Photolysis of chlorophenols

Figure 1. Photolysis of chlorophenols in estuarine water. Chlorophenols were added to the water sample at a concentration of 25 ig L in December. Temperature was 14 C and pH was 7.6. Transformation equations were In c - 3.22-0.01 t (p-chloro-phenol). In c 3.22-0.44 t (2,4-dichlorophenol), In c 3.22-0.65 t (2,4,5-trichlorophenol), and In c 3.22-0.27 t (penta-chlorophenol), where c is the concentration of the chlorophenol at time t. Vertical bars represent 1 standard deviation with n 3.

Table III. Effect of pH on Photolysis of Chlorophenols in Water a 2,4-Dichlorophenol ...

An important aspect of our studies were the effect of cloud cover, pH, chloride ion, and season on the photolysis of phenol and various chlorophenols. The effect of pH on the photolysis of chlorophenols is due to the higher rate of photolysis of the phenoxlde ion relative to the nonionized compound, due to stronger absorbance by the phenoxide ion ( ). The pK of dichlorophenol, trichlorophenol, and pentachlorophenol are 7.6, 7.0 and 4.8, respectively (.22, 24). Low photolysis rates of chlorophenols in both estuarine and distilled water were obtained at pH below the pK (Table III). At pH 7.6 found in estuarine water 50Z, 80Z, and 99.82 of the dichlorophenol, trichlorophenol, and pentachlorophenol, respectively, is in the form of the phenoxide ion. The photolysis rate of pentachlorophenol in estuarine water was lower than in distilled water (Table I). Addition of chloride ion to distilled water containing pentachlorophenol resulted in a decrease in the photolysis rate. Miille and Crosby ( 5) found that pentachlorophenol had a lower photolysis rate in seawater compared to distilled water due to the photonucleophilic... 

A small amount of octachlorodibenzo-p-dioxin, amounting to approximately 0.03%, was formed during the photolysis of sodium penta-chlorophenol buffered at pH 8 as shown by the data in Table IV. 

Whereas photolysis of 2- and 4-chlorophenols in aqueous solution produced catechol and hydroquinone, in ice the more toxic dimeric chlorinated dihydroxybiphenyls were formed (Blaha et al. 2004). 

Hwang, H.-M., Hodson, R.E., Lee, R.F. (1987) Photolysis of phenol and chlorophenols in estuarine water. In Photochemistry of Environmental Aquatic Systems. American Chemical Society, Washington DC. 

Because of their importance in pesticide manufacturing, chlorophe-nols are one of the most studied types of phenols. Thus, Esplugas et al. [142] studied the direct photolysis of p-chlorophenol with a 250-W, medium-pressure vapor arc lamp with and without ozone. They followed the process of mineralization from total organic carbon (TOC) data. They observed that UV photolysis hardly affected the process (5% mineralization after 30 min). When ozone was fed in at a mass flow of 23 mg/min under similar conditions, they observed almost complete mineralization. The synergism between ozone and UV light was also much more effective than ozonation alone, which, under the same operating conditions, resulted in roughly 45% mineralization. For kinetic purposes, they assumed a first-order reaction for both ozonation and UV photolysis of TOC. For the latter, they used the linear spherical model to define the radiation field inside the photoreactor. 

The carbene mechanism of heterolytic dehalogenation of 4-chlorophenol was subsequently confirmed by studies using flash photolysis [21] and FT-EPR [22], A detailed account of the EPR measurements was published later [23], in which it was shown that the spin polarization of the phenoxyl-propanoyl radical pair produced in the photolysis of 4-chlorophenol in 2-propanol is consistent with a triplet state precursor. The proposition that this precursor is the above-mentioned carbene was proved by generating the same radical pair, with identical spin polarization, by photolysis of p-benzo-quinone diazide [22,23]. 

The photolysis of aqueous 3-chloroaniline was found to proceed via a clean photohydrolysis step to give 3-aminophenol with a quantum yield of

hydrolysis process or a fast reaction of a primarily formed aminophenyl cation with H20 (see Scheme 5). 

