Directory UMM :Data Elmu:jurnal:O:Organic Geochemistry:Vol31.Issue10.Oct2000:
Organic Geochemistry 31 (2000) 147±158
www.elsevier.nl/locate/orggeochem
Sorption of crude oil from a non-aqueous phase onto silica:
the in¯uence of aqueous pH and wetting sequence
Christopher J. Daughney*
Department of Earth and Ocean Sciences, University of British Columbia, 6339 Stores Road Vancouver, B.C., V6T 1Z4, Canada
Received 8 March 1999; accepted 23 August 1999
(Returned to author for revision 11 May 1999)
Abstract
Sorption of crude oil by mineral surfaces may aect its ¯ow and recovery in porous media. Here, crude oil sorption
from a toluene±heptane phase onto quartz and silica gel has been investigated both in the presence and absence of an
aqueous phase, as a function of time, oil-to-sorbent ratio, pH, and wetting sequence. Steady-state is attained in roughly
24 h, with sorption following Freundlich isotherms. Maximum sorption capacity of initially dry quartz is roughly 2 mg
oil per g; that of initially dry silica gel is greater than 8 mg per g. Sorption capacities of pre-wetted quartz and silica gel
are approximately 0.5 and 0 mg oil per g, respectively. Aqueous pH only aects sorption by pre-wetted quartz, where
sorption is greatest at pH 4, roughly 25% less at pH 7, and roughly 50% less at pH 2. The eect of pH and wetting
sequence on oil sorption can be qualitatively described by calculating the electrostatic interaction energy between the
mineral±water and oil±water interfaces. # 2000 Elsevier Science Ltd. All rights reserved.
Keywords: Oil; pH; Silica; Sorption; Wetting sequence
1. Introduction
The sorption of crude oil onto mineral surfaces may
aect its migration in the subsurface (Yamamoto, 1992;
Thomas and Clouse, 1995), its recovery from reservoir
rocks (Morrow et al., 1986; Morrow, 1990), and its
remediation in the event of environmental contamination (Domenico and Schwartz, 1990). Thus, an understanding of the factors aecting the extent of sorption is
critical to the prediction of the fate, or mobility, of
crude oil in natural environments. In this study, laboratory experiments are conducted to monitor the sorption
of crude oil from a non-aqueous phase liquid onto
quartz and silica gel, both in the presence and the
absence of an aqueous phase. This study provides
insight into the processes that control sorption in
* Current address: Department of Earth Sciences, University of
Ottawa, 140 Louis Pasteur, Ottawa, Ontario, K1N 6N5, Canada.
Tel.: +613-562-5800, ext. 6848; fax: 613-562-5192.
E-mail address: [email protected]
natural environments, and also provides information
that can be used to control the extent of crude oil sorption in the laboratory.
Many previous laboratory studies have investigated
crude oil sorption from a non-aqueous phase onto
mineral surfaces, both in the presence and the absence of
water, and several controlling variables have been identi®ed. In the absence of water, the extent of sorption is
observed to depend upon (1) the chemistry of the crude
oil, (2) the concentration of the crude oil, (3) the composition of the non-aqueous liquid used to solubilize the
crude oil, (4) the concentration, type, surface area and
roughness of the mineral, (5) temperature, and (6) equilibration time (Czarnecka and Gillot, 1980; Crocker and
Marchin, 1988; Jadhunandan and Morrow, 1991; GonzaÂlez and Tavalioni-Louvisse, 1993; Akhlaq et al.,
1997). If both aqueous and non-aqueous phase liquids
are present, the sorption of crude oil by mineral surfaces
depends not only on the six parameters listed above, but
also on (7) the composition of the aqueous liquid, particularly its pH and salinity, and (8) the order, or wetting
sequence, in which the aqueous and non-aqueous liquids
0146-6380/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0146-6380(99)00130-8
148
C.J. Daughney / Organic Geochemistry 31 (2000) 147±158
Nomenclature
aHb Proton activity in the bulk solution (mol lÿ1)
aHs Proton activity at the silica±water or oil±
water interface (mol lÿ1)
C
Equilibrium concentration of oil in organic
phase (mg lÿ1)
d
Separation distance between silica±water and
oil±water interfaces (m)
e
Elementary charge (1.610ÿ19 C)
F
Faraday constant (96485 C eqÿ1)
k
Boltzmann constant (1.3810ÿ23 J Kÿ1)
K
Empirical constant in Freundlich isotherm
(units vary)
K1 Stability constant for protonation of silanol
functional groups (mol lÿ1)
K2 Stability constant for deprotonation of silanol functional groups (mol lÿ1)
Ka Acid dissociation constant for crude oil
organics (mol lÿ1)
Kb Basicity constant for crude oil organics (mol lÿ1)
n
Empirical constant in Freundlich isotherm (ÿ)
ne
Number of electrolyte ion pairs per unit
volume (mÿ3)
R
Gas constant (8.3145 J Kÿ1 molÿ1)
T
Absolute temperature (K)
x
Distance between shear plane and silica±
water or oil±water interface (m)
"
Relative dielectric permittivity of water (80)
"0
Permittivity of free space (8.85410ÿ12 C
Vÿ1 mÿ1)
ÿ
Sorption density (mg gÿ1)
0
Electric charge at the silica±water or oil±
water interface (C mÿ2)
Electric potential at the silica±water or oil±
0
water interface (V)
Zeta potential (V)
x
Ge Electrostatic component of Gibbs free energy
change (J molÿ1)
are equilibrated with the sorbent (Brown and Neustadter, 1980; Crocker and Marchin, 1988; Buckley et al.,
1989; Ardebrant and Pugh, 1991; Skauge and Fosse,
1994; Valat et al., 1994; Liu and Buckley, 1997).
Of the eight variables listed above, the chemistry of
the crude oil is the most dicult to constrain. Crude oil
is a complex mixture of hydrocarbons with wide ranges
of molecular weights and structures that are experimentally dicult, if not impossible, to characterize completely
(Speight and Moschopedis, 1981; Semple et al., 1990).
Thus eorts have been made to describe the extent of oil
sorption using parameters that are related to oil chemistry
but are more easily measured. The eect of crude oil
composition on sorption can be related to the oil±water
contact angle (Brown and Neustadter, 1980; Barranco et
al., 1997; Liu and Buckley, 1997), the oil±water interfacial tension (Skauge and Fosse, 1994; Barranco et al.,
1997), or the zeta potential of oil droplets in water
(Buckley et al., 1989; Ardebrant and Pugh, 1991; Dubey
and Doe, 1993).
The zeta potential is a particularly useful proxy variable for oil composition, because it can be used to
develop a molecular-scale model of the oil±water interface (Takamura and Chow, 1985; Buckley et al., 1989;
Doe, 1994; Liu and Buckley, 1997). The zeta potential
can be related to the electric potential at the oil±water
interface through a number of electric double layer
models (Westall and Hohl, 1980). The electric potential
at the oil±water interface is assumed to depend on the
protonation and deprotonation of speci®c functional
groups (i.e. carboxyl, amine, etc.) present at the interface. Thus, zeta potential data collected as a function of
pH can be used to determine the concentration and
deprotonation constant of each type of interfacial functional group (Takamura and Chow, 1985; Buckley et
al., 1989). Potentiometric titration data can also be used
to determine the concentrations and deprotonation
constants of interfacial functional groups (American
Society for Testing and Materials, 1987a, b; Dubey and
Doe, 1993). Combination of zeta potential and potentiometric titration data therefore permits the development of a robust model of the chemistry of the oil±water
interface. This approach can also be applied to the
mineral±water interface (Menon and Wasan, 1986;
Braggs et al., 1994). Unlike the zeta potential, contact
angle and interfacial tension are macroscopic measurements that are not easily related to potentiometric
titration data or to interfacial chemistry at the molecular level. Similarly, many bulk properties of crude oil
commonly reported in the literature (i.e. viscosity, density, and hydrogen±carbon±nitrogen ratio) cannot be
directly related to interfacial chemistry.
Molecular-scale models of the oil±water and mineral±
water interfaces can be useful for resolution of the
components of the Gibbs free energy of sorption. It is
well established that crude oil sorption occurs through
physico-chemical (van der Waals, hydrophobic and
structural) and electrostatic interactions (Aronson et al.,
1978; Brown and Neustadter, 1980; Buckley et al., 1989;
Jadhunandan and Morrow, 1991; Dubey and Doe,
1993; Liu and Buckley, 1997). With the concentrations
and deprotonation constants of all surface functional
groups known, it becomes possible to calculate the
interfacial potentials at any pH and salinity (Takamura
and Chow, 1985), and determine the electrostatic component of the Gibbs free energy of sorption (Hogg et al.,
1966). Thus if the overall Gibbs free energy of sorption
is measured experimentally (e.g. Ardebrant and Pugh,
C.J. Daughney / Organic Geochemistry 31 (2000) 147±158
1991), and if the electrostatic component is calculated
using a model of interfacial chemistry, then the physicochemical component of sorption free energy can be
determined by dierence (Stumm and Morgan, 1981).
An ability to quantify the physico-chemical component
of sorption free energy is desirable because it can be
incorporated into many common geochemical speciation codes. Ultimately, such speciation codes can be
used to predict the fate of crude oil in a variety of geochemical environments.
Several workers have developed molecular-scale
models of the mineral±water and oil±water interfaces,
and related these models to oil sorption, but the relationship between pH, wetting sequence and extent of
sorption remains to be addressed. Buckley et al. (1989),
Dubey and Doe (1993) and Skauge and Fosse (1994)
present adhesion maps that show the pH and salinity
conditions under which oil adhesion (sorption) is
observed or expected, but the extent of sorption (mg oil
sorbed per g sorbent) is not reported. Additionally,
previous research on the importance of wetting sequence
is equivocal. Some authors have reported that a mineral
pre-wetted with an aqueous phase will sorb more oil (or
similar organic) than a dry mineral surface (Brown and
Neustadter, 1980; Lagerge et al., 1993), whereas other
authors have reported the opposite (Crocker and
Marchin, 1988). It is possible that these contradictory
reports arise due to dierences in the pH of the aqueous
phase and its eect on the electrostatic component of
sorption free energy. For example, under some pH conditions, oil sorption may be favoured due to a strong
electrostatic attraction between the oil±water and
mineral±water interfaces, and thus the presence of an
aqueous phase may increase sorption relative to that
observed in the absence of an aqueous phase.
