Environ. Sci. Technol. 2001, 35, 4933-4935
Effects of Quartz Addition on the Mechanochemical Dechlorination of Chlorobiphenyl by Using CaO QIWU ZHANG,* FUMIO SAITO, TADAAKI IKOMA, AND SHOZO TERO-KUBOTA Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan KIYOTAKA HATAKEDA National Industrial Research Institute of Tohoku, Sendai 983-8551, Japan
Grinding a mixture of 3-chlorobiphenyl (BP-Cl) and CaO with or without the addition of quartz was conducted in air by a planetary ball mill to investigate the mechanochemical dechlorination of BP-Cl. The dechlorinating reaction proceeds with an increase in grinding time, and over 99% of BP-Cl is decomposed at 360 min. Washing the ground sample with different solvents results in different products. Addition of quartz to the grinding mixture facilitates dechlorination efficiency, especially in the case of a high weight ratio of BP-Cl to CaO.
Introduction It is widely recognized that hazardous chlorinated organic compounds, polychlorinated biphenyl (PCB) and dioxins in particular, are the most dangerous pollutants to our environment. This concern has led to intensive investigations for the complete decomposition of such toxic compounds. Numerous methods (1-11) have been studied for decomposing PCB using different chemical reagents with the aid of temperature or pressure. There seems to be greater difficulty when a great deal of sample contaminated by PCB or dioxins at low concentrations has to be treated. A mechanochemical method has recently attracted attention as a simple route for treating chlorinated organic compounds (12-15). In this case, inorganic additives such as CaO and Ca metal play important roles in the dissociating reactions. The use of CaH2 has been tested to facilitate the reactions (12, 13). However, grinding operations have to be carefully conducted because of the potential occurrence of an explosive type reaction when CaH2 is used. Thus, there is a pressing need to develop an alternative mechanochemical route by which cheap and easily available CaO can be used as an additive with satisfactory grinding efficiency, namely, a satisfactory decomposing rate of PCB. Furthermore, the reaction mechanism needs to be more clearly understood in regard to the unclearness remaining in recent reports. The main purpose of this study is to improve the mechanochemical process by employing the addition of quartz to mixed grinding and to provide information on the products of dissociating reaction of PCB with CaO by washing the ground samples with different solvents. Although our preliminary experiments have confirmed that other hard * Corresponding author phone: +81-22-217-5137; fax: +81-2227-5211. 10.1021/es010638q CCC: $20.00 Published on Web 11/15/2001
2001 American Chemical Society
materials exhibiting high stability during grinding, e.g., corundum (R-Al2O3), are also suitable as additives, quartz was used because of its low cost and easy availability.
Experimental Section 3-Chorobiphenyl (BP-Cl, Lancaster, 98% purity) was used as a PCB sample. Reagents of ethyl acetate (CH3COOC2H5), Ca(OH)2, and quartz (SiO2) were obtained from Wako Pure Chemicals and used as received. CaO was prepared by heating the Ca(OH)2 at 800 °C for 2 h before grinding operation. Two starting mixtures for the grinding were prepared: one is a mixture of BP-Cl and CaO (5 g), and the other is a mixture of BP-Cl, CaO (2.5 g), and SiO2 (2.5 g). The weight ratio of BP-Cl to the inorganic powder samples was regulated in a range of 5% to about 30%. As a reference, the use of Ca(OH)2 instead of CaO was also compared at the same grinding and washing conditions. A planetary ball mill (Pulverisette-7, Fritsch, Germany) was used to grind the starting mixtures at approximately 700 rpm in air. The mixtures were charged in a stainless steel pot of 45 cm3 inner volume with 7 stainless steel balls of 15 mm diameter and were ground for different periods of time. A zirconia pot and balls were specially used to prepare samples for electron spin resonance (ESR) analysis under the same grinding conditions. Samples for ESR measurements were charged in a quartz tube of 5 mm diameter, and measurements were carried out with an X-band ESR spectrometer (Bruker ESP-380E). Washing operations for the ground samples were conducted to extract the remaining BP-Cl by two methods: ethyl acetate washing and water washing followed by ethyl acetate washing. In the case of the two-step washing, the filtrate was separated into organic and inorganic phases. These samples were subjected to GC/MS analysis using a Hewlett-Packard gas chromatograph (model 6890) equipped with a mass selective detector (model 5973) to determine the remaining BP-Cl concentration and other possible products.
