G Model
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ARTICLE IN PRESS
S.J. Ki et al. / Catalysis Today xxx (2016) xxx–xxx
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dimensions of 470 × 550 × 235 in mm) in which the photocatalytic
reaction took place. Fig. 1e and f also show two snapshots of exper-
imental configuration inside the microwave cavity when the MDEL
(see bottom image in Fig. 1a) installed outside of the photocat-
alytic reactor (see top image in Fig. 1a) was idle and activated by
the microwave radiation, respectively. Note that the MDEL is used
instead of conventional UV lamps to provide an UV radiation to
the reactor. This is because traditional lamps have metal electrodes
that cannot emit the UV light inside the cavity. The MDEL which was
made of quartz had a double tube structure with a vacuum between
the two sides of the glass, where an extremely small amount of mer-
cury was contained. Specifically, the MDEL with an inside diameter
of 36 mm, an outside diameter of 55 mm, and a length of 170 m was
used in the degradation experiments.
Fig. 2. The absorption spectra of the reactant solution monitored at selected time
intervals (0, 10, 20, 40, 70, 90, and 100 min), obtained using ultraviolet-visible spec-
trophotometer.
2.4. Experimental procedure
Below is a detailed description of the experimental procedure
employed for the photocatalytic degradation of 4-chlorophenol.
A 0.15 mM aqueous solution of 4-chlorophenol was prepared by
dissolving 4-chlorophenol in 500 mL mL of distilled water. The reac-
tant aqueous solution was then transferred to a stainless steel
beaker in a constant temperature water bath and passed through
the photocatalytic reactor at flow rates of 200–600 mL/min using a
roller pump. Note that the water bath which keeps the solution at
298 K is used to eliminate the thermal effects of the microwave
radiation on the degradation of 4-chlorophenol. Before starting
the designed experiments, the UV light which was emitted from
the MDEL was provided to the reactor in 15 s and the reactant
aqueous solution was also circulated for 10 min to maintain a con-
stant solute concentration in the reactor. During the experiments,
the microwave power delivered to the cavity varied from 0.2 to
0.6 kW, which was evenly distributed throughout the reactor using
a stirrer on the rear side of the cavity. Samples were taken from
the stainless steel beaker at various time intervals to estimate the
decomposition rate of 4-chlorophenol under different experimen-
tal conditions. The concentrations of 4-chlorophenol in the samples
were quantified by UV–vis (UV–vis) spectrophotometer (UV-1801,
Shimadzu Co. Ltd.) with the absorption maximum at 225 nm. Gas
chromatography mass spectrometry equipped with a headspace
auto sampler (GC/MS, QP2000, Shimadzu Co. Ltd.) was also used
to identify a degradation mechanism of a parent compound (i.e.,
4-chlorophenol) and its intermediates produced during a series of
photocatalytic experiments. The capillary column was a HP-5 MS
with dimensions of 30 m (in length) × 0.25 mm (in internal diam-
eter) × 0.25 m (in film thickness), which was operated initially at
323 K for 30 s and then maintained at 553 K for 5 min after increas-
ing its temperature at a rate of 10 K/min.
about 280 nm for exposure times of 10 and 20 min. The intermedi-
ates generated during the photocatalytic process probably caused a
temporary increase of absorbance at around 280 nm. Nonetheless,
3.2. Effect of microwave intensity
Fig. 3a shows the variation in the decomposition rate of 4-
chlorophenol as a function of the microwave intensity, from 0.2 to
0.6 kW. As shown in the figure, the pseudo first-order kinetic model
was shown to successfully describe the experimental data on the
photocatalytic degradation of 4-chlorophenol obtained under dif-
ferent conditions. The first-order kinetic model can be expressed
as:
C/C0 = exp(-kt)(1)
where C0 and C are the concentrations of 4-chlorophenol at t = 0 and
time t (in units of M/L3), respectively. k indicates the overall decom-
position rate constant (in unit of 1/T). Note that the non-thermal
effects produced from the microwave radiation are only involved
in this photocatalytic process as we constantly maintain the reac-
tant solution at a particular temperature using the water bath, as
discussed earlier. It was shown from the figure that the degrada-
tion rate of 4-chlorophenol increased significantly with increasing
the microwave intensity from 0.2 to 0.5 kW. Beyond this limit, the
degradation of 4-chlorophenol was, however, accelerated slightly
by the microwave intensity. In fact, the MDEL was found to emit
three types of UV radiation, namely UV-A, UV-B, and UV-C. Accord-
ing to our previous study [27], the emission level of UV-C which
was remarkably higher than those of UV-A and UV-B was increased
sharply up to 0.4 kW and then very slowly after that. It should be
noted that the higher the intensity of the UV radiation, the more
reactive radical species (e.g., hydroxyl radical) are available for the
photocatalytic reaction, and vice versa. Accordingly, we observed
with the microwave intensity kept above 0.4 kW a slow increase in
3.1. Photocatalytic degradation of 4-chlorophenol
Fig. 2 illustrates the change of the absorption spectrum in differ-
ent times (from 0 to 100 min) when the reactant solution circulated
lowing operating conditions: the microwave intensity of 0.4 kW
and the circulating fluid velocity of 400 mL/min. In fact, the reac-
tant aqueous solution of 4-chlorophenol was found to have strong
absorbance at 225 and 280 nm [23,24]. From the figure, it was
determined that the absorbance at wavelengths of around 225 and
280 nm decreased considerably, as the reaction time increased.
However, a slight increase in absorbance was also observed at
3.3. Effect of pH
Fig. 3b describes the effect of pH levels (between 2 and 10)
on the decomposition rate of 4-chlorophenol. The same operat-
ing conditions, as described in Section 3.1, were applied to these
experiments, but the degradation of the parent compound was
determined for pH levels of 2, 4, 7, and 10. As can be seen in
the figure, as pH values extends from 2 to 10, the degradation
rate of 4-chlorophenol declines quite rapidly. The decomposition
Please cite this article in press as: S.J. Ki, et al., Improving removal of 4-chlorophenol using a TiO2 photocatalytic system with microwave