The objective of this research is to examine the relationship between pH, wetting sequence, and extent of oil
sorption. Crude oil sorption is investigated for three
dierent wetting sequences: (1) the sorbents are equilibrated with excess water before the organic phase is
introduced to the system, (2) the sorbents are equilibrated with the organic phase prior to the addition of
water, and (3) water is absent entirely. In each case,
sorption is investigated as a function of equilibration
time, and where applicable, as a function of aqueous
pH. Silica gel and powdered quartz are used as analogues for natural mineral surfaces, Cold Lake crude oil
dissolved in a mixture of toluene and heptane is used as
the organic phase, and 0.01 M NaCl is used as the aqueous phase. Potentiometric titrations and zeta potential
measurements are used to characterize the relationship
between aqueous pH and electric charge distribution at
the mineral±water and oil±water interfaces. Batch
adsorption experiments are conducted to investigate the
eect of wetting sequence, pH, and equilibration time
on the extent of oil sorption.
149
2. Experimental
2.1. Materials
Reagent grade silica gel and powdered quartz were
obtained from B. D. H. and Alpha-Aesar, respectively.
Both solids were successively rinsed in 10% HCl, 10%
NaOH and distilled, de-ionized (DDI) water (18 M
),
then oven-dried, in order to remove impurities and
homogenize the surfaces. BET N2-adsorption surface
areas for the silica gel and the powdered quartz were 356
and 0.2 m2/g, respectively (Micromeretics Inc., Atlanta,
Georgia). Cold Lake crude oil was obtained from Imperial
Oil (Calgary, Alberta). Reagent grade 1 M HCl and 1 M
NaOH (Aldrich) were used to adjust pH, and 0.01 M NaCl
(Aldrich) was used as the background electrolyte.
2.2. Zeta potential measurement
Colloidal dispersions of silica gel, powdered quartz or
crude oil in 0.01 M NaCl were prepared using an ultrasonic disaggregator. The pH of the electrolyte was
adjusted with HCl or NaOH, and the dispersions were
allowed to stand for 2 h at 25 C in order to allow for
equilibration and settling of larger particles. For each
dispersion, zeta potential was measured for 25 particles
using a Zeta-Meter 3.0+ ®tted with a UVA-II cell
(Zeta-Meter, Inc., N. Y.). The pH of each dispersion
was determined immediately after the zeta potentials
were measured.
2.3. Potentiometric titrations
Proton binding by silica gel and powdered quartz was
studied in a 0.01 M NaCl electrolyte solution. Suspensions containing 5±10 g silica gel or powdered quartz in
100 ml electrolyte were titrated using standardized HCl
and NaOH, in order to determine the deprotonation
constants and the concentrations of functional groups
present on the solid surfaces. The titrations were conducted at 25 C in polypropylene reaction vessels, with
suspensions mixed by a magnetic stirrer. All solutions
were bubbled with N2 before and during the titrations in
order to purge them of dissolved CO2. Titrations were
performed sequentially in up-pH and down-pH directions and repeated in triplicate to evaluate the reversibility of the reactions and the reproducibility of the
experimental method.
Proton binding by the organic molecules present in
the crude oil was studied in non-aqueous solution using
ASTM methods D664-87 and D4739-87 (American
Society for Testing and Materials, 1987a, b). Brie¯y, 2±3
g of oil were dissolved in 100 ml of a mixture of toluene,
isopropyl alcohol, chloroform and a small amount of
water. The solutions were titrated using standardized
HCl and/or KOH in isopropyl alcohol. The titrations
150
C.J. Daughney / Organic Geochemistry 31 (2000) 147±158
were conducted at 25 C in glass reaction vessels, and
were mixed and purged of CO2 as described above.
Titrations were performed sequentially in up-pH and
down-pH directions and repeated in triplicate.
2.4. Sorption experiments
Two sets of batch experiments were conducted to
study sorption as a function of equilibration time.
Identical Te¯on reaction vessels were used, each containing 1.5±2 g oven-dried silica gel or powdered quartz
and 25 ml of an oil±toluene±heptane mixture (65 mg oil/
l, 1:1 toluene:heptane by volume). In the ®rst set of
experiments, the solids and the organic phase were
mixed and placed in a rotary tumbler at 25 C for
between 15 min and 4 days, after which time the concentration of oil remaining in the toluene±heptane phase
was determined by spectrophotometry at 402 nm, using
the method of GonzaÂlez and Middea (1987). In the second set of experiments, the solids were equilibrated with
10 ml of 0.01 M NaCl at pH 2, 4 or 7 for 3 days prior to
the addition of the oil±toluene±heptane phase. The
reaction vessels were tumbled for an additional 15 min
to 4 days and analyzed as above.
Three sets of batch experiments were conducted to
investigate sorption as a function of wetting sequence,
oil-to-sorbent ratio, and aqueous pH. Identical Te¯on
reaction vessels were used, each containing 1.5±2 g
oven-dried silica gel or powdered quartz and 25 ml of a
toluene±heptane mixture (1:1 by volume) with dierent
initial concentrations of oil (5±500 mg/l). First, based on
the results of the kinetic experiments, the suspensions were
tumbled for three days at 25 C, then the oil±toluene±
heptane phase was analyzed for oil as above. In the second
set of experiments, the solids were equilibrated with the
oil±toluene±heptane mixtures for 3 days, after which
time 10 ml of 0.01 M NaCl were added to each reaction
vessel. The reaction vessels were tumbled for an additional
three days before the organic phase was analyzed for oil.
In the third set of experiments, the solids were equilibrated
with 10 ml of 0.01 M NaCl for 3 days before the addition of the oil±toluene±heptane mixtures. After tumbling for an additional three days, the toluene±heptane
phase was analyzed as above. The second and third sets
of experiments were repeated at aqueous pH 2, 4 and 7.
One set of batch experiments was conducted to investigate the reversibility of the sorption reaction. Between 1.5
and 2 g oven-dried silica gel or powdered quartz were
equilibrated with 35 ml of the oil±toluene±heptane mixture containing varying initial concentrations of oil.
After 3 days, 10 ml of 0.01 M NaCl were added to each
vessel, and 10 ml of the oil±toluene±heptane mixture
were removed and analyzed by spectrophotometry.
After tumbling for an additional 3 days, the oil±toluene±
heptane mixtures were reanalyzed. These experiments
were repeated at aqueous pH 2, 4 and 7.
Fig. 1. Experimental data collected during potentiometric
titration of (a) silica gel and (b) powdered quartz in 0.01 M
NaCl. Open triangles represent down-pH titration; ®lled circles
represent up-pH titration. Model ®t to experimental data is
indicated by the solid line; dashed line represents titration of
background electrolyte. Note the dierence in scale.
3. Results and discussion
3.1. Characterization of the silica±water interface
The experimental data collected during potentiometric titration of silica gel indicate that it imparts a
signi®cant buering capacity to the suspensions, over
and above that of the electrolyte alone (Fig. 1a). Powdered quartz imparts a lesser buering capacity to the
solution per unit weight (Fig. 1b). For both solids, buffering is most signi®cant for pH >7. The data in Fig. 1
were collected as titrations were conducted sequentially
in both up-pH and down-pH directions, indicating that
the reactions are reversible and that equilibrium is
attained. This reversibility is not representative of solid
dissolution or surface modi®cation, and therefore the
dierence between the titration curves of the silica gel or
powdered quartz and the electrolyte can be attributed
completely to proton binding by the surfaces.
Several researchers have shown that amphoteric silanol functional groups (SiOH0) are responsible for
proton binding at the silica±water interface (Dugger et
C.J. Daughney / Organic Geochemistry 31 (2000) 147±158
al., 1964; Schindler et al., 1976; Yates and Healy, 1976;
Sermon, 1980; Dove and Rimstidt, 1994; Osthols, 1995):
0
*
SiOH
2 ) SiOH H
1
SiOH0 *
) SiOÿ H
2
The mass action equations corresponding to the
above equilibria are:
SiOH0 aHs
K1
SiOH
2
3
SiOÿ aHs
K2
SiOH0
4
The square brackets represent the concentration of
the enclosed surface species (mol/kg solution) and aHs
represents the proton activity at the solid surface, which
is related to the proton activity in the bulk solution,
aHb :
aHs aHb exp ÿF
0 =RT
5
The variables F; 0 ; R and T represent Faraday's
constant, the electric potential of the surface, the gas
constant, and the absolute temperature, respectively.
The electric potential of the surface ( 0 ) cannot be
measured directly, but can be related to the experimentally determined surface charge (0 ) by several double
layer models (Westall and Hohl, 1980). Here, the GouyChapman model is applied:
F 0
0 8""0 ne KT1=2 sin h
6
2RT
where "; "0 ; ne and k represent the relative dielectric
permittivity of water, the permittivity of free space, the
number of electrolyte ion pairs per unit volume, and
Boltzmann's constant, respectively. In the Gouy±Chapman model, d ÿ0 , where d is the charge of the diffuse portion of the double layer.
In this study, the computer program FITEXP is used
to determine the concentration (mol of sites per m2) and
protonation constants (K1 and K2 ) of the silanol functional groups, in order to identify the model which best
describes the experimental data. FITEXP is a version of
FITEQL 2.0 (Westall, 1982a,b) modi®ed by Johannes
LuÈtzenkirchen (Department of Inorganic Chemistry,
Umea University, Sweden; pers. comm.) to allow
simultaneous modeling of titrations conducted at dierent solid-to-solution ratios. All model calculations apply
conventional standard states, and all stability constants
are referenced to 25 C, the ionic strength of the background electrolyte, and zero surface potential. The stability constants describing the dissociation of water, the
151
electrolyte, the acid and the base are included in all calculations.
The silica gel titration data are well described by a
model with 2.310ÿ6 mol of silanol sites per m2, with
stability constants K1 =10ÿ1.5 and K2 =10ÿ8.0. Powdered quartz titration data are described by a similar
model, with 4.210ÿ6 mol of silanol sites/m2, K1 =10ÿ1.5
and K2 =10ÿ8.3. These model parameters are in reasonable agreement with previous work, where surface site
concentrations determined by titration vary from
710ÿ6 to 210ÿ5 mol/m2 (James and Parks, 1982), K1
varies from 10ÿ2.3 (Schindler and Stumm, 1987) to
10ÿ1.0 (Riese, 1982), and K2 ranges from 10ÿ7.5 to 10ÿ4.6
(James and Parks, 1982; Anderson and Benjamin, 1990).
