Journal of Contaminant Hydrology 144 (2013) 46–57
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Journal of Contaminant Hydrology
journal homepage: www.elsevier.com/locate/jconhyd
Wettability contrasts between fresh and weathered diesel fuels
Stephanie S. Drake 1, Denis M. O'Carroll ⁎, Jason I. Gerhard
Department of Civil & Environmental Engineering, The University of Western Ontario, London, ON, Canada N6A 5B8
a r t i c l e
i n f o
Article history:
Received 19 May 2012
Received in revised form 11 September 2012
Accepted 28 September 2012
Available online 10 October 2012
Keywords:
Wettability
NAPL
Remediation
Retention functions
a b s t r a c t
The remediation of non-aqueous phase liquid (NAPL) contaminated sites is impeded due to
subsurface complexities, including wettability. Wettability quantifies which of two immiscible fluids
preferentially coats a solid. At most contaminated sites water-wetting conditions are typically
assumed despite mounting evidence that this is not always the case. In this study, wettability was
examined for two NAPL samples of contrasting origin: a fresh and a field sample. Wettability was
assessed through (i) cyclical, ‘cumulative elapsed contact time’ intrinsic contact angle measurements, (ii) interface jar tests, and (iii) cyclical, pseudo-static capillary pressure–saturation curves.
The work as a whole demonstrated that while the fresh diesel sample was consistently water-wet,
the field diesel sample exhibited repeatable cycles of wettability reversal between water drainage
and imbibition. And while wettability hysteresis increased with contact time for the field diesel, the
occurrence of wettability reversal at each change of saturation direction was independent of contact
time. Such behavior is not easily assessed by standard wettability indices. Moreover, it contrasts
with the permanent wettability alteration observed for complex organics (e.g., coal tar) observed in
most studies. It is hypothesized that the cyclical wettability reversal is related to cyclical changes in
intermediate pore wettability due to sorption of surface active compounds (causing NAPL-wetting
imbibition) and rupturing of the soil grain water film (causing water-wet drainage). The wettability
differences between the two NAPLs may be due to additives (i.e., a surfactant) in the original
formulation and/or byproducts from subsurface weathering. These results support better
characterization of site-specific wettability, improved model development and more realistic site
conceptual models for improved remediation efforts.
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
Remediation efforts at Non-Aqueous Phase Liquid (NAPL)
contaminated sites often do not reduce the contamination to
health-based standards (National Research Council, 1999). This
may be due to an incomplete understanding of subsurface
heterogeneities, including wettability (Dwarakanath et al.,
2002). Wettability is the affinity of one fluid for a given solid
in the presence of another immiscible fluid (Craig, 1971). Many
aquifers are composed primarily of quartz sand or other
naturally hydrophilic materials (Powers et al., 1996), therefore
⁎ Corresponding author. Tel.: +1 519 661 2198; fax: 519 661 3942
E-mail address: [email protected] (D.M. O'Carroll).
1
Now at WorleyParsons, Calgary, AB, Canada T3B 5R5.
0169-7722/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jconhyd.2012.09.008
it is commonly assumed in conceptual models of contaminated
sites that the porous medium is strongly water-wetting. This
strongly water-wet condition is herein referred to as ‘ideal
wettability’. There is mounting evidence, however, that the
assumption of ideal wettability may not be appropriate at a
significant fraction of contaminated sites (Dwarakanath et al.,
2002; Harrold et al., 2001, 2003; Jackson and Dwarakanath,
1999; Powers et al., 1996; Zheng and Powers, 1999).
Wettability can be quantified at the scale of a single interface
by measuring the intrinsic contact angle (θint), which is
dependent on the interfacial tension (IFT) between the immiscible fluids. A surface is typically defined as water-wetting for
(approximately) θint b 60° to 75° (measured through the aqueous
phase), NAPL-wetting for θint >105° to 120° and intermediatewet for 75°b θint b 105° (Anderson, 1986). For air/water/quartz,
advancing contact angles (i.e., air displacing water) are small
S.S. Drake et al. / Journal of Contaminant Hydrology 144 (2013) 46–57
(e.g., 2°). Other materials such as shales, coal, graphite, talc,
dolomite and calcite exhibit significantly higher air/water
contact angles (e.g., 33° to 82°) (Ryder and Demond, 2008).
And, while wettability is partly dependent on porous medium
mineralogy, it has been suggested that fluid composition has the
greatest influence on wettability in the saturated zone (Harrold
et al., 2001; Ryder and Demond, 2008). For example, the addition
of surfactants to an air/water/quartz system can significantly
increase measured contact angles (e.g., ~47°) (Desai et al., 1992;
Lord et al., 2000). Pure NAPL/water/quartz contact angles
reported in the literature range from 5° to 72° (Barranco et al.,
1997; Demond and Roberts, 1991; Powers et al., 1996) while
NAPLs containing surfactants exhibit larger contact angles
(e.g., 15° to 180°) (Demond et al., 1994; Lord et al., 1997a;
Molnar et al., 2011; Powers and Tamblin, 1995). Contact angles
measured for field or waste NAPL samples on quartz have also
shown a wide range of contact angles: from 12° to 163° (Harrold
et al., 2001; Powers et al., 1996). Wettability is further dependent
on the order of fluid contact: a typically water-wet system may
be NAPL-wetting if the NAPL phase contacts the solid first (Ryder
and Demond, 2008).
