minerals to precipitate more readily upon freezing,
sand than in pure solutions. Baker and Osterkamp
causing an unfavorable Na/(Ca+Mg) ratio in the
(1988) also found that the amount of salt rejection
solution, which could result in the dispersion of clay
from the freezing region increased with decreas-
minerals and a sharp decrease in infiltration, prob-
ing freezing rate. Solute pockets that initially form
lems generally associated with Na-dominated soils.
in soils during freezing may ultimately freeze, re-
Actually, at 0C, epsomite (MgSO4 . 7H20) is four
sulting in alternating bands of high and low salt
times as soluble as mirabilite (Richardson et al.
concentrations (Romanov and Levchenko 1989). On
1990). At freezing temperatures, CaSO4 . 2H2O (gyp-
the other hand, Baker and Osterkamp (1988) found
sum) should first precipitate from solution during
no evidence for salt banding in their freezing
freeze-concentration of equal molal solutions, fol-
experiments. Solute exclusion that leads to alter-
lowed by mirabilite and finally epsomite. De Jong
nating bands of high and low salt concentrations
(1981) examined the freeze-purification of saline
may not be a significant mechanism for macroscale
groundwater as a source of water for reclaiming
solute redistribution in soils (Kay and Groenevelt
saline soils. The initial 30% of the meltwater had a
1983). A more thorough discussion on solute ex-
higher salt content than the original solution, and
clusion is given in the section on Chemical trans-
the last 70% had a lower salt content. The results
port.
suggest that freeze-purification of saline waters
Iskandar and Jenkins (1985) examined the poten-
could be used in reclaiming saline soils by:
tial use of artificial ground freezing for contami-
Collecting and disposing of the initial meltwa-
nant immobilization. Frozen metal-contaminated
ter and leaching with the later meltwater; or
soils eliminated metal leaching to groundwater.
Using all the meltwater, which is a procedure
Freezing the soil from the bottom apparently en-
similar to the high-salt-water dilution method
hanced upward movement of volatile organics to
of reclaiming saline soils (De Jong 1981).
the soil surfaces, where losses occurred by volatil-
Freezethaw processes may have both adverse
ization. The amount lost depended on the mobility
and beneficial effects on contaminant transport
of specific organics and ranged from 90% for chlo-
through soils (Iskandar and Jenkins 1985, Iskandar
roform, benzene and toluene to as low as 45% for
1986, Henry 1988). Adverse effects include frost
tetrachloroethylene.
heaving and reductions in the durability of clay lin-
Ayorinde and Perry (1990) examined the utility
ers, both of which can cause increased leakage from
of freezing to move explosive compounds through
hazardous waste landfills. Additionally, freezing
soils. For one freeze cycle, they found a 40%
causes solute exclusion, which pushes contaminants
reduction in concentration for meta-nitrotoluene
out of soils, potentially contaminating groundwa-
(M-NT) and orthonitrotoluene (O-NT) and less than
ters. Ways in which freezing can be used for benefi-
a 20% reduction for 2,6-dinitrotoluene (2,6-DNT)
cial uses include dewatering hazardous materials,
in frozen soil layers compared to unfrozen controls.
At an average freeze rate of 0.4 cm day1, statisti-
building ice walls to contain contaminants, freez-
ing of entire soil blocks to immobilize contaminants
cally significant movement was observed for M-
NT and O-NT but not for 2,6-DNT. They postulated
and freezing to push contaminants out of soils.
that for a given freeze rate, number of freezethaw
Whether solute exclusion of contaminants is a ben-
cycles, type of soil and level of soil moisture, the
eficial or adverse effect of freezing depends on the
ability to move contaminants through soils by freez-
control of the unfrozen effluent.
ing strongly depends on the type of contaminant,
Since solute exclusion has been suggested as a
the initial concentration level and soilcontaminant
possible mechanism for decontaminating soils
interactions.
(Iskandar and Jenkins 1985, Iskandar 1986, Ayor-
In another laboratory study, Ayorinde et al.
inde et al. 1988), factors controlling the process are
(1988) showed significant mobility of volatile
critical for evaluating practical applications. A crit-
organics, such as benzene, chloroform and toluene,
ical factor controlling the exclusion of solutes from
through a Lebanon silty soil frozen from the bot-
ice is the rate of freezing. Romanov and Levchenko
tom up. They found a 2567% reduction in contam-
(1989) noted a marked increase in the effectiveness
inant concentration in the frozen soil but no
of exclusion with a decrease in the cooling rate from
0.5 to 0.25C/day. These workers also found that
corresponding increase above the freezing front.
They attributed this discrepancy to contaminant
the effectiveness of solute exclusion increased with
losses through volatilization, biodegradation and
increasing solute concentration. In addition, the ef-
sorption.
fectiveness of solute exclusion was much less in a
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