Superior nanofiltration membranes with gradient cross-linked selective layer fabricated via controlled hydrolysis
Introduction
With the increasing demands for water derived from the rapid development of economy and growth of population, many countries and regions pay more attention to recycling wastewater and using meagre water resources efficiently. Low-cost and effective technologies dealing with wastewater and desalination are in urgent need [1]. Because of its high efficiency and low energy consumption [2], nanofiltration (NF) has been extensively applied in decontaminating wastewater for removing heavy metal ions, separating low-molecular-weight organics and water softening [3,4]. The pore size of a typical NF membrane is around 1 nm. Compared with reverse osmosis (RO) membranes, NF membranes exhibit high efficiency in separating divalent ions from monovalent ions under lower operating pressure [5]. Currently, thin-film composite (TFC) membrane is the most extensively used form of NF membranes, which is fabricated by interfacial polymerization (IP) on the top of microfiltration or ultrafiltration substrate [6].
A representative polyamide NF membrane is synthesized by interfacial polymerization of an acyl chloride monomer, trimesoyl chloride (TMC), and an amine monomer, piperazine (PIP), at the oil-water interface. The trade-off between permeance and selectivity limits the capacity to increase membrane performance using single materials significantly [4,7]. Prevailing strategy involves incorporating novel functional materials such as water channel proteins [[8], [9], [10]], metal-organic frameworks (MOF) [11,12], organic/inorganic nanoparticles [[13], [14], [15]], and carbon-based nanomaterials (e.g., graphene oxide [[16], [17], [18]], and carbon nanotubes [19,20]) leads to increased water pathways in membrane selective layer. However, these functional nanofillers lead inevitably to complexity and increasing cost in membrane preparations. Some methods are also faced with difficulties in industrialization [4,21].
Fabricating the polyamide layer with an optimized structure is favored for industry. This strategy involves lowing the thickness or increasing the surface area in a selective layer [[22], [23], [24], [25]]. For example, introducing an interlayer between the substrate and polyamide layer has been shown to reduce the thickness of the polyamide layer and thus enhance its permeance [[26], [27], [28]]. Greater effective filtration area can be created by the use of sacrificial templates [29,30] and the creation of nanosized gas bubbles [31]. Wu et al. [27] introduced hybrid interlayer comprising polydopamine and covalent organic framework onto PAN substrate and then the polyamide active layer was fabricated on the interlayer. Owning to excellent hydrophilicity and high porosity of the hybrid interlayer, the thickness of the polyamide active layer was reduced to 11 nm and the resulting NF membrane exhibited outstanding permeability. Jiang et al. [32] developed the aqueous templated onto the surface of the substrate, which resulted in the formation of surface nanostructures on the polyamide NF membrane and simultaneously enlarged the permeable area. The water permeance of the optimal membrane is 21.3 L m−2 h−1 bar−1. Recently, gradient cross-linked structure for the active layer was reported. Guo et al. [33] fabricated a gradient cross-linked polyvinyl alcohol (PVA) active layer by introducing the crosslinker to the surface of the substrate before PVA coating. Owning to this gradient structure, the water permeance of the optimized membrane was significantly improved. Inspired by this strategy, the gradient cross-linked structure can also be applied in polyamide active layer by controlled hydrolysis. Jun et al. [34] investigated the effect of acid-catalyzed hydrolysis on the performance of a commercial NE70 polyamide membrane. Do et al. [35] reports the possibility of simultaneous enhancement in membrane permeability and rejection by hypochlorite-induced hydrolysis under mild chlorine and alkaline conditions. However, these hydrolysis procedures are generally hard to optimize and the small to moderate improvements in separation properties have been reported (e.g., 10% enhancement in water permeance by acid-catalyzed hydrolysis [34]).
Conventionally, a nascent polyamide membrane is placed in an oven to allow more complete polymerization between the monomers. In our previous research, the influence of heat treatment conditions on polyamide NF membrane performance was systematically investigated [36]. The results show that two accompanying problems, over cross-linking of the polyamide active layer and shrinkage of membrane pores, will inevitably arise from the drying process. Heat treatment in air leads to a great loss of membrane permeability. Consequently, alternative curing strategies need to be developed to avoid the permeability loss for high-performance polyamide NF membranes.
Herein, we report a single-step method to fabricate polyamide NF membranes with superior performance. Water heat treatment is conducted immediately after fabricating the nascent polyamide NF membrane, which effectively avoids the drawbacks caused by heat treatment in air. More importantly, the adjustment in the pH value of a water bath is applied to control the hydrolysis of membrane selective layer. This single-step method resulting in a gradient cross-linked structure of polyamde active layer, maintaining the fast water pathway and regulating the crosslinking density in the NF membrane. Our study provides a facile and industrially feasible strategy to improve the performance of the NF membrane.
Section snippets
Materials
Polyethersulfone (PES, pure water permeability (PWP) = 180 L m−2 h−1 bar−1) was kindly provided by Development Center for the Water Treatment Technology. Inorganic salts, neural organics (glucose, sucrose, raffinose and carbamide, AR), piperazine (PIP, GR), HCl (AR), NaOH (AR), n-hexane (AR), and poly(ethylene glycol) (PEG, AR) were all provided from Sinopharm Chemical Reagent Co., Ltd. Trimesoyl chloride (TMC, ≥98%) used for IP was obtained from Qingdao Benzo Chemical Company.
Preparation of polyamide NF membrane
According to our
Surface element composition
Surface element compositions were analyzed by XPS with a photoelectron takeoff angle of 30° and a detection depth around 5 nm. Table 1 exhibits the surface element composition and crosslinking density of the PA@A, PA@W-7, PA@W-0, and PA@W-14 NF membranes, respectively. The crosslinking density of the polyamide active layer is calculated by the atomic ratio of O and N [25,39]. The crosslinking density of PA@A, PA@W-7, PA@W-0, and PA@W-14 are 66.7%, 64.3%, 20.6%, and 27.3%, respectively (Table 1
Conclusion
In this work, we have developed a facile strategy to NF membranes with gradient cross-linking polyamde layer for enhanced NF performance. Heat treatment in water is carried out, which effectively avoids the over cross-linking and shrinkage of water pathways in a polyamde active layer compared to conventional heat treatment in air. With the hydrolysis of amido bond adjusted by the strong acid solution or the strong alkali solution, the polyamide NF membrane with gradient cross-linked structure
CRediT authorship contribution statement
Zi-Ming Zhan: Data curation, Investigation, Writing - original draft, Visualization. Zhen-Liang Xu: Supervision, Writing - review & editing. Ka-Ke Zhu: Supervision, Writing - review & editing. Shuang-Mei Xue: Visualization. Chen-Hao Ji: Writing - review & editing. Ben-Qing Huang: Writing - review & editing. Chuyang Y. Tang: Writing - review & editing. Yong-Jian Tang: Conceptualization, Methodology, Validation, Supervision, Writing - review & editing.
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.
Acknowledgement
This work was financially supported by the National Natural Science Foundation of China (Grant No: 21808060), the Fundamental Research Funds for the Central Universities (222201814009).
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