CSIRO PUBLISHING
www.publish.csiro.au/journals/ajsr
Australian Journal of Soil Research, 2005, 43, 261–267
Short Communication
Effects of heating and post-heating equilibration times
on soil water repellency
S. H. DoerrA,D , P. DouglasB , R. C. EvansB , C. P. MorleyB , N. J. MullingerB , R. BryantC ,
and R. A. ShakesbyA
A Department
of Geography, University of Wales Swansea, Singleton Park, Swansea SA2 8PP, UK.
of Chemistry, University of Wales Swansea, Singleton Park, Swansea SA2 8PP, UK.
C School of Chemical Engineering, University of Wales Swansea, Singleton Park, Swansea SA2 8PP, UK.
D Corresponding author. Email: [email protected]
B Department
Abstract. The effects of variation in heating temperature T (50–300◦ C), heating duration (20–60 min), and postheating equilibration times (24–168 h at 20◦ C and 50% relative humidity) on the wettability, as measured by the
Critical Surface Tension (CST) method, of 4 initially water repellent soils from Canada, Portugal, and the UK are
reported. All soils show an increase in water repellency following heating at temperatures in the range of 50 to
150◦ C, followed by a considerable decline after heating to 200–250◦ C, and, except for one soil, the eradication of
repellency after heating to 300◦ C. For two soils with a comparatively high organic carbon content and fine texture,
water repellency levels were also affected by the length of the post-heating equilibration period.
The results demonstrate that (i) the common practice of heating samples to 105◦ C does not provide a viable
standard procedure for the measurement of water repellency as it may alter repellency to different degrees, and (ii)
where heat treatment is required, a post-heating equilibration time of 24 h is not necessarily sufficient for sample
repellency levels to adjust to atmospheric laboratory conditions; therefore it is advisable to prolong equilibration to
at least one week prior to measurement.
Additional keywords: hydrophobicity, water repellence, water repellent.
Introduction
It has long been established that heating of potentially
water repellent soils, whether in situ or in the laboratory,
can influence their wettability. For example, drying soils at
temperatures (T) considerably above room T may increase
water repellency (e.g. Crockford et al. 1991; Dekker and
Ritsema 1994; Franco et al. 1994) and whereas some workers
dried soil samples at 20 or 25◦ C to avoid such effects
(Dekker et al. 1998; Doerr 1998), others have preferred to
use the standard procedure of drying soils at 105◦ C before
assessing repellency (e.g. Carter et al. 1994; Roy et al. 1999;
Franco et al. 2000).
Heating soils to temperatures in excess of 105◦ C prior to
assessments of water repellency is not a standard procedure,
but soils can be heated to much higher temperatures
during wildfires or prescribed fires. For a slightly water
repellent Californian chaparral soil heated for periods of 5–
20 min, Krammes and DeBano (1965) found that heating
up to 175◦ C caused generally little alteration in repellency,
whereas it increased considerably following heating between
175 and 200◦ C, but was destroyed by heating between
280 and 400◦ C. For lower temperatures, repellency increased
© CSIRO 2005
when samples were subjected to heating for increasing
periods (DeBano and Krammes 1966). Other studies have
also generally agreed that repellency disappears in this hightemperature range (e.g. Savage 1974; Scholl 1975; DeBano
et al. 1976; DeBano 2000; Robichaud and Hungerford
2000; Doerr et al. 2004). Nakaya (1982) found that the
repellency of 4 unspecified Japanese brown forest soils
increased to some extent after heating to 105 < T < 200◦ C,
increased steeply for 200 < T < 250◦ C, and declined
for T > 250◦ C.
It is not entirely clear why heat-induced increases in
repellency occur. It has been suggested that heating may
lead to improved alignment of hydrophobic molecules (Valat
et al. 1991), a more even distribution of repellent substances
over soil grains (Savage et al. 1972), or the migration of
repellent material from interstitial organic matter onto soil
grains (Franco et al. 1994). The latter has been supported
by micromorphological investigations showing an increased
formation of organic coatings on sand grains, induced by
sample drying at 105◦ C (Dekker et al. 1998). Increases in
repellency at higher temperatures have been attributed to the
polymerisation of organic molecules to produce materials
10.1071/SR04092
0004-9573/05/030261
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Australian Journal of Soil Research
S. H. Doerr et al.
that are both more hydrophobic (Giovannini 1994) and that
bond better to soil grains (Savage 1974). The reduction and
elimination of repellency near or above 300◦ C is thought
to be caused by the volatilisation and oxidation of organic
compounds (DeBano 1981; Chandler et al. 1983).
