Electroreduction of hexachlorobenzene in micellar aqueous solutions of Triton-SP 175

Article abstract of DOI:10.1021/es970969w

The electrochemical reduction of hexachlorobenzene (HCB) has been studied in micellar aqueous solutions using Triton-SP 175 which, unlike conventional surfactants, is acid-labile. At pH <3, the hydrophobic residue cleaves from the hydrophilic chain, leaving a solution without surface- active properties and allowing recovery of the electrolysis products from the solution. A micellar solution containing 0.1% v/v Triton-SP 175 and 1% v/v heptane as cosolvent was indefinitely stable in the presence of 0.05 M sodium sulfate as an environmentally friendly supporting electrolyte. Electrolytic dehalogenation to less chlorinated benzenes was studied at a wide variety of cathodes; in all cases a quantitative material balance of phenyl residues was achieved. Lead was the preferred cathode in terms of both the degree of dechlorination achieved and the current efficiency. The electrochemical reduction of hexachlorobenzene (HCB) has been studied in micellar aqueous solutions using Triton-SP 175 which, unlike conventional surfactants, is acid-labile. At pH<3, the hydrophobic residue cleaves from the hydrophilic chain, leaving a solution without surface-active properties and allowing recovery of the electrolysis products from the solution. A micellar solution containing 0.1% v/v Triton-SP 175 and 1% v/v heptane as cosolvent was indefinitely stable in the presence of 0.05 M sodium sulfate as an environmentally friendly supporting electrolyte. Electrolytic dehalogenation to less chlorinated benzenes was studied at a wide variety of cathodes; in all cases a quantitative material balance of phenyl residues was achieved. Lead was the preferred cathode in terms of both the degree of dechlorination achieved and the current efficiency.

Full text of DOI:10.1021/es970969w

Environ. Sci. Technol. 1998, 32, 1509-1514
•-
chloride ion (6, 7, 8). An intermediate radical anion (ArCl )
Electroreduction of
-
•
formed in the first step expels Cl to give an aryl radical (Ar ).
This rapidly accepts a second electron to yield the reduced
Hexachlorobenzene in Micellar
Aqueous Solutions of Triton-SP 175
-
species Ar , protonation of which completes the reduction
to ArH.
Our goal in this research was to evaluate electrolytic
methods for the removal of hexachlorobenzene (HCB, a
pesticide intermediate) from aqueous waste streams. We
previously showed that HCB could be sequentially dechlo-
rinated to penta-, tetra-, tri-, and dichlorobenzenes in
methanol solution, using a mercury pool cathode and
tetraethylammonium chloride as the supporting electrolyte
S I M O N A G . M E R I C A ,
C L A U D I A E . B A N C E U ,
W O J C I E C H J E¸ D R A L ,
J A C E K L I P K O W S K I , A N D
N I G E L J . B U N C E *
Department of Chemistry and Biochemistry,
University of Guelph, Guelph, Ontario, Canada N1G 2W1
(
8). A quantitative material balance was obtained, and a
current efficiency of 60% was achieved in the most favorable
case. We now report that we have overcome three of the
remaining obstacles to developing a practical electroreduc-
tion technology. First, solutions of practical interest are
The electrochemical reduction of hexachlorobenzene
-
8
aqueous, but HCB is poorly (∼10 M) soluble in water.
Second, mercury is inappropriate for use as a cathode, as
shown by experience in the chlor-alkali industry (9). Third,
tetraethylammonium chloride is too costly to use as a
(HCB) has been studied in micellar aqueous solutions using
Triton-SP 175 which, unlike conventional surfactants, is
acid-labile. At pH <3, the hydrophobic residue cleaves from
the hydrophilic chain, leaving a solution without surface-
active properties and allowing recovery of the electrolysis
products from the solution. A micellar solution containing
supporting electrolyte and would give Cl
reaction.
2
in the anodic
0.1% v/v Triton-SP 175 and 1% v/v heptane as cosolvent was
Experimental Methods
indefinitely stable in the presence of 0.05 M sodium sulfate
as an environmentally friendly supporting electrolyte.
