Dechlorination of hexachlorobiphenyl by using potassium-sodium alloy

Article abstract of DOI:10.1016/S0045-6535(99)00557-3

2,2',4,4',5,5'-Hexachlorobiphenyl (HCB) was dechlorinated with potassium-sodium (K-Na) alloy under an inert gas atmosphere. Solvent effect was observed in the reaction. Dechlorination yields in benzene and cyclohexane were 99.9998% and 99.99996%, respecti

Full text of DOI:10.1016/S0045-6535(99)00557-3

Chemosphere 41 (2000) 819±824
Dechlorination of hexachlorobiphenyl by using potassium±
sodium alloy
a
a
b,
b
Kumiko Miyoshi , Takehiko Nishio , Akio Yasuhara *, Masatoshi Morita
a
Department of Chemistry, University of Tsukuba, Tennodai, Tsukuba, Ibaraki 305-0006, Japan
Regional Environment Division, National Institute for Environmental Studies, 16-2 Onogawa, Tsukuba, Ibaraki 305-0053, Japan
b
Received 30 July 1999; accepted 18 November 1999
Abstract
2,20,4,40,5,50-Hexachlorobiphenyl (HCB) was dechlorinated with potassium±sodium (K±Na) alloy under an inert gas
atmosphere. Solvent e€ect was observed in the reaction. Dechlorination yields in benzene and cyclohexane were
99.9998% and 99.99996%, respectively. The reaction was exothermic and proceeded at room temperature. In benzene,
trace amounts of polychlorinated biphenyls (PCBs) as products by stepwise dechlorination and polychlorinated
quarterphenyls as product of Wurtz±Fittig reaction were detected as reaction intermediate. Reaction products were
biphenyl, cyclohexylbenzene, and dicyclohexyl. In cyclohexane, there were no products of Wurtz±Fittig reaction.
Dechlorination at para-position preferred to that at ortho-position, judging from analysis of PCBs as intermediates of
stepwise dechlorination. Ó 2000 Elsevier Science Ltd. All rights reserved.
Keywords: Reaction mechanism; By-products; PCBs; Reduction
1. Introduction
exceeded more than 20 yr. Therefore, decomposition of
PCBs in storage is desired urgently.
The waste polychlorinated biphenyls (PCBs) must be
stored under closed condition in Japan until the Minister
of Japan Environment Agency directs decomposition
methods. At present, about 150 000 t of waste PCBs
including PCB-containing insulating oil, as well as
436 000 t of various materials contaminated with PCBs,
including electrical equipment, pressure-sensitive paper
containing PCBs, PCB-containing sludge and waste, are
reported to be in storage awaiting decomposition. The
Ministry of Health and Welfare has reported that the
amount of waste PCBs lost or missing during storage is
increasing, because the storage period has already
Recently, the Japan Environment Agency has de-
cided that waste PCBs should be destroyed by chemical
treatments in place of incineration or high-temperature
pyrolysis. Lots of investigation has been carried out on
chemical decomposition. Representative methods are
potassium tert-butoxide method (Ono and Hirata,
1996), degradation method by using alkali±metal hy-
droxide and polyethylene glycol (DesRosiers, 1989; De
Filippis et al., 1997), decomposition method with alkali±
metal hydroxide in aprotic solvent such as 1,3-dimethyl-
2-imidazolidinone (Toma and Dohmoto, 1997), base
catalyzed decomposition (BCD) method (Taniguchi et
al., 1996; Taniguchi, 1997) developed by the US Envi-
ronmental Protection Agency, supercritical water oxi-
dation method (Anjoh et al., 1997), catalytic extraction
method (Chanenchuk et al., 1996) developed by Molten
Metal Technology in the US, metallic sodium method
(Ariizumi et al., 1997; Suzuki, 1997), dehalogenation
method with sodium/naphthalene (Oku et al., 1978),
* Corresponding author. Tel.: +81-298-50-2544; fax: +81-
298-50-2570.
E-mail address: yasuhara@nies.go.jp (A. Yasuhara).
0045-6535/00/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved.
PII: S 0 0 4 5 - 6 5 3 5 ( 9 9 ) 0 0 5 5 7 - 3

