Abstract
Lignocellulosic biomass was delignified by combining physical and chemical pretreatment techniques. Then, a polysulfone-polyethylene glycol blend, which was compatible with the lignin-free biomass (0 wt% to 3.0 wt%), was used to fabricate composite membranes. The presence of hydroxyl groups after the pretreatment was evaluated via Fourier transform infrared spectroscopy. The rheology of the polymer solutions was assessed via the viscometric method. Also, the hydrophobicity of the fabricated membranes was determined using contact angle and porosity measurements. The fabricated membranes with near superhydrophobic properties (a contact angle of approximately 140°) based on this study revealed that contactor systems and biomedical applications would benefit from this modification.
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Study on Lignin-free Lignocellulosic Biomass and PSF-PEG Membrane Compatibility
Abiodun A. Amusa,a Abdul L. Ahmad,a,* and Jimoh K. Adewole b
Lignocellulosic biomass was delignified by combining physical and chemical pretreatment techniques. Then, a polysulfone-polyethylene glycol blend, which was compatible with the lignin-free biomass (0 wt% to 3.0 wt%), was used to fabricate composite membranes. The presence of hydroxyl groups after the pretreatment was evaluated via Fourier transform infrared spectroscopy. The rheology of the polymer solutions was assessed via the viscometric method. Also, the hydrophobicity of the fabricated membranes was determined using contact angle and porosity measurements. The fabricated membranes with near superhydrophobic properties (a contact angle of approximately 140°) based on this study revealed that contactor systems and biomedical applications would benefit from this modification.
Keywords: Lignin; Solubilization; Polysulfone; Polyethylene glycol; Membrane; Compatibility
Contact information: a: School of Chemical Engineering, Engineering Campus, Universiti Sains Malaysia, Acme University, 14300 Nibong Tebal, Seberang Perai Selatan, Pulau Pinang, Malaysia; b: Process Engineering Department, International Maritime College, Suhar Port Falaj Al Qabail, Suhar 332 Sultanate of Oman; *Corresponding author: chlatif@usm.my
INTRODUCTION
Agro-based biomass constitutes lignocellulosic waste materials in the form of wood, grass, and forestry left-overs. From the total biomass, approximately 60% of it is plant biomass (lignocellulose). Lignocellulosic biomass (LCB) serves as an excellent feedstock resource (both renewable and nonrenewable) for modern industrially evolving societies. The valorization of LCBs is not overemphasized, depending on the source of the biomass, e.g., grasses, hardwoods, or softwoods, for various applications, e.g, gas separation, energy sources, biofuels, and incorporation into the production of fine chemicals (Irshad et al. 2013). Date seed is one of the lignocellulosic materials that continues to attract attention from research scientists.
The date palm, known as Phoenix dactylifera L, belongs to the Arecaceae family. Dates could be relevant to upcoming generations based on their remarkable nutritional value, environmental friendliness, and economic prowess. Global date palm cultivation is estimated to include approximately 2000 different species. Every species provides date seed and the fleshy pericarp (Farooq et al. 2018).
One of the top three countries in the world in terms of producing date fruits is Saudi Arabia. This region accounts for approximately 15% of the total amount of annual global dates cultivated (approximately 100 million) (Zhang et al. 2017). Al-Jasass et al. (2015) reported that even in the United States, dates that are bright in color are favored by customers. However, studies by Bouhlali et al. (2017) reported that the types of dates and stages of maturity can affect the color of the date fruit. Sukkari date seeds are one of the varieties of the discarded by-products of date palm fruits. The seed weighs approximately 11% to 18% of the total weight of the date palm fruit. Therefore, Sukkari (which means sweet) dates, which are gold in color and therefore draw the attention of the consumer and the most popular, were chosen for this work (Azeem et al. 2016).
Researchers have analyzed date seeds to determine their constituents and nutritional value, which has led to date seeds being used as raw materials for various applications, e.g., the synthesis of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (Purama et al. 2018), poly(3-hydroxybutyrate) (Yousuf and Winterburn 2017), polyhydroxybutyrate, polyhydroxyalkanoates (Yousuf and Winterburn 2016), and activated carbon (Ogungbenro et al. 2017). Besides, date seeds are been used to produce nanocomposites (Adewole 2016), cosmetic products (Adeosun et al. 2016), oil (Habib et al. 2013), fats (Ben-Youssef et al. 2017), biodiesel (Azeem et al. 2016), for enhanced oil recovery (Adewole and Sultan 2013), and other products (Adewole and Ahmad 2016; Blaisi 2018).