The macroscopic effects of non-ideal wettability (i.e., at the
scale of a representative elementary volume, REV), are apparent
in the capillary pressure–saturation (Pc–S) relationships. As a
fluid/fluid/solid system becomes less-water wet, the capillary
pressure required to achieve a given saturation decreases,
compressing the Pc–S curve (Bradford and Leij, 1995a;
O'Carroll et al., 2005; Powers and Tamblin, 1995; Powers et al.,
1996). This compression is typically accounted for by modifying
standard Leverett (1941) Pc–S scaling to include a contact angle
term (Bradford and Leij, 1995a,b; Demond and Roberts, 1991;
O'Carroll et al., 2005). This contact angle is defined as the
operative contact angle (θop) and is typically less than the
intrinsic contact angle. Pc–S scanning curves are less commonly
explored, particularly with respect to wettability, but can reveal
valuable characteristics associated with governing surface
chemistries and pore-filling behavior on saturation reversals
(Gerhard and Kueper, 2003; Kovscek et al., 1993).
NAPLs that are used as solvents and fuels typically contain
compounds added to improve their performance. These additives can impact interfacial behavior, specifically those with an
amphiphilic structure (i.e., surfactants), which can sorb onto the
solid surface, altering surface charge. For example, polybutene
amine, the active ingredient in products added to fuels to clean
the carburetor and fuel injector, has rendered quartz surfaces
neutral wet in the presence of synthetic fuel and water (Powers
and Tamblin, 1995). Furthermore, spent solvents and fuels are
likely composed of a wide range of constituents that they came
into contact with during their useful life. Chlorinated solvents
have been used as degreasers and in the dry cleaning industry
where surfactant use is common to improve detergency. These
additional constituents can have a significant impact on solid
phase wettability, rendering the solid surface intermediate to
NAPL-wet (Demond et al., 1994; Harrold et al., 2001, 2005; Hsu
and Demond, 2007; Jackson and Dwarakanath, 1999; Molnar et
al., 2011). Similarly crude oils, coal tars and creosotes, which are
comprised of a wide range of constituents, frequently wet
quartz surfaces. This has been attributed to the presence of high
molecular weight asphaltenes with polar functional groups that
reduce adhesive forces (Barranco and Dawson, 1999; Dong et al.,
2004; Hugaboom and Powers, 2002; Nelson et al., 1996; Powers
47
and Tamblin, 1995; Powers et al., 1996; Ren et al., 2009; Zheng
and Powers, 2003; Zheng et al., 2001).
Weathering processes such as leaching, volatilization and
biological decay are all mechanisms by which a NAPL can
change composition in the subsurface and potentially induce
non-ideal wettability. For example, weathering processes
may degrade lighter compounds of petroleum-derived products, leaving behind heavier hydrocarbons that are known to
alter wettability (Cuiec, 1990; Powers et al., 1996). As well,
oxidation of NAPL systems has been shown to shift waterwetting systems to intermediate- and NAPL-wetting conditions
(Cuiec, 1990; Ren et al., 2009).
These studies provide insight into how additives in fuels or
weathering of waste/field NAPLs can lead to non-ideal wettability behavior. However a direct correlation between constituents present (or absent) in waste/field NAPLs and wettability
alterations is still unclear. This is complicated by the significant
number of constituents typically present and the potential for
synergistic effects of these constituents impacting wettability.
For example, the presence of both organic acid and base
surfactants in a NAPL/water/quartz system has been found to
alter interfacial activity more than the sum of the individual
effects (Hsu and Demond, 2007; Spildo et al., 2001). With
multiple additives and impurities in field systems, as well as
complex release histories for many field NAPLs, it is therefore a
significant challenge to a priori predict the wetting behavior of a
given NAPL.
The purpose of this study was to investigate the wettability
of a diesel/water/quartz system and, in particular, the contrast
between a weathered field diesel and a fresh diesel. Wettability
was investigated at (i) the pore scale via cyclical advancing and
receding intrinsic contact angle measurements, (ii) at the scale
of an interface through time lapse and equilibrium bottle tests,
and (iii) at the REV scale through multiple, hysteretic, Pc–S
experiments including detailed scanning curves. The interfacescale analysis was complemented by testing of fluid and
interfacial matter composition. The REV scale analysis was
augmented through Leverett scaling of different fluid-fluid
pairs and Pc–S curve fitting. This comprehensive analysis of this
single fluid pair aims to provide a holistic picture of (i) the
wettability that may be expected for a petrochemical recovered
from the field, (ii) the types of measurements that are most
indicative of the observed behavior, and (iii) the potential
underlying causes of that behavior. The potential applications
of the information obtained include improving both numerical
simulations and remediation of contaminated sites.
2. Materials and methods
2.1. Materials
Two diesel fuel oils were used as the non-aqueous phases in
the experiments. A field diesel was obtained from a monitoring
well at a contaminated site in London, Ontario. The NAPL was
stored in a tightly sealed container in absence of light at 5 °C
from time of sampling until time of equilibration for testing to
ensure the sample maintained its properties at representative
aquifer conditions. The fresh diesel was obtained from a gas
station (Drummond Fuels Ltd., Ottawa, ON). Equilibration of the
diesels with the aqueous phase occurred at room temperature
(25°), the same temperature employed for experiments. The
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S.S. Drake et al. / Journal of Contaminant Hydrology 144 (2013) 46–57
specific gravity of the NAPLs was measured with a pycnometer.
The aqueous phase used in all experiments was distilled,
de-ionized water (Milli-Q filters, Millipore, Billerica, MA). As
aqueous phase ionic strength has been known to impact IFT
(Barranco et al., 1997; Lord et al., 1997b; Standal et al., 1999), IFT
measurements were additionally taken for the field diesel and
0.01 M solution of NaCl. The average measured IFT of the field
diesel/0.01 M NaCl solution was within the 95% confidence
interval of the average measured IFT for field diesel/Milli-Q
water.