The reported values of these parameters vary due to
dierences in the type of solid used, the pre-treatment
method, the background electrolyte, and the type of
double layer model applied.
Zeta potential of silica gel is approximately zero
below pH 5 and negative above pH 5. Powdered quartz
zeta potential is positive below pH 2 and negative above
pH 2 (Fig. 2). Using the Gouy±Chapman model, the
potential at the surface ( 0 ) can be related to the zeta
potential ( x , where x represents the distance between
the shear plane and the surface):
x
2RT 1
0 exp ÿkx
ln
F
1 ÿ
0 exp ÿkx
7
where
0
exp F
exp F
ÿ1
=2RT
1
0
0 =2RT
2e2 ne =""0 KT1=2
8
9
and e is the electron charge. The location of this shear
plane is not known, but it is generally assumed to lie
0.5±1 nm from the surface (Takamura and Chow, 1985).
Fig. 2. Zeta potential of colloidal dispersions containing silica
gel, powdered quartz or Cold Lake crude oil. Lines indicate
model ®t to experimental data.
152
C.J. Daughney / Organic Geochemistry 31 (2000) 147±158
The surface potential can be related to the surface
charge by Eq. 6, and the surface charge arises due to
pH-dependent protonation and deprotonation of the
functional groups [Eq. (1) and (2)], making it possible to
solve for the functional group concentration and the
values of the stability constants K1 and K2 .
The silica gel zeta potential data are best described by
a model with 2.410ÿ6 mol of sites/m2, K1 =10ÿ1.5 and
K2 10ÿ7:6 . Powdered quartz zeta potential data are
best ®t by a model with 4.410ÿ6 mol of sites/m2, K1
10ÿ1:5 and K2 10ÿ3:8 . Literature values for silanol site
concentration determined from zeta potential data
range from 4.210ÿ6 moles/m2 (Buckley et al., 1989) to
roughly 110ÿ5 mol/m2 (Doe, 1994). Buckley et al.
(1989) report K1 10ÿ1:0 and K2 10ÿ4:0 for quartz.
Few other researchers have used zeta potential data to
calculate K1 and K2 , but zeta potential vs pH curves
commonly show points of zero charge at pH 1.5±3.0 and
nearly identical negative charge development at pH >5,
in agreement with the models presented here (Li and De
Bruyn, 1966; Gonzalez and Middea, 1987; Buckley et
al., 1989; Ardebrant and Pugh, 1991; Doe, 1994).
When the titration data and the zeta potential data
are modeled simultaneously using FITEXP, the silica
gel model parameters are 2.310ÿ6 mol of sites/m2,
K1 10ÿ1.5 and K2 =10ÿ7.8, in good agreement with the
literature values. The powdered quartz models developed from titration data and from zeta potential data
have similar silanol site concentrations and values for
K1 , which are also in good agreement with the literature,
but computed values of K2 dier by several orders of
magnitude. Of the two models, that based on zeta
potential data is more sensitive to K2 , because the buffering capacity (measured by potentiometric titration) is
small relative to the background electrolyte. The value
of K2 determined from zeta potential data is not signi®cantly aected by the position assigned to the shear
plane. In contrast, the position assigned to the shear
plane does aect the model silanol site concentration, so
the site concentration is more reliably computed from
the titration data. Thus when the titration data and the
zeta potential data are treated simultaneously using
FITEXP, the powdered quartz model parameters are
4.710ÿ6 mol of sites/m2, K1 =10ÿ1.5 and K2 =10ÿ3.8.
These best-®tting model parameters are used in all subsequent calculations (Table 1). The model ®t is compared to the experimental data in Figs. 1 and 2.
3.2. Characterization of the oil±water interface
The experimental data collected during non-aqueous
titration of Cold Lake crude oil indicate a signi®cant
capacity to neutralize both acid and base relative to the
solvent alone (Fig. 3). Titrations were rapid and rever-
Fig. 3. Electromotive force (EMF, mV) data collected during
non-aqueous potentiometric titration of Cold Lake crude oil by
(a) HCl and (b) KOH. Dashed line represents titration of
background solvent.
Table 1
Model parameters used to characterize interfacial chemistry
Interface
Reaction
Site concentration
Ka
Gel±water
0
+
*
SiOH+
2 )SiOH +H
)SiOÿ+H+
SiOH0*
0
+
*
SiOH+
2 )SiOH +H
)SiOÿ+H+
SiOH0*
AH0*
)Aÿ+H+
)B0+H+
BH+*
2.310ÿ6 mol/m2
10ÿ1.5
10ÿ7.8
10ÿ1.5
10ÿ3.8
10ÿ4.4
10ÿ1.0
Quartz±water
Oil±water
a
4.710ÿ6 mol/m2
7.910ÿ6 mol/g
2.910ÿ6 mol/g
Stability constant referenced to zero surface potential and the ionic strength of the background electrolyte.
C.J. Daughney / Organic Geochemistry 31 (2000) 147±158
sible, suggesting that equilibrium is attained. Several
researchers have suggested that the electric charge distribution at the oil±water interface is due to protonation
and deprotonation of organic molecules present in the
oil. The chemical structure of the organic molecules is
often highly variable and dicult to identify precisely
(Speight and Moschopedis, 1981; Semple et al., 1990),
but carboxylic acid and pyridine base groups are often
assumed responsible for the majority of the interfacial
charge (Dutta and Holland, 1984; Takamura and Chow,
1985; Buckley et al., 1989; Dubey and Doe, 1993; GonzaÂlez and Travalioni-Louvisse, 1993; Buckley, 1994;
Doe, 1994; Liu and Buckley, 1997). The deprotonation
of the generalized acidic group AH0 can be represented
by the following reaction and mass action law:
are well described by a model with Ka 10ÿ4:4 and
Kb 10ÿ1:0 . Ka and Kb values can range from 10ÿ12 to
10ÿ2 (Dutta and Holland, 1984). Based on the magnitudes of the stability constants determined here, the
acidic functional groups are likely carboxylic, and the
basic groups may represent pyrazine or sulfoxide structures. Although the charging behaviour of each crude
oil is dierent, Brown and Neustadter (1980), Takamura
and Chow (1985), Chow and Takamura (1988), Buckley
et al. (1989), Dubey and Doe (1993) and Doe (1994)
have reported comparable zeta potential vs pH trends in
similar electrolytes. The model ®t is compared to the
experimental data in Fig. 2, with parameters listed in
Table 1.
3.3. Sorption experiments
AH0 *
) A ÿ H
10
Aÿ aHs
Ka
AH0
11
where Ka is the acid dissociation constant for the
reaction, and other symbols are as de®ned above. The
deprotonation of the generalized basic group BH+ can
be represented by:
BH *
) B0 H
12
B0 aHs
Kb
BH
153
13
where Kb is the basicity constant. The organic phase
may contain several types of organic molecules, each
with dierent absolute concentrations of acidic and
basic functional groups and corresponding stability
constants.
In this study, titration of Cold Lake crude oil by base
shows one in¯ection point, corresponding to an acidic
functional group with a concentration of 7.910ÿ6 mol/
g oil. Literature values for acidic functional group concentration of dierent varieties of oil vary from roughly
3.310ÿ6 (Takamura and Chow, 1985) to 410ÿ4 mol/
g. (Dutta and Holland, 1984). Titration of Cold Lake
crude oil by acid also reveals one in¯ection, corresponding to a basic functional group with a concentration of 2.910ÿ6 mol of sites/g oil. Literature values,
again for dierent varieties of oil, range from 1.410ÿ6
mol/g (Dubey and Doe, 1993) to 1.810ÿ4 mol/g (Dutta
and Holland, 1984). Dubey and Doe (1993) have noted
that the ASTM standard used here (American Society
for Testing and Materials, 1987b) may underestimate
the concentration of weakly basic substances in the oil.
Due to the inaccuracy of pH measurement in the nonaqueous titration solvent, values of Ka and Kb cannot be
determined from these data alone.
Using the site concentrations determined by titration,
zeta potential measurements of oil dispersions (Fig. 2)
Experiments conducted as a function of time indicate
that sorption of oil by powdered quartz reaches a
steady-state in approximately 24 h, both in the presence
and absence of an aqueous phase (Fig. 4). The sorption
of oil by oven-dry silica gel occurs at a similar rate (data
not shown). The rate of sorption is not signi®cantly
aected by the oil-to-solid ratio, wetting sequence or the
pH of the aqueous phase, except in the case of pre-wetted silica gel, where sorption is negligible even after 4
days. Literature values for equilibration time vary from
less than 1 min (Valat et al., 1994) to more than 10 days
(Liu and Buckley, 1997), though the observed equilibration time may be strongly controlled by the nature of
the system investigated and the experimental methods
applied. In order to allow for experimental errors and
trials involving desorption, all experiments were permitted to equilibrate for 3 days.
Sorption of crude oil by oven-dry powdered quartz
plateaus at approximately 1.5±2.0 mg/g (Fig. 5a). Most
minerals with surface areas of 1±50 m2/g have comparable
Fig. 4. Representative data showing sorption of oil by powdered quartz as a function of equilibration time, both in the
presence and absence of an aqueous phase (type of sorbent, oilto-sorbent ratio, initial wetting state, and aqueous pH do not
aect the approach to steady-state).
154
C.J. Daughney / Organic Geochemistry 31 (2000) 147±158
maximum sorption capacities for crude oil organics
(GonzaÂlez and Middea, 1987; Crocker and Marchin,
1988; GonzaÂlez and Moreira, 1991; GonzaÂlez and Travalioni-Louvisse, 1993). Due to its greater surface area,
the maximum sorption capacity of oven-dry silica gel is
greater than that of the powdered quartz, but it is not
attained under these experimental conditions (Fig. 5b).
In the absence of an aqueous phase, oil sorption can be
represented by the following schematic reaction:
SiOH0 R *
) SiOH ÿ R0
14
where R represents the organic sorbate. Note that the
stoichiometry of the reaction and the chemistry of the
organic sorbate are unknown.
