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1、Journal of Hazardous Materials 252 253 (2013) 220 226Contents lists available at SciVerse ScienceDirectJournal of Hazardous Materialsjou rnal h om epa ge: kinetics, products and mechanism of timolol under simulatedsunlightYong Chena, Qi Liangb, Danna Zhoub, Zongping Wanga, Tao Taoa, Yuegang ZuocaSc
2、hoolof Environmental Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China bCollegeof Material Science and Chemical Engineering, China University of Geosciences, Wuhan 430074, China cDepartmentof Chemistry and Biochemistry, University of Massachusetts Dartmouth,
3、 285 Old Westport Road, North Dartmouth, MA 02747, USAh i g h l i g h t s?Theindirect degradation of timolol is first investigated in fulvic acid solution. ?3FA*and1O2accountedfor the degradation of timolol in the aerated FA solutions. ?Thepresence of halides inhibited the degradation in the order o
4、f Cl 400 nm, Shanghai Seagull Colored Optical Glass Co., Ltd) to avoid the direct photodegradation. Fig. 3b illustrates the different1O2reaction rates at various pH. The bimolecular rate constants ranged from (0.65 0.02) to (6.2 0.1) 107M1s1 at pH 6.0 10.0 (Table A1). The results indicated that timo
5、lol markedly reacted with1O2, as has been documented by Criado et al. 2628. The1O2reaction was more quickly in the slightly acid or alkaline solutions and increased in the order of pH 6.0 10.0 9.0 7.0 8.0. Timololis a weak base with a pKavalue of 9.2 due to the dissociation of the amine functional g
6、roup 40. Besides, it was reported that an intramolecular hydrogen bond is formed between the terminal amino group and the ?-hydroxyl group on the alkanolamine side- chain 44. Thus, the different1O2reaction rates were likely related to both the speciation of timolol and intramolecular hydrogen bond.
7、The detailed mechanisms still need further investigation.3.3.Reaction mechanismA series of quenching experiments were carried out to inves- tigate the photosensitized degradation mechanism of timolol in the FA solutions. Table 1 illustrates the quenching experiments of photodegradation of timolol in
8、 the presence of dissolved oxy- gen,OHinhibitor isopropanol, and1O2quencher NaN3or DABCO 39. The degradation drastically increased by approximately 7-fold after removing the dissolved oxygen (Table 1). This was consistent with the previous studies for the photosensitized degradation of the other ?-b
9、lockers 16,17. It is well known that oxygen is the quencher of most excited triplet states of organic compounds 29. Therefore, the enhancement effect suggested that the photodegra- dation of timolol was associated with3FA*.The addition ofOHinhibitor isopropanol showed little effect on the photodegra
10、dation in the aerated and deoxygenated FA solu- tions, indicating thatOHdid not contribute to the degradation of timolol (Table 1). Although the bimolecular rate constant for the reaction of timolol withOHwas (5.3 0.1) 109M1s1, the OHwas almost nonselective and thus it was readily quenched by FA 45.
11、 Thus, compared to the other reactive species (e.g.3FA*),the role ofOHin the photodegradation was negligible. In con- trast, the pseudo-first-order rate constants for photodegradation of timolol in aerated FA solutions in the absence and presence of 1O2quencher NaN3or DABCO were 0.15 0.002, 0.070 0.
12、003, and 0.072 0.005 h1, respectively. This suggested that approxi- mately half of the photodegradation of timolol was attributed to the reaction with1O2,which was different from the photosensi- tized degradation of atenolol, metoprolol, and propranolol 16,17. Itallows one to speculate that approxim
13、ately 90% of3FA*was ineffectively quenched by dissolved oxygen, considering that the removal of dissolved oxygen accelerated the degradation of timolol by approximately 7-fold. Althoughthe1O2reaction rate constant of timolol was (1.7 0.1) 107M1s1at pH 9.0 (Table A1), lower than that ofY. Chen et al.
14、 / Journal of Hazardous Materials 252 253 (2013) 220 226223Fig. 4. Photodegradation rate constants of timolol (5.0 ?M) in deoxygenated and aeratedFA solutions (16.0 mg/L) in the presence of halide ions (100 ?M) at pH 9.0.OH,the contribution of1O2to the degradation of timolol was appreciable in the a
15、erated FA solutions. This was ascribed to the selectivity of1O2to substrates. It was suggested that timolol is attacked by1O2on the terbutyl methyl group 26. According to the quenching experiments, both3FA*and1O2were the main reactive species responsible for the photodegradation of timolol in the ae
16、rated FA solutions. There was no production of1O2in the deoxygenated FA solutions andOHalso did not contribute to the photodegradation of timolol (Table 1). Therefore, the degradation of timolol was exclusively attributed to3FA*in the deoxygenated solutions. In our previous studies, the electron transfer from the non- bonding electrons on nitrogen (N-electrons) of the amine drugs
