 , 8 ) . However, relatively low solubility in purely aqueous surfactant dispersions limits practical applications. Recently, conductive bicontinuous microemulsions of didodecyldimethylammonium bromide (DDAB) were applied as media for catalytic dehalogenations (8a, 9). Bicontinuous microemulsions are clear, thermodynamically stable mixtures ofwater, oil, and surfactant. Oil and water are continuous throughout with surfactant residing at interfacial regions (10). Microemulsions can dissolve significantlylargeramounts of PCBs than micellar solutions ( 1 1 ) . Microemulsions made with ionic surfactants are often conductive enough for electrochemical experiments. Because of adsorption of the surfactant on the electrode, the reduction of water is partly blocked, and the working reduction window is extended ( 1 2 ~ ) .This is useful for reductive dechlorination of PCBs because their reduction potentials are relatively negative. Electrochemical catalytic dechlorination of PCBs was recently achieved by using lead electrodes in a bicontinuous microemulsion (8a, 9b). Microemulsions can also release PCBs from soils and sediments for subsequent electrolysis (8a, 12b). The reactivity of polyhalogenated biphenyls in reductive processes can be predicted from formal potentials (E"') of the first one-electron transfer. These fundamental parameters have been estimated so far only in organic media such as DMF (7, 13). Direct measurement of formal potentials for polyhalogenated biphenyls is difficult because of a chemical cleavage following the initial electron transfer. Scheme 1 describes the general pathway for PCB reduction. ArX represents a PCB (7, 13, 1 4 ) . An electron is first transferred to ArX (eq l),followed by cleavage of the anion radical AiY- to yield radical Ar and halide ion (eq 2). The radical Ar has a more positive formal potential than ArX and thus may be reduced by an electron from an electrode (eq 3) or by ArP- (eq 4). The product Ai- can be protonated (eq 5). Ar may also abstract a hydrogen from solvent (eq 6). Solvent radical S' can be reduced at the electrode (eq 7) or in solution (eq 8).
0013-936X/95/0929-0098$09.00/0
Q 1994 American
Chemical Society
SCHEME 1
ArX + e = Arx-
(E"')
Ar*+e-ArAr'+ArX--Ar-+ArX
Ar- + (H') -ArH Ar'
+ SH -.ArH + S' S' + l e = S-
Arx-+s'-Arx+s-
(1)
(3)
(4)
(5) (6) (7)
(8)
Scheme 1 appears in voltammetry as a two-electron irreversible peak, provided the lifetime of the Ar?? radical is short compared to the experimental time scale. Under such conditions, equilibriumbetweenArX andArX'- cannot be achieved, and estimation of the formal potential by voltammetry is not possible. Reduction processes of halobiphenylsare quite relevant to their interactions in the environment. A more positive formal potential correlates with greater toxicity (2, 7). Conversions of highly chlorinated PCB congeners by anaerobic bacteria to less chlorinated congeners give products containing chlorines on ortho positions of the biphenyl ring (3, 4). These ortho-chloro congeners have more negative formal potentials and are thought to be less toxic than isomers having ortho positions substituted with hydrogen. In this paper, we report the electrochemicalproperties of 16 PCBs and PBBs (polybrominated biphenyls) in a bicontinuous microemulsion of water, dodecane, and DDAB. Emphasis was placed on estimating formal potentials and on elucidating the rate-determining step in electrode reactions. Microemulsions containingwater, oil, and monolayers of surfactant may provide media with more similarities than organic solvents to sites at which halobiphenyls may reside during their environmental interactions. Thus, the present studies may yield information of more relevance to mechanismsof toxicityand bioreductions of halobiphenyls.
