Applications of Chemical Kinetics in Environmental Systems - Elements of Environmental Engineering: Thermodynamics and Kinetics - page 393

species and that of the truly dissolved species in the aqueous phase. If both are first
order, r sorbed
v

ρ b is the soil density, [S] ads is
the sorbed mass of S, k s and k w are the respective rate constants. We can write the
following expressions for the fraction sorbed and truly dissolved:

= ρ b k s [S] ads and r w = k w [S], where

k sw ρ s

f sorbed = ,

1 + k sw ρ s

(6.263)

1

f w

=

k sw ρ s ,

1 +

where [S] ads is the soil sorbent concentration. A composite rate is defined as

r ∗ = k ∗ [ S ] tot , (6.264)

where [S] tot is the total substrate concentration and

k ∗ = f sorbed k s + f w k w .

(6.265)

The above equation satisfactorily represents the effect of sorption on the overall
biodegradation if it is modeled as a first-order process. Apart from sorption, bio-
transformations can also be affected by other environmental factors such as soil pH,
temperature, soil moisture, clay content, and nutrients.Valentine and Schnoor (1986)
have summarized these issues in an excellent review, to which the reader should refer
to for more details.
Since bioavailability is a primary limitation in bioremediation, considerable effort
has been expended to facilitate desorption of chemicals from sediment and soil sur-
faces. Mobilization via solubilization using surfactant solutions is one of the concepts
under study. Several workers have suggested that nonionic surfactant solutions used in
conjunction with specific microbes can affect the mineralization of several hydropho-
bic organics. There are, however, several conflicting reports regarding this approach;
in particular the long-term viability of the microbes in the presence of surfactants, the
presence of residual surfactants in the subsurface soil environment, and the degree of
mineralization in the presence of surfactants have been disputed.

6.5.2 K INETICS OF B IOACCUMULATION OF C HEMICALS IN THE A QUATIC
F OOD C HAIN
In Chapter 4, we introduced the bioconcentration factor that defined the equilib-
rium partitioning of chemicals between the biota and water. This section is to model
the kinetics of chemical uptake and dissipation in organisms. Connolly and Thomann
(1992) summarized the state-of-the-art in this area.What follows is a concise descrip-
tion of their model. There are three important subareas that have to be included in
such a kinetic model, namely, (i) the growth and respiration rates of each organism
included in the food chain, (ii) the efficiency of chemical transfer across the biological
membranes (gills, gut, etc.), and (iii) the rate of dissipation and excretion of chemicals
by the organism.