Construction of a composite microporous polyethylene membrane with enhanced fouling resistance for water treatment
Graphical abstract
Introduction
The scarcity of freshwater has been magnified by the population explosion and urbanization process in the last hundreds years [1,2]. However, issues of water pollution that human activities raise further worsen this embarrassing predicament. Sewages from industry, agriculture and domestic living contaminate surface water and groundwater if discharged without treatment [3], which curtails human direct access to freshwater. Membrane separation technology has been convinced as one of the most efficient and least energy-consuming means to address water problems [[4], [5], [6]]. As a branch of this technology, ultrafiltration (UF) membrane technology has earned widespread attention in wastewater treatment and other industries, such as textile dyeing and pharmaceutical [[7], [8], [9], [10], [11]]. Combining other techniques, UF membranes with proper separating properties can retain toxic or harmful matters (heavy metals, dyes, bacteria and so forth) from effluents [[12], [13], [14]], leaving slight but acceptable hazard to the aquatic ecosystem.
Common polymers utilized for UF membranes include polysulfone (PSf), poly (ether sulfone) (PES), poly (vinylidene fluoride) (PVDF), and so forth [15]. Through electrospinning, phase inversion or sol-gel method [[16], [17], [18]], UF membranes can be fabricated with pore sizes ranging from 2–100 nm. The outstanding mechanical strength and chemical stability of UF membrane endow it the capability to meet the need of various service environments [15]. Currently, one of the most versatile techniques in fabricating UF membranes is non-solvent induced phase separation (NIPS) [19,20]. During the NIPS process, the casting solution (polymer organic solution) is transferred into a non-solvent coagulation bath to induce phase inversion and polymer precipitation [18]. Membranes prepared in this technique generally exhibit asymmetric porous morphology and low permeating resistance [21]. Meanwhile, the facility in operation and rich design in optimizing components and conditions render great popularity and applications among kinds of preparation techniques.
However, further commercialization is bound, to a certain extent, by the undesired expenses in UF membrane production [22]. The mainstream polymers, such as PVDF and PSf, are pretty costly to manufacture membranes in industrial scales. Meanwhile, massive dosage of organic solvents in the NIPS process mix with water and produce effluent, most of which are toxic and hardly permitted for direct discharge [23,24]. For instance, N, N-Dimethylformamide (DMF) is a typical solvent for the abovementioned polymers in fabricating UF membranes. After NIPS, the DMF-containing wastewater would lead to severe damage to environment and human health without post-processing [23]. Though solvent extraction can reduce the DMF content in the effluent to meet the drainage standard, it may result in a secondary pollution as well. Therefore, exploring low-cost polymers and green recyclable technology conduce to further popularity in UF membrane industrialization [25].
Microporous polyolefin membranes are extensively applied as the lithium-ion battery separators [26]. According to the differences in fabrication, they are classified into two types: dry process and wet process. Wet-process separators generally exhibit relatively homogeneous pore distribution, excellent mechanical durability and chemical tolerance [27], resulting from their high molecular weight, high crystallinity and fabricating superiority. Several studies [[28], [29], [30], [31]] have demonstrated success of applying microporous battery separators as the support layers of pressure-driven membranes. Park et al. [29] reported a thin-film-composite (TFC) NF membrane using a commercialized PE separator as the support. Prior to the interfacial polymerization (IP), the separator surface was treated with O2 plasma. Similarly, novel TFC RO membranes were prepared via IP on plasma-treated polyolefin separators, which demonstrated promising performance and mechanical durability at 15.5 bar [31]. These cases verify the feasibility of polyolefin separators being a promising candidate applied as UF membrane substrate. Table 1 shows the distinctions from costing and techniques between polyolefin separators and conventional UF membranes. From polyolefin separators’ perspective, the maturity of techniques and less consumption to fabricate products with identical scales facilitate cost saving obviously [32]. Besides, paraffin oil, the pore-foaming agent in fabricating wet-process separators, can be recycled in industrial production, which meets a green and sustainable concept; while the NIPS process adopting massive toxic solvent may be less environmentally friendly.
