Applications of Chemical Kinetics in Environmental Systems - Elements of Environmental Engineering: Thermodynamics and Kinetics - page 387

of species S. [S] tot is the total concentration of substrate S in the aqueous phase. k E is
the effective complex formation rate constant. For a given ionic strength and pH, k E
is fixed. From the steady-state balance of cell growth rate and uptake rate, we obtain

k E [

S ] tot

μ = [ E ] tot (6.249)

[ S ] cell

or

ln 1
k E

ln

.

μ [

S ] cell

ln [ S ] tot = + (6.250)

[ S ] tot

If both [S] cell and [E] tot are constant, and k E is fixed for a given pH and ionic strength,
then a plot of ln [S] tot versus ln (1/ k E ) should yield a linear plot (Hudson and Morel,
1990). An interesting conclusion of that work is that “ ... complexation kinetics may
be one of the keys to marine ecology ... .” (Morel and Herring, 1993).
The next two sections will describe some characteristics of bioreaction kinetics
for wastewater treatment and in situ biodegradation of subsurface contaminants. It
is important in these cases to facilitate the contact of pollutant with bioorganisms so
that the reaction is completed in a short time. In the case of wastewater treatment, the
enzymes can be isolated from the organisms and used in a completely mixed reactor.
This can be made more efficient by attaching the isolated enzymes or biomass on
a solid support and using it as a bed reactor. This also facilitates the separation of
biomass from solution after the reaction. For in situ biodegradation, the organisms
should have easy access to the substrate in the subsurface. In soils and sediments this
is a major impediment. Moreover, nutrient and oxygen limitations in the subsurface
environment will limit the growth and activity of organisms. We shall describe first
the immobilized enzyme reactor for wastewater treatment. Selected aspects of in situ
subsoil bioremediation will follow this.

6.5.1.4 Immobilized Enzyme or Cell Reactor

Enzymesaregenerallysolubleinwater.Hencetheirreuseafterseparationfromareac-
tor is somewhat difficult. It is, therefore, useful to isolate and graft it onto surfaces
where they can be immobilized.The surface can then act as a fixed-bedreactor similar
to a packed column or ion-exchange column. The enzyme or cell can be easily regen-
erated for further use. The reaction can be carried out in a continuous mode where the
substrate(pollutant)ispassedthroughthereactorbedandtheproductsrecoveredatthe
effluent end. Both chemical and physical methods can be used to immobilize enzymes
onto solid substrates (see Table 6.18). The same methods are also useful in immobi-
lizing living cells onto solid substrates. Both immobilized enzyme and cell reactors
have been shown to have applications in wastewater treatment (Trujillo et al., 1991).
The use of an immobilized enzyme or cell reactor involves the consideration of
some factors that are not necessarily addressed in a conventional CSTR. Specifically,
we have to consider the different resistances to mass transfer of substrates toward
the reaction site on the immobilized enzyme. The situation is very similar to that
described for sorption and reaction in a natural porous medium (Section 6.1.2.1).
There are three sequential steps before a substrate undergoes transformation; these