Regular ArticleConstructing dense and hydrophilic forward osmosis membrane by cross-linking reaction of graphene quantum dots with monomers for enhanced selectivity and stability
Graphical abstract
Introduction
Nowadays, with the acceleration of global industrialization and the increase of population, industrial wastewater rises sharply and fresh water used for drinking is in short supply and deterioration. The global water demand trend has a rise of about 1% every year, and the growth rate will continue to increase in the next 20 years [1]. Fresh water only accounts for 2.5% of the total global water resources, whereas the rest is saline [2]. In order to satisfy people's demand for potable water, extensive researches on the treatment and purification of water resources have been done. Among them, forward osmosis (FO) technology is a promising new-type membrane separation technology. In the FO process, the osmotic pressure difference on both sides of the membrane provides the driving force to spontaneously extract water in the feed solution into the draw solution. With no external pressure applied, it is not easy for pollutants to form the cake layer on the membrane surface. Therefore, the low energy consumption and good anti-fouling property promote the forward osmosis technology widely used in petrochemical [3], liquid food processing [4], wastewater treatment [5] and other fields.
As the most critical factor in FO separation technology, the FO membrane has a similar structure with reverse osmosis membrane, which consists of a porous support layer and a thin active layer (i.e. substrate and polyamide layer). At present, the support layer used for FO is mainly based on the ultrafiltration membrane obtained by phase inversion method, and the active layer is usually prepared on the substrate surface by interfacial polymerization (IP) reaction. Remarkably, the performance of FO membrane does not reach the ideal value. The driving force generated by difference of osmotic pressure decreases continuously due to the reverse diffusion of solute in the FO process, resulting in a reduction in the productivity of water. In addition, the traditional permeability–selectivity trade-off effect and internal concentration polarization (ICP) also significantly weaken the desalination efficiency of the FO membrane [6], [7].
In recent studies, nanoparticles have been widely used as additives to remold the properties of the membrane itself. And these nanomaterials including oxides (e.g. SiO2 [8], TiO2 [9], and graphene oxide (GO) [10], [11], [12]), carbon nanotube [13], [14], mental organic frameworks (MOFs) [15] endow the membrane surface with good hydrophilicity and improved separation performance. As one representative, GO with abundant oxygen-containing functional groups and flexibility in chemical functionalization is widely used for the construction of nanochannels within active layer. Shen et al.[16] studied the effect of GO loading on FO performance and confirmed that GO nanoparticles boosted water permeation and reduced reverse salt flux. Nevertheless, some research found that the lateral size of GO significantly affected its distribution in the polyamide (PA) layer and the degree of improvement of membrane separation performance [17], [18]. Compared to GO with large lateral dimension, small GO flakes provided shorter transmission paths for water molecules. Graphene oxide quantum dots (GOQDs), as a novel carbon nanomaterial, has single-layer or few-layer graphene structure with the lateral size of only several nanometers [19]. GOQDs can effectively improve the surface hydrophilicity of membrane and reduce the thickness of active layer. Zhao et al. [20] coated GOQDs on the surface of polyamide nanofiltration membrane by pressure-assisted filtration. The results showed an improved hydrophilic surface and stronger electronegativity. At the pressure of 100 psi, the water flux increased by 15 LMH and higher salt retention was maintained. S. Fatemeh Seyedpour et al. [21] embedded GOQDs into the PA layer of thin-film composite (TFC) membrane, and the novel thin-film nanocomposite (TFN) membrane showed excellent FO performance and anti-fouling property. However, the physical blending of nonreactive additive in the polymer matrix has little effect on the chain structure [22] and the density of the membrane surface, even deteriorates interfacial adhesion force between substrate and PA layer [23], [24]. Furthermore, the nanoparticles will gradually be washed away in the cleaning process, so the degree of improvement for permeation and separation performance by this method is limited to some extent.
