Chlorophenols photolysis rates

The objectives of this study were to determine the photolysis rates of phenol and some chlorophenols in estuarine and distilled water under natural sunlight conditions. Effects of sunlight irra-diance, pH, and chloride ions concentration on photolysis rates were determined. Spectral data and sunlight photolysis rate constants of both the actinometer and the chlorophenols were used to calculate the apparent quantum yields of the various chlorophenols. 

Apparent Quantum Yield (0 ) of the Direct Photolysis of Chloro-phenols. The spectral data and sunlight photolysis rate constants of both actinometer valerophenone and the compound, and reaction quantum yield of valerophenone were used to calculate 0 of chlorophenols using GCSOLAR program (13). 

Photolysis of various chlorophenols as a function of time are shown in Fig. 1. The photo-transformation and photo-mineralization of the compounds followed a first-order equation In (Cg /C) kpt, where Cg and C are the concentrations of the compound at time zero and time t, while kp is the first-order photolysis rate constant. The photolysis half-lives of the compounds were calculated using the equation ti/2 = 0.693/kp. The relative rates of photolysis in estuarine water decreased in tne order 2,4,5-trichlorophenol, 2,4-dichlorophenol, pentachlorophenol, p-chlorophenol, phenol (Table 1). 

As noted above photolysis rates in estuarine water decreased in the order trichlorophenol, dichlorophenol, pentachlorophenol, chlor-ophenol, and phenol. With the exception of the transformation rates for p-chlorophenol and the photolysis rates for pentachlorophenol, for all compounds the photolysis rates including both photo-transformation and photo-mineralization were higher in the sumner than in the winter (t-test, p 0.05 Table I, Fig. 2). For example, the photo-transformation rate constant of dichlorophenol increased from 0,38 to 1.00 hr going from winter to summer, and the photo-mineralization rate constant of phenol increased from 0.006 to 0.04 day going from winter to sumner. 

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. 

Formaldehyde has been shown to influence photolysis rates for some compounds. In our experiments formaldehyde at a concentration of 0.4Z did not show absorption above 295 nm and did not affect the photolysis rates of chlorophenols in distilled water (data not presented). ... 

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... 

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