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The rate constants of reactions 3-5 and the standard errors were determined by a nonlinear least-squares regression of our experimental data using the fit curve function of SigmaPlot 2000 software
Chlorination Of Phenols: Kinetics And Formation Of Chloroform
ENVIRONMENTAL SCIENCE & TECHNOLOGY, no. 5 (2002): 884-890
The kinetics of chlorination of several phenolic compounds and the corresponding formation of chloroform were investigated at room temperature. For the chlorination of phenolic compounds, second-order kinetics was observed, first-order in chlorine, and first-order in the phenolic compound. The rate constants of the reactions of HOCl with ...More
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- Since the work of Rook in 1974 (1), it is well documented that chlorinated byproducts detected in drinking water are produced through the reactions of chlorine with natural organic matter (NOM) in the presence or absence of bromide.
- For pKa ) -3 and for the values in Table 1 rate constants of H2OCl+ with substituted phenols would be between 3.7 × 102 M-1 s-1 (4-cyanophenol, no.
- Since the work of Rook in 1974 (1), it is well documented that chlorinated byproducts detected in drinking water are produced through the reactions of chlorine with natural organic matter (NOM) in the presence or absence of bromide
- Because of the complexity of natural organic matter, the mechanisms of THM formation were studied in synthetic waters with model organic compounds
- We showed in a previous paper that the rate of THM formation from resorcinol is too fast to account for all THM precursors contained in natural organic matter (11)
- The rate constants of reactions 3-5 and the standard errors were determined by a nonlinear least-squares regression of our experimental data using the fit curve function of SigmaPlot 2000 software (21)
- Kinetic results suggest that the first pathway might be similar to resorcinols, and the second pathway would involve similar byproducts to the others chlorophenols because their rate constants are identical
- For most of substituted phenols, the rate constants for O3, HOI, and ClO2 are several orders of magnitude higher than rate constants for HOCl. These results explain why iodinated-organic compounds can be formed when iodide-containing waters are chlorinated (22) and that mixtures of HOCl and ClO2 lower the concentrations of trihalomethanes when disinfecting drinking waters (28).
- Except for 4-iodophenol, a linear regression was found between the apparent rate constants k3 for the acid-catalyzed chlorination of para-substituted phenols (Table 1, nos.
- Both formation of chloroform and chlorine consumption were followed when solutions of phenol, resorcinol, 4 para-substituted phenols, and 7 chlorophenols were chlorinated at pH 8.0 ([phenol] ) 4 μM, [HOCl]To ) 140 μM).
- Phenol and chlorophenols substituted in ortho and para position show similar rate constants (k around 0.24 M-1 s-1) and similar yields of chloroform.
- The rate constants of chloroform formation for phenol and para-substituted phenols (Table 2, nos.
- 3) that exhibits an activating substituent in para position exhibits lower yield of chloroform formation and rate constant than phenol and presents higher initial chlorine demand than the other phenolic compounds (Table 2 and Figure 4).
- To further investigate the rate-limiting step for the formation of chloroform, both phenol and 2,4,6-trichlorophenol were chlorinated at pH 8.0.
- When 2,4,6-trichlorophenol is chlorinated, the depletion of the parent compound and the formation of chloroform occur within a similar time frame (Figure 6b).
- The differences for phenols and resorcinols with respect to chloroform formation cannot be only explained by the higher reactivity of resorcinol compounds with chlorine.
- It does not explain the initial fast formation of chloroform from meta chlorinated phenols.
- Table1: Rate Constants for the Initial Chlorination of Mono- and Dihydroxybenzenes in Aqueous Media rate constants no
- Table2: Molar Yields and Rate Constants of Chloroform Formation from Chlorination of Phenols and Resorcinol at pH 8.0
- THM formation from monohydroxybenzenes can be an explanation for the kinetics of slowly reacting THM precursors, which can account for 70% of the whole THM precursors (11)
- For example for a reaction time of 13 h (vertical line), 90% of 2,4,6-trichlorophenol was consumed, and only 65% of chloroform was formed
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