Adsorption of enhanced oil recovery polymer, schizophyllan, over carbonate minerals

Carbohydrate Polymers 240 (2020) 116263

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Carbohydrate Polymers
journal homepage: www.elsevier.com/locate/carbpol

Adsorption of enhanced oil recovery polymer, schizophyllan, over carbonate
minerals

T

Mohammad Shoaiba, Syed Mohamid Raza Quadrib, Omar Bashir Wania, Erin Bobickia,
Gerardo Incera Garridoc, Ali Elkameld,e, Ahmed Abdalaf,*
a

Department of Chemical Engineering and Applied Chemistry, University of Toronto, Canada
Abu Dhabi National Oil Company, Abu Dhabi, United Arab Emirates
c
BASF, Ludwigshafen Am Rhein, Germany
d
Department of Chemical Engineering, University of Waterloo, Ontario, Canada
e
Department of Chemical Engineering, Khalifa University, Abu Dhabi, United Arab Emirates
f
Chemical Engineering Program, Texas A&M University at Qatar, Doha, Qatar
b

A R T I C LE I N FO

A B S T R A C T

Keywords:
Schizophyllan
EOR
Polysaccharides
Adsorption
Carbonate minerals

Schizophyllan is a natural polysaccharide that has shown great potential as enhanced oil recovery (EOR)
polymer for high-temperature, high-salinity reservoirs. Nevertheless, the adsorption behavior of schizophyllan
over carbonate minerals remains ambiguous element towards its EOR applications. Here, we investigate the
adsorption of schizophyllan on different carbonate minerals. The effect of mineral type, salinity, and background
ions on adsorption is analyzed. Our results indicate the adsorption capacity is higher on calcite and dolomite
compared to silica and kaolin and the adsorption capacity decreases with salinity. Moreover, the adsorption
kinetics follows pseudo-second order mechanism regardless of the mineral type. Adsorption over calcite is diminished in presence of water structure making ions and enhanced in presence of structure breaking ion and in
presence of urea. Gel permeation chromatography results reveal the preferential adsorption of longer chains. The
adsorption over carbonate minerals proceed via complex formation between polymer molecule and mineral
surface.

1. Introduction
High-performance water-soluble polymers are used in Enhanced Oil
Recovery (EOR) applications to increase oil recovery from depleted oil
reservoirs. However, applications of polymer EOR in high-temperature,
high-salinity carbonate reservoirs typical of the Arabian Gulf reservoirs
remains very challenging due to the stringent requirement of thermal
stability, salt tolerance, and low adsorption on the carbonate rocks that
are difficult to meet with even the latest generations of acrylamide or
saccharide-based polymers (Al Mahrouqi, Vinogradov, & Jackson,
2011; Han et al., 2013; Sheng, 2014; Sheng, Leonhardt, & Azri, 2015).
In polymer EOR, polymer solution in formulation brine or seawater

with higher viscosity than brine viscosity provide mobility control via a
piston-like flow of the displacing fluid. However, as the polymer slug
propagates in the reservoir, it interacts with the porous media. Such
interaction could result in polymer retention making the propagating
polymer slug lean which increases the required amount of injected
polymer to achieve a specific recovery goal (Berg, Danilova, & Liu,

⁎

2019; Szabo, 1975). This leads to an increase in the overall cost of
polymer EOR technology and decreases its competitiveness with other
EOR techniques (Berg et al., 2019; Manichand & Seright, 2014;
Uzoigwe, Scanlon, & Jewett, 1974). Apart from the direct economic
consequences, adsorption of EOR polymer on the reservoir mineral can
also alter the reservoir permeability and may lead to advancement of
water bank ahead of the polymer solution which greatly reduces the
effectiveness of the process. In contrast, for very heterogeneous reservoirs, the injected polymer solution follows the high permeability
path and when high adsorption is experienced, blockage of high permeability zones takes place (Grattoni et al., 2004; Parsa & Weitz, 2017;
Sheng, 2014; Wani et al., 2020). This makes a positive impact on the
overall process as it diverts more of the fluid to the low permeability
zones resulting in better vertical sweep efficiency as demonstrated for
the polymer flooding project in Minnelusa, Wyoming (Chris, Galas,
Jaafar, & Jeje, 2011). Therefore, to optimize the polymer slug, it is
important to predict and manage the polymer adsorption for a costeffective polymer EOR project (Bondor, Hirasaki, & Tham, 1972).

Corresponding author.
E-mail address: (A. Abdala).