The differences in temperature thresholds for associated
effects on repellency reported in the literature may be a result
of differences in measuring methods, duration of exposure
to heat, type of organic matter, initial moisture content and
thermal properties of soil, or intrinsic differences in the
nature of water repellency between samples. For example,
the cooling effect of evaporating moisture prevents the
temperature from rising much above 100◦ C until most soil
water has been removed (Chandler et al. 1983). This limits the
portion of a prescribed heating period during which samples
tend to reach, and remain at, the prescribed temperature. The
effective thermal conductivity of soils also varies between
coarse and fine textured soils (Chandler et al. 1983). Similar
exposure to heat may thus lead to significantly different
thermal histories for, and within, different soil samples.
The observation that even at relatively low temperatures
water repellency increased with the length of time that
samples were held at the temperatures studied (DeBano
and Krammes 1966) suggests that equilibrium conditions
had not been reached, but rather that there is an approach
to equilibrium, which may take some time and probably
occurs at a temperature-dependent rate. Keeping soils at high
temperatures may allow the following processes to occur:
enhanced migration; redistribution; and structural changes
to the hydrophobic materials or other soil constituents.
Some of these processes may also be relevant and affect
repellency levels as the soil is allowed to cool, then progresses
towards a new equilibrium at the atmospheric humidity and
temperature of the laboratory at which water repellency tests
are carried out.
In order to explore these processes, this study investigates
the effect of: (i) duration of exposure to various temperatures,
and (ii) the duration of exposure to ambient atmospheric
conditions in which water repellency is assessed, on the
wettability of soils of diverse origins as measured by the
Critical Surface Tension (CST) method.
Materials and methods
Four bulk soil samples of different soil types and vegetation covers
from Canada (CA), Portugal (PT), and the United Kingdom (UK-T and
UK-D) providing a range of different water repellency levels and textures
were investigated (Table 1). These soils are naturally water repellent,
except for soil CA, which had been contaminated with crude oil in 1973
and developed repellency some time after contamination (see Roy et al.
1999 for details).
Preliminary sample treatment
Samples were air-dried at 20◦ C to equilibrium weight and passed
through a 2-mm sieve before further analysis. The coning and quartering
method (Jackson 1958) was used to homogenise the material and provide
subsamples of minimum variability.
Heat treatment and subsequent equilibration
Samples of soil (4 g) were placed in porcelain crucibles and placed in
a preheated muffle furnace in air at temperatures of 50, 100, 150, 200,
250, and 300◦ C for fixed times of 20, 30, or 60 min. After removal from
the furnace they were left to equilibrate at 20 (±1)◦ C and 50 (±5)%
relative humidity for 24, 72, 96, 144, or 168 h. Samples were then
subjected to CST tests in a controlled environment (20◦ C ±1, relative
humidity 50% ±5).
Water repellency assessments
Water repellency was determined using the Critical Surface Tension
(CST) test (Letey 1969; Roy and McGill 2002). The CST test as
carried out in this study involved placing a series of droplets of ethanol
solutions with decreasing surface tensions onto separate areas of the
soil surface until an ethanol concentration with a sufficiently low
surface tension was reached that allowed the droplet to infiltrate in
less than 5 s. The surface tension of this solution was then used to
classify the repellency of the sample. If distilled water (surface tension
∼0.073 N/m at 20◦ C) infiltrated in less than 5 s, the soil was classed as
wettable. King (1981) provided interpretation guidelines for repellent
soils according to which soils with approximate CST > 0.056 N/m
are classed as slightly, 0.056 < CST > 0.047 N/m as moderately, and
CST < 0.047 N/m as severely water repellent. Thus, repellency increases
with decreasing CST. In this study, a series of ethanol/water mixtures
with surface tension increments of ∼0.001 N/m was used. Selected
subsamples were examined in triplicate, and CST tests were carried
out in triplicate on heat-treated and on control samples, to provide an
assessment of random errors in procedures.
A summary of the main experimental procedures and data on
variability of results is given in Table 2. It may be useful to note
that sample CA had been investigated first and other samples were
then included in the experiments. Limited sample material and further
Table 1. Sample characteristics (TOC, total organic carbon) and water repellency levels (Critical Surface Tension,
CST and associated severity rating) for air-dry samples prior to treatments
It should be noted that CST levels of samples PT and UK-D are not very different, but nevertheless fall into different
categories of repellency severity
Sample
code
Soil type
(FAO)
Vegetation
type
Sampled
depth (m)
Texture
TOC
(% wt)
CST
(N/m)
Repellency
severityA
CA
PT
UK-D
UK-T
Black Chernozem
Humic Cambisol
Arenosol
Anthrosol
Agriculture
Eucalypt forest
Dune grassland
Sports turf
0–0.15
0–0.1
0–0.1
0–0.1
Silt clay loam
Sandy loam
Medium sand
Medium sand
6.8
6.2
0.6
0.4
0.037
0.048
0.045
0.054
Severe
Moderate
Severe
Moderate
A
Based on ethanol molarity data and associated classification of King (1981).
