Anti-fouling and permeable polyvinyl chloride nanofiltration membranes embedded by hydrophilic graphene quantum dots for dye wastewater treatment
Graphical abstract
Introduction
One of the most severe environmental issues is water pollution that contains various contaminants. To overcome such a problem, purification methods could be applied to provide usable water from wastewater [[1], [2], [3], [4]]. As the globe's population increases, the demand for water on limited water resources increases, hence creating a technology to recycle the wastewater is inevitable. More importantly, the filtration process, such as membranes, has attracted the attention of many researchers recently. Water scarcity causes some drastic issues, such as human and animal mortality and environmental pollution [5,6].
While resources can be optimally used and restored, the wastewater, which is composed of industrial, household, and agricultural wastewater, can be a potential resource to overcome water shortages. There are various filtration methods like coagulation, sedimentation, absorption, adsorption, and membrane processes to treat wastewater from drilling companies, agricultural runoff, and municipal sewage [[7], [8], [9], [10]].
Membranes are one of the physicochemical purification methods that can be used to filter out pollutants. Compared to the chemical treatment methods such as evaporation and thermal treatment, membrane technology is inexpensive and faster [11,12]. Incorporating various materials into the membrane matrix could improve the membrane performance depending on operating conditions. Membrane fabrication methods can be classified, such as TIPS, NIPS, VIPS, and EIPS [[13], [14], [15], [16]].
Hydrophilic nanoparticles are utilized to improve the hydrophilicity of the membranes. Physical-chemical strategies can be shown in two various forms, such as physical and chemical properties [17]. Also, the nanoparticles could improve the anti-fouling performance. Due to their high flexibility, polymer membranes play an important role in many areas such as biosensing [18], ion exchange [19], water purification [20], membrane distillation [21,22] and biomedical [23], and have achieved better performance.
Polyvinyl chloride (PVC) is one of the most popular polymers because of its benefits such as availability, inexpensive (10% PSf price), decent physicochemical, and mechanical properties such as good tensile strength. Due to the hydrophobic nature of PVC, its usage as a filtration system causes some problems, including the reduction of the permeation flux, decrement of the system efficacy, and increase in the membrane washing expenses. Hence, such a membrane should be modified in order to be used for industrial applications. [[24], [25], [26]].
Nanoparticles, because of their unique features, can be used to modify and improve the filter properties such as hydrophilicity and porosity [27]. Carbon nanotubes (CNTs) have been used in various applications due to their unique features, including high mechanical strength, large aspect ratios, high surface area, distinct optical characteristics, high thermal and electrical conductivity. These properties introduce them for a broad range of utilization in the field of electronics (energy storage), biotechnology (sensors, and drug delivery) and other applications (multi-functional coatings/films) [28]. Like for instance, because of high thermal conductivity and surface area, making them useful as electrode catalyst supports in a fuel cell that is utilized in transport application, where durability is predominant [29]. Moreover, since a significant part of the human body is made up of carbon, it could be considered as a biocompatible material. CNTs are polymer containing pure carbon, which can be manipulated utilizing the extremely rich chemistry of carbon. This gives an excellent opportunity to change their structure and to maximize their solubility. More importantly, CNTs are perfect molecularly that make them free of property-damaging defects in the nanotube structure [28].
CNTs have succeeded in many challenges in the field of filtration and show an excellent anti-fouling property. Specifically, it has been used in water purification processes along with polyethersulfone (PES), polyvinylidene fluoride (PVDF), polysulfone (PSf), and PVC membranes. Using NF, UF, RO, PV mechanisms, and mixed matrix and thin films, nanocomposites structures contribute significantly to improve performance [[30], [31], [32]]. Another point is that the quantum dots (QDs) are made of nanocrystals composing a semiconductor core material [33]. QD particles have a wider bandgap and therefore require extra excitement energy for light emission at shorter wavelengths. QDs possess a wide absorption spectrum but limited fluorescent emission spectra. Such a spectrum range makes QD particularly appropriate for complex recognition, especially by employing an exciting light supplier. The QD emitting spectrum is 10-20 times brighter compared to an organic fluorophore and thousands of times more durable than traditional dyes [34]. QDs are novel nanocarbon-based materials that have been used in various fields, such as chemical assays, biosensing/bioimaging, photocatalysis, and electrocatalysis [35].