Electrolytic dehalogenation to less chlorinated benzenes
was studied at a wide variety of cathodes; in all cases
a quantitative material balance of phenyl residues was
achieved. Lead was the preferred cathode in terms of
both the degree of dechlorination achieved and the current
efficiency.
Chem icals. Chlorinated benzenes were obtained from
Aldrich, and their purities were checked by HPLC and GC-
MS. Tetraethylammonium chloride (TEACl) and sodium
sulfate were used as supporting electrolytes (0.05 M TEACl
2 4
and 0.05 M Na SO ). HPLC-grade solvents were obtained
from Fisher. Triton-SP 175 was supplied by Union Carbide.
Microemulsions containing 0.1% Triton-SP 175 and 1%
heptane as cosolvent were prepared using Millipore water
and were sonicated for at least 4 h.
Chrom atography. HPLC analyses were performed on a
Waters system containing a model 486 tunable absorbance
detector, U6K injector, and 600 E system controller. The
system was computer controlled using Millennium 2010
chromatographic software. Absorbances were monitored
at λ ) 227 nm. Analytical separations were performed on
a Waters µBondapack C18, 3.9 × 300 mm column, which was
eluted with 90/ 10 methanol/ water at flow rate of 2 mL/ min.
Solvents used were HPLC grade and were filtered under
vacuum before use through 0.45 µm Nylon 66 membranes
to remove microparticulates.
Reference Electrodes. An internal Ag/ AgCl reference
electrode was used when TEACl was the supporting elec-
2 4
trolyte. For experiments with Na SO as the supporting
electrolyte, an external reference electrode was used (satu-
rated Ag/ AgCl or SCE). All potential data were corrected to
the standard hydrogen electrode (SHE) as a common point
of reference.
Introduction
Chlorinated aromatic compounds such as polychlorinated
benzenes and polychlorinated biphenyls (PCBs) owe their
environmental persistence to their low chemical and bio-
logical reactivity. Aqueous waste streams or leachates
containing these compounds are both recalcitrant to con-
ventional biological treatment and toxic to the microorgan-
isms in the biological reactor. Electrochemical technologies
show promise for treating these materials due to the relative
simplicity of the equipment, environmental friendliness, and
the possibility of high-energy efficiency as compared with
thermal and photochemical processes (1-3).
The low-chemical reactivity of chlorinated aromatic
compounds is reflected in very negative potentials for
reduction and very positive potentials for oxidation. The
potential for dehalogenation of chlorinated benzenes in
aprotic solvents such as dimethylformamide, dimethyl sul-
foxide, and acetonitrile becomes increasingly negative with
successive removal of chlorine substituents (4). This ob-
servation suggests that practical technologies should limit
electrolysis to partial dechlorination to afford a product that
is less toxic and more amenable to biological treatment; this
approach contrasts with that of Petersen et al. (5), who
exhaustively electrolyzed chlorinated benzenes to maximize
dechlorination.
Working Electrodes. The anode was composed of Ti/
2
IrO
2
and had an area of 27 cm . The mercury pool cathode
2
had an area of 6 cm (1.7 mL) and almost filled the cathodic
compartment of the cell. The glassy carbon (GC) electrode
was prepared from a plate that was sealed in epoxy resin and
polished using 0.3 µm alumina. Other metallic cathodes were
prepared from the relevant metal wire (2 m × 0.5 mm). For
the experiments involving lead cathodes with large surface
area, the most successful material was obtained by cathodic
deposition of ∼4 g of lead on a lead wire support, using 0.015
Electrolytic reduction of polychlorobenzenes requires two
one-electron transfers for each chlorine atom removed as
3 2
M Pb(NO ) and 0.15 M sodium citrate at pH 6.5. Lead was
deposited in the form of long bright needles.
Exhaustive Electrolyses. These were performed using a
sandwich-type flow-through cell that was previously de-
*
Corresponding author e-mail address: bunce@chembio.uoguelph.
ca; fax: (519)766-1499; telephone: (519)824-4120, ext. 3962.
