Pentachlorophenol photolysis

Our studies indicated rapid photolysis of trichlorophenol, di-chlorophenol, and pentachlorophenol in both distilled and estuarine water. We can compare our results of pentachlorophenol with those of other investigators who have studied the photolysis of this compound in both fresh and marine waters O, 17-20). The photolysis rate constant kp for pentachlorophenol in a freshwater stream was 0.29 hr ( 1/2 2.4 hr) at 3.8 cm in the summer ( W), while we found a kp of 0.37 hr ( 1/2 " 2 hr, light hours) at a depth of 3.0 cm in the summer (Table I). The half-life of pentachlorophenol in a 1 m deep freshwater pond was 1.5 to 3 days (17) while in 5.5 m deep marine mesocosm the half-life was 22 days (18). Using lamps to simulate sunlight the pentachlorophenol in surface seawater was found to have a half-life of 2.4 hr. ( 5), Thus our rate constant and half-life for pentachlorophenol photolysis was similar to one determined by others in surface waters. Due to attenuation of light by substances in the water longer half-lives, i.e., days rather than hours, are found for pentachlorophenol when distributed throughout the water column. 

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. 

PCP presents a different picture from that of the lower chlorophenols and their derivatives. The corresponding dioxin shows much more stability to light than does TCDD, enough to permit its prolonged existence at low concentrations in a photoreactor. As a phenol it can directly yield dioxins, a process favored by its normal mode of application as the sodium salt. Although octachlorodibenzo-p-dioxin has much lower mammalian toxicity than TCDD (6), its formation, properties, and effects demand additional investigation. Technical preparations of PCP are frequently mixtures of tetra- and pentachlorophenols consequently, hepta-and possibly hexachlorodibenzo-p-dioxins might be expected as photolysis products in addition to the octachloro derivative. 

Pignatello JJ, MM Martinson, JG Steiert, RE Carlson, RL Crawford (1983) Biodegradation and photolysis of pentachlorophenol in artificial freshwater streams. Appl Environ Microbiol 46 1024-1031. 

Lamparski, L.L., R.H. Stehl, and R.L. Johnson. 1980. Photolysis of pentachlorophenol-treated wood. Chlorinated dibenzo-p-dioxin formation. Environ. Sci. Technol. 14 196-200. 

Wong, A.S. and D.G. Crosby. 1978. Photolysis of pentachlorophenol in water. Pages 119-25 in K.R. Rao (ed.). Pentachlorophenol Chemistry, Pharmacology, and Environmental Toxicology. Plenum Press, New York. 

Photochemical decomposition can also be carried out in the presence of a suspension of photoactive material such as Ti02 where substrate absorption onto the uv activated surface can initiate chemical reactions e. g. the oxidation of sulphides to sul-phones and sulphoxides [37]. This technology has been adapted to the destruction of polychlorobiphenyls (PCB s) in wastewater and is of considerable interest in environmental protection. Using pentachlorophenol as a model substrate in the presence of 0.2 % TiOj uv irradiation is relatively efficient in dechlorination (Tab. 4.5) [38]. When ultrasound is used in conjunction with photolysis, dechlorination is dramatically improved. This improvement is the result of three mechanical effects of sonochemistry namely surface cleaning, particle size reduction and increased mass transport to the powder surface. 

Tab. 4.5. Photolysis of aqueous pentachlorophenol (2.4x10 M) containing 0.2% Ti02).

In another study, the irradiation (X = 290-310 nm) of hexachlorobenzene in an aqueous solution gave only pentachlorobenzene and possibly pentachlorophenol as the transformation products. The photolysis rate increased with the addition of naturally occurring substances (tryptophan and pond proteins) and abiotic sensitizers (diphenylamine and skatole) (Hirsch and Hutzinger, 1989). 

When an aqueous solution containing pentachlorophenol (45 pM) and a suspension of titanium dioxide (2 g/L) was irradiated with UV light, carbon dioxide and HCl formed in quantitative amounts. The half-life for this reaction at 45-50 °C is 8 min (Barbeni et al, 1985). When an aqueous solution containing pentachlorophenol was photooxidized by UV light at 90-95 °C, 25, 50, and 75% degraded to carbon dioxide after 31.7, 66.0, and 180.7 h, respectively (Knoevenagel and Himmelreich, 1976). The photolysis half-lives of pentachlorophenol under sunlight irradiation in distilled water and river water were 27 and 53 h, respectively (Mansour et al., 1989). 

Donaldson, S.G. and Miller, G.C. Transport and photolysis of pentachlorophenol in soils subject to evaporating water. J. Environ. Qual, 26(2) 402-409, 1997. 

P 15.5 Estimating the Direct Photolysis Half-Life of Pentachlorophenol in Chesapeake Bay (adaptedfrom Roberts, 1995)... 

The direct photolysis of nitrobenzene was found to be slow and it did not occur at a significant rate. The naphthalene/pentachlorophenol system showed that the degradation rate of each compound was affected by the presence of the other compounds. In addition, both compounds when subjected to direct photolysis produced significant amounts of hydrogen peroxide. Thus, shortly after the initiation of the photolysis, the system became a UV/H202 process. 

Lamparski LL, Stehl RJ, Johnson RL. 1979b. Photolysis of pentachlorophenol-treated wood ... 

Liu, P.Y., Zheng, M.H. and Xu, X.B. (2002) Phototransformation of polychlorinated dibenzo-p-dioxins from photolysis of pentachlorophenol on soils surface. Chemosphere 46, 1191-1193. 

Vollmuth S, Zajc A, Niessner R (1994), Environ. Sci. Technol. 28 1145-1149.. .Formation of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans during photolysis of pentachlorophenol-containing water"... 

Figure 10. Proton-nmr spectra recorded (A) before and (D) during tne photolysis of 4-phenylacetophenone and pentachlorophenol in isopropanol. The methyl protons spectrum at 6 2.0 was recorded at a much lower sensitivity than that of the aromatic protons at...

Photolysis aqueous photolysis L, 1-336 h, based on aqueous data for 24,5- 2,4,6-trichlorophenols and pentachlorophenol (Howard et al. 1991) ... 

An interesting result has been obtained in a study of the dechlorination of polychlorobenzenes by direct irradiation in acetonitrile in addition to dechlorination, products resulting from rearrangement of the chlorine atoms to positions mefa to their original point of attachment are observed. Thus orfho-dichlorobenzene was found to yield mainly chlorobenzene along with small amounts of para-dichlorobenzene the authors speculate that the rearrangement products are formed by recombination of an aryl-chlorine radical pair. In another novel result photolysis of pentachlorophenol in acetonitrile is found to give small amounts of the benzoxazole (160). ... 

Preservatives Pentachlorophenol (PCP) Phenols, quinones, acids, C02, CD Initiated by direct photolysis of PCP Complex... 

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