Facile preparation of antifouling nanofiltration membrane by grafting zwitterions for reuse of shale gas wastewater
Graphical abstract
Introduction
The “shale gas revolution” was first successfully practiced in the USA to ensure energy security. China is currently accelerating the pace of shale gas development and exploration [1], [2]. According to evaluations made by the Chinese government, China has 25.08 × 1012 m3 of technically recoverable shale gas reserves, positioning it as one of the most promising countries in the world for shale gas development [3], [4]. However, the shale gas exploration process consumes large amounts of freshwater resources and generates significant quantities of shale gas wastewater (SGW) [5], [6]. In the Sichuan Basin, China, the average freshwater demand of a shale gas well is 23,650–34,000 m3, and 8–70% of the shale gas flowback and produced water is returned to the ground [7], [8], [9].
SGW in the Sichuan Basin contains low to moderate salinity, complex and heterogeneous organic compounds, and its treatment to safe standard is costly and challenging [10]. Currently, most SGW both in China and USA are reused for fracturing in new wells, since the recycling of the fracturing fluid can achieve both cost savings and environmental pollution reduction [11], [12], [13]. The effective removal of divalent ions is critical for SGW reuse to avoid scaling on production equipment and in the shale formation [14], [15], [16]. Based on the salinity and composition of the Sichuan Basin SGW, nanofiltration (NF) membrane technology is a suitable and promising candidate for purification with the aim to remove divalent ions and maintain a stable production of shale gas [9], [17], [18]. However, membrane fouling is a key drawback that restricts the application of NF membranes and can lead to increased energy consumption, alongside increased frequency of chemical cleaning and shortened membrane life [19], [20], [21]. In general, membrane fouling is the result of a complex series of physicochemical interactions between the surface of membrane and the fouling agents [21], [22], [23], [24]. The complex organic mixture in SGW is arguably the main cause of NF membrane fouling in this application [25]. Effective pretreatments are often used to improve the NF performance and ensure its sustainable operation [13], [26]. The combination of coagulation [27], adsorption [28], ozone pre-oxidation [29], or biological treatment technology [30] with UF has been applied for pre-treatment. However, NF fouling is still inevitable and few studies have investigatedthe properties of the NF membrane itself to improve their resistance to toux from SGW foulants.
The synthesis of new antifouling membranes through surface modification is an ideal approach to control fouling [22], and direct modification of the membrane surface is ideal for large-scale processing applications [31], [32], [33]. Antifouling membrane modification materials are mainly polymers, and the introduction of hydrophilic monomers or polar groups can significantly improve the membrane antifouling performance [34], [35]. Commonly used modifiers include: poly(vinyl alcohol) (PVA) [36], polyethylene glycol (PEG) [37], MXene [38], polydopamine (PDA) [39], and polyurethane [40]. Compared to other hydrophilic materials, zwitterionic polymer brushes (e.g., sulfobetaine methacrylate, SBMA) comprising both anionic and cationic end groups exhibit excellent antifouling performance at high salt concentrations [41], because of their overall electrical neutrality and strong hydration ability. SBMA is able to form a tightly hydrated layer on the surface of membrane materials, which weaken the interaction force between organic pollutants and the membrane surface [21], [31], [41], [42], [43], [44], [45], [46], [47], [48]. In particular, this mechanism is effective against hydrophobic organic substances, such as protein-like and humic-like matter, which accounts for a relatively high proportion of the organics in SGW and which can easily adhere to the membrane surface and cause membrane fouling [28], [29], [35], [49]. Typical modification methods include chemical grafting, physical coating, and polymer modification, where chemical grafting regulates the separation mechanism of the membrane by grafting specific groups to the surface, which not only achieves specific selectivity of the membrane, but also is an effective way to minimize fouling [44], [45], [47]. Modification through activators regenerated by electron transfer–atom transfer radical polymerization (ARGET-ATRP) is suitable to modify the membrane surface at large scale under routine industrial conditions in an easy and quick way, because it only requires a low dosage of copper catalyst and has high tolerance to oxygen [50], [51]. Our previous study used ARGET-ATRP method to graft [2-(methacryloyloxy) ethyl] dimethyl (3-sulfopropyl) ammonium hydroxide (DMAPS) on the surface of self-made green ultrafiltration membranes showing highly promising results also for other membrane processes and applications [52].
In this work, we tune and apply this procedure to fabricate a high-performance NF membrane deployed to treat SGW with the goal of reuse. The relationship between membrane surface modification and improved antifouling performance is investigated upon grafting a zwitterionic polymer brush PSBMA onto the surface of a commercial NF membranes via an ARGET-ATRP method. The water flux decline rate is analyzed and evaluated in the light of the membrane surface characteristics and the degree of organic deposition. The effect of membrane fouling is discussed using the XDLVO theory. The purpose of this paper is to provide valuable information on ways to reduce NF membrane fouling by designing NF membranes suitable for shale gas wastewater treatment.
Section snippets
Pretreatment of the SGW
The SGW utilized in this experiment was acquired from a reservoir of the Weiyuan shale gas sites, Sichuan, China. The water characteristics and preceding steps in nanofiltration have been thoroughly summarized in our previous study [13]. In short, the ideal experimental condition for coagulation involved addition of 900 mg/L ferric chloride hexahydrate (FeCl3·6H2O) followed by 30 min settling. The supernatant was then fed to the tank of an ultrafiltration system comprising a hollow fiber poly
Membrane surface properties
Membrane surface characteristics, such as charge and roughness, hydrophobicity and chemical composition determine the selective separation properties and the anti-fouling performance of the membrane [42], [58]. Fig. 1A presents that the ATR-FTIR spectra determined between 500 cm−1 and 4000 cm−1 for the pristine NF90 and VNF1 membranes, the BiBB-initiation-PDA mobilized membranes (NF90-PDA, VNF1-PDA), and the PDA-g-PSBMA modified NF membranes (NF90-PSBMA, VNF1-PSBMA). The stretching vibration
Conclusion
Zwitterionic polymer brushes SBMA were grafted onto the surface of commercial NF membranes, greatly improving the membrane antifouling ability and organics removal while retaining high values of water permeability and only slightly decreasing the rejection rate of conventional monovalent and divalent ions. The VNF1-PSBMA membrane displayed extremely high performance with a pure water flux of 86.3 LMH, sodium sulfate removal rate of 95.67%, and with the J/J0 ratio upon fouling 73.5% higher than
CRediT authorship contribution statement
Minli Hu: Methodology, Data curation, Formal analysis, Validation, Visualization, Writing – original draft. Qidong Wu: Validation, Formal analysis, Investigation. Chen Chen: Writing - review & editing. Songmiao Liang: Writing - review & editing. Yuanhui Liu: Writing - review & editing. Yuhua Bai: Writing - review & editing. Alberto Tiraferri: Formal analysis, Writing - review & editing. Baicang Liu: Conceptualization, Supervision, Formal analysis, Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (52070134, 51678377), Sichuan University and Yibin City People's Government strategic cooperation project (2019CDYB-25), and Xinglin Environment Project (2020CDYB-H02). We would like to thank the Institute of New Energy and Low-Carbon Technology, Sichuan University, for AFM and SEM, and the Analytical & Testing Center of Sichuan University for XPS work and we would be grateful to Suilin Liu for his help of XPS analysis.
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