2.4- Dichlorophenol, irradiation

Chlorinated dibenzo ip-dioxins are contaminants of phenol-based pesticides and may enter the environment where they are subject to the action of sunlight. Rate measurements showed that 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is more rapidly photolyzed in methanol than octachlorodi-benzo-p-dioxin. Initially TCDD yields 2,3,7-trichlorodiben-zo-p-dioxin, and subsequent reductive dechlorination is accompanied by ring fission. Pure dibenzo-p-dioxin gave polymeric material and some 2,2 -dihydroxybiphenyl on irradiation. Riboflavin-sensitized photolysis of the potential precursors of dioxins, 2,4-dichlorophenol and 2,4,5-trichloro-phenol, in water gave no detectable dioxins. The products identified were chlorinated phenoxyphenols and dihydroxy-biphenyls. In contrast, aqueous alkaline solutions of purified pentachlorophenol gave traces of octachlorodibenzo-p-dioxin on irradiation. 

The irradiation of aqueous solutions of 2,4-dichlorophenol and 2,4,5-trichlorophenol with UV light at wavelengths above 280 nm gave little... 

Photolytic. Photolysis of 2,4-D in distilled water using mercury arc lamps (A, = 254 nm) or by natural sunlight yielded 2,4-dichlorophenol, 4-chlorocatechol, 2-hydroxy-4-chlorophenoxyacetic acid, 1,2,4-benzenetriol, and polymeric humic acids. The half-life for this reaction is 50 min (Crosby and Tutass, 1966). A half-life of 2 to 4 d was reported for 2,4-D in water irradiated at 356 nm (Baur and Bovey, 1974). 

Bell (1956) reported that the composition of photodegradation products formed were dependent upon the initial 2,4-D concentration and pH of the solutions. 2,4-D undergoes reductive dechlorination when various polar solvents (methanol, butanol, isobutyl alcohol, ferf-butyl alcohol, octanol, ethylene glycol) are irradiated at wavelengths between 254 to 420 nm. Photoproducts formed included 2,4-dichlorophenol, 2,4-dichloroanisole, 4-chlorophenol, 2- and 4-chlorophenoxy-acetic acid (Que Hee and Sutherland, 1981). 

Irradiation of a 2,4-D sodium salt solution by a 660-W mercury discharge lamp produced 2,4-dichlorophenol within 20 min. Continued irradiation resulted in continued decomposition. The irradiation times required for 50% decomposition of the 2,4-D sodium salt at pH values of 4.0, 7.0, and 9.0 are 71, 50, and 23 min, respectively (Aly and Faust, 1964). 

CASRN 1836-75-5 molecular formula C13H7F3N2O5 FW 284.10 Chemical/Physical. When nitrofen as an aqueous suspension was irradiated using UV light (A. = 300 nm), 2,4 -dichloro-4 -aminodiphenyl ether formed as the major products (>80% of total product formation). In addition, 4-nitrophenol and 2,4-dichlorophenol formed as minor products (<10%). In cyclohexanone, the major photooxidation product was aminonitrofen (Ruzo et al., 1980). 

For example, 2,4-dichlorophenol (C6H4CI2O) mineralizes stoichiometrically according to Eq. 1-2 (Bolton and Cater, 1994). Even pollutants containing inorganic carbon such as cyanide (CN ) can be oxidized to less toxic compounds by adequate irradiation methods. 

Fig. 7.14 Oxidation and mineralization of 2,4-dichlorophenol (2,4-DCP) in aqueous solution by the H2O-VUV AOP using a Xe2 excimer flow-through photoreactor evolution of 2,4-DCP, TOC and chloride ion concentrations. Conditions photoreactor see Figure 4-16, Pel (lamp) = 142 W, irradiated volume V = 2L,...

Kemp et al. (92) operated in the close proximity mode to study the concentration of chloride ions generated near the surface of UV-irradiated titanium dioxide in the presence of aqueous 2,4-dichlorophenol. The Cl concentration was measured potentiometrically with a 50 pan diameter Ag/ AgCl microdisk. Experiments were performed at several tip-substrate distances, and the concentration of chloride was recorded as a function of time after turning on and turning off the UV irradiation. 

When the molecular form of 2,4-dichlorophenol was irradiated, the S2une photoproducts were formed but the quantum yield was only 0.01-0.02. No formation of dlphenol was detected. 

Photohydrolysis of 3-chlorophenol, 3,5- and 3,4-dichlorophenol Our previous work (30) indicated that when 3-chlorophenol in anionic or molecular form was irradiated at 254 nm or 296 nm ( in aerated or degassed solution ( 5kI0"5—sxl0 3 mol.l l ) ), a specific conversion into resorcinol was observed. For a conversion rate of 5%, edsout 80% of the chlorophenol transformed were converted into resorcinol according to the HPLC analysis. Chlorine was quantitatively converted into Cl with an equal formation of H" ". The quantum yield of photoconversion of 3-chlorophenol was significantly higher with the anionic form ( 0.13 0.02 ) than with the molecular form ( 0.0910.01 ). The photochemical behavior of 3-chlorophenol is similar to the behavior of chlorobenzene. 

Vllth 3,5-dichlorophenol, the photbhydrolysis could be sensitized by phenol with a specific formation of 5-chlororesorcinol (35). But the Irradiation of a mixture phenol/ 3,4-dichlorophenol produced a complex mixture of products ( including 4-chlororesorcinol ). A competition between sensitization and reidical induction appeared. 

Kemp, T. J., Unwin, P. R., Vincze, L., Photogenerated chloride-ion concentration near-the-surface of UV-irradiated titanium-dioxide in the presence of aqueous 1,4-dichlorophenol probed using an ultramicroelectrode. J Chem Soc Earaday Trans 1995, 91(21), 3893-3896. 

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