Artigo Achinta Bera Adsorption
Transcript of Artigo Achinta Bera Adsorption
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Applied Surface Science 284 (2013) 87–99
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Applied Surface Science
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Adsorption of surfactants on sand surface in enhanced oil recovery:Isotherms, kinetics and thermodynamic studies
Achinta Bera, T. Kumar, Keka Ojha, Ajay Mandal∗
Department of PetroleumEngineering, IndianSchool of Mines, Dhanbad 826004, India
a r t i c l e i n f o
Article history:
Received 15May 2013
Received in revised form 5 July 2013Accepted 7 July 2013
Available online 16 July 2013
Keywords:
Surfactant
Adsorption isotherm
Isothermmodel
Adsorption kinetics
Thermodynamics of adsorption
a b s t r a c t
Adsorption of surfactants onto reservoir rock surfacemay result in the loss and reduction of their concen-
trations in surfactant flooding,whichmayrender themless efficient or ineffective inpractical applications
of enhanced oil recovery (EOR) techniques. Surfactant flooding for EOR received attraction due to itsabil-
ity to increase the displacement efficiency by lowering the interfacial tension between oil andwater and
mobilizing the residual oil. This article highlights the adsorption of surfactants onto sand surface with
variation of different influencing factors. It has been experimentally found that adsorption of cationic
surfactant on sandsurface ismore and less for anionic surfactant, while non-ionic surfactant shows inter-
mediate behaviour. X-ray diffraction (XRD) study of clean sand particles hasbeenmade to determine the
maincomponentpresent in the sand particles. The interaction between sand particles and surfactanthas
been studied by Fourier Transform Infrared (FTIR) Spectroscopy of the sand particles before and after
aging with surfactant. Salinity plays an important role in adsorption of anionic surfactant. Batch experi-
mentswere also performed to understand the effects of pH and adsorbent dose onthe sorption efficiency.
The sand particles exhibited high adsorption efficiency at low pH for anionic and nonionic surfactants.
But opposite trendwas found for cationic surfactant. Adsorption datawere analyzed by fittingwithLang-
muir, Freundlich, Redlich-Peterson, and Sips isothermmodels. Results show that the Langmuir isotherm
and pseudo-second order kineticsmodels suit the equilibrium and kinetics of adsorption on sand surface.
Thermodynamics feasibility of the adsorption process was also studied to verify the spontaneity of the
process.© 2013 Elsevier B.V. All rights reserved.
1. Introduction
Adsorption of surfactants on rock/clay/sediment solid matrix
may result in the loss and reductionof their concentrations, which
may render them less efficient or ineffective in practical appli-
cations of EOR techniques. Surfactants are also widely used in
various industrial processes for their favourable physicochemical
characteristics likedetergency, foaming,emulsification, dispersion
and solubilization effects [1–4]. Due to extreme ability to reduce
oil-water interfacial tension (IFT), surfactants are very important
materials in chemical flooding for EOR methods. Adsorption of surfactants from aqueous solutions in porous media is a funda-
mental issue in EORfrom oil reservoirs because surfactant loss due
to adsorption on the reservoir rocks impairs the effectiveness of
the chemical solution injected to reduce the IFT of oil-water and
may turn into the process economically unfeasible [5–8]. Surfac-
tantadsorptionat solid/liquid interfacehas beenstudiedforseveral
∗ Corresponding author. Tel.: +91 3262235485; fax: +913262296632.
E-mail address:mandal [email protected](A.Mandal).
decades. A number of studies havebeen conducted on the adsorp-
tion of ionic andnonionic surfactants onto reservoir rocks [8–22].
The solid surfaces are eitherpositively or negatively charged in
theaqueousmediumbyionization/dissociationofsurfacegroupsor
bytheadsorptionof ionsfromsolutionontoa previouslyuncharged
surface. At low surfactant concentrations, the charge on the elec-
trical double layer (proposed by Helmholtz in 1879, and modified
by Stern in 1924) of the solid surface largely determines the sur-
factant adsorption. The surfactant molecules are adsorbed on rock
surface or sediments as a single monomer and form monomeric
layer at low concentration of surfactant solution. As the surfac-tant concentration increases, the adsorbed surfactant monomers
tend to aggregate and form micelles [13,23]. This aggregate can
form one layer (ad micelles) or two layer (hemi micelles). The
onsetof hydrophobic interaction between the adsorbed surfactant
molecules leads to a substantial increase in the adsorption level-
ling off at the criticalmicelle concentration (CMC) [24,25]. In order
to lower theadsorption, negatively charged surfactants are usually
considered as themain surfactant species of theslugandso anionic
surfactantsarebelieved tobe themostusedtypeof chemicals inthe
flooding of sandstoneoil reservoirs [26]. The adsorption of surfac-
tants fromthe solutionisaffectedby itsphysicochemicalproperties
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Nomenclature
EOR enhanced oil recovery
XRD X-ray diffraction
FTIR Fourier Transform Infrared
IFT interfacial t ension
CMC criticalmicelle concentration
SDS sodium dodecylsulphate
CTAB cetyltrimethylammoniumbromideDOE design of experiments
COD chemical oxygen demand
MSE Mean Square Error
MINAPE Minimum absolute percentage error
MAXAPE Maximum absolute percentage error
Variables
C 0 Initial concentrations of surfactants (mg/g)
C e Equilibrium aqueous concentration of surfactants
(mg/L)
V Volumeof the surfactant solution (L)
m Weight of the sandparticles (g)
RL Separation factor or equilibrium parameter
K R Redlich-Peterson isotherm constants (L/mg)K S Sips isotherm constant [(L/mg)ms]
ˇ Exponential factorK L Langmuir equilibrium constant (L/mg)
K F Freundlich adsorption constants related to sorption
capacity (mg/g)
t Amount of surfactantsadsorbedonsandparticlesat
time t (mg/g)
k1 Rate constant of the pseudo-first-order adsorption
(min−1)
k2 Rate constant of the second-order equation
(g/mg/min)
G◦ Change in Gibbs energy (J/mol)S ◦ Change in entropy (J/mol)H ◦ Change in enthalpy (J/mol) Amount of adsorbateadsorbed (mg/g) max Maximum amount adsorbed (mg/g)n Sorption intensity
˛R Redlich-Peterson isotherm constants [(L/mg)ˇ]
ms Empirical constant in Sips isotherm
T Temperature (K)
R2 Regression coefficient
R UniversalGas Constant (8.314J/K/mol)
K id Rate constant of intraparticular diffusion
(mg/g/min)
C Intercept
such as pH [27–29], temperature [30,31], ionic strength [27,31],adsorbentdose [32] andelectrolyteconcentration[27,30,33]. These
physicochemical properties of solutions can also influence in the
dissolution behavior of minerals resulting significant changes in
theprecipitationbehaviorof thesurfactants [34]. A slight variation
in one of the above factors or the other can result in a significant
change in the adsorption characteristics of the system.