820
K. Miyoshi et al. / Chemosphere 41 (2000) 819±824
catalytic hydrogenation (Marques et al., 1994; Liu et al.,
1998), dechlorination (Epling et al., 1988; Roth et al.,
1994; Liu et al., 1995) with reductants such as NaBH4
and the method (Chaudhary et al., 1984; Hawari et al.,
1991, 1992; Zhang et al., 1993; Lin et al., 1995; De Felip
et al., 1996; Huang et al., 1996; Lin et al., 1996; Kinbara,
1997; Yao et al., 1997; Schmelling et al., 1998) using
ultraviolet irradiation. Two methods have been reported
in regard to the metallic sodium method. One is sodium
dispersion method, where sodium powder is dispersed in
non-reactive oil, and the other sodium naphthalene
method, where sodium is dissolved with naphthalene in
tetrahydrofuran.
2.2. General procedure for dehalogenation with K±Na
alloy
Sodium (1.15 g) and potassium (1.95 g) are put into
three-necked ¯ask (300 ml) under atmosphere of argon
and heated lightly until liquid alloy is produced. All
operations are done under atmosphere of argon until
methanol is added into the reaction mixture for re-
moval of unreacted K±Na alloy. Thirty-®ve milliliters
of benzene or cyclohexane is poured into the ¯ask as
reaction solvent. Then, 0.6347 g of HCB in 35 ml of
solvent is added drop by drop into the ¯ask under
stirring for 15 min. Ten milliliters of solvent is further
added to the reaction mixture after addition of HCB.
The reaction mixture is re¯uxed for 2 h after 30 min
keeping at room temperature. After cooling, residual
K±Na alloy is destroyed with methanol. The reaction
mixture is concentrated to ca. 50 ml with a rotary
evaporator and the residual solution is diluted with
20 ml of water. Organic substances in the solution are
extracted with hexane (20 ml) and the aqueous layer
and the hexane layer are separated. The hexane layer is
washed with 20 ml of water. The aqueous layers are
combined and are used for measurement of chloride
anion. The organic layer is dried with anhydrous so-
dium sulfate and concentrated to ca. 1 ml. The con-
centrated solution is cleaned up by column
chromatography using silica gel (10 g) and hexane
(200 ml). The elute is concentrated to 0.5 ml by using
rotary evaporator and successive nitrogen-blowing.
Details of reaction conditions are shown in Table 1.
The method developed in this paper is a modi®cation
of metallic sodium method. Potassium±sodium (K±Na)
alloy in place of sodium powder. The alloy is reactive
liquid at room temperature. In this paper, perfect de-
chlorination of 2,20,4,40,5,50-HCB has been accom-
plished and toxic by-products have not been detected.
Reaction mechanism for dechlorination is discussed.
2. Experimental
2.1. Chemicals
2,20,4,40,5,50-hexachlorobiphenyl (HCB) was synthe-
sized from 2,20,5,50-tetrachlorobenzidine according to
the literature (Safe and Hutzinger, 1972). Purity of HCB
was 99.0% by gas chromatography equipped with fame
ionization detector. Hexane, benzene, cyclohexane,
methanol, and dichloromethane, which were of special
grade, were purchased from Wako Pure Chemical In-
dustries, Osaka, Japan. Solvents for dechlorination re-
action were dried with small slices of sodium before use.
Sodium and potassium, of practical grade , were pur-
chased from Wako Pure Chemical Industries, Osaka,
Japan, and Aldrich Chemical Company, Milwaukee,
USA, respectively. Silica gel for column chromatogra-
phy, whose commercial name was Wakogel C-200, was
obtained from Wako Pure Chemical Industries, Osaka,
Japan. The silica gel was used without any acitivation
procedure. Pure water was generated with the Milli-Q
System (Millipore, USA).
2.3. Measurement of chloride anion
Chloride anion was measured with an instrument,
Model IM-40S ion meter purchased from the TOA
Electronics, Tokyo, Japan. Analytical principle was a
measurement with an electrode for chloride anion.
2.4. Gas chromatography/mass spectrometry
An aliquot of the concentrated solution was injected
into a gas chromatograph±mass spectrometer. The
Table 1
Reaction conditions for dechlorination of 2,20,4,40,5,50-HCB
Run 1
Run 2
Run 3
Amount of HCB (mmol)
Dropping rate of HCB solution (ml/min)
Amount of potassium (mol)
Amount of sodium (mol)
Solvent
1.76
2.5
1.76
2.9
1.76
2.9
0.05
0.05
Benzene
80
0.05
0.05
0.05
0.05
Cyclohexane
Cyclohexane
Volume of solvent (ml)
Reaction time before heating (min)
Period of heating (min)
80
30
80
15
0
30
120
120

