0가 금속 철을 이용한 Perchloroethylene과

Carbon Tetrachloride의 환원적 탈염소화

이 승 우, 장 덕 진

명지대학교 화학공학과

Reductive Dechlorination of Perchloroethylene and Carbon Tetrachloride

with Zero-Valent Iron

Seungwoo Lee and Deokjin Jahng

Department of Chemical Engineering, Myongji University

Introduction

Chlorinated aliphatic compounds, majority of which are chlorinated methanes, ethanes, and ethenes, are mainly used as solvents and degreasing agents in industries.  Due to a long-term usage of these compounds and their concomitant environmental contaminations, many of them are frequently detected pollutants in groundwater and soil. U. S. EPA has designated perchloroethylene (PCE), trichloroethylene (TCE), dic- hloroethylenes (DCEs), monochloroethylene (vinyl chloride: VC), carbon tetrachloride (CT), and others as top priority pollutants because of their toxicity/carcinogenecity.  Extensive efforts are being made to develope technologies to treat chlorinated aliphatics contaminated environments.

Remediation or schemes commonly involve pumping of the contaminated groundwater to the surface further to a treatment system, which either degrades the chlorinated compunds, as in the case of advanced oxidation systems, or transfers them to another medium, as in the case of air stripping and granular activated carbon adsorption.

Reduction of chlorinated methanes and ethenes in the aqueous solution by iron metal was studied in batch and column systems under anaerobic and middle aerobic conditions. All experimental evidences were found to support a direct reduction mechanism in which electron transfer from Fe0 to the adsorbed chloromethane and chloroethene occurs at the metal/water interface.

Materials and Methods

Batch test

In this tests, each 26.8 mL serum bottle (Labtech) received 1.0 g of iron (100 mesh, >99% iron, Fisher Scientific, U.S.A.), weighed dry to the nearest milligram. Oxides and other surface coatings were removed by exposing the iron sample to 10 mL of 3% HCl for 1 h and then rinsing three times with deoxygenated distilled water while pursing the open bottle with N2. Serum bottles containing acid-washed iron were then filled completely (anaerobic condition) or 10 mL (middle-aerobic condition) with deoxygenated distilled water and crimp-sealed with a Teflon-lined septum(Wheaton, U.S.A.). To initiate a dechlorination experiment, 2 mL of a saturated aqueous halocarbon stock solution was added by injection through the septum. Initial concentrations of system were 30, 50, 80, and 100 □M and then each vials were placed on a rotary shaker (200 rpm) at a room temperature (20∼25℃).

Column test

Deoxygenated distilled water containing various concentrations of PCE and CT was pumped (SMP-23, Eyela, Japan) through a pyrex glass column (20 cm long × 4 cm i.d.) containing a mixture of silica sand (60 mesh, Aldrich, U.S.A.) and iron powder (10 wt%). Four sampling ports were located at 4 cm intervals along the column, and two additional ports were located at inlet and out let. Each sampling port was constructed using a thread top cap vial(12×32 mm, National Scientific, U.S.A.) with Teflon-lined septum. Water samples were collected through these ports and analyzed for organic and inorganic parameters. A control column was identical except that no iron was added to the silica sand. Experiments were conducted using the same deoxygenated distilled water but with PCE or CT concentrations of 30, 50, 80, and 100 □M. The water containing PCE or CT was held in a 2 L collapsible Teflon bag to minimize losses due to volatilization as the source water was consumed. Source water was pumped from the collapsible bag to the column using an Eyela variable speed, peristaltic pump at a flow rate of approximately 0.5, 2.0, 4.0, 5.0 mL/min. Actual flow rates, as determined by measuring the discharge, varied from 0.4∼4.2 mL/min. Viton tubing(1.6 mm, Cole-Parmer, U.S.A.) used in the delivery systems.

