Novel sandwich-like membrane with hydrophilic-omniphobic-hydrophilic structure for sustainable water recovery in membrane distillation
Introduction
Recently, the worldwide excessive exhaustion of conventional non-renewable energy sources (e.g. coal, oil, and natural gas) stimulates the development of shale gas (Sun et al., 2019). However, the rapid growth in shale gas industry has resulted in a dramatic increase in horizontal drilling and hydraulic fracturing related wastewaters (Hoelzer et al., 2016). For instance, it was estimated that over 1.3 billion m3 of shale gas wastewaters were produced in the United States in 2016 (Kondash et al., 2018). These wastewaters are usually characterized by high salinity, fluctuated organic oils and surfactants, making them unsuitable directly re-rejected into deep basement rocks and challenging treated by conventional wastewater treatment technologies (Boo et al., 2016). In comparison, water recovery from these wastewaters could not only diminish the wastewater production but also reduce the freshwater consumption.
Membrane distillation (MD) as a hybrid membrane-thermal process, is an emerging water recovery technology. In MD, the vapor (but not the liquid) could transport through the hydrophobic microporous membrane from hot feed stream to cold distillated permeate, driven by the vapor gradient across membrane induced by the transmembrane temperature difference (Wang et al., 2019). As an energy-intensive technology, MD is usually regarded to be more meaningful in treating hypersaline wastewater and achieving zero liquid discharge (Yao et al., 2020). Recently, MD is also utilized in recovering water from industrial wastewaters, especially in the industry with waste heat energy generation (Feng et al., 2021; Yadav et al., 2021). The characteristics such as high salinity tolerance, almost complete non-volatile component rejection, low temperature/pressure demand and low-grade heat energy utilization endow MD with great potential for water recovery from shale gas wastewater. Cho et al. (2018) compared the water recovery performance of three commercial hydrophobic polyvinylidene fluoride (PVDF), polyethylene (PE), and polypropylene (PP) hollow fiber membranes in MD dealing with shale gas wastewater, which had TDS over 120,000 mg L − 1 and dissolved organic matters of 249 mg L − 1. Obvious reductions ranging from 13.6% to 27.7% were observed on water flux during 50% water recovery with highest initial water flux of ∼5 kg m − 2 h − 1. The commercial flat hydrophobic polytetrafluoroethylene (PTFE) membrane was also used (Kim et al., 2017), and results showed that the residual oil and grease induced severe membrane wetting with significantly reduced water flux and deteriorated permeate quality, thereby leading to relatively low water recovery about 25% in consideration of permeate quality. Boo et al. (2016) presented a novel omniphobic PVDF membrane via alkaline treatment, (3-aminopropyl)triethoxysilane grafting, silica nanoparticles coating and perfluorodecyltrichlorosilane coating to treat shale gas produced wastewater. This omniphobic membrane showed much alleviated water flux decline and enhanced salt rejection. However, the compromise on initial water flux (13.6 kg m − 2 h − 1) was inevitable in comparison with the control commercial PVDF membrane (23.5 kg m − 2 h − 1) due to the partially blocked membrane pore. These practices indicated that sustainable water recovery from surfactant-stabilized oil-in-water emulsion (O/W emulsion) was still challenging in MD.
Appropriate MD membrane plays a critical role in addressing the membrane fouling and wetting issues, which is essential to not only maintain the stable water flux but also guarantee the permeated water quality (Eykens et al., 2017). Recently, electrospinning is considered an effective method to prepare hydrophobic/superhydrophobic MD membranes (Lu et al., 2019). Kinds of electrospun membranes have been successfully implemented in the rejection for inorganic salts rather than organic oils or surfactants (Eykens et al., 2017). In comparison, omniphobic membranes are more prone to remaining unwetted in presence of or surfactants (Boo et al., 2018; Horseman et al., 2021b). It is facile to prepare omniphobic membranes based on the electrospun substrates mainly via enhancing re-entrant structure and lowering surface energy (Huang et al., 2017; Lee et al., 2016). Unfortunately, the improvement on water vapor transmembrane transport is still needed due to the narrowed membrane pore after omniphobic modification.