/>Received 12 March 2020; Received in revised form 30 March 2020; Accepted 5 April 2020
Available online 14 April 2020

0144-8617/ © 2020 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license
( />

Carbohydrate Polymers 240 (2020) 116263

M. Shoaib, et al.

solid or mass of polymer per unit surface area of the solid. Polymer
adsorption is generally classified based on the energy of the interaction
between the surface and polymer as chemisorption if the energy change
during adsorption is ≥ 2 kBT and physisorption if adsorption energy is
of the order of kBT (Vavylonis, 2005). Chemisorption involves the formation of a chemical bond between polymer and the surface, which
holds the polymer firmly and is considered irreversible due to the high
desorption energy. On the other hand, in physisorption, the polymer
interacts with the surface through Van der Waal forces, electrostatic
interactions or hydrogen bonding (Pashley & Karaman, 2004). For large
polymer molecules, the sum of the adsorption energies of monomers
lead to a high-energy change, which make even physisorption irreversible. This leads to isotherms which have a high-affinity nature, i.e., at
low concentrations, the adsorption density rises sharply, while at higher
concentrations it reaches a pseudo-plateau (Vavylonis, 2005). Adsorption of polymer over surface only proceeds if the attractive interaction
exceeds the entropy loss associated with adsorption (Parfitt &
Rochester, 1983). The adsorption process of a polymer molecule from
the bulk solution to being fully attached on to the surface involves
several steps that take place over time. Initially, the molecules diffuse
toward the surface and make a stagnant layer. Then the molecules
approache the surface and are attracted towards it resulting in adsorption which is faster than the bulk diffusion. After this, the polymer
molecules collapse and spread onto the surface (Källrot & Linse, 2007).
The polymer molecules change their conformation to reach minimum
free energy (Fleer et al., 1993). It is understood that the polymer adsorbs in the form of trains, loops and trails over the solid surface as
depicted in the graphical abstract (Al-Hashmi, 2008; Fleer et al., 1993).

During the adsorption of polymer chains, several thermodynamic
interactions between the segment-surface, segment-solvent and segment-segment take place. The change in enthalpy during adsorption
and the change in conformational entropy, which is a measure of the
number of chain configurations available to the adsorbed molecule to
the number available for a free molecule in solution, govern the adsorption process. The entropy change by the displacement of solvent
molecules from the surface by the polymer molecules is also responsible
for the thermodynamics of the process. It is also worth noting that the
adsorbed polymer does not have a fixed configuration; rather different
segments of the polymer chain are in constant motion changing from
attached train segments to unattached loop or tail segments (AlHashmi, 2008).
For a polymer to be an EOR candidate in harsh carbonate reservoir

Conventional synthetic EOR polymers including HPAM and Xanthan
have been utilized in some of the most successful polymer projects, but
their application is still limited in high-temperature, high-salinity reservoirs due to their hydrolysis at high temperature and precipitation in
high-salinity conditions (Davison & Mentzer, 1982). This limits the use
of these polymers to only low temperature and low salinity reservoirs
(Vermolen et al., 2011). In fact, few studies recommended other EOR
methods over polymer flooding for high-temperature, high-salinity reservoirs due to the unavailability of a suitable polymer candidate (AlBahar et al., 2004; Alkafeef & Zaid, 2007; Liu et al., 2018; Song et al.,
2019; Wani et al., 2018).
To overcome these challenges in high-salinity, high-temperature
reservoirs, a new biopolymer ‘Schizophyllan’ (SPG) has attracted ample
attention due to its exceptional thermal stability stemmed from its triple
helical structure which is stabilized by intramolecular hydrogen
bonding (Al-Ghailani et al., 2018; Beeder et al., 2018; Joshi et al., 2016;
Leonhardt et al., 2011; Mukherjee et al., 2018; Urkedal et al., 2017).
Moreover, its non-ionic character makes it a suitable candidate for high
salinity reservoirs. SPG was first evaluated by Udo Rau et al. in 1992 for
its potential in high-temperature, high-salinity reservoirs (Rau,
Haarstrick, & Wagner, 1992). A similar polysaccharidic polymer