Short Communication
Australian Journal of Soil Research
Table 2. Summary of experimental treatments and replications
used
Sample
Heating
Equilibration CST-test Replicate samples for
code
times (min) periods (h)
repl.A
heating exptsB
CA
PT
UK-T
UK-D
A
B
20, 60
30, 60
30, 60
30, 60
24, 72, 144
24, 96, 168
24, 96, 168
24, 96, 168
No repl.
3
3
3
No repl.
No repl.
3
3
Max. standard deviation for replicate CST tests was 0.0017 N/m.
Max. standard deviation in CST for replicate heating tests was
0.0025 N/m.
development of the experimental protocol during the study resulted in
the differences in sample treatment and replication detailed in Table 2.
These differences are highlighted in the discussion where they may be
important in the interpretation of the results. In reading the following
sections, it should also be borne in mind that high CST levels indicate
low levels of water repellency and vice versa.
repellent soil (CA; initial CST ∼0.037 N/m), the drop in
CST is only c. 0.005 N/m, whereas for the least water
repellent soil (UK-T; CST ∼0.054 N/m), the drop in CST
is c. 0.010 N/m. There are, however, notable differences
in the detailed behaviour of the 4 soils when exposed to
higher temperatures.
Behaviour of the individual soils
Soil CA (Fig. 1a, b) shows the most consistent behaviour of
the 4 soils examined, with CST values differing by less than
c. 0.008 N/m for any of the individual temperature exposures
regardless of the time the sample is held at the elevated
temperature or the equilibration time prior to measurement.
However, there is some difference in the behaviour of the
samples that are equilibrated for 24 h compared with those
24 h
Results
Data quality
Variability between the replicated CST tests on untreated
subsamples did not exceed ±0.002 N/m. This level of
variability in CST results was not exceeded between
repeat treatments and between subsamples, and is of little
significance compared with changes in water repellency
arising from the experimental treatments described below.
Maximum standard deviations of CST results were
0.0017 N/m for replicate CST tests and 0.0025 N/m for
replicate heating tests (see Table 2).
70
144 h
65
60
55
50
45
40
CST (mN/m)
CST temperature curves for all soils and equilibration
periods and for the 2 different heating times used for each
soil are given in Figs 1–4 (a and b). For all soils studied the
general behaviour with respect to heating is similar to that
observed in previous studies summarised in the introduction.
Thus, for the heating durations examined here, an increase
in water repellency (i.e. a reduction in CST) occurs as the
temperature is increased from 20◦ C to c. 200◦ C, which is
followed by a significant reduction in water repellency for
samples heated in the temperature range 200◦ C to 300◦ C.
For samples CA, UK-T, and UK-D (60-min heating), this
reduction equates to the complete elimination of water
repellency at 300◦ C.
For all soils studied the CST temperature curves across
the temperature range 50–200◦ C (50–100◦ C for soil PT)
show little dependence upon either the time of exposure to
elevated temperatures or the equilibration period prior to
measurement. However, those soils with the lowest initial
repellency (i.e. moderate repellency, samples PT and UK-T)
show the greatest change in CST (i.e. increase in repellency)
across this temperature range. For the most severely water
72 h
(a)
75
General observations
263
35
30
(b)
75
70
65
60
55
50
45
40
35
30
0
50
100
150
200
250
300
Temperature (°C)
Fig. 1. CST as a function of drying temperature for (a) 20-min and
(b) 60-min exposures and for various post-heating and equilibration
periods (24, 72, and 144 h) for sample CA.
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Australian Journal of Soil Research
S. H. Doerr et al.
equilibrated for longer times. For the former the increase
in repellency associated with exposure to temperatures in
the 50–200◦ C range is not so apparent as for the latter
conditions. In addition, exposure to temperatures in the range
200–300◦ C followed by equilibration for only 24 h gives
samples with higher CST values than those undergoing the
same heat treatment but equilibrated for >24 h. Exposure
to a temperature of 300◦ C eliminated water repellency
irrespective of the treatment details.