S0013-936X(97)00969-3 CCC: $15.00
Published on Web 03/28/1998

1998 Am erican Chem ical Society
VOL. 32, NO. 10, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
1 5 0 9

scribed (8). The cells were constructed from high-density
polypropylene or Plexiglas; they were mounted horizontally
for use with the Hg pool cathode and vertically for use with
solid cathodes for a better removal of hydrogen gas that forms
during electrolysis. The anode and cathode compartments
of the cell had volumes of 3.3 mL and were separated by a
Nafion 417 membrane; the distance between the electrodes
was ∼0.1 cm. The cell was controlled by a EG&G model 273
potentiostat/ galvanostat.
Continuous Flow Experim ents. Experiments were car-
ried out under amperostatic polarization, with the various
cathode materials compared at the same current density. A
-
5
3
0-mL sample of 3 × 10 M HCB in Triton-SP 175 micro-
emulsion/ 0.05 M Na SO was continuously recirculated
2
4
through the cathodic compartment by a peristaltic pump at
flow rate of 2.5 mL/ min, and 25 mL of electrolyte (micro-
emulsion/ 0.05 M Na SO ) was pumped through the anodic
2 4
compartment. Before applying any current, the solutions
were recirculated for 1 h, and samples were collected during
this time to ensure that no degradation of the starting material
occurs under open circuit conditions. The catholyte was
deaerated by purging with argon for 30 min before elec-
trolysis, and a slow Ar flow was maintained throughout the
experiment. In order not to break the microemulsion by
evaporating the heptane, one drop of heptane was added
periodically to the catholyte, and the contents of the flask
were mixed for a few minutes. Later, it was discovered that
the electrolyses could be carried out without loss of efficiency
even in the presence of air. The experiments were analyzed
by HPLC, after removing 1.0-mL aliquots of microemulsion
from the catholyte, breaking the emulsion with dilute
hydrochloric or sulfuric acid, and partitioning the products
into heptane. Lower chlorinated benzenes were formed, and
a quantitative material balance (>98%) was achieved. The
yields were corrected for the change in volume due to removal
of aliquots during electrolysis.
FIGURE 1. Differential pulse voltammograms of 0.1 mM HCB in
MeOH/water mixture using a sessile drop Hg electrode.
when the ratio of methanol/ water was 70/ 30 (v/ v), and at a
5
0/ 50 methanol/ water ratio, the recorded DPV closely
resembled that of the electrolyte alone. These experiments
made clear that it was impractical to raise the aqueous
concentration of HCB by adding small amounts of methanol
as a cosolvent.
Surfactant media have previously been used to increase
the solubility of hydrophobic chloro compounds in water
(
10-13). Surfactants with a single hydrophobic chain, such
as cetyltrimethylammonium bromide, afford conventional
microemulsions. Those with two hydrophobic chains, such
as didodecyl dimethylammonium bromide, give lamellar
solutions in water or salt solutions. In homogeneous mixtures
of oil and water, didodecyl dimethylammonium bromide
gives interweaved strands of continuous aqueous and organic
phases, known as bicontinuous microemulsions. The organic
phase not only increases the solubility of the hydrophobic
chloro compound but also brings the substrate into close
proximity with the cathode. A difficult problem has been to
find a method to break the microemulsion and to quanti-
tatively recover the electrolysis products from this medium;
recently, it has been found that the reaction products can be
separated by passing the product mixture twice through silica
gel columns and evaporating the solvent (14).
Single Flow Experim ents. A microemulsion containing
-
5
3
× 10 M HCB/ 0.05 M Na
2 4
SO was pumped from a 0.5-L
flask through the cathodic compartment at 0.4 mL/ min, and
the output from the cathode was directed to waste. Ap-
proximately 25 mL of electrolyte (microemulsion/ 0.05 M Na
2
-
SO ) was continuously circulated through the anodic com-
4
partment. The catholyte was deaerated by purging with argon
for 30 min before electrolysis. Electrolysis was carried out
under amperostatic polarization for 30 min before any
samples were collected for analysis to achieve a steady state.
The analytical procedure was similar to that used in
continuous flow experiments. Measured potentials were
corrected for IR drop by the current interrupt method.