To obtain reproducible primary drainage Pc–S data, the
field diesel was pre-filtered through a 0.45 μm filter to
remove particulate matter. Initial experiments with the field
diesel were conducted with unfiltered samples, causing the
upper hydrophobic membrane (through which the NAPL
must pass before it enters the porous medium) to act as a
filter, removing any particulate matter. This resulted in
partial clogging of the upper membrane, thus NAPL pressure
readings measured by the pressure transducers on primary
drainage were not representative of pressures in the porous
medium. Once the diesel had entered the porous medium it
was essentially filtered and therefore imbibition data was
consistent for both filtered and unfiltered samples.
Quartz was used as the solid phase as it is a ubiquitous
aquifer mineral and has been widely studied. This ensured
the focus of the investigation was on wettability alterations
due to NAPL properties alone. Two silica sand size distributions (Opta Minerals Inc., Waterdown, ON) were used in the
capillary pressure–saturation experiments: (i) 50% by weight
mixture of F32 and F50 for experimental system validation
(mean grain size = 0.42 mm, uniformity index = 2.3), and
(ii) F70 for detailed analysis (mean grain size = 0.18 mm,
uniformity index = 1.6) (Camps-Roach et al., 2010). Contact
angles were completed on smooth quartz plates (Alfa Aesar,
Ward Hill, MA). All glassware and apparatus were cleaned
using established procedures (Lord et al., 1997b) to eliminate
the presence of impurities that may impact interfacial
behavior.
2.2. Weathering
GC–MS analysis was used to establish the degree of
weathering by determining the presence of diesel constituents.
An Agilent 7890 GC with a DB-5 column (30 m × 0.25 mm i.d.,
0.25 μm thickness) was coupled with an Agilent 5675 MSD.
The GC–MS method used was based on the weathered diesel
analysis method of Lang et al. (2009) for GC–MS analysis. Fresh
and field diesel samples were diluted with dichloromethane
(DCM) using a 5:1 ratio.
2.3. Interfacial analysis
Bottle tests were performed to observe temporal changes
in the diesel–water interface in the absence of a porous
medium. Vials were suspended over the camera (Nikon D80)
so the interface was directly above the field of view. The 3 mL
vials contained 1 mL of de-ionized water and 2 mL of diesel
that was filtered through a 0.2 μm syringe filter. Photographs
were taken at two-hour intervals over a period of two weeks.
Additional tests were performed to determine if microorganisms were accumulating in the vials. An inoculation loop was
used to streak samples from the jar tests onto LB-Agar plates
and left to incubate at room temperature for 13 days.
2.4. Interfacial tension and contact angle
Interfacial tension and intrinsic contact angle measurements
were obtained using an Axisymmetric Drop-Shape Analysis
system (First Ten Angstroms, Portsmouth, VA). The system
employs Young–Laplace drop shape analysis applied to high
resolution photographs of light non-aqueous phase liquid
(LNAPL) drops in water; a j-shaped needle was used to float an
LNAPL bubble in water(for IFT) or to introduce an LNAPL bubble
against the bottom of a quartz plate (for contact angle). All
measurements for both IFT and contact angle were conducted
with systems equilibrated in a consistent manner. A beaker with
30 mL of the aqueous phase containing a quartz plate and an
inverted vial with 2 mL of LNAPL was allowed to equilibrate for
at least 72 h. This setup allowed equilibration between all three
phases while still allowing the quartz plate to be removed from
the aqueous phase without contacting the NAPL. This setup
ensured the contact angle measurements were analogous to the
initially water-saturated Pc–S experiments (corresponding to
NAPL encountering an uncontaminated aquifer).
LNAPL contact angles were achieved by repeatedly advancing and receding an LNAPL drop on the same area of a quartz
plate. Between advancing and receding contact angle measurements, LNAPL drops were left to equilibrate on the quartz
plate. Advancing and receding contact angle measurements
were taken at multiple equilibration time intervals to investigate temporal contact angle hysteresis. At each time interval, at
least five measurements for each of the advancing and receding
contact angles were taken and averaged. In cases where the
measurements were not consistent (e.g., the receding contact
angle varied as the drop receded) or a single value was not
easily determined (e.g., due to imperfect resolution of the
intersection of the contact line and surface) the maximum and
minimum possible contact angles are reported. This cyclical
and ‘cumulative elapsed contact time’ contact angle measurement method was developed specifically to imitate the cyclical
saturation reversals experienced by the quartz sand in the Pc–S
experiments (i.e., NAPL contact times in the porous media
varied from experiment to experiment, particularly when
conducting scanning curve experiments).
2.5. Capillary pressure–saturation experiments
Capillary pressure–saturation experiments were conducted
using a membrane-based stainless steel pressure cell based on
an established design (Salehzadeh and Demond, 1999). An air
pressure regulator (Model 44-2, Moore Products, Spring House,
PA) was used to control NAPL pressure in a reservoir connected
to the top of the pressure cell (Fig. 1). Stainless steel porous
plates (20 μm pores and 1.6 mm thickness, Mott Corp.,
Farmington, CT) served as structural support for the upper
hydrophobic membrane (PTFE with 0.45 μm pores, Pall Corp.,
Port Washington, NY) and the lower hydrophilic nylon
membrane (0.2 μm pores). The cells were wet-packed in
three layers using Milli-Q water that had equilibrated with
the NAPL for at least 72 h using a method similar to that used
by Chen et al. (2007). 25 pore volumes of the aqueous solution
S.S. Drake et al. / Journal of Contaminant Hydrology 144 (2013) 46–57
49
Fig. 1. Experimental setup for running two parallel, pseudo-static capillary pressure–saturation experiments.
were then flushed through the packed cell using a syringe
pump (kd Scientific, Holliston, MA).