Adsorption is a unit operation in which dissolved constituents
are removed from the solvent by interphase transfer to the sur-
face of an adsorbent particle. In chemical flooding, surfactants
are inevitably adsorbed on the surface of reservoir rock by the
rock/oil/brineinteraction. Surfactant adsorptionin porousmedia is
a typically complexphenomenon (e.g.,masstransferandreaction).
Adsorption in porous media is a phenomenon in which trans-
port of surfactant molecules takes place from bulk phase onto the
interface at rock-fluid boundary. This process can be explained as
the interface is energetically favoured by the surfactant molecules
compared to the bulk phase [35,36]. It has been shown that the
nature of the adsorption isotherm depends to a large extent on
the type of surfactant used, the morphological and mineralogical
characteristics of the rock, and the type of electrolytes present in
solution [37]. The adsorption of surfactants can be affected by the
surface chargeon the rock surface andfluid interfaces [38–41]. Pos-
itivelychargedcationicsurfactant isattracted tonegatively charged
surfaces,while negatively chargedanionic surfactant is attracted to
positively charged surfaces. The salinity and pH of brine strongly
affect the surface charge. When the effects of brine chemistry are
removed, silica tends to adsorb simple organic bases (cationic sur-
factant),while the carbonates tend to adsorb simple organic acids
(anionic surfactant). This occurs because silicanormallyhasa neg-
atively chargedweak acidic surface inwaternear neutral pH,while
the carbonates have positively charged weak basic surfaces. Loss
of surfactants owing to their interactions with reservoir rocks and
fluid is possibly the most important factor that can determine the
efficiency of a micellar flooding process [42].
Studies of adsorption kinetics and equilibrium of different
surfactants are very practical tests in laboratory for study of sur-
factant adsorption onto rock surface. These phenomena depend
on the nature of the surfactants and also the solid-liquid interface[36,43–45]. Recently Ahmadi et al. [46] have studied the adsorp-
tion behavior of the Glycrihiza Glabra, a novel nonionic surfactant,
onto carbonate rock andAhmadi and Shadizadeh [47] have inves-
tigated the effect of nanosilica on adsorption behavior of Zyziphus
Spina Christi onto rock surface. Ahmadi et al. [46] concluded that
adsorption isotherm follows the Langmuir model. On the other
hand when nanosilica is used the Linear, Langmuir, and Temkin
equilibriumadsorptionmodelswerenotsuitable forpredicting the
surfactant adsorption, but the Freundlich equilibrium adsorption
was in good agreement between the experimental data. They also
studied the kinetics of the adsorption and showed that the pro-
cess follows the second order kinetic model. Gogoi [48] reported
theeffect of NaCl concentration and pH on the adsorption equilib-
rium of Na-lignosulfonate onto reservoir rocks. He demonstratedthat adsorption increases with increasing NaCl concentration but
decreaseswith increasing pH.
The net adsorption of surfactant in an EOR process strongly
depends on the presence of oil and the flow field. When a surfac-
tant slug is injected as displacing fluid, it undergoes partitioning
into oil and water and lowers the interfacial tension between oil
andwater thereby increasing thecapillarynumber. As a result, the
trapped immobile oil becomes mobile. At the same time, an oil-
in-water emulsion is formedwhichblocks the largerpores leading
to an improvement in the effective mobility ratio. Otherwise the
injected surfactant solution flows through the highly permeable
zone bypassing the trapped oil in smaller pores. The injected sur-
factant continues to mobilize oil, until the surfactant is diluted or
otherwise lost dueto adsorption on therock surface. Consequentlythe surfactant solutionswithlower concentration could notbeable
to lower the interfacial tension and mobilize oil. At that point, the
processdegenerates intoa waterflood.Hencetodesigna surfactant
flooding forEOR, it isvery importantto havea completeknowledge
of adsorption of the specific surfactant on the reservoir rock under
the reservoir conditions.
In the present paper the adsorptions of three different surfac-
tants namely anionic,cationic, andnonionicby clean sandparticles
have been investigatedwith variation of different parameters i.e.,
salinity, pH, temperature, and adsorbent dose. Adsorption data
have been analysed by fittingwith Langmuir, Freundlich, Redlich-
Peterson,and Sips isothermmodels. Kineticsof adsorption hasalso
been carried outwithanionic surfactant. Thermodynamic feasibil-
ity of the adsorption process has also been studied to verify the
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Fig. 1. Molecular structures of thesurfactantsused in thepresent study.
spontaneityof theprocess.Semi-quantitativeanalysisof cleansand
particles has been done by X-ray diffraction (XRD) study.
