Chitosan/polyvinylpyrrolidone/polyvinyl alcohol/carbon nanotubes dual layers nanofibrous membrane constructed by electrospinning-electrospray for water purification

Highlights

• Dual layers nanofibrous membranes were constructed by electrospinning electrospray.

• The thickness and roughness of active layer could be controlled by varying electrospray time.

• The nanofibrous membranes can efficiently remove heavy metals and dyes.

• The pure water flux could reach up 1533.26 L·m⁻²·h⁻¹.

• The membranes exhibited a long-term stability and good antifouling property.

Abstract

Nanofibrous membrane have great potential in the field of water purification due to the high porosity and large specific surface area. Herein, a dual layers nanofibrous membrane was prepared by combining an active layer containing carbon nanotubes (CNTs) with a porous chitosan (CS)/polyvinylpyrrolidone (PVP)/polyvinyl alcohol (PVA) nanofibrous support layer via electrospinning-electrospray technique for highly efficient heavy metal and organic pollutants removal. Incorporating CNTs into the active layer offered additional nanochannels which significantly enhanced pure water permeate flux (1533.26 L·m⁻²·h⁻¹) and heavy metal ions/dyes rejection (Cu²⁺ 95.68 %, Ni²⁺ 93.86 %, Cd²⁺ 88.52 %, Pb²⁺ 80.41 %, malachite green 87.20 %, methylene blue 76.33 %, and crystal violet 63.39 %). The optimal membranes were formed with a thickness of 20 μm and a roughness of 142 nm while still showing good perm-selectivity compared with commercial PVDF membrane. Moreover, the constructed membrane exhibited good antifouling property and long-term stability during filtration process. This work provides a new strategy to fabricate advanced separation membranes for water treatment.

Introduction

Water scarcity is one of the most challenges facing the world in the 21st century (Greve et al., 2018; Mekonnen & Hoekstra, 2016). With the growth of the world's population and rapid economic development, a large number of pollutants has been discharged into natural waters. Among them, heavy metals and organic dyes contamination pose a serious threat to humans and the environment due to their high toxicity and carcinogenicity (Bolisetty & Mezzenga, 2016; Feng et al., 2018; Liao et al., 2018). Therefore, it is urgent to develop an efficient and feasible water purification technique for heavy metal and dye pollutants elimination. Currently, various physical, chemical and biological methods, such as sedimentation, filtration, disinfection, ion exchange, chemical oxidation, anaerobic and aerobic digestion, have been used to remove or reduce pollutants in water (Farzaneh et al., 2021; Ren et al., 2020). Remarkably, polymer-based membrane technologies have attracted intensive interest from academia and industry because of their high efficiency, easy operation, low energy consumption and good effluent quality (Sharma et al., 2020; Tagliavini & Schäfer, 2018; Zhang et al., 2021). In the past few decades, synthetic polymeric membranes such as polyamide (PA) (Chowdhury et al., 2018; Liang et al., 2020; Tan et al., 2018), poly(vinylidene fluoride) (PVDF) (Chen et al., 2020; Kharraz & An, 2020), polysulfone (PS) (Duan et al., 2018; Mondal & Majumder, 2020), and polyacrylonitrile (PAN) (Wen et al., 2017; Zahed et al., 2019), have been widespread use in desalination and water purification, which possess good thermal stability, and high processability. However, those synthetic polymeric materials are usually derived from petroleum-based products that do not biodegrade in a landfill or in the environment and pose serious long-term environmental problems. Besides, the separation abilities of polymeric membrane such as ultrafiltration (UF) membrane for heavy metals and organic compounds still need to be further improved. Therefore, the development of an environmentally friendly alternative with high separation performance to synthetic polymeric membrane is highly important for sustainable water treatment.

