Research Paper
A novel carboxylated polyacrylonitrile nanofibrous membrane with high adsorption capacity for fluoride removal from water

https://doi.org/10.1016/j.jhazmat.2021.125113Get rights and content

Highlights

  • The C-PAN NFM is a novel porous membrane with unique layered stack structure firstly used for fluoride removal.

  • The C-PAN NFM is mass produced using simultaneously biaxial stretching and carboxylation for the first time.

  • The C-PAN NFM exhibits high fluoride adsorption capacity, outstanding selectivity and excellent reusability.

  • The C-PAN NFM has broad application potential to be a new fluoride removal strategy without secondary pollution.

  • The C-PAN NFM adsorption mechanism is mainly hydrogen bonding and ion exchange.

Abstract

To deal with the drinking water safety caused by fluoride, a novel carboxylated polyacrylonitrile nanofibrous membrane (C-PAN NFM) is designed and fabricated massively for the first time by adopting synchronously biaxial stretching and carboxylation. The C-PAN NFM is composed of the layered stack structure by cross-linked nanofibers. Due to its high specific surface area, excellent hydrophilicity, a large amount of carboxyl and amine groups, C-PAN NFM owns high fluoride adsorption capacity and outstanding selectivity. Both the carboxylation and acid treatment of C-PAN NFM improved the fluoride adsorption capacity remarkably. Specifically, C-PAN NFM shows excellent reusability without secondary pollution. The fluoride adsorption behavior of C-PAN NFM is dominated by chemical adsorption, and the adsorption mechanism is mainly driven by hydrogen bonding and ion exchange. The mass-produced C-PAN NFM is a novel polyacrylonitrile-based porous membrane that shows a great application potential for fluoride removal with good efficiency and recyclability.

Introduction

Fluoride is one of the essential trace elements for human normal growth. However, the excessive intake of fluoride can lead to endemic fluorosis, which seriously affects people's health (Bhatnagar et al., 2011). As one of the most abundant anion in groundwater worldwide, the safety problems of drinking water caused by fluoride have drawn wide attentions. The tolerance limit of fluoride content in drinking water is 1.5 mg/L as specified by the World Health Organization (WHO) (Singh et al., 2018). People in countries, such as China, India, Sri Lanka and Rift Valley countries in Africa, have suffered from various health problems severely due to the drinking water of high fluoride concentration (Singh et al., 2018, Jagtap et al., 2012, Li et al., 2018, Teshome et al., 2018, Dhillon et al., 2016). Accordingly, there is an urgent demand for the deep treatment of fluoride-contaminated water.

Numerous attempts have been applied for fluoride removal. Currently, the main remediation options for fluoride removal are precipitation-coagulation, membrane-based processes, ion-exchange method and adsorption. Among these methods, the adsorption using porous materials shows great potential due to the low cost, easy operation, high adsorption capacity and reusability (Jagtap et al., 2012, Dhillon et al., 2016, Barathi et al., 2019, Tomar V, 2013). Many novel adsorbents are developed for fluoride removal in recent years, such as metal oxides-based materials (Ali et al., 2015, Cai et al., 2018, Chaudhary and Maiti, 2019, Chaudhary et al., 2019, Dhillon et al., 2018, Dong and Wang, 2016, Gao et al., 2020b, Haddad et al., 2019b, Jiang et al., 2019, Kumar et al., 2011, Mahapatra et al., 2013, Muthu Prabhu and Meenakshi, 2014, Prathna and Raichur, 2018, Wu et al., 2018, Yin Q L and Asuha, 2019), carbonaceous materials (Abe et al., 2004), soils, minerals and other low-cost materials (Iriel et al., 2018, Zhang et al., 2019, Yu et al., 2018, Xia et al., 2019, Xu et al., 2015), polymer-based materials (Robshaw et al., 2019, Mondal et al., 2018) and biopolymer-based materials (Raghav and Kumar, 2019, Viswanathan et al., 2009). Nevertheless, currently developed adsorbents have faced some drawbacks, such as low adsorption efficiency, crammed reaction vessel during adsorption, or separation problems after adsorption due to its powdery block-bodied morphology, poor mechanical property, and expensive cost as well as secondary pollution during the practical application (Wang et al., 2019). Key factors closely related to enhance the adsorbent efficiency and the industrial applicability, includes large surface area, appropriate thermal properties, high selectivity, excellent stability and low production cost (Ma et al., 2017). However, the currently developed adsorbents cannot fulfill these requirements and most of the adsorbents cannot be used for groundwater treatment due to the defluoridation capacity lost in low fluoride concentration solution (Yu et al., 2018). Therefore, developing an efficient adsorbent for the deep treatment of low fluoride concentration solutions on a large scale is required. The polymer-based adsorbent can be used as an ideal adsorbent among various candidate adsorbents due to its advantages in adsorption efficiency and recyclability, reducing secondary pollution and price.