Linear Sweep Voltam m etry. These experiments were
The recent introduction of the Triton-SP series of sur-
factants by Union Carbide offered a prospective novel
solution to the problem of breaking the emulsion. These
nonionic surfactants consist of a proprietary hydrophobe
attached to a polyethoxylate hydrophilic chain. The sig-
nificant advance is that the hydrophobe can be cleaved from
the polyethoxylate chain by brief treatment with dilute acid
-
5
performed using 3 × 10 M HCB in microemulsion/ 0.05 M
Na SO without prior deoxygenation of the solutions. Several
different cathode materials were used, each with an area of
2
4
(
pH < 3), upon which the cleaved Triton-SP 175 loses its
2
0
.1 cm . A scan rate of 10 mV/ s was used.
surfactant properties irreversibly and becomes compatible
with subsequent biological treatment (15).
Results and Discussion
Concentrations of HCB up to at least 50 µM were attainable
in 0.1% Triton-SP 175 with cosolvent 1% heptane: the
solutions were milky white and stable toward breaking the
emulsion for (at least) weeks. Our objective was to employ
aqueous media with the miminum of additives. HCB could
not be solubilized in Triton-SP 175 alone, even at 1%
surfactant, and a stable emulsion was not formed below 0.05%
surfactant. The use of 0.1% surfactant and 1% cosolvent
offered a successful compromise. HCB was successfully
reduced in 0.1% Triton-SP 175 at the mercury pool cathode
in a recirculating flow-through cell under amperostatic
conditions that closely duplicated our previous experiments
in methanol (8) (Figure 2A). Penta-, tetra-, and trichlo-
robenzenes were formed, as in methanol, and a quantitative
material balance (> 98%) was achieved.
-
8
Solubility. The solubility of HCB in water is very low (∼10
-
3
M), but in methanol, concentrations of up to 10 M can be
attained (8). We attempted to increase the solubility of HCB
by the use of methanol/ water mixtures, reasoning that
methanol is inexpensive and is also nontoxic toward mi-
croorganisms in subsequent biological treatment of partly
dechlorinated HCB. Differential pulse voltammograms (DPV)
were recorded on a sessile drop Hg electrode, starting with
0
.1 mM HCB/ 0.05 M TEACl in methanol and gradually adding
water/ 0.05 M TEACl. DPV were recorded after each such
addition (Figure 1). The diminution of the peak height was
greater than can be explained simply because of diluting the
solution with water/ TEACl (compare the peak heights at 95%
and 100% methanol in Figure 1). Precipitation of HCB began
1
5 1 0
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 32, NO. 10, 1998

FIGURE 3. Linear sweep voltammograms of microemulsion/0.05 M
Na SO using various cathode materials.
2
4
Choice of Cathode. Because mercury is undesirable as
an electrode for industrial processes, we investigated a series
of other metals as possible cathodes for comparison against
Hg. The most important property sought was a high
overvoltage for hydrogen evolution, otherwise the current
efficiency is severely reduced.
Voltam m etry. As implied by Figure 1, the detection of
reduction currents of HCB in mostly aqueous solutions is
not possible. We examined the electrochemical properties
of several cathode materials in 0.1% Triton-SP 175 with
cosolvent 1% heptane and supporting electrolyte 0.05 M Na
2
-
SO using linear sweep voltammetry. A complication in these
4
experiments was that the solubility of oxygen in the micro-
emulsion is higher than in methanol and is at least 2 orders
of magnitude larger than that of HCB. Complete removal of
oxygen by purging with argon would require such a long
purging time that it would also remove the heptane used as
a cosolvent in the microemulsion. Consequently, Figure 3
portrays the performance of the cathode materials rather
than the behavior of HCB at these cathodes. To interpret
Figure 3, we note that the potential required for the first
three dechlorinations of HCB are about -1.1, -1.3, and -1.6
V vs SHE (8) and that the anodic current is presented as a
negative current. The objective was to find cathode materials
that would exhibit only residual current (no hydrogen
evolution) when the cathode was strongly negatively polar-
ized.