Chitosan (CS) is a promising candidate widely used in polymeric membrane fabrication owing to its abundance, renewability, environmental friendliness, and excellent absorption (Chatterjee et al., 2018; Jana et al., 2011; Long et al., 2020; Sharma et al., 2018). As molecular chains of CS contain a large amount of active functional groups such as amino and hydroxyl groups, they can not only chelate with positively charged metal ions and cationic dyes to form stable complexes, but also adsorb negatively charged metal acid ions through electrostatic interaction, therefore, CS-based membranes have been extensively studied in recent years for the separation and absorption of heavy metal ions and organic dyes in wastewater. Gharbani et al. reported an adsorptive CS membrane fabricated for Rhodamine B dye removal from water and found rejection of 72.47 % (Gharbani & Mehrizad, 2022). Huo et al. synthesized CS-based acid-resistant composite membranes with highly efficiency and selectivity for removing toxic anionic dyes from wastewater (Huo et al., 2021). Long et al. fabricated a positively charged CS nanofiltration membranes by using film casting method for enhanced removal of dyes and salts in textile water. The as-fabricated CS membranes exhibited considerable water permeate flux and high filtration capacities for conventional dyes and salts due to their suitable thickness and tensile strength (Long et al., 2020). However, these CS-based membranes are mainly constructed by solvent casting or chemical cross-linking strategies, which usually result in membrane microstructures such as pore size, porosity, roughness and thickness that cannot be precisely controlled, thus limiting the further improvement of water flux and separation accuracy of CS membranes. Consequently, it is highly desirable to develop a flexible and versatile strategy for the synthesis of advanced CS membrane with superior heavy metal/dye separation performance.

Compared to other membrane preparation methods, electrospinning has a wide selection of materials, easy multi-functionality, simple and convenient processes, and can prepare high porosity and high specific surface area membranes without pore-forming agents (Khairnar et al., 2021; Liao et al., 2018; Siddique et al., 2021; Wang et al., 2018). Combining electrospinning with electrospray offers a facile and fast fabrication method for controlled nanostructured thin film composite (TFC) membranes for water purification (Kang et al., 2018; Shen et al., 2016; Xue et al., 2019; Yoon et al., 2018). During electrospinning/electrospraying, the polymer solution is charged in the presence of a high voltage and forms a hemispherical droplet (Taylor cone) at the tip of the spinneret. Coulombic repulsion forces the ejected droplets to distribute with nanoscale fibers or microspheres on the collecting device. The structure and performance of electrospun fibers and electrospray particles can be adjusted via the synthesis parameters such as applied voltage, liquid flow rate, surface tension, etc. These features attracted Chowdhury et al. to use electrospray for synthesizing TFC membranes. The thickness of the resulting TFC film is controllable in increments of 4 nm and roughness down to 2 nm, while still showing good permeable selectivity (Chowdhury et al., 2018). To better control thin selective layer thickness and pore size, Kang et al. applied electrospray interfacial polymerization method to build the loose ultrafiltration (UF) membranes with high dye/salt rejection properties and excellent antifouling abilities (Kang et al., 2020). In order to enhance filtration efficiency of nanofibrous TFC membranes, the incorporation of nanoparticles such as carbon nanotubes (CNTs) into TFC membranes can provide additional nanochannels that could greatly improve the membrane selectivity without compromising water permeability (Dong et al., 2015; Geng et al., 2014; Huang et al., 2020; Tunuguntla et al., 2017). However, the CNTs usually aggregate into bundles and/or entangle together because of the intrinsic van der Waals attraction of the flexible nanotubes. High-energy sonication over prolonged periods of time is usually necessary to produce uniformly dispersed CNTs suspensions. Polyvinylpyrrolidone (PVP), a polymeric surfactant with excellent properties such as soluble, non-toxic and stable, is commonly used for the dispersion of CNTs (Alimohammadi et al., 2018; Udomsin et al., 2021; Zhang et al., 2015). It has also been reported that CS has a facilitative effect on the dispersion of CNTs (Wang et al., 2005).

In this study, CS/PVP/PVA nanofibrous membrane as a substrate was first prepared by electrospinning. Then, CNTs incorporated into a CS/PVP matrix were coated via electrospray for constructing the nanochannels that provided perm-selectivity. The preparation strategy of the CS-based nanofibrous membrane was illustrated in Fig. 1. The microstructure and physicochemical properties of the synthesized membranes were analyzed by using FE-SEM, TEM, AFM, EDS, XRD, FT-IR, and XPS. The fabrication conditions were optimized for water permeate flux and heavy metal ions/dyes rejection. Furthermore, the surface composition and structure, hydrophilicity, and antifouling ability of the resulting membranes were also evaluated. The obtained CS-based nanofibrous membrane exhibited remarkable pure water permeate flux of 1533.26 L·m⁻²·h⁻¹ and satisfied heavy metal ions/dyes rejection. Noteworthily, no environmentally harmful and volatile organic solvents are used in the synthesis of nanofibrous membranes, which is in line with the concept of sustainable development and green chemistry. This work could provide a practical and cost-effective strategy for designing and synthesizing advanced water purification membranes.