A date seed, like any other LCB, is made up of cellulose, hemicellulose, and lignin as the primary components. One of the challenges is the complex structure of the lignin in the biomass, which limits its processability. The proposal to use LCB as a filler is gaining an increased amount of interest for various applications in membrane engineering (Amusa et al. 2020). The two divisions of membranes are polymeric and inorganic. However, unlike inorganic membranes, polymeric membranes are flexible, easily prepared, and have other desirable properties, which has influenced their wide application in gas and liquid separation processes. The properties of ultrafiltration membranes can be improved by using polymer blends to incorporate certain missing properties into a polymer matrix (Gullinkala and Escobar 2008).
Polysulfone (PSF) is an excellent polymer for membrane fabrication, with unique properties and high stability (chemical or mechanical). The biocompatibility properties of PSF are better than those of other materials (Kubala et al. 2002). In comparison to polyacrylonitrile and polymethylmethacrylate membranes, a study by Yamashita and Tomisawa (2009) established that α-chymotrypsinogen A and cytochrome C (with molecular weights of 25,000 and 12,400, respectively) did not show any affinity towards PSF. The hydrophilicity of the membrane has been improved by blending it with biocompatible polymers, e.g., polyethylene glycol (PEG) (Ostuni et al. 2001). Sinha et al. (2013) blended PSF and dimethylacetamide (DMAc), and Chakrabarty et al. (2008) used N-methyl-2-pyrrolidone (NMP) as the solvent.
In this study, LCB was pretreated and incorporated into a PSF-PEG blend to fabricate a flat sheet, mixed matrix membrane. Then, its compatibility was investigated via Fourier Transform Infrared Spectrometry (FTIR), as well as determining its contact angle and the polymer solution flow was studied using a viscometer. Detailed delignification process evaluation and membrane characterization and performance study were not covered as part of the scope of this study as they are parts of the authors’ ongoing research.
EXPERIMENTAL
Materials
The date pits (Sukkari, LCB) were sourced from Riyadh, Saudi Arabia. The hydrogen peroxide (H2O2, 30%), n-hexane (99%), boric acid (H3BO3, 99.5%), sodium hydroxide (NaOH, 99%), potassium hydroxide (KOH, 99%), sodium chloride (NaCl, 99%), ethanol (C2H5OH, 99%), hydrochloric acid (HCl, 37 %), and N-methyl-2-pyrrolidone (NMP, 99%) were used without any further purification, i.e., as purchased from Sigma Aldrich (St. Louis, MO). The polysulfone (PSF pellets) and polyethylene glycol (PEG) were purchased from Amoco Chemicals (Chicago, IL).
Methods
Lignin-free pretreatment techniques
The date pits were processed into fine powder by adapting the methods reported by Karinkanta et al. (2018) and Mayer-Laigle et al. (2018) with some modifications. The date pits were washed, soaked in hot water for 72 h, ground, dried under a vacuum at a temperature of 50 °C, and then re-ground to a fine powder. Approximately 20 g of finely powdered samples were then de-oiled to remove the extractives in a Soxhlet apparatus using n-hexane as the solvent, and the recovery of the solvent was achieved using a rotary evaporator. Then, the deoiled samples were dried at a temperature of 80 °C in a vacuum oven until the weight became constant, which indicated the removal of traces of residual solvents. The lignins were solubilized using the Van Soest method (Van Soest et al. 1991) with some modifications. First, a NaOH alkaline solution was prepared at 7 wt.%. Then, the de-oiled sample (DP) and the NaOH alkaline solution were mechanically stirred for 3 h at 80 °C (in a solid to solvent ratio of 1 to 10). Distilled water was used to wash the treated samples to a neutral pH after the treatment was performed three times. Additional lignification steps to remove the remaining lignins, accompanied by bleaching, were performed using acidified sodium chlorite (1.7 wt%). The treated samples were transferred to 10 mL of a freshly prepared (11%, v/v) hydrogen peroxide solution. At a temperature of 45 °C, the acidified sodium hydroxide solution was used to adjust the pH of the mixture to 11 via vigorous stirring for 3 h. The residue was then treated with 10 wt.% KOH, which contained 1% H3BO3, at room temperature and a 1 to 25 solid to liquid ratio. This was stirred for 10 h. After each treatment, the powdered-particles were washed with distilled water. This process was repeated twice while maintaining the same conditions, resulting in effective lignin solubilization. Finally, the sample was filtered and freeze-dried, and the dried date pits cellulose (DPC) samples were kept in a glass bottle for further analysis. Afterward, the particle size of the synthesized DPCs was examined using a particle size analyzer (CILAS 1180, CILAS, Orléans, France). The composition and functional groups of the synthesized DPC samples were investigated via Fourier transform infrared spectroscopy (FTIR) with an IRAffinity-1S (Shimadzu Co., Kyoto, Japan).