Results and Discussion

Batch test

Since dehalogenation apparently occurs at the Fe/H2O interface, transport of reactants to the iron surface and chemical reaction on the surface must be involved in this dehalogenation reaction. A general model for surface reactions consists of five steps: (ⅰ)mass transport of the reactants to the Fe0 surface from the bulk solution; (ⅱ)adsorption of the reactants to the surface; (ⅲ)chemical reaction at the surface; (ⅳ)desorption of the product(s); and (ⅴ)mass transport of the product(s) to the bulk solution. Any one or a combination of these steps may be rate limiting. To properly interpret trends in the reaction rate, it is especially important to distinguish between transport- and reaction- limited kinetics. A common criterion for detecting mass transport-limited kinetics is variation in reaction rate with intensity of mixing. Rates that are controlled by a chemical reaction step should not be affected, whereas vigorous mixing usually accelerates diffusion-controlled rates by reducing the thickness of the diffusion layer at particle surfaces. Due to the uncertain form of the relationship between rate constant and rpm, regression had not been performed on data. However, the trend in Fig. 1 shows the importance of mixing as an experimental variable in batch studies of dehalogenation reaction by iron. The most striking features of the degradation of PCE (Fig. 2) was that no degradation products detected in the headspace (lower than detection limit). This suggested that at least at the spatial and temporal scales of sampling almost all of the PCE was completely dechlorinated in a single step(simultaneous transfer of six electrons) but this is a highly improbably event. It may be a result of covalent bonding included by the first electron transfer. Sivavec and Horney [5] suggested that a strong chloroethene Fe0-□ bonding may prevent desorption until dechlorination is complete. This differed considerably from our studies of carbon tetrachloride degradation in which stepwise dechlorination occurred. CT was found to be degraded by reductive dechlorination to chloroform typically accounted for about 40% of the carbon tetrachloride lost. After the carbon tetrachloride concentration had decreased to lower than 7% of intial concentration, methylene chloride was detected from further reductive dehalogenation. Methylene chloride disappearance was not observed. In these results, plots of lnC/C0 versus time for PCE and CT gave straight lines from their intial concentrations to their respective minimum detection limits. From this, it was concluded that the kinetics of these reactions are pseudo-first-order in substrate. The slope of lines regressed to natural logarithm of relative concentration versus time data were used to obtain first-order rate constants k. The results of these calculations are presented in Table 1. The disappearance rate constants for PCE and CT were typical of unbuffered experiments run at 200 rpm using 1.00 g of acid-washed Fisher electrolytic iron, under these conditions, methylene chloride was not measurably degraded.

Column test

Fig. 3 shows the results degraded and plotted as lnC/C0 versus time for CT, where time was calculated using the measured flow rates, the porosity of the reactive material, and the cross-sectional area of the column. A good fit of the linear model to the data provides a strong evidence that the reaction is pseudo-first-order with respect to CT concentration. In column test, rate constant k was much larger than batch test.    Understanding dehalogenation by iron as reduction of the halocarbon coupled with oxidative dissolution of the metal suggests several ways in which pH may influence the reaction rate. The requirement for H+ participation in the overall reaction suggests the possibility that protons may appear in one or more elementary steps that influence the reaction rate directly. Fig. 4 shows unbuffered systems with consistant rise up to  pH 7∼11. Although this pH increase did not appear to affect dehalogenation rates, further investigation required to evaluate the long-term performance in groundwater treatment, .

Acknowledgments

This work has been supported by the Korea Science Engineering Foundation (KOSEF) under the Grant 971-1105-031-2

Reference

1. Mackay, D. M., and J. A. Cherry, Environ. Sci. Technol., 23(6), 630-636(1989).

2. Matheson, L. J. and P. G. Tratnyek, Environ. Sci. Technol., 28(12), 2045-2053(1994).

3. Orth, W. S. and R. W. Gillham, Environ. Sci. Technol., 30(1), 65-71(1996).

4. Robers, A. L., L. A. Totten, W. A. Arnold, D. R. Burris, and T. J. Campbell, Environ. Sci. Technol., 30(8), 2654-2659(1996).

5. Sivavec, T. M. and Horney, D. P., 209th ACS National Meeting, 35(1), 796-799(1995)

Fig. 1 Effects of mixing on carbon tetrachloride degradation by metallic iron in batch vials.		Fig. 2 Degradation of perchloroethylene by different amounts of metallic iron in batch vials.

Fig. 3 Degradation of carbon tetrachloride and generation of chloroform in a column packed with metallic iron and sand.		Fig. 4 pH increase due to carbon tetrachloride degradation by metallic iron.

RX	[RX]max(□M)	k(min-1)
CCl4	131.37	0.2542
C2Cl4	89.03	0.0393

Table 1 Kinetic parameters of carbon tetrachloride and perchloroethylene disappearance