Graphene oxide quantum dots (GQDs) were selected because of their nanostructure, hydrophilic feature, and proper dispersion. As an attractive carbon-based material, GQDs have attracted significant interest in a wide range of applications because of their size, notable mechanical properties, frictionless surface, and stability [36]. One of the applications of GQDs is in the capability of targeting and imaging tumor cells simultaneously [37,38]. The broad application of GQDs was reported by Zhang et al. [39]. They researched scanning and transmission electron microscopy, and fluorescence microscopy. Zeng et al. [40] reported the covalent bonding of the GQDs onto amino-modified PVDF membranes that have anti-fouling and bactericidal better performance. There was another study on amino-functionalized GODs (aGQDs) encapsulated in thin-film nanocomposites as solvent resistance NF membranes, which were fabricated using interfacial polymerization and subsequent steps [41]. Song et al. fabricated GQDs based membranes as a chlorine resistance and anti-fouling membrane [42]. There are several nanoparticles embedded in the PVC polymer membranes in the literature. A number of these studies are summarized in Table 1.
Based on our knowledge, the effect of the addition of GQDs into PVC polymer for the fabrication of polymeric membrane has not been studied yet. In this study, our primary goal was to fabricate a novel membrane to enhance the flux and reduce the fouling of polyvinyl chloride NF membranes. The morphology and surface roughness of the membranes were investigated by scanning electron microscope (SEM) and atomic force microscopy (AFM). Anti-fouling properties were measured by BSA protein and rejection of the modified membranes by dye solution permeation that was Reactive Blue 19. This dye is one of the most common dyes used in different fiber dyeing procedures, such as dying of cotton, viscose fiber disseminated, roll dye, knot dyeing piled up, and continuous pad staining.
Section snippets
Materials
All components were used in their analytical grade. Doubly distilled deionized water was utilized. BSA protein was bought from Sigma-Aldrich Co., USA. Citric acid monohydrate (CA), sodium hydroxide (NaOH), and glucose were purchased from Merck (Darmstadt, Germany). Suspension PVC (SPVC) was obtained from Arvand Petrochemical Co., Iran. N-methyl-2-pyrrolidone (NMP) and polyethylene glycol (PEG, MW=4000 g/mole) were received from Merck Co., Germany.
Synthesis of graphene quantum dots
Based on our previous work, CA-derived GQDs were
Nanomaterials Characterization
In the present study, a facile carbonization technique was utilized for the preparation of GQDs using citric acid as a carbon source. The TEM image of achieved GQDs is illustrated in Fig. 2a. The TEM image shows that GQDs are almost monodispersed nanoparticles. Moreover, the size distribution of GQDs is also displayed in the inset of Fig. 2a, which shows the mean sizes of ∼6 nm for GQDs. Furthermore, the dynamic light scattering analysis (Fig. 2b) confirmed the results of the TEM image and
Conclusion
In this study, the potential of using graphene quantum dots (GQDs) as a hydrophilic nanofiller was examined for enhancing the PVC-based nanofiltration membranes. The membrane hydrophilicity was improved by increasing the GQDs amount due to the existence of functional groups on the GQDs surface, helping in improving permeability. Blending a suitable amount of the GQDs to the casting solution enlarged the porosity and mean pore diameter. The SEM images showed that the fabricated membranes
Declaration of Competing Interest
The authors report no declarations of interest.
Acknowledgement
The authors would like to acknowledge Kharazmi University and the University of Tehran, Iran, for providing financial support from Kharazmi Membrane Research Core (Grant number: H/4/360). We also give special thanks to Pamela Reynolds at Oklahoma State University for editing the manuscript.
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