However, as far as our knowledge goes, no studies have reported the application of polyolefin separators on UF membranes yet. In this work, we developed a composite membrane used for BSA sieving, with a commercialized PE separator as the substrate for the first time. In order to modulate pore features and construct hydrophilic channels for water transport, hydrophilic poly (vinyl alcohol) (PVA) was decorated into the cavities of microporous PE separators by filtrating PVA aqueous solutions of various concentrations through the separators’ cross-sections, then crosslinking the PVA retentate by glutaraldehyde (GA). Other than surface modification or dynamic membrane (DM) method that generates a dynamic layer over the membrane surface during filtration [35], this method, named as “membrane-forming filtration” (MFF), guarantees that PVA is immobilized on both the surface and pores, and combine with the separator to form a stable composite membrane. Fang et al. decorated the internal pores of PES UF membranes via a circular filtration of self-polymerized dopamine, which endowed membranes with enhanced heavy metal removal [36].
However, BSA is not only the impurity in the feed water, but a typical protein model foulant that constantly impairs membranes' performance. In consideration of the inevitable fouling on membrane surface during filtration, additional anti-fouling assistance is essential for a satisfactory permeability. Zwitterions are a type of hydrophilic materials containing both anionic and cationic moieties [37], which have been increasingly applied on surface modification of water treatment membranes to mitigate surface fouling. Construction of a zwitterionic layer on membrane surface conduces to forming a firm and compact hydration shell against foulants' adsorption by polarization and steric hindrance [[38], [39], [40]]. Therefore, l-cysteine, a zwitterionic material, was further immobilized onto the composite membranes via “thiol-ene” click chemistry. Instead of zwitterionic polymers, grafting l-cysteine on UF membranes produces negligible loss or, perhaps, even promotion on the permeability [38,41]. Through click chemistry, the zwitterionic groups (-NH2 and –COOH) of l-cysteine were not offended during the synthesis, which facilitates better fouling resistance for membrane surface. The chemical structures, surface morphologies and properties of various membrane samples were investigated. Performances of composite membranes and conventional UF membranes were explored and compared by a crossflow filtration instrument, in which BSA solution was served as feed solution; and the capacity of retaining BSA was verified via Coomassie Bright Blue (CBB) staining. In addition, a short-term BSA-fouling test was conducted to evaluate the effect of grafting zwitterion on the time-dependent antifouling properties of various membranes. The fabrication of this low-cost UF membrane was as depicted in Scheme 1.
Section snippets
Materials
PE separator (ultrahigh molecular weight polyethylene, fabricated by wet process) was kindly provided by Shenzhen Senior Technology Material Co., Ltd. The thickness of the separator is 17 μm. PVA 1788, sodium dodecyl sulfonate (ACS, ≥ 99.0%), acryloyl chloride (96.0%, containing 200 ppm stabilizer), tris(hydroxymethyl) aminomethane hydrochloride (reagent grade, ≥ 99.0%), l-cysteine (99.0%) and brilliant blue G (BR) were purchased from Shanghai Aladdin Biochemical Technology Co. Ltd. Bovine
Chemical structure analysis
XPS was employed to analyze the chemical compositions on top-surface of membranes, and the wide scanning results are demonstrated in Fig. 2a. The PE separator only exhibits one peak along its entire curve, since it is purely made of Carbon (C, 283 eV) and Hydrogen (H, not shown on spectra). However, a sharp and strong peak appears at 535 eV after filtrating PVA solutions through PE separators, which verified the existence of Oxygen (O) due to the retention of PVA. As summarized in Table 2, the
Conclusion
This study reported a new composite UF membrane by applying PE separators as the substrate for the first time. The “membrane-forming filtration” method allowed the preservation of PVA on both the surface and cavities of PE substrate. Besides, zwitterionic l-cysteine was grafted on membrane surface to resist foulants' adsorption. The composite PE/PVA membranes exhibited superior hydrophilicity, smoothness, and narrowed pores as PVA chains decorated the microfibers of PE separators steadily after
Author statement
- 1.
The authors declare no competing financial interest.
- 2.
The authors guarantee that the manuscript has not been published previously, or submitted somewhere else.
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.
Acknowledgements
This work was supported by National Natural Science Foundation of China (NSFC 51503134, 51721091), the Fundamental Research Funds for the Central Universities and State Key Laboratory of Polymer Materials Engineering (Grant No. SKLPME 2017-3-02).
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