“Scleroglucan” also possessing a triple helical structure has also been
proposed for EOR under extreme reservoir conditions (Fortenberry
et al., 2017; Jensen et al., 2018; Rivenq, Donche, & Nolk, 1992).
SPG is a non-ionic, homoglucan polysaccharide, which is an extracellular product of Schizophyllum Commune. It has a backbone of β
glucopyranose residue units linked at 1–3 position with a single β
glucopyranose linked via 1–6 linkage to every third unit of the backbone as shown in Fig. 1(a). In aqueous solutions, SPG adopts a triple
helical structure which is the basis for its well-known viscosifying
properties and thermally stable thickening effect up to 140 °C (Grisel &
Muller, 1997). Its triple helix has a pitch (per residue) of
0.30 ± 0.02 nm and diameter of 2.6 ± 0.4 nm and its structure is
stabilized by interchain hydrogen bonds (Fig. 1(b)) (Yanaki, Norisuye,
& Fujita, 1980; Norisuye, Yanaki, & Fujita, 1980; Zhang et al., 2013).
SPG can dissociate into a single chain structure in dimethyl sulfoxide
(DMSO) and regains its triple helical structure if DMSO is exchanged
with water (Koumoto et al., 2001). SPG forms physical gels with borate
ions B(OH)4 as a result of chelation of borate ions through the hydroxyl
groups (Grisel & Muller, 1997).
Polymer adsorption is characteristic of polymer-solid interaction
and is usually reported as either mass of polymer per unit mass of the

Fig. 1. Schizophyllan (a) monomer unit (National Library of Medicine Data, 2020) (b) triple helical structure Reproduced with permission from Reference (Okobira
et al., 2008).
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M. Shoaib, et al.

50 mL centrifuge tube. The tube was flushed repeatedly with nitrogen

and then sealed. The tube was then placed on a reciprocating water
bath shaker operating at 200 rpm at the desired temperature. At the end
of the experiment, the centrifuge tubes were kept static overnight and
then centrifuged at 3000 rpm for 10 min to precipitate the rock. A
control experiment was carried in the same manner but without the
addition of rock. After centrifugation, three samples were drawn from
the supernatant and the adsorbed polymer amount was determined by
measuring the residual polymer concentration using the PhenolSulphuric Acid method (Dubois et al., 1956), the detials of which are
presented in the supplementary information. The pH of solution was
measured before and after the adsorption test and no change was observed for any of the tests.

3. Results and discussion

Fig. 2. X-ray Photoelectron Spectroscopy (XPS) analysis results of different
synthetic minerals.

3.1. Schizophyllan structure

conditions, it should exhibit high salinity tolerance, long term thermal
stability, good injectivity and low/manageable adsorption. The first
three aspects of SPG have been studied thoroughly in the past (Joshi
et al., 2016; Leonhardt et al., 2014; Ogezi et al., 2014; Sheng et al.,
2015) without any extensive focus on its adsorption characteristics. In
this article, we investigate the adsorption characteristics of SPG on
different minerals. The effect of parameters such as overall salinity,
background ions, urea, etc. have also been investigated.

The polymer structure was analyzed using FTIR and the spectrum is
provided in Fig. S3 of the supplementary information. The FTIR spectrum for SPG indicates the appearance of OH stretching/vibration peak
at 3300 cm−1 (Abdel-Mohsen et al., 2014), CeH stretching/vibrations

peak at 2930 cm−1, associated water band near 1640 cm−1, CeH
variable angle vibration of β-pyranoside band at 890 cm−1, and CeO
stretching peak at 1034 cm−1 typical of polysaccharide (Wang et al.,
2009). The presence of these bands confirms the β-glycosidic bond and
pyranose rings in SPG.

2. Experimental: materials and methods
2.1. Materials

3.2. Mineral composition and morphology

SPG polymer solution was supplied by Wintershall as a stock solution of 5600 ppm and was further diluted using brine to reach the required polymer and salt concentrations. In addition, a solid SPG sample
was supplied by Invivogen. Synthetic brine (TDS ∼167 g/L) containing
NaCl (134.7 g/L), KCl (1.4 g/L), CaCl2 (25.6 g/L), and MgCl2 (5.6 g/L)
was prepared and other brine compositions were prepared by dilution.
Synthetic carbonate minerals including calcite (Alfa Aesar), dolomite
(Vital Earth Resources), kaolin (Sigma-Aldrich) and silica (U.S silica
Company) was used for the static adsorption study.

The atomic composition of all the minerals were measured using
XPS and the results are shown in Fig. 2. The Calcite sample is confirmed
to be pure CaCO3 as the XPS spectrum contains calcium (Ca), oxygen
(O) and carbon(C) with no traces of other elements and dolomite was
also found to be pure CaMg(CO3)2 containing Ca, Mg, O and C without
any traces of other impurities. On the other hand, kaolin
(Al2Si2O5(OH)4) contained aluminum (Al), silicon (Si), oxygen and
some carbon impurities possibly in the form of carbonate. Silica (SiO2)
contains O, Si and small traces of C. Moreover, the concentration of
divalent ions in calcite is 15.78 % (Ca2+) compared to 17.57 % (6.92 %
Mg2+ + 10.65 % Ca2+) in dolomite.

