Sorption Kinetics - Transport and Fate of Chemicals in Soils: Principles and Applications - page 56

in disk
supported
on membrane
lter

Fraction
collector

Pump

Reservoir

FIGURE 2.18
The thin disk flow method experimental setup. Background solute solution is pumped from
the reservoir into the thin disk and the effluent is collected by a fraction collector.

flow rate. Nevertheless, the pulse is often terminated when a change in
the concentration in the effluent is no longer observed. Once terminated,
desorption commences using a continuous pulse of a solute-free solution.
This procedure is often repeated using different initial or input concentra-
tion ( C i ) in order to achieve rates of reactions over a wide range of solute
concentrations.
There are several alternative ways for the use of the thin disk method to
quantify solute sorption under different constraints, including ionic strength
and pH. For example, pulses of increasing concentrations ( C i s ) may be intro-
duced consecutively to the thin disk in order to achieve a wide range of con-
centrations for the same soil. Multiple solutes may also be introduced to the
thin disk either concurrently (i.e., a pulse of several solutes) or consecutively
where each pulse is identified by a different solute species.
There are several advantages of thin disk methods over the batch methods
described above. First, the reactions associated with adsorption as well as
release or desorption take place under conditions of transport or when flow
is taking place. As such, thin disk methods represent an open system where
incoming solute is being introduced to the soil during pulse application.
The same is true during desorption or release where the incoming solution
contains no solute. Another advantage is that this method closely resembles
field situations compared to batch methods with their inherent artificial con-
straints of unrealistic ratios of soil to solution, stirring, and centrifugations.

2.3.5 Stirred-Flow Method

A schematic of the stirred-flow method is given in Figure 2.19. This method
was originated by chemical engineers in the 1980s and was later adopted by