Elsevier

Journal of Membrane Science

Volume 610, 1 September 2020, 118111
Journal of Membrane Science

Thin-film nanocomposite membrane doped with carboxylated covalent organic frameworks for efficient forward osmosis desalination

https://doi.org/10.1016/j.memsci.2020.118111Get rights and content

Highlights

  • COOH-COF was firstly utilized to prepare a TFN FO membrane with improved hydrophilicity and negatively charge property.

  • Ultrahigh water flux, lower reverse flux selectivity were synchronously achieved, breaking a trade-off effect to an extent.

  • High performance kept consistent during both osmosis- and pressure-driven membrane process.

  • Internal concentration polarization was greatly mitigated.

Abstract

Compared to the inorganic nanofillers, an organic one might be more appropriate to control the aggregation and interfacial microvoids between nanofiller and polymeric matrix for an efficient mixed matrix membrane. In this study, a hydrophilic carboxylated covalent organic frameworks (COF-COOH) as an organic nanofiller, for the first time, was conveniently incorporated to fabricate a novel thin-film nanocomposite (TFN) membrane via interfacial polymerization. Thanks to the abundant carboxyl groups and pure organic nature of COF-COOH, the TFN membrane was endowed with an improved hydrophilicity and negative charge property. With COF-COOH content of 0.5 mg ml−1 in aqueous solution, the elaborately designed TFN membrane rendered a smaller mean pore size of 0.45 nm and an ultralow structural parameter of 58.6 μm, exhibiting a 4-fold water flux (Jv) of 64.2 L m−2 h−1 with a synchronously improved selectivity (Jv/Js as high as 10.0 L g−1), compared to a pristine TFC membrane (Jv = 15.9 L m−2 h−1, Jv/Js = 2.6 L g−1) in forward osmosis (FO) mode with 1 mol L−1 NaCl as draw solution and DI water as feed solution. The intriguing results shed light on the potential of COFs in the highly efficient membranes for water desalination and purification.

Introduction

Membrane technologies provide promising solutions to address the growing fresh water scarcity and contamination issues [1]. Forward osmosis (FO), as an emerging membrane technology, has received ever-increasing attention in water treatment [2]. Different from pressure-driven reverse osmosis (RO) process [3], FO utilizes the inherent osmosis pressure between feed solution (FS) and concentrated draw solution (DS) as driving force to achieve water transport across a semipermeable membrane. Attractive advantages of FO process, including negligible operation pressure, superior efficacy, low fouling propensity and small footprint, make it having extensive potential applications, such as municipal/agricultural wastewater reclamation for irrigation [4,5], landfill leachate treatment [6], emergency water bag for expedition [7], food and drug concentration [8].

Enlarged applications of membranes have come with a renewed focus on membrane materials research. One of current technical obstacles for FO development was the high-performance FO membranes. Thin-film composite (TFC) FO membranes comprised of an ultrathin active polyamide (PA) layer and a porous ultrafiltration (UF) substrate have been dominating the FO membrane market, due to their merits of relatively high water permeability and wide pH tolerance. However, permeability resistance arising from amorphous PA layer with non-uniform pore, limited porosity and poor inter-connectivity, severely hinders the desalination efficiency. Besides, internal concentration polarization (ICP) attributed to UF substrates drastically reduces the effective osmotic driving force and further deteriorates the separation efficiency [9]. To exert the potential application of FO process, advanced membrane materials and well-designed structures are crucial to develop. Some strategies have been applied to regulate the pore size and interconnected voids of PA layer by adding nanofillers with defined cavities [10], such as aquaporin [11], metal organic frameworks [12], graphene [13], nanotubes [14], and self-assembled small molecules [15], fabricating the thin-film nanocomposite (TFN) membranes with a high water permeability. However, the nanofiller aggregation derived from the weak affinity with PA matrix, as well as the incompatible nanofiller size with the thickness of the PA layer, might lead to the interfacial microvoids and thus deteriorate the selectivity [4,10]. Therefore, adopting a new porous organic-nanofiller with chemical similarity and affinity to PA matrix is important for an advanced FO membrane with both high water permeability and selectivity.

