Reduction of CCl4 by iron powder in aqueous solution

Article abstract of DOI:10.1002/ejic.200400979

^.CCl3 radicals formed in the first step of the CCl₄ degradation react with iron powder to form an intermediate with an Fe-C σ bond. In the absence of iron powder, ^.CCl₃ radicals formed by γ-irradiation react with 2-propanol in a chain reaction toward dechlorination. When the irradiated solution contains iron powder, the iron scavenges the ^.CCl₃ radicals, thus inhibiting the chain reaction. The minor product of the dechlorination of CCl₄ by iron powder in aqueous solution is CH₄, contrary to the results reported in the literature for the electrochemical reduction of CCl₄ on iron electrodes. Wiley-VCH Verlag GmbH & Co. KGaA.

Full text of DOI:10.1002/ejic.200400979

SHORT COMMUNICATION
Reduction of CCl by Iron Powder in Aqueous Solution
4
[a]
[a,b]
and Dan Meyerstein*[a,b]
Irena Rusonik, Haim Cohen,
Keywords: Carbon tetrachloride / Reduction / Iron / Mechanism
·CCl
react with iron powder to form an intermediate with an Fe–
radicals formed in the first step of the CCl
degradation
inhibiting the chain reaction. The minor product of the de-
3
4
chlorination of CCl
CH , contrary to the results reported in the literature for the
electrochemical reduction of CCl on iron electrodes.
4
by iron powder in aqueous solution is
·
C σ bond. In the absence of iron powder, CCl
3
radicals
4
formed by γ-irradiation react with 2-propanol in a chain reac-
4
tion toward dechlorination. When the irradiated solution con-
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2005)
·
tains iron powder, the iron scavenges the CCl
3
radicals, thus
of CHCl
the latter compounds are reduced slower than CCl by the
, CH Cl , and CH
Cl as intermediates,[16] though
3 2 2 3
Introduction
4
The widespread use of chlorinated solvents in commerce
has led to considerable groundwater contamination. Halo-
carbons, including carbon tetrachloride, have a cumulative
toxicity and are carcinogenic. Research on the biological
degradation of carbon tetrachloride has received much at-
0
[9,17–19]
Fe electrode.
Iron is a reasonably strong reducing
agent, thus it is not surprising that iron powder can reduce
[
20]
chloro-organic compounds.
Therefore it was of interest
to investigate the mechanism of the dechlorination reaction
of CCl by iron powder, as this method is economically
[
1,2]
4
tention.
Nevertheless, the microbial degradation of CCl4
preferable to the electrochemical one.
can be relatively slow[ and may be inhibited by the toxicity
3]
of CCl4.
Many recent studies have focused on the abiotic transfor-
[
4,5]
[6,7]
mation of CCl4
and other ground-water pollutants
by naturally occurring soil minerals such as iron sulfide
FeS). This weak reducing agent reacts slowly with some
Results and Discussion
(
The mechanism proposed to explain the latter results, in-
[8]
[16]
halo-aliphatic compounds. However, the mechanism of volves the formation of carbenes.
However, this mecha-
the process has not been elucidated.[ Consequently, abiotic nism seems to be too complicated, and alternative simpler
8]
reductive dechlorination with reduced metals has received mechanisms can be proposed. In principle, the following
increased attention. The use of zero-valent metals has been mechanism is proposed (Scheme 1).
an active research area[
9–11]
, and most efforts have been con-
centrated on iron because of the success encountered with
iron subsurface permeable walls[ and the acceptance of
zero-valent iron as being safe for the environment. The re-
12]
sults indicate that significant CCl reduction occurred at
4
[
13]
pits rather than on the passive oxide film on the metal.
Least-oxidized Fe is the most reactive in CCl degrada-
0
4
[
14]
tion.
0
The combination of ultrasound and Fe has a positive
synergistic effect on the dehalogenation of CCl . A mecha-
4
·
nism involving the formation of CCl radicals was pro-
3
[
15]
Scheme 1. Plausible mechanism of reduction of CCl by iron pow-
4
posed.
der immersed in aqueous solution.
0
The electrochemical reduction of CCl on Fe electrodes
4
was reported to yield CH directly without the formation
4
The first reaction in this scheme is identical to that pro-
[
[
a] Department of Chemistry, Ben-Gurion University of the Negev,
Beer-Sheva, Israel
[16]
posed in the literature.