2. Experimental
2.1. Materials used
Threedifferent categoriesof surfactantssuchas anionic,cationic
and nonionic were used to determine the adsorption isotherms
on the clean sand particles (60–70mesh size). Anionic surfactant,
Sodium dodecylsulphate (SDS) (with 98% purity) was purchased
fromFisher Scientific, India and cationicsurfactant, Cetyltrimethy-
lammoniumbromide (CTAB)of98%purewasprocuredfromMerck,
India, both were used in the present study. Tergitol 15-S-7 (99.5%
pure) from sigma-Aldrich, Germany was used as nonionic surfac-
tant. Themolecular structures of the surfactant have been given inFig.1. SodiumChloride(NaCl)procuredfromQualigens FineChem-
icals, India, was used for preparation of brine. Reverse osmosis
water fromMilliporewater system(Millipore SA,67120Molshein,
France) was used for preparation of solutions.
2.2. Design of experiments (DOE)
DOE refers to the process of planning, designing and analyz-
ing the experiment so that valid and objective conclusions can
be drawn effectively and efficiently. Three different surfactants
namely SDS (anionic), CTAB (cationic) and Tergitol 15-S-7 (non-
ionic)havebeenused fortheadsorptionstudiesatdifferentsalinity,
pH, temperature and adsorbent dose. Semi-quantitative analysis
of clean sand particles has been done by X-ray diffraction (XRD)study to determine the main component present in the sand
particles. CMCs of thesurfactantswere determined by surface ten-
sion method. Adsorption data have been analysed by fitting with
Langmuir, Freundlich, Redlich-Peterson,andSips isothermmodels.
Kinetics of adsorption has also been carried out with anionic sur-
factant. Effect of temperature on surfactant adsorption has been
investigated. Thermodynamic feasibility of the adsorption process
has also been investigated to verify the spontaneity of theprocess.
2.3. Experimental procedures
2.3.1. Preparation of clean sand particles (adsorbent)
Sandswhichareusedformakingbuildingwerefirstsievedto get
60–70mesh sized sand particles andwashed with doubledistilled
water for several times followed by settling and decanting. After
removing the dust particles the residual wet sand particles weredried at353K for 18h. The clean dried sandparticles wereused for
the experimental purposes.
2.3.2. XRD study of clean sand powder
The clean sands were ground to prepare powder sample. X-ray
diffractogram of prepared sample were recorded in a wide range
of Bragg angle 2 (10◦≤2 ≤90◦) using Bruker D8 advanced XRDmeasuring instrument with Cu target radiation (=0.154056nm).The datawere analysedwith the help of the JCPDS files.
2.3.3. FTIR study
The apparatus used for measuring the FTIR spectra of the sand
particles before and after surfactant treatment in the range of
450–4000cm−1,wasaPerkinElmerSpectrumversion10.03.07FTIR spectrometer. The instrument is operated by Spectrum two soft-
ware supplied by PerkinElmer (USA). For the FTIR analysis, 4mgof
dried samplewasmixedwith potassiumbromide (KBr) (∼300mg),
whichwas used as a reference standard sample. The mixture was
compressedbyhydraulic pump toprepare palletand the palletwas
placed in a desiccator to remove moisture content of the sample.
The dried sample thenwas used for experimental purpose.
2.3.4. Determination of critical micelle concentration (CMC)
Measurement of surface tension is very much useful supple-
mentary test method for determination of CMC of surfactant. It is
particularly useful when only very small quantities of an experi-
mentalsurfactant areavailable. Inthepresentstudy surfacetension
of the different concentrated surfactant solutions were measuredby a programmable tensiometer (Kruss GmbH, Germany, model:
K20 EasyDyne) under atmospheric pressure by the Du Noüy ring
method. CMCs of the surfactants were determined from plot of
surface tension and surfactant concentration. The concentrationat
the inflexion point of the curve is termed as CMC. During the mea-
surement,the experimental temperaturewasmaintainedat 298K.
Theplatinumringwasthoroughlycleanedwithacetoneandflame-
dried beforeeachmeasurement.In all cases thestandarddeviation
did not exceed±0.1mN/m.
2.3.5. Adsorption isotherms
A series of batch experiments were carried out to determine
the adsorption isotherms of different types of surfactants on the
adsorbent. 8g of clean sand particles were added to a set of 50ml
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surfactant solutions in a 100ml glass vials and allowed to con-
duct the experiments by constant shaking at 303K for 24h on
a temperature controller horizontal shaker machine (Model No.
NovaShake BB03) at 120 rpm speed. After adsorption, the surfac-
tantsolutionswereisolatedbycentrifugationwithRemi centrifuge
instrument (Model No.RemiR-8C). Theequilibrium concentration
of the surfactant solutions were determined by Chemical Oxygen
Demand (COD) measurement of the solution. The amount of sur-
factant adsorbed on the adsorbent, (mg/g), was calculated by a
mass balance relation (1):
= (C 0 − C e)V
m (1)
where, C 0 and C e are the initial andequilibrium concentrations
of surfactants (mg/g) respectively,V is the volumeof the surfactant
solution(L),andm is theweightof thesandparticles(g) (adsorbent)
used.
