Environ. Sci. Technol. 1992,26, 553-556
Electrochemical Decomposition of Chlorinated Benzenes in Dimethylformamide Solvent John J. Lee, Chaln P. Yao, Yung Y. Wang," and Chi C. Wan Department of Chemical Engineering, Tsing Hua University, Hsinchu, Taiwan, R.O.C.
The decomposition of C6H2C14,C6HC1,, and c&16 by 02'-was investigated. We verified that O2 was reduced into 02'-on the platinum electrode in an aprotic DMF solvent with the cyclic voltammetric technique. We also found that process quasi-reversible. When the water content in DMF was between 20 mM and 1.95 M, the reaction product of 02'-and c&16 was identified as c6Cl,OH. Two reactions which occur simultaneously are proposed, and the reaction process can be predicted quantitatively.
Introduction In general, organic chemicals containing halogen are poisonous. For instance, hexachlorobenzene is suspected of being a carcinogen. It also causes cutaneous porphyria on human skin following prolonged contact. Thus it is important to find useful and effective means to eliminate this pollutant (1-3). The destruction of halogenated carbon compounds by superoxide ions (02'-) has been found to be a promising technique for pollution control in recent years. In a series of reports (4-12), Sawyer and co-workers found that polychlorinated biphenyls (PCBs) and other highly halogenated aromatic compounds could be converted to bicarbonate and chloride by treatment with 02'-.
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CI
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CI-
(2)
CI
CI
-
+
3C20:-
CI
+ o2 +
202
+
4CI-
(3)
I
I
k%
CI
6HOC(0)0- + 1 . 5 0 2
The overall reaction is as follows: c6c16
+ 1202'--
3c2062- + 6C1-
+ 302
(4)
However, the proposed fragmentation steps have not been supported by detection of any intermediate species. Thus, the purpose of this paper is to study further the feasibility of using 02'-,which is generated in situ by electrolytic reduction of dissolved 02,to react with the halogenated aromatic compounds C6H2C14,C6HC15,and C&&. We also try to identify the reaction products and 0013-936X/92/0926-0553$03.00/0
propose a new mechanism via various analytic techniques.
Experimental Section (1) Electrolytic Procedure. A typical electrochemical reactor was made up of two cells of 4-cm diameter and 7-cm height. The cells were made of Pyrex glass and were separated by sintered glass. Both the anode and cathode were 5 cm x 3 cm platinum electrodes. A 60-mL aliquot of dimethylformamide (DMF) containing 0.1 M tetraethylammonium perchlorate (TEAP) and 60 mL of DMF containing 0.1 M TEAP, 6 mM C6H2C14,6 mM C&C&, and 6 mM C6H6 were used as anolyte and catholyte, respectively. High-purity oxygen was passed into the catholyte during electrolysis. The cathode potential was controlled a t -1.1 V vs a silver-silver chloride electrode (SSCE) with an EG&G 362 potentiostat. (2) Analysis Method. The oxidation-reduction of oxygen on the platinum electrode in DMF + 0.1 M TEAP solvent was observed using a cyclic voltammetric (CV) produced via electrolytic reduction of technique. 02*dissolved oxygen under -1.1V vs SSCE was confirmed with EPR spectroscopy (Bruker 200 D). From the measured signal (Figure l),we found that the value of the g factor was approximately 2. Thus we are certain that we can generate 02*by the electrolytic method (13). The color of the solution changed from colorless to orange during electrolysis. The absorption maximum was located at 260 nm (Figure 2). This is another indication that 02'is produced (14). The decomposition of C6H2C14,C6HC1,, and c&16 was analyzed quantitatively by HPLC (Shimadzu LC-SA). Finally, the reaction products were identified with IC (Dionex 10) and IR (Perkin-Elmer 843), mass (JEOL Jms-DlOO), and NMR (Bruker AM 400) spectrometries. Results and Discussion (1) CV Studies. Figure 3 shows the CV results for DMF and DMF/C6C16 solutions in the absence of oxygen. For the DMF system, there is no reduction reaction between -0.5 and -1.9 V. For the DMF/C&16 solution, two cathodic peaks from -1.6 to -1.8 V presumably correspond to the successive reduction of the chlorine atom of c&& (12). Figure 4 illustrates the results obtained when oxygen was present in the DMF or DMF/C6C&solution. For the DMF solution, there is a cathodic peak at -1.1 V, which corresponds to the reduction of oxygen. An obvious cathodic peak also appears at the same -1.1 V in the DMF/C6Cl6 solution system. However, the cathodic peaks from -1.6 to -1.8 V disappear. This clearly shows that C&16 undergoes a different reaction depending on the presence of oxygen. Figure 5 shows the CV results for the DMF/02 solution when c&&, C6HC15, and C6H2C1,were added into the system, curves a-c, respectively. From these curves, we have observed that the cathodic peak current (I,,) increases while the anodic peak current (I,,) decreases. The change of the current with the c&& reactant is larger than those with C6HC1, and C6H2C14.This means that c6c16 reacts more easily with 02'-. Figure 6 shows the CV results of the DMF/02 solution at different scan rates. We found that I,, increases as a
0 1992 American Chemical Society
Environ. Sci. Technol., Vol. 26, No. 3, 1992
553
A Scan rate 100mV/s
(b)
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Flgure 1. EPR signal of Oz*-.