However the second one is dif-
b] Department of Biological Chemistry, College of Judea and Sa- ferent; it is proposed that the radical formed in the first
maria,
Ariel, Israel
reaction, that is, the one near the iron surface, reacts with
+
the resultant 1 particle to form an intermediate having an
Fax: +972-3-906-7440
E-mail: danmeyer@bgumail.bgu.ac.il
Fe–C σ bond, that is, a –CCl bound to the iron powder.
3
Eur. J. Inorg. Chem. 2005, 1227–1229
DOI: 10.1002/ejic.200400979
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1227

I. Rusonik, H. Cohen, D. Meyerstein
SHORT COMMUNICATION
iron powder and an aqueous solution containing CCl and
4
2-propanol. One of these samples was irradiated, and the
second served as a thermal blank. The light alkanes formed
in the latter samples are due to traces of carbon in the ana-
[
21]
lytical iron.
The main idea was to study the reaction of
·
CCl radicals with the iron powder, where, in the nonirradi-
3
Analogous transients are formed in the fast reaction of
ated system, the source of radicals is the dehalogenation
0
[21]
+
Fe powder with CH .
The intermediate 1 –CCl thus
3
3
·
process, and in the irradiated system, CCl radicals are de-
3
formed might decompose by two alternative mechanisms:
. Heterolysis of the Fe–C bond, reaction (3), as ob-
rived from γ-irradiation in addition to the dehalogenation
process. The third system contained the aqueous solution
1
[
21]
+
served for 1 –CH .
3
of CCl and 2-propanol in the absence of iron powder and
4
2
. The iron particle can be considered as a microelectrode
that can supply many electrons, which can reduce the –CCl3
substituent directly to form methane, reaction (4).
was irradiated. The latter blank system contributes to the
·
investigation of the reaction of CCl radicals with 2-propa-
3
nol in aqueous solution. The results of this blank experi-
ment clearly demonstrate (Table 1) that indeed the chain
In order to check the relative contribution of reactions
·
(
3) and (4) it was decided to study the reaction of CCl3
–
process occurs, as the yield of Cl is G ~ 50–60, even though
radicals with iron powder and measure the relative yields of
[
25]
the total primary radical yield is G(radicals) ~ 6.
G is
–
Cl and CH . The radicals were formed by γ-irradiating an
4
defined as the number of molecules of each product per 100
eV of radiation absorbed by the solution. When the same
solution is brought in contact with the iron powder, the
yield of methane is approximately 1% of the Cl– yield
aqueous solution containing CCl (0.01 m) and 2-propanol
4
(
2.6 m). This solution was placed between the grains of iron
powder.
·
CCl radicals formed in the first step of the CCl degra-
3
4
(
Table 1). This result clearly demonstrates that CH is not
4
dation might react by two pathways:
the major product of the thermal dechlorination reaction
and suggests that this process involves reactions (1), (2), and
1
. Initiate a chain reaction by hydrogen abstraction from 2-
[
22–24]
propanol.
(
3) in Scheme 1. This result clearly differs from that re-
·
·
CCl + HC(CH ) OH Ǟ HCCl + C(CH ) OH
0
[16]
3
3 2
3
3 2
ported for the reduction of CCl on Fe electrodes.
4
·
·
–
+
C(CH ) OH + CCl Ǟ C(CH ) O + CCl + Cl + H O
3
2
4
3 2
3
3
When the solution is irradiated in contact with the iron
–
powder, the results (Table 1) point out that the Cl yield is
2
. React with the iron powder surface to form CHCl or
3
considerably smaller than the sum of the two blank experi-
directly CH , as proposed in Scheme 1.
4
0
ments. The irradiation of the Fe -containing system adds
Therefore, dehalogenation of CCl was investigated in
4
only about 3×10–4 m to the Cl yield when compared to the
–
three different systems (Table 1). Two of them contained
–
Table 1. The relative yields of the organic gases and Cl formed in the reduction of CCl
4
(radiation 3.3 Gy/min, 120 min).