K. Miyoshi et al. / Chemosphere 41 (2000) 819±824
821
operational conditions were as follows: Instrument,
Hewlett Packard 5972 mass selective detector equipped
with Hewlett Packard 5890 Series II Plus gas chro-
matograph; column, PTETM-5 fused silica capillary (30
m length, 0.25 mm i.d., 0.25 lm ®lm thickness); oven
temperature, 50°C for 1 min, 20°C/min ramp to 220°C,
followed by a second ramp of 5°C/min to 300°C for 5-
min hold; injector temperature, 280°C; interface tem-
perature, 300°C; carrier gas (He) head pressure, 82 kPa;
carrier gas ¯ow rate, 1.4 ml/min; linear velocity of car-
rier gas, 43.0 cm/s; injection mode, splitless; purge time,
60 s; ion-source temperature, 192°C; ion-source pres-
¯ow after evaporation of solvent. The incineration ex-
haust gas was passed through an electrolytic solution,
made of 85% acetic acid and 0.136% sodium acetate,
after removal of water in the gas by passing the gas
through conc. H2SO4. Concentration of chloride anion
in the electrolytic solution was determined by electrol-
ysis titration using silver electrode using a Mitsubishi
Kagaku Titration Model TOX-10. The lower quantita-
tion limit was 0.3 lg/ml.
3. Results and discussion
5
sure, 6:4 Â 10 Torr; ionizing energy, 70 eV. Quanti-
tation was carried out by using SIM mode on mass
spectrometry. Monitor ions were shown in Table 2.
Runs 1 and 2 were performed for investigating sol-
vent e€ect and run 3 was carried out for detecting re-
action intermediates in dechlorination. Therefore,
heating was not done in run 3 and reaction time was
shorter than in run 2.
2.5. Total organic chlorine (TOX) measurement
Reaction between HCB and K±Na alloy was exo-
thermic. Temperature increases in reaction mixture be-
fore heating were 8°C for run 1 and 15°C for runs 2 and
3. This di€erence is due to ability as a hydrogen donor
which solvent has in this reaction. Cyclohexane is better
than benzene as a solvent.
One hundred microliters of sample solution was
combusted at 900°C in a quartz container under oxygen
Table 2
Monitor ions on gas chromatography/mass spectrometry
Compound
Ion 1
Ion 2
Insoluble sludge was generated with dechlorination
proceeding. The sludge was composed of inorganic salt
and adsorbed a small amount of HCB on the surface.
The rate of HCB adsorbed on the sludge was around
0.00003% based on starting amount. Dechlorination
yields in run 1 were 94.33% before heating and 99.9998
after ®nal treatment. On the other hand, dechlorination
yield in run 2 was 99.99996% before heating and this
yield was equal to the yield after ®nal treatment. The
decrease of parent HCB in the reaction is shown in
Fig. 1. This means that the dehalogenation in cyclo-
hexane proceeds rapidly and quantitatively within 30
min at room temperature. Namely, cyclohexane seems
to be an excellent solvent in this reaction. The reason is a
powerful hydrogen-donating ability of cyclohexane,
compared with benzene.
Monochlorobiphenyl
Dichlorobiphenyl
Trichlorobipheny
Tetrachlorobiphenyl
Pentachlorobiphenyl
Hexachlorobiphenyl
Biphenyl
188
222
256
290
326
360
154
166
160
190
224
258
292
328
362
Dicyclohexyl
Cyclohexylbenzene
Material balance of chlorine is shown in Table 3.
Chlorine in other chlorinated compounds except for
parent HCB and polychlorobiphenyls newly formed was
calculated by subtracting total chlorine of parent HCB
and polychlorobiphenyls newly formed from total or-
ganic chlorine determined experimentally. Judging from
this result, chlorine atoms in HCB changed almost
Fig. 1. Decrease of concentration of parent HCB.
Table 3
Material balance of chlorine in dechlorination reaction
Chloride
anion (%)
Parent HCB
remaining (%)
Polychlorobiphenyl newly
formed (%)
Other chlorinated compounds
calculated from TOX (%)
Total
(%)
Run 1
Run 2
97.7
102.0
0.6
0.0
0.1
0.0
0.9
0.0
99.3
102.0

822
K. Miyoshi et al. / Chemosphere 41 (2000) 819±824
completely to chloride anion both in runs 1 and 2. This
conclusion is in accordance with the observation that
by-products containing chlorine have not been detected
or a little detected.
Small amounts of PCBs having 1±5 chlorine atoms
were detected. These compounds are thought to be in-
termediates in dechlorination. Intermediate formations
are showed in Figs. 2 and 3. Various kinds of PCBs were
observed in run 1, but most of them were almost de-
halogenated in ®nal stage. On the other hand, a few
PCBs were observed in run 2 and their concentrations
were extremely low. Quantity of monochlorobiphenyl
was more than other chlorinated biphenyls both in runs
1 and 2.
Fig. 3. Concentrations of parent HCB and dechlorinated PCBs
in cyclohexane.
By-products detected in run 1, which contained
chlorine atoms, were polychlorinated terphenyls and
quaterphenyls, where the number of chlorine atoms
was 3±5, except for PCBs. They were produced by the
Wurtz±Fittig reaction. They disappeared after heating
reaction mixture. In run 2, they were not detected
at all.
Reduction products detected were biphenyl, cyclo-
hexylbenzene, and cyclohexen-1-ylbenzene in run 1 and
dicyclohexyl, biphenyl, cyclohexylbenzene, and cycloh-
exen-1-ylbenzene in run 2. Production situation is shown
in Fig. 4. Amounts of reduction products analyzed by
gas chromatography were not so much, because most of
dechlorinated products or reduction products seemed to
polymerize to insoluble amorphous materials which
were organic substances not containing any chlorine
atoms as demonstrated by TOX measurement. Amounts
(ca. 0.3 g) of amorphous materials were nearly equal to
the calculated amount (minimum 0.27 g) of perfectly
dechlorinated products.
Fig. 4. Formation of reduction products in dechlorination of
HCB.
Intermediates in this dechlorination were determined
in run 3. The analytical results are shown in Table 4.
Dechlorination mechanism is considered in Fig. 5.
Chlorine atom located at para-position in PCBs was
more reactive than at ortho-position because of steric
hindrance. This mechanism is di€erent from the de-
chlorination mechanism in photolysis where dechlori-
nation at ortho-position is more preferable.
Fig. 2. Concentrations of parent HCB and dechlorinated PCBs
in benzene.
Fig. 5. Estimated dechlorination mechanism of 2,20,4,40,5,50-
HCB.

K. Miyoshi et al. / Chemosphere 41 (2000) 819±824
823
Table 4
PCB isomers detected as intermediates in experiment run 3a
Compound
Retention time (min)
Concentration (nmol/ml)
2-Chlorobiphenyl
3-Chlorobiphenyl
8.35
8.80
0.077
0.036
0.007
0.095
0.145
0.066
0.015
0.04
4-Chlorobiphenyl
2,20-Dichlorobiphenyl
8.85
9.09
2,4-/2,5-Dichlorobiphenyl
2,30-Dichlorobiphenyl
2,40-Dichlorobiphenyl
3,30-Dichlorobiphenyl
9.37
9.50
9.56
9.94
3,4-/3,40-Dichlorobiphenyl
2,20,5-Trichlorobiphenyl
2,4,5-Trichlorobiphenyl
2,30,5-Trichlorobiphenyl
2,40,5-/2,4,40-Trichlorobiphenyl
2,30,40-Trichlorobiphenyl
3,30,4-Trichlorobiphenyl
3,4,40-Trichlorobiphenyl
2,20,5,50-Tetrachlorobiphenyl
2,20,4,50-Tetrachlorobiphenyl
2,20,4,40-/2,20,4,5-Tetrachlorobiphenyl
2,30,4,5-Tetrachlorobiphenyl
2,4,40,5-Tetrachlorobiphenyl
2,30,40,5-Tetrachlorobiphenyl
2,30,4,40-Tetrachlorobiphenyl
2,20,4,5,50-Pentachlorobiphenyl
2,20,4,40,5-Pentachlorobiphenyl
2,30,4,40,5-Pentachlorobiphenyl
10.02
10.05
10.45
10.50
10.61
10.76
11.34
11.46
11.09
11.16
11.22
11.80
11.95
12.01
12.07
12.50
12.59
13.69
0.16
0.588
0.752
0.147
0.028
0.840
0.300
0.038
0.901
0.187
8.86
1.11
0.186
2.59
0.17
38.2
6.96
14.5
a Concentration of 2,20,4,40,5,50-HCB (parent compound) was 770 nmol/ml in this sample.
DesRosiers, P.E., 1989. Chemical detoxi®cation of dioxin-
contaminated waste using potassium polyethylene glycolate.
Chemosphere 18, 343±353.
Acknowledgements
We thank Mr. Takashi Yamamoto for measurement
of total organic chlorine.
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J.D., 1988. Borohydride, micellar, and exciplex-enhanced
dechlorination of chlorobiphenyls. Environ. Sci. Technol.
22, 952±956.
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