Applications of Chemical Kinetics in Environmental Systems - Elements of Environmental Engineering: Thermodynamics and Kinetics - page 388

TABLE 6.18
Immobilization Techniques for Enzymes or Cells
Chemical Physical

Covalent bonding to inactive supports Adsorption onto supports

Copolymerization with support structure Entrapment in cross-linked polymers

Crosslinking multifunctional groups Microencapsulation

are indicated in Figure 6.70. The first step, namely, the diffusion in the bulk fluid
toward the liquid boundary layer and the subsequent diffusion through the boundary
layer to the support surface, constitutes the external mass transfer resistance. The last
step, namely, the diffusion to the enzyme reaction site, constitutes the internal mass
transfer resistance.
If the enzyme or cell is grafted on the surface of an insoluble support, only the
first two steps contribute to mass transfer resistance and reaction. In other words, the
external resistance controls the substrate decomposition. In such a case, the rate of
transfer of solute to the surface of the support is balanced exactly by the substrate
reaction at the enzyme site:

V max [ S ]

k L a v [ S ] ∞ −[ S ]= , (6.251)

K m +[ S ]

where [S] is the substrate concentration at the surface, [S] ∞ is the concentration in
the bulk solution, and k L is the overall liquid-phase mass transfer coefficient for
substrate diffusion across the boundary layer. a v is the total surface area per unit
volume of the liquid phase. To simplify further discussion, we define the following

Boundary
layer

Bulk
solution

Surface

Solid
support

2

3

1

Reaction
site

Immobilized
enzyme on
solid support

FIGURE 6.70 Steps involved in diffusion and reaction of a substrate at an immobilized
enzyme support.