Degradation rate constant

The examples given below exclude changes in ka that can be attributed to modifications of the enzyme structure. Accordingly, mutant enzymes, enzymes in which certain residues are replaced by unusual amino acids, or enzymes which have been altered by posttranslational modifications are not considered. This restriction in scope has been made because any meaningful explanation of changes in ka must assume that the enzyme being studied under diverse conditions has in fact the same primary structure. 

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Nielsen PH, PL Bjerg, P Nielsen, P Smith, TH Christensen (1996) In situ and laboratory determined first-order degradation rate constants of specific organic compounds in an aerobic aquifer. Environ Sci Technol 30 31-37. 

Figure 5 shows the plots of the two major photooxidative processes versus photolysis time and temperature. Table IV summarizes the two major photooxidative degradation rate constants and activation energies. 

Table IV, Sunmary Data on the Two Major Photooxldative Degradation Rate Constants...

Table 1. Some degradation rate constants and half-life times taken from literature.

Fig. 5.14. The effect of the enthalpy of vaporisation on the degradation rate constant of dextran solutions. A - Water ...

Surface Water. In filtered lake water at 29 °C, 90% of 2,4-D (1 mg/L) mineralized to carbon dioxide. The half-life was <5 d. At low concentrations (0.2 mg/L), no mineralization was observed (Wang et ah, 1984). Subba-Rao et al. (1982) reported that 2,4-D in very low concentrations mineralized in one of three lakes tested. Mineralization did not occur when concentrations were at the picogram level. A degradation rate constant of 0.058/d at 29 °C was reported. At original concentrations of 100 mg/L and 100 pg/L in autoclaved water, the amount of 2,4-D remaining after 56 d of incubation were 81.6 and 79.6%, respectively (Wang et ah, 1994). 

Fig. 16.2 Degradation rate constant for methyl parathion as a function of pH, in aqueous 5.0 mM hydrogen sulfide with and without natural organic matter (NOM), at 25°C. Reprinted with permission from Guo XF, Jans U (2006) Kinetics and mechanism of the degradation of methyl parathion in aqueous hydrogen sulfide solution Investigation of natural organic matter effects. Environ Sci Technol 40 900-906. Copyright 2006 American Chemical Society...

In order to derive a first-order degradation rate constant, fcNXA, for NTA in Greifensee (Illustrative Example 21.1), you had to make several restrictive assumptions regarding the characteristics of the lake and the behavior of NTA. Identify possible sources of uncertainty and formulate the necessary conditions (1) to keep the uncertainty of NTA below 10%, and (2) to prove that NTA is not conservative (nonreactive) in Greifensee. Note Here we do not consider the analytical uncertainty of the measurement of NTA in water. 

What is surprising is the catalytic activity under an ordinary fluorescent white light bulb. The UV intensity was only 11 fiW cm-2, which was ca. 1/30 that of the fluorescent black light, but the first-order degradation rate constant with the former amounts to ca. 1/3 that of the latter. The quantum efficiency at 11 juW cm-2 light intensity, calculated under the assumption that 6 holes are used to degrade... 

Hammett s equation was also established for substituted phenols from the elementary hydroxyl radical rate constants. The Hammett resonance constant was used to derive a QSAR model for substituted phenols. The simple Hammett equation has been shown to fail in the presence of electron-withdrawing or electron-donating substituents, such as an -OH group (Hansch and Leo, 1995). For this reason, the derived resonance constants such as o°, cr, and o+ were tested in different cases. In the case of multiple substituents, the resonance constants were summed. Figure 5.24 demonstrates a Hammett correlation for substituted phenols. The least-substituted compound, phenol, was used as a reference compound. Figure 5.24 shows the effects of different substituents on the degradation rates of phenols. Nitrophenol reacted the fastest, while methoxyphenol and hydroxyphenol reacted at a slower rate. This Hammett correlation can be used to predict degradation rate constants for compounds similar in structure. 

To evaluate the effect of the number of chlorines on the degradation rate constants of different chlorophenols, Table 6.2 shows the rate constants of elementary, oxidation, and dechlorination for the ratios of k2 CP/k2/l DcP and 2,4,6-tcp/ 2,4-dcp The relative rate constants are plotted against the number of sites unoccupied by chlorine atoms on the chlorinated phenols in Figure 6.3.A linear correlation between the rate constants and the number of sites available is found with a standard deviation of 0.132. Clearly, the more chlorine atoms the aromatic rings contain, the fewer sites are available for hydroxyl radical attack however, the correlation should not be used for... 

Table 8.3 shows QSAR models between the degradation rate constants of chloroethanes and EHOMO ar d ELUMO under different ozone dosages and UV intensities. Table 8.3 also shows that EHOMO exhibits better correlation with the kinetic rate constants than does Elumo. The correlation increases as the UV intensity increases. Because EHOMO is a measure of the energy necessary... 

Excellent correlations were obtained between the degradation rate constants of triazin herbicides and EHOmo/ as shown in Figure 8.15 and Figure 8.16. This suggests that amines subject to nucleophilic oxidation therefore, EHOMO... 

PCBs available for various uses. For this reason, 2-chlorobiphenyl and a mixture of Aroclor were studied. Samples (100 mL) of the mixtures were prepared with 265 (ig/mL of PCB and 25 pg/mL of Ti02. The pH of the samples was adjusted to 3, 7, and 10. The samples were irradiated under a UVA-340 light source for 120 min. The concentrations of the PCBs were determined using GC. The degradation rate of 2-chlorobiphenyl increased with decreasing pH values. After 1 hour of irradiation, more that 90% of the PCBs had been degraded at pH 3 and pH 7. The degradation rate constant, k, can be determined from the first-order reaction model ... 

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