Sorption Kinetics - Transport and Fate of Chemicals in Soils: Principles and Applications - page 55

volume of the centrifuge tubes varies from 40 to 50 mL where a soil-to-solution
ratio of 3 g to 40 mL solution (or 4 g to 40 ml solution) is used. The solution con-
tains the solute at the desired concentration dissolved in a background solu-
tion. Commonly used are 0.005 M CaCl 2 , KNO 3 , or KCl background solution.
Duplicate or triplicate samples (or tubes) are used for each initial solute con-
centration. For other matrices, such as organic material or plant material, 1 g
of matrix to 30 or 40 mL of solution is used. The range of initial concentrations
to be used varies dependent on the objectives of each study.
The mixtures or slurries in the centrifuge tubes are then shaken continu-
ously, and after each reaction time (or sampling time), the tubes are centri-
fuged at 500 × g for 15 min. An aliquot is then sampled from the supernatant
at the specified reaction time. The volume to be decanted can be as little
as 0.2 mL if radionuclides are used. The volume of the decanted solution
is normally 3 mL. The slurries are then vortex mixed and returned to the
shaker after each sampling. These steps are repeated for each adsorption
time. Generally, initial reaction times may vary from 2 to 4 hours followed
by daily sampling for 1 to 7 days, and weekly for 4 to 6 weeks or longer. To
avoid excessive changes in the soil-to-solution ratio, the number of samples
should be limited to three to four if large aliquots are needed. In contrast as
many as 12 or more reaction times may be carried out if radionuclides are
used.
Desorption is carried out using the method of successive dilutions and
commences immediately after the last adsorption time step. Each desorp-
tion step is carried out by replacing as much of the supernatant with the
background solution. The amount of decanted solution and that of the back-
ground solution added must be recorded for mass balance calculations. A
desorption step often consists of 24 to 36 hours of shaking for each step.
Desorption is repeated for several steps as desired. The total desorption
or release time depends on the number of desporption steps and the time
intervals between each desorption step. Examples of adsorption-desorption
results based on this method were described earlier and are presented in
Figure 2.13.
The decanted solution from each adsorption and desorption step is ana-
lyzed for the solute and the amount retained by the soil matrix is calculated.
Moreover, the remaining soil following the last step can be used for specia-
tion based on different fractionation procedures (see Zhang and Selim, 2005).
2.3.4 Thin Disk Method
In this method, a thin disk of dispersed soil (1 to 2 g) deposited on a porous
membrane is placed in a holder and connected to a piston-flow pump
to maintain a constant flow rate (see Figure 2.18). A fraction collector is
used to collect sample effluents. During adsorption, a pulse of solution
containing a solute having a concentration ( C i ) is introduced. The dura-
tion of the pulse varies and often is a function of sorption affinity and the