Sorption Kinetics - Transport and Fate of Chemicals in Soils: Principles and Applications - page 52

soil moisture or intermittent periods of wetting and drying, adsorption dur-
ing solute application (or spill) and subsequent desorption, and so on. These
laboratory measurements often involve mixing a few grams of soil with a
volume of solution (30 mL to 2 L) having a solute of known concentration for
a given equilibration time (hours to weeks).
In several methods, adsorption and desorption kinetics are quantified dur-
ing flow or transport rather than where mixing is dominant. The degree of
complexity of laboratory analyses and data interpretation varies extensively
among the different methods. Advantages and limitation of each method
and assumptions made are given in detail elsewhere (e.g., Sparks, 2003;
Amacher, 1991; Selim and Amacher, 1997).
It is important to stress that data from column experiments are distinctly
different from data obtained batch experiments (Miller, Sumner, and Miller,
1989; Hodges and Johnson 1987). Therefore, prediction of reactive solute
transport in soil columns based on batch experiments may yield inaccurate
results (Persaud, Davidson, and Rao, 1983), and it is thus recommended that
kinetic retention be quantified in flow systems.
Inasmuch as batch-type experiments are commonly carried out at high
solution-to-soil ratios, the data may not be applicable to transport processes
(Green and Obien, 1969; Dao and Lavy, 1978). An alternative to batch experi-
ments are short columns or thin disk flow experiments. Miller, Sumner, and
Miller (1989), Hodges and Johnson (1987), and Akratanakul et al. (1983) showed
that batch results underestimated the extent of ion sorption on soils and miner-
als when compared with short columns. The use of long columns for determi-
nation of equilibrium parameters has been practiced extensively by Schweich,
Sardina, and Gaudet (1983) and Selim and Amacher (1997) for ion exchange.
Furthermore, solutes that are highly reactive require a long leaching time in
order to reach the input concentration at the effluent. The mixing effects in the
flow direction (hydrodynamic dispersion) need to be accounted for.
In this section, selected methods of solute adsorption and desorption are
described briefly. Emphasis here is on the simplicity of each method and
frequency of its use.
2.3.1 One-Step Batch
This is the classical method of measuring adsorption parameters such as
K d , K f , and Langmuir parameters. Here predried and ground soil sample
(or sediment) is mixed with a tenfold volume of solution with the solute of
interest in a test tube or a centrifuge tube (40 to 100 mL). The suspension is
subsequently placed in a shaker for some time (hours or days). The solution
is separated by filtering or centrifugation and the concentration of the sol-
ute in solution and in the solid phase is measured. This procedure is often
repeated for different solute concentrations in order to provide a representa-
tive adsorption isotherm similar to those presented earlier. The range of con-
centrations and number of input (or initial concentrations) C i used vary from