Cellulose-based electrospun nanofiber membrane with core-sheath structure and robust photocatalytic activity for simultaneous and efficient oil emulsions separation, dye degradation and Cr(VI) reduction
Graphical abstract
Introduction
In recent years, due to the drastic urbanization and industrialization, numerous hazardous contaminants, e.g. waste oils, organic dyes, heavy metal ions, etc., have discharged into water, resulting in various streams of complex wastewater, which are hard to remediate and seriously threaten human’s health (Miara et al., 2017). Various treatment technologies, including adsorption (Duan et al., 2019), flotation (Etchepare, Oliveira, Azevedo, & Rubio, 2017), flocculation (Wang, Cheng et al., 2020; Wang, Gao, Al-Enizi, Nafady, & Ma, 2020; Wang, Tang et al., 2020), biological treatment (Oberoi, Jia, Zhang, Khanal, & Lu, 2019), and membrane filtration (Chen, Huang, Liu, Meng, & Ma, 2020; Fane, Wang, & Hu, 2015), have been developed to eliminate various contaminants from water. Among them, membrane separation technology is particularly effective due to its high efficiency, low space footprint, easy operation and industrial-scale application without many extraneous chemicals (Fane et al., 2015). Due to the increasing complexity of wastewater, traditional single-function membranes, for example, can only demulsify sewage or absorb dyes and heavy metal ions, has been unable to adapt to the corresponding conditions (Prasannan, Udomsin, Tsai, Wang, & Lai, 2020). Moreover, the performance of the membrane deteriorates gradually during long-term use (Ge, Zong, Jin, Yu, & Ding, 2018). Therefore, developing versatile membranes with multifunction of superior hydrophilicity, self-cleaning capacity and purification performance during the treatment of the complex wastewater is highly desired.
Electrospinning is a versatile method that utilizes high electrostatic forces to manufacture ultrathin nanofibrous membranes (Zhang, Liu, Si, Yu, & Ding, 2020). The as-fabricated electrospun nanofiber membranes (ENMs) are advantageous due to the tunable pore structure, high porosity and flexibility, large surface areas and microporous structure, thus allowing an excellent separation efficiency and water flux (Cui et al., 2020). In addition, the smart electrospinning technology has good feedstock adaptiveness to accommodate different polymers, including natural (e.g. cellulose acetate, lignin, etc.), synthetic polymers (e.g. polyacrylonitrile (PAN), polyvinylidene fluoride (PVDF), polyvinyl alcohol (PVA) etc.) or their combination (Jamshidifard et al., 2019). Among them, cellulose acetate (CA) is one of the derivatives of natural polymer cellulose with sustainability and stability, which can be converted to cellulose (Ce) by deacetylation. In addition, the addition of a small part of hydrophilic polyvinyl pyrrolidone (PVP) not only has the dual functions of self-sacrificing stomata and additional binding sites, but also can appropriately improve the analytical strength of the fiber (Jiang et al., 2019).
Moreover, ENMs can be readily tuned and modified by combining with some functional materials to improve the membrane properties and performances with respect to the wettability, self-cleaning capacity and separation efficiency, via different post-modification methods (Capilli, Calza, Minero, & Cerruti, 2019; Yi et al., 2019). For instance, Yi et al. (2019) fabricated a polystyrene (PS)/PAN ENM with superhydrophilic and underwater superoleophobic surface by grafting acrylic acid. Liu et al. adopted a seed-assisted hydrothermal method to prepare a core-shell structured PVDF-HFP/CuO ENM and the hybrid ENM showed an excellent anti-oil-fouling performance and separation efficiency in the process of emulsified oily wastewater purification (Liu, Cao, Wei, Zhao, & He, 2019). However, it remains a challenge to develop desirable ENM, which can be simultaneously and effectively eliminate complex ingredients, e.g. emulsified oils, organic dyes and heavy metal ions, in the wastewater (Yin et al., 2020; Zhao et al., 2019).