Experiments conducted with various wetting sequences indicate that less oil sorption occurs when water is
present than when it is absent. Where the solids are
allowed to equilibrate with the oil±toluene±heptane phase
before the water is introduced, oil sorption by powdered
quartz is reduced by approximately 10% relative to the
oven-dry case (Fig. 5a). Under the same conditions, oil
sorption by silica gel is reduced by roughly 50% (Fig. 5b).
These data may re¯ect the exchange of previously sorbed
oil molecules [Eq. (14)] for water molecules, as represented
by the following schematic reaction:
SiOH ÿ R0 H2 O *
) SiOH ÿ H2 O0 R
15
The displaced oil molecules return to the organic
phase, but again, the stoichiometry of the reaction is
unknown. The experimental data indicate that water
molecules are more able to cause organic desorption
from silica gel than from powdered quartz.
Where the solids are equilibrated with the aqueous
phase before the oil±toluene±heptane phase is added,
organic sorption by powdered quartz is 50±80% less
than the oven-dry case (Fig. 5c), and no organic sorption by silica gel occurs (Fig. 5d). These data may
represent the ability of the oil to displace previously
sorbed water molecules, as shown by the following
schematic reaction:
* SiOH ÿ R0 H2 O
SiOH ÿ H2 O0 R )
16
Where the sorbent is initially wetted powdered quartz,
the extent of sorption is aected by the pH of the aqueous
phase (Fig. 5c). In this system, sorption is greatest at pH 4,
roughly 25% less at pH 7, and roughly 50% less at pH 2.
Organic sorption by pre-wetted silica gel is not detectable.
The relationship between the extent of oil sorption
(ÿ, mg oil sorbed per g sorbent) and the equilibrium
concentration of oil in the organic phase (C, mg/l) is
best described by the Freundlich isotherm:
Fig. 5. Sorption of oil by (a) powdered quartz and (b) silica gel, with the aqueous phase either absent or introduced after the organic
phase, and sorption of oil by (c) powdered quartz and (d) silica gel equilibrated with the aqueous phase prior to the addition of the
organic phase.
155
C.J. Daughney / Organic Geochemistry 31 (2000) 147±158
ÿ KCn
desorption involve dierent interactions, and thus sorption cannot be considered a reversible process. Several
other researchers have noted irreversibility in oil sorption reactions (Brown and Neustadter, 1980; Ardebrant
and Pugh, 1991; Thomas et al., 1993; Yan and
Masliyah, 1994; Liu and Buckley, 1997).
17
where K is a constant related to the overall change in
Gibbs free energy of the reaction and n is a constant.
The Langmuir isotherm (Stumm and Morgan, 1981)
also provides a good ®t to the data (R2 >0.85). However, because many of the experimental trials do not
display a sorption plateau, many of the Langmuir
parameters obtained by regression do not provide reasonable estimates of maximum sorption capacity (ÿmax ).
Site speci®c surface complexation models cannot be
applied because the stoichiometries of the sorption
reactions are unknown. Hence, only the Freundlich isotherm parameters K and n are compiled in Table 2. The
experimental data indicate that the amount of sorbed oil
is strongly dependent on wetting sequence and pH, and
thus the sorption reactions must be considered irreversible. Where the sorbents are equilibrated with water
prior to the addition of the organic phase, little or no oil
sorption occurs. In contrast, if the sorbents are equilibrated with the organic phase before the water is introduced, signi®cantly more oil sorption results, even
though the ®nal system composition is identical. This
suggests that oil molecules do not displace sorbed water
eciently, but that sorbed oil is very dicult to replace
with water. In thermodynamic terms, this indicates that
the Gibbs free energies of sorption (Eq. 16) and desorption (Eq. 15) are not equal in magnitude but opposite
in sign. The varying eect of pH also suggests that
sorption is irreversible. Oil sorption is strongly dependent on pH, whereas desorption is not. The pH aects
the charge distribution at the mineral-water and oil±
water interfaces, and subsequently aects the electrostatic interaction between them. The data therefore
suggest that electrostatic interactions are pH-dependent
during sorption, but not during desorption. This may
indicate that the organic sorbates are non-ionizable,
such that once sorbed, an electric double layer cannot
develop adjacent to the solid surface. Regardless, the
varying eect of pH indicates that sorption and
Fig. 6. Electrostatic interaction energy (a) between the quartz±
water and oil±water interfaces and (b) between the gel±water
and oil±water interfaces, both as a function of pH and interfacial separation. Negative values indicate interfacial attraction; positive values indicate interfacial repulsion, normalized
to the concentration of silanol sites.
Table 2
Model parameters used to characterize oil sorption
Sorption by gel
Wetting technique
pH
Ka
No aqueous phase
Aqueous phase added after
organic phase
±
2
4
7
2
4
7
10ÿ1.40
10ÿ1.15
10ÿ1.18
10ÿ1.28
Aqueous phase added before
organic phase
a
b
Freundlich isotherm parameters [Eq. (17)].
Correlation coecient.
Sorption by quartz
na
(R2)b
Ka
na
(R2)b
1.05
0.78
0.80
0.83
No sorption
No sorption
No sorption
1.00
0.99
0.99
1.00
10ÿ0.43
10ÿ1.37
10ÿ1.48
10ÿ1.42
10ÿ2.92
10ÿ3.17
10ÿ2.53
0.27
0.66
0.71
0.69
0.95
1.14
0.79
0.98
0.99
0.97
0.98
0.89
0.93
0.88
156
C.J. Daughney / Organic Geochemistry 31 (2000) 147±158
Although the irreversibility of the sorption reactions
prevents the development of a quantitative thermodynamic model, calculation of the electrostatic component of sorption free energy (Ge ) provides a qualitative
means of describing some of the data. It is possible to
calculate Ge at any pH, because interfacial potentials
can be calculated using the model parameters compiled
in Table 1. Speci®cally, as two ¯at, parallel interfaces
with unequal surface potentials 0a and 0b move from
in®nite separation to separation d; Ge is (Hogg et al.,
1966):
""o
Ge
8
2
0a 20b 1 ÿ coth2d 2
18
0a
0b cosech2d
providing the potentials remain small and constant
during the interaction. Assuming that the quartz±water
and oil±water interfaces must approach to within 0±1
nm to permit oil sorption (Takamura and Chow, 1985),
the electrostatic interaction between them can be either
negative (attractive) or positive (repulsive), depending
on aqueous pH (Fig. 6a). For interfacial separations of
0.8±0.9 nm, the electrostatic interaction energy is mildly
attractive at pH 4, less attractive at pH 7, and mildly
repulsive at pH 2. Under the same conditions, the electrostatic interaction between the gel±water and oil±
water interfaces is strongly repulsive at pH 4 and 7, and
mildly repulsive at pH 2 (Fig. 6b). Thus, if the physicochemical energy of sorption is constant at all pHs, then
the pH trends displayed in Figs. 5c and d may be
explained by the eect of the electrostatic interaction.
However, it is not possible to calculate the magnitude of
the physico-chemical component of sorption free energy,
because the stoichiometry and chemistry of the sorption
reactions are unknown. Hence, calculation of the electrostatic interaction between the mineral±water and oil±
water interfaces provides, at best, a qualitative means of
assessing the relationship between pH and oil sorption.
oil sorption by pre-wetted quartz, where sorption is
greatest at pH 4, less at pH 7, and least at pH 2.
A quantitative or thermodynamic model of sorption
cannot be developed because the reaction stoichiometries and sorbate chemistries are unknown, and because
the reactions are not reversible. However, the trends in
oil sorption observed here can be qualitatively described
by considering the electrostatic interaction between the
mineral±water and oil±water interfaces, as controlled by
pH and wetting sequence. In the absence of an aqueous
phase, electrostatic forces are negligible, because all
interfacial functional groups are neutrally charged.
Where both aqueous and organic phases are present, the
extent of sorption is reduced, because the oil and water
molecules compete for available surface sites. Where an
aqueous phase is introduced after the sorbents have
equilibrated with the organic phase, desorption results,
indicating that water molecules are able to displace previously sorbed oil. The eect of aqueous pH on this
exchange reaction is negligible, suggesting that electrostatic forces are either constant or insigni®cant, perhaps
because the sorbed molecules are non-ionizable and
curtail the establishment of an electric double layer.
Where the sorbents are equilibrated with an aqueous
phase before the organic phase is introduced, sorption is
more limited still. In such systems, sorption is controlled, in part, by electrostatic forces. The magnitude of
the electrostatic force can be calculated using the models
of interfacial chemistry developed from potentiometric
and zeta potential data. For an interfacial separation of
roughly 0.8±0.9 nm, there is mild to strong electrostatic
repulsion between the gel±water and oil±water interfaces
at all pHs examined here, which may explain the lack of
sorption observed. At the same interfacial separation, the
electrostatic force between the quartz±water and oil±water
interfaces is most attractive at pH 4, less at pH 7, and least
at pH 2, in agreement with the observed trend in sorption. This indicates that chemical models of the
mineral±water and oil±water interfaces can be used
qualitatively to describe the eects of pH and wetting
sequence on crude oil sorption.
4. Conclusions
Associate EditorÐG.A. Wol
Sorption of crude oil from an organic phase onto
silica gel or quartz is aected by equilibration time, oilto-sorbent ratio, wetting sequence, and in some cases,
aqueous pH. In all systems studied here, sorption
reaches steady-state in approximately 24 h, regardless of
sorbent, initial or ®nal water content, oil-to-sorbent
ratio, or aqueous pH. Crude oil sorption is most extensive in the absence of an aqueous phase; the sorption
capacity of silica gel is at least four times that of powdered quartz. Sorption is reduced in the presence of an
aqueous phase, by an amount that is dependent on the
order in which the aqueous and organic liquids are
equilibrated with the sorbent. Aqueous pH only aects
Acknowledgements
This research was supported in full by funding to
Rosemary Knight under Grant No. DE-FG0796ER1471, Environmental Management Science Program, Oce of Science and Technology, Oce of
Environment Management, United States Department
of Energy (DOE). However, any opinions, ®ndings,
conclusions or recommendations expressed herein are
those of the author and do not necessarily re¯ect the
views of DOE. The author is grateful to Rosemary
C.J. Daughney / Organic Geochemistry 31 (2000) 147±158
Knight and Traci Bryar for their valuable comments, to
Paul Harrison for the use of his spectrophotometer, and
to Janusz Laskowski for the use of his zeta potential
apparatus. This manuscript was greatly improved by the
comments of three anonymous reviewers.
