FreezeThaw Processes and Soil Chemistry
GILES M. MARION
INTRODUCTION
dT/dmB ≈ RT2/(55.5 ∆H)
(1)
This review will broadly examine the interac-
where mB = molality of solute B (mol kg1)
tions between freezethaw processes and soil
T = temperature (K)
chemistry, relying primarily on the empirical
R = gas constant (J K1 mol1)
record; the theoretical foundations for freezing and
∆H = enthalpy of fusion of ice (J mol1).
chemical interactions were discussed in another
report (Grant, in prep.). In the first part of this re-
This equation simplifies to:
port, I examine the effect of soil chemical proper-
ties on physical factors associated with freezing
∆T ≈ 1.860 (νmB)
(2)
and thawing, such as freezing-point depression,
where ∆T is in C and ν is the number of aqueous
unfrozen water content and frost heaving. The
second part of this report will examine the effect
species resulting from the dissolution of the solute
of physical factors such as freezethaw processes
(e.g., 1 for sucrose, 2 for NaCl, 3 for CaCl2) (Lewis
and subzero temperatures on chemical reactions
and Randall 1961). Therefore, the freezing point of
a 0.1-mol kg1 NaCl solution would be 0.37C lower
and chemical transport in soils. Then the effect of
freezethaw processes and subzero temperatures
than the corresponding freezing point of pure
on soil biology and soil remediation will be exam-
water.
ined. And finally I will examine theoretical mod-
Equation 2 is general for "dilute" solutions and
els for freezing and thawing processes that
holds for all chemical species regardless of their
explicitly include chemistry.
degree of dissociation. However, departures from
the limiting slope of 1.86C/νm are apparent at
concentrations > 0.2 νm (Fig. 1). These departures
EFFECTS OF CHEMISTRY ON
are most significant for electrolytes that dissociate
SOIL FREEZING AND THAWING
into ions of higher valences (i.e., MgSO4 > Na2SO4
> NaCl). Part of the explanation for the deviations
Two important ways in which soil chemical
from the limiting slope are attributable to ion pair-
properties influence soil physical properties in
ing between oppositely charged species, which de-
freezing and thawing environments are through
creases the true concentration of chemical species
freezing-point depression and unfrozen water con-
in solution (e.g., one MgSO0 ion pair instead of two
tent. These properties, in turn, affect water and
4
2+ and SO2 ions). In general, the
separate Mg
solute movement in frozen soils, which influenc-
4
strength of ion pairing is proportional to the prod-
es soil strength, frost heaving and contaminant
uct of the ionic charges (Davies 1962). Therefore,
transport.
di-divalent ion pairs such as MgSO0 are much
4
stronger than uni-univalent ion pairs such as
Soil moisture
NaCl0.
Solutes lower the freezing point of water. The
For a few salts, primarily highly soluble chlo-
freezing-point depression in dilute solutions is
rides and sulfates, it is possible to construct com-
given by the Van't Hoff equation (Lewis and Ran-
plete phase diagrams at subzero temperatures. A
dall 1961):