The effects of the pH, temperature, NaCl concentrations and
adsorbent dose on the adsorption capacity of the adsorbent to
the anionic surfactant, SDS were also investigated. To adjust the
required pH values of the solutions, HCl (0.1N) and NaOH (0.1N)
solutionswereused.Thethermodynamicstudyhasbeenconducted
by change in temperaturewith abovementionedspeed andproce-
dure.
2.3.6. Adsorption kinetics and thermodynamics
8gofclean sandparticleswereputinto50ml ofSDSsolutions at
threedifferentconcentrations of400ppm, 800ppm, and1000ppm
respectively. Theadsorption kineticsexperimentswerecarriedout
at 303K and the concentration of SDS in the solutions were deter-
mined at regular intervals until an equilibrium concentration was
achieved.
The effect of temperature (thermodynamic study) was carried
out by shaking 8g of clean sandparticles in 50ml surfactant solu-
tion atdifferenttemperatures (303,313, and 323K) in temperature
controlled shaker. After 6h, the sample was centrifuges and the
concentrations of the solutionswere determined.
3. Results and discussion
3.1. Characterization of used sand particles and their interaction
with surfactant
3.1.1. XRD study
Thesand particleshave been characterizedby XRDstudy. Fig. 2
shows the X-ray diffractogram of the powder sample. The single
headed peak indicates that there is no impurity in the sample and
only one phase is present. The characteristic peaks are obtained
at 21◦, 27.74◦, 28.57◦, 47.23◦, 60◦ and 76◦, etc. The main peak
was obtained at 27.47◦. JCPDS (file no. 861630) record indicates
the presence of silica in the pure sand. The other peaks show the
presence of quartz in lowquantity.
3.1.2. FTIR study of sand particles
The main application of this technique is to detect the struc-
ture of chemical species and provide qualitative measurement,
basedon theadsorption andmolecularvibration peaks.Theresults
of the FTIR test for pure sand before and after treatment with
different surfactants are presented in Figs. 3(a)–(d). The infrared
spectra of pure sand shows adsorption peaks at 776.33cm−1
and 1080.17cm−1, in the region of stretching vibration for Si-
O symmetric and asymmetric bond vibration respectively. Again
absorption bands at 521.27cm−1, 693.91cm−1 are related to the
bending vibration of Si-O group in asymmetric and symmetric
vibration. So it is clear that the used sand particles contain pure
silica as main composition. The sand sample also shows peaks at
Fig. 2. XRDstudy of thecrushed sand particlesused in thepresentwork.
2851.85cm−1 and 2924.64cm−1. These two peaks indicates the
symmetric andasymmetric –CH2 stretching.In Fig. 3(b) the results of the sample treated with SDS has
been shown. In general, SDS exhibits bands due to symmetric
andasymmetric stretching and deformation ofmethylenechainat
2851.85cm−1 and2922cm−1. The2851.85cm−1 and2920.73cm−1
bandsoverlappedwithpure sandpeaks.It isalso seen that thesam-
ple treated with SDS, the stretching vibration of the S O bond is
observedat1360.23which is overlappedwithdifferent smallpeaks
ofpure sand.The stretching vibrationofalkylC Hbond is indicated
in SDS treated sand as a strong and sharp peak at 2920.73cm−1,
which shows that SDS is adsorbed on the sand surface.
Fig. 3(c) shows the results of the sample treated withCTAB. The
CH2 group to peak at 1637.18 cm−1 for SDStreated sand shifted
to 1627.98cm−1 and therefore CTAB adsorption on sand particles
also takes place.Theintense bandsnear 2850.52and2919.33cm−1can be assigned to the C H stretching and deformation vibrations
of CTABwhich are overlappedwith spectra of pure sand. The shif-
ting of methylene chain at 2850.52 cm−1 and 2919.33cm−1 from
2852.1cm−1 is due to adsorption of CTAB on sand surface in solu-
tion phase.
In Fig. 3(d) FTIR spectra of Tergitol 15-S-7 treated sand is pre-
sented. In this caseC H stretchingvibration of surfactant shows at
2927.16cm−1 instead of 2920cm−1. This is due to in solution Ter-
gitol 15-S-7 with ethoxylated group gets adsorbed on sand surface
andabsorptionband is shifted. Anadditional band at1888.05cm−1
is appeared due to adsorption of this ethoxylated nonionic surfac-
tant.
In all cases after treatment with surfactants, bands at
3468.68cm−1
and1637.18cm−1
, 3468.86cm−1
and 1627.98 cm−1
,and3469.04cm−1 and1626.73cm−1 forSDS, CTAB, Tergitol 15-S-7
respectively originated from the stretching vibration of OH group
of interlayer water molecule during surfactant adsorption.
3.2. Critical micelle concentration and effectiveness of the
surfactants
It is well known that the surfactants reduce the surface ten-
sion of water by getting adsorbed on the liquid–gas interface. The
critical micelle concentration (CMC), one of the main parameters
for surfactants, is the concentration at which surfactant solutions
begin to form micelles in large amount [49]. Surface tensions of
the above three surfactants (SDS, CTAB, and Tergitol 15-S-7) solu-
tions at different concentrations were measured and plotted as a
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Fig. 3. FTIR spectra of sand particlesbefore andafter surfactant treatmentin brine (2wt% NaCl):(a) pure sand; (b) treated with SDS; (c) treated with CTAB; (d)treatedwith
Tergitol 15-S-7.
functionof concentration in Fig. 4. Theconcentrationat the inflex-
ion point of the curve is critical micelle concentration. The lowest
surface tension value achieved by Tergitol 15-S-7 is 30mNm−1
whichis significantlylower thanthesurfacetension valueof water.
