Photoconversion of 2-methylphenol, 4-methylphenol, and 2-amino-4-methylphenol in water

Abstract

We have used fluorescence spectroscopy to study photoconversions of 2-methylphenol (2-MP), , in water when excited by UV radiation from different sources. We discuss the effect of the acidity of the medium and dissolved oxygen on photoconversions of the indicated methylphenols. The photodegradation efficiency for 2-MP and 4-MP decreases in the absence of oxygen. The photoconversion efficiency for methylphenols in alkaline medium is higher when irradiated by light with λ rad = 283 nm, while in neutral medium it is higher when irradiated by light with λ rad = 222 nm.Introduction. This work is devoted to study of the fluorescent properties of some substituted phenols after UV irradiation, and is a continuation of a cycle of investigations on photodecomposition of organic molecules in the phenol class [1]. It was shown earlier [2] that the highest photodegradation efficiency for phenol in water is detected when it is exposed to radiation from a KrCl lamp, and for 4-chlorophenol when exposed to radiation from a XeBr lamp. The aim of this work was to study the luminescent spectral properties of 2-methylphenol (2-MP), 4-methylphenol (4-MP), and 2-amino-4-methylphenol (2-A-4-MP) (Fig. 1) in water for different pH values when exposed to UV light, and also to select a UV radiation source for most efficient photoconversion of the indicated compounds.According to the results of quantum chemical calculations in [3], we can identify the following differences between the photophysical properties of these molecules. When an NH 2 group is introduced into the 2 position of the 4-MP molecule, the energies of the electronic states of the 2-A-4-MP molecule are lowered (see Fig. 1), the oscillator strengths of the S 0 → S n transitions increase compared with 2-MP and 4-MP. Electronic transitions in 2-MP and 4-MP molecules from the ground state S 0 to the excited states S 1 , S 2 , S 3 are accompanied by transfer of electron density from the oxygen atom of the OH group to the carbon atoms of the aromatic ring. Such transfer of electron density promotes an increase in the probability of abstraction of a proton upon excitation. The S 0 → S 1 and S 0 → S 2 transitions in the 2-A-4-MP molecule also occur with participation of the oxygen atom from the OH group, while upon excitation to the S 3 state, electron density transfer from the oxygen atom of the OH group to a carbon orbital no longer occurs. Therefore we should expect differences between the properties of the molecule when it is excited to different electronic states. The most likely mechanism for breaking of the OH bond in methyl-substituted phenols upon excitation up to 50,000 cm -1 (200 nm) is predissociation. The fluorescence quantum yield for 2-A-4-MP in water is lower due to the high efficiency of S 1 -T 3 conversion compared with 4-MP (0.004 and 0.170 respectively).The proposed photolysis products of the selected molecules have been rather well studied by pulsed photolysis and are described in [4][5][6][7][8][9]. It has been established that the OH radicals formed during pho...

Degradation and Defluorination of Per- and Polyfluoroalkyl Substances by Direct Photolysis at 222 nm

“…The krypton chloride (KrCl*) excimer lamps emit narrowly at 222 nm which falls in the so-called far-UVC range of 200–230 nm . Compared to conventional LPUV lamps, the KrCl* lamps have several advantages, including higher photon energy due to the shorter wavelength, the absence of mercury, minimal harm to exposed human tissues and eyes, and output stability at cold temperatures. − The 222 nm irradiation has been demonstrated to be highly effective in inactivating pathogens − and degrading some organic pollutants. − While the VUV also has the advantage of higher photon energy, the strong light absorption of water at 185 nm (ε = 1.61–1.62 cm –1 ) substantially limits light penetration. In contrast, the light absorption of water at 222 nm (ε ∼ 0.001 cm –1 ) is only slightly higher than that at 254 nm .…”

“… 22 − 24 The 222 nm irradiation has been demonstrated to be highly effective in inactivating pathogens 25 − 27 and degrading some organic pollutants. 28 − 30 While the VUV also has the advantage of higher photon energy, the strong light absorption of water at 185 nm (ε = 1.61–1.62 cm –1 ) 31 substantially limits light penetration. In contrast, the light absorption of water at 222 nm (ε ∼ 0.001 cm –1 ) is only slightly higher than that at 254 nm.…”

Direct Photolysis of Chlorophenols In Aqueous Solution By Ultraviolet Excilamps

“…Excilamps can be considered as attractive alternatives to conventional mercury lamps for photochemical purification of water, especially in situations where mercury content is restricted or undesirable. In particular, the photodegradation efficiency of methylphenols with emphasis on possible electronic transitions of the excited molecules was studied using KrCl (222 nm), XeBr (282 nm) and XeCl (308 nm) excilamps [10]. Direct UV photolysis with KrCl and XeBr excilamps has been proven to be very effective for degrading herbicides 2,4-D and 2-methyl-4chlorophenoxyacetic acid in pure water [11], and 2,2',4,4',5,5'-hexachlorobiphenyl (PCB 153) in hexane [12].…”

KrCl and XeCl excilamps and LP-Hg lamp for UV and UV/H2O2decolourization of dyes in water

“…It has been determined that KrCl and Xe 2 excilamps are the most promising excilamps for dye decolourization, achieving higher decolourization efficiency in shorter reaction times [26,28]. Although XeCl excilamps have been reported for the removal of organic pollutants in water [29], to our knowledge this UV source has not been reported in the literature for dye decolourization in water.…”