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www.elsevier.nl/locate/orggeochem
Sorption of crude oil from a non-aqueous phase onto silica:
the in¯uence of aqueous pH and wetting sequence
Christopher J. Daughney*
Department of Earth and Ocean Sciences, University of British Columbia, 6339 Stores Road Vancouver, B.C., V6T 1Z4, Canada
Received 8 March 1999; accepted 23 August 1999
(Returned to author for revision 11 May 1999)
Abstract
Sorption of crude oil by mineral surfaces may aect its ¯ow and recovery in porous media. Here, crude oil sorption
from a toluene±heptane phase onto quartz and silica gel has been investigated both in the presence and absence of an
aqueous phase, as a function of time, oil-to-sorbent ratio, pH, and wetting sequence. Steady-state is attained in roughly
24 h, with sorption following Freundlich isotherms. Maximum sorption capacity of initially dry quartz is roughly 2 mg
oil per g; that of initially dry silica gel is greater than 8 mg per g. Sorption capacities of pre-wetted quartz and silica gel
are approximately 0.5 and 0 mg oil per g, respectively. Aqueous pH only aects sorption by pre-wetted quartz, where
sorption is greatest at pH 4, roughly 25% less at pH 7, and roughly 50% less at pH 2. The eect of pH and wetting
sequence on oil sorption can be qualitatively described by calculating the electrostatic interaction energy between the
mineral±water and oil±water interfaces. # 2000 Elsevier Science Ltd. All rights reserved.
Keywords: Oil; pH; Silica; Sorption; Wetting sequence
1. Introduction
The sorption of crude oil onto mineral surfaces may
aect its migration in the subsurface (Yamamoto, 1992;
Thomas and Clouse, 1995), its recovery from reservoir
rocks (Morrow et al., 1986; Morrow, 1990), and its
remediation in the event of environmental contamination (Domenico and Schwartz, 1990). Thus, an understanding of the factors aecting the extent of sorption is
critical to the prediction of the fate, or mobility, of
crude oil in natural environments. In this study, laboratory experiments are conducted to monitor the sorption
of crude oil from a non-aqueous phase liquid onto
quartz and silica gel, both in the presence and the
absence of an aqueous phase. This study provides
insight into the processes that control sorption in
* Current address: Department of Earth Sciences, University of
Ottawa, 140 Louis Pasteur, Ottawa, Ontario, K1N 6N5, Canada.
Tel.: +613-562-5800, ext. 6848; fax: 613-562-5192.
E-mail address: [email protected]
natural environments, and also provides information
that can be used to control the extent of crude oil sorption in the laboratory.
Many previous laboratory studies have investigated
crude oil sorption from a non-aqueous phase onto
mineral surfaces, both in the presence and the absence of
water, and several controlling variables have been identi®ed. In the absence of water, the extent of sorption is
observed to depend upon (1) the chemistry of the crude
oil, (2) the concentration of the crude oil, (3) the composition of the non-aqueous liquid used to solubilize the
crude oil, (4) the concentration, type, surface area and
roughness of the mineral, (5) temperature, and (6) equilibration time (Czarnecka and Gillot, 1980; Crocker and
Marchin, 1988; Jadhunandan and Morrow, 1991; GonzaÂlez and Tavalioni-Louvisse, 1993; Akhlaq et al.,
1997). If both aqueous and non-aqueous phase liquids
are present, the sorption of crude oil by mineral surfaces
depends not only on the six parameters listed above, but
also on (7) the composition of the aqueous liquid, particularly its pH and salinity, and (8) the order, or wetting
sequence, in which the aqueous and non-aqueous liquids
0146-6380/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0146-6380(99)00130-8
148
C.J. Daughney / Organic Geochemistry 31 (2000) 147±158
Nomenclature
aHb Proton activity in the bulk solution (mol lÿ1)
aHs Proton activity at the silica±water or oil±
water interface (mol lÿ1)
C
Equilibrium concentration of oil in organic
phase (mg lÿ1)
d
Separation distance between silica±water and
oil±water interfaces (m)
e
Elementary charge (1.610ÿ19 C)
F
Faraday constant (96485 C eqÿ1)
k
Boltzmann constant (1.3810ÿ23 J Kÿ1)
K
Empirical constant in Freundlich isotherm
(units vary)
K1 Stability constant for protonation of silanol
functional groups (mol lÿ1)
K2 Stability constant for deprotonation of silanol functional groups (mol lÿ1)
Ka Acid dissociation constant for crude oil
organics (mol lÿ1)
Kb Basicity constant for crude oil organics (mol lÿ1)
n
Empirical constant in Freundlich isotherm (ÿ)
ne
Number of electrolyte ion pairs per unit
volume (mÿ3)
R
Gas constant (8.3145 J Kÿ1 molÿ1)
T
Absolute temperature (K)
x
Distance between shear plane and silica±
water or oil±water interface (m)
"
Relative dielectric permittivity of water (80)
"0
Permittivity of free space (8.85410ÿ12 C
Vÿ1 mÿ1)
ÿ
Sorption density (mg gÿ1)
0
Electric charge at the silica±water or oil±
water interface (C mÿ2)
Electric potential at the silica±water or oil±
0
water interface (V)
Zeta potential (V)
x
Ge Electrostatic component of Gibbs free energy
change (J molÿ1)
are equilibrated with the sorbent (Brown and Neustadter, 1980; Crocker and Marchin, 1988; Buckley et al.,
1989; Ardebrant and Pugh, 1991; Skauge and Fosse,
1994; Valat et al., 1994; Liu and Buckley, 1997).
Of the eight variables listed above, the chemistry of
the crude oil is the most dicult to constrain. Crude oil
is a complex mixture of hydrocarbons with wide ranges
of molecular weights and structures that are experimentally dicult, if not impossible, to characterize completely
(Speight and Moschopedis, 1981; Semple et al., 1990).
Thus eorts have been made to describe the extent of oil
sorption using parameters that are related to oil chemistry
but are more easily measured. The eect of crude oil
composition on sorption can be related to the oil±water
contact angle (Brown and Neustadter, 1980; Barranco et
al., 1997; Liu and Buckley, 1997), the oil±water interfacial tension (Skauge and Fosse, 1994; Barranco et al.,
1997), or the zeta potential of oil droplets in water
(Buckley et al., 1989; Ardebrant and Pugh, 1991; Dubey
and Doe, 1993).
The zeta potential is a particularly useful proxy variable for oil composition, because it can be used to
develop a molecular-scale model of the oil±water interface (Takamura and Chow, 1985; Buckley et al., 1989;
Doe, 1994; Liu and Buckley, 1997). The zeta potential
can be related to the electric potential at the oil±water
interface through a number of electric double layer
models (Westall and Hohl, 1980). The electric potential
at the oil±water interface is assumed to depend on the
protonation and deprotonation of speci®c functional
groups (i.e. carboxyl, amine, etc.) present at the interface. Thus, zeta potential data collected as a function of
pH can be used to determine the concentration and
deprotonation constant of each type of interfacial functional group (Takamura and Chow, 1985; Buckley et
al., 1989). Potentiometric titration data can also be used
to determine the concentrations and deprotonation
constants of interfacial functional groups (American
Society for Testing and Materials, 1987a, b; Dubey and
Doe, 1993). Combination of zeta potential and potentiometric titration data therefore permits the development of a robust model of the chemistry of the oil±water
interface. This approach can also be applied to the
mineral±water interface (Menon and Wasan, 1986;
Braggs et al., 1994). Unlike the zeta potential, contact
angle and interfacial tension are macroscopic measurements that are not easily related to potentiometric
titration data or to interfacial chemistry at the molecular level. Similarly, many bulk properties of crude oil
commonly reported in the literature (i.e. viscosity, density, and hydrogen±carbon±nitrogen ratio) cannot be
directly related to interfacial chemistry.
Molecular-scale models of the oil±water and mineral±
water interfaces can be useful for resolution of the
components of the Gibbs free energy of sorption. It is
well established that crude oil sorption occurs through
physico-chemical (van der Waals, hydrophobic and
structural) and electrostatic interactions (Aronson et al.,
1978; Brown and Neustadter, 1980; Buckley et al., 1989;
Jadhunandan and Morrow, 1991; Dubey and Doe,
1993; Liu and Buckley, 1997). With the concentrations
and deprotonation constants of all surface functional
groups known, it becomes possible to calculate the
interfacial potentials at any pH and salinity (Takamura
and Chow, 1985), and determine the electrostatic component of the Gibbs free energy of sorption (Hogg et al.,
1966). Thus if the overall Gibbs free energy of sorption
is measured experimentally (e.g. Ardebrant and Pugh,
C.J. Daughney / Organic Geochemistry 31 (2000) 147±158
1991), and if the electrostatic component is calculated
using a model of interfacial chemistry, then the physicochemical component of sorption free energy can be
determined by dierence (Stumm and Morgan, 1981).
An ability to quantify the physico-chemical component
of sorption free energy is desirable because it can be
incorporated into many common geochemical speciation codes. Ultimately, such speciation codes can be
used to predict the fate of crude oil in a variety of geochemical environments.
Several workers have developed molecular-scale
models of the mineral±water and oil±water interfaces,
and related these models to oil sorption, but the relationship between pH, wetting sequence and extent of
sorption remains to be addressed. Buckley et al. (1989),
Dubey and Doe (1993) and Skauge and Fosse (1994)
present adhesion maps that show the pH and salinity
conditions under which oil adhesion (sorption) is
observed or expected, but the extent of sorption (mg oil
sorbed per g sorbent) is not reported. Additionally,
previous research on the importance of wetting sequence
is equivocal. Some authors have reported that a mineral
pre-wetted with an aqueous phase will sorb more oil (or
similar organic) than a dry mineral surface (Brown and
Neustadter, 1980; Lagerge et al., 1993), whereas other
authors have reported the opposite (Crocker and
Marchin, 1988). It is possible that these contradictory
reports arise due to dierences in the pH of the aqueous
phase and its eect on the electrostatic component of
sorption free energy. For example, under some pH conditions, oil sorption may be favoured due to a strong
electrostatic attraction between the oil±water and
mineral±water interfaces, and thus the presence of an
aqueous phase may increase sorption relative to that
observed in the absence of an aqueous phase.
