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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88 A.Bera et al. / AppliedSurface Science 284 (2013) 87–99

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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A. Bera et al. / Applied Surface Science 284 (2013) 87–99 89

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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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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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


10/13


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


11/13


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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