Adsorption of enhanced oil recovery polymer, schizophyllan, over 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.

Carbohydrate Polymers 240 (2020) 116263 Contents lists available at ScienceDirect 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 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: ahmed.abdala@qatar.tamu.edu (A Abdala) https://doi.org/10.1016/j.carbpol.2020.116263 Received 12 March 2020; Received in revised form 30 March 2020; Accepted 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 (http://creativecommons.org/licenses/BY/4.0/) 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 ≥ 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 Schizophyllan (a) monomer unit (National Library of Medicine Data, 2020) (b) triple helical structure Reproduced with permission from Reference (Okobira et al., 2008) Carbohydrate Polymers 240 (2020) 116263 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 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 Results and discussion Fig 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 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 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 μm to μm, while kaolin has a small particle size and disc-shaped particles with very small diameters and silica’s particle size is ∼ 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 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 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 Carbohydrate Polymers 240 (2020) 116263 M Shoaib, et al Fig 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 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 t = + qt k2 qe 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 3.4 Adsorption on different minerals Fig 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 Pseudo-second order plot of SPG adsorption on different minerals at (a) 25 °C and (b) 75 °C Carbohydrate Polymers 240 (2020) 116263 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 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 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 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 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 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 Carbohydrate Polymers 240 (2020) 116263 M Shoaib, et al 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 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 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 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 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 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 × 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 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 Molecular weight distribution of SPG before and after adsorption over calcite at 25 °C Carbohydrate Polymers 240 (2020) 116263 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 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 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 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 online version, at doi:https://doi.org/10.1016/j.carbpol.2020.116263 References Abdel-Mohsen, A., et al (2014) Preparation, characterization and cytotoxicity of schizophyllan/silver nanoparticle composite Carbohydrate Polymers, 102, 238–245 Al Mahrouqi, D., Vinogradov, J., & Jackson, M D (2017) 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Conference Wang, J., & Somasundaran, P (2005) Adsorption and conformation of carboxymethyl ... solution SPE Improved Oil Recovery Conference Joshi, S., et al (2016) Production and application of schizophyllan in microbial enhanced heavy oil recovery SPE EOR Conference at Oil and Gas West Asia... linkage SPG is a non-ionic polymer, therefore, electrostatic interactions not govern its adsorption over calcite However, adsorption of SPG over calcite in the presence of salinity decreases to... Improved Oil Recovery Leonhardt, B., et al (2014) Field testing the polysaccharide schizophyllan: Results of the first year SPE Improved Oil Recovery Symposium Liu, Q (1988) The role of mineral