Effects Of Cyclodextrins On Photodegradation Of Phenol Compounds Under UV Light: Kinetics And Mechanisms

Phenol compounds are aromatic compounds containing hydroxyl group. Today, Phenolic wastewater is one of the most serious water pollution in the world which endangers the environment widely and dramatically, and it is the main origin of the water pollution. As bisphenols may cause potential adverse effects on humans and animals, it concerns the public seriously. Consequently, the study of disposal of this type of material has arisen an issue of environmental pollution control which involves a large number of professional researches. It is accepted that photochemical transformations in environmental systems play a major role in determining the behavior and fate of organic pollutants. And direct photolysis of these pollutants is an important transformation route in the aquatic environment. However, the study of utilizing direct photolysis reaction as a way of organic wastewater treatment is not adequate, and it can only be applied in some limited fields. Cyclodextrins are cyclic oligosaccharides which have an external hydrophilic surface and an inner hydrophobic cavity; they are able to include a variety of objects such as organic molecules, inorganic ions, gas molecules and other compounds. Cyclodextrins have been widely proven as a medium for chemical control and photochemical reactions.Therefore, the photodegradation behavior and corresponding mechanisms of phenol compounds mainly bisphenol comoounds in the present of cyclodextrins were studied, Used host compounds wereβ-cyclodextrin,α-cyclodextrin andγ-cyclodextrin. Irradiation experiments were carried out in a self-made cylindrical reactor, with a 15 W UV sterilization lamp (λ=254nm).The experiments were carried out in the following areas of work.1. The effect ofβ-CD on the degradation of 17 phenol compounds under UV light was studied with the initial concentration of phenol compounds andβ-CD being both 4.0E-5mol·L-1. The result showed thatβ-CD could inhibit the photodegradation of 1,3,5-benzenetriol、o-Nitrophenol、4,4’-thiodiphenol (TDP)、4,4’-dihydroxy-benzophenone (DHBP)、2-phenylphenol, but promote the photodegradation of bis(4-hydroxyphenyl) sulfone (BPS)、4,4-bis(4-hydroxyphenyl)pentanoic acid (DPA)、Bisphenol A (BPA)、bis(4-hydroxyphenyl)methane (BPF)、4,4’-ethylidene-bisphenol (BPE), and it has essentially no effect on the photodegradation of p-chlorophenol、phenol、p-dihydroxybenzene、4-acetamidophenol、2,2’-dihydroxybiphenyl、4,4’-dihydroxy-bipheny1、2-isopropyl-5-methyl phenol. Moreover, the kenetics of photodegradation of TDP、DHBP、BPS、DPA、BPA、BPF、BPE in the presence ofα-CD、β-CD、γ-CD and in the absence of CDs were studied. The photodegradation of these bisphenol compounds both in the presence and in the absence of CDs proceeds is in accord with the pseudo-first order reaction kinetics. The photodegradation rate constant of TDP、DHBP、BPS、DPA、BPA、BPF、BPE in the absence of CDs are 0.429 min-1、0.102min-1、0.0164min-1、0.00848min-1、0.00451min-1、0.00406min-1、0.00343min-1 respectively, while the photodegradation rate constant of TDP、DHBP、BPS、DPA、BPA、BPF、BPE in the absence ofβ-CD are 0.276min-1、0.126min-1、0. 156min-1、0. 0658min-1、0.0184min-1、0.0261min-1、0.00273min-1 respectively. The photodegradation rate constant of BPS, DPA, BPA, BPF, BPE after p-CD inclusion catalysis can reach 1.2、9.5、7.8、4.1、6.4 fold increase respectively, but the photodegradation rate constant of TDP、DHBP after p-CD inclusion catalysis will be 1.6、1.3 fold decrease respectively.2. The effects of initial substrate concentration,β-CD concentration and pH value on the photodegradation of TDP, BPS, DPA, BPA, BPF, BPE and DHBP were studied. The results showed that when the amount ofβ-CD on the bisphenols photodegradation varied from 0.5 to 10 fold, the effect ofβ-CD on these bisphenols photodegradation increased with an increase in the amount ofβ-CD concentration. The pheotodegradation of these bisphenol compounds is essentially decrease with the increasing of