Results and Discussion Figure 1 shows changes in the yield of the remaining BP-Cl with grinding time, with 5% BP-Cl added to the solid samples. Whether ground with CaO only or with the mixture of CaO and SiO2, the remaining percentage of BP-Cl in the ground sample decreases rapidly with an increase in grinding time within 20 min and diminishes gradually with further grinding. It has been found that less than 0.5% of BP-Cl remains at 360 min grinding, indicating that over 99.5% BP-Cl has been decomposed in both cases. The prominent effect due to the addition of SiO2 lies in the quick decrease of the remaining BP-Cl in the early stage of grinding as compared with CaO only. It indicates that the addition of SiO2 facilitates the mechanochemical reaction. Figure 2 shows changes in the yield of the remaining BPCl in the samples ground for 360 min with the added percentage. When the percentage is less than 10%, the remaining BP-Cl is decreased below 1% in both cases, and no large difference is observed. With an increase in added percentage up to 30%, although the remaining amount of BP-Cl is observed to increase in both cases, the remaining yield ultimately differs. About 16% BP-Cl remains in the case of CaO use only. This value is reduced to about 4% when SiO2 is added. Especially the high percentage of oily BP-Cl in the mixture induces heavy agglomeration during grinding, worsening the grinding efficiency. The addition of SiO2 allows breakdown of the agglomerates and therefore improves the decomposing reaction. VOL. 35, NO. 24, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Yield of BP-Cl remaining in the ground mixture with CaO and SiO2.
FIGURE 3. GC/MS analyses of 360-min ground sample washed under different conditions. S1, ethyl acetate washing; S2, organic phase from water and ethyl acetate washing; S3, water phase from water and ethyl acetate washing.
FIGURE 4. GC/MS spectrum of the organic phase from water and ethyl acetate washing of 360-min ground mixture of Ca(OH)2/SiO2/ BP-Cl.
FIGURE 2. Change in yield of BP-Cl remaining in the ground mixture with ratio of BP-Cl added. Figure 3 shows the GC/MS spectra of 360-min ground sample of CaO and SiO2 added with 5% BP-Cl, washed with ethyl acetate only (S1) as well as with water and ethyl acetate (S2, organic phase) (S3, water phase). Although a peak positioned at 8.88 min corresponding to the original BP-Cl is observed in S1 and S2 spectra, the areas of the peaks of BP-Cl existing in the ground samples correspond to less than 0.5% of that of the starting BP-Cl, indicating the BP-Cl has been decomposed effectively irrespective of the washing conditions. However, the patterns of the spectra are quite different, suggesting that the products are different under different washing conditions. The many weak peaks in spectrum S1 indicate the formation of various organic products when the ground sample was washed by ethyl acetate only. It is difficult to determine the chemical compositions for all the peaks. It is worthwhile to consider the peak size. Since all these peaks are in the same order in area as that of the remaining BP-Cl, it could be deduced that the amount of each product should be less than 1 mol % based on the starting BP-Cl. This means that the total amount of the products shown in spectrum S1 does not match that of the starting BP-Cl. This phenomenon has been reported in refs 13 and 14. It is considered that most of the organic 4934
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products in the ground sample are not soluble in ethyl acetate, although BP-Cl is easily soluble in it. In contrast, two large peaks corresponding to biphenyl (BP-H) and hydroxybiphenyl (BP-OH) are observed together with several small ones in spectrum S2 with another two BP-OHs in spectrum S3. Considering an aqueous solution with a pH value as high as about 12 from the CaO washing, two kinds of BP-OH are dissolved in water and concentrated in the aqueous phase; therefore, the existence of a considerable amount of BP-OHs can be confirmed in the solution shown in spectrum S3. Although the two small peaks corresponding to BP-H and BP-OH appear in spectrum S1, a much greater increase in area of these peaks is observed in spectra S2 and S3. This result indicates that these products are formed from a chemical reaction between water and the organic compounds that are barely dissolved in ethyl acetate. CaO sample has a tendency to absorb moisture from air to form Ca(OH)2. A mixture of Ca(OH)2/SiO2 and BP-Cl was ground as a comparative test. After the same washing procedure of water washing and ethyl acetate washing, a GC/MS spectrum of organic phase is shown in Figure 4. It is clear that BP-Cl remains as the main phase unchanged, although a small peak corresponding to BP-H is observed. This means that Ca(OH)2 cannot dissociate BP-Cl effectively. There exists a radical difference between the oxide and the hydroxide in the dechlorination of BP-Cl.