Pc–S data was obtained using an automated pseudo-static
system (Chen et al., 2007). The NAPL pressure was gradually
increased by increasing the air pressure above the NAPL surface
(Fig. 1) at a slow and constant rate (0.2 cm water/min). Pressure
readings were collected every 10 s using gauge pressure
transducers (FP2000, Honeywell, Morristown, NJ) connected
to a datalogger (CR3000, Campbell Scientific, Edmonton, AB).
The pressure transducers were used to quantify both phase
pressure and saturation (Chen et al., 2007). Pc values were
averaged over 0.5% saturation bins, and each resulting Pc data
value (composed of, on average, 12–15 measurements) was
reported at the midpoint of each saturation bin. A complete Pc–S
primary drainage/main imbibition curve was completed in
approximately 15 h. The system was validated by demonstrating that the Pc–S curve for a silicone oil/water/F32-50 sand
system determined by a standard static experiment lay within
the 95% confidence interval about the mean of four repeat
pseudo-static Pc–S curves (Drake, 2010).
Six experiments consisting of primary drainage and main
imbibition curves were completed for the fresh diesel with F70
sand. In addition, two of the six experiments included scanning
loops (6 loops over 3 unique saturation paths and 21 loops over
9 unique saturation paths, respectively). For the field diesel,
eight experiments with F70 sand were completed: five with
unfiltered and three with filtered field diesel; this resulted in
reliable data for three drainage and eight imbibition Pc–S
curves. One of the eight experiments included scanning curves
(13 scanning loops over 11 unique saturation paths).
All data was plotted as apparent saturation (Bradford et al.,
1998) to accommodate curve-fitting and comparison between
data sets; this is reasonable because the focus in this work is on
significant differences in capillary pressure values characterizing the majority of the Pc–S main branches and not on the
details of the residual water or NAPL at the curve endpoints. A
modified form of the van Genuchten (1980) equation, incorporating a translation parameter (η) that satisfies (Pc + η) ≥0
for all Pc , enabled fitting of both the positive and negative Pc
values observed (Bradford and Leij, 1995a). The best-fit van
Genuchten parameters were determined by the method of
O'Carroll et al. (2005), which minimized the root mean square
difference of saturation values, putting importance on the
intermediate saturation range.
Operative contact angles, quantifying the hysteresis between
drainage and imbibition for the measured Pc–S imbibition
curves, were determined through modified Leverett scaling.
The fresh diesel/water drainage curve, taken as the reference
curve, was scaled to the imbibition curves via the modified
Leverett–Cassie equation (O'Carroll et al., 2005):
αβ
Pc
app Sα
¼
αβ
γ α
ref
app cos θop P c Sw
ref
γ
ð1Þ
where Pcαβ(Sαapp) is the Pc for a given imbibition curve (fluids α
and β) at apparent saturation of the α phase corresponding to
app
the reference Pc at the same saturation (Pcref(Sw
)),γαβ is the IFT
α
between fluids α and β, γref is the reference system IFT and θop
is
the operative contact angle through the α (wetting) phase. The
Leverett–Cassie equation was developed for systems with
heterogeneous wettability, however it reduces to Eq. (1) when
the porous medium is homogeneous.
3. Results and discussion
3.1. Weathering analysis of diesels
The GC/MS chromatograms for the two diesel samples are
consistent with expectations given their origins. The fresh
diesel chromatogram (Fig. 2) reveals the strong presence of
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S.S. Drake et al. / Journal of Contaminant Hydrology 144 (2013) 46–57
Fig. 2. Fresh diesel GC/MS chromatogram illustrating major alkane species occurring at regular intervals across chromatogram.
straight-chained alkanes (n-alkanes) that peak at regular intervals over the chromatogram range (Christensen and Larsen,
1993); in particular, the major components present are the
twelve dominant alkanes ranging from n-C10 to n-C21, which
are typical of a diesel fuel (Galperin and Kaplan, 2008). In
contrast, the field diesel chromatogram (Fig. 3) exhibits
prominent peaks for pristane and phytane while the n-C17
peak is absent and the n-C18 peak is relatively small. While
n-alkanes such as n-C17 and n-C18 are subject to biodegradation, their closely related isoprenoid compounds – pristane and
phytane – are not; for this reason, the ratio of n-C17 to pristane is
typically correlated to the age of a diesel spill (Christensen and
Larsen, 1993). Conservative estimates suggest that n-C17 is
expected to be completely degraded after two decades in the
subsurface, although this can occur in as little as 5–6 years under
ideal, aerobic conditions (Hurst and Schmidt, 2005). The absence
of n-C17 in the field diesel chromatogram (Fig. 3) suggests that
this diesel was significantly weathered. Oxidation products were
not observed in either chromatogram. Refined petroleum
products commonly have stabilizer additives, included to inhibit
oxidation of the fuels during storage (Song et al., 2000). These
additives may be present in either or both diesel samples.
3.2. Interfacial analysis
Bottle tests conducted in the absence of porous media
revealed that a white matter accumulated at the interface
between diesel and water for both diesel types. No interfacial
matter was observed for control samples (i.e., diesel not in
contact with water) at the same temperature. Samples were
filtered prior to each test and no interfacial matter was initially
present. Moreover, results revealed that the field diesel sample
developed significantly more interfacial matter than the fresh
diesel and the matter appeared sooner. The first appearance
of the interfacial matter was at 7.5 h for the field diesel,
contrasted with 5 days for the fresh diesel. The interfacial
matter developed at the lowest point of the interface (visible as
a bright white dot) but spread out over the interface with time
(appearing as a white film and resulting in the diesel looking
cloudy). On the timescale of a Pc–S experiment (20 h to
S.S. Drake et al. / Journal of Contaminant Hydrology 144 (2013) 46–57
51
Fig. 3. Field diesel GC/MS chromatogram illustrating major alkanes at regular intervals across the chromatogram with C17 notably absent, and pristine and
phytane exhibiting significant peaks.