In order to solve the above problems, many scholars have conducted designs on the polyamide structure of active layer. As is known to all, the traditional PA layer is built by the polymerization reaction of ammonia monomer and acid chloride monomer at the interface between organic phase and water phase. The content of amino groups or acyl chloride groups can greatly affect the degree of IP reaction. In order to increase the permeability of water during FO process, Jiang et al.[25] prepared the active layer with a low degree of cross-linking, which was obtained by dopamine (DA) as ammonia monomer. Compared with the traditional TFC membrane, the DA/TMC composite membrane exhibited an improved water flux, but the salt interception was poor. In order to avoid compromising salt rejection, Xu et al. [26] mixed DA into the MPD aqueous solution and prepared a novel FO membrane with a proper cross-linking degree by adjusting the mass ratio of DA and MPD. Under the optimal mass ratio of DA/MPD, the FO membrane exhibited excellent water flux and comparable salt retention. Although decreasing the cross-linking degree can improve the water flux, this method is often accompanied by high salt flux. Xu et al. [27] studied the influence of monomer concentration on the physical and chemical structure of polyamide in a wide range. The experimental results showed that degree of cross-linking of the PA layer increased with the increase of ammonia monomer concentration, which led to the rise of salt rejection rate. Interestingly, wang et al. [28] found that polydopamine particles with small size were beneficial to increase the cross-linking degree of PA layer, thus improving the membrane selectivity. Therefore, in order to break the traditional trade-off effect (i.e. increasing water flux and reducing salt flux), the hydrophilic nanoparticle as reactive monomer which contains active functional groups is expected to chemically cross-link with PA molecular chain and then improve the FO performance.
In this study, we prepared graphene oxide quantum dots containing amino groups (NH2-GOQDs) with ammonia and citric acid through the hydrothermal process (as shown in Fig. S1). Then NH2-GOQDs were introduced to the MPD aqueous solution as reactant to participate in IP reaction. The abundant amino groups could chemically cross-link with acid chloride groups and carboxyl groups, thereby forming a dense, flat and defect-free PA layer. Furthermore, the chemical reaction promoted the connection between the substrate and the PA layer, and improved the stability of the nanoparticles in the filtration process and the durability of the PA layer. In the process of interfacial polymerization, all possible reactions are shown in Fig. 1. In addition, considering that there are more hydrophilic functional groups at the edge of NH2-GOQDs, the driving force for water transmission across the membrane could be improved. Combined with the unique advantages of quantum dots, a novel TFN membrane with atypical PA structure by chemical cross-linking method was prepared.
Section snippets
Materials
Polyethersulfone (PES) obtained from Solvay S.A. was used to fabricate support layer. N, N-dimethylformamide (DMF) provided by Fuyu Chemical Co., Ltd. (Tianjin) was employed as the solvent of polymer. Polyvinylpyrrolidone (PVP) provided by Kermel Reagent Co., Ltd. (Tianjin) was used as pore-forming agent for the membrane support layer. Citric acid (CA) was purchased from Tianjin Yongda Chemical Reagent Co., Ltd.. Ammonia was supplied from Tianjin Jinbohua Chemical Co., Ltd. and used without
Structure and morphology of NH2-GOQDs
The chemical structures of NH2-GOQDs were investigated by XPS and FTIR. As observed in Fig. 2a, the XPS survey spectrum shows three main peaks at 284 eV, 400 eV and 532 eV, respectively corresponding to C 1s, N 1s and O 1s signals. Furthermore, the high-resolution spectra of C 1s (Fig. S3a) presents four energy peaks located at 284.6 eV, 285.4 eV, 287.7 eV and 288.4 eV which were attributed to CC/CC in graphene, CN, CO and CO groups [31]. In addition, the deconvolution of N 1s XPS spectra is
Conclusion
Herein, NH2-GOQDs were synthesized by hydrothermal reaction and chemically cross-linked in the PA active layer via IP reaction. This reinforced polyamide nanostructure endowed the PA layer with a small-scale ridge structure (i.e. flat “leaf-like” morphology), compact and smooth membrane surface observed by FIB-SEM, SEM and AFM images. The change of molecular chain spacing in the crystal structure and zeta potential proved the successful incorporation of NH2-GOQDs. Remarkably, the introduction
CRediT authorship contribution statement
Zhiwei Xu: Conceptualization, Methodology, Software, Data curation, Investigation, Writing - original draft. Peng Li: Methodology, Software, Formal analysis, Writing - review & editing. Nan Li: Resources, Writing - review & editing. Wei Wang: Validation, Visualization. Changsheng Guo: Resources, Writing - review & editing. Mingjing Shan: Resources, Supervision. Xiaoming Qian: Software, Supervision.
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
The work was funded by the Qaidam Salt Chemical Joint Fund of National Natural Science Foundation of China - People's Government of Qinghai Province (U1607117) and the Science & Technology Development Fund of Tianjin Education Commission for Higher Education (2019ZD02)
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