In addition to aliphatic chain attack, hydroxyl radicals may also directly attack the aromatic ring. This is believed to be the case for the iron-mediated photo degradation of 3-chlorophenol (3CP) [12] with hydroxyl radicals that are formed on photolysis of FeOH2+ rapidly reacting with 3CP to form rad-... 

Jakob L, Hashem T, Burki S, Guindy NM, Braun AM (1993) Vacuum-Ultraviolet (VUV) Photolysis of Water Oxidative Degradation of 4-Chlorophenol, J. Photochem. Photobiol. A Chem. 75 97-103. 

Kawaguchi, H. (1992) Determination of direct and indirect photolysis of 2-chlorophenol in humic acid solution and natural waters. Chemosphere 25, 635-641. 

Photocatalytic degradation of 4-chlorophenol in TiOz aqueous suspensions produces 4-chlorocatechol, an ortho hydroxylated product, as the main intermediate. This result disagrees with data reported by other researchers, who proposed the formation of a para-hydroxylated product, hydroquinone, as the major intermediate. Results also indicated that further oxidation of 4-chlorocatechol yields hydroxy hydroquinone, which can readily be oxidized and mineralized to carbon dioxide. Complete dechlorination and mineralization of 4-chlorophenol can be achieved. In contrast, direct photolysis of 4-chlorophenol produces hydroquinone and p-benzoquinorie as the main reaction products. The photocatalytic oxidation reaction, initially mediated by TiO2, is generated by an electrophilic reaction of the hydroxyl radical attacking the benzene ring. 

Comparison with Direct Photolysis Process. The Ti02-mediated photocatalytic oxidation reaction involves a complex free-radical reaction mechanism in which OH radicals are responsible for the oxidation of 4-chlorophenol. The initial reaction step produces 4-chlorocatechol as the main product. In contrast, the direct photolysis of 4-chlorophenol produces a different set of reaction products. Figure 8 shows that the direct photolysis of... 

Figure 8. Direct photolysis of 4-chlorophenol. Hydroquinone and 4-chloroca-techol were analyzed by GC-MS p-benzoquinone was analyzed by HPLC with UV-visible detector at a wavelength of254 nm. Experimental conditions 4-chlo-rophenol = 10 3 M, pH = 4.0, I = 5 X 10 2 M NaN03, oxygen atmosphere, and temperature = 25 °C.

In addition to the difference in forms and the distribution of reaction products, the photocatalysis and photolysis processes exhibit different extents of mineralization. As shown in Figure 1C, the generation of carbon dioxide in direct photolysis of 4-chlorophenol is almost negligible. In contrast, pho-tocatalytic oxidation leads to total mineralization of 4-chlorophenol to carbon dioxide. There is, however, no evidence that further ring-cleavage reaction takes place in direct photolysis of 4-chlorophenol. 

The Ti02-mediated photocatalytic oxidation process can readily degrade 4-chlorophenol in aqueous solutions, with a complete mineralization to carbon dioxide and chloride ions, whereas the direct photolysis of 4-chlorophenol generates only a small amount of carbon dioxide. The distribution of intermediates during the course of the reaction shows that the reaction mechanism of the photocatalytic oxidation process is clearly different from that of the direct photolysis reaction. 

The Ti02-mediated photocatalytic oxidation reaction can be described by the radical mechanism involving OH as the major reaction species. The reaction mechanism follows the ortho pathway, so that the main intermediate found is 4-chlorocatechol, whereas the formation of 4-chlororesorcinol and hydroquinone is only a minor pathway. Further degradation of 4-chloroca-techol leads to production of hydroquinone, which can be further oxidized and mineralized to carbon dioxide. In contrast, the direct photolysis of 4-chlorophenol follows the para pathway, which leads to the formation of hydroquinone and p-benzoquinone as the major products. 

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