Electro-enhanced removal of cobalt ions from aqueous solution by capacitive deionization
Graphical abstract
Introduction
Heavy metals are recognized as the environmental pollutants because of their toxicity and bioaccumulation and persistence, which are harmful to the ecosystem and human health (Wang and Chen, 2009, Wang and Chen, 2014). The main sources of heavy metals are wastewater discharge from various industrial activities including electroplating, battery manufacturing, mining, nuclear and power industries (Wang and Zhuang, 2017; Chen and Wang, 2011). Therefore, the industrial wastewater must be treated to remove the heavy metal before discharging.
Cobalt (Co) as a heavy metal has been widely used in industries, such as cobalt-base alloys, cemented carbides, magnetic materials, electronic, chemical and ceramic uses because its ferromagnetic property, anti‑oxygenic property, appearance and hardness (Cojocaru et al., 2009; Gómez et al., 2018; Wołowicz and Hubicki, 2018). Co radioisotope (60Co) is frequent in medicine for radiotherapy (Cojocaru et al., 2009) and radioactive wastewater from the nuclear power plants (Wang and Zhuang, 2019a, Wang and Zhuang, 2019b; Wang et al., 2018; Liu et al., 2017). However, these industrial applications become the potential source of cobalt-containing wastewater. Although trace amounts of Co ions are essential for life (e.g. important component of vitamin B12), high concentration of Co ions impacts human health and causes respiratory illness (Liu et al., 2017), cardiac diseases (Gherasim et al., 2015), vision and hearing problems (Jamiu et al., 2017), thyroid damage, and even death (Wołowicz and Hubicki, 2018). The permissible level for Co ions in potable water and livestock wastewater has been fixed at 0.05 mg L−1 (Mahmoud et al., 2016; Anirudhan et al., 2019) and 1.0 mg L−1 (Mahmoud et al., 2016). As a result, it is necessary to treat cobalt-containing wastewaters before they discharge to the environment. Various technologies have been studied for recovery and removal of Co ions from aqueous solution, including chemical precipitation (Anirudhan et al., 2019), electrochemical reduction (Mahmoud et al., 2016), chelation (Anirudhan et al., 2019), electrodialysis (Anirudhan et al., 2019), coagulation (Anirudhan et al., 2019), ion-exchange (Wołowicz and Hubicki, 2018; Anirudhan et al., 2019), solvent extraction (Gómez et al., 2018; Anirudhan et al., 2019), evaporation (Anirudhan et al., 2019), adsorption (Chen and Wang, 2012a, Chen and Wang, 2012b; Zhu et al., 2012, Zhu et al., 2014; Jamiu et al., 2017; Gómez et al., 2018; Anirudhan et al., 2019), membrane separation (Mahmoud et al., 2016; Jamiu et al., 2017; Jia et al., 2018; Anirudhan et al., 2019). Among these technologies, adsorption is considered as one of the most efficient and potential techniques because of its simplicity and relatively low cost (Xing et al., 2016; Jamiu et al., 2017; Xu and Wang, 2017; Gómez et al., 2018; Taka et al., 2018; Anirudhan et al., 2019). During adsorption process, adsorption capacity and rate are closely related to the surface area, pore size distribution and surface functional group of adsorbents (Huang and Su, 2010). Therefore, many porous materials have been used as adsorbents to remove Co ions, including activated carbon, ion-exchange resins, graphene oxide, magnetic graphene oxide/chitosan composite, maleic acid-grafting chitosan, silica gel, chitin, biosorbents and zeolite (Mahmoud et al., 2016; Zhuang and Wang, 2018; Zhuang et al., 2018; Anirudhan et al., 2019). For the past few years, electrosorption has drawn more and more interest for the removal of metal ions (Huang and Su, 2010). In which, materials with high specific surface and conductivity are taken as the electrodes. The ions in electrolyte solution can be adsorbed on the surface of electrodes by induced polarization potential (Huang and Su, 2010).