Apparatus and Procedures. An EG&G PAR 270 potentiostat was used for cyclic and square wave voltammetry. The electrochemical cell was kept at 25 & 2 "C. The microemulsionwas purged with nitrogen to remove oxygen before each measurement. A three-electrode cell was used with a mercury-film working electrode, a platinum wire counterelectrode, and an aqueous Ag/AgBr reference (- 150 mVvs SCE). The reference electrode contained 0.10 M KBr and was prepared in a glass tube ending in a vycor tip. The resistance of the cell was compensated at 290%. All reported CV and SWV results were averages of three or more replicate measurements. Platinum, glassy carbon, silver, silver amalgam, lead, pyrolytic graphite, nickel, and mercury were tested as working electrodes in preliminary experiments. Mercury was the best in sensitivity, peak resolution, and width of potential window. The Hg-film electrode was prepared on a silver disk of 1.6 mm diameter (16). The silver electrode was polished with alumina (0.05pm) and then placed into a cell containing 0.10 M HN03and a drop of liquid mercury on the bottom. Using AglAgCl reference and Pt counterelectrodes, a potential of -1.0 V was applied. Hydrogen gas formed and activated the silver surface. After 2-min electrolysis, the silver electrode was placed in contact with the liquid mercury, maintaining the same potential. A film of mercury formed on the silver electrode. This electrode was rinsed with water and carefully wiped with a kimwipe before use. The Hg-film electrode was used for 6-8 h before resurfacing. Bicontinuous microemulsions were prepared by titrating the appropriate mixture of DDAB and dodecane with water until a clear solution with relatively high conductivity was obtained (11). The composition of the microemulsion by weight was DDAB 20%,dodecane 56%,and water 24%.The bicontinuous nature of this system was established previously (11). The conductivity of the microemulsions was 1.5-1.7 mQ-' cm-l. Micellar solutions of CTAB were ultrasonicated to dissolve PCBs followed by equilibration at 30 "C for 2 days.
Results and Discussion
Voltammetry of Halobiphenyls in Microemulsions. PCBs and PBBs in microemulsions showed well-defined cyclic and square wave voltammograms. Figure 1 shows voltammograms of 4,4'-dichlorobiphenyl and 3,3',4,4',5,5'hexachlorobiphenyl in the DDAB microemulsion. Two dechlorination peaks of 4,4'-PCB, each involving twoExperimental Section electron transfer, can be identified. SWV showed better resolution than CV. The height of the first reduction peak Chemicals and Solutions. 4-Chlorobiphenyl(4-CB)from in SWV (Figure IC)is twice that of all the other peaks. Since Aldrich was recrystallized twice from ethanol [mp 77.579.0 "C, lit. 77.7 "C (1.31. 4,4'-Dichlorobipheny1(4,4'-PCB) SWV peak heights are proportional to the number of electrons transferred per molecule (17b cf. eq 9), this first was from Alfa. 2-Bromobiphenyl(2-BB),4-bromobiphenyl peak most probably involves simultaneous removal of two (4-BB),2,2',5,5'-tetrachlorobiphenyl(2,2',5,5'-PCB),2,2',5,5'chlorines. tetrabromobiphenyl (2,2',5,5'-PBB), 3,3',5,5'-tetrachlorobiphenyl (3,3',5,5'-PCB),and 2,2',3,3',4,4',5,5',6,6'-decachloCI. ,CI robiphenyl (2,2',3,3',4,4',5,5',6,6'-PCB) were from AccuStandard (New Haven, CT). All other PCBs and PBBs were from Ultra Scientific (North Kingstown, RI). Tetraethylammonium bromide (TEAB) from Eastman Kodak was CI' %I recrystallized from ethanol. Didodecyldimethylammonium bromide (DDAB,99+%) and hexadecyltrimethylammonium 4,4'-dichlorobiphenyl 3,3',4,4',5,5'bromide (cetyltrimethylammonium bromide, CTAB, 99+%) hexachlorobiphenyl were from Eastman Kodak. Distilled water was purified by For comparison with behavior in the microemulsions, a Sybron-Barnstead Nanopure system to a specific resiscyclic voltammetry in aqueous 0.10 M CTABlO.10 M TEAB tance '15 MQ-cm. VOL. 29, NO. 1. 1995 /ENVIRONMENTAL SCIENCE & TECHNOLOGY
1
99
9 Y
2.0
-
1.5
-
1.0
-
0.5
-
l-4
0.0
-1.0
-1.2
-1.4
-2.0
-2.2
-1.6
-1.8
E (V
vs. Ag/AgBr)
-2.4
-2.6
-1.0
25 4
20
1
I
-1.2
I
I
-1.4
-1.6
I
-1.8
I
-2.0
I
-2.2
I
-2.4
I
-2.6
1
3 -
10 -
15
3
2 -
w
1 -
5 -
0-
r
0 -0.8
-1.2
-1.6
E (V
-7s.
