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 contamina-tion (Domenico and Schwartz, 1990). Thus, an under-standing 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, labora-tory 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
natural environments, and also provides information that can be used to control the extent of crude oil sorp-tion 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 iden-ti®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 com-position 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) equi-libration time (Czarnecka and Gillot, 1980; Crocker and Marchin, 1988; Jadhunandan and Morrow, 1991; Gon-zaÂlez 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, parti-cularly 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.
P I I : S 0 1 4 6 - 6 3 8 0 ( 9 9 ) 0 0 1 3 0 - 8
www.elsevier.nl/locate/orggeochem
* 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.
are equilibrated with the sorbent (Brown and Neus-tadter, 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 experimen-tally 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 inter-facial 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 vari-able for oil composition, because it can be used to develop a molecular-scale model of the oil±water inter-face (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 inter-face. 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 func-tional 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 poten-tiometric titration data therefore permits the develop-ment 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 measure-ments that are not easily related to potentiometric titration data or to interfacial chemistry at the mole-cular level. Similarly, many bulk properties of crude oil commonly reported in the literature (i.e. viscosity, den-sity, 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 com-ponent 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, Nomenclature
aH
b Proton activity in the bulk solution (mol l
ÿ1) aH
s 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 (1.610ÿ19C) F Faraday constant (96485 C eqÿ1) k Boltzmann constant (1.3810ÿ23J 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
sila-nol 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 Kÿ1molÿ1) 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 (8.85410ÿ12C
Vÿ1mÿ1)
ÿ Sorption density (mg gÿ1)
0 Electric charge at the silica±water or oil±
water interface (C mÿ2)
0 Electric potential at the silica±water or oil±
water interface (V)
x Zeta potential (V)
Ge Electrostatic component of Gibbs free energy
1991), and if the electrostatic component is calculated using a model of interfacial chemistry, then the physico-chemical 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 specia-tion codes. Ultimately, such speciaspecia-tion codes can be used to predict the fate of crude oil in a variety of geo-chemical 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 rela-tionship 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 con-ditions, 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 rela-tionship between pH, wetting sequence, and extent of oil sorption. Crude oil sorption is investigated for three dierent wetting sequences: (1) the sorbents are equili-brated with excess water before the organic phase is introduced to the system, (2) the sorbents are equili-brated 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 analo-gues 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 aqu-eous 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.
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 ultra-sonic 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. Suspen-sions 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 con-ducted at 25C in polypropylene reaction vessels, with suspensions mixed by a magnetic stirrer. All solutions were bubbled with N2before and during the titrations in
order to purge them of dissolved CO2. Titrations were
performed sequentially in up-pH and down-pH direc-tions and repeated in triplicate to evaluate the reversi-bility of the reactions and the reproducireversi-bility of the experimental method.
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 con-taining 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 con-centration 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 sec-ond 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 addi-tion of the oil±toluene±heptane mixtures. After tum-bling 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 investi-gate 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 mix-ture 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.
3. Results and discussion
3.1. Characterization of the silica±water interface
The experimental data collected during potentio-metric 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). Pow-dered quartz imparts a lesser buering capacity to the solution per unit weight (Fig. 1b). For both solids, buf-fering 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 sila-nol functional groups (SiOH0) are responsible for
al., 1964; Schindler et al., 1976; Yates and Healy, 1976; Sermon, 1980; Dove and Rimstidt, 1994; Osthols, 1995): SiOH
2 *) SiOH0H 1
SiOH0*) SiOÿH 2
The mass action equations corresponding to the above equilibria are:
The square brackets represent the concentration of the enclosed surface species (mol/kg solution) and aH s represents the proton activity at the solid surface, which is related to the proton activity in the bulk solution,
aH b:
aH
s 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 experimen-tally determined surface charge (0) by several double
layer models (Westall and Hohl, 1980). Here, the Gouy-Chapman model is applied:
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±Chap-man model,d ÿ0, wheredis the charge of the
dif-fuse 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 andK2) of the silanol
func-tional 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 dier-ent 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 back-ground electrolyte, and zero surface potential. The sta-bility constants describing the dissociation of water, the
electrolyte, the acid and the base are included in all cal-culations.
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.
Pow-dered quartz titration data are described by a similar model, with 4.210ÿ6mol of silanol sites/m2,K
1=10ÿ1.5
andK2=10ÿ8.3. These model parameters are in
reason-able agreement with previous work, where surface site concentrations determined by titration vary from 710ÿ6to 210ÿ5mol/m2(James and Parks, 1982),K
1
varies from 10ÿ2.3 (Schindler and Stumm, 1987) to
10ÿ1.0(Riese, 1982), andK
2ranges from 10ÿ7.5to 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):
x2RT
andeis 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).
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 constantsK1andK2.
The silica gel zeta potential data are best described by a model with 2.410ÿ6mol of sites/m2,K
1=10ÿ1.5and K210ÿ7:6. Powdered quartz zeta potential data are
best ®t by a model with 4.410ÿ6mol of sites/m2,K 1
10ÿ1:5andK
210ÿ3:8. Literature values for silanol site
concentration determined from zeta potential data range from 4.210ÿ6moles/m2(Buckley et al., 1989) to
roughly 110ÿ5 mol/m2 (Doe, 1994). Buckley et al.
(1989) report K110ÿ1:0 and K210ÿ4:0 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, K110ÿ1.5andK2=10ÿ7.8, in good agreement with the
literature values. The powdered quartz models devel-oped 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 toK2, because the
buf-fering capacity (measured by potentiometric titration) is small relative to the background electrolyte. The value of K2 determined from zeta potential data is not
sig-ni®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,K
1=10ÿ1.5 andK2=10ÿ3.8.