Fabrication of the cellulose embedded composite
The polysulfone pellets were dried in an oven at 120 °C overnight before being used for moisture removal. A mixing-casting process was used for the fabrication of membrane samples, as reported by Ismail and Lai (2004), with some modifications. For the fabrication of pristine PSF membranes, the casting solution was prepared by dissolving 20 wt.% of a PSF-PEG blend with NMP on a hot plate at 60 °C for 24 h, stirring until a clear solution was obtained. For the composite membranes (CMBs), a calculated amount of DPC (0 wt% to 3 wt%) was dispersed into NMP and stirred for 3 h; then the suspension was placed in an ultrasonic bath for 1 h to obtain a homogeneous dispersion. Then, PSF and PEG were gradually added, and the suspension was stirred overnight until the solution became homogeneous. Next, the solution was degassed for 4 h to remove any trapped bubbles. Thereafter, the pure PSF solution and the CMB suspensions were cast at room temperature with a casting knife on a flat glass plate with a gap setting of 25 μm. Solvent evaporation was expected while allowing a free-standing time of 30 s, since other free-standing times (10 s and 60 s) did not yield the desired morphology. Next, the samples were immersed in a water-containing coagulation bath for 48 h at room temperature; the water was changed after 24 h for the complete phase separation process. The membranes were removed and dried for 72 h in the open air and labelled PC-0, PC-0.5, PC-1, PC-2, and PC-3 for the pristine PSF membrane and the CMBs with DPC loads of 0.5 wt%, 1 wt%, 2 wt%, and 3 wt%, respectively. Then, the fabricated membranes were analyzed to determine their wettability, porosity, and mean pore size.
RESULTS AND DISCUSSION
Delignification and Dope Solution Retrospect
The removal of the non-cellulosic components via the pretreatment of LCB is aimed at maximizing the accessibility to the hydroxyl groups. However, lignin functionalization is closely related to the delignification of LCB because the solubility of the process liquid is due to the formation of functional groups. This implies polymer dissolution via the addition of hydrophobic groups. Hansen and Björkman (1998) postulated that hydrogen bonds, dipole-dipole, and nonpolar interactions must be broken for effective lignin solubilization. The use of organosolv (Salapa et al. 2017) or solvents (dioxane and acetone) (Saha et al. 2019) to solubilize lignins have their drawbacks. The alkaline delignification method is better in terms of ease, safety, and scalability (Cavali et al. 2020).
Other varieties of date pits (DP) possessed similar chemical composition as found in this study (Habib and Ibrahim 2009; Essa and Elsebaie 2018). The obtained physicochemical properties of the collected DP and the properties of other agro-based waste products were comparable (Fernández-López et al. 2004; Alfredo et al. 2009). As shown in Fig. 1, the lignin-free sample was approximately 20 µm.
Fig. 1. Lignin-free particle size analysis
The dope solution formed by a polymer and/or additive with a solvent in this study was a homogeneous polymer solution without agglomeration. A thermodynamically unstable polymer solution was avoided by ensuring that demixing did not occur as the synthesized DPC was dispersed in the polymer solutions. The materials were uniform, as reflected in the FTIR and other analyses to be discussed.
