Agriculture Reference
In-Depth Information
as tritium or bromide. These methods are also used by scientists dealing
with transport phenomena of various reactive chemicals such as agricultural
chemicals, organics (pesticides and industrial compounds), various inorgan-
ics, radionuclides, and explosives.
Since the transport of reactive solutes is directly influenced by its affin-
ity and various chemical reactions with matrix surfaces, miscible displace-
ment methods are best regarded as indirect means to quantify the affinity of
adsorption and desorption processes in porous media. The experimental set-
up for miscible displacement experiments is similar to that for the thin disk
method. The only exception is that a column of length L of soil or porous
media is packed prior to its attachment to the pump and fraction collector as
shown in Figure 2.20. Acrylic columns are commonly used and uniformly
packed with air-dried soil then slowly water-saturated with a background
solution at a low Darcy flux using a variable-speed piston pump. Once satu-
ration is achieved, the flux is adjusted to the desired flow rate. A background
solution is used to maintain constant ionic strength, prior to the introduction
of a solute. In most studies, the application of solute pulse is maintained for
an extended period. However, it is much more desirable to introduce a finite
volume of the input solute pulse (i.e., for a specified duration), which is sub-
sequently followed by solute-free solution. Introduction of free-solute solu-
tion induces release or desorption of retained solute. Solute concentrations
Soil
Column
Heavy Metal Solution
Piston Pump
Fraction Collector
FIGURE 2.20
Miscible displacement solute transport experimental setup. Background solution and solute
are pumped upward from the reservoir through a water-saturated soil column and are col-
lected as aliquots by the fraction collector. Separation of solid and aqueous phases is accom-
plished by a filter at the outlet end of the soil column.
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