Flgure 3. Cyclic voltammograms of (a) DMF and (b) DMF/CBClssolutions in the absence of oxygen. scan rate 100mV/s ( a ) - DMF + OlMTEA" +02
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(nm)
Figure 2. UV-vis spectrum of Oz'-.
nonlinear function of the square root of the scan rate. E , moves toward the negative potential as the scan rate increases. Furthermore, AEpincreases with increase of the scan rate and is larger than 0.059 V. Thus, the reduction of oxygen on the platinum electrode is a quasi-reversible reaction. The same conclusion was drawn by Maricle (13) for studies in which DMSO was the solvent. (2) Reaction Mechanism Studies. The HPLC results of c6c16 oxygenated by 02'are shown in Figure 7. The retention times of DMF and c&& were 1.8 and 5.0 min, respectively. A new peak was found, which coincided with the retention time of C6C150Hat 1.5 min. It was found 554
-05
-1
-1.5
403
Environ. Sci. Technol., Vol. 26, No. 3, 1992
Potential vs SSCE ( V )
Figure 4. Cyclic voltammograms of (a) DMF and (b) DMF/CsClsSOlutions in the presence of oxygen.
that C&16 was completely destroyed after 3 h of electrolysis. If we add HC1 to the solution after reaction, we can obtain a needlelike substance. This unknown substance was recrystallized and identified by mass, 13C NMR, and IR spectrometries. The results were compared with the previous reports (15,16).We can conclude that the unknown substance is C6C150H. Further experimental evidence shows that there is a failure to form C6C1,0H if the water content in DMF is either less than 20 mM or over 1.95 M. In the former case,
DMF
1
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DMF
10
0
05
-05
Potential vs SSCE
I
-1 5
-10
(v)
1
Flgure 5. Cyclic voltammograms of (a) DMF/0,/C6CI, (b) DMF/O,/ C6HCI, (c) DMF/O,/C,H,CI,, and (d) DMF/O, solutions. (a) scan rate 500 mV/s (b) scan rate 200 mV/s
( c ) scan rate 100 m V / s ( d ) scan rate 50 mV k
(e) scan rate 10 m V
DMF ( c ) after electrolysis 1 hr
% b;
/S
Retention time ( m i n '
Flgure 7. HPLC results of
c&&
oxygenated by 0,'-
is consistent with our experimental result.
W
-0 5
0
-1 0
-1.5
Potential vs SSCE ( V )
Figure 6. Cyclic voltammograms of DMFIO, solutions at different scan rates.