Inspiringly, incorporating photocatalytic nanomaterials into the membrane matrix, thus obtaining photocatalytic hybrid ENMs, is an effective strategy to meet the demand (Dou, Zhang, & Kaiser, 2020). By combination of ultrafine nanofibrous ENMs with unique photocatalytic properties, the as-prepared photocatalytic ENMs can not only enhance the degradation of organic pollutants, thus improving fouling resistance of membranes, but also facilitate the dispersion and recyclability of nano- or micro- catalysts, which in turn improves the catalysis and reusability of membranes (Xu et al., 2020). Among these photocatalysts, metal-organic frameworks (MOFs), as emerging 3D porous crystals formed by connecting metal ions and organic linkers, have captured remarkable attention in environmental remediation due to their large surface areas, fine-tuned structure and well-defined pore size (Liu, Cao et al., 2019; Liu, Liu et al., 2019). Many MOFs, such as MOF-5(Zn) (Hao et al., 2018), UiO-66(Zr) (de Lima et al., 2021) and MIL-100(Fe) (Duan et al., 2020; Lu et al., 2020) have semiconductor-like behavior and show great advantages in photocatalysis due to their flexible structure design and unique physicochemical properties compared to traditional photocatalysts (Wang, Gao et al., 2020). Moreover, numerous studies have proven that the photocatalytic activity of MOFs can be further enhanced via combining some semiconductor-like nanomaterials, e.g. TiO2 (Fathi Achachlouei & Zahedi, 2018), ZnO nanoparticles (Xiao et al., 2020), g-C3N4 nanosheets (Sadjadi, Heravi, & Malmir, 2018) and β-FeOOH nanorods (Zhao et al., 2020), etc. In particular, in addition to cost-efficiency, photocatalysts made from iron-based MOFs or nanomaterials, i.e. MIL-100(Fe) or β-FeOOH, not only have a strong adsorption effect on dyes (Aslam et al., 2017), but also can give rise to a photocatalysis-Fenton synergy system, enhancing catalytic activity towards degradation of pollutants.
On the basis of the above discussion, we herein demonstrate a combined method consisting of electrospinning and in-situ synthesis to fabricate novel cellulose-based core-sheath structured photocatalytic hybrid ENMs, namely β-FeOOH@MIL-100(Fe)/CeP, for the remediation of complex wastewater containing emulsified oils, organic dyes and heavy ions. After deacetylation, the Cellulose/PVP (CeP) hybrid electrospun nanofibers serve as scaffolding cores, and the beta hydroxyl oxidize iron (β-FeOOH) decorated MOF (denoted as β-FeOOH@MIL-100(Fe)) heterojunctions act as catalytic sheaths. The hypotheses are 1) the deacetylation of CA and partial leaching of PVP porogen can lead to the electrospun CeP nanofibers with rough hydrophilic surfaces and active sites, thus improving the ultrahigh MIL-100(Fe) loading via coordination interactions; 2) the ultrahigh loading of porous and hydrophilic MIL-100(Fe), along with the well-dispersed β-FeOOH nanorods, can endow the ENMs with excellent properties in terms of underwater oil repellency, dye adsorption/enrichment and photocatalysis-Fenton synergy. Consequently, the as-prepared self-cleaning photocatalytic ENM can deliver robust separation efficiency, water flux and membrane reusability even under a complex wastewater system.
Section snippets
Materials
Sodium hydroxide (NaOH), acetone, N,N-dimethylacetamide (DMAc), iron(III) chloride hexahydrate (FeCl3·6H2O), hydrochloric acid (HCl), hydrogen peroxide (H2O2, 30 wt%), methylene blue (MB), iron chloride tetrahydrate (FeCl2·4H2O), tween-80, potassium dichromate (K2Cr2O7), five different oils (petroleum ether, toluene, dichloroethane, cyclohexane and colza oil) were purchased from Sinopharm Chemical Reagent Co. Ltd., China. Polyvinylpyrrolidone (PVP, Mw 130000), trimesic acid (H3BTC),
Synthesis and characterization of core-sheath structured β-FeOOH@MIL-100(Fe)/CeP ENM
The morphological changes of the electrospun nanofibers in each step are shown in Fig. 1(b–e). The pristine cellulose acetate/PVP (CAP) nanofibers show very smooth surfaces and ultrafine fibers (Fig. 1b, b-1, b-2). After deacetylation and porogen leaching, the surface of the Cellulose/PVP (CeP) nanofibers becomes much rougher and some mesoporous pores are evident (Fig. 1c, c-1, c-2). Upon in-situ immobilization of MIL-100(Fe), a thick layer of octahedral and well-separated MIL-100(Fe) crystals
Conclusions
In summary, a core-sheath structured photocatalytic cellulose-based β-FeOOH@MIL-100(Fe)/CeP ENM has been successfully fabricated using combined electrospinning and in-situ synthesis method. The β-FeOOH@MIL-100(Fe)/CeP ENM showed ultrahigh MIL-100(Fe) loading (78 wt%) and super wettability (WCA = 0° in air) due to the deacetylation of CA and the partial leaching of PVP porogen, thus facilitating the immobilization of hydrophilic MIL-100(Fe) crystals. The as-prepared ENM demonstrated excellent
CRediT authorship contribution statement
Wanli Lu: Methodology, Visualization, Writing - original draft. Chao Duan: Conceptualization, Supervision, Writing - review & editing. Yanling Zhang: Resources, Investigation. Kun Gao: Visualization. Lei Dai: Investigation, Validation. Mengxia Shen: Conceptualization, Validation. Wenliang Wang: Formal analysis. Jian Wang: Methodology, Formal analysis. Yonghao Ni: 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.
Acknowledgements
This work was financially supported by the National Natural Science Foundation of China (31700510), the Natural Science Foundation of Shaanxi Province (2018JQ3013), and the China Scholarship Council (201908610074).
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