3 days), and specifically the primary drainage curve (7 h), it is
expected that only the field diesel Pc–S results may be impacted
by the development of this interfacial matter.
The formation of interfacial matter has also been observed
in other studies. Field samples of black diesel and coal tar
formed semi-rigid interfacial films impeding accurate contact
angle measurements (Powers et al., 1996). Similar films have
been observed for waste trichloroethylene (Harrold et al.,
2001), crude oils and field coal tars (Nelson et al., 1996).
Nelson et al. (1996) concluded that the binding of water
molecules to coal tar molecules created this film which acted
as a very thin emulsion layer. They did not see any functional
groups typical of surfactant compounds in GC/MS, NMR and
IR spectra, however they note that very small concentrations
would not be detected. The white interfacial matter in this
study did not form a continuous, semi-rigid film as observed
with the heavier NAPL samples discussed above. Considerable effort was expended attempting to identify the precipitate. Isolating the matter from the diesel proved to be
very difficult. Neither FTIR nor SEM/EDX analyses provided a
definitive answer regarding composition.
The diesel, interfacial matter, and water from the bottle
tests were additionally streaked on LB-Agar plates to investigate the presence of microorganisms. For both diesels, there
was microbial growth for those samples taken from both the
water phase and the interfacial area but not the diesel. The
numbers of colonies from the water and interfacial matter from
the field diesel sample were substantially higher than those
from the fresh diesel sample, possibly indicating that the
presence and accumulation rate of the interfacial matter may
be related to microorganisms.
There are at least two possible other explanations for the
more significant rate of accumulation of interfacial matter
observed in the field diesel. First, some difference in the
original composition of the two diesels may play a significant
role. Composition variations may have been due to different
intended uses of each diesel and/or the formula of additives
used. Hundreds of additives, which vary between supplier,
may be used to improve performance and these often have
surface active behavior (Song et al., 2000). Analysis for these
compounds is challenging as the target compound(s) must be
known in advance. Second, the final composition of the diesels
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S.S. Drake et al. / Journal of Contaminant Hydrology 144 (2013) 46–57
Table 1
Physical properties of non-aqueous phases.
NAPL
Density
(g/mL)
IFT with water
(25 °C) (mN/m)a
θAdvancing
(°)a
θReceding
(°)
Fresh diesel
Field diesel
0.808
0.825
28.1 ± 1.5
17.2 ± 1.0
10.7 ± 0.3
29.5 ± 2.2
b33b
180c
a
Error reported as 95% confidence interval about the mean.
Visual obstruction made measurement difficult therefore the maximum
possible angle is reported here.
c
Pinning of contact angle taken as 180°.
b
(i.e., weathering) may have been significant. As previously
discussed, GC–MS chromatograms indicated the field diesel
was significantly weathered. In such cases, the biodegradation
and oxidation of hydrocarbons may produce more polar
compounds (Cuiec, 1990; Lang et al., 2009) reducing hydrophobicity. Therefore, when the diesel was left in contact with
water these compounds could preferentially partition to the
water phase, changing the diesel phase chemistry and resulting
in heavier, non-polar molecules that precipitated out. In
addition, the weathering of the field diesel may have resulted
in NAPL components binding with water molecules as
observed for Nelson et al. (1996).
3.3. Interfacial tension and contact angle
Contact angle (through water phase)
The fresh diesel exhibited a greater IFT than the
field diesel (Table 1), however both IFT values were relatively
low compared to an analogous, single component NAPL;
for example, the IFT between pure o-xylene and water is
39 mN/m (Lord et al., 2000). The measured IFT values are
consistent with diesel fuels commonly containing additives
for improved performance. It is possible that the reduced
IFT of the field diesel is associated with the white interfacial
matter observed in the jar tests. However, weathered
NAPLs do not necessarily exhibit IFT values less than that of
un-weathered samples. Harrold et al. (2003) found that
surface active constituents present in NAPL sorbed to soil,
thereby decreasing their concentration in the NAPL phase
and increasing interfacial tension.
Intrinsic contact angles measured with the fresh diesel
were consistent with strongly water wet conditions (Table 1,
Fig. 4). No significant hysteresis was observed between
advancing and receding contact angles and no sensitivity to
elapsed contact time (up to 300 min.) was observed (Fig. 4).
The maximum θint = 33° (with mean θint = 10.7 ± 0.3) indicated that fresh diesel remained water-wetting even after
significant contact time with the solid and repeated reversals
of contact line direction. In contrast, the field diesel exhibited
considerable contact angle hysteresis that was time-dependent.
The field diesel advancing θint was consistent at 29.5°±2.2° over
700 min. of total elapsed contact time (Table 1, Fig. 4). The field
diesel receding contact angles, however, exhibited intermediate
wetting behavior immediately (θint =146° at 0 min.) and
increasing hydrophobicity with elapsed contact time. From
20 min to 110 min of elapsed contact time, observed receding
contact angles were not consistent since erratic, “temporary
pinning” led to NAPL drop recession that was not smooth and
continuous. In Fig. 4, the range between the maximum and
minimum observed receding contact angles are indicated by the
error bars. After 110 min of elapsed time, the contact line pinned
completely – i.e., the baseline of the drop remained static as the
drop volume was reduced – and the receding intrinsic contact
angles for the field diesel approached 180°.
