Photodegradation step

In this part, the effect of H2O2 addition on 5 consecutive photo-oxidation cycles after preliminary adsorption step.

a. Kinetic of photodegradation cycles of phenol

The study of the reaction kinetics was performed according to all conditions previously used in the photocatalytic oxidation experiment, without H2O2. After the step of adsorption without UV, 50 mL of H2O2 (30 %) was added to the reaction solution in the reactor and the UV lamps were switched on. The decrease in phenol concentration with the time variation during 5 photooxidation cycles is represented in Figure IV-31. It was observed that when H2O2 was added and the solution was irradiated, the phenol was totally eliminated as compared to the initial concentration used during a period of time of almost 200 min in the first and second oxidation cycles. The phenol degradation then slightly decreased to 97 % in the next oxidation cycles during 360 min. It was also observed that the rate of phenol degradation in the 1^st cycle is faster than that in the next cycles. This may be due to the fact that the initial concentration of phenol in the first cycle (0.05 g/L) was smaller than those in the next cycles and also probably due to the lower intermediates concentrations in the first cycle than that in the next oxidation cycles.

164 Fig. IV-31. Kinetics of phenol photocatalytic degradation cycles onto tissue in the presence of H2O2 at 25 °C.

When comparing with the results of photocatalytic oxidation of phenol onto tissue previously obtained without H2O2, it is very clear that phenol degradation is much faster with H2O2, It was found that the phenol concentration decreased to 0.0039 g/L during a period of time of 4700 min in the first run without H2O2, while phenol was totally photodegraded to reach a very small concentration of 0.0001 g/L within 200 min in the presence of H2O2. Therefore, the combination of the TiO2 catalyst in tissue with H2O2 leads to an important increase of the photooxidation rates. Similar findings were found with Zhu and Zou (2009b) who reported that the organic compounds were similarly removed by TiO2 coated activated carbon system in presence of H2O2 after 4 hrs and 16 hrs using TiO2/AC alone. In the present work, successive cycles have also been performed with H2O2 showing the elimination percentage of phenol to range from 99 % in cycle 1 to 97 % in the last cycle of photocatalytic oxidation when H2O2/phenol equal to 10.

As reported in the literature (Herrmann et al., 1993; Poulios et al., 1999; Hachim et al., 2001; Sauer et al., 2002, Barakat et al., 2005) the increase in concentration of ^•OH radicals by the addition of hydrogen peroxide leads to the increase in the rate of degradation.

0

165 These can easily attack the adsorbed organic molecules or those located close to the surface of the catalyst, thus finally leading to their complete mineralization.

It is known that hydrogen peroxide can enhance the reaction of phenol degradation by providing additional hydroxyl radicals as shown in equations (IV-7-a) to (IV-7-e) by trapping photogenerated electrons produced by TiO2 whatever the wavelength. This trapping would also help to suppress recombination of electron–hole pair produced at the activated TiO2

catalyst surface, which would lead to the increased rate of phenol degradation (Dionysiou et al., 2004; Yano et al., 2005).

b. Model application for photodegradation of phenol in the presence of H2O2

As mentioned above, the photocatalytic degradation process follows a first order model. The first order kinetic constant (Kap) and R² of all the oxidation cycles are reported in Table IV-9.

Table IV-9

Pseudo first-order apparent constants (Kap ) and correlation coefficients (R²) during the consecutive phenol photocatalytic oxidation runs in the presence of H2O2

Cycles No * C`0,i

(g.L^-1) Kap

(min^-1) R²

1^st run 0.054 0.057 0.99

2^nd run 0.093 0.017 0.98

3^rd run 0.159 0.011 0.98

4^th run 0.332 0.010 0.99

5^th run 0.365 0.008 0.99

* The initial phenol concentration after dark adsorption for each run, i.

200-280 nm

(IV-7-a) (IV-7-b) (IV-7-c) (IV-7-d) (IV-7-e)

166 From the results, it was found that the first order model having R² values 0.99, were fitted well in terms of the photocatalysis kinetics of phenol degradation with AC/TiO2 tissue in the presence of H2O2. The comparison of the experimental data with the theoretical values of phenol concentration calculated from the kinetic of pseudo first order confirmed that data are well described by this model for all cycles (Fig.IV-32). The results in table IV-10 show that even with H2O2, cycle 1 has a much greater Kap value (0.057 min^-1) than the next consecutive cycles (0.01 min^-1). Thus the apparent “deactivation” of the sequential process adsorption- regenerative photo-degradation is still very important.

This decrease in Kap may be again due to the increase in the concentration of phenol and its intermediates from one cycle to other. Here, this decrease in Kap during the cycles is less than that observed in the previous case using the same TiO2/AC tissue without H2O2

Fig.IV-32. Comparison between the experimental (symbols) and theoretical (lines) data during the photocatalytic degradation cycles of phenol onto tissue in the presence of H2O2 (T:

25 °C, C0; 0.88 g/L).

In addition, our study agrees with several authors (Reeves et al., 1992; Augugliaro et al., 2002; Saquib and Muneer, 2003; Stylidi et al., 2003) who report that the experimental

167 photocatalyst was reached (in previous section, Fig.IV-19a).

Therefore, the synergetic effect of H2O2 addition was tested using cycle 1 as indicator for this effect. First order kinetic model constant (Kap) is used for this calculation (Table IV-10). Moreover, this table shows also Kapfor homogeneous photocataysis using H2O2 alone.

Table IV-10

Apparent first order rate constants detected in the photodegradation of phenol by UV irradiation of tissue (TiO2/AC) and Synergetic factor (S) in the presence and in the absence of H2O2.

addition (0.46 x 10^-2 min^-1). Therefore, the presence of H2O2 creates a kinetic synergetic effect (^S) of the system with increase of the rate constant by a factor of 7.5 in the first photocatalytic run. This is similar to Terzian ^{et al}. (1991) who reported that the combination of TiO2 catalyst with H2O2 leads to an increase of the photooxidation rates. Moreover, it is clear from the table that ^Kap of homogeneous photocatalysis using H2O2 alone is lower than those obtained when TiO2 /AC is coupled with H2O2.

These results confirm that a major practical problem when using TiO2 as photocatalyst is electron-hole recombination which, in the absence of proper electron acceptors. One