Agriculture Reference
In-Depth Information
Ion association
Multivalent ion hydrolysis
Gas-water
Ion exchange
Sorption
Mineral-solution
Mineral crystallization
usec
sec
min
hr dy
Time Scale
mo
yr
mil
FIGURE 2.2
Time ranges required to attain equilibrium by different types of reactions in soil environ-
ments. (From H. M. Selim and Amacher, M. C. 1997. Reactivity and Transport of Heavy Metals in
Soils . Boca Raton, FL: CRC Press. With permission.)
consecutively. The complexity of reactions in soils that occur over a time
continuum defies a simple analysis of the kinetics involved (Sparks, 2003).
Numerous methods have been developed to isolate and study the various
types of reaction that occur in soils. The choice of method largely depends
on the type of reaction to be studied, although some methods are applicable
to more than one type of reaction.
Selim and Amacher (1997) developed the following sequence of steps that
may be followed in a typical series of kinetic studies on heterogeneous sys-
tems such as soils:
1. Select the kinetic method(s) to be used for the reaction to be studied
(batch, flow, stirred flow methods, etc.).
2. Obtain the kinetic data under varying reactant concentrations, tempera-
ture, pH, ionic strength, and composition of other solution components.
3. Determine the rate function(s) from the experimental data using
initial rate, isolation, graphical, rate coefficient constancy, fractional
lives, or parameter optimization methods.
4. Propose mechanism(s) from experimental rate function and other data.
5. Test mechanism(s) by conducting experiments designed to eliminate
alternative mechanisms.
6. Refine or reject mechanism(s).
7. Perform additional experiments as needed to validate or eliminate
revised mechanism(s).
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