-2.0
-2.4
-0.8
-1.2
-1.6
-2.0
-2.4
Ag/AgBr)
FIGURE 1. 1. CV and SWV of 1.0 mM 4A'-dichlorobiphenyl and 1.0 mM 3,3',4,4,5,5'-hexachlorobiphenyl with mercury-film electrode in the bicontinuous microemulsion. CV at scan rate 200 mV/s, SWV with frequency 5 Hz, step height 2 mV, and pulse height 25 m V potentials vs Ag/AgBr. (a) CV for 4.4'-PCB; (b) SWV for 4,4'-PCB; (c) CV for 3,3',4,4',5,5'-PCB; (d) SWV for 3,3',4,4,5,5'-PCB. 30
I
I -1.2
I
-1.4
I
I
I
I
-1.6
-1.8
I
1
1
I
-2.0
-2.2
by using the mercury-film electrode in DDAB microemulsions. Monohalobiphenyls. The remainder of this paper is devoted to presentation and interpretation ofvoltammetric data for PCBs and PBBs. The simple halobiphenyls 4-CB and 4-BB were studied most extensively. We previously showed that theory for homogeneous media applies to voltammetry in bicontinuous microemulsions (11). We employed this theory (17b) for analyzing the data. For CV, the peak current (i,) for an irreversible reaction under diffusion control is
I
I
-2.4
I -2.6
E (V =. Ag/AgBr) FIGURE 2. 2. CV of 4-chlorobiphenyl in the bicontinuous microemulsion. 1.0 mM 4-chlorobiphenyl at scan rates of (a) 100. (bl200, (c) 500, and (d) 1 V/s.
was done. In this micellar solution, 4-CB did not show clear peaks but only a large background current. Alternatively, 4-CB in microemulsion has a well-defined cyclic voltammogram in the scan rate range from 5 mV/s to 10 V/s (Figure 2 ) . Reduction of PCBs is more difficult as the number of chlorines decreases (7). This is illustrated in Figures 1and 2 . As 4-CB is one of the most difficult PCB congeners to reduce, its well-definedvoltammograms in microemulsions suggested that the entire family of PCBs can be investigated 100 a ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, NO. 1, 1995
where n is the number of electrons transferred per molecule, n, is the number of electrons transferred in the ratedetermining step, a is the electrochemical transfer coefficient,A is the electrode area, C,* is the bulk concentration of electroactive species, 0, is the diffusion coefficient of the reactant, and v is the scan rate. At a given temperature (25 "C), all parameters except for scan rate are constant. Thus, the relationship between the peak current and scan rate describes the diffusion behavior of the electroactive species. CVpeak currents were proportional to the concentration of 4-CB. Peak currents of 4-CB (Figure 3) show a linear relationship with the square root of the scan rate from 5 to 10Vls. Plotsoflogipvslogvshowedalinearrelationship of i, vs for 1.0mM 4-CB and ipvs for 2.0 mM 4-CB.
140 120
a
TABLE 1
-
100
-
80
-
60
-
First Reduction Peak Potentials of CV and SWV for ArX/ArX’- Redox Couples of Polyhalogenated Biphenyls in DDAB Bicontinuous Microemulsiona 4 4 PCB or PBB
20 -
40
0~
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
FIGURE 3. 3. Dependence of peak current (ip) on the square root of scan rete (0) in the bicontinuousmicroemulsion.4-Chlorobiphenyl at 1.0 (0)and 2.0 m l (0).