-
5
FIGURE 2. Electrolysis of 3 × 10 M HCB in microemulsion using
2
continuous flow, F ) 2.5 mL/min, current density ) 2.5 mA/cm , Hg
pool: (A) 0.05 M TEACl; (B) 0.05 M Na SO .
2 4
Supporting Electrolyte. The experiment reported in
Figure 2A employed TEACl as the supporting electrolyte, but
this has already been noted as unsuitable for practical
technology. We considered sodium sulfate to be an ideal
supporting electrolyte for this application because sodium
ions and sulfate ions are neither electroactive nor pose an
environmental threat (no limits are placed on their con-
centrations in aqueous discharges).
Experimentally, Hg showed the largest cathodic range
before hydrogen evolution commenced, but Pb and Sn also
exhibited only residual current beyond the potential required
for the first reduction of HCB. This suggested that Pb and
Sn might be promising cathode materials for practical
electroreductions of HCB. At Pb and Sn cathodes, current
peaks were recorded at potentials of -0.5 and -0.9 V vs SHE,
respectively. These were attributed to the reduction of metal
oxide films formed during more anodic polarization or at no
polarization (open circuit). (During bulk electrolysis, these
oxide films would be reduced as soon as the cathode became
negatively polarized.)
The Triton-SP 175 microemulsion (Triton-SP 175:heptane:
water ) 0.1:1:98.9, v/ v) was stable in the presence of 0.05 M
Na SO . No significant reduction or oxidation current was
2 4
observed by DPV in the potential range -2.0 to 1.4 V vs SHE,
showing that this medium had suitable electrochemical
properties for the purpose of reducing HCB (it could
presumably also be useful for electrooxidations). Electro-
2 4
chemical reduction of HCB with Na SO as supporting
electrolyte gave the same dechlorination products and
occurred with similar current efficiency as with tetraethy-
lammonium chloride (Figure 2B). In our previous work in
methanol, the efficiency of electrolysis was lower when NaCl
replaced TEACl as the supporting electrolyte (8). The latter
Metals such as iron and stainless steel, which have low
overvoltage for hydrogen evolution, had very restricted ranges
of usable potential. We had expected glassy carbon (GC) to
be an useful material based on its behavior in methanol, in
which GC performed almost as well as a sessile drop Hg
electrode (Figure 4). In practice, GC had little usable potential
range in the surfactant medium, showing a large current of
hydrogen ion reduction at modestly negative potentials. We
suggest that this phenomenon may be caused by the presence
+
effect was ascribed to preferential adsorption of TEA ions
at the surface of the cathode, thereby altering the electrical
double layer so as to increase the useful potential range of
the electrode (16). We speculate that in the microemulsion
the surfactant itself is the chief adsorbent to the cathode
surface. All subsequent experiments were carried out using
0
2 4
.05 M Na SO as the supporting electrolyte.
VOL. 32, NO. 10, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
1 5 1 1

FIGURE 4. Differential pulse voltammograms of 1 mM HCB in
methanol/0.05 M Na SO at Hg, lead, and glassy carbon cathodes.
2
4
TABLE 1. Pseudo-First-Order Rate Constants of 30 µM HCB in
2 4
Microemulsion/0.05 M Na SO (Continuous Flow Electrolysis,
F ) 2.5 mL/min)
k (s- ) × 10
1
6
electrode
0.5 mA/cm2
2.5 mA/cm
2
6.7 mA/cm2
Hg
Pb
Zn
Sn
Cd
SS
Fe
65
55
43
37
43
37
32
28
53
48
48
42
72
63
48
48
42
32
23
Ni
GC
-
5
FIGURE 5. Electrolysis of 3 × 10 M HCB in microemulsion/0.05
of surfactant on the electrode, causing the GC to lose its high
overvoltage for hydrogen evolution.