The objective of this research is to examine the relationship between pH, wetting sequence, and extent of oil
sorption. Crude oil sorption is investigated for three
dierent wetting sequences: (1) the sorbents are equilibrated with excess water before the organic phase is
introduced to the system, (2) the sorbents are equilibrated with the organic phase prior to the addition of
water, and (3) water is absent entirely. In each case,
sorption is investigated as a function of equilibration
time, and where applicable, as a function of aqueous
pH. Silica gel and powdered quartz are used as analogues for natural mineral surfaces, Cold Lake crude oil
dissolved in a mixture of toluene and heptane is used as
the organic phase, and 0.01 M NaCl is used as the aqueous phase. Potentiometric titrations and zeta potential
measurements are used to characterize the relationship
between aqueous pH and electric charge distribution at
the mineral±water and oil±water interfaces. Batch
adsorption experiments are conducted to investigate the
eect of wetting sequence, pH, and equilibration time
on the extent of oil sorption.
149
2. Experimental
2.1. Materials
Reagent grade silica gel and powdered quartz were
obtained from B. D. H. and Alpha-Aesar, respectively.
Both solids were successively rinsed in 10% HCl, 10%
NaOH and distilled, de-ionized (DDI) water (18 M
),
then oven-dried, in order to remove impurities and
homogenize the surfaces. BET N2-adsorption surface
areas for the silica gel and the powdered quartz were 356
and 0.2 m2/g, respectively (Micromeretics Inc., Atlanta,
Georgia). Cold Lake crude oil was obtained from Imperial
Oil (Calgary, Alberta). Reagent grade 1 M HCl and 1 M
NaOH (Aldrich) were used to adjust pH, and 0.01 M NaCl
(Aldrich) was used as the background electrolyte.
2.2. Zeta potential measurement
Colloidal dispersions of silica gel, powdered quartz or
crude oil in 0.01 M NaCl were prepared using an ultrasonic disaggregator. The pH of the electrolyte was
adjusted with HCl or NaOH, and the dispersions were
allowed to stand for 2 h at 25 C in order to allow for
equilibration and settling of larger particles. For each
dispersion, zeta potential was measured for 25 particles
using a Zeta-Meter 3.0+ ®tted with a UVA-II cell
(Zeta-Meter, Inc., N. Y.). The pH of each dispersion
was determined immediately after the zeta potentials
were measured.
2.3. Potentiometric titrations
Proton binding by silica gel and powdered quartz was
studied in a 0.01 M NaCl electrolyte solution. Suspensions containing 5±10 g silica gel or powdered quartz in
100 ml electrolyte were titrated using standardized HCl
and NaOH, in order to determine the deprotonation
constants and the concentrations of functional groups
present on the solid surfaces. The titrations were conducted at 25 C in polypropylene reaction vessels, with
suspensions mixed by a magnetic stirrer. All solutions
were bubbled with N2 before and during the titrations in
order to purge them of dissolved CO2. Titrations were
performed sequentially in up-pH and down-pH directions and repeated in triplicate to evaluate the reversibility of the reactions and the reproducibility of the
experimental method.
Proton binding by the organic molecules present in
the crude oil was studied in non-aqueous solution using
ASTM methods D664-87 and D4739-87 (American
Society for Testing and Materials, 1987a, b). Brie¯y, 2±3
g of oil were dissolved in 100 ml of a mixture of toluene,
isopropyl alcohol, chloroform and a small amount of
water. The solutions were titrated using standardized
HCl and/or KOH in isopropyl alcohol. The titrations
150
C.J. Daughney / Organic Geochemistry 31 (2000) 147±158
were conducted at 25 C in glass reaction vessels, and
were mixed and purged of CO2 as described above.
Titrations were performed sequentially in up-pH and
down-pH directions and repeated in triplicate.
2.4. Sorption experiments
Two sets of batch experiments were conducted to
study sorption as a function of equilibration time.
Identical Te¯on reaction vessels were used, each containing 1.5±2 g oven-dried silica gel or powdered quartz
and 25 ml of an oil±toluene±heptane mixture (65 mg oil/
l, 1:1 toluene:heptane by volume). In the ®rst set of
experiments, the solids and the organic phase were
mixed and placed in a rotary tumbler at 25 C for
between 15 min and 4 days, after which time the concentration of oil remaining in the toluene±heptane phase
was determined by spectrophotometry at 402 nm, using
the method of GonzaÂlez and Middea (1987). In the second set of experiments, the solids were equilibrated with
10 ml of 0.01 M NaCl at pH 2, 4 or 7 for 3 days prior to
the addition of the oil±toluene±heptane phase. The
reaction vessels were tumbled for an additional 15 min
to 4 days and analyzed as above.
Three sets of batch experiments were conducted to
investigate sorption as a function of wetting sequence,
oil-to-sorbent ratio, and aqueous pH. Identical Te¯on
reaction vessels were used, each containing 1.5±2 g
oven-dried silica gel or powdered quartz and 25 ml of a
toluene±heptane mixture (1:1 by volume) with dierent
initial concentrations of oil (5±500 mg/l). First, based on
the results of the kinetic experiments, the suspensions were
tumbled for three days at 25 C, then the oil±toluene±
heptane phase was analyzed for oil as above. In the second
set of experiments, the solids were equilibrated with the
oil±toluene±heptane mixtures for 3 days, after which
time 10 ml of 0.01 M NaCl were added to each reaction
vessel. The reaction vessels were tumbled for an additional
three days before the organic phase was analyzed for oil.
In the third set of experiments, the solids were equilibrated
with 10 ml of 0.01 M NaCl for 3 days before the addition of the oil±toluene±heptane mixtures. After tumbling for an additional three days, the toluene±heptane
phase was analyzed as above. The second and third sets
of experiments were repeated at aqueous pH 2, 4 and 7.
One set of batch experiments was conducted to investigate the reversibility of the sorption reaction. Between 1.5
and 2 g oven-dried silica gel or powdered quartz were
equilibrated with 35 ml of the oil±toluene±heptane mixture containing varying initial concentrations of oil.
After 3 days, 10 ml of 0.01 M NaCl were added to each
vessel, and 10 ml of the oil±toluene±heptane mixture
were removed and analyzed by spectrophotometry.
After tumbling for an additional 3 days, the oil±toluene±
heptane mixtures were reanalyzed. These experiments
were repeated at aqueous pH 2, 4 and 7.
Fig. 1. Experimental data collected during potentiometric
titration of (a) silica gel and (b) powdered quartz in 0.01 M
NaCl. Open triangles represent down-pH titration; ®lled circles
represent up-pH titration. Model ®t to experimental data is
indicated by the solid line; dashed line represents titration of
background electrolyte. Note the dierence in scale.
3. Results and discussion
3.1. Characterization of the silica±water interface
The experimental data collected during potentiometric titration of silica gel indicate that it imparts a
signi®cant buering capacity to the suspensions, over
and above that of the electrolyte alone (Fig. 1a). Powdered quartz imparts a lesser buering capacity to the
solution per unit weight (Fig. 1b). For both solids, buffering is most signi®cant for pH >7. The data in Fig. 1
were collected as titrations were conducted sequentially
in both up-pH and down-pH directions, indicating that
the reactions are reversible and that equilibrium is
attained. This reversibility is not representative of solid
dissolution or surface modi®cation, and therefore the
dierence between the titration curves of the silica gel or
powdered quartz and the electrolyte can be attributed
completely to proton binding by the surfaces.
Several researchers have shown that amphoteric silanol functional groups (SiOH0) are responsible for
proton binding at the silica±water interface (Dugger et
C.J. Daughney / Organic Geochemistry 31 (2000) 147±158
al., 1964; Schindler et al., 1976; Yates and Healy, 1976;
Sermon, 1980; Dove and Rimstidt, 1994; Osthols, 1995):
0
*
SiOH
2 ) SiOH H
1
SiOH0 *
) SiOÿ H
2
The mass action equations corresponding to the
above equilibria are:
SiOH0 aHs
K1
SiOH
2
3
SiOÿ aHs
K2
SiOH0
4
The square brackets represent the concentration of
the enclosed surface species (mol/kg solution) and aHs
represents the proton activity at the solid surface, which
is related to the proton activity in the bulk solution,
aHb :
aHs aHb exp ÿF
0 =RT
5
The variables F; 0 ; R and T represent Faraday's
constant, the electric potential of the surface, the gas
constant, and the absolute temperature, respectively.
The electric potential of the surface ( 0 ) cannot be
measured directly, but can be related to the experimentally determined surface charge (0 ) by several double
layer models (Westall and Hohl, 1980). Here, the GouyChapman model is applied:
F 0
0 8""0 ne KT1=2 sin h
6
2RT
where "; "0 ; ne and k represent the relative dielectric
permittivity of water, the permittivity of free space, the
number of electrolyte ion pairs per unit volume, and
Boltzmann's constant, respectively. In the Gouy±Chapman model, d ÿ0 , where d is the charge of the diffuse portion of the double layer.
In this study, the computer program FITEXP is used
to determine the concentration (mol of sites per m2) and
protonation constants (K1 and K2 ) of the silanol functional groups, in order to identify the model which best
describes the experimental data. FITEXP is a version of
FITEQL 2.0 (Westall, 1982a,b) modi®ed by Johannes
LuÈtzenkirchen (Department of Inorganic Chemistry,
Umea University, Sweden; pers. comm.) to allow
simultaneous modeling of titrations conducted at dierent solid-to-solution ratios. All model calculations apply
conventional standard states, and all stability constants
are referenced to 25 C, the ionic strength of the background electrolyte, and zero surface potential. The stability constants describing the dissociation of water, the
151
electrolyte, the acid and the base are included in all calculations.
The silica gel titration data are well described by a
model with 2.310ÿ6 mol of silanol sites per m2, with
stability constants K1 =10ÿ1.5 and K2 =10ÿ8.0. Powdered quartz titration data are described by a similar
model, with 4.210ÿ6 mol of silanol sites/m2, K1 =10ÿ1.5
and K2 =10ÿ8.3. These model parameters are in reasonable agreement with previous work, where surface site
concentrations determined by titration vary from
710ÿ6 to 210ÿ5 mol/m2 (James and Parks, 1982), K1
varies from 10ÿ2.3 (Schindler and Stumm, 1987) to
10ÿ1.0 (Riese, 1982), and K2 ranges from 10ÿ7.5 to 10ÿ4.6
(James and Parks, 1982; Anderson and Benjamin, 1990).