CVs of 4-BB showed similar behavior, with a linear relationship of ip vs These exponents are consistent with the theoretical relation ip vs v0,50(eq 9). AU these results indicate an irreversible diffusion-controlled reduction of 4-CB and 4-BB. We found no evidence for adsorption of 4-CB or 4-BB on the Hg-film electrode under these conditions. Since all PCBs and PBBs were reduced irreversibly in the microemulsion, their electrochemical transfer coefficients (a)were estimated from (17b)
a = -RT 1.857 n w p / z - Ep
(CV)*
(SWV)
2-CB 2-BB 4-CB 4-BB 4,4‘-PCB 4,4‘-PBB 3,5-PCB 2,2’,5,5’-PCB 2,2’,5,5’-PBB 3,3’,4,4‘-PCB 3,3’,5,5’-PCB
-2.233 -1.878 -2.187 -1.788 -2.08 -1.773 -2.101 -1.926 -1.60 -1.720 -1.85
-2.098 -1.627 -2.064 -1.648 -1.958 -1.617 -1.979 -1.801 -1.43 -1.591 -1.78
2,2’,6,6‘-PCB 3,3’,4,4‘,5,5’-PCB 2,2’,3,3’,4,4‘-PCB 2,2’,4,4‘,6,6‘-PCB 2,2‘,3,3’,4,4‘,5,5’,6,6’-PCB
-2.15 -1.407 -1.61 -1.88 -1.235
first peak assignment
2-CI peak 2-Br peak 4-CI peak 4-Br peak 4-CI peak 4,4‘-Br peaks overlapped one CI peak two CI peaks overlapped peaks overlapped two CI peaks overlapped one CI peak partly overlapped peaks overlapped -2.04 -1.297 two CI peak overlapped -1.497 peaks overlapped -1.80 peaks overlapped -1.134 two CI peaks overlapped
a CV with scan rate 200 mV/s; S W with frequency 5 Hz, step height 2 mV, and pulse height 25 mV. All potentials (l4 vs Ag/AgBr reference electrode.
’*2
-2.6
1
(10)
where E p is peak potential, EPf2is half-peak potential, R is gas constant, Tis absolute temperature, and Fis Faraday’s constant. The a v d u e of 4-CB was estimated as 0.51 f0.02 from 40 measurements from 5 to 10 V/s and as 0.34 & 0.02 for 4-BB in 26 measurements. These values were similar to those found for chloro- and bromobenzenes,respectively
J
I
I
-1.4
-1.6
-1.8
-2.0
-2.2
(14).
cm2/s Apparent diffusion coefficients were 5.1 x for 4-CB and 5.6 x cm2/s for 4-BB from slopes of ip vs Y ~plots / ~ using eq 9. These values are close to those of other nonpolar organic compounds in similar bicontinuous microemulsions (111,such as ferrocene (6.3 x cm2/s), perylene (5 x cm2/s),pyrene (6 x cm2/s),and 9-phenylanthracene (8 x cm2/s) and to the selfdiffusion coefficient (6 x cm2/s) of dodecane (lob). These data suggest that the halobiphenyls reside in the oil phase (11). Formal Potendals of PCBs and PBBs. The first cathodic peak potentials for 16 PCBs and PBBs are listed in Table 1. Peaks were often better resolved in the microemulsion than in an organic solvent such as DMF. For example, 2,2’,6,6’-tetrachlorobiphenyl had two SWV peaks in the microemulsion, but only one peak in DMF (7). For corresponding derivatives, PCB peaks were better resolved than PBB peaks. Similar shifts of peak potentials for congeners of different structures were observed by CV and SWV in the microemulsion and in DMF. No reverse peak in CVs up to scan rates of 50 VIS was observed for all PCBs and PBBs, consistent with fast chemical cleavage of the initial reduction products (eq 2). A few derivatives with overlapped first peaks in the microemulsion were investigated in more detail. For
Ep vs. Ag/AgBr in DMF
FIGURE 4. 4. Correlationbetween most positive SW peak potentials of PCBs end PBBs in DMF and in the bicontinuous microemulsion. Experimentswith frequency 5 Hz, step height 2 mV, and pulse height
25 mV. example, 1.0 mM 3,3’,4,4‘,5,5’-hexachlorobiphenyl gave a linear plot of the first cathodic peak current vs the square root of scan rate from 5 mV/s to 10 V/s. Logarithm data analysis showed that the peak current and scan rate have alinear relationship of ip vs No evidence of adsorption was found. The positively charged head group of DDAB may interact with the radical ArF- (Scheme 11, which could affect the Gibbs free energy (AGO) of the electron transfer and shift the peak potentials of PCBs and PBBs positively (94. In general, congeners with more halogens had larger positive shifts in peak potentials in the microemulsion compared to DMF. A roughly linear correlation was obtained for corresponding CV and S W V peak potentials of PCBs and PBBs in DMF vs those in the microemulsion (Figure4). The linear regression lines had slopes of 1.23for S W V and 1.18for CV. This correlation allowed estimation of formal potentials of PCBs and PBBs by using those reported for DMF (7). VOL. 29, NO. 1, 1995 /ENVIRONMENTAL SCIENCE &TECHNOLOGY
