Surface-crumpled thin-film nanocomposite membranes with elevated nanofiltration performance enabled by facilely synthesized covalent organic frameworks

Highlights

• Hydrophilic and nanoscale COFs were synthesized under mild conditions.

• CTN membranes were prepared via pre-deposition of NCOFs followed by IP reaction.

• NCOFs induced formations of regular crumpled PA surface morphologies.

• The CTN membranes exhibited elevated water flux and high rejection capacity.

• The CTN9 membrane behaved long-term stability and good antifouling ability.

Abstract

Hydrophilic and porous nanofillers have shown great benefits in the attainment of high-performance nanofiltration (NF) membranes. In this work, a novel nanoporous covalent organic framework (NCOF) was facilely synthesized under mild conditions and deposited onto a polyethersulfone substrate to induce formations of crumpled polyamide layers via interfacial polymerization. With the incorporation of NCOFs, the surface of the COF-based thin-film nanocomposite (CTN) membranes gradually transformed from tent-like to mesh morphology, which significantly enhanced the water permeance owing to the increased effective membrane filtration area and additional water transport channels provided by NCOFs. Besides, the favorable compatibility between NCOFs and the polyamide layer endowed the CTN membranes with high rejection capacities to a variety of solutes. The optimal membrane with the NCOF deposition density of 19.27 μg cm⁻², CTN9 membrane, showed nearly twice higher water permeance of 15.5 L m⁻² h⁻¹ bar⁻¹ while maintaining high rejections of Na₂SO₄ and MgCl₂ (98.9% and 94.2%, respectively). In addition, the CTN membranes exhibited a satisfying stability during the long-term filtration of saline solutions as well as an improved antifouling ability to the model foulants of bovine serum albumin. The superior NF performance rendered the NCOF functionalized membranes highly promising for water treatment.

Introduction

Nanofiltration (NF) membranes with nanoscale pore sizes and charge properties are efficient in retaining multivalent salts and small organics, which has showcased prospective applications in water treatment such as drinking water purification, wastewater reuse, desalination, etc. [[1], [2], [3]]. Numerous approaches including interfacial polymerization (IP), dip-coating, graft polymerization, lay-by-layer (LBL) deposition, and surface modification, have been adopted for the development of high-performance nanofiltration membranes to satisfy different treatment requirements [[4], [5], [6], [7]]. The thin-film composite (TFC) membrane with a polyamide (PA) selective layer atop a porous substrate constructed by the IP reaction has pervasively got into the production market of commercial NF membranes owing to the facile fabrication procedure and stable membrane performance [2,8]. Nonetheless, the PA-based TFC membranes normally experience a low water permeability and are subjected to the trade-off effect between permeability and selectivity [9,10]. Therefore, massive studies have been dedicated into heightening the membrane separation efficiency so as to lower the operational costs [11,12].

Recently, the incorporation of nanomaterials with ordered and tunable pore structures has emerged as a facile and effective method to develop the thin-film nanocomposite (TFN) membranes with elevated separation performance, which have shown the potential to break through the constrain of TFC membrane permselectivity [[13], [14], [15], [16], [17]]. Among various porous nanomaterials [[18], [19], [20], [21], [22]], the metal-organic frameworks (MOFs) and covalent-organic frameworks (COFs) have attracted more interest due to their outstanding advantages including advanced pore structures, large surface area, uniform porosity, as well as designable functionality [[23], [24], [25]]. Although MOF materials have demonstrated certain superiority in membrane preparation over other conventional nanofillers, the partial inorganic nature still compromises their compatibility with polymeric membranes and inevitably gives rise to some defects in the membrane. Moreover, most of the reported MOFs are unstable in aqueous solution, which restricts their application in the design of TFN membranes via the IP process for water treatment [14,24].

COFs are a novel class of crystalline porous nanomaterials composed of organic building units connected by strong covalent bonds [26]. They have shown vastly superior as the nanofillers of membranes at the following aspects: (i) COFs have a completely organic composition with excellent affinity to the PA layer as well as high adhesion to the polymeric substrate, which ensures good interfacial compatibility within the composite membrane [27]; (ii) The pore size of most reported COFs ranges from 1 nm to 4 nm, which is suitable for nanofiltration separation processes [24]; (iii) COFs can be facilely functionalized by the decoration of organic linkers with functional groups, which endows the COF-incorporated TFN membranes with desired properties (e.g., antifouling ability, catalytic property, and high permselectivity) for specific applications in water treatment [25,28,29]; (iv) The membrane surface morphology can be flexibly tailored via the controlled positioning of COF nanoparticles. Specifically, the formation of a crumpled PA layer is conducive to providing an increased effective filtration area and consequently enhancing the water permeability. Meanwhile, since the membrane surface morphology is possible to be regulated without much impact on the crosslinking degree of the PA layer, the rejection capacity of the membrane can be maintained [[30], [31], [32]]. For instance, Chang et al. incorporated a porous amine-rich COF (SNW-1) into the PA layer, which increased the membrane water permeability by 92.5% with a Na₂SO₄ rejection of around 80% benefited from the water channels provided by the COF [16]. Wu et al. obtained ultrathin PA films by means of a polydopamine (PDA)-COF interlayer, and the water permeance was significantly facilitated due to the increased surface roughness and thus the enlarged filtration area of the membrane [29].

Despite the enhanced membrane performance achieved by the incorporation of COF nanomaterials, some problems still need to be solved for further promoting the application of COFs in the construction of water treatment membranes. Above all, most reported COF sheets are two-dimension (2D) in shape and inhomogeneous in size distribution, which is unfavorable for the formation of PA films with regular surface morphologies. Secondly, the water permeabilities of the established COF-based NF membranes were acceptable, while the rejection capacities were still not satisfying [16,28,[33], [34], [35]]. Thus, the membrane fabrication methods should be adjusted to prepare the TFN membrane with simultaneously high water permeability and high solute selectivity. In addition, the synthesis of COFs normally suffers from a complicated and time-consuming solvothermal procedure, which could further restrict their large-scale applications in the membrane preparation [36,37]. Consequently, the selection of appropriate COFs plays a decisive role in the development of high-performance TFN membranes.

Herein this research, a kind of novel hydrophilic COF material, nanoscale TPB-DMTP-COF (NCOF), was judiciously adopted for the preparation of crumpled PA membranes [38]. The NCOF nanospheres have a uniform size distribution with abundant hydrophilic functional groups (e.g., amine, imine) inside the porous channels (mean pore size of 3.2 nm, see Fig. S1), which can provide additional water transport channels and induce the formation of regularly crumpled PA layers and thus potentially enhance the membrane water permeance. Furthermore, the NCOFs can be synthesized under relatively mild conditions without severe solvothermal reactions, which is crucial for practical applications. In specific, the NCOF nanoparticles were first dispersed in the aqueous solution of piperazine (PIP) and then transferred onto a polyethersulfone (PES) substrate membrane via pressure-assisted deposition, followed by the IP reaction with trimesoyl chloride (TMC) monomers. The porous and hydrophilic NCOFs were expected highly effective in enhancing the separation performance of the COF-based TFN (CTN) membranes. The effect of NCOF incorporation on the morphology and chemical properties of the CTN membranes was systematically investigated through comprehensive characterizations. The nanofiltration performance of the CTN membranes, including water permeance, solute rejections, long-term separation stability, as well as antifouling properties, were further evaluated and compared with that of the control TFC membrane.