The reported values of these parameters vary due to
dierences in the type of solid used, the pre-treatment
method, the background electrolyte, and the type of
double layer model applied.
Zeta potential of silica gel is approximately zero
below pH 5 and negative above pH 5. Powdered quartz
zeta potential is positive below pH 2 and negative above
pH 2 (Fig. 2). Using the Gouy±Chapman model, the
potential at the surface ( 0 ) can be related to the zeta
potential ( x , where x represents the distance between
the shear plane and the surface):
x
2RT 1
0 exp ÿkx
ln
F
1 ÿ
0 exp ÿkx
7
where
0
exp F
exp F
ÿ1
=2RT
1
0
0 =2RT
2e2 ne =""0 KT1=2
8
9
and e is the electron charge. The location of this shear
plane is not known, but it is generally assumed to lie
0.5±1 nm from the surface (Takamura and Chow, 1985).
Fig. 2. Zeta potential of colloidal dispersions containing silica
gel, powdered quartz or Cold Lake crude oil. Lines indicate
model ®t to experimental data.
152
C.J. Daughney / Organic Geochemistry 31 (2000) 147±158
The surface potential can be related to the surface
charge by Eq. 6, and the surface charge arises due to
pH-dependent protonation and deprotonation of the
functional groups [Eq. (1) and (2)], making it possible to
solve for the functional group concentration and the
values of the stability constants K1 and K2 .
The silica gel zeta potential data are best described by
a model with 2.410ÿ6 mol of sites/m2, K1 =10ÿ1.5 and
K2 10ÿ7:6 . Powdered quartz zeta potential data are
best ®t by a model with 4.410ÿ6 mol of sites/m2, K1
10ÿ1:5 and K2 10ÿ3:8 . Literature values for silanol site
concentration determined from zeta potential data
range from 4.210ÿ6 moles/m2 (Buckley et al., 1989) to
roughly 110ÿ5 mol/m2 (Doe, 1994). Buckley et al.
(1989) report K1 10ÿ1:0 and K2 10ÿ4:0 for quartz.
Few other researchers have used zeta potential data to
calculate K1 and K2 , but zeta potential vs pH curves
commonly show points of zero charge at pH 1.5±3.0 and
nearly identical negative charge development at pH >5,
in agreement with the models presented here (Li and De
Bruyn, 1966; Gonzalez and Middea, 1987; Buckley et
al., 1989; Ardebrant and Pugh, 1991; Doe, 1994).
When the titration data and the zeta potential data
are modeled simultaneously using FITEXP, the silica
gel model parameters are 2.310ÿ6 mol of sites/m2,
K1 10ÿ1.5 and K2 =10ÿ7.8, in good agreement with the
literature values. The powdered quartz models developed from titration data and from zeta potential data
have similar silanol site concentrations and values for
K1 , which are also in good agreement with the literature,
but computed values of K2 dier by several orders of
magnitude. Of the two models, that based on zeta
potential data is more sensitive to K2 , because the buffering capacity (measured by potentiometric titration) is
small relative to the background electrolyte. The value
of K2 determined from zeta potential data is not signi®cantly aected by the position assigned to the shear
plane. In contrast, the position assigned to the shear
plane does aect the model silanol site concentration, so
the site concentration is more reliably computed from
the titration data. Thus when the titration data and the
zeta potential data are treated simultaneously using
FITEXP, the powdered quartz model parameters are
4.710ÿ6 mol of sites/m2, K1 =10ÿ1.5 and K2 =10ÿ3.8.
These best-®tting model parameters are used in all subsequent calculations (Table 1). The model ®t is compared to the experimental data in Figs. 1 and 2.
3.2. Characterization of the oil±water interface
The experimental data collected during non-aqueous
titration of Cold Lake crude oil indicate a signi®cant
capacity to neutralize both acid and base relative to the
solvent alone (Fig. 3). Titrations were rapid and rever-
Fig. 3. Electromotive force (EMF, mV) data collected during
non-aqueous potentiometric titration of Cold Lake crude oil by
(a) HCl and (b) KOH. Dashed line represents titration of
background solvent.
Table 1
Model parameters used to characterize interfacial chemistry
Interface
Reaction
Site concentration
Ka
Gel±water
0
+
*
SiOH+
2 )SiOH +H
)SiOÿ+H+
SiOH0*
0
+
*
SiOH+
2 )SiOH +H
)SiOÿ+H+
SiOH0*
AH0*
)Aÿ+H+
)B0+H+
BH+*
2.310ÿ6 mol/m2
10ÿ1.5
10ÿ7.8
10ÿ1.5
10ÿ3.8
10ÿ4.4
10ÿ1.0
Quartz±water
Oil±water
a
4.710ÿ6 mol/m2
7.910ÿ6 mol/g
2.910ÿ6 mol/g
Stability constant referenced to zero surface potential and the ionic strength of the background electrolyte.
C.J. Daughney / Organic Geochemistry 31 (2000) 147±158
sible, suggesting that equilibrium is attained. Several
researchers have suggested that the electric charge distribution at the oil±water interface is due to protonation
and deprotonation of organic molecules present in the
oil. The chemical structure of the organic molecules is
often highly variable and dicult to identify precisely
(Speight and Moschopedis, 1981; Semple et al., 1990),
but carboxylic acid and pyridine base groups are often
assumed responsible for the majority of the interfacial
charge (Dutta and Holland, 1984; Takamura and Chow,
1985; Buckley et al., 1989; Dubey and Doe, 1993; GonzaÂlez and Travalioni-Louvisse, 1993; Buckley, 1994;
Doe, 1994; Liu and Buckley, 1997). The deprotonation
of the generalized acidic group AH0 can be represented
by the following reaction and mass action law:
are well described by a model with Ka 10ÿ4:4 and
Kb 10ÿ1:0 . Ka and Kb values can range from 10ÿ12 to
10ÿ2 (Dutta and Holland, 1984). Based on the magnitudes of the stability constants determined here, the
acidic functional groups are likely carboxylic, and the
basic groups may represent pyrazine or sulfoxide structures. Although the charging behaviour of each crude
oil is dierent, Brown and Neustadter (1980), Takamura
and Chow (1985), Chow and Takamura (1988), Buckley
et al. (1989), Dubey and Doe (1993) and Doe (1994)
have reported comparable zeta potential vs pH trends in
similar electrolytes. The model ®t is compared to the
experimental data in Fig. 2, with parameters listed in
Table 1.
3.3. Sorption experiments
AH0 *
) A ÿ H
10
Aÿ aHs
Ka
AH0
11
where Ka is the acid dissociation constant for the
reaction, and other symbols are as de®ned above. The
deprotonation of the generalized basic group BH+ can
be represented by:
BH *
) B0 H
12
B0 aHs
Kb
BH
153
13
where Kb is the basicity constant. The organic phase
may contain several types of organic molecules, each
with dierent absolute concentrations of acidic and
basic functional groups and corresponding stability
constants.
In this study, titration of Cold Lake crude oil by base
shows one in¯ection point, corresponding to an acidic
functional group with a concentration of 7.910ÿ6 mol/
g oil. Literature values for acidic functional group concentration of dierent varieties of oil vary from roughly
3.310ÿ6 (Takamura and Chow, 1985) to 410ÿ4 mol/
g. (Dutta and Holland, 1984). Titration of Cold Lake
crude oil by acid also reveals one in¯ection, corresponding to a basic functional group with a concentration of 2.910ÿ6 mol of sites/g oil. Literature values,
again for dierent varieties of oil, range from 1.410ÿ6
mol/g (Dubey and Doe, 1993) to 1.810ÿ4 mol/g (Dutta
and Holland, 1984). Dubey and Doe (1993) have noted
that the ASTM standard used here (American Society
for Testing and Materials, 1987b) may underestimate
the concentration of weakly basic substances in the oil.
Due to the inaccuracy of pH measurement in the nonaqueous titration solvent, values of Ka and Kb cannot be
determined from these data alone.
Using the site concentrations determined by titration,
zeta potential measurements of oil dispersions (Fig. 2)
Experiments conducted as a function of time indicate
that sorption of oil by powdered quartz reaches a
steady-state in approximately 24 h, both in the presence
and absence of an aqueous phase (Fig. 4). The sorption
of oil by oven-dry silica gel occurs at a similar rate (data
not shown). The rate of sorption is not signi®cantly
aected by the oil-to-solid ratio, wetting sequence or the
pH of the aqueous phase, except in the case of pre-wetted silica gel, where sorption is negligible even after 4
days. Literature values for equilibration time vary from
less than 1 min (Valat et al., 1994) to more than 10 days
(Liu and Buckley, 1997), though the observed equilibration time may be strongly controlled by the nature of
the system investigated and the experimental methods
applied. In order to allow for experimental errors and
trials involving desorption, all experiments were permitted to equilibrate for 3 days.
Sorption of crude oil by oven-dry powdered quartz
plateaus at approximately 1.5±2.0 mg/g (Fig. 5a). Most
minerals with surface areas of 1±50 m2/g have comparable
Fig. 4. Representative data showing sorption of oil by powdered quartz as a function of equilibration time, both in the
presence and absence of an aqueous phase (type of sorbent, oilto-sorbent ratio, initial wetting state, and aqueous pH do not
aect the approach to steady-state).
154
C.J. Daughney / Organic Geochemistry 31 (2000) 147±158
maximum sorption capacities for crude oil organics
(GonzaÂlez and Middea, 1987; Crocker and Marchin,
1988; GonzaÂlez and Moreira, 1991; GonzaÂlez and Travalioni-Louvisse, 1993). Due to its greater surface area,
the maximum sorption capacity of oven-dry silica gel is
greater than that of the powdered quartz, but it is not
attained under these experimental conditions (Fig. 5b).
In the absence of an aqueous phase, oil sorption can be
represented by the following schematic reaction:
SiOH0 R *
) SiOH ÿ R0
14
where R represents the organic sorbate. Note that the
stoichiometry of the reaction and the chemistry of the
organic sorbate are unknown.