The CMCs of the surfactants are found to be 0.23wt%, 0.0345wt%,and 0.0051wt% for SDS, CTAB, and Tergitol 15-S-7 respectively.
3.3. Adsorption isotherms of surfactants on sand particles
The Langmuir adsorption isotherm and the Freundlich adsorp-
tion isotherm are two common isotherms used to describe the
equilibrium adsorption isotherm. Another two isotherms such as
Redlich-Peterson and Sips are considered here to describe the
experimentaldataandfindoutthe best fittedmodel foradsorption
of surfactant on sand surface.
The Langmuir equation relates the amount of solid adsorbate
adsorbed, , to theequilibriumliquid concentration at a fixed tem-perature. The equation was developed by Irving Langmuir [50] in
1916 and is expressed in this nonlinear formas follows:
= maxK L C e1+ K L C e
(2)
where, is theamount of adsorbateadsorbed (mg/g); max is themaximum amount adsorbed (mg/g); K L is the Langmuir equilib-
riumconstant (L/mg);C e is the equilibrium aqueous concentration
(mg/L). It is well-known that the Langmuir isotherm is applicable
formonolayer adsorption because of the homogeneous surface of
a finite number of identical sites. Another important parameter of
the Langmuir isotherm model is the term “RL ” which is a nondi-
mensional constant and called as separation factor or equilibrium
parameter, andit is represented by thefollowingequation[51,52]:
RL =1
1+ K L C 0
(3)
where,C 0 (mg/L)expresses initialadsorbateconcentration inaque-
ous solution.K L (L/mg) is theLangmuir constant. TheRL parameter
gives important signs on the compatibility of adsorption for the
selected adsorbent–adsorbate pair. There are four possibilities for
the RL value:
• In the case 0< RL 1, adsorption is unfavorable.• RL =1 indicates linearity of adsorption.• In the case RL =0, adsorption is irreversible.
The values of RL obtained in this study were between 0.0445
to 0.3507, indicating that the adsorption of surfactant onto sand
surface is favourable.TheFreundlich isothermassumesthatif theconcentrationof the
solute in the solution at equilibrium,C e, is raised to the power 1/n,
the amount of the solute adsorbed being , the C 1/ne is constant
at given temperature and the nonlinear form of the equation is
expressed as:
= K FC 1/ne (4)
where,K F (mg/g) andn are theFreundlich adsorptionconstants
related to sorption capacity and sorption intensity, respectively.
Freundlich isotherm has been derived by assuming an exponen-
tially decaying sorption site energy distribution. The Freundlich
isotherm assumes that the surfactant adsorption occurs on a het-
erogeneous surface bymultilayer sorption.
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Fig. 4. Plot of surface tension vs. surfactant concentration forfinding theCMCs of thesurfactants: (a) SDS, (b) CTAB and (c)Tergitol 15-S-12.
TheFreundlich constant(1/n) is related to theadsorption inten-
sityoftheadsorbent.When,0.1
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Fig. 5. Adsorption isothermsof differenttypes of surfactants at 303K.
nonlinear regression method and are given in Table 1. If the value
of K S approaches 0, the Sips isotherm will become a Freundlich
isotherm.While the valueof ms =1 or closer to1, the Sips isotherm
equation reduces to the Langmuir equation; that is, adsorption
takes place on homogeneous surface [57].
Fig. 5 shows the adsorption of different types of surfactants
on sand surfaces at 303K. To quantify the adsorption capacity of
the sand particles for surfactant adsorption, Langmuir Freundlich,
Redlich-Peterson, andSips adsorption isothermmodels have been
used. Fig. 6 depicts the different adsorption models for SDS sur-
factant. Curve fittings for Langmuir, Freundlich, Redlich-Peterson,
and Sips adsorption isothermmodels for theother two surfactants
like CTAB and Tergitol 15-S-7 have been given in Figs. S1 and S2
(supplementary information) respectively. The calculated results
fromthecurvesofLangmuir,Freundlich,Redlich-Peterson, andSips
isotherm adsorption models for the surfactants have been sum-marized in Table 1. The values of regression coefficient (R2), mean
square error(MSE),minimumabsolutepercentage error(MINAPE),
and maximum absolute percentage error (MAXAPE) indicate that
the Langmuirmodel is well fitted with the adsorption isotherm of
the surfactants on sand surface. The details of these values have
been given in supplementary information (Table S1).
Fig. 6. Different isothermmodelsfit foradsorption of SDSsurfactant on sand parti-
clesat 303K.
The equilibrium amount of surfactant adsorbed on the sand
particles depends on their structures and nature of head groups.
It is clear from Fig. 5 that the amount of SDS adsorbed on the
adsorbent shows the lowest value compared to the others. The
equilibriumamountof CTAB and Tergitol 15-S-7 adsorbed on sand
particles are considerably higher than SDS. For all the surfac-
tants, it was found that there is a sudden increase in adsorption
isotherm as concentration of the surfactant increases. The sud-
den increase in adsorption isotherm may be described in terms
of formation of surface aggregates, known as “hemi micelles” of
the surfactant molecules on the sand surface due to lateral inter-
action between hydrocarbon chains. This lateral attraction force
generatesan additional driving force,which superimposes existing
electrostaticattractioncausinga sharp increaseinadsorption. Inall
cases the increaseof adsorptionwith concentrationup to a certain
point and then no increase have been observed. In case of CTAB
whensurfactantconcentrationreachesCMC,micellesstartsto form
and exist in the bulk solution and act as chemical potential sink
for additional surfactant added to the system. As a result, surfac-
tants cannot adsorbonto the surface andplateau of theadsorption
isotherm shown in Fig. 5 is characterized by little or no increases
in surfactant adsorption with increasing surfactant concentration.