The SEM images of the four minerals is provided in Fig. S4 of the
supplementary information and shows calcite has a uniform particle
size distribution from 2−3 μm, dolomite has a broad particle size distribution extending from 1 μm to 8 μm, while kaolin has a small particle
size and disc-shaped particles with very small diameters and silica’s
particle size is ∼ 5 μm.
The BET surface area, zeta potential and particle size distribution of
the four synthetic minerals are presented in Table S2 of the supplementary information. The four minerals have particle size ranged between 1.9 and 5 μm. The BET results show that calcite, dolomite and
silica have low surface area (1–1.7 m2/g), while kaolin has a significantly higher surface area. These results are in close agreement with
the SEM imaging results.
The zeta potential results (Table S2) indicate that in reservoir brine,
silica and kaolin are negatively charged (-1.5 mV and -4.6 mV), while
dolomite and calcite are positively charged (+11 mV and +7.4 mV).
Shehata et al. reported zeta potential results for these four minerals
(calcite 13 mV, dolomite 19.7 mV, kaolinite −11 mV, silica −2.5 mV)
at 55 g/L salinity (Shehata & Nasr-El-Din, 2015). The low magnitude
obtained in our experiments is due to the higher salinity of the system
which compresses the electrical double layer resulting in lower magnitude of zeta potential values. (Brown, Goel, & Abbas, 2016).

2.2. Material characterization
The chemical structure of SPG was determined by analyzing the
FTIR spectra acquired in the range of 4000–525 cm−1 with a resolution
of 4 cm−1 using FTIR instrument (FT-IR Nicolet™ iS™ 50). The surface
chemistry of the carbonate minerals was analyzed by X-ray
Photoelectron Spectroscopy (XPS) analysis using SSX-100 system
(Surface Science Laboratories, Inc.). The morphology of the carbonate
minerals was examined using SEM (FEI Quanta 250 FEG SEM) operating at 20 kV using powder samples fixed on the surface of a standard
aluminium SEM stub using conductive super glue.
The specific surface area of minerals was measured via nitrogen
adsorption using BET technique (Quantachrome Autosorb-3b). Zeta
Potential and particle size of the minerals were measured after equilibrating overnight in reservoir brine using Malvern Zetasizer Nano-ZS

(Malvern Instruments).
2.3. Adsorption measurements
To study the effect of polymer concentration, salinity and temperature on the adsorption of SPG on various minerals, a biopolymer
solution of the desired salinity and concentration was prepared by diluting the stock solution of 5600 ppm with brine solution. A known
amount of the mineral was added to 40 mL of the polymer solution in
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M. Shoaib, et al.

Fig. 3. Kinetics of adsorption at (a) 25 °C and (b) 75 °C studied at an initial polymer to mineral ratio of 20 mg/g and salinity of 167 g/L.

equilibrium and is a function of the type of salts present in the solution.
Previous studies suggest the zeta potential equilibration time for carbonaceous surfaces may extend from days to months (Heberling et al.,
2011; Alroudhan, Vinogradov, & Jackson, 2016; Cicerone, Regazzoni, &
Blesa, 1992; Ruiz-Agudo et al., 2010; Somasundaran & Agar, 1967).

3.3. Adsorption kinetics
The kinetics of adsorption is very important in designing and carrying the adsorption experiments as well as in determining the affinity
of the polymer towards the surface and its propagation in porous media.
Fig. 3 shows the adsorption kinetics of SPG on different synthetic minerals at 75 °C and 25 °C. Regardless of the mineral type, the adsorption
equilibrium is reached faster at 75 °C compared to at 25 °C and the
adsorption over dolomite at 25 °C does not reach equilibrium even after
13 days, consistent with what was reported by Tempio et al. for adsorption of xanthan over calcite (Tempio & Zatz, 1981) and by El’tekov,
El’tekova and Roldughin (2007) for the adsorption of dextran over sibunit powder (El’tekov et al., 2007). El’tekov et al. (2007) further
concluded that adsorption of polymers over minerals proceeds quickly
during the initial stage of adsorption, which is characterized by adsorption capacity equal to about 60–70 % of the maximum (equilibrium
capacity), while subsequent stages may last for days. In our study,

