Quaternized graphene oxide modified PVA-QPEI membranes with excellent selectivity for alkali recovery through electrodialysis
Graphical abstract
Composite anion exchange membranes are prepared from incorporating QGO into PVA or PVA/QPEI matrix. The OH groups of PVA can selectively accelerate the transport of OH− ions through hydrogen bonding. Meanwhile, QGO nanosheets, which are irregularly dispersed in the polymer matrix to form narrow-sized nanochannels, can hinder the transport of ions with large hydrated ionic radii. As a result, the composite membranes exhibit high recovery ratio and selectivity when applied in ED process to recover NaOH from NaOH/Na2WO4 mixture.
Introduction
Rapid advancement of modern industries like paper, leather, artificial fiber, dying, printing, aluminum, and tungsten ore processing, causes severe damage to the environment by discharging huge amounts of alkali wastewater (Liu et al., 2014; Luo et al., 2011). For instance, the tungsten ore smelting industry generally produces a large amount of feed solution containing OH− and WO42−, and since China is the world’s largest producer of tungsten products, the quantity of this feed solution is extremely high.
The conventional methods for treating alkaline solution include neutralization with acids, concentration and burning (mainly used in the paper industries). Huge quantities of acids, or a high quantity of energy is consumed and subsequently an enormous amount of sludge or waste gas is generated, causing further pollution (Mondal et al., 2015; Wang et al., 2011; Gu et al., 2012). On this basis, studies have been carried out to recover alkali by newly developed methods, such as diffusion dialysis (DD) process (Liu et al., 2014; Pan et al., 2014; Dai et al., 2017; Tong et al., 2017). Nevertheless, the application of DD in industrial alkali treatment is still difficult because of the slow diffusivity of alkali. Electrodialysis (ED) with excellent water treatment capacity has been widely used in metallurgy, seawater desalination, etc (Garrido, 2011; Roczanski et al., 2012; Chang et al., 2015; Ibanez, 2012). However, ED has rarely been reported in treating alkaline solution such as Na2WO4/NaOH mixture, for the common anion exchange membrane (AEM) has low selectivity to OH−. The availability of AEMs with high OH− selectivity as well as excellent thermal/alkali stability is required to extend the application of ED in processing alkaline wastewater.
High stability and favorable selectivity are both required for ED membranes. The membrane stability may be mainly determined by polymer matrix and cross-linking. For example, the poly(vinylidene difluoride) (PVDF) is one of candidate materials due to its high thermal stability. However, its insufficient alkali-resistance induced by the degradation of HF bonds (Danks et al., 2002) limits its usage for ED membranes. Similarly, the swelling and weight loss of polyethylene oxide (PEO) based hybrid membranes in an alkaline solution is obvious owning to the lack of sufficient cross-linking (Wu et al., 2008b). However, the PVA polymer backbone has favorable flexibility and alkaline resistance. What’s more, the presence of plenty of OH groups (Wu et al., 2012) leads to high affinity and adsorption ability for OH− ions, and hence the transportation of OH− through PVA-based membranes may be significantly accelerated (Dai et al., 2017; Grew and Chiu, 2010; Yang et al., 2008). Unfortunately, pure PVA materials contain no ion exchange groups, which may lead to high resistance during ED process.
Graphene is a flexible two-dimensional material exhibiting good chemical stability and mechanical strength (Perreault et al., 2015; Kim et al., 2013; Shi et al., 2019; Zhu et al., 2018), as well as a certain hindrance to the removal of high valence ions (Hu et al., 2017; Yan et al., 2017). The thermal stability (Xu et al., 2001), electrical properties (Wu et al., 2008a) and hydrophilicity of polymers could be greatly improved by the incorporation of graphene oxide (GO) nanosheets (Xu et al., 2009). Furthermore, the functionalization of GO can also provide a specific functional group for the composite. For example, the incorporation of quaternized GO (QGO) can improve the density of quaternary ammonium ions in the composite membranes.
Hence in this work, QGO will be incorporated in PVA or the mixture of PVA and quaternized polyethyleneimine (QPEI). QPEI is a polyelectrolyte and thus is expected to increase the number of anion exchange groups. The combination of PVA with GO and QPEI may yield optimized membranes in mechanical and thermal stability, swelling and alkaline resistances. The composite membranes are used in ED process for separating NaOH/Na2WO4 mixture and are expected to show improved selectivity than commercial membrane AMV. The selectivity is further investigated by separating NaCl and Na2SO4 mixture, which can show the monovalent anion selectivity of the optimized membranes.
Section snippets
Material
Cation-exchange membrane CMV and antion-exchange membrane AMV were obtained from Asahi Glass Company (Japan). The monovalent anion exchange membrane ACS was obtained from Tokuyama Corporation (Japan). Graphite powders were purchased from Shandong Qingdao Laixi graphite company in China. Glycidyltrimethylammonium chloride (GDTMAC), 3-aminopropyltriethoxysilane (APTES), PVA (MW 85 000–124 000 g mol−1), dimethyl sulfoxide (DMSO) and polyethyleneimine (PEI) were purchased from Aladdin Industrial
FT-IR spectra andmorphologies
The FT-IR spectra of GO, AGO and QGO are shown in Fig. 3. For the GO, the peak at 1720 cm−1 is from CO stretching vibration, and the peak at 1625 cm−1 corresponds to ether/epoxy stretching vibration, indicating the successful oxidization of graphite. For the AGO, the stretching vibration peak of O-H (3200–3400 cm−1) is obviously weakened, and some new peaks appear to prove the successful synthesis of AGO: the absorption band at 2925 cm−1 is due to the CH2 stretching vibration of APTES segments in
Conclusions
Composite anion exchange membranes are prepared from incorporating QGO into PVA or PVA/QPEI matrix. The membranes have excellent alkali stability and thermal stabilities. The weight losses of membranes are in the range of 0.5%–8.9% after immersion in 60 °C 2 mol/L NaOH alkaline solution for 32 days, and the initial degradation peaks of the membranes are at a high temperature of about 280 °C. The PVA-QGO series of membranes have low ion exchange capacities (IECs) of 0.10–0.37 mmol/g and low water
Declaration of interests
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 project was supported by National Natural Science Foundation of China (No. 21476056 and No. 21606063).
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