Harrold et al. (2001) also observed contact angles that
increased with the time that the quartz plates were immersed
in the NAPL phase prior to measurement. Immersion for 1 min
was sufficient to change an initially strongly water-wet plate to
weakly NAPL-wet (Harrold et al., 2001). The impact of prior
exposure in their study was more significant for waste NAPLs
(i.e., from a solvent recovery company) than for pure NAPLs
with surfactant additives. Molnar et al. (2011) also observed
similar tetrachloroethylene/water contact angle hysteresis due
to sorption of a cationic surfactant onto an iron oxide-coated
plate at neutral pH. Lam et al. (2002) identifies a number of
fluid-fluid–solid systems that exhibited similar “stick/slip”
behavior, attributed to chemical bonding between the receding
180
165
150
135
120
105
90
75
60
45
30
15
0
Fresh Advancing
Fresh Receding
Field Advancing
Field Receding
0
200
400
600
800
Elapsed contact time (minutes)
Fig. 4. Advancing and receding contact angles plotted against total (cumulative) elapsed time of contact between the NAPL drop and solid surface. Data points
without error bars represent average values with deviation between the measurements less than the size of the symbol. Data points with error bars represent the
entire range of possible values for a given contact time, where the data point serves as the median value between the maximum and minimum. The horizontal
line at 90° represents the distinction between water-wetting and NAPL-wetting regions of the plot.
S.S. Drake et al. / Journal of Contaminant Hydrology 144 (2013) 46–57
60
80
70
50
Drainage pseudo-static average (6 curves)
60
Drainage van Genuchten fit
Imbibition van Genuchten fit
50
Drainage pseudo-static average (3 curves)
Imbibition pseudo-static average (8 curves)
Imbibition pseudo-static average (6 curves)
Capillary Pressure (cm water)
Capillary Pressure (cm water)
53
40
30
20
Drainage van Genuchten fit
40
Imbibition van Genuchten fit
30
20
10
10
0
0
0
0
- 10
0.2
0.4
0.6
0.8
0.2
0.4
0.6
0.8
1
1
- 10
Apparent Water Saturation
Apparent Water Saturation
Fig. 5. Fresh diesel/water/F70 pseudo-static capillary pressure–saturation
primary drainage and main imbibition curves (each an average of 6 experiments).
95% confidence intervals plotted for every fifth point (averaged over that point
and the 4 nearest neighbors). Also plotted are best-fit van Genuchten functions
(parameters in Table 2).
Fig. 6. Field diesel/water/F70 pseudo-static capillary pressure–saturation
primary drainage and main imbibition curves (averages of 3 drainage and
8 imbibition experiments). 95% confidence intervals plotted for every fifth
point (averaged over that point and the 4 nearest neighbors). Also plotted
are best-fit van Genuchten functions (parameters in Table 2).
drop and the surface. That work further quantified the time
dependence of receding contact angles with liquid molecule
penetration of the surface and/or sorption and, when the liquid
molecules anchored themselves sufficiently firmly on the solid,
the drop would not recede whatsoever (Lam et al., 2002). Thus,
the time-dependent and pinning behavior observed here is
likely due to surface active constituents present in the field
diesel sorbing to the solid surface.
strongly water-wetting conditions through advancing contact
angle quantification, qualitative bottle tests and USBM/Amott–
Harvey indices calculated from Pc–S curves (Powers et al.,
1996).
Fig. 6 presents the averaged field diesel Pc–S data, including
three primary drainage and eight main imbibition curves. The
field diesel primary drainage data was consistent with intrinsic
(advancing) contact angle measurements that indicated strongly
water-wet behavior (Table 1, Fig. 4). Capillary pressures on
imbibition, however, were negative above an apparent water
saturation of 5%, consistent with intermediate- or NAPL-wetting
conditions. This indicates a reversal of the fluid phase that
preferentially wetted the quartz surfaces once the field diesel
contacted the porous medium. This wettability reversal is also
consistent with the measured (receding) intrinsic contact angles
(Table 1, Fig. 4).
Fig. 7 plots the primary Pc–S data for both diesels, with
that of the fresh diesel and its 95% confidence intervals scaled
by the ratio of their interfacial tensions. The scaled fresh
diesel data lies slightly above that of the field diesel, however
the 95% confidence intervals overlap suggesting that both
systems are strongly water-wet and the operative contact
angle is approximately 0°.
Fig. 8 illustrates the Leverett scaling of imbibition curves
from the reference fresh diesel drainage curve. An operative
contact angle of 71.7° for the fresh diesel imbibition curve was
determined using Eq. (1). Strongly water-wet systems typically
exhibit capillary pressures on imbibition that are 40–60% of
their magnitude on drainage (Gerhard and Kueper, 2003;
O'Carroll et al., 2005). In this case, cos(71.7°) = 0.31 indicating
slightly greater hysteresis than typical, but suggestive that
relatively strong water-wet conditions prevailed. Eq. (1)
determined an operative contact angle of 107.7° for the field
3.4. Capillary pressure–saturation main curves
The average of the six fresh diesel/water system primary
drainage and imbibition experiments, with 95% confidence
intervals, is presented in Fig. 5. The curves are consistent with
strongly water-wet conditions, with positive capillary pressures exhibited on both Pc–S pathways at almost all saturations. The imbibition curve reaches negative Pc values at
water saturations above 96.5% This is in agreement with the
measured advancing and receding intrinsic contact angles
(Table 1, Fig. 4). van Genuchten fits reproduced the measured
mean behavior except for a slight discrepancy near residual
and high water saturation (parameters in Table 2). A similar
study on a diesel fuel obtained from a retail outlet also showed
Table 2
van Genuchten best-fit parameters for LNAPL/water/F70 sand.
Experiment
N[]
α [cmwater]−1
η [cmwater]
Fresh diesel primary drainage
Fresh diesel main imbibition
Field diesel primary drainage
Field diesel main imbibition
14.04
5.18
14.90
5.69
0.032
0.071
0.061
0.024
0
5.88
0
10.84
54
S.S. Drake et al. / Journal of Contaminant Hydrology 144 (2013) 46–57
80
60
70
Fresh Diesel Drainage Scaled by IFT
Field Diesel Drainage
40
60
30
20
10
0
0
0.2
0.4
0.6
0.8
1
Capillary Pressure (cm water)
Capillary Pressure (cm water)
50
40
30
5
20
Fig. 7. Fresh diesel average pseudo-static drainage curve and error bars, both
scaled by the ratio of field diesel IFT to fresh diesel IFT, compared to the field
diesel experimental average pseudo-static drainage curve.