Fourier Transform Infrared Spectroscopy Analysis (FTIR)
The generation of a superoxide radical that is highly reactive resulted in the formation of carboxylic acid compounds via lignin aromatic rings and some hemicellulose oxidation. The oxidative mechanism results in the removal of an electron from the lignin phenolic units, followed by other fast reactions involving oxygen and carboxyl. Under these conditions, the cellulose is not degraded; rather, the lignin structure is subject to attack from the aggressive oxidizing agent (Sun et al. 2004; Peng et al. 2010; Sun et al. 2015). Therefore, hemicellulose and cellulose became predominant. The structural transformation of the LCB samples is revealed in Fig. 2a; the lignin absorbance was at 1230 cm-1 and the aromatic parts were associated with an absorbance peak at 1517 cm-1, which all hardwood LCB spectra contain. The C=O stretching, and skeletal vibrations of the aromatic rings were represented by the peak at 1614 cm-1. The -CH3 and -CH4 asymmetry and deformation of C-H can give rise to the peak at 1444 cm-1 (Scholze and Meier 2001). The band at 1745 cm-1 was associated with the hemicellulose C=O stretching vibration which formed broadband at 1590 cm-1 to 1745 cm-1 after pretreatment, which was likely due to the overlap of the ester stretching of C=O at 1650 cm-1 (Liang et al. 2006). These bands disappeared on the DPC absorbance spectra. The lignin carbonyl groups, ester linkages, and acetyl groups were represented with the peak at 1240 cm-1 and the decrease indicated lignin dissolution. The methyl and phenol OH aliphatic C-H stretching gave rise to peaks at 1370 cm-1 and 2349 to 2923 cm-1. The hemiacetal and carbonyl groups correspond to the enhanced peak at 1630 cm-1. Cellulose contributed to the alcoholic hydroxyl group of the C-O stretching vibration peak at 1000 cm-1. The presence of OH groups was indicated by the O-H stretching broadband appearance at 3000 cm-1 to 3500 cm-1 for the pretreated samples. The lignocellulosic biomass absorption bands reported agreed with the data from the following studies: Gandini and Belgacem (2008) and Briones et al. (2011).
Fig. 2. The FTIR spectra of (a) the date pits (DP) and lignin-free samples (DPC) and (b) the PSF-PEG, PSF-PEG-DP, and PSF-PEG-DPC membranes
As shown in Fig. 2b, the incorporation of DPC into PEF-PEG did not affect the PSF absorption spectra from being retained. A polysulfone membrane is characterized by an aromatic framework (C=C) and an O=S=O group for the spectra in the range of 1400 cm-1 to 1600 cm-1 and 1000 cm-1 to 1300 cm-1, respectively. The aromatic stretching of the C-H group corresponded to the peak at 2930 cm-1 (Farrokhara and Dorosti 2020). The incorporation of DP into PSF-PEG did not yield different results in terms of the spectra. However, the appearance of a broad peak at 3100 to 3500 cm-1 for the CMB membrane, which was missing on the PSF-PEG and PSF-PEG-DP membrane spectra (the lignin recalcitrance effects in the DP), was attributed to the O-H band stretching of the DPC, which indicated the accessibility of the OH groups in the mixed matrix membrane. This result was also in agreement with the PSF incorporated with commercial cellulose as reported by Bai et al. (2015).
Surface Energy and Water Contact Angle Analyses
Different measurement types, e.g., sessile drop (or static) contact angle, tilting plate method, an add and remove method, time-dependent dynamic studies, and Wilhelmy plate method were reported by Drelich (2019). The static contact angle method was used in this study using a Model 590 H automated goniometer/tensiometer (Ramé-Hart Instrument Co., Succasunna, NJ) using DROPimage Advanced software (version Model: 250-F1). The incorporation of the synthesized DPC into the PSF-PEG membrane was investigated to measure the changes in surface wettability. Figure 3 represents the image for the contact angle measurement for the samples. Sample 3a shows a flattening water droplet, which signifies hydrophilicity, and which conformed with the obtained measurement (72.0°), contrary to the modified samples (3b to 3e).
Fig. 3. Pictograms of the water contact angles for (a) PC-0, (b) PC-0.5, (c) PC-1, (d) PC-2, and (e) PC-3, for the pristine membrane and the CMB’s with the DPC loads of 0.5 wt%, 1 wt%, 2 wt%, and 3 wt%, respectively.
The analysis of the water contact angle, porosity, and mean pore size for the PSF-PEG and the CMB membranes are shown in Table 1. This study showed that the contact angle values drastically increased from 72.0°, which was in accordance with other reports (Benkhaya et al. 2020; Camacho et al. 2020), with values ranging from 94.5° and 153.3° for the PSF-PEG and CMB membranes, respectively (hydrophobicity is any value greater than 90°). This was attributed to the fundamental theory related to the wetting action via changes in the surface roughness of a material. In general, the higher contact angle is consistent with increased fine-scale roughness of the membrane material upon the incorporation of the DPC (Hubbe et al. 2015). The incorporation of DPC to the PSF-PEG polymer matrix increased the heterogeneity of the surface of the composite membrane. The contact angle increased by a certain roughness factor which is associated with the roughness area ratio of the surface heterogeneity (roughness) of the composite membrane. The surface roughness factor is independent of the surface profile. However, it is difficult to quantitatively evaluate the roughness area ratio, which would be needed to fully confirm this concept (Wenzel 1949).
