kaolin > soil > montmorillonite. They reported stan-
and Vlasov 1973, Sletten 1988). Sletten (1988)
attributed the formation of aragonite, a relatively
dard Gibbs energies, enthalpies and entropies for
rare polymorph of CaCO3 (Doner and Lynn 1977),
ion exchange.
to a more favorable Mg2+/Ca2+ ratio in solution
brought about by the differential solubilities of Ca
Chemical transport
and Mg carbonates. Hallet (1976) demonstrated
When aqueous solutions freeze, solutes are large-
that freezing strongly concentrates solutes in
ly excluded from ice. As a consequence, solute con-
centrations are generally highest at the freezing
of CaCO3. Based on temperature-dependent solu-
front (Fig. 3). Kay and Groenevelt (1983) derived a
bility data, Hallet estimated the eutectic tempera-
simple equation:
ture for pure CaCO3 (0.34C). The relative
insolubility of minerals such as CaCO3 and CaSO4
Cf = Ci + 80 k Ci
(8)
make them difficult to study at subzero tempera-
tures because of the limited temperature range in
where Ci = original solute concentration
which their single-salt solutions can co-exist in
Cf = solute concentration at the frost front
equilibrium with solid phase ice. This is in marked
k = solute inclusion coefficient.
contrast to soluble chloride salts such as NaCl,
which has a eutectic temperature of 21.2C (Fig.
If there is no solute inclusion in the frozen zone (k
2), or CaCl2, which has a eutectic temperature of
= 1), then the solute concentration at the freezing
50.4C (Spencer et al. 1990, Marion and Grant
front could rise to 80 times the original concentra-
1994).
tion (assuming no salt precipitation). Such high
Ion exchange reactions play a major role in con-
concentrations can cause the freezing front to leap
trolling many physical and chemical properties of
over solute pockets (Hallet 1978, Kay and Groen-
soils, such as aggregation, pH buffering and ion
evelt 1983, Romanov and Levchenko 1989). These
transport. Hinman (1970) found that alternate
solute pockets may ultimately freeze, resulting in
freezing and thawing increased exchangeable NH4-
alternating bands of high and low concentrations
N and decreased exchangeable K; there was no
in frozen soils (Romanov and Levchenko 1989). Kay
change in the cation exchange capacity (CEC) or
and Groenevelt (1983) have argued that solute ex-
exchangeable Ca and Mg. Pulubesova and Shir-
clusion, which leads to narrow alternating high-
shova (1992) found no significant change in CEC
and low-solute bands in soils, may not be a signif-
or exchangeable Ca, Mg, K and Na in two soils and
icant mechanism for macroscale solute redistribu-
two clays (kaolinite and bentonite) following pro-
tion in soils.
longed freezing (two months) and prolonged thaw-
The dominant gradients that control the move-
ing (two months). Alternate cycles of freezing and
ment of solutes through freezing and frozen soils
thawing have led to both fixation of fertilizer K
are concentration, temperature and hydrostatic
pressure (Table 1, eq 7) (Cary and Mayland 1972,
and release of crystal lattice K from K-depleted soils
(Graham and Lopez 1969). Freezing increased ad-
Baker and Osterkamp 1988, Qiu et al. 1988, Perfect
sorbed bases (the pH increased), while thawing
et al. 1991). Solutes will diffuse from zones of high
increased soil acidity (the pH decreased) (Fedorov
concentration (e.g., the freezing front) to zones of
and Basistyi 1974). Deep freezing of soils with liq-
low concentration (Ficks Law, Table 1). In soils, sol-
uid N2, which boils at 196C, led to significant
utes will move from warm to cold regions (Soret
increases in the concentration of Ca, Mg and K in
Effect, Table 1) (Cary and Mayland 1972, Qiu et al.
solution following thawing (Iskenderov 1976).
1988). However, the direct effect of temperature
Surprisingly little work in soil science has ex-
gradients on solute movement in soils is generally
amined the role of subzero temperatures on chem-
insignificant (Cary and Mayland 1972). Much more
ical thermodynamic equilibrium constants for
significant is the movement of solutes with water
cation exchange. Tyutyunova and Antipov-
along hydrostatic gradients from warm zones to the
Karatatyev (1965) examined the Ca2+-K+ and the
freezing front (reverse osmosis, Table 1). Freezing
Mg2+- K+ exchange on montmorillonite, kaolin and
of water creates a strong thermodynamic sink, and
soil over the temperature range from +20 to 17C.
water will move in both the vapor and liquid (car-
Equilibrium constants at negative temperatures
rying solutes) phases to the freezing front (Cary and
were determined in wateralcohol mixtures. There
Mayland 1972, Gray and Granger 1986, Hofmann
was an increased adsorption of K with decreasing
et al. 1990). In this process, solutes may move
temperature, which fell in the relative order:
against a concentration gradient.
8
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