Capacitive deionization (CDI) as an electrosorption process has been used in metal ions removal and water purification (Huang et al., 2016), which only needs a low electrical field (0.6–2 V) to respectively attract the cations and anions onto the electric double layer (EDL) of cathode and anode (Huang et al., 2016; Zhang et al., 2018; Choi et al., 2019). Meanwhile, the CDI electrodes are able to be regenerated through short-circuiting or inverting the cell voltage when the electrodes reaches saturation (Huang et al., 2016; Tian et al., 2019). Therefore, the CDI is proven an energy-saving and easy-to-operate process (Hu et al., 2018; Oladunni et al., 2018). At present, CDI process has been widely investigated to remove various heavy metal ions, including chromium (Cr3+) (Rana et al., 2004; Rana-Madaria et al., 2005; Huang et al., 2016), cadmium (Cd2+) (Huang et al., 2016), copper (Cu2+) (Huang and Su, 2010; Huang et al., 2014), ferric (Fe3+) (Li et al., 2010), nickel (Ni2+) (Dermentzis, 2010), Vanadium (V5+) (Bao et al., 2018) and lead (Pb2+) (Huang et al., 2016) from aqueous solution and recycle these metals by regeneration, but few studies explore the CDI performance in removing the Co ions from aqueous solution.
According to previous studies, CDI performance is usually evaluated by its electrosorption capability or removal efficiency, which strongly depends on various influencing factors, such as cell geometry (Agartan et al., 2018; Oladunni et al., 2018), electrode material (Agartan et al., 2018; Byles et al., 2018; Oladunni et al., 2018; Saleem and Kim, 2018), operational conditions (Agartan et al., 2018; Oladunni et al., 2018; Saleem and Kim, 2018) and feed solution composition (Oladunni et al., 2018). The removal efficiency can be further improved through the selection and optimization of the operational modes, such as constant-current operation (Zhao et al., 2012; Porada et al., 2013) and stop-flow operation (Bouhadana et al., 2011; Porada et al., 2013). Additionally, new materials and various modifications have been explored for novel and improved electrodes (Porada et al., 2013; Byles et al., 2018; Saleem and Kim, 2018). In CDI process, charged ions were entrapped at the surface of polarized electrodes by the formation of electric double layers (EDLs) (Porada et al., 2013; Han et al., 2014). EDLs formation is the basis of capacitive energy storage (Porada et al., 2013). Traditional EDL theory suggests that electrosorption only happens in mesopores (Ahmed and Tewari, 2018). However, modern EDLs theory considers ion hydration and water-water interaction, capable of explaining the electrosorption happening in micropores, which is also significant (Han et al., 2014; Ahmed and Tewari, 2018). Therefore, the ratio of mesoporosity to microporosity of the electrode materials should be optimized and characterized for improving the removal of targeted ions and establishing the precise CDI modeling (Han et al., 2014). At present, it is important to explore the novel electrode materials and establish a solid connection between pore structure and charge efficiency.
This study used commercial activated carbon cloth (ACC) as the electrode material for preliminary research because of its high conductivity and specific surface area (Tian et al., 2019). The effect of applied voltage and initial Co ions concentration, as well as coexisted ions on removal efficiency of Co ions was determined to verify the feasibility of CDI in removing Co ions from aqueous solution. Co ions adsorption performance was also evaluated by kinetic models, isotherm models and three mass transfer models. The changes of ACC after electrosorption separation were characterized.
Section snippets
Chemicals
The commercial ACC (Tianxiang, Shanghai, China) was used as electrode material. In this study, the electrodes were circular and effective surface area was 7.1 cm2 (weight ~0.1 g). Titanium plates (Baotai, China) attached with the electrodes were employed as current collector.
The surface morphology and structure of the new and used ACC electrodes were observed by field emission scanning electron microscopy equipped with an energy dispersive spectrometer (EDS) (FESEM; SU8010, Hitachi, Japan).
The
Effect of applied voltