Research PaperThe effect of Nanocrystalline cellulose/Gum Arabic conjugates in crosslinked membrane for antibacterial, chlorine resistance and boron removal performance
Graphical abstract
Introduction
Boron is extensively present in the environment either by natural (e.g. geothermal processes and mineral weathering) and anthropogenic (e.g. mining industries and agriculture) contamination, primarily in the form of boric acid, borosilicate and borax [1]. It can be found in sea water (approximately 5 mg/L as boron), brackish and freshwater supply. The concentration of boron in the fresh water supply is governed by the geochemical nature of the area i.e. by municipal or industrial effluents [2].
Even though the boron is considered as one of the vital micronutrients for plants, animals and humans for certain metabolic activities, there exists a narrow range between the over consumption and deficiency [3]. According to the World Health Organization (WHO) guidelines for drinking water suggested the limit of boron concentration should lie below the concentration of 2.4 mg/L. Therefore the boron and its compounds are in desperate need to be eliminated from water with elevated concentrations [4].
In seawater, boron is mainly present as boric acid B(OH)3 while partially as borate ions BO33− according to dissociatin reaction shown in Equation 1 as a function of pH [5].B(OH)3 + H2O ↔ B(OH)4 + H+ pKa = 9.2 at 25°C
Boric acid is highly weak acid (pKa∼ 9.2). At pH < 7, boron exists as boric acid (non-ionized form), while at pH < 10, it exists as borate (ionized form). The rejection rate of non-dissociated boric acid via reverse osmosis (RO) membranes is usually low because of its insignificant size and absence of electric charge. On contrary, the ionized form will be fully hydrated following to the increased in size and negative charge. As a result rejection rate increased by both size exclusion and charge repulsion of the negatively charged membrane [6], [7].
Lately, the polymeric membrane for boron removal has gained global attention due to low energy expenditure, reduced maintenance cost, ease of synthesis and modification flexibility [8], [9], [10]. PVA is highly used polymer due to its properties such as hydrophilicity, solubility in aqueous media, biodegradability and film forming property. It is considered as one of the best choices to fabricate pressure driven membrane for separation process includes ultrafiltration (UF), RO, microfiltration (MF) and pervaporation (PV) [11], [12], [13], [14], [15], [16], [17]. One of the main disadvantages of PVA membrane is the swelling that occurs in aqueous media due to high hydrophilic nature that can affect its performance properties, mainly the solute rejection phenomena. Hence, the crosslinking of PVA is required to stabilize the hydrophilic-hydrophobic interaction of the membrane [18], [19], [20]. The hydroxyl group present in PVA could be cross-linked by multifunctional compounds [21], [22], [23], [24], [25], [26].
Polyethylene oxide–polypropylene oxide–polyethylene oxide (PEO–PPO–PEO) triblock copolymers also termed as Pluronics consist of both hydrophilic and hydrophobic segments. Among various types, PEO-PPO-PEO block copolymer has elevated molecular weight (Mw ∼12,600) excellent extractability in the aqueous phase and moderate hydrophilic and lipophilic balance (HBL = 22) [27], [28], [29]. The addition of PEO-PPO-PEO block copolymer in PVA membranes enhance performance, by altering the rate of diffusion across the membrane, thus resulting in increased flux rate [28], [30], [31].
Gum Arabic (GuA) is recognized as natural, non-toxic polysaccharide having negatively charged, hydrophilic nature. It is naturally obtained from the secretion of Acacia Senegal and Vachellia Seyaltrees [32], [33]. It consists of > 97% carbohydrates and < 3% proteins in their distinct arabinogalactan polysaccharide structure [34], [35]. Due to its tremendous antibacterial properties, it is mostly used in cosmetic and pharmaceutical industries [34], [36].
Cellulose is the widely available and adequate biopolymer raw material. It has gained research attention as one of the most favorable polysaccharides, as they can be tailored according to its application [37], [38]. Lately, nano crystalline cellulose (NCC) has attracted keen attention as green nano biosorbent owned to its high mechanical strength, low density, high aspect ratio, biocompatible nature and crystallinity [39].
In this study, cross-linked PVA membranes integrated with PEO-PPO-PEO block copolymer and incorporated with NCC/GuA conjugates for boron removal were prepared. The NCC was first synthesized from microcrystalline cellulose by acid hydrolysis process. The fabricated membranes were then analyzed and characterized by employing different techniques like Attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), contact angle measurements, X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscope (TEM), atomic force microscopy (AFM) and antibacterial testing. The performance testing of the fabricated membranes includes permeability test, boron selectivity and chlorine resistivity, was analyzed using a dead-end RO filtration unit.
Section snippets
Materials
Hydrophilic polymer PVA with average molecular weight 30,000–70,000, Cross-linking agent (Tetraethylorthosilicate −TEOS) of reagent grade (98%), Gum Arabic (Mw = 250,000 Da), sodium hypochlorite solution (NaClO, with 15 wt.% active chlorine content) and PEO-PPO-PEO block copolymer (average molecular weight 12.6 kDa) were purchased from Sigma Aldrich (USA). Microcrystalline cellulose and Sulphuric acid (H2SO4) was purchased from BDH Laboratory supplies (Poole, England) and AnalaR respectively. Boric
Fourier transform infrared spectroscopy
An IR Prestige-21 (Shimadzu) employing attenuated total reflectance (ATR) accomplice with zinc selenide (ZnSe) crystal was employed to achieve spectra for the fabricated membrane samples. The range of frequency was from 4000 to 400 cm−1 with an average of 70 scans per spectrum with a resolution of 4 cm−1.
Surface contact angle
Goniometer (Digidrop, KSV Instruments) was employed to measure the surface water contact angle values of the RO conjugated membranes. The achieved value was mean of left and right angles of the
Fourier transform infrared spectroscopy
FTIR analysis was done to propose the interactions between PVA, NCC and GuA in the presence of the crosslinker (TEOS) and PEO-PPO-PEO block copolymer, as shown in Fig. 1.
The strong band at 3305 cm−1 is ascribed to OH stretching vibrations, existed in PVA and TEOS. The band in the range of 1000–1100 cm−1 confirmed the formation of Si-O- bonds (1083 cm−1) resulted from the condensation reaction of the hydroxyl groups of silanol (–Si-OH) and PVA in the prepared membrane. The Si-OC covalent bond
Conclusion
Series of crosslinked PVA membranes with different NCC/GuA conjugates loadings were successfully developed by solution casting method and used for boron (hazardous salt) removal. It was confirmed that the incorporation improved the hydrophilic character of the crosslinked membranes. The cross-linking among PVA and TEOS was confirmed by FTIR, while SEM showed successful incorporation of NCC/GuA conjugates. The crosslinked membranes presented an excellent boron rejection up to 92.4%. The chlorine
Acknowledgments
The authors are thankful to Higher Education Commission of Pakistan for funding under the project (HEC-USAID) of “Development of Innovative Technical and Medical Textile Products”.
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