Myun-Joo Lee, Jinho Jung, Jeong-Hyo Yoon and Hung-Ho Chung
Radioisotope & Radiation Application Team, Korea Atomic Energy
Research Institute 150 Dukjin-dong, Yusong-gu, Taejon 305-353, Korea
Address for correspondence: Dr. Myun-Joo Lee
Radioisotope & Radiation Application Team, Korea Atomic Energy Research Institute
150 Dukjin-dong, Yusong-gu, Taejon 305-353, Korea
Phone: 82-42-868-2333 Fax: 82-42-868-2292
E-mail: mjlee@kaeri.re.kr
Abstract: Radiation treatment showed efficient removal of TCE
and PCE. Almost all TCE and PCE were removed at an irradiation dose of 300 Gy in
the presence of O3 and TiO2, where TiO2 showed
explicit enhancement of decomposition. The catalytic activity of TiO2
was characterized by EPR spectroscopy and spin-trapping method. The EPR signal
of TiO2 was increased up to 15 % after gamma-irradiation, and this
results in the increase of hydroxyl radical production by 5 %. Except for Cu2+,
TCE and PCE decomposition in the presence of metal ions was lineally dependent
on the standard reduction potential of the metal ions. The extraordinary
inhibition of Cu2+ was caused by the action of Cu2+ as a
strong hydroxyl radical scavenger that was directly confirmed by EPR
spectroscopy.
Keywords: EPR, gamma-ray, ozone, PCE, TCE, titanium oxide
The pollution of groundwater with chlorinated ethylenes is becoming a serious problem in industrialized areas of Korea. As a result, the government limited TCE (trichloroethylene) and PCE (perchloroethylene) concentrations in groundwater to less than 0.03 and 0.01 mg/L, respectively, since 1993. However, the contamination of groundwater with these pollutants was not decreased due to increasing industrialization and poor groundwater conservation. Thus, many techniques have been proposed for the reclamation of groundwater. Among them, adsorption onto activated carbon and air-stripping are found to be efficient and economic, however, they just remove the contaminants but do not destroy them (Proksch et al., 1987). As noted by Gehringer et al. (1994a), this results in a mere displacement of the problem from groundwater to the carbon and atmosphere.
An attractive solution of TCE and PCE pollution is radiation-induced decomposition. The radiation treatment of the pollutants has been thoroughly studied by Gehringer et al. (1988a, 1988b, 1994a, 1994b, 1994c). They showed that the organic pollutants were completely decomposed by gamma-rays or electron-beams, and the decomposition was more efficient in the presence of ozone (O3). However, there are few reports that investigate the effect of catalysts such as titanium oxide (TiO2) and the effect of dissolved metal ions on the decomposition of TCE and PCE. Moreover, their catalytic or inhibitory activities in radiation treatment were not yet characterized. Electron paramagnetic resonance (EPR) spectroscopy and spin-trapping method were used to characterize the gamma-irradiated TiO2 and to determine the efficiency of hydroxyl radical production in this work. EPR spectroscopy is a powerful tool to characterize paramagnetic metal ions and defects in oxide catalysts (Bensimon et al., 1999; Jung et al., in press), and the combination of EPR and spin-trapping method is a sensitive and selective technique to detect unstable radicals such as hydroxyl radicals (Han et al., 1998).
Irradiation was performed at room temperature (around 20 OC) in a 60Co source (Paranomic, UK). The radioactivity of the source is around 270 Ci. Irradiation samples were prepared in 120 ml glass bottles with TCE and PCE stock solutions (about 100 mg/L, Aldrich) in distilled and deionized water (g-rays alone) or in ozone-saturated water (around 3 mg O3/L, g-rays/O3). For irradiation combined with O3 and TiO2 (g-rays/O3/TiO2), four TiO2-coated glass tubes were immersed into the g-rays/O3 samples. The TiO2-coated glass tubes were made using a Sol-Gel method described in detail elsewhere (Jeong et al., in press) and each glass tube contains around 2 mg of TiO2 (anatase, Aldrich). The effect of metal ions was investigated by adding metal nitrate solutions into g-rays alone samples. The irradiation bottles were filled up to zero-headspace in order to minimize air contamination. TCE and PCE contents after gamma-irradiation were measured by an HP 5890 II gas chromatograph equipped with an electron capture detector. The column was a 30-m HP-5 (cross-linked 5 % phenyl methyl siloxane) from Hewlett Packard.
The change of titanium oxide by gamma-rays was determined using EPR spectroscopy in the X-band on a Bruker EMX spectrometer at 77 K. For the measurement of hydroxyl radicals, a nitron spin-trapping reagent, DMPO (5,5-dimethyl-pyrroline-N-oxide, Aldrich) was used. TiO2 and Cu2+ solutions were mixed with DMPO in phosphate buffer (0.1 M, pH 7.4) just before gamma-irradiation. Immediately after irradiation, around 25 ml of the sample solution was transferred into a capillary tube, and EPR spectra were recoded in the X-band on a Bruker EMX spectrometer at room temperature.