50–60 % of the equilibrium was achieved during the first 50 h and the
rest of adsorption capacity proceeded slowly during the next 192 h (8
days). At 75 °C, the adsorption process is much faster, and the time
required to reach 90 % of the equilibrium capacity is reached within
30 h. Furthermore, the adsorption on dolomite is characterized by
higher equilibrium capacity compared to the other minerals regardless
of its lower surface area. On the contrary, kaolin has the lowest adsorption capacity. Silica is the fastest to reach equilibrium requiring
approximately 150 h at 25 °C and 12 h at 75 °C and kaolin takes slightly
longer time than silica to reach equilibrium even though the adsorption
capacity for kaolin is low, possibly due to the high specific surface area
of kaolin (26.6 m²/g) or small particle size used in the experiment even
though the adsorption capacity on kaolin is low. One factor which may
contribute to the large time required to reach equilibrium on calcite and
dolomite as compared to quartz and silica is the stabilization of zeta
potential for calcite and dolomite, which takes time to reach

3.3.1. Pseudo-second order kinetic model
Several models such as film diffusion, intra-particle and pseudofirst-order were tried to fit the kinetic data. A model which fits both the
25 and 75 °C is the pseudo-second order model. This model is best
suited for a system where chemical reaction is the significant ratecontrolling step (Ho & McKay, 1998, 1999). The model is represented as
per the following equation.

dqt
= k2 (qe − qt )2
dt
m2

where k2 ( mg − h ) is the rate constant for pseudo-second-order reaction
rate. The equation can be transformed further into the following
equation:

t
1
t
=
+
qt
k2 qe 2
qe
The slope and intercept of the equation enables one to calculate the
rate constant (k2 ). Fig. 4(a,b) shows the pseudo-second-order fit to kinetics data at 25 and 75 °C and the adsorption rate constant is provided
in Table 1.
3.4. Adsorption on different minerals
Fig. 5 shows the adsorption plateau for all the minerals at 75° C. At
these experimental conditions, calcite and dolomite have positive zeta
potential whereas kaolin and silica have slightly negative zeta potential
(Al Mahrouqi et al., 2017; Besra et al., 2004; Ho & McKay, 1998;

Fig. 4. Pseudo-second order plot of SPG adsorption on different minerals at (a) 25 °C and (b) 75 °C.
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M. Shoaib, et al.

the SEM imaging, the smallest particle size is of kaolin, while dolomite
has the largest particle and their adsorption capacity are minimum and
maximum respectively.Moreover, calcite and silica have nearly the
same particle size and their adsorption is also close to each other with

calcite plateau slightly higher than silica possibly due to the chemistry
of calcite which prefers SPG molecules more than silica. The adsorption
values for calcite and dolomite support this hypothesis as these two
minerals have similar surface chemistry yet the adsorption over dolomite is higher than calcite possibly due to larger particle size of dolomite. The Schizophyllan chains are 0.1−0.5 μm in length (Ferretti
et al., 2003) which are of the order of particle’s dimensions signifying
the importance of particle size in relating the adsorption value. An
additional factor for low adsorption on kaolin may be due to the anisometric and anisotropic character of kaolinite. The kaolinite particles
are disc-type particles with two basal and an edge plane. The surface
chemistry of the three planes are dissimilar which may be an additional
reason for the low adsorption.

Table 1
Pseudo-second order rate constant at 22 and 75 °C for different minerals.
Mineral

Pseudo-second order
25 °C

calcite
dolomite
kaolin
silica

75 °C

k2 x103

R²

k2 x103

R²

3.4
1.1
18.9
7.6

0.99
0.98
0.99
0.99

66.8
57.5
1494.5
249.6

0.97
0.97
0.99
0.99

3.5. Effect of salinity and background ions
Fig. 6(a) shows the effect of overall salinity on adsorption isotherm.
As apparent from the figure, the adsorption level on calcite decreases
significantly with salinity. The adsorption plateau on calcite in DI water
is around 3.6 mg/m2which reduces to 2.2 mg/m2 in 125 g/L salinity. A
similar trend is observed for 167 g/L and 252 g/L brine where adsorption level decreases significantly in the initial points of the isotherm.
However, the adsorption plateau value for all the three salinities on