101
TABLE 2
Estimated formal Potentials for ArxIArx.- Redox Couples of Polykrhtgsaated Bip#enyls in DDAB Bicontinuous Microemulsio3 PCB or PBB 2-CB 2-BB 4-CB 4-BB 4,4'-PCB 4,4'-PBB 3,5-PCB 2,2',5,5'-PCB 2,2',5,5'-PBB 3,3',4,4'-PCB 3,3',5,5'-PCB 2,2',6,6'-PCB 3,3',4,4',5,5'-PCB 2,2',3,3',4,4-PCB 2,2',4,4',6,6'-PCB 2,2',3,3',4,4',5,5',6,6'-PCB
P' (CV)) -2.171 - 1.E97 -2.136 - 1.E27 -1.87 -1.742 -2.069 - 1.E08 -1.67 -1.719 -1.78 - 1.90 -1.585 -1.67 - 1.79 -1.511
P S ~ ( C V(SW) ~
Spc~mr,
0.039 0.029 0.037 0.030 0.022 0.021 0.033 0.019 0.028 0.023 0.020 0.025 0.038 0.027 0.020 0.047
0.034 0.032 0.033 0.031 0.030 0.030 0.030 0.024 0.041 0.032 0.024 0.028 0.050 0.037 0.024 0.062
-2.120 -1.741 -2.092 - 1.757 -2.007 -1.700 -2.024 - 1.778 - 1.62 - 1.689 - 1.77 - 1.E8 -1.563 - 1.649 -1.78 - 1.494
P' (av) -2.146 -1.819 -2,114 - 1.792 - 1.94 -1.721 -2.047 - 1.793 - 1.65 - 1.704 -1.78 - 1.89 - 1.574 -1.66 - 1.79 -1.503
a Obtained by using correlation method in ref 7 via eq. 11. In cyclic voltammetry, slope = 0.773 and intercept = -0.571 for resolved first peaks; slope = 0.430 and intercept = 1.157 for overlapped first peaks. In square wave voltammetry, slope = 0.805 and intercept = 0.552 for resolved first peaks; slope = 0.426 and intercept = 1.189 for resolved peaks. These parameters were referred to potentials vs SCE, so that all calculations were based on the potentialsvs. SCE and then converted to the potentials vs NHE after calculation. bAll potentials (V) vs NHE reference. The estimated standard deviation of E"'.
In previous work, we estimated E"' values of PCBs and PBBs in DMF by a kinetic catalytic method (7). Linear correlations for CV and SWV were found: E"' = (slope)E,
+ intercept
(11)
where E"' is the kinetically estimated formal potential, Ep is peak potential, and slope and intercept are parameters estimated from data obtained in DMF. Taking the correlation in Figure 4 as justification, formal reduction potentials were estimated from eq 11. Slopes and intercepts for resolved and overlapped first peaks were used separately (Table 2). The formal potentials of PCBs and PBBs and their standard deviation are listed in Table 2. The values of formal potentials calculated from CV peaks agree with the values from SWVpeakswithin the estimated standard deviations. The calculated formal potential of 2,2',3,3',4,4',5,5',6,6'-PCB has a large difference from its peak potential. This value may have a larger error because the slope and intercept for the correlation were computed for the mono- to hexahalobiphenyls in DMF. For PCB congeners with a given number of halogens, isomers without o-halogens have more positive formal potentials and also tend to be the most toxic. 3,3',4,4'-PCB and 3,3',4,4',5,5'-PCB without o-halogens are the most toxic PCB congeners. They elicit biological and toxic effects similar to those reported for 2,3,7,8-tetrachlorodibenzop-dioxin (2). They are also the most easily reduced derivatives among their isomers. On the basis of PCB structure-activity relationships, the most toxic PCB congeners are nearly coplanar (2). Decreased coplanarity between the two phenyl rings results from the steric interaction between the o-chlorines and hydrogens on the adjacent ring. The most toxic PCBs are 102
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, NO. l , 1995
