179 In order to clarify the byproducts of the photochemical oxidation of phenol, Figure 60 shows the FTIR spectrum of phenol irradiated at different periods at the same temperatures for 2 h only under UV light irradiation. The FTIR experiment was conducted only to verify the complete mineralization of phenol during the photocatalytic degradation process. In the Figure 60 the evolution of 2 bands, and CO2 was identified during each photodegradation period. The presence of two bands at 2364 and 2324 cm
-1; specifies the mineralization peaks at different stages of the photocatalytic reactions.
These results are clearly in accordance with other references showed earlier [435-437].
The presence of the two bands obviously specifies the availability of CO2 in the early stage of phenol photodegradation.
4.8
Degradation mechanism of phenol involving the O-H bond (Formation mechanism)In order to better understand the degradation mechanism, the major degradation byproducts were studied independently upon photoirradiation.
Figure 61: Possible formation of phenoxide ion in water from phenol
The reaction begins with the O-H group attached the aromatic (benzene) ring bonds breaking as it was explained earlier, giving up a proton or releasing a proton, thus forming an anion stabilized by the aromatic ring (phenoxide ion), as shown in Figure 61.
Figure 62: Possible degradation mechanism of phenol and resonance stability of phenoxide, and the formation of benzoquinone intermediate
Activation of the phenol by the generation of •OH will produce phenoxy radical which is in resonance with radical structures in the ortho and para positions as it shown in Figure 61 [438].
The three mesomeric forms of the radical are the initial formation of the different intermediates. These radicals can react with the •OH to form compounds such as HQ, BQ and Cat as it was explained earlier.
From the previous analysis it is shown that the photodegradation of phenol involves two stages, the first is the formation of the intermediate products, while the second is completion of the intermediates and the mineralization to CO2 and H2O. Results also confirmed the presence of Cat, BQ, Res and HQ with the continuous degradation peak of phenol. The presence of these intermediates decreased slightly over time.
Benzene ring is a stable structure, but the bond on the benzene ring can be potentially weakened when the •OH attacks the ring, resulting to the accumulation of one or more O-H group at the ortho or para position as it explained earlier.
Continuous photodegradation of phenol lead to the ring opening and the decomposing of phenolic structures to show up of diverse organic compounds. This reaction accounts to the continuous decrease in the values (measured by different instrumentations such as UV-Vis, HPLC, GC, CE, TOC and COD after only two h of photodegradation) and
4.8 Degradation mechanism of phenol involving the O-H bond (Formation mechanism)
181 the appearance of different intermediates until finally (AA) will form and mineralization of phenol.
Figure 63: Different chemical photoproducts produced from phenol photodegradation before its mineralization by CO2 and H2O build up from results given by UV-Vis, HPLC, CE, GC and FTIR with the reports from the literature [190, 196]
Figure 63 shows the current experimental conditions following the observations made from HPLC, CE and other studies and with the reports in the literature [190, 196]. With the continuous effects of UV, solar or visible light irradiation hydrocarbon chains were mineralized completely to CO2 and H2O. The current experimental conditions are proposed in Figure 63, where the photocatalytic degradation of phenol involves the generation of intermediate compounds and a mineralization stage.
4.8.1 Hydroquinone (HQ)
During phenol degradation HQ appeared after 30 min, reaching its maximum after 45 min of photoirradiation at a rate of 0.0025 ppm/min, which was detected by UV-Vis spectrophotometry, in CE, and in HPLC. At 60 min of phenol photodegradation, HQ concentrations decreased sharply and further slow reduction was observed [439], followed by degradation to different short chain organic acids until all phenol was removed from the solution [440]. The concentration of HQ product obtained was moderate, but it is more toxic than phenol itself even at this low level [441]. The presence of HQ can lead to reasonably high value of ecotoxicity, consequently it is very important to follow the evolution of HQ in phenol photodegradation, until almost complete vanishing of the byproduct can be recorded [442]. It is shown from Figure 1 and 55 that HQ is a para directed product but it produced less than Res. HQ formation can be done from substitution at the position (C4) at the para carbon atom. Possibility for higher concentrations of ortho and para directing in aromatic electrophile substitution reactions could be assumed, but the results shown that concentrations of HQ were less produced than of Cat which is ortho directed and Res which is (meta) directed substitutions. The reason for this may be due to the nonselectivity nature of attack of the •OH in the aqueous media. This effect is much stronger than the ortho and para directing effect. Also possibility of ortho and meta positions present two new attack phenomena’s as it been explained above.
4.8.2 Benzoquinone (BQ)
BQ was traced for the first 30 min of photodegradation and it was found to be lower than HQ. The formation rate of BQ was 0.0107 ppm/min, and the degradation rates were calculated from the reduction of the BQ peak at 0.00357 ppm/min. BQ showed maximum concentrations at 40 min and remained the same up to 60 min, after which its concentrations decreased gradually due to the photocatalytic degradation. The very low concentrations of BQ were detected by UV-Vis spectrophotometry, GC, and also by CE.
4.8 Degradation mechanism of phenol involving the O-H bond (Formation mechanism)
183 4.8.3 Catechol (Cat)
Higher concentrations of Cat were produced than any other byproduct, which may be related to the ortho directed properties group of phenol. Cat was formed within 15 min of phenol degradation; its concentration increased to a maximum of 0.7 ppm, and kept increasing until it reached 3 ppm after 45 min at a rate of 0.1249 ppm/min. After 75 min, the concentration of Cat decreased at a rate of 0.0292 ppm/min. Figure 45 shows that Cat was produced during all phenol degradation process. Cat possibility formation comes from substitution at the positions (C4) and (C6) at the carbon atom.
4.8.4 Resorcinol (Res)
Res appeared from the beginning, and its concentration increased from 0.3 ppm until it reached 1.5 ppm and became stable at that point, after 75 min at a rate of 0.0075 ppm /min. The almost stable concentration of Res production in the mixture analysis suggested that Cat would be produced by the same rate until all phenol could be removed from the solution. It was also noticed that Res concentrations were higher than HQ ones.
It seems that the substitution rules of •OH radicals attacking the phenol molecule with higher probability in positions 1 and 4 (ortho and para directing effect) are weaker than the non-selective nature of attack of •OH group of the aromatic ring. Res possibility formation comes from substitution at the positions (C3) and (C3) at the carbon atom.
4.8.5 Acetic acid (AA)
AA appears more intractable to photodegradation, but this has no significance in conditions of ecotoxicity elevation. Ring opening of the byproducts leads to the formation of AA, and pH will also increase (results not shown). AA appeared in the reaction medium after 45 min at a rate of 0.0015 ppm/min.
4.8.6 Isopropanol (2-P)
2-P has a neutral pH level of approximately 7 (results from our previous analysis, not shown), and it is very similar to pure water. It seems that propionic acid competes with AA, which explains why traces of this compound sometimes appear before AA and on
other occasions after AA. 2-P appeared in the solution after 75 min at a rate of 0.0813 ppm/min.