Construction of omniphobic PVDF membranes for membrane distillation: Investigating the role of dimension, morphology, and coating technology of silica nanoparticles
Graphical abstract
Introduction
The drinking water supply is far from meeting its consumption owing to the population growth, flourishing of industry, and surface water pollution [1]. Owing to the increasing water demand, desalination technologies have been used for converting seawater and hypersaline wastewater from chemical industries to freshwater [2], [3]. Considering that simplified processes and theoretically immaculate rejections to non-volatile solutes are of key concern in current research, membrane distillation (MD), which is a promising desalination technology driven by a vapor pressure gradient across a hydrophobic microporous membrane [4], [5], has attracted extensive attention both in academia and industry.
Although low-grade waste heat and other renewable thermal energy are favorable application prospects in the MD process as an external heat supply [6], the implementation of this technology still suffers from an inherent drawback, i.e., membrane pore wetting. The primary reasons for membrane wetting are excessive liquid entry pressure and membrane scaling, resulting in the formation of mineral crystals and eventually depositing onto the membrane surface. Therefore, the MD performance would be largely deteriorated, showing either an acute decrease in flux or a deterioration in the permeate quality [7]. To address the above issue, researchers have devoted great efforts over the past decades to develop new membrane alternatives that can resist inorganic fouling or low-surface-tension contaminants. Inspired by self-cleaning features in nature, membrane surface characteristics with superhydrophobicity [8], superoleophobicity [9], and omniphobicity [10], [11] have been proposed, which repels a number of liquids with different surface tensions, and thus, is of great promise for MD applications [12].
Omniphobicity has facilitated the progress of surface engineering of conventional hydrophobic membranes, and it has been widely used in recent years. Owing to the micro- or nano-reentrant texture with low interfacial energy, the constructed omniphobic membrane with a metastable Cassie–Baxter state was shown to largely enhance the wetting resistance [13], [14], [15]. For instance, Lin et al. [10] prepared an omniphobic surface by coating a glass fiber membrane with silica nanoparticles, followed by surface modification with a fluorinated alkyl silane. Zheng et al. [16] fabricated a hierarchical surface by synthesizing multiscale microspheres (SiNPs@polystyrene) to mimic the lotus surface and improve the performance of the membrane in treating saline emulsion solutions. The enclosed air gaps between the liquid and solid are crucial for Cassie–Baxter state, and the usage of nanoparticles has become ubiquitous in the membrane fabrication process. On one aspect, as the nanoparticles, e.g., SiNPs, generally cover a wide range of diameters (10–450 nm) [11], [16], [17], while their morphologies would also exert an impact on forming the reentrant structures. On the other aspect, the conventional fabrication approach, i.e., solution deposition, is believed to pose an inhomogeneous distribution of silica spheres, undermining the impeccable surface architecture, which in turn impairs the MD performance likewise. Nevertheless, researches on a systematic and comprehensive understanding were yet in their infancy, which appeals sufficient information.
The lack of insight on the physical properties of nanoparticles and the coating approaches motivated us to systematically evaluate their mechanistical influences. In this study, silica nanoparticles of different sizes (30 and 200 nm in diameter) and morphologies (solid and hollow) were employed as the coating layer on a commercial polyvinylidene fluoride (PVDF) substrate. Specifically, all these SiNPs were deposited on the membrane surface by either electrospray or conventional solution deposition methods, and their MD performances were compared thoroughly. In addition, other properties, including surface hydrophobicity, morphologies, anti-scaling behaviors, and corresponding mechanisms, were investigated in detail.
Section snippets
Materials and chemicals
Commercial hydrophobic PVDF membranes (HVHP) with a 0.728 μm nominal pore size (experimentally measured and listed in Table 1) and an average thickness of 125 μm were purchased from Millipore (Tullagreen, Carrigtwohill Co. Cork, Ireland), named as C-PVDF. 3-Aminopropyl-triethoxysilane (APTES 99%), 1H,1H,2H,2H-perfluorodecyl-triethoxysilane (FDTS, 97%), tetraethyl orthosilicate (TEOS), and hexadecyl trimethyl ammonium bromide (CTAB) were purchased from Shanghai Energy Chemical Co. Ltd. NaOH,
Synthesis and characterization of HM-SiNPs
Fig. 1 schematically illustrates the synthetic route of the hollow mesoporous silica spheres, which were synthesized by ammonia-catalyzed hydrolysis and condensation of TEOS [18]. The addition of ethanol improved the stability of the emulsion system (oil-in-water/ethanol) to obtain a monodispersed nanoparticle solution. Notably, the ratio of ethanol-to-water directly determined the size of oil-in-water droplets, i.e., the diameter and shell thickness of the resulting HM-SiNPs. The hydrolyzed
Conclusions
In this work, solid and hollow SiNPs with different dimensions were used to construct omniphobic MD membranes. An electrospray coating method was employed for the membrane fabrication process and compared with the conventional solution deposition approach. The results showed that the WCAs of the electrospray-coated membranes (S30E-PVDF, S200E-PVDF, and H200E-PVDF) were higher than those of the solution-deposited membranes (S30D-PVDF, S200D-PVDF, and H200D-PVDF), while lower sliding angles were
CRediT authorship contribution statement
Hui Feng: Methodology, Validation, Characterization, Data analysis, Writing-Original draft preparation.
Huijuan Li: Characterization, Methodology, Data analysis.
Meng Li: Data analysis, Funding acquisition, Writing-review and editing, Supervision.
Xuan Zhang: Conceptualization, Methodology, Funding acquisition, Writing-reviewing and editing, Supervision.
Declaration of competing interest
The authors declare no competing financial interest.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (22178178, 52100046, 21774058), the Natural Science Foundation of Jiangsu Province (BK20210359), the Fundamental Research Funds for the Central Universities (NUST 30920021119), and the China Postdoctoral Science Foundation (2021M691594).
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