Soils UK-T and UK-D are considered together because
of their similarity in behaviour (Figs 2a, b and 3a, b). For
these 2 soils the major difference in the CST temperature
curves is a consequence of the time the sample is exposed
to the elevated temperature rather than the length of the
equilibration period. Thus differences between equilibration
times for the same soil do not exceed c. 0.002 N/m for
24 h
96 h
temperatures between 50 and 200◦ C, and ∼0.005 N/m
for 250◦ C or 300◦ C. However, the time for which the
sample is exposed to the elevated temperature makes a
large difference to the resulting CST values, with exposure
for 60 min resulting in much higher CST values in this
latter temperature range. For 250◦ C, 60 min exposure is
sufficient to nearly or fully eliminate water repellency of
soil UK-T (CST 0.068–0.073 N/m), whereas for 30-min
exposure, a considerable level of repellency remains (CST
0.053–0.056 N/m). At this temperature, differences in
heating times on sample CST are of a similar order of
magnitude for soil UK-D (∼0.015 N/m).
Soil PT (Fig. 4a, b) shows distinctly different behaviour
compared with the other 3 soils studied and also shows
the most complex behaviour with respect to the various
treatments used. Relevant points to note are (i) a general
24 h
168 h
168 h
(a)
75
75
70
70
65
65
60
60
55
55
50
50
45
45
40
40
35
35
CST (mN/m)
CST (mN/m)
(a)
96 h
30
(b)
30
(b)
75
75
70
70
65
65
60
60
55
55
50
50
45
45
40
40
35
35
30
30
0
50
100
150
200
250
300
Temperature (°C)
Fig. 2. CST as a function of drying temperature for (a) 30-min and
(b) 60-min exposures and for various post-heating and equilibration
periods (24, 96, and 168 h) for sample UK-T.
0
50
100
150
200
250
300
Temperature (°C)
Fig. 3. CST as a function of drying temperature for (a) 30-min and
(b) 60-min exposures and for various post-heating and equilibration
periods (24, 96, and 168 h) for sample UK-D.
Short Communication
Australian Journal of Soil Research
24 h
96 h
168 h
(a)
75
70
65
60
55
50
45
CST (mN/m)
40
35
30
(b)
75
70
65
60
55
50
45
40
35
30
0
50
100
150
200
250
300
Temperature (°C)
Fig. 4. CST as a function of drying temperature for (a) 30-min and
(b) 60-min exposures and for various post-heating and equilibration
periods (24, 96, and 168 h) for sample PT.
decrease in CST values as the exposure temperature is
increased from 50◦ C to 200◦ C, although the data show greater
variability than that for the other soils studied; (ii) for the
measurement at 250◦ C, those 2 samples equilibrated for 96 h
give much higher CST values than those equilibrated for
longer; and (iii) even exposure to a temperature of 300◦ C
does not fully eliminate water repellency.
Discussion
The influence of heating observed by Crockford et al.
(1991), Dekker and Ritsema (1994), and Franco et al. (1994)
who treated soils at 105◦ C, and by Krammes and DeBano
(1965) and Nakaya (1982) who treated them also at higher
temperatures, generally falls within the domain of the
present data: exposure to temperatures <200◦ C tends to
265
increase water repellency by <0.010 N/m on the CST
scale, followed by a decline in repellency for exposure
to T ∼ 250◦ C and its elimination for T ∼ 300◦ C. Shorter
heating periods than those examined here may result in
a higher elimination temperature for water repellency, as
demonstrated by Doerr et al. (2004) in a study on some
Australian soils.
The observation that the largest relative increase in
repellency following heating in the range 20–200◦ C range
is associated with soils of initially lower levels of water
repellency (PT and UK-T) suggests that measurement after
drying at 105◦ C may artificially minimise differences in water
repellency levels for any soil samples under study.
For the 2 soils with the lower total organic carbon
(TOC) content and coarser textural composition (UK-T
and UK-D; see Table 1), the results show differences
primarily arising from the time the sample is held at
the elevated temperature; whereas for the other 2 soils the
major differences in behaviour arise from differences in
the post-heating equilibration time at 20◦ C and 50% relative
humidity prior to CST measurement. However, because
of the limited dataset available, without further study, we
cannot ascertain whether this correspondence in behaviour
of samples with related textural/TOC properties is of
wider applicability.