calcite is around 2 mg/m2 indicating the important impact of salinity on
adsorption. Overall, the adsorption decreases with an increase in salinity as compared to DI Water. But, there is little difference in adsorption plateau for different salinities. Fig. 6(b) shows the effect of salinity
on maximum adsorption on calcite, dolomite, kaolin and silica. The
adsorption on dolomite, kaolin and silica also decreases with salinity.
The maximum adsorption capacity in DI water on dolomite, kaolin and
silica is 5.5, 0.2 and 1.5 mg/m2 which reduces to 3.3, 0.10, and 1.3 mg/
m2 respectively, in 125 g/L. However, further increase in salinity to
252 g/L has little effect on the maximum adsorption capacity, which
drops very little to 3.0, 0.10, 1.1 mg/m2 for dolomite, kaolin and silica,
respectively. Ma (2007) reported similar effect of NaCl on the adsorption of guar gum onto hematite, alumina and titania. They observed a
decrease in the adsorption density of guar gum at high NaCl concentrations. Chen et al. with their molecular dynamic simulation study
attributed the decrease in adsorption of carbohydrates over calcite in
the presence of salt to the formation of salt layers on the calcite surface
which act as a screen for the carbohydrate–calcite interaction (Chen,
Panagiotopoulos, & Giannelis, 2015).
To further understand this phenomenon, the effect of individual
ions on adsorption was also studied. Fig. 6(c) shows the adsorption
values in the presence of different background ions. The individual
concentration of salts was kept equal to their concentration in

Fig. 5. Adsorption of SPG over different minerals at 75 °C in 167 g/L brine.

Jackson, Al-Mahrouqi, & Vinogradov, 2016; Kim & Lawler, 2005;
Yukselen & Kaya, 2003). Adsorption is maximum on dolomite with a
plateau value of 1.58 mg/m². For silica and calcite, the adsorption
plateau value is 1 and 1.30 mg/m² respectively, while for kaolin this
value is less than 0.05 mg/m².The adsorption level measured for carbonate minerals (calcite and dolomite) is in the same range as reported
by Somasundaran (1969) for the adsorption of starch over calcite
(1.3 mg/m²). Rinaudo and Noik (1983) also reported the adsorption of
amylopectin on calcite (13 mg/g). Xia and Marek (Ma & Pawlik, 2005)

reported the adsorption of guar gum (a polysaccharide) on the quartz
surface, the maximum value of adsorption attained in their case was
0.4 mg/m². Ma (2008) in another study reported the adsorption level of
starch on quartz to be around 1.5 mg/m². The results reported by these
authors are at different salinities.
The maximum adsorption capacity (plateau value) may also depend
on the particle size of mineral as previously reported (Chodanowski &
Stoll, 2001). As confirmed from the particle size measurement as well as

Fig. 6. Effect of (a) salinity on adsorption of SPG over calcite, (b) salinity on adsorption of SPG over different minerals, and (c) background ions on the adsorption of
SPG on calcite at 25 °C.
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chains is faster than the longer chains due to which most of the chains
which remain adsorbed are longer ones and the chains which remain in
solution are shorter chains. Preferential adsorption of longer chains as
compared to shorter chains has also been observed for synthetic polymers such as HPAM, polystyrene and PEO over silica (Bessaies-Bey
et al., 2019; Fu & Santore, 1998; Vander Linden & Van Leemput, 1978).

represented reservoir brine (167 g/L). The adsorption plateau in DI
water which is 3.6 mg/m² decreases to 1.7, 2.6 and 2.4 mg/m² in the
presence of NaCl, CaCl2 and MgCl2, respectively. However, the adsorption increases to 5 mg/m² in the presence of KCl. This can be attributed to structure breaking (chaotropes) properties of poorly hydrated K+ ions. The K+ ions are capable of breaking the interfacial
water structure at the calcite-solution surface thus allowing the SPG
molecules to approach more closely to the mineral surface which results
in enhanced adsorption whereas the other ions Na+, Ca+2and Mg+2

are all structure makers and thereby decrease the adsorption capacity
(Ma & Pawlik, 2005, 2006; Ma & Pawlik, 2007). Therefore, even though
SPG is a neutral molecule, its adsorption behavior is influenced by the
presence of different ions due to the interaction between ions and
surface.

3.7. Adsorption mechanism
Many polysaccharides are used for adsorption application. For example, starch is used in mineral flotation as a selective adsorbate for
removal or depression of phosphate from quartz (Lange, 2020). Carboxymethyl cellulose and guar gum are used to depress hydrophobic
gangue minerals such as talc and graphite. Guar gum is also used in
potash flotation process to preferentially adsorb over slimes such as
clays, carbonates and quartz so that these particles do not adsorb cationic amine collectors meant for the flotation of potash (Ma, 2007).
Despite the extensive use of polysaccharides in several industries, their
adsorption mechanism over mineral surfaces remains elusive. Several
mechanisms have been proposed, but none of the mechanisms is widely
accepted (Laskowski, Liu, & O’connor, 2007; Liu, Zhang, & Laskowski,
2000). In this section, we discuss the possible adsorption mechanism of
SPG over calcite.