the reaction should proceed according to eq 4 since the mole ratio of C1- to c&16 is 6. In the latter case, the decomposition of c&16 does not occur. On the basis of the study of Sawyer et al. the 02'species which is generated on the platinum electrode is very susceptible to reaction with the water remaining in DMF as follows:
(In,
202'-
+ HzO
-
02
+ HOO- + HO-
(5)
However, the OH- does not react with C&& to form C&l@H under our conditions. Hence, the product c6C1,OH in our system should come from HOO-. When the water content is between 20 mM and 1.95 M, 10 mM c&16 reacts with 02'-to produce 32 mM C1- and about half of the c6c16 would become C6C150H. If the reaction solely follows eq 4,the C1- concentration should be 60 mM. On the other hand, if the reaction proceeds to form C6C150Hcompletely, the theoretical C1- concentration should be around 5 mM. Therefore, these two processes are in parallel and approximately equal and the final C1- concentration should be around 35 mM, which
Conclusion From the present investigation, the oxidation-reduction of O2 on the platinum electrode in a DMF + TEAP solution is shown to be quasi-reversible. I t has been shown that 02'-produced on the platinum electrode reacts with c&&to form C6C1,0H, if the water content in DMF is between 20 mM and 1.95 M. The reaction mechanism proposed by Sawyer has been modified and it has been shown that these two parallel reaction mechanisms fit the experimental results more accurately. Registry No. DMF, 68-12-2; 02, 11062-77-4; C6H2C1,, 12408-10-5;C6HC15,608-93-5; C&3,118-74-1;C&&OH, 87-86-5.
Literature Cited (1) Karasek, F. W.; Dickson, L. C. Science 1987, 237, 754. (2) Creaser, C. S.; Fernandes, A. R.; Ayres, D. C. Chem. Ind. 1988, 15, 499. (3) Quensen, J. F., 111;Tiedje, J. M.; Boyd, S. A. Science 1988, 242, 752. (4) Sawyer, D. T.; Roberts, J. L., Jr. U.S. Patent 4,410,402,1983. (5) Sawyer, D. T.; Calderwood, T. S. US. Patent 4,468,297, 1984. (6) Merritt, M. V.; Sawyer, D. T. J. Org. Chem. 1970,35,2157. (7) Roberts, J. L., Jr.; Calderwood, T. S.; Sawyer, D. T. J . Am. Chem. SOC.1983,105, 7691. (8) Sawyer, D. T.; Stamp, J. J.; Menton, K. A. J. Org. Chem. 1983,48, 3733. (9) Roberts, J. L., Jr.; Calderwood, T. S.; Sawyer, D. T. J. Am. Chem. SOC.1984,106, 4667. (10) Sawyer, D. T.; Valentine, J. S. Acc. Chem. Res. 1987, 14, 393. (11) Sugimoto, H.; Matsumoto, S.; Sawyer, D. T. J. Am. Chem. Soc. 1987, 109, 8081. (12) Sugimoto, H.; Matsumoto, S.; Sawyer, D. T. Enuiron. Sci. Technol. 1988, 22, 1182. Environ. Sci. Technol., Vol. 26,
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Environ. Sei. Technol. 1992, 26, 556-560
Maricle, D. E.; Hodgson, W. G. Anal. Chem. 1965,37,1562. Sawyer, D. T.; Calderwood, T. S.; Yamaguchi, K.; Angelis, C. T. Inorg. Chem. 1983,22, 2577. Kalinowski, H. D.; Berger, S.; Rraun, S. Carbon-13 NMR Spectroscopy, 1st ed,; John Wiley & Sons: New York, 1988; p 331. Pouchert, C. J. T h e Aldrich Library of Infrared Spectra;
Aldrich Chemical Co., Inc.: Milwaukee, WI, 1981. (17) Chin, D. H.; Chiericato, G., Jr.; Nonni, E. J.; Saawyer, D. T. J . Am. Chem. SOC.1982, 104, 1296. Received for review M a y 29, 1991. Revised manuscript received October 3, 1991. Accepted October 8, 1991.
A New Perspective (Sorption/Desorption) on the Question of Chlorolignin Degradation to Chlorinated Phenolics Brian I . O’Connor” and Ronald H. Voss Pulp and Paper Research Institute of Canada, 570 St. John’s Boulevard, Pointe Claire, Quebec, Canada H9R 3J9
A “monomer-free” (Le., predominantly high molecular weight chlorolignin) solution prepared from spent liquor collected from the alkali extraction stage of a softwood kraft pulp mill bleach plant was examined for its ability to release monomeric chlorinated phenolic compounds when stored under sterile conditions at pH 7. The major chlorinated phenolics released from the chlorolignin solution, after 28 days of storage at 50 OC, were 4,5-dichloroguaiacol,3,4,5-trichloroguaiacol, 6-chlorovanillin, and 5,6-dichlorovanillin. These four compounds were found to reach maximum concentrations over the storage period, which corresponded to only 2.3-3.6% of their concentrations in the original E-stage effluent. The remaining chlorinated phenolics normally present in spent bleach liquors did not reach concentrations of >1 pg/L over the course of the experiment. A spiking experiment using I3C-labeled 4,5-dichloroguaiacol demonstrated that the chlorinated phenolics which are released from the chlorolignin over time may be the result of the slow desorption of chlorinated phenolics which had become associated with the chlorolignin during the bleaching process and not necessarily due to chlorolignin degradation as previously hypothesized.