The reduction and elimination of water repellency at
T > 250◦ C is likely to be associated with the volatilisation
and oxidation of organic compounds (DeBano et al. 1976;
Chandler et al. 1983). The reason why heating to 300◦ C
did not eliminate water repellency from soil PT is not
easily explicable in terms of its bulk properties, as these
are intermediate to those of the other samples investigated
(Table 1). Its bulk properties also suggest that the thermal
conductivity of soil PT is likely to be intermediate to those
of the other samples. It might be that the organic compounds
responsible for water repellency in this particular soil,
which are thought to be derived mainly from eucalypt trees
(E. globulus; Doerr et al. 1998), are unusually heat- and
oxidation-resistant. We note, however, that previous work
by Doerr et al. (2003) has shown that exposure to 300◦ C
for 2 h did eliminate water repellency from a similar soil,
taken from the same area and with similar vegetation cover,
texture, and initial water repellency, which suggests that for
soil PT, a somewhat longer heating time would eliminate
repellency at 300◦ C.
The higher CST values (i.e. lower water repellency) of
CA samples equilibrated with ambient conditions (20◦ C and
50% relative humidity) for 24 h in comparison with those
equilibrated for 72 and 144 h, suggest that some heat-affected
aspects of the water repellent behaviour may be recovered
during the equilibration period. In contrast, the elevated CST
values (i.e. reduced repellency) of soil PT equilibrated for
the intermediate post-heating period (96 h) in comparison
with those obtained from both shorter and longer periods
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Australian Journal of Soil Research
(Fig. 4), and equivalent to CST ∼0.020 N/m suggest a nonlinear process occurring during equilibration, which warrants
further examination.
Although it might appear reasonable to assume that
24 h is a sufficient time period for samples to cool and
equilibrate with humid air, morphological rearrangement of
hydrophobic and hydrophilic compounds, and reorientation
of hydrophobic moieties (i.e. the non-polar tails of longchained hydrocarbons) on, and between, soil particles may
be slow, and affected by the intrusion and adsorption of
water vapour (Doerr et al. 2002). This may even result
in a non-linear ‘recovery behaviour’ of water repellency.
The samples investigated here display a wide variety
of heating and equilibration time dependent behaviour
that does not reflect their bulk properties but instead
reflects more subtle influences that may encompass their
detailed physical structure and the particular nature, and
perhaps, age and history of their organic constituents (see
also Doerr et al. 2005, this issue; Morley et al. 2005,
this issue).
Conclusions
Results from this study of the effects of exposure to elevated
temperatures for 20–60 min on the wettability of 4 water
repellent soil samples from Canada, Portugal, and the United
Kingdom are in general agreement with previously reported
observations in that there is an increase in water repellency
as the temperature to which they are exposed increased from
20◦ C to c. 200◦ C, which can be followed by a significant
reduction in water repellency for samples heated for such
durations in the temperature range 200–300◦ C.
Assessments of water repellency were also found to
be affected by the length of period for which heattreated samples were left to equilibrate to atmospheric
conditions present in the laboratory prior to water repellency
measurements. These effects were found to be dependent
on the temperatures used in the heat treatment, with
those resulting from T > 150◦ C being most pronounced. It
appears that following heat-treatments, a period of 24 h is
not necessarily sufficient for sample repellency levels to
equilibrate to atmospheric laboratory conditions. From these
results, the following implications are drawn.
(i) The common method of heating soil samples to 105◦ C
prior to water repellency assessments can lead to both an
overestimation of repellency levels as they would occur
naturally in dry soils in the field and also a reduction in
soil-to-soil variation in repellency levels.
(ii) The effects of heating repellent soils to higher
temperatures (T ≥ 150◦ C) described in previous studies
based mainly on soil samples taken in the western
USA (DeBano and Krammes 1966; Savage 1974; Scholl
1975; DeBano et al. 1976; Robichaud and Hungerford
2000) appear to be of wider applicability.
S. H. Doerr et al.
(iii) Where an experimental procedure requires heat
treatments, it is advisable to prolong the postheating equilibration period to controlled atmospheric
conditions prior to water repellency measurements to
at least 1 week. The changes in soil water repellency
that may occur during a post-heating equilibration period
could also be of importance where soils are examined
for wettability immediately following a wildfire.
Acknowledgments
This study was supported by EU grant FAIR-CT98-4027,
NERC Advanced Fellowship NER/J/S/2002/006622 and
Australian Research Council International Linkage Scheme
Grant LX0211202. This work does not necessarily reflect the
European Commission’s views and in no way anticipates its
future policy in this area. The authors are grateful to Julie
Roy, Imperial Oil Resources, Alberta, Canada, for kindly
providing sample material ‘CA’ and related information.
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Manuscript received 25 June 2004, accepted 24 December 2004
http://www.publish.csiro.au/journals/ajsr