With increase in SDS concentration strong repulsion takes place
between sand surface and surfactant molecules due to negativehead groups of SDS surfactant. Therefore before CMC no increase
in adsorption also takes place with increasing concentration of
surfactant. In case of Tergitol 15-S-7 after CMC small increase of
adsorption takesplacedueto weak hydrophobicandH-bondinter-
action.
Theadsorption of an ionic surfactant at solid–liquid interface is
strongly influenced by the compositions of the sandwhich makes
the sand surface negatively charged therefore, weak interaction
takes place with anionic surfactant (SDS) having their negatively
charged head part. So the SDS adsorption capacity on sand parti-
clesisnotsignificantlyhigh.However,CTABis a cationicsurfactant,
and the adsorption takes place mainly due to presence of some
charged components of sand particles such as silicawhichare neg-
ative in nature at neutral pH or in water. The high adsorptioncapacity of CTAB on sand particles may be explained on the basis
of electrostatic interaction that exists between negatively charged
adsorbentandpositively chargedhead groupof surfactant.Adsorp-
tionof nonionicsurfactantoccurredon solidadsorbent duetoweak
hydrophobic andhydrogenbond interactions between surfactants
and the adsorbent. Since no positive and negative charge can exist
on nonionic surfactants so the adsorption capacity of Tergitol 15-
S-7 is also low.
3.4. Effect of salt concentration on adsorption isotherm of SDS
Adsorption isotherms for SDS surfactant solution at different
salinities have been shown in Fig. 7. At the interface between sur-
factant and sand particles, there is always an unequal distributionof electrical charges. This unequal charge distribution gives rise
to a potential across the interface and forms a so-called electrical
double layer [58]. With increase in NaCl concentration, the elec-
trical double layer on the surface of adsorbent is compressed and
electrostatic repulsion between the adsorbed surfactant species
decreases, which results in the increase of adsorption capacity.
The surfactant adsorption capacity increases with the increase
in salinity of the system at a constant temperature of 303K.
These facts imply that the adsorption of SDS on sand particle
adsorbent is favored at high salinity and therefore the adsorption
process is seen to be a chemical process with increasing salin-
ity.
The curve fittings for Langmuir and Freundlich adsorption
isothermmodels have been depicted in Figs. 8a andb respectively
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94 A.Bera et al. / AppliedSurface Science 284 (2013) 87–99
Fig. 7. Adsorption isothermof SDSsurfactant on sand surface at differentsalinities
of brine at 303K.
at 303K. Table 2 shows the parameters obtained the two mod-els used. In case of Langmuir model, the regression coefficients
(R2) for the linear equation fittings at different salinities are found
to be greater than 0.950 and at high salinity it is above 0.980
whereas the vales of R2 for the Freundlich isotherm model are
found to less than 0.950. Therefore, in presence of salt adsorp-
tion of surfactants on sand surface follow the Langmuir isotherm
model.
Fig. 9. Theeffectof theamountof sand onthe adsorption process ofthe surfactants.
3.5. Effect of adsorbent dose on the extent of surfactants
adsorption
Adsorption of the surfactant on sand depends on its dose as
shown in Fig. 9. 1000ppm concentration of different surfactants
(SDS, CTAB, and Tergitol 15-S-7) has been used for adsorption
study at 303K. From Fig. 9 it has been found that adsorption
increases with adsorbent dose and then remains constant after
certain dose for each surfactant. As the amount of adsorbent
Fig. 8. Adsorption isothermsof SDSat differentNaCl salt concentrations at 303K: (a) Langmuir equation fitting; (b) Freundlich equation fitting.
Table 2
Adsorption isothermparameters of SDS at different salinities.
Salinity (wt% NaCl) Langmuir parameters Freundlich parameters
max (mg/g) K L×102 (L/mg) R2 K F (mg/g) 1/n R2
0 0.763 2.141 0.951 0.231 0.173 0.937
2 1.011 1.472 0.993 0.216 0.218 0.894
4 1.031 1.642 0.988 0.345 0.151 0.899
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A. Bera et al. / Applied Surface Science 284 (2013) 87–99 95
Fig. 10. Theeffect pH on theadsorption process of thesurfactants.
increases the adsorption sites also increase and the adsorption
process takes place easily with increase in order. After a certain
adsorbentdose there is no further adsorption because of gathering
of huge adsorption sites and produced particle interaction among
the sand particles in the system. Particle-particle interaction takes
place fromhigh adsorbent concentration which leads to a decrease
in total surface area of the adsorbent and an increase in diffused
path length [59].
3.6. Effect of pH on adsorption of surfactants
The pH of the aqueous solution is one of the important control-
ling parameters in the adsorption of surfactant on reservoir rocks.