As a new porous polymer membrane, the nanofibrous membrane has been widely applied in several fields, such as water treatment and desalination (Wan et al., 2018). Compared with other fluoride adsorption materials, the nanofibrous membrane has a large surface area, controllable pore size, low curvature factor and high porosity which can alleviate the effect on water flux caused by internal concentration polarization effectively. Moreover, the nanofibrous membrane has a great potential application in water treatment, as it can be removed from the water easily, and the membrane-based separation is more energy-efficient (Sholl and Lively, 2016). P. Pal et al. proved that nanofiltration is an economic and effective method for fluoride removal, which can be used on a large scale based on an experiment and simulation study (Chakrabortty et al., 2013). Some other researchers have studied fluoride removal's performance and mechanism using the ceramic membrane (Ghosh et al., 2013) and mixed matrix membrane (Mondal et al., 2015). The researchers found that the nanofiltration cannot treat wastewater with higher fluoride concentration and requires additional pretreatment like adsorption (Xu et al., 2015). Combining the membrane-based processes and adsorption method, the research on a functional nanofibrous membrane adsorbent are in line with the research trend of fluoride removal adsorbent. At present, studies on adsorbents based on membrane materials are rarely reported.

In this work, we developed a novel carboxylated polyacrylonitrile nanofibrous membrane (C-PAN NFM) used for fluoride removal. Herein, polyacrylonitrile nanofibrous membrane (PAN NFM) is prepared by the biaxially stretching of PAN membrane, instead of using its conventional preparation method (electrospinning, spinning and centrifugal spinning). After then, the PAN NFM has been modified through a carboxylation reaction to prepare the C-PAN NFM. The subsequent fluoride absorption test shows that C-PAN NFM exhibits high fluoride adsorption capacity, outstanding selectivity and excellent reusability without secondary pollution. With the aid of the Fourier Transform Infrared (FTIR), X-ray Photoelectron Spectroscopy (XPS) and Energy Dispersive Spectrometer (EDS) characterization, it is proposed that the C-PAN NFM adsorption mechanism is mainly driven by hydrogen bonding and ion exchange. The C-PAN NFM with high fluoride removal efficiency and good recyclability exhibits a great potential of application for the deep treatment of wastewater containing fluoride.

Section snippets

Materials

Polyacrylonitrile (PAN, Mw = 200 kg/mol) is purchased from Kaierda plastic raw materials Co., Ltd. (Suzhou, China). Dimethyl sulfoxide (DMSO), HCl, NaOH, NaF, NaCl, NaNO3, Na2CO3, Na2SO4 and Na3PO4 are supplied by Sinopharm chemical reagent Co., Ltd. (Shanghai, China). All chemicals and deionized water were used without further purification.

PAN nanofibrous membrane

PAN-DMSO-H2O solution was prepared by mixing PAN powder (10 g) in DMSO solvent (72.5 mL) and H2O non-solvent (10 mL) with vigorous magnetic stirring for 3 h

Macroscopic properties difference of PAN and C-PAN nanofibrous membrane

The morphology of PAN NFM and C-PAN NFM was analyzed by scanning electron microscope (SEM) as shown in Fig. 1c-d and Fig. S2. The original PAN casting membrane has a dense and uniform structure (Fig. 1b). The PAN NFM is composed of the characteristic layered stack structure by cross-linked nanofibers. And the PAN nanofibers obtained after synchronously biaxial stretching did not show obvious preferred orientation (Fig. 1c and Fig. S2b). The diameter of PAN nanofibers is less than 200 nm (Fig.

Conclusion

In summary, we prepared a novel carboxylated polyacrylonitrile nanofibrous membrane (C-PAN NFM) for the first time by the combination of synchronously biaxial stretching and carboxylation. The C-PAN NFM is composed of the characteristic layered stack structure by the cross-linked nanofibers after modification. According to the high specific surface area, excellent hydrophilicity and a large amount of carboxyl and amine groups, it exhibits high fluoride adsorption capacity and outstanding

CRediT authorship contribution statement

Xin Chen: Data curation, Writing - original draft, Software. Caixia Wan: Investigation, Software. Rui Yu: Investigation, Data curation. Lingpu Meng: Writing - review & editing. Daoliang Wang: Writing - review & editing. Wei Chen: Writing - review & editing. Tao Duan: Conceptualization, Methodology. Lingbin Li: Conceptualization, Methodology, 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.

Acknowledgments

This work is financially supported by the National Key R&D Program of China (2018YFB0704200); the National Natural Science Foundation of China (U1732151) and Guangdong Provincial Key Laboratory of functional soft condensed matter.

Conflicts of Interest

There are no conflicts of interest to declare.

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