3.6. Effect of adsorption on MWD of SPG
Gel Permeation Chromatography (GPC) of the fresh polymer solution and supernatant after the adsorption experiment were performed
to characterize any change in polymer molecular weight distribution on
a qualitative basis. GPC experiments were performed following the
same protocol reported by Horvathova et al. for a β-1,3-glucans including that of schizophyllan (Horváthová et al., 1990). Briefly, an
Agilent 1260 HPLC System with a differential refractometric detector
and PL aquagel-OH MIXED-M 8 μm column was used. A flow rate of
0.5 mL/min was used for the experiment. The sample volume injected
was 100 μL after proper dilution. The system was stabilized for 24 h
before use and all the experiments were carried out at 25 °C. Fig. 7
shows the comparison of GPC profiles between the fresh polymer solution and the same polymer sample from the supernatant after the

adsorption. The high molecular weight regime or fraction is absent in
the supernatant indicating the preferential adsorption of bigger chains
compared to the smaller chains which remain unadsorbed.
When a polymer solution is mixed with mineral particles, the
smaller macromolecules diffuse more swiftly and bind to the surface
first, but later they are displaced by the larger ones. This has been explained in the literature due to the entropy changes which occur when
short chains adsorb and desorb as compared to the longer chains
(Devotta & Mashelkar, 1996). A short chain accounts for bigger entropy
changes during adsorption compared to the entropy changes of longer
chain(Dijt, Cohen Stuart, & Fleer, 1994). Hence, desorption for smaller

3.7.1. Electrostatic interactions and Salt linkage
SPG is a non-ionic polymer, therefore, electrostatic interactions do
not govern its adsorption over calcite. However, adsorption of SPG over
calcite in the presence of salinity decreases to a certain level after which
salinity has very little effect on adsorption. This initial decrease may be
attributed to the screening of calcite surface by adsorption of salt ions
preventing the non-electrostatic interactions between SPG and calcite
surface.
3.7.2. Hydrogen bonding
The role of hydrogen bonding in the adsorption of SPG over calcite
can be confirmed by the addition of urea. Urea is known to be a strong
hydrogen bonding acceptor and thus can affect the adsorption process
of polysaccharides over minerals if the adsorption is taking place
through hydrogen bonding (Maeda et al., 1988; McQueen-Mason &
Cosgrove, 1994; Wang & Somasundaran, 2005; Wang, Somasundaran,
& Nagaraj, 2005). Urea is expected to decrease the hydrogen bonding
between the mineral and the polymer in solution by preferential formation of hydrogen bonds between polysaccharides and water. Despite
the low strength of hydrogen bond which is of the order of 2 × 104
joule/mole, the cumulative energy of adsorption becomes significant

for polysaccharides with high molecular weight (Steenberg, 1982).
Hydrogen bonding occurs between the hydrogen atom of polysaccharide and oxygen atoms present on the mineral surface. If the
polysaccharide is capable to form hydrogen bonds with mineral surface
oxygen atoms, then before the adsorption, each of the species should be
involved in hydrogen bonding either with water or internally so that
the formation of a polysaccharide-mineral hydrogen bond is a result of
two hydrogen bonds split-up. Since the energetics of such a process is
not easily justified, this mechanism can only be applied if there are
factors available which contribute to the polysaccharide-mineral hydrogen bond stability (Liu, 1988). Adsorption isotherm of SPG on calcite in the presence and absence of urea is shown in Fig. 8. Both the
experiments in the presence and absence of urea were conducted in
167 g/L brine. The adsorption in the presence of urea increases to
3.9 mg/m2 as compared to 1.9 mg/m2 when no urea was present. This
may be ascribed to the interaction of urea with intermolecular or intramolecular hydrogen bonds of SPG. Urea may disrupt the intermolecular hydrogen bonding network of polysaccharides and can provide more molecules for adsorption as compared to the original
molecule (Southwick, Jamieson, & Blackwell, 1982; Zhang, Zhang, &
Cheng, 2000). Jaishankar et al. (2015) studied the impact of urea on the

Fig. 7. Molecular weight distribution of SPG before and after adsorption over
calcite at 25 °C.
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M. Shoaib, et al.