Fig. 10 shows the effect of pH on the extent of adsorption of dif-ferent surfactants (anionic, cationic and nonionic) on clean sand
surface. The sand particles exhibited high adsorption efficiency at
low pH for anionic and nonionic surfactants. As pH increases the
adsorption decreases for anionic surfactant. The adsorption capac-
ity at alkaline solution is lower due to the decrease of positively
charged sites on adsorbent and thecompetition between OH− and
anionic surfactant for the adsorption site. A number of research
works has been reported regarding the effect of pH of solution
on adsorption of surfactants on rock surfaces [27,60–63]. At low
pH, SDS adsorption capacity of sand is high due to acidic nature
of the solution which makes the sand surface more positive and
that is why the interaction of sand surface with anionic surfactant
SDS is high and hence adsorption capacity is high. In case of Ter-
gitol 15-S-7 (nonionic), adsorption decreases up to neutral pH andremains almost constant at alkaline pH region. This canbedemon-
strated that the presence of lone pair of electrons of the oxygen
atom of the ethylene oxide group of ethoxylated nonionic surfac-
tant which is broadly attracted by the positively charged surfaces
of sand particlesatpHvalues lower than 7. Theloweradsorption of
the surfactant at alkaline region is due to hydrophobic interaction
only.As pH of thesolution increases adsorption of CTAB surfactant
(cationic) also increasesbecausepositively chargedheadgroups of
the cationic surfactant are strongly attracted at high pH with neg-
atively charged sand surfaces. So from this study theadsorption of
the surfactants on rock surfaces can be reduced or alter by fixing
the solution pH for nonionic and ionic surfactants which are very
important issue regarding the economic feasibility for surfactant
flooding.
3.7. Adsorption kinetics
Adsorption is a physicochemical process that involves themass
transferofadsorbate fromthe liquidphase totheadsorbent surface.
A study of kinetics of adsorption is desirable as it provides infor-
mation about themechanism of adsorption, which is important to
evaluateefficiencyof theprocess.Theexperimentaldataofadsorp-
tion of surfactants on sand particles have been analysed by three
differentmodels viz. Lagergren-first-order equation, second-order
equation and intraparticle diffusionmodel.
3.7.1. Lagergren-first-order kinetic equation
Lagergren-first-orderequation is verywellknownkinetic equa-
tion. It was first proposed by Lagergren in 1988 to determine the
kinetic process of liquid-solid phase adsorption. Thecommon form
of the equation is
d t dt = k1( e − t ) (7)
On integration of this equation for the boundary condition t =0
to t = t and e =0to e = t , gives:
ln( e − t ) = ln e − k1t (8)
where, e (mg/g) and t (mg/g) are the amount of surfac-
tants adsorbed on sand particles at equilibriumandat time t (min)
respectively. k1 (min−1) is the rate constant of the pseudo-first-
orderadsorption. Thevaluesof k1 can becalculated experimentally
fromthe slope of the linear plot of ln( e− t ) versus t .
In Fig. 11(a), adsorption kinetics of SDS surfactant on sandpar-
ticle at different concentration at 303K has been depicted. The
parameters are calculated from the model have been summarized
in Table 3.
3.7.2. Pseudo-second-order kinetic equation
The pseudo-second-order kinetic model equation is expressed
as follow:
d t dt = k2( e − t )
2 (9)
and rearranging the Eq. (9) gives
d t
( e − t )2 = k2dt (10)
where, k2 (g/mg/min) is the rate constant of the second-order
equation.
Nowapplying theboundaryconditions t =0to t = t and e =0to e = t , the integrated linear formof Eq. (10) can be rearranged toobtain Eq. (11).
t
t =
1
k2 2e+
t
e(11)
The plot of t / t versus t has been shown in Fig. 11(b). The val-ues of equilibrium adsorption capacity e and rate constant k2,
calculated from the intercept and the slope of the linear plot of
t / t versus t , alongwith the valueof regression coefficient R2, MSE
values are listed in Table 3.
3.7.3. Intraparticle diffusion model
The intraparticle mass transfer diffusion model was proposed
byWeber and Morris [64]. For determination of rate constant and
reaction type,first-orderandsecond-order kineticmodels aregen-
erally used. To understand the diffusionmechanismof adsorption
process it is very important to introduce intraparticle diffusion
model. In this model the fractional approach to the equilibrium
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96 A.Bera et al. / AppliedSurface Science 284 (2013) 87–99
Fig.11. Thekineticsmodels foradsorption of SDSsurfactant on sand particle at different concentrations at 303K: (a)Lagergren-firstorderkinetics; (b)pseudo secondorder
kinetics; (c) interparticle diffusion kinetics.
changes according to a function of (Dt/r 2)0.5, where D is the dif-
fusion coefficient within the solid adsorbent and r is the particle
radius.
Theintraparticlediffusionrate constantcanbedetermined from
the following equation [65–68]:
t = K idt 0.5
+ C (12)
where,K id (mg/g/min) is therateconstantof intraparticular dif-
fusion and C is the intercept. A plot of t versus t 0.5 should bestraightlinewitha slopeK id andinterceptC whenadsorptionmech-
anism follows the intraparticle diffusion process. Ho [69] pointed
out that in case of intraparticle diffusion the t versus t 0.5 plotmust go through the origin and that is sole rate-limiting step. In
the present study,no plot passed through the origin. This indicates
thatalthough intraparticlediffusionwasinvolvedin theadsorption
process, it was not sole rate-controlling step. This also confirms
that adsorption of surfactant on sand was a multi-step process;
involvingadsorption on theexternal surface anddiffusion into the
interior [70]. It can be demonstrated from Fig. 11(c) and Table 3
that other adsorption mechanisms along withdiffusion contribute
in the interactions between thesurfactantmolecules and sandpar-
ticles.The highvalue of R2 and low valueMSE obtained from thethree
models suggest theapplicabilityof the second-order kineticmodel
to describe the adsorption kinetics data of surfactants onto sand
surface and the calculated e values are in good agreement withthe experimental one.
3.8. Thermodynamic parameters of adsorption
Both enthalpy and entropy are the key factors to be considered
in anyprocess design[71]. The feasibility of theadsorption process
Table 3
Kinetics parameters for the adsorption of surfactant on sand particles at different surfactant concentrations.