M Na SO
2
4
(continuous flow, flow rate ) 2.5 mL/min, current ) 75
2
mA, Pb cathode): (A) wire with area 30 cm : (B) ∼4 g of Pb deposited
Bulk Electrolysis. Bulk electrolyses were carried out in
the flow-through cell with amperostatic polarization (see
Table 1). Unlike our previously reported experiments in
methanol (8), which were always deoxygenated, the elec-
trolyses described in this paper could be carried out in the
presence of air. In all cases a quantitative material balance,
based on phenyl residues, was observed throughout the
reactions. Most of the cathodes gave distributions of the
electrolysis products similar to those recorded in Figure 2B,
but dehalogenation was more efficient at the Pb cathode, at
which trichlorobenzene was the major product (Figure 5A).
The order of usefulness of the different cathodes generally
followed that expected from the voltammetric experiments;
lead, zinc, and tin (with mercury as a reference point) showed
the largest overpotentials for hydrogen evolution in this
medium (cf. Figure 3) and also had greater efficiencies for
electrochemical degradation of HCB by the crude measure
of the pseudo-first-order rate constant for its disappearance.
on wire frame.
in the flow-through cell. In the case of Pb and Sn cathodes,
an initial period of cathodic polarization was required for
oxide film reduction before steady-state conditions were
attained (compare Figure 3). Current efficiencies of elec-
trolysis for these cathode materials are presented in Figure
6
as a function of applied current density. Table 2 records
the experimental conditions for electroreduction corre-
sponding to the best current efficiency as well as the product
distributions observed for each cathode material. Lead was
the best choice of cathode, in terms of both current efficiency
(
∼5%) and extent of dechlorination. Its superior performance
can be explained because the potential of -1.4 V vs SHE was
achieved at very low current density, allowing the successive
reduction of HCB to penta-, tetra-, and trichlorobenzenes.
The optimal current efficiencies were achieved at the lowest
current densities to minimize the extent of cathodic polar-
ization and hence limit the production of hydrogen.
(
We note that toxic metals such as lead and cadmium may
be employed as cathodes because there is no tendency for
them to oxidize under these conditions to soluble M2
species.) Glassy carbon was the least effective cathode
material, again consistent with its voltammetric behavior.
These amperostatic experiments were carried out at
+
We showed, using different lengths of the Pb wire cathode
at equal current densities, that the rate of reduction of HCB
was directly proportional to the surface area of the electrode
(data not reported). Knowing that the current efficiency is
optimized at low current density, we concluded that practical
dechlorination technology will require the cathode to have
the largest possible surface area to achieve rapid reduction
at the same time as passing a high current through the
solution. Large area Pb cathodes were prepared in three
ways: by depositing Pb from solution on to spongy iron, by
using a piece of an automotive battery, and by depositing Pb
-
2
current densities (0.5-6.7 mA cm ) at which hydrogen
evolution was copious. The overall current efficiency was
very low (∼1%), and the rate of reduction scarcely increased
with increasing current density on account of competing
evolution of hydrogen.
Single flow experiments with amperostatic polarization
were carried out using lead, zinc, tin, and mercury cathodes
1
5 1 2
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 32, NO. 10, 1998

would be Cr(II). The use of CrCl
3
was unsuccessful due to
reduction of Cr3 directly to the metal, and so the EDTA
complex of Cr(III) was used. The reduction of the Cr(III)
complex was easily observed by the discharge of its purple
color, and no deposition of metallic Cr occurred, but the rate
of reduction of HCB was not enhanced as compared with
experiments in which Cr(III)-EDTA was absent. Due to
account of the lack of success in these preliminary experi-
ments and toxicity concerns in the specific case of chromium
salts as mediators, we abandoned further exploration of
mediated electrolysis in our system.
+
Finally, we examined the effect of temperature on the
rate of electrodehalogenation using the Pb cathode. No
change in current efficiency was observed when the micellar
solution was electrolyzed at 40 and 50 °C, compared with
2
room temperature, at a current density of 2.5 mA/ cm .
Experiments at higher temperature were tried (60 and 70
°
C), but under these conditions the microemulsion was not
stable. From the practical perspective, these observations
show that the efficiency of dechlorination is indifferent to
temperature.
FIGURE 6. Current efficiency of HCB reduction in microemulsion/
.05 M Na SO as a function of applied current density.
