자외선 또는 태양광에 조사된 TiO2 필름상에서 광촉매 반응에 의한 다이옥신의 분해

최원용, 홍수진

포항공과대학교 환경공학부

Photocatalytic Degradation of Polychlorinated Dibenzo-p-Dioxins on TiO2 Film under UV or Solar Light Irradiation

Wonyong Choi, Soo Jin Hong

School of Environmental Engineering, Pohang University of Science and Technology

Introduction

Many physicochemical processes have been studied in order to degrade PCDDs.  The present paper reports the study of photocatalytic PCDDs degradation on TiO2 film under UV or solar irradiation.  A few investigations on photocatalytic degradation of dioxins (e.g., 2-MCDD, 2,7-DCDD) on TiO2 have been performed in aqueous solution [1], organic solvents [2], and soils [3].  However, the degradation of highly chlorinated dioxin congeners such as OCDD and HpCDD has not been reported.  The photocatalytic degradation mechanism of PCDDs is not well understood.  In this work, investigations into the photocatalytic degradation of PCDDs as a function of various experimental parameters, their structure-reactivity relation, and the mechanistic pathways of their degradation are discussed.

Experimental

PCDD samples were loaded on TiO2 film, which was coated on a glass plate, and subsequently exposed to UV light in the ambient air.  Coating the glass plate (18mm x 18mm) was performed by dipping it into a 5 wt% TiO2 suspension, drying at room temperature and then, heating at 400 ℃ for 30 min.  The coating procedure was repeated five times.  A typical coating mass of TiO2 obtained by this method was ~600 □g/cm2.  An aliquot of 200 □L of the stock solution was dropped on the TiO2-coated glass plate, then the solvent was evaporated to get a dioxin concentration of ~6.2 □g/cm2.

The light was irradiated from a 200 W mercury lamp through a Pyrex filter cover (□ > 300 nm) to the samples (PCDD adsorbed on TiO2 film).  The average light intensity in the range of 300 < □ < 400 nm was about 3.1 mW/cm2.  All experiments were carried out at ambient temperature and pressure.  Dark control samples were carried along with the photolysis samples through the entire experimental procedure in order to check any loss from thermal degradation or evaporation.  No such effects were detected.

Samples for solar photolysis experiments were prepared by the same procedure.  The samples were placed on a solar panel (25 cm x 25 cm), which was adjusted to face the sun every 30 min.  Solar photolysis experiments were carried out on the roof of the environmental engineering building at POSTECH (located in the city of Pohang, 36 oN latitude) in the mid-September days between 11 AM and 3 PM under clear or partly cloudy sky conditions.  The intensities of the active portion of the solar light (300 < □ < 400 nm) that excite the photocatalyst varied between 1.8 and 2.5 mW/cm2 with an average of 2.1 mW/cm2.

After irradiation, the residual parent PCDDs and any stable intermediates on the TiO2 film were extracted with a soxhlet extractor in toluene for 12 hours and concentrated to 800 □L.  A blank run under the same conditions ensured that 95□5% could be recovered through this procedure.  Extracted PCDDs and intermediates were analyzed by using a Hewlett-Packard (HP6890A) gas chromatograph equipped with a 63Ni-electron capture detector (ECD) and a HP-5 column.  Nitrogen was used as a carrier gas.

Results and Discussion

Figure 1 compares the photocatalytic and the direct photolytic degradation of OCDD under a 200W mercury lamp irradiation.  While the light with □ > 300 nm (Pyrex filtered) did not induce any direct photolytic degradation, a noticeable photolytic degradation was observed under naked UV lamp irradiation.  The absorption peak of OCDD around 250 nm, which is coincident with a major mercury lamp emission at 254 nm, is responsible for the direct photolysis.  On the other hand, the photocatalytic degradation on TiO2 under □ > 300 nm illumination was greatly enhanced, and resulted in 85% conversion within 15 hour.  It should be also noticed that an extremely high concentration of OCDD in TiO2 (closed to 10000 ppm by weight) was employed in this study.  Considering that a typical concentration of PCDDs found in environmental samples is in the order of sub-ppb, the remedial action of the dioxin-contaminated TiO2 is phenomenal.