[a]
[
d]
[ppm] Cl–[d] [m]
Sample
Radiation CH
+
4
[ppm]
CH
4
[m]
C
2
H
4
[ppm]
C
2
H
6
[ppm]
C
C
C
3
3
3
H
6
[ppm]
C
C
C
3
3
3
H
H
H
8
G
0
Fe (activ.), CCl
4
,
,
2.0×10–5
1.9×10–3
143
7.2
6.4
21.7
8.9
–
2
-propanol
0
Fe (activ.), CCl
4
1.5×10–5
1.1×10–5
1.6×10–3
2.2×10–3
–
110
5.2
–
4
10
3.4
–
–
2
CCl
-propanol
4
, 2-propanol
+
80.6
1.5
6.6
55.6
[b]
[d]
[ppm] Cl–[d] [m]
Sample
Radiation CH
+
4
[ppm]
CH
4
[m]
C
C
2
H
4
[ppm]
C
C
2
H
6
[ppm]
8
H
6
[ppm]
8
G
0
Fe (activ.), CCl
4
,
,
1.8×10–5
1.7×10–3
127
7
24
8.9
–
2
-propanol
0
Fe (activ.), CCl
4
1.3×10–5
9.0×10–6
1.3×10–3
2.0×10–3
–
91.9
64.3
6.4
–
5.9
1
4
3
–
–
2
CCl
-propanol
4
, 2-propanol
+
4.2
50.5
[c]
[d]
[ppm] Cl–[d] [m]
Sample
Radiation CH
+
4
[ppm]
CH
4
[m]
2
H
4
[ppm]
2
H
6
[ppm]
H
6
[ppm]
8
G
0
Fe (activ.), CCl
4
,
,
2.6×10–5
2.3×10–3
184
8.9
8.5
21.6
12
–
2
-propanol
0
Fe (activ.), CCl
4
1.5×10–5
9.7×10–6
2.0×10–3
2.6×10–3
–
110.4
69.5
7.4
–
7.4
1.2
5.6
11
7.5
1.6
–
2
CCl
-propanol
4
, 2-propanol
+
65.7
0
[
4
a] Activated Fe powder (10 g), solution (2.5 mL) containing CCl (0.01 m) and 2-propanol (2.6 m), phosphate buffer (0.01 m, pH 7.0),
0
He sat., t = 3 h. [b] Activated Fe powder (10 g), solution (2.5 mL) containing CCl
0.01 m, pH 3.0), He sat., t = 3 h. [c] Activated Fe powder (10 g), solution (2.5 mL) containing CCl
4
(0.01 m) and 2-propanol (2.6 m), phosphate buffer
0
(
4
(0.01 m) and 2-propanol (2.6 m,
pH 7.0), He sat., t = 3 h. [d] Error limit ±15%.
1228
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
Eur. J. Inorg. Chem. 2005, 1227–1229

Reduction of CCl
4
by Iron Powder in Aqueous Solution
–
SHORT COMMUNICATION
nonirradiated sample, that is, G(Cl ) under these conditions Acknowledgments
–
0
is G(Cl ) = 7.6. This result clearly demonstrates that the Fe
·
powder scavenges the ·CCl and partially the C(CH ) OH This study was supported in part by a grant from the Budgeting
3
3 2
and Planning Committee of The Council of Higher Education and
the Israel Atomic Energy Commission. D. M. wishes to express his
thanks to Mrs. Irene Evens for her ongoing interest and support.
radicals, thus inhibiting the chain reaction. Also under
·
–
these conditions where G( CCl ) Ն 2.5, as all the e
is
3
aq
–
expected to react with CCl , G(CH ) Յ 2% of G(Cl ). This
4
4
result clearly proves that reaction (4) in Scheme 1 is at most
a side reaction in the process, and that reaction (3) is the
major pathway.
[
1] G. R. Chaudhry, S. Chapalamadugu, Microbiol. Rev. 1991, 55,
59–79.
[
2] W. W. Mohn, J. M. Tiedje, Microbiol. Rev. 1992, 56, 482–507.
3] E. J. Bouwer, P. L. McCarty, Appl. Environ. Microbiol. 1983,
45, 1286–1294.
The results are pH-independent in the range pH 3–7
[
(Table 1). Therefore, it was decided to omit the phosphate
buffer from the solution. This action slightly increased the [4] E. Lipczynska-Kochany, S. Harms, R. Milburn, G. Sprah, N.
–
Nadarajah, Chemosphere 1994, 29, 1477–1489.
radiolytic yield of Cl . This result suggests that one of the
[
5] J. F. Devlin, D. Muller, Environ. Sci. Technol. 1999, 33, 1021–
reactions,
1027.
·
·
–
·–
C(CH ) OH + H PO Ǟ (CH ) CHOH + HPO
3
2
2
4
3 2
4
[6] E. C. Butler, K. F. Hayes, Environ. Sci. Technol. 1999, 33, 2021–
–
·–
CCl + H PO Ǟ HCCl + HPO
2027.
