Environ. Sci. Technol. 2001, 35, 2122-2127
Radiolytic and Thermal Dechlorination of Organic Chlorides Adsorbed on Molecular Sieve 13X YUKIO YAMAMOTO* AND SEIICHI TAGAWA The Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan
Reductive dechlorination of chlorobenzene (PhCl), trichloroethylene (TCE), tetrachloroethylene (PCE), 1- and 2-chlorobutanes, chloroform, carbon tetrachloride, and 1,1,1and 1,1,2-trichloroethanes adsorbed on molecular sieve 13X was investigated. The molecular sieve adsorbing the organic chlorides was irradiated with γ-rays, heated, or allowed to stand at room temperature in a sealed ampule and was then soaked in water. The dechlorination yields were determined from the Cl- concentrations of the supernatant aqueous solutions. It was found that the chlorinated alkanes adsorbed on the molecular sieve are readily dechlorinated on standing at room temperature. The dechlorination at room temperature was limited for TCE and PCE. PhCl was quite stable even at 200 °C. γ-Radiolysis was examined for PhCl, TCE, and PCE at room temperature. The radiation chemical yields of the dechlorination, G(Cl-), were 1.9, 40, and 30 for PhCl, TCE, and PCE, respectively. After 5 h of heating at 200 °C, the dechlorination yields for TCE and PCE were 24.5 and 4.3%, respectively. TCE is much more reactive than PCE in the thermal dechlorination, whereas their radiolytic dechlorination yields are comparable. The pH of the supernatant solutions decreased along with the dechlorination.
Introduction Reductive dechlorination of organic chlorides is an important research subject because of the increasing environmental accumulation of harmful organic chlorides, such as trichloroethylene (TCE), polychlorinated biphenyls (PCBs), and a dioxin group. Zeolites have been used as decomposition catalysts for organic chlorides including chlorofluorocarbons (CFCs) (1-11). Investigations have also been done on the adsorption behaviors of organic chlorides on zeolites (1, 1223). We have previously reported on the radiolytic and thermal dehalogenation of 1,1,2-trichloro-1,2,2-trifluoroethane (CFC-113) adsorbed on molecular sieve 13X (24, 25). The dehalogenation yields for CFC-113 were determined by measuring the Cl- and F- concentrations of the supernatant aqueous solutions obtained by soaking the irradiated or heated samples in water. It was revealed that radiolytic dehalogenation of CFC-113 is promoted on the molecular sieve as is thermal dehalogenation. Although a number of studies have been done on the primary processes of irradiated zeolites adsorbing organic compounds (26), catalytic effects of zeolites on the radiolytic decomposition of organic compounds have not yet been * Corresponding author phone: +81-6-6879-8501; fax: +81-66876-3287; e-mail:
[email protected] 2122
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 35, NO. 10, 2001
investigated. The result of the previous study suggests that the combination of the radiolytic initiation and the catalytic action of zeolites is available for the reductive dechlorination of harmful organic chlorides. The study on the radiolytic and thermal dehalogenation on the molecular sieve was extended to organic chlorides, such as chlorobenzene (PhCl), TCE, tetrachloroethylene (PCE), 1- and 2-chlorobutanes, chloroform, carbon tetrachloride, and 1,1,1- and 1,1,2-trichloroethanes (1,1,1- and 1,1,2-TCAs). In general, the decomposition of organic chlorides on zeolites has been studied using flow reactors at elevated temperatures by analyzing the products in the effluent gases (2-9), but it has rarely been studied at room temperature, probably because of the difficulty of the desorption of the decomposition products to be analyzed. In the present study the reductive dechlorination of the organic chlorides adsorbed on the molecular sieve was investigated in a sealed ampule. Soaking the samples in water has enabled the determination of the yields of the reductive dechlorination at room temperature by analyzing the Cl- ions, eluted in water together with Na+, the charge-balancing cations of the negatively charged aluminosilicate framework of the molecular sieve. It was found that the chlorinated alkanes adsorbed on the molecular sieve are readily dechlorinated on standing at room temperature. The pH of the supernatant aqueous solutions is correlated to the amount of the released Cl- ions. A comparison is made of the dechlorination reactivity of the organic chlorides in the radiolytic and thermal dechlorination. The effect of 2-propanol adsorbed on the molecular sieve was tested for the radiolytic dechlorination of PhCl, TCE, and PCE. No appreciable acceleration of the dechlorination was observed in the presence of 2-propanol in contrast to the case of CFC-113 (24, 25). According to the literature, CFCs predominantly decompose to carbon dioxide in flow reactors in the presence of zeolites involving molecular sieve 13X (1, 6-9). On the other hand, it has been reported that chlorinated alkanes decompose on zeolites through dehydrochlorination, resulting in olefinic products (2-5). Thus, the reaction mechanisms for CFCs and chlorinated alkanes are different. Although the dehydrochlorination is considered to be important for the organic chlorides in the present systems, there is a contribution of other dechlorination processes especially for PCE and carbon tetrachloride having no hydrogen. Unfortunately, the organic products could not be analyzed, because the adsorbates, reactants and products, were not removed from the molecular sieve by evacuation under the moderate conditions, where no further reactions of the primary products occur.
Experimental Section The experimental procedures were almost the same as those in the previous study (24, 25). Sample preparation was carried out by using a vacuum line (