hydroxyl groups. The overall adosprtion mechanism is greatly influenced by the presence of different ions as the external ions affect the
polymer reach to the surface.
4. Conclusions
The adsorption of schizophyllan on different minerals follows
pseudo-second-order kinetic and suggests the adsorption phenomenon

is slow over calcite and dolomite requiring around 10 days at 25° C and
3 days at 75° C because the slow surface equilibration of carbonaceous
minerals. Moreover, adsorption capacity on calcite and dolomite is
more significant than on silica and kaolin and the adsorption over
calcite decreases in presence of structure making ions
(Na+,Ca+2,Mg+2) and increases in presence of structure breaking ion
(K+). Furthermore, an inverse relationship is observed between brine
salinity and adsorption on calcite. This relationship can be utilized to
tune the polymer adsorption as required for polymer flooding projects.
Adsorption over calcite also increases in the presence of urea due to the
disruption of the schizophyllan intermolecular hydrogen bonding. We
also observed selective adsorption of longer schizophyllan chains over
calcite .We propose acid-base driven complexation between schizophyllan and the mineral surface. The adsorption of schizhophyllan on
calcite (2 mg/m²) is lower than the adsorption of polyacrylamide-based
polymer on silica (0.5 mg/m²) (Masalmeh et al., 2019; Quadri et al.,
2015; Zhang & Seright, 2013). Therefore, the low adsorption of schizhophyllan on calcite along with its outstanding thermal stability makes
it an excellent candidate for polymer flooding applications in high
temperature, high salinity carbonate reservoirs.

Fig. 8. Effect of urea on adsorption of SPG on Calcite.

intermolecular hydrogen bonding network of Mamaku Gum which is a
polysaccharide. They concluded that the addition of urea disrupts the
intermolecular hydrogen bonding network of Mamaku Gum which
leads to its altered rheological properties as compared to the native
form. Tako (1996) found that the addition of urea to SPG prevents the
intermolecular hydrogen bonding between different SPG molecules.
This suggests that hydrogen bonding although present as intermolecularly and intramolecularly in SPG solution does not contribute to
its adsorption over calcite.

CRediT authorship contribution statement
Mohammad Shoaib: Investigation, Formal analysis, Visualization,
Writing - original draft. Syed Mohamid Raza Quadri: Validation.
Omar Bashir Wani: Writing - review & editing. Erin Bobicki: Formal
analysis, Writing - review & editing. Gerardo Incera Garrido:
Resources, Formal analysis. Ali Elkamel: Supervision, Writing - review
& editing. Ahmed Abdala: Funding acquisition, Conceptualization,
Methodology, Supervision, Writing - review & editing.

3.7.3. Acid-base interaction
The hydroxyl groups (eOH) present in polysaccharide and over the
mineral surfaces in the presence of water have been proposed to play an
important role in the adsorption mechanism. The hydroxyl groups on
the mineral surface depending upon the metal ion to which it is attached can act as a Bronsted acid (proton donor) or Bronsted base
(proton acceptor). It has been proposed that mineral surface donates an
eOH group to form a five-membered polysaccharide-metal ring complex with two protons from the polysaccharide hydroxyl groups.
According to this mechanism, during the interaction with polysaccharide mineral surface, metal-hydroxylated species act as a
Bronsted base. The strong basic character of mineral surface results in
higher adsorption. Due to this, the natural polysaccharides such as
dextrin, guar gum and starch adsorb strongly over metal oxides/hydroxides of Pb, Ni, Ca, and Mg as compared to Si attributed to the acidic
character of Si surface (Liu et al., 2000). This mechanism is also supported by the fact that glucose, a neutral molecule, adsorbs in a considerably large amount on normal alumina surface than on acidic alumina surface (Nakatani, Ozawa, & Ogino, 1990). Somasundaran also
proposed the adsorption of starch over calcite occurring via a complex
formation (Somasundaran, 1969). Apart from the basicity of the surface, the density of surface metal hydroxyl groups also affects the adsorption with higher densities of hydroxyl groups resulting in stronger
adsorption. Adsorption of Baker dextrin on Pb-coated quartz is reported
to be lower than on the galena surface because of the lower density of
hydroxylated groups on the Pb-coated quartz surface (Liu & Laskowski,
1989a, 1989b). In our case, this may be an additional reason for higher
adsorption on dolomite as compared to calcite as the dolomite is reported to have a higher density of hydroxylated species as compared to
calcite (Pokrovsky et al., 2000). Based on above analyses, the possible
mechanism of SPG adsorption over carbonate mineral surfaces may be

the acid-base interaction mechanism which proceeds through complex
formation between hydroxylated metal centers and polysaccharidic

Acknowledgement
We acknowledge Abu Dhabi National Oil Company, UAE for
funding this research. We also acknowledge Wintershall for providing
Schizophyllan sample.
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
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