initial concentration in the absence of CDs and in the presence ofα-CD,γ-CD, while in the presence of β-CD, there was no significant relationship between the DPA、BPF、BPE pheotodegradation efficiency and initial concentration. The effect of p-CD on the photodegradation of these bisphenol compounds in alkaline solution was less than in acidic and neutral solutions. And a similar trend of the effect ofβ-CD on the photodegradation of these bisphenol compounds was found in acidic and neutral solutions. The mineralization of these bisphenol compounds was followed by total organic carbon analysis (TOC) in the absence of CDs and in the presence of p-CD, the results showed thatβ-CD can not enhance the mineralization of these bisphenol compounds, but reduce the BPS, TDP, DHBP mineralization.3. The inclusion complex of bisphenols with CDs was characterized by ultraviolet spectroscopy and fluorescence spectroscopy. Ultraviolet absorption and fluorescence spectra show that absorption peaks of BPS, TDP in inclusion withβ-CD significantly weaken, while fluorescence intensity of DPA、BPA、BPF、BPE、DHBP in inclusion withβ-CD significantly strengthen. The degree of weakening or strengthening of these bisphenol compounds spectrum except DHBP in the presence ofβ-CD is far greater than in the presence ofα-CD andγ-CD, which is in accordance with the results of the formation constants of inclusion complex measured by modified Benesi-Hildebrand method. According to a modified Bensi-Hildebrand equation, the formation constant of inclusion complex between TDP、BPS、DPA、BPA、BPF、BPE、DHBP andβ-CD is 8.21×103L·mol-1、1.48×104L·mol-1、3.80×104 L·mol-1、5.27×104 L·mol-1、1.18×104L·mol-1、3.89×104L·mol-1、187L·mol-1 respectively, the molar ratio is 1:1. The large constant values found suggest significant interaction between the guest and the host molecules. The formation constant of inclusion complex in the presence ofβ-CD does not show significant changes when the pH rose from 3.5 to 10.0. The bisphenols/β-CD complex shows a little smaller ultraviolet absorption than in the absence ofβ-CD when the concertation ritio of bisphenols/β-CD is 1:2. The Ultraviolet absorption spectra of these bisphenols compounds and bisphenols/β-CD complex are very similar at both acid and neutral pH. While at alkaline pH, the bisphenol compounds and bisphenols/β-CD complex change the shape of the ultraviolet spectrum. The UV absorption peaks of TDP, DPA, BPA, BPF, BPE, DHBP and these bisphenols/β-CD complex significantly increased at 254nm.4. Photochemistry and photophysics of both BPE in the absence of CDs and inclusion complex of BPE withβ-CD was studied by nanosecond laser flash photolysis and time-correlated single photon counting techniques. For both systems the primary photochemical process was found to be photoionization with formation of hydrated electron and phenoxyl radical. The main difference between photochemistry of free BPE ((?)(266 nm)= 3.7x10-3) and BPE/β-CD complex ((?)(266 nm)= 1.9x10-2) is a 5-fold increase in photoionization quantum yield in the case of the complex. Inclusion of BPE in cyclodextrin cavity leads to great increase of photoionization and fluorescence quantum yield as well as fluorescence lifetime due to decreasing of relaxation processes in the singlet excited state of complexed BPE. This is in the good agreement with lower photochemical stability of BPE/p-CD complex observed in stationary photochemical experiments.5. Quantitative structure-property relationship (QSPR) model was developed for photodegradation rates of phenol compounds. Molecule structure parameters were investigated with the help of the GAUSS98 software and related literatures. The photodegradation initial rates of these phenol compounds can be fitted by QSPR models. The photodegradation initial rates depend on the logP, pka, the gap of frontier molecular orbital energies (△E). Increasing logP of phenol compounds leads to an increase in photodegradation initial rate, increasing pka,△E leads to a decrease in photodegradation initial rate.