Similar photolytic and photocatalytic behavior were observed for HpCDD, TCDD, and MCDD: no direct photolytic degradation and a greatly enhanced photocatalytic degradation on TiO2 under □ > 300 nm irradiation for all of them.  Their photocatalytic degradation rates are compared in Figure 2, which shows the first-order kinetics for each PCDD congeners with their rate constants of 1.67, 0.46, 0.15, 0.10 hr-1 for MCDD, TCDD, HpCDD, and OCDD, respectively.  The photocatalytic degradation rate exponentially decreases with increasing the number of Cl atoms on PCDD.  The slower degradation rates of the highly chlorinated PCDDs are ascribed to the lack of unsubstituted carbons that are vulnerable to an OH radical attack.  The degradation rates increased with the light intensity and the TiO2 coating weights up to 2 mW/cm2 (300 < □ < 400 nm), and 200 □g TiO2/cm2, respectively.  Further increase in either the light intensity or the TiO2 coating weight resulted in little increase of the degradation rate.  The natural solar light under a clear sky condition was as effective as a 200 W mercury lamp irradiation (filtered through a Pyrex filter) in degrading OCDD on TiO2.

When TiO2 is illuminated by photons with energy larger than 3.2 eV (388 nm), electron-hole pairs are generated within the lattice.  A fraction of charge carriers that survive the fast recombination process diffuse to the TiO2 surface, and transfer to the adsorbed species with initiating redox reactions.  The degradation pathways of organic compounds on illuminated TiO2 surface involve a series of electron or hole transfer reactions.

Figure 1. Direct photolytic and photocatalytic degradation of OCDD as a function of irradiation time under a 200W mercury lamp.

Figure 2. Comparison of the photocatalytic degradation of four PCDD congeners as a function of irradiation time under a 200W mercury lamp.

Among 75 congeners of PCDDs, OCDD is the most recalcitrant for degradation since there are no C-H bonds that are vulnerable to OH radical attacks.  The degradation of OCDD might be therefore initiated not only by an OH radical attack but also by a CB electron transfer:

C12O2Cl8 + e-cb   □   [C12O2Cl7]□ + Cl-

A cyclic voltammetry was performed in order to determine the reduction potential of OCDD.  The cyclic voltammogram of OCDD showed its reduction potential to be -1.42 V vs SEC.  Compared with the TiO2 CB edge potential (-0.74 V vs SCE at pH7), the reduction of OCDD by a CB electron is not thermodynamically feasible.  Therefore, the photocatalytic degradation of all PCDD congeners should be initiated by an OH radical attack on the aromatic ring structure.  The addition of OH radicals to the aromatic ring of polychlorinated dioxins takes place preferentially at unsubstituted carbons [4]. The trend is clearly demonstrated in the photocatalytic degradation of polychlorinated dioxin whose degradation rates are in the order of MCDD > TCDD > HpCDD > OCDD.

From a diffuse reflectance FTIR study, the degradation of OCDD was found to involve the cleavage of the aromatic ring of OCDD with the accompanying appearance of the aliphatic -C=C- bond.  No stable intermediates or no less chlorinated PCDDs were detected during the photocatalytic degradation.  Since PCDDs and their degradation products reside on TiO2 surface in the air throughout the photoirradiation, stable degradation intermediates, if any, seemed to be volatilized into the air.

References

(1)	E. Pelizzetti; M. Borgarello; C. Minero; E. Pramauro; E. Borgarello; N. Serpone, Chemosphere 1988, 17, 499-510.

(2)	E. Pelizzetti; V. Carlin; C. Minero; M. Gr□tzel, New J. Chem. 1991, 15, 351-359.

(3)	E. Pelizzetti; C. Minero; V. Carlin; E. Borgarello, Chemosphere 1992, 25, 343-351.

(4)	Olimmer, K. R.; Klingbiel, U. I.; Crosby, D. G.; Wong, A.S., Adv. Chem. Ser. 1973, 120, 44.