The DPC loading relationship with the membrane porosity and mean pore size were estimated using bubble point measurements (Tylkowski and Tsibranska 2015) and are presented in Table 1. The relative spreading of water on the membrane surface based on the unbalanced interaction of molecules, i.e., the wettability, was further examined. It was observed that the membrane porosity increased after the DPCs were added while the mean pore size decreased. This could be attributed to the viscous nature of the dope solutions, which resulted in pore growth restrictions (Said et al. 2017). This necessitated an investigation into the flow of the dope solution, as shown in the next section.
Table 1. Effect of the Content of Lignin-Free Particles in Composite Membranes on the Contact Angle, Surface Energy, Porosity, and Mean Pore Size
Rheological Investigations
The behavior of the polymers during processing are widely connected to their viscosity, which is related to their shear rate, shear stress, and torque. In this study, an RVDV-IIIU programmable rheometer (Brookfield, Middleboro, MA), with spindle number 27, was used. After turning on the motor and maintaining the temperature (at 21.5 °C), the speed of rotation was set between 10 and 250 rpm, and the values were recorded. The average of three experimental data measurements for viscosity, torque, shear stress, and shear rate were used for the PSF-PEG and CMB polymer solutions.
Figure 4a depicts the steady-state viscosity as a function of shear rate for the polymer solutions. The illustrated trend is a non-Newtonian behavior, since the plots are non-linear and do not pass through the origin, with shear-thickening as the shear rate increases. The same pattern is obtained for a log-log plot of the viscosity against the shear rate. In contrast to the polymer solutions being pseudoplastic, the viscosity increases as the shear rate increases (Hubbe et al. 2017). As shown in Fig. 4, the addition of DPC to the PSF-PEG solution shifts the viscosity curve to higher values. This can be attributed to the viscoelastic nature of the incorporated DPC with coil conformation, which is flexible in solution. It can also be attributed to the formation of hydrogen bonds between the NMP and PSF molecules, as well as between the PEG and the accessible hydroxyl groups from the DPC, as confirmed by the FTIR spectra. Also, anisotropic chain alignment due to the reorganization of polymer solutions can cause shear thickening behavior, as well as an increasing shear value between the particles as the lubrication regimes change (Fernandez et al. 2013).
Fig. 4. Viscoelastic properties of the (a) polymer solutions and (b) flow curves of polymer solutions at 21.5 °C
Figure 4b confirms the non-Newtonian properties of these polymer solutions in agreement with Fig. 4a. The PSF-PEG plot shows certain deformation at an approximate shear rate of 180 s-1, which can be attributed to a form of deviation from its linear viscoplastic nature to the closeness of exhibiting some non-linearity properties related to chain entanglements. This linear pattern is attributed to the micro-scallop average velocity of the shear thickening fluid, which increases due to the interactions among the polymer chains and becomes a definite, linear relationship, i.e., does not pass through the origin for shear stress deformation, with the incorporation of DPC, which is in line with the Power Law rheology model (Martínez-Pedrero and Tierno 2018). This finding will be helpful during the process of piping systems predicting flow behavior.
CONCLUSIONS
- Polysulfone-poly(ethylene glycol) (PSF-PEG) with incorporation of different wt.% of date pits cellulose (DPC) were fabricated as flat sheet CMB membranes and investigated for compatibilities. The obtained results confirmed that the OH groups of the DPC were present in the fabricated CMB membranes (via FTIR). The hydrophobic nature of the membranes was enhanced, as shown by contact angle measurements.
- The membrane materials exhibiting shear-thickening properties and achieved the highest porosity with a 1 wt% DPC load. The stability of the composite membranes can be guaranteed based on the reaction mechanism of the alkaline pretreatments of the date pits (DP) and the obtained near superhydrophobic values from the contact angle measurements.
- Overall, the pretreated lignin-free LCB was successfully incorporated into the PSF-PEG polymer matrix to modify its pore and resistance to wetting, which could be used for various applications in contactor systems and biomedical devices (Amusa et al. 2018).
ACKNOWLEDGMENTS
The authors acknowledge the Ministry of Higher Education (MOHE), under the TRGS grant (Grant no.: 203/PJKIMIA/67612001, project code: TRGS/1/2018/USM/01/5/1) and USM-RUI grant (Grant no. 1001/PJKIMIA/8014063, project code: U01631) for this research work.
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Article submitted: September 1, 2020; Peer review completed: November 15, 2020; Revised version received: December 8, 2020; Accepted: December 11, 2020; Published: December 17, 2020.
DOI: 10.15376/biores.16.1.1063-1075