Kinetics model Kinetics parameters Surfactant concentration (ppm)
400 800 1000
Lagergren-first-order k1 (min−1) 1.249×10−2 1.344×10−2 2.03×10−2
e (mg/g) 0.4667 0.4409 0.4857
R2 0.9657 0.9755 0.9471
MSE 2.6354 2.7661 1.9178
Pseudo-second-order k2 (g/mg/min) 2.501×10−2 3.230×10−2 4.139×10−2
e (mg/g) 0.810 0.875 0.864
R2 0.991 0.994 0.992
MSE 0.8343 0.6421 0.6678
Intraparticle diffusion kid (mg/g/min) 3.896×10−2 3.893×10−2 3.318×10−2
C 0.1313 0.2319 0.3197
R2 0.988 0.981 0.990
MSE 1.2678 1.1523 1.1235
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A. Bera et al. / Applied Surface Science 284 (2013) 87–99 97
Fig.12. Langmuir equationfitting foradsorptionisotherms of SDSat differenttem-
peratures.
is clarified by the value of change inGibbs energy,G◦ (J/mol) andit is estimated by applying thermodynamic equation [72,73]:
G◦ = −RT lnK L (13)
where, R is theuniversal gas constant (8.314J/K/mol), T is the tem-
perature (K) and K L is the Langmuir constant at temperature T .
Again the feasibility and endothermic nature of the adsorption
process are determined by the entropy change, S ◦ (J/mol) andenthalpy change,H ◦ (J/mol). The dependence of temperature on
adsorption of surfactant onsand particlewas evaluatedusing van’t
Hoff equation by calculating the values of H ◦ andS ◦.
ln K L =S◦
R −
H ◦
RT (14)
G◦ =H ◦ − TS◦ (15)
The effect of temperature on adsorption of surfactant on sand
particle atdifferent temperature hasbeen depicted inFig. 12. Tem-
perature plays an important role on the adsorption of surfactant
onto sand particles. The variation of adsorption with temperature
has been explained with help of the thermodynamic parameters
suchas changein standardGibbsfreeenergy,enthalpyandentropy.
The variation of Langmuir constant with temperature has been
shown in Fig. 13. The values of S ◦ andH ◦ were calculated fromthe intercept and slope of plot between lnK L versus 1/T . The cal-
culated values of all the thermodynamic parameters have been
reported inTable4. Thenegativevaluesof G◦ indicatethesponta-neousandfeasibilitynatureofsurfactantadsorptionprocess.It may
also benoted thatwith increase in temperature from303 to 333K,
the negative values of the Gibbs free energy decrease. This sug-
gests that with increase in temperature spontaneity andfeasibility
of the process are decreased and resulting the weaker adsorptive
force. In general, the value of Gibbs free energy for physisorption
lies between −20kJ/mol and 0kJ/mol and that for chemisorptions
lies between −400kJ/mol and −80kJ/mol value [74]. The high
Table 4
Thermodynamic parameters for the adsorption of SDS on clean sand particles at
different temperatures.
Temperature(K) K L (L/mol) G◦ (kJ/mol) H ◦ (kJ/mol) S ◦ (kJ/mol/K)
303 6.113 −4.562
313 4.959 −4.167 −24.846 −0.0667
323 3.316 −3.217
Fig. 13. Relationship between Langmuir constant and temperature for SDS adsorp-
tion on sand surface.
adsorption of surfactant at low temperature attributed to the fact
that theadsorption interactions areexothermic innature. Theneg-ative value of enthalpy change confirmed the exothermic nature
of thesorptionprocess. Negativevalue of standard entropy change
confirmedthatwithincrease in temperature therandomnessof the
molecules at the solid–solution interface decreases during the fix-
ationof the surfactantmoleculeson theactive site of sand surfaces.
Temperature significantly influences the adsorption of surfac-
tant on reservoir rock surface. In the present study temperature
plays an important role. From Fig. 12, it is clear thatwith increase
in temperature adsorption capacity decreases. Twomain impacts
of temperature are generally found. Firstly, when temperature
increases the rate of diffusion of the adsorbate across the exter-
nal boundary layer and interior pores of the reservoir rocks is
decreased because of the solution viscosity declines as tempera-
ture increases. Secondly, temperature influences the equilibriumadsorption capacity of the sand particles depending on whether
theadsorption process is exothermic or endothermic.
Pressure can also play an important role on adsorption of gases
or liquids when physisorption has taken place onto solid surface.
The amount of adsorption will increase with increase in pressure.
The increasedadsorption capacity is due to reduction in adsorbate
volumeduringadsorptionwith increaseinpressure. It is important
to note that the effect of pressure on adsorption of gas is stronger
than liquid on solid surface.
4. Conclusions
Theadsorption of the three types of surfactants namely anionic
(SDS), cationic (CTAB), and nonionic (Tergitol 15-S-7) onto cleansand particles from aqueous solutions was systematically stud-
ied. Experimental investigations were carried out to examine the
adsorption equilibrium, isotherm, kinetic behaviors, and thermo-
dynamics of adsorption of these surfactants. XRDstudy shows the
presence of silica in the pure sand which provides in active sites
for adsorption of different surfactants. FTIR of the sand particles
again indicates thepresence of silica. After treatmentof surfactant
spectral changes are found and adsorption is confirmed from the
result. Accordingto theresults obtained in the present study,as we
move from cationic to anionic via nonionic surfactant, adsorption
of surfactants on sand particles decreases. With increasing salin-
ity of the solution adsorption of SDS increases on sand surface
due to lowelectrostaticrepulsionbetween theadsorbedsurfactant
species. With increase in the surfactant concentration, adsorption
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