7
2
0
0
0.5
1
Effective Water Sautration
-10
diesel imbibition curve, indicating intermediate-wetting to
NAPL-wetting conditions. The receding contact angle measured on the quartz slide for the fresh diesel system (b 33°) was
less than the operative contact angle scaled based on the Pc–S
experiments (71.7°). This possibly indicates that the intrinsic
contact angle contact angle, measured on a quartz slide, may
not adequately quantify the degree of hysteresis present in
porous media. In the field diesel system the intrinsic contact
angle (~180°) was considerably larger than that found based
on scaling the Pc–S curves (107.7°). The reason for this
discrepancy could be that the residency time of the NAPL in
Fig. 9. Fresh diesel/water/F70 capillary pressure–saturation experiment
with scanning curves; 21 scanning curves over 9 unique saturation paths.
Numbers near the curves represent the number of overlapping curves at that
location.
the Pc–S experiments may not have been adequately long to
render the porous media extremely hydrophobic.
3.5. Capillary pressure–saturation scanning curves
Two representative experiments, one for each of the fresh
and field diesel (Figs. 9 and 10, respectively) that included
scanning curves demonstrate the reproducibility of the data.
The number of repeat scanning curves that overlay each
80
Fresh Diesel Drainage Experimental Curve
Fresh Diesel Imbibition Experimental Curve
70
50
Fresh Drainage Scaled to Fresh Imbibition
60
Field Diesel Imbibition Experimental Curve
Capillary Pressure (cm water)
Capillary Pressure (cm water)
6
10
Effective Water Saturation
-10
50
Fresh Drainage Scaled to Field Imbibition
50
40
30
20
10
30
20
10
2
5
2
3
0
0
0.2
0.4
0.6
0.8
1
- 10
0
0
-10
40
0.2
0.4
0.6
0.8
1
Effective Wetting Phase Saturation
Fig. 8. Leverett–Cassie scaling for fresh and field diesel imbibition curves.
The fresh diesel main drainage curve is scaled to the imbibition curves by an
operative contact angle of 71.7° for fresh diesel imbibition and 107.7° for
field diesel imbibition.
-20
Effective Water Saturation
Fig. 10. Field diesel/water/F70 capillary pressure–saturation experiment
with scanning curves; 13 scanning curves over 11 unique saturation paths.
Numbers near the curves represent the number of overlapping curves at that
location.
S.S. Drake et al. / Journal of Contaminant Hydrology 144 (2013) 46–57
other, the variety of saturation pathways explored, and noting
that each complete data set was measured in approximately
89 h speaks to the strength of the pseudo-static method.
On primary drainage both system exhibited water wetting
behavior since they scale well based solely on interfacial
tension. It is important to note that both porous media systems
have been flushed with numerous pore volumes of aqueous
phase that was pre-equilibrated with either fresh or field
diesel. As such, water/diesel interfacial tensions were not
equivalent in both systems and soluble diesel constituents
would have been exposed to the surface of the quartz grains.
Upon saturation reversal, either at residual water saturation or
at some intermediate saturation, the shape of the imbibition
curves were distinctly different (Figs. 9 and 10). Moreover, the
field diesel exhibited cyclical, repeatable wettability reversals
(Fig. 10). Wettability reversal phenomena are discussed in
significant detail by Kovscek et al. (1993) for a crude oil system.
Similar phenomena are likely occurring in the field diesel
system however wettability reversal is not permanent as is the
case with crude oil. Permanent wettability reversal in crude oil
is due to the presence of asphaltenes. On water drainage diesel
displaces the aqueous phase as capillary pressure increases and
the thickness of the water film surrounding the porous media
decreases. When the critical capillary pressure is achieved in a
pore the water film surrounding the quartz grains ruptures,
leaving monolayer coverage of water molecules surrounding
the sand grains (Kovscek et al., 1993). At this point constituents
from the field diesel can partition to the solid surface rendering
the sand organic-wetting. As suggested by Kovscek et al.
(1993), this wettability reversal occurs primarily in the
intermediate pore size class as the critical capillary pressure
may not be achieved in the largest pores and no NAPL invades
the smallest pores. Wettability reversal was not observed in the
fresh diesel system, consistent with measured contact angle
and the main Pc–S curves.
With decreased capillary pressure little water imbibed
into the field diesel system initially as the intermediate pore
size range was organic-wet and the largest pores required
a significant decrease in capillary pressure prior to water
reinvasion. As illustrated in Fig. 10, some water entered the
largest pores at positive capillary pressures but negative
capillary pressures are required to displace the majority of the
NAPL. In the fresh diesel system on water imbibition, water
would enter the intermediate sized pores followed by the
largest pores as there was no wettability reversal (Fig. 9). For
this reason, the water imbibition curves in the fresh diesel
system exhibited a reduced slope relative to the field diesel
system (i.e., dPc/dSw for field diesel > dPc/dSw for fresh diesel
at saturation reversal). In the field diesel system, however, the
intermediate sized pores were organic wetting at the end of
drainage. As such, it is postulated that, similar to water
drainage in an initially water wet system, a critical (minimum, in this case) capillary pressure must be achieved to
rupture the NAPL film surrounding the quartz sand grains. At
this point the NAPL molecules on the quartz sand grains are
displaced by water and pores where this critical capillary
pressure has been achieved in the field diesel system become
water wetting again.