crossing biological barriers (e.g., the gills of fish) limiting their bioavailability (6). Nonetheless, concern has been expressed (6) about the possibility of chlorolignin being broken down in the recipient waters to form low molecular mass chlorinated compounds which may give rise to detrimental biological effects. Such a concern has been heightened by recent reports by Swedish researchers that a portion of the chlorolignin material may undergo chemical (11) and microbial degradation (11-14) to low molecular weight chlorinated organic compounds. For example, Eriksson et al. (11) have reported that chlorolignin, from C- and E-stage bleaching liquors of softwood kraft pulp, when held under sterile conditions for up to 40 days at pH 7.2, appears to undergo chemical degradation to various chlorinated guaiacols and catechols. The present study was undertaken to investigate whether the release of chlorinated phenolics under sterile conditions at neutral pH from chlorolignin material, which had previously been interpreted (11) as being due to “chemical decomposition”, may be accounted for by other pathways, such as simple sorption/ desorption type mechanisms.
Introduction The use of chlorine bleaching agents for the manufacture of bleached chemical pulp inevitably leads to the formation of a wide range of chlorinated organic compounds in the spent bleach liquors. These compounds are produced primarily as a result of complex reactions occurring between the chlorine bleaching agent and the residual (5-10%) lignin remaining in the wood pulp after the preceding chemical (kraft or sulfite) pulping process. Approximately 10% of the chlorine applied to the pulp in the first bleaching stage appears in the effluent as organically bound chlorine [measurable, for example, as adsorbable organic halogen (AOX)] while the remainder (90%) ends up as chloride ions (1-51. The bulk, as much as 80% or more, of the chlorinated organic matter which is dissolved in the spent wash liquors during the bleaching of softwood kraft pulp comprises relatively high molecular weight (MW >1000) chlorinated material, commonly referred to as chlorolignin (6). The chlorolignin which remains after bleaching is decidedly different in nature from unchlorinated lignin. Extensive structural analyses (4,6-10) of chlorolignin have shown it to be virtually nonaromatic with a high carbonyl and carboxyl content, low methoxyl and phenolic hydroxyl content, and only 10% chlorine content by weight. The high molecular weight chlorolignin is generally not believed to be of immediate concern for aquatic organisms since the size of the molecules precludes them from
Materials and Methods Alkali Extraction (E-Stage) Liquor. E-Stage liquor was obtained from a pulp mill producing bleached softwood kraft pulp using a conventional CdEoDED bleaching sequence with 10% chlorine dioxide substitution in the chlorination stage. The E-stage liquor was sampled on two separate occasions. The liquor from the first sampling date was used for a storage experiment at 27 “C. The second sample of E-stage liquor was utilized for a storage experiment at 50 “C and for the 13C-labeled4,5-dichloroguaiacol spiking experiment. Preparation of Chlorolignin Solutions from EStage Liquor. Two different methods were used to remove as much as possible of the low molecular weight material from the E-stage liquors in order to prepare a “monomer-free” (i.e., predominantly high molecular weight chlorolignin) liquor fraction for the subsequent storage experiments. Method 1: Diafiltration and Solvent Extraction. E-Stage bleaching liquor (500 mL) was adjusted to pH 7 and diafiltered (2.5-L wash volume) with deionized water using an Amicon YM2 membrane (nominal molecular weight cutoff 1000). To ensure a more complete removal of the low molecular weight material, the diafiltered Estage liquor (400 mL) was then adjusted to pH 2 and solvent extracted (5 X 200 mL) with methyl tert-butyl ether (MTBE). After the last solvent extraction, residual quantities of MTBE in the E-stage liquor were removed
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