RADIATION TREATMENT. The removals of TCE and PCE as a function of irradiation dose are given in Figure 1. At a dose of 300 Gy, more than 99 % of TCE was removed. There was no significant difference between g-rays alone, g-rays/O3 and g-rays/O3/TiO2, but the combined g-rays/O3/TiO2 process increased the decomposition of TCE. The effect of O3 and TiO2 on decomposition was more evident at low irradiation doses. Similar to TCE decomposition, more than 99 % of PCE was removed at a dose of 300 Gy. However, contrary to TCE decomposition, there is a significant difference among g-rays alone, g-rays/O3 and g-rays/O3/TiO2. The combined process of g-rays/O3/TiO2 showed the most efficient PCE decomposition where TiO2 explicitly enhanced decomposition.
The effect of dissolved metal ions such as Cr3+, Mn2+, Fe3+, Co2+, Ni2+, Cu2+, and Zn2+ on TCE and PCE removals is given in Figure 2. The efficiency of metal ions in TCE removal was in the order of Fe3+ > Ni2+ > Cr3+ > g-rays alone > Co2+ > Zn2+ > Cu2+ > Mn2+. Similar to TCE removal, the efficiency of metal ions in PCE removal was in the order of Fe3+> Ni2+ > Co2+ > g-rays alone > Zn2+ > Cr3+ > Mn2+ > Cu2+. Except for Cu2+, TCE and PCE removals were largely dependent on the standard reduction potentials of metal ions (data not shown). This can be explained by the fact that metal ions usually act as electron scavengers (Chaychian et al., 1998). The extraordinary inhibition of Cu2+ may be caused by the action of Cu2+ as a strong hydroxyl radical scavenger. The above result indicates that the composition of metal ions in groundwater has an important role in radiation treatment, and thus this should be considered when the treatment is applied to real system.
EPR STUDY. The enhancement of TCE and PCE decomposition in the presence of TiO2 may be caused by the formation of electron/hole pairs and defects (activated centers) in TiO2 by gamma-rays (Krapfenbauer and Getoff, 1999). It was reported that TiO2 catalyzes the gamma-ray destruction of EDTA and the catalytic efficiency increases upon each reapplication (Su et al., 1998; Krapfenbauer et al., 1999). EPR spectroscopy was used to characterize the change of TiO2 by gamma-rays. Figure 3 shows the EPR spectra of TiO2 powder recorded at 77 K. Four peaks in the spectra increased up to 15 % after irradiation. According to Lu et al. (1994), the peaks with g║ = 1.951 and g⊥ = 1.972 can be assigned as the axial defect (Ti3+), g = 2.005 as an electron trapped at an oxygen vacancy (F-center) and g = 1.991 as Ti3+ in TiO2 formed after degassing at 200 OC or reduction with 50 Torr of H2 at the same temperature. These four peaks may be deeply related to the enhancement of pollutant decomposition.
Hydoxyl radicals were measured by EPR/spin-trapping method since the radicals have a dominant role in the decomposition of TCE and PCE. Figure 4 shows the typical EPR spectra of DMPO-OH adduct that was composed of quartet lines having peak height ratio of 1:2:2:1 (Utumi et al., 1998). Gamma-rays increased the DMPO-OH signal around 5 %. This directly indicates that TiO2 enhanced the production of hydroxyl radicals in water radiolysis, and thus increased the decomposition of TCE and PCE. In the presence of Cu2+ ions, the DMPO-OH signal was reduced by 90 %. This strongly confirms the action of Cu2+ ions as a hydroxyl radical scavenger.
The radiation treatment of TCE and PCE in the presence of O3 and TiO2 removed the pollutants near 100 %, where TiO2 catalyst showed explicit enhancement of decomposition. The catalytic activity of TiO2 was characterized by EPR spectroscopy. The four peaks of the EPR spectra that were increased by gamma-rays may be assigned as activated centers. The increase of these centers enhanced the production of hydroxyl radicals and thus increased the decomposition of TCE and PCE. Though the relationship between the change of TiO2 and hydroxyl radical production was clearly explained in this work, further investigation is needed to identify the catalytic mechanism of TiO2. The effect of dissolved metal ions on TCE and PCE decomposition was explained in a relatively simple system. In order to apply radiation treatment to real system, the interaction between various metal ions should be clarified. Considering the possible inhibition of dissolved metal ions in radiation treatment, more efficient catalysts should be developed.
Acknowledgements
This study was supported by the Nuclear R & D program of MOST, Korea.
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FIGURES
Fig.1 TCE and PCE decomposition as a function of irradiation dose. The concentration of TCE and PCE was about 10 mg/L. Legend: g-rays alone; p g-rays/O3; ¢ g-rays/O3/TiO2.
Fig.2 The effect of dissolved metal ions on TCE and PCE decomposition. The concentration of TCE and PCE was about 20 mg/L. The concentration of metal ions was 0.18 mM and irradiation dose applied was 100 Gy. Legend: No (black bar) is g-rays alone.
Fig.3 The EPR spectra of TiO2 powder at 77 K before and after gamma-irradiation. Irradiation dose applied was 70 kGy.
Fig.4 EPR spectra of DMPO-OH adduct at room temperature. The conditions are (a) gamma-irradiation alone, (b) Cu2+ ions added and (c) TiO2 powder added. The concentrations of DMPO, Cu2+ and TiO2 were 0.1 M, 10 mg/L and 4 g/L, respectively. Irradiation dose applied was 100 Gy.