Upon subsequent saturation reversal (i.e., main drainage
scanning curve) the water-wet pores in the field diesel once
again followed the main drainage path consistent with a
55
water-wet system (Fig. 10). The shapes of the water drainage
scanning curves between the fresh and field diesels are
revealing. In general, at the start of a drainage event, a high
Pc–S slope and the presence of an entry capillary pressure is
evidence that the NAPL was nonwetting and discontinuous in
the largest pore bodies (Gerhard and Kueper, 2003). For the
fresh diesel (Fig. 9), typical water-wetting behavior was
observed in that the primary drainage curve exhibited a
distinct entry pressure, but as scanning curve reversal
points approach Sw = 0.5, Pc–S slopes were observed to
reduce (i.e., become shallower) (Haines, 1930; Morrow,
1976; Parlange, 1976). This is expected since, at intermediate
saturations the NAPL was continuous and no entry pressure
needed to be overcome. However, in the field diesel system,
all Pc–S drainage curves were equally steep, exhibiting a
distinct entry pressure for any NAPL invasion regardless
of reversal saturation, further underscoring the cyclic wettability reversal (Fig. 10).
Studies employing laboratory grade NAPLs and controlled
concentrations of surfactant additives have observed wetting
phase reversal (Demond et al., 1994; Desai et al., 1992; Powers
and Tamblin, 1995). In these studies, cationic surfactants sorbed
to the quartz surface, reducing hydrophilicity and, at high
surfactant concentration, reversed wettability. Lenhard and
Oostrom (1998) discuss how a NAPL with a sorbing surfactant
can create mixed wettability – where the pores exposed to NAPL
become NAPL-wetting and the pores continuously filled with
water remain water-wetting. These previous studies provide
insight but do not adequately explain the cyclical wettability
reversal behavior observed in this study. The interfacial matter
observed in the field diesel after a short time, potentially
associated with diesel additives (Song et al., 2000), may play a
role. In addition, the weathered composition of the diesel fuel
may play a role as may any compounds added to the diesel to
inhibit degradation during storage. Typical antioxidants include
alkylated phenols and secondary amines (Song et al., 2000),
both of which may act as surfactants. The a priori knowledge of
specific analytes combined with the extremely low detection
limits required for these compounds made the identification of
any additives not practical in this study.
4. Conclusions
Examination of wettability at the pore scale, interface scale
and REV scale suggests that NAPLs recovered from a field site
may not be adequately represented by similar fresh NAPLs due
to wettability differences. Increased contact time between a
field diesel and quartz in the presence of water significantly
increased the hydrophobicity of the solid, from intermediate
wetting to NAPL wetting. Moreover, a field diesel exhibited
cyclical reversals of wettability, with water-wetting behavior
on drainage and NAPL-wetting behavior on imbibition; such
behavior at the REV-scale is linked to cyclical hysteresis of the
intrinsic contact angle. The shape of the Pc–S scanning curves in
such cases reveals that the invading fluid must overcome the
porous medium entry pressure at every saturation reversal,
unlike an identical system that is strongly water-wetting
irrespective of saturation history.
It is not surprising that a diesel fuel may exhibit NAPLwetting behavior after contact with the soil, e.g., due to the
presence of a surfactant additive in the formulation or polar
56
S.S. Drake et al. / Journal of Contaminant Hydrology 144 (2013) 46–57
compounds produced via weathering. Such compounds, even
in small quantities, can precipitate at the interface, sorb to
solid surfaces, and alter the soil–liquid interfacial forces. It is
hypothesized that the cyclical wettability reversal is related
to cyclical changes in intermediate pore wettability due to
rupturing of the film surrounding the soil grain at a critical
capillary pressure. It is possible that cyclical wettability
reversals have not previously been observed because of the
challenge of conducting multiple, rapid Pc–S experiments
with the small volumes of NAPL typically available from field
sites. Towards this end, the pseudo-static method for Pc–S
curves combined with the “elapsed contact time” method for
measuring cyclical intrinsic contact angles, may be a valuable
approach to determine the wettability of field soil/NAPL
systems. The wettability observed for the field system would
need to be incorporated into numerical models to properly
predict NAPL migration and remediation at the field site.
Remediation plans must be developed with knowledge of
field NAPL wettability to achieve source zone remediation goals.
Wettability changes will impact source zone architecture —
sub-regions of pools and residual may have significantly
different spatial distribution and NAPL/water interfacial area
than expected. Thus, at the field scale, this may have implications
on local mass flux and, in turn, the potential longevity of a NAPL
release under natural or engineered conditions. Additionally,
remediation techniques that involve waterflooding are based on
understanding and manipulating Pc–S relationships; wettability
impacts on Pc–S imbibition curves must be understood in order
to adequately manipulate capillary pressure for NAPL displacement by water.
It is noted that the conclusions in this study are based
upon a single detailed comparison of two similar NAPLs. The
fresh sample examined in this study exhibited water-wetting
behavior however other retail formulations will vary and
may possess compounds that will create non-ideal wettability. In addition, only quartz was employed which neglects the
potential impacts on wettability of other minerals or organic
coatings. The aqueous phase chemistry was simplified by
using deionized water at neutral pH. However, this study
does isolate and illustrate the impacts of a field NAPL on
wettability and the significant differences that may exist
between a field and a fresh NAPL.
Acknowledgments
This research was supported by Natural Sciences and
Engineering Research Council of Canada (NSERC) and Ontario
Graduate scholarships to the first author. Additional support
has been provided by an NSERC Strategic Grant and the
Canadian Foundation for Innovation. The authors would like
to thank Dr. Kela Weber, at the Royal Military College, for his
help with the microbial analysis and Drs. Tohren Kibbey and
Lixia Chen, at the University of Oklahoma, for their significant
support related to the Pc–S apparatus.
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