Experiments conducted with various wetting sequences indicate that less oil sorption occurs when water is
present than when it is absent. Where the solids are
allowed to equilibrate with the oil±toluene±heptane phase
before the water is introduced, oil sorption by powdered
quartz is reduced by approximately 10% relative to the
oven-dry case (Fig. 5a). Under the same conditions, oil
sorption by silica gel is reduced by roughly 50% (Fig. 5b).
These data may re¯ect the exchange of previously sorbed
oil molecules [Eq. (14)] for water molecules, as represented
by the following schematic reaction:
SiOH ÿ R0 H2 O *
) SiOH ÿ H2 O0 R
15
The displaced oil molecules return to the organic
phase, but again, the stoichiometry of the reaction is
unknown. The experimental data indicate that water
molecules are more able to cause organic desorption
from silica gel than from powdered quartz.
Where the solids are equilibrated with the aqueous
phase before the oil±toluene±heptane phase is added,
organic sorption by powdered quartz is 50±80% less
than the oven-dry case (Fig. 5c), and no organic sorption by silica gel occurs (Fig. 5d). These data may
represent the ability of the oil to displace previously
sorbed water molecules, as shown by the following
schematic reaction:
* SiOH ÿ R0 H2 O
SiOH ÿ H2 O0 R )
16
Where the sorbent is initially wetted powdered quartz,
the extent of sorption is aected by the pH of the aqueous
phase (Fig. 5c). In this system, sorption is greatest at pH 4,
roughly 25% less at pH 7, and roughly 50% less at pH 2.
Organic sorption by pre-wetted silica gel is not detectable.
The relationship between the extent of oil sorption
(ÿ, mg oil sorbed per g sorbent) and the equilibrium
concentration of oil in the organic phase (C, mg/l) is
best described by the Freundlich isotherm:
Fig. 5. Sorption of oil by (a) powdered quartz and (b) silica gel, with the aqueous phase either absent or introduced after the organic
phase, and sorption of oil by (c) powdered quartz and (d) silica gel equilibrated with the aqueous phase prior to the addition of the
organic phase.
155
C.J. Daughney / Organic Geochemistry 31 (2000) 147±158
ÿ KCn
desorption involve dierent interactions, and thus sorption cannot be considered a reversible process. Several
other researchers have noted irreversibility in oil sorption reactions (Brown and Neustadter, 1980; Ardebrant
and Pugh, 1991; Thomas et al., 1993; Yan and
Masliyah, 1994; Liu and Buckley, 1997).
17
where K is a constant related to the overall change in
Gibbs free energy of the reaction and n is a constant.
The Langmuir isotherm (Stumm and Morgan, 1981)
also provides a good ®t to the data (R2 >0.85). However, because many of the experimental trials do not
display a sorption plateau, many of the Langmuir
parameters obtained by regression do not provide reasonable estimates of maximum sorption capacity (ÿmax ).
Site speci®c surface complexation models cannot be
applied because the stoichiometries of the sorption
reactions are unknown. Hence, only the Freundlich isotherm parameters K and n are compiled in Table 2. The
experimental data indicate that the amount of sorbed oil
is strongly dependent on wetting sequence and pH, and
thus the sorption reactions must be considered irreversible. Where the sorbents are equilibrated with water
prior to the addition of the organic phase, little or no oil
sorption occurs. In contrast, if the sorbents are equilibrated with the organic phase before the water is introduced, signi®cantly more oil sorption results, even
though the ®nal system composition is identical. This
suggests that oil molecules do not displace sorbed water
eciently, but that sorbed oil is very dicult to replace
with water. In thermodynamic terms, this indicates that
the Gibbs free energies of sorption (Eq. 16) and desorption (Eq. 15) are not equal in magnitude but opposite
in sign. The varying eect of pH also suggests that
sorption is irreversible. Oil sorption is strongly dependent on pH, whereas desorption is not. The pH aects
the charge distribution at the mineral-water and oil±
water interfaces, and subsequently aects the electrostatic interaction between them. The data therefore
suggest that electrostatic interactions are pH-dependent
during sorption, but not during desorption. This may
indicate that the organic sorbates are non-ionizable,
such that once sorbed, an electric double layer cannot
develop adjacent to the solid surface. Regardless, the
varying eect of pH indicates that sorption and
Fig. 6. Electrostatic interaction energy (a) between the quartz±
water and oil±water interfaces and (b) between the gel±water
and oil±water interfaces, both as a function of pH and interfacial separation. Negative values indicate interfacial attraction; positive values indicate interfacial repulsion, normalized
to the concentration of silanol sites.
Table 2
Model parameters used to characterize oil sorption
Sorption by gel
Wetting technique
pH
Ka
No aqueous phase
Aqueous phase added after
organic phase
±
2
4
7
2
4
7
10ÿ1.40
10ÿ1.15
10ÿ1.18
10ÿ1.28
Aqueous phase added before
organic phase
a
b
Freundlich isotherm parameters [Eq. (17)].
Correlation coecient.
Sorption by quartz
na
(R2)b
Ka
na
(R2)b
1.05
0.78
0.80
0.83
No sorption
No sorption
No sorption
1.00
0.99
0.99
1.00
10ÿ0.43
10ÿ1.37
10ÿ1.48
10ÿ1.42
10ÿ2.92
10ÿ3.17
10ÿ2.53
0.27
0.66
0.71
0.69
0.95
1.14
0.79
0.98
0.99
0.97
0.98
0.89
0.93
0.88
156
C.J. Daughney / Organic Geochemistry 31 (2000) 147±158
Although the irreversibility of the sorption reactions
prevents the development of a quantitative thermodynamic model, calculation of the electrostatic component of sorption free energy (Ge ) provides a qualitative
means of describing some of the data. It is possible to
calculate Ge at any pH, because interfacial potentials
can be calculated using the model parameters compiled
in Table 1. Speci®cally, as two ¯at, parallel interfaces
with unequal surface potentials 0a and 0b move from
in®nite separation to separation d; Ge is (Hogg et al.,
1966):
""o
Ge
8
2
0a 20b 1 ÿ coth2d 2
18
0a
0b cosech2d
providing the potentials remain small and constant
during the interaction. Assuming that the quartz±water
and oil±water interfaces must approach to within 0±1
nm to permit oil sorption (Takamura and Chow, 1985),
the electrostatic interaction between them can be either
negative (attractive) or positive (repulsive), depending
on aqueous pH (Fig. 6a). For interfacial separations of
0.8±0.9 nm, the electrostatic interaction energy is mildly
attractive at pH 4, less attractive at pH 7, and mildly
repulsive at pH 2. Under the same conditions, the electrostatic interaction between the gel±water and oil±
water interfaces is strongly repulsive at pH 4 and 7, and
mildly repulsive at pH 2 (Fig. 6b). Thus, if the physicochemical energy of sorption is constant at all pHs, then
the pH trends displayed in Figs. 5c and d may be
explained by the eect of the electrostatic interaction.
However, it is not possible to calculate the magnitude of
the physico-chemical component of sorption free energy,
because the stoichiometry and chemistry of the sorption
reactions are unknown. Hence, calculation of the electrostatic interaction between the mineral±water and oil±
water interfaces provides, at best, a qualitative means of
assessing the relationship between pH and oil sorption.
oil sorption by pre-wetted quartz, where sorption is
greatest at pH 4, less at pH 7, and least at pH 2.
A quantitative or thermodynamic model of sorption
cannot be developed because the reaction stoichiometries and sorbate chemistries are unknown, and because
the reactions are not reversible. However, the trends in
oil sorption observed here can be qualitatively described
by considering the electrostatic interaction between the
mineral±water and oil±water interfaces, as controlled by
pH and wetting sequence. In the absence of an aqueous
phase, electrostatic forces are negligible, because all
interfacial functional groups are neutrally charged.
Where both aqueous and organic phases are present, the
extent of sorption is reduced, because the oil and water
molecules compete for available surface sites. Where an
aqueous phase is introduced after the sorbents have
equilibrated with the organic phase, desorption results,
indicating that water molecules are able to displace previously sorbed oil. The eect of aqueous pH on this
exchange reaction is negligible, suggesting that electrostatic forces are either constant or insigni®cant, perhaps
because the sorbed molecules are non-ionizable and
curtail the establishment of an electric double layer.
Where the sorbents are equilibrated with an aqueous
phase before the organic phase is introduced, sorption is
more limited still. In such systems, sorption is controlled, in part, by electrostatic forces. The magnitude of
the electrostatic force can be calculated using the models
of interfacial chemistry developed from potentiometric
and zeta potential data. For an interfacial separation of
roughly 0.8±0.9 nm, there is mild to strong electrostatic
repulsion between the gel±water and oil±water interfaces
at all pHs examined here, which may explain the lack of
sorption observed. At the same interfacial separation, the
electrostatic force between the quartz±water and oil±water
interfaces is most attractive at pH 4, less at pH 7, and least
at pH 2, in agreement with the observed trend in sorption. This indicates that chemical models of the
mineral±water and oil±water interfaces can be used
qualitatively to describe the eects of pH and wetting
sequence on crude oil sorption.
4. Conclusions
Associate EditorÐG.A. Wol
Sorption of crude oil from an organic phase onto
silica gel or quartz is aected by equilibration time, oilto-sorbent ratio, wetting sequence, and in some cases,
aqueous pH. In all systems studied here, sorption
reaches steady-state in approximately 24 h, regardless of
sorbent, initial or ®nal water content, oil-to-sorbent
ratio, or aqueous pH. Crude oil sorption is most extensive in the absence of an aqueous phase; the sorption
capacity of silica gel is at least four times that of powdered quartz. Sorption is reduced in the presence of an
aqueous phase, by an amount that is dependent on the
order in which the aqueous and organic liquids are
equilibrated with the sorbent. Aqueous pH only aects
Acknowledgements
This research was supported in full by funding to
Rosemary Knight under Grant No. DE-FG0796ER1471, Environmental Management Science Program, Oce of Science and Technology, Oce of
Environment Management, United States Department
of Energy (DOE). However, any opinions, ®ndings,
conclusions or recommendations expressed herein are
those of the author and do not necessarily re¯ect the
views of DOE. The author is grateful to Rosemary
C.J. Daughney / Organic Geochemistry 31 (2000) 147±158
Knight and Traci Bryar for their valuable comments, to
Paul Harrison for the use of his spectrophotometer, and
to Janusz Laskowski for the use of his zeta potential
apparatus. This manuscript was greatly improved by the
comments of three anonymous reviewers.
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