Janus membrane with opposite surface wettability is another approach to improve MD water recovery performance (Feng et al., 2021; Hou et al., 2018). Li et al. (2020) prepared a hydrophilic-hydrophobic Janus membrane via assembling of Teflon® AF1600 and polydopamine (PDA) on commercial PTFE/PP substrate, which exhibited enhanced wetting resistance for salinity, surfactant, crude oil and humic acid. Compared with PTFE/PP membrane (17.2 kg m − 2 h − 1, 45.3%), this membrane showed higher initial water flux (19.2 kg m − 2 h − 1) and only experienced mild water flux decline (12.0%). Huang et al. further fabricated a hydrophilic-omniphobic Janus membrane via integrating a skin hydrophilic layer on omniphobic substrate (Huang et al., 2017). The membrane resisted fouling and wetting simultaneously, and exhibited stable MD performance toward saline O/W emulsion and 0.4 mM sodium dodecyl sulfate (SDS). In addition, a few literature investigated the membrane performance with hydrophobic-hydrophilic structure in MD. Zhao et al. (2020) developed a dual-layer membranes with electrospun hydrophobic PVDF layer on commercial hydrophilic Nylon 6,6 membrane. Tests for 3.5 wt.% NaCl solution showed that its water flux increased by 180–275% compared with the single-layer hydrophobic PVDF membrane. Besides, Zuo et al. (2017) prepared a hydrophobic-hydrophilic Janus composite hollow fiber membrane for desalination of produced water, which exhibited enhanced water flux and improved energy efficiency than the neat hydrophobic membrane. Therefore, it seems that the sandwich-like hydrophilic-omniphobic-hydrophilic membrane would exhibit better water recovery performance both on water flux and permeate quality in dealing with surfactant-stabilized O/W emulsion. Though it is generally accepted that the hydrophilic layer of Janus membrane is beneficial to reduce the propensity for oil-induced fouling, its effect on water flux still remained controversial. As mentioned above, some literature reported that the hydrophilic layer could enhance the water flux due to the decreased mass transfer resistance, heat loss and improved energy efficiency (Li et al., 2020; Zuo et al., 2017). However, some other literature found that the hydrophilic layer would induce additional heat transfer resistance to amplify the temperature polarization, which would lead to the reduced water flux instead, even the hydrophilic layer could reduce the mass transfer resistance of water vapor (Chen et al., 2022; Zhao et al., 2020). Thus, the effects of hydrophilic layer of hydrophilic-omniphobic-hydrophilic membrane on mass and heat transfer (such as water flux, mass transfer coefficient, heat loss, temperature polarization and transmembrane temperature distribution) need to be clarified. In addition, although the anti-fouling mechanism of Janus membrane (especially hydrophobic-hydrophilic membrane) has been widely investigated, the underlying anti-fouling mechanism (such as foulant interaction analysis, interfacial free energy calculation via extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory) of this three-tiered membrane still needs to be further investigated.
In light of these knowledge gaps, the main objectives of this study were to (i) develop a robust sandwich-like membrane with hydrophilic-omniphobic-hydrophilic structure; (ii) evaluate the water recovery performance of this newly developed membrane for surfactant-stabilized O/W emulsion; (iii) clarify the anti-fouling mechanism of this newly developed membrane, and (iv) investigate the effect of hydrophilic layer on water flux and explore the underlying mechanism via transmembrane mass and heat transfer analysis. The findings can provide a guide for developing suitable MD membrane for sustainable water recovery from surfactant-stabilized O/W emulsion.
Section snippets
Materials and chemicals
PVDF (Solef 6010) was provided by Solvay (USA). Acetone (AR) and N,N-dimethyl acetamide (DMAc, AR) were purchased from Shanghai Lingfeng (China). Dopamine, tris(hydroxymethyl) aminomethane (Tris, ≥99.9%), SDS and hexadecane were purchased from Sigma-Aldrich (USA). Zinc acetate (Zn(CH3COO)2, AR), ethanol (AR), sodium hydroxide (AR), zinc nitrate (AR), dichloromethane (AR), 1H,1H,2H,2H-perfluorodecyltriethoxysilane (PDTS, 97%), and polyetherimide (PEI, Mn=600) were obtained from Sinopharm
Surface structure
Fig. 2a–c depict the surface morphology and roughness of pristine PVDF, omniphobic FZP and hydrophilic-omniphobic-hydrophilic PFZP membranes. The color of pristine membrane was white, and it gradually changed to wheat (FZP) and brown (PFZP) after omniphobic modification and hydrophilic coating, respectively. Compared to the highly-porous, non-woven and overlapping nanofibrous structure of pristine membrane, FZP membrane had a significant different surface morphology due to the in-situ growth of
Conclusion
The study developed a facile method to prepare a novel hydrophilic-omniphobic-hydrophilic sandwich-like MD membrane for water recovery from surfactant-stabilized O/W emulsion for the first time. The top PDA skin layer facing feed induced the formation of hydration shell, endowing membrane with enhanced repellence for free hexadecane deposition due to the relatively high energy barriers. Bottom PDA skin layer facing permeate significantly alleviated the temperature polarization to maintain high
CRediT authorship contribution statement
Long-Fei Ren: Conceptualization, Formal analysis, Funding acquisition, Methodology, Visualization, Writing – original draft, Writing – review & editing. Jun Li: Data curation, Formal analysis, Investigation, Visualization, Writing – original draft. Yubo Xu: Data curation, Investigation, Visualization. Jiahui Shao: Conceptualization, Funding acquisition, Resources, Supervision, Writing – review & editing. Yiliang He: Funding acquisition.
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 sponsored by National Natural Science Foundation of China (no. 52000129, 21737002), Shanghai Sailing Program (no. 20YF1419500), Natural Science Foundation of Chongqing, China (no. cstc2021jcyj-msxmX0383), National Key R&D Program of China (no. 2019YFC0408201), and Sinopec Zhenhai refining and chemical company.
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