0
2
4
In conclusion, we have achieved the three goals stated in
the introduction of this paper. We have shown that practically
useful concentrations of HCB can be obtained in aqueous
solution using 0.1% Triton-SP 175 and 1% heptane as
surfactant and cosolvent. Electrolytic dehalogenation of HCB
can be achieved at a variety of cathodes, of which lead was
the best under our conditions. The electrolysis products
can be recovered following mild acid hydrolysis of the
surfactant. We contrast our approach with the studies of
Rusling et al. (13, 19) on the electrochemical dechlorination
of PCBs using lamellar solutions containing water, oil, and
didodececyl ammonium bromide, the latter acting as both
surfactant and supporting electrolyte. The latter system, with
high concentrations of hydrocarbon phase, is well suited to
the remediation of PCB-mineral oil mixtures. Since our
objective was to remediate aqueous streams or leachates
containing HCB, we sought conditions that required minimal
additives to the water phase. Our use of a nonionic surfactant
requires an added supporting electrolyte: in our case, the
environmentally friendly Na SO . So far, the current ef-
TABLE 2. Relative Current Efficiencies for Various Cathode
Materials (Reference Point: Pb Cathode with Current
Efficiency ∼5%)
current
relative
HCB:PeCB:TeCB:TrCB current
density
-E
2
electrode (mA/cm ) (V vs SHE)
(mol %)
efficiency
Hg
Pb
Zn
Sn
0.06
0.01
0.15
0.05
1.47
1.40
1.25
1.58
87.0:5.7:5.2:2.1
85.3:0.6:4.8:9.3
81.0:2.8:7.8:8.4
91.8:2.1:3.7:2.4
53
100
10
10
from solution on to Pb wire. The last of these was the most
successful, although we were unable to measure the surface
area of the cathode so prepared. Figure 5 compares the rate
of reduction of HCB in continuous flow electrolysis at 75 mA
2
on plain Pb wire of area 30 cm (panel A) with the
corresponding experiment on Pb deposited from citrate
solution (panel B). Reduction at the cathode of larger surface
area occurred an order of magnitude faster. One difference
noted was in product distribution: whereas the major product
at plain Pb wire was trichlorobenzene, the reaction at lower
current density proceeded only to the tetrachlorobenzene
stage because the lower current density caused the cathode
to be less negatively polarized.
2
4
ficiency we have achieved is an order of magnitude poorer
than that attained in methanol (8); however, the latter
experiments employed a much larger concentration of HCB
2
and were carried out in the absence of O , reduction of which
will undoubtedly worsen current efficiency. It is also clear
that higher current efficiency is achievable by operating at
the smallest practical current density, although with some
reduction in the overall extent of dechlorination. To achieve
both high efficiency and rapid rate of reaction in practical
remediations, cathodes must be developed having the
maximum possible surface area.
The literature contains numerous reports of reductions
(
“
17-20) and oxidations (21, 22) that can be carried out by
mediated” electrolysis. The principle underlying mediated
electrolysis is that the mediator is a substance having one-
electron redox properties. In the case of mediated reduction,
for example, the electrochemical stage of the reaction is the
Acknowledgments
We thank the Natural Sciences and Engineering Research
Council of Canada for financial support through the Strategic
Grants Program and Mark McDonough of Union Carbide
Canada for supplying us with Triton-SP 175.
•
-
reduction of the mediator (Med) to Med at the cathode,
•
-
followed by chemical reduction of the substrate by Med in
the bulk solution, with concomitant regeneration of Med.
Rusling and co-workers have described the mediated elec-
troreduction of polychlorinated biphenyls at impressive
current efficiency in surfactant media using zinc phthalo-
cyanine (ZnPc) as the mediator (12); however, high con-
centrations of ZnPc were needed relative to the concentration
of the reducible substrate. In the micellar systems employed
in the present work, we were able to achieve only micromolar
concentrations of ZnPc, and no increase in the rate of
reduction was observed by comparison with solutions not
containing ZnPc. We also explored the use of Cr(III) as a
water-soluble mediator, in which case the active reductant
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