those with chlorines on the para and at least one meta position of both phenyl rings. Related PCBs show the same tendency of formal potentials in the microemulsion: 3,3',4,4'-PCB (2 para and 2 meta) > 3,3',5,5'-PCB (4 meta) > 2,2',5,5'-PCB (2 metaand 2 ortho) > 2,2',6,6'-PCB (4 ortho). In the series of hexachlorobiphenyls, a similar tendency was observed: 3,3',4,4',5,5'-PCB (2 para and 4 meta) > 2,2',3,3',4,4'-PCB (2para,2meta, and2orthol > 2,2',4,4',6,6'PCB (2 para and 4 ortho). This trend of formal potentials is also consistent with the patterns of anaerobic reduction of PCBs in contaminated lake and river sediments (3, 4 ) . Thus, structural features controlling the formal reduction potentials of PCBs may also control the biological reductions and toxicity. There appears to be a correlation between E"' values in microemulsions to toxicity and ease of reduction by anaerobic bacteria. Mechanistic Considerations. For the mechanism in Scheme 1, the pathway in eqs 1-3 is known as an ECE mechanism (electron transfer, chemical step, electron transfer), while the pathway in eqs 1, 2, and 4 is called DISPl mechanism (disproportionationwith first-orderratedetermining step) (18). The voltammetric process is kinetically controlled either by the electron transfer (eq l), by the chemical cleavage (eq 21, or by both. In cases where no reverse peak is found in CV, information concerning the rate-determining step (rds) in ECE or DISPl mechanisms can be obtained from the shift of cathodic peak potential with scan rate. If electron transfer is fast, and the electrode process is under the kinetic control of the cleavage reaction in ECE mechanism (19):
E,=,??'+-
yT[
In-
-078
1
RT +-ln2F
kchem Y
(
(12)
where kchem is the rate constant of the cleavage (eq 2). For cleavage reaction control in the DISPl mechanism:
[
[y)
E,=,??'+- RT ln-
- 0.781
+ gin(%)
(13)
Both equations indicate that the slope -aEp/a In v of an Ep vs In v plot should have a value of 12.8 mV at 25 "C if the mechanism is under the kinetic control by either eq 2 or eq 4. If the cleavage rate of the anion radical is much faster than the reverse electron transfer, electron transfer in the forward direction (eq 1)becomes the rate-determining step. The followingequation describes such an electron transfer control mechanism (20):
Equation 14 indicates a slope - a h l a In v of 12.8/amV at 25 "C for slow electron transfer. The a values estimated for 4-CB and 4-BB can be used to compute the theoretical -&??,/a In v for an ECE or DISPl reaction where the first electron transfer is the rate-determining step. CVpeak potentials of 4-CB and 4-BB depended linearly on ln(1/VI, in agreement with theoretical predictions. The linear regression line for 4-CB was Ep = -2.240 0.03315 ln(l/v)indicating a slope -aEp/a In v of 33 mV. 4-BB gave a slope -&??,/a In v of 43 mV with regression line Ep = -1.864 0.04286 In ( l / v ) . The theoretical value of -aEp/a In v is 12.8 mV if the cleavage reaction is the rate-
+
+
determining step. For electron transfer as the ratedetermining step, the theoretical slopes of -%!&/a In v were calculated as 25.1 mV for 4-CB and 37.6 mV for 4-BB using the estimated a values. Comparing these values to experimental slopes suggests that the voltammetry of these compounds is consistent with an ECE or DISPl mechanism where the first electron transfer is rate-determining. Both experimental slopes are a little larger than the predicted theoretical values, suggesting that the cleavage reaction may still have some influence on the electrode reaction (20, 211, although slow electron transfer is the main controlling step.
Conclusion For the direct electrochemicaldehalogenation of PCBs and PBBs in a bicontinuous DDAB microemulsion,the reduction kinetics are controlled by the rate of the first electron transfer. The double-layer structure at the surface of the electrode and the surfactant in the microemulsion may influence PCB and PBB formal reduction potentials, shifting them positive compared to homogeneous DMF. This suggests that the reactivity of PCBs toward reduction may depend on the chemical nature of their surroundings. Formal potentials are correlated with toxicity and reactivity in anaerobic biodegradation of PCBs.
Acknowledgments This work was supported by U.S.PHS Grant ES03154 awarded by the National Institute of Health, National Institute of Environmental Health Science.
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Received for review March 28, 1994. Revised manuscript received September 6, 1994. Accepted September 15, 1994.@
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@Abstractpublished in Ad~anceACSA~strac~s, November 1,1994.
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