These best-®tting model parameters are used in all sub-sequent calculations (Table 1). The model ®t is com-pared 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-Table 1
Model parameters used to characterize interfacial chemistry
Interface Reaction Site concentration Ka
Gel±water SiOH2+*)SiOH0+H+ 2.310ÿ6mol/m2 10ÿ1.5
SiOH0*)SiOÿ+H+ 10ÿ7.8
Quartz±water SiOH2+*)SiOH0+H+ 4.710ÿ6mol/m2 10ÿ1.5
SiOH0*)SiOÿ+H+ 10ÿ3.8
Oil±water AH0*)Aÿ+H+ 7.910ÿ6mol/g 10ÿ4.4
BH+*)B0+H+ 2.910ÿ6mol/g 10ÿ1.0
a Stability constant referenced to zero surface potential and the ionic strength of the background electrolyte.
sible, suggesting that equilibrium is attained. Several researchers have suggested that the electric charge dis-tribution 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; Gon-zaÂlez and Travalioni-Louvisse, 1993; Buckley, 1994; Doe, 1994; Liu and Buckley, 1997). The deprotonation of the generalized acidic group AH0can be represented
by the following reaction and mass action law:
AH0*)AÿH 10
Aÿa
H
s
AH0 Ka 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*)B0H 12
B0a H
s
BH Kb 13
whereKb 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 7.910ÿ6mol/
g oil. Literature values for acidic functional group con-centration of dierent varieties of oil vary from roughly 3.310ÿ6(Takamura and Chow, 1985) to 410ÿ4mol/
g. (Dutta and Holland, 1984). Titration of Cold Lake crude oil by acid also reveals one in¯ection, corre-sponding to a basic functional group with a concentra-tion 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 1.810ÿ4mol/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 non-aqueous titration solvent, values ofKaandKbcannot be
determined from these data alone.
Using the site concentrations determined by titration, zeta potential measurements of oil dispersions (Fig. 2)
are well described by a model with Ka10ÿ4:4 and Kb10ÿ1:0.KaandKbvalues can range from 10ÿ12to
10ÿ2 (Dutta and Holland, 1984). Based on the
magni-tudes of the stability constants determined here, the acidic functional groups are likely carboxylic, and the basic groups may represent pyrazine or sulfoxide struc-tures. 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
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-wet-ted 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 equili-bration 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 per-mitted 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
maximum sorption capacities for crude oil organics (GonzaÂlez and Middea, 1987; Crocker and Marchin, 1988; GonzaÂlez and Moreira, 1991; GonzaÂlez and Tra-valioni-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: SiOH0R*) SiOHÿR0 14
where Rrepresents the organic sorbate. Note that the stoichiometry of the reaction and the chemistry of the organic sorbate are unknown.
Experiments conducted with various wetting sequen-ces 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:
SiOHÿR0H2O*) SiOHÿH2O0R 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 sorp-tion 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:
SiOHÿH2O0R*)SiOHÿR0H2O 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:
ÿKCn 17
where Kis 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).
How-ever, because many of the experimental trials do not display a sorption plateau, many of the Langmuir parameters obtained by regression do not provide rea-sonable 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 iso-therm parametersKandnare 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 irrever-sible. 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 equili-brated with the organic phase before the water is intro-duced, 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 deso-rption (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 depen-dent on pH, whereas desorption is not. The pH aects the charge distribution at the mineral-water and oil± water interfaces, and subsequently aects the electro-static 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
desorption involve dierent interactions, and thus sorp-tion cannot be considered a reversible process. Several other researchers have noted irreversibility in oil sorp-tion reacsorp-tions (Brown and Neustadter, 1980; Ardebrant and Pugh, 1991; Thomas et al., 1993; Yan and Masliyah, 1994; Liu and Buckley, 1997).
Table 2
Model parameters used to characterize oil sorption
Sorption by gel Sorption by quartz
Wetting technique pH Ka na (R2)b Ka na (R2)b
No aqueous phase ± 10ÿ1.40 1.05 1.00 10ÿ0.43 0.27 0.98
Aqueous phase added after 2 10ÿ1.15 0.78 0.99 10ÿ1.37 0.66 0.99
organic phase 4 10ÿ1.18 0.80 0.99 10ÿ1.48 0.71 0.97
7 10ÿ1.28 0.83 1.00 10ÿ1.42 0.69 0.98
Aqueous phase added before 2 No sorption 10ÿ2.92 0.95 0.89
organic phase 4 No sorption 10ÿ3.17 1.14 0.93
7 No sorption 10ÿ2.53 0.79 0.88
a Freundlich isotherm parameters [Eq. (17)]. b Correlation coecient.
Although the irreversibility of the sorption reactions prevents the development of a quantitative thermo-dynamic model, calculation of the electrostatic compo-nent of sorption free energy (Ge) provides a qualitative
means of describing some of the data. It is possible to calculateGe 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 0aand 0bmove from
in®nite separation to separationd;Ge is (Hogg et al.,
1966):
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 elec-trostatic 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 physico-chemical 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 elec-trostatic interaction between the mineral±water and oil± water interfaces provides, at best, a qualitative means of assessing the relationship between pH and oil sorption.
4. Conclusions
Sorption of crude oil from an organic phase onto silica gel or quartz is aected by equilibration time, oil-to-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 exten-sive in the absence of an aqueous phase; the sorption capacity of silica gel is at least four times that of pow-dered 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
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 stoichiome-tries and sorbate chemisstoichiome-tries 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 pre-viously sorbed oil. The eect of aqueous pH on this exchange reaction is negligible, suggesting that electro-static 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 con-trolled, 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 sorp-tion. 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.
Associate EditorÐG.A. Wol
Acknowledgements
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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