where none of the ESR signal is observed. It can be deduced that the dechlorinating reaction has close relation with radical formation. Taking the above results and other related references into account, it is understood that BP-Cl can be easily decomposed by simply grinding it with CaO powders with the aid of quartz coexistence and that radical reactions play an important role in dechlorinating BP-Cl. It is also worth mentioning that the moisture of the sample should be carefully controlled when this process is applied to treat practical wastes because formation of Ca(OH)2 due to the existence of moisture significantly reduces the effect of CaO sample.
Acknowledgments The authors are grateful to Dr. Kaoru Shimme, Sumitomo Metal Industries, Ltd., Japan, for his invaluable comments to this work.
Literature Cited FIGURE 5. ESR spectra of 360-min ground samples of CaO only, CaO/SiO2/BP-Cl mixture, and Ca(OH)2/SiO2/BP-Cl mixture, respectively. To look for what causes the difference and the relation with products from GC/MS analysis, the following reports are worthy of notice. Charge transfer occurs from the surface of the solid CaO to the absorbed molecules (16-19), and the biphenyl radical reacts with water to form BP-OH during the photocatalytic degradation for organic toxic substances such as chlorobiphenyl (20-23). We therefore performed ESR measurements of the various ground samples. The samples for ESR analysis were prepared with a ZrO2 pot and balls to prevent metal contamination. The GC/MS analysis has shown that there is not a great difference between the products created from the stainless steel and the ZrO2 mills. In case of the mixed grinding of CaO/SiO2 and BP-Cl, the remaining yield of BP-Cl after grinding for 360 min was 1.5% in the case of ZrO2, which is a little higher than the value of 0.5% by the stainless steel mill (Figure 1). Figure 5 shows the ESR spectra of 360-min ground samples of CaO only, CaO/SiO2/BP-Cl mixture, and Ca(OH)2/SiO2/BP-Cl mixture, respectively. The intensities of the spectra of ground CaO and Ca(OH)2/SiO2/ BP-Cl mixture were 3 and 10 times multiplied for easier observation. Even in the case of CaO ground separately there are several peaks observed. Unpaired electrons are produced in the ground sample. When BP-Cl is ground with CaO, quite different patterns with high intensity are obtained. The addition of SiO2 does not change the pattern of ESR spectrum, except for the intensity of peaks. The discussion on the compositions in detail has been reported (24). The sharp peak is assigned to a trapped electron (e-) in an oxygen vacancy on the CaO powder surface, and the broad signal is attributed to aromatic hydrocarbon radicals. On the other hand, when BP-Cl is ground with Ca(OH)2, there is none of the ESR signal observed. The effective dechlorination is achieved with CaO where ESR signal is detected. On the other hand, no effective dechlorination is achieved with Ca(OH)2
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Received for review February 14, 2001. Revised manuscript received September 12, 2001. Accepted September 21, 2001. ES010638Q
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