Rates of Biotransformations Uptake

The first process that can limit initial biotransformation rates involves delivery of the organic substrates from the environment to the microorganism s exterior. This 

Kid is the equilibrium solid-water distribution constant [L3 M 1 solid] (see Chapter 9) 

When kdesorb is very slow (or even zero as when the compound is encapsulated in an authigenic mineral), [z]sorbed Ald[z]w so we can neglect the second term in the gradient driving transfer. In this case, we refer to the compound as experiencing sequestration. The parameter, (1 -fw), quantifies the extent of a compound s sequestration in a particular case of interest when we are justified to assume that the dissolved fraction is equal to the bioavailable fraction. Quantitative evaluation of desorb is taken up in Section 19.5. 

When transfers of molecules from media where the microorganisms do not occur (e.g., solids, NAPLs, gases) to phases where they are present (e.g., in water) are not rate-limiting, then it is possible that uptake by the microorganisms can be the slowest step in the sequence. Such a situation implies that the chemical fiigacity of the chemical of interest in that system is not equal to the chemical fugacity inside the cells where the relevant enzymatic apparatus occurs. Now we have a case in which the rate of biodegradation is expressed  

KtUo is the equilibrium biota-water distribution constant [L3 Mb i] (See Chapter 10) 

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