Degradation first-order kinetics

Figure 7.8 Arrhenius plot of the global rate constant of glycerol degradation (first-order kinetics, 45MPa, water/ glycerol ratio 199) from experimental results and from model calculations. In addition, the ionic product for water at 45 MPa is given [31].

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. 

Unusual reactivities of mechano-radicals have been reported in a few instances. To explain the pseudo first-order kinetics and the high yield of linear block copolymers formed during the mechanochemical degradation of a mixture of... 

Photolysis of all brominated dibenzodioxins investigated occurs very fast in n-hexane. The rate of degradation of all compounds follows a good first-order kinetic scheme. In Table 4 the calculated first order rate constants k are summarised along with the quantum-yields. The corresponding results for three brominated dibenzofurans are also included. 

Studies of chlorophyll degradation in heated broccoli juices over the 80 to 120°C range revealed that chlorophylls degrade first to their respective pheophytins and then to other degradation products in what can therefore be described as a two-step process. Both chlorophyll and pheophytin conversions followed a first-order kinetics, but chlorophyll a was more heat sensitive and degraded at a rate approximately twice that of chlorophyll This feature had been observed by other authors. Temperature dependence of the degradation rate could adequately be described by the Arrhenius equation. ... 

Degradation rates were determined for the reported data using a nonlinear regression of conventional first-order kinetic equations. The software used for this fitting procedure was Model Manager, Version 1.0 (Cherwell Scientific, 1999). 

Fractional Order. In the decomposition of pure solids, the kinetics of reactions can often be more complex than simple zero- or first-order processes. Carstensen [88] has reviewed the stability of solids and solid dosage forms as well as the equations that can be used in these cases. In addition to zero- and first-order kinetics, solid-state degradations are often described by fractional-order equations. 

For some organic compounds, such as phenols, aromatic amines, electron-rich olefins and dienes, alkyl sulfides, and eneamines, chemical oxidation is an important degradation process under environmental conditions. Most of these reactions depend on reactions with free-radicals already in solution and are usually modeled by pseudo-first-order kinetics ... 

A dry, thin lycopene layer heated at 50°C, 100°C, and 150°C showed first-order kinetic decay (Lee and Chen 2002). At 50°C, isomerization dominated in the first 9h however, degradation was favored afterward. On the other hand, at 100°C and 150°C degradation proceeded faster than isomerization. Although cis isomer identification was not confirmed by standards, the mono-civ lycopene isomers, 5-cis-, 9-cis-, 13-cis-, and 15-m-, degraded at the same rate as did all-trans-lycopene,... 

Aqueous solutions of (z))-penicillamine are comparatively stable at pH 2-4, and it degrades slowly by first order or pseudo-first order kinetics in aqueous solutions. A 3% solution of penicillamine hydrochloride stored under nitrogen in a sealed container at 20 °C decomposed to the extent of about 10% per year [2]. 

Fig. 28.4. Degradation of phenol by a consortium of methanogens, as observed in a laboratory experiment by Bekins et al. (1998 symbols), and modeled using the Michaelis-Menten equation (solid line). Inset shows detail of transition from linear or zero-order trend at concentrations greater than KAy to asymptotic, first-order kinetics below this level. Broken line is result of assuming a first-order rather than Michaelis-Menten law.

Main characteristics of biodegradation experiments and first order kinetic parameters (degraded LAS(%) = A e kt) obtained for LAS primary biodegradation in different estuarine and coastal media at different initial concentrations and for different homologues (t — 20-25°C)... 

First-order kinetics is easier to apply to transport and degradation models because they do not require knowledge about particle geometry. 

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