Covalent organic frameworks (COFs), as a burgeoning class of crystalline porous polymers via strong covalent bonds, have been attractive candidates for advanced water-treatment membranes owing to their high porosity and well-organized channel structures [16]. Based on their orderly arranged pore structures with diameter of 0.7–4.7 nm [[17], [18], [19]] and versatile tailored functionalities, COFs showed a great potential in gas separation [20]. Our group conducted a pioneering research on the water-stable COFs (TpPa-2) as an organic nanofiller to fabricate a novel efficient UF membrane [21], achieving synchronous improvements in water permeability and humic acid rejection. In order to match the thickness of PA layer, TpPa-2 was exfoliated into nanosheets and then incorporated into PA layer, performing over three times improvement on water permeability and an excellent H2O/NaCl selectivity, which shed light on the great potentials of COFs in water treatment [22]. Recently, COFs have been employed in the fields of dye sieving [23] and desalination application [[24], [25], [26], [27]]. However, relatively hydrophobic backbone of COFs hampered water affinity and transport through PA layer [19,27]. And the complicated and time/cost-consuming fabrication of COFs hindered the scale of COF-based TFN membranes.

Commonly, the hydrophilicity and charge property of PA layer are attributed to the carboxylic groups (-COOH) on the membrane surface, which derives from the hydrolysis of unreacted acyl chloride groups. If the COFs introduced into the PA layer is rich in -COOH groups, the membrane would exhibit a satisfactory hydrophilicity and an electrostatic repulsion effect to hydrated anion [19]. Such a speculation has been confirmed by a mixed matrix UF membrane containing carboxyl-functionalized COFs (COF-COOH), exhibiting a more hydrophilic and negatively charged surface [27]. While, the research on the TFC membranes containing COF-COOH for desalination is lacking. In this study, a novel TFC membrane incorporated with COF-COOH was fabricated on a macro-porous substrate via interfacial polymerization. COF-COOH was obtained from commercially available raw chemicals of trimesoyl chloride (TMC) and p-phenylenediamine (PPD) [28], probably rendering an excellent compatibility with PA layer due to their similar chemical structure [28,29]. In addition, compared to the NH2-functionalized COFs (COF-NH2) [25], COF-COOH also yields a lower susceptibility to chlorine attack [22]. Effects of COF-COOH on membrane morphology, surface charge, hydrophilicity, mechanical robustness and pore size distribution of PA layer were comprehensively investigated. Separation performance of the TFN membrane was tested under both osmotic-driven FO and pressure-driven RO processes, further exploring the potential of COFs in a highly efficient membrane for desalination and water purification.

Section snippets

Materials

Unless stated otherwise, all solutions and reagents were used without further purification. MPD (99%, Sigma-Aldrich), PPD (99%, Sigma-Aldrich), TMC (98%, Sigma-Aldrich), n-hexane (≥97%, Sinopharm chemical Reagent Co., Ltd), ethyl acetate (EA), absolute alcohol, acetone, ethylene glycol and polyethylene glycol (PEG, molecular weight of 200, 400, 600, Sinopharm Chemical Reagent Co., Ltd). Mixed cellulose ester (MCE) membrane with average pore size of 0.1 μm and average thickness of 135.2 μm was

Characterization of COF-COOH

The COF-COOH was obtained from the polymerization reaction of TMC and PPD as shown in Scheme 1a. From SEM image, COF-COOH showed a regular spherical-like morphology with a particle size distribution of 60–72 nm (Fig. 1a and b). Apparent Tyndall effect further affirmed its size in colloid scale (within 100 nm). It indicated the COF-COOH particle size was well-matched with the thickness of PA layer (100 - 300 nm), making it possible that COF-COOH could manipulate defect-free PA layer. The

Conclusions

From the perspective of membrane structure and chemistry, this work intended to explore the science and engineering of constructing TFN desalination membranes with high water permeability and selectivity by incorporating hydrophilic COF-COOH. The abundant -COOH groups in COF-COOH originated from the commercially available raw materials (TMC and PPD), rendering PA layer with improved hydrophilicity (WAC decline from 93.7° to 53.5°), more negatively surface charge (Zeta potential increase from

Author statement

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.

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 NSFC (No. 21878279), Natural science fund of Shandong Province Project (No. ZR2018MB032), Fundamental Research funds for the Central Universities (No. 201841012). 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.

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