3
2
4
3
4
[
7] R. A. Doong, S. C. Wu, Chemosphere 1992, 24, 1063–1075.
[8] E. C. Butler, K. F. Hayes, Environ. Sci. Technol. 2000, 34, 422–
29.
probably the latter, shortens the chain reaction. The effect
of phosphate on the reduction of CCl by Fe is probably
due to the precipitation of Fe (PO ) or Fe(PO ) on the
surface of the iron particles, which decreases their ac-
tivity.
Finally it is suggested that, in the electrochemical re-
duction of CCl4 on Fe , the potential of the cathode was
negative enough to reduce the –CCl groups bound to the
surface by reaction (4). The redox potential of the iron par-
ticle is smaller; therefore CHCl is obtained as the main
product of the reduction of CCl by the iron powder. This
conclusion is highly complementary to the early studies of
the reductive dehalogenation of CCl by elementary iron.
0
4
4
3
4 2
4
[9] L. J. Matheson, P. G. Tratnyek, Environ. Sci. Technol. 1994, 28,
2045–2053.
[
21]
[10] A. L. Roberts, L. A. Totten, W. A. Arnold, D. R. Boris, T. J.
Campbell, Environ. Sci. Technol. 1996, 30, 2654–2659.
11] B. Deng, T. J. Campbell, D. R. Burris, Environ. Sci. Technol.
[
[
16]
0
1997, 31, 1185–1190.
3
[12] R. W. Gillham, Natl. Meet. Am. Chem. Soc., Div. Environ.
Chem. 1995, 35, 691–693.
[
13] D. J. Gaspar, A. S. Lea, M. H. Engelhard, D. R. Baer, R.
Miehr, P. G. Tratnyek, Langmuir 2002, 18, 7688–7693.
3
4
[
14] M. L. Tamara, E. C. Butler, Environ. Sci. Technol. 2004, 38,
1866–1876.
[
9]
4
[15] H. M. Hung, M. R. Hoffmann, Environ. Sci. Technol. 1998, 32,
3011–3016.
[
[
16] T. Li, J. Farrell, Environ. Sci. Technol. 2000, 34, 173–179.
17] R. Muftikian, Q. Fernando, N. Korte, Water Res. 1995, 29,
2434–2439.
Experimental Section
[
[
[
18] B. R. Helland, P. J. J. Alvarez, J. L. Schnoor, J. Hazardous Ma-
ter. 1995, 41, 205–216.
19] K. D. Warren, R. G. Arnold, T. L. Bishop, L. C. Lindholm,
E. A. Betterton, J. Hazardous Mater. 1995, 41, 217–227.
20] W. A. Arnold, A. L. Roberts, Environ. Sci. Technol. 2000, 34,
1794–1805.
0
The metal powder used in this study was Fe powder Merck 99%,
Յ10 μm. The solution was deaerated by bubbling He through it for
15 min by the syringe technique. The solution (2.5 mL) was then
added over 2 min to a glass bulb (15 mL) sealed with a rubber
septum and containing the Fe powder (10 g), which was previously
0
[21] I. Rusonik, H. Polat, H. Cohen, D. Meyerstein, Eur. J. Inorg.
Chem. 2003, 4227–4233.
2 4
activated by H SO (0.1 m), and then washed with water (7–
[
[
22] D. Brault, P. Neta, J. Phys. Chem. 1983, 87, 3320–3327.
23] W. C. Choi, M. R. Hoffmann, Environ. Sci. Technol. 1995, 29,
1646–1654.
8×8 mL). The bulb was also deaerated with He prior to the injec-
tion of the solution. Some of the samples were irradiated for 2 h
in a 60Co γ source with a dose rate of 3.3 Gy/min, within a 3-h
[24] W. C. Choi, M. R. Hoffmann, J. Phys. Chem. 1996, 100, 2161–
reaction time span. After 3 h of reaction, the gas phase above the
metal was analyzed with an HP 5890 GC fitted with an FID detec-
tor [Porapak QS GC column, 10 ft, 1/8 in; He (30 mL/min), T =
2169.
[
[
25] J. C. Russell, G. R. Freeman, J. Phys. Chem. 1968, 72, 808–815.
26] T. M Florence, Y. J. Farrar, Anal. Chim. Acta 1971, 54, 373–
377.
7
0 °C]. Concentrations of chloride ions were analyzed by a colori-
[
26]
metric method.
Received: November 25, 2004
Eur. J. Inorg. Chem. 2005, 1227–1229
www.eurjic.org
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1229