Constant degradation kinetics

For those pesticides that are cometabolized, ie, not utilized as a growth substrate, the assumption of first-order kinetics is appropriate. The more accurate kinetic expression is actually pseudo-first-order kinetics, where the rate is dependent on both the pesticide concentration and the numbers of pesticide-degrading microorganisms. However, because of the difficulties in enumerating pesticide-transforming microorganisms, first-order rate constants, or half-hves, are typically reported. Based on kinetic constants, it is possible to rank the relative persistence of pesticides. Pesticides with half-hves of <10 days are considered to be relatively nonpersistent pesticides with half-hves of >100 days are considered to be relatively persistent. 

In water, carbofuran reacted with OH radicals at a first-order rate constant of 2.2 x 10 /M-sec (Mabury and Crosby, 1996a). Benitez et al. (2002) reported an apparent pseudo-first-order rate constant ranging from 5.1 x 10to 19.5 x lO Vsec for the reaction of carbofuran with ozone in water. When ozone and UV radiation was used to study the degradation kinetics, the pseudo-first-order rate constant was 22.8 x 10 Vsec. Similarly, the oxidation of carbofuran by Fenton s reagent and UV radiation ranged from 17.2 x 10 to >200 x lO Vsec. 

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...

Based on this equation, when the pseudo-first-order kinetic constant ( ga) was estimated at 150 Lg of (TSS)J, the half-life of E2 was established to be 0.2 h, with nearly all of the E2 being converted to El. El was removed more slowly at a half-life of 1.5 h and a kinetic constant of approximately 20 L g of (TSS)J, and EE2 was not significantly degraded under those same conditions. By comparison, in similar experiments conducted by Layton et al. (2000) at higher temperamres (30°C), at least 40% of the EE2 was mineralized in activated sludge within 24 h. 

Any chemical reaction, whether involving the main chain or side groups, results in a change of composition of one or more groups and consequently in the IR spectrum. This makes it possible to study oxidation, thermal degradation, cyclization, grafting, and other reactions of polymers [2,4]. Evaluation of both qualitative and quantitative changes, as well as determination of kinetic constants of the reaction, is possible [2]. 

The UV/ozone process can treat pesticides without little generation of refractory products. One of the most common pesticides found in water supplies is Propoxur. The oxidation kinetics was developed in terms of the reaction orders and apparent kinetic constants (Benitez et al., 1994). The amount of chemical removed (Xp) is inversely proportional to the amount of initial Propoxur concentration (Cp), ozone partial pressure (kPa), temperature, and pH. When Propoxur is degraded by UV /ozone, the reactions can be represented by the following general reactions ... 

The degradation kinetics of the target chemical compounds (TCB, DCB, or PA) was assumed to be first order in terms of the concentration of target compound (C). The conditional rate constant (k) can be expressed as ... 

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... 

Because ELUMO is a measure of the ability of a compound to accept electrons (i.e., act as an electrophilic species or undergo reduction), the above correlations show that the kinetic rates and activation energy increase as ELUM0 increases. Therefore, the ability of the compound to behave as an electrophilic species increases as ELUMO increases. The increased reactivity of organic compounds may have a direct influence on its degradation rate constants as follows ... 

The QSAR models can be used to estimate the treatability of organic pollutants by SCWO. For two chemical classes such as aliphatic and aromatic compounds, the best correlation exists between the kinetic rate constants and EHOMO descriptor. The QSAR models are compiled in Table 10.13. By analyzing the behavior of the kinetic parameters on molecular descriptors, it is possible to establish a QSAR model for predicting degradation rate constants by the SCWO for organic compounds with similar molecular structure. This analysis may provide an insight into the kinetic mechanism that occurs with this technology. 

It is important to remember that in the calculation of property degradation kinetics, the constants Ea and A do not imply any information regarding the chemical processes occurring. Multiple chemical reactions may give rise to an effective "activation energy" that is useful for comparison purposes only. 

As has already been made clear, there are many factors that affect the kinetics and mechanisms of Fenton-based degradation. Rate constants, hydroxyl radical concentration, pollutant concentration, and the presence of other species are all among the factors previously discussed. Additional important factors include those related to solubility and homogeneity. 

The decomposition of an active drug in an aqueous solution is investigated over a period of time at a constant temperature. The resulting data of the concentration vs. time are shown in Table 5.4. What is the approximate order of the degradation kinetics and its rate constant ... 

Degradation kinetics of mcthyl-fram-cinnamate in the presence of hydroxide ions follows second-order reaction kinetics. It was shown that the drug forms a complex with theophylline. The following kinetic data were obtained in the presence of theophylline and a great excess of hydroxide ion. Calculate the stability constant. 

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