Biodegradation first-order

In the RT3D simulation, advective/dispersive transport of each contaminant is assumed. Sorption is modeled as a linear equilibrium process and biodegradation is modeled as a first-order process. Due to the assumed degradation reaction pathways (Fig. 2) transport of the different compounds is coupled. In the study, four reaction zones were delineated, based on observed geochemistry data. Each zone (two anaerobic zones, one transition zone, and one aerobic zone) has a different value for the biodegradation first-order rate constant for each contaminant. For example, since PCE is assumed to degrade only under... 

Using literature values for the various biodegradation first-order rate constants, concentrations over space and time for all the contaminants (as well as methane, chloride, and ferrous iron) were simulated. BioRedox includes a visualization module that simplifies comparison between model predictions and field measurements for the numerous compounds of interest. While the authors give an example of predicted versus measured results at several locations on the site, no attempt is made to quantify the quality of the overall prediction. 

Biodegradation biodegradation first-order rate of hydroxylation of 0.008 min by Pseudomonas putida FI (Spain et al. 1989 quoted, Neilson et al. 1991). 

Biodegradation first order microbial degradation rate constant k = 7.4 x 10 h in sediment and water (Yoshida Kojima 1978, quoted, Addison et al. 1983) ... 

Kinetics of Pesticide Biodegradation. Rates of pesticide biodegradation are important because they dictate the potential for carryover between growing seasons, contamination of surface and groundwaters, bio accumulation in macrobiota, and losses of efficacy. Pesticides are typically considered to be biodegraded via first-order kinetics, where the rate is proportional to the concentration. Figure 2 shows a typical first-order dissipation curve. 

For those pesticides which are utilized as microbial growth substrates, sigmoidal rates of biodegradation are frequentiy observed (see Fig. 2). Sigmoidal data are more difficult to summarize than exponential (first-order) data because of their inherent nonlinearity. Sigmoidal rates of pesticide metabohsm can be described using microbial growth kinetics (Monod) however, four kinetics constants are required. Consequentiy, it is more difficult to predict the persistence of these pesticides in the environment. 

The rates of each of the environmentally important chemical processes are influenced by numerous parameters, but most processes are described mathematically by only one or two variables. For example, the rate of biodegradation varies for each chemical with time, microbial population characteristics, temperature, pH, and other reactants. In modeling efforts, however, this rate can be approximated by a first-order rate constant (in units of time). 

Finally, degradation processes which are usually assumed to be first order are not. Degradation in soil has been shown by Hamaker (27) to often behave in a biphasic manner. Biodegradation in water has been shown to more closely follow second order kinet-ics(28). Photolysis in solution is highly dependent on antenua-tion of light in the water body which will depend on water quality... 

DMP and DBP with the short alkyl-side chain phthalates can be degraded, whereas the DOP degradation under the same experimental conditions appeared to be relatively slow. It is noteworthy that both the ester groups and the phthalate ring were mineralized at a significant rate. The kinetics study demonstrated that biodegradation of the three phthalates conformed to a first-order model with respect to their concentrations. 

The first-order and fractional power kinetics were also used to describe the behavior of DEHP biodegradation in the thermophilic phase, including the initial mesophilic phases (phase I) and the phase thereafter (phase II), respectively [62]. The fractional power kinetic model parameters, i.e., K and N. were calculated by (l)-(3) and derived from a plot of log (C/Co) versus log(f). The half time (f0.s) of DEHP degradation in phases I and II was calculated using first-order and fractional power kinetic equations (3), respectively. 

As shown in Table 1, many FMs meet the biodegradation criteria of a ready or inherent test. If a FM meets the criteria of a ready test, with or without acclimation, a first-order biodegradation rate of 3 h 1 in activated sludge can be assumed [ 1 ]. For FMs that show extensive biodegradation but fail the ready test criteria, a first-order rate of 0.3 h 1 can be assumed for activated sludge treatment [1]. 

Generally, LAS biodegradation processes were previously monitored either as primary biodegradation (a decreasing LAS concentration) or as mineralisation (formation of biomass and/or C02). Both processes have usually been fitted to a first order kinetics, but this approximation is only valid if there has been no prior acclimatisation phase an initial lag phase is usually present in biodegradation tests [4]. Nevertheless, first order kinetic constants may be used for special cases or for a preliminary characterisation of the process. 

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)... 

Many common organic chemicals are degraded at a first-order rate, which assumes that the rate of biodegradation depends only on the concentration of the contaminant and the rate coefficient (k). The rate coefficient can be calculated by the equation ... 

The yield (y) of a biomass production process is defined as the moles of biomass formed per mole of substrate consumed. Aerobic conditions are more conducive to higher biomass formation (and therefore also to biofilm formation) than anaerobic conditions. Empirically, under aerobic conditions, a yield of 0.05 - 0.6mol biomass/mol carbon can be obtained, while under anaerobic conditions the attainable yield falls to 0.04 -0.083mol. The reaction kinetics of biodegradation processes can be approximated by the first-order reaction rate constant k as follows ... 

Biological. Using the experimentally determined first-order biotic and abiotic rate constants of chlorpyrifos in estuarine water and sediment/water systems, the estimated biodegradation half-lives were 3.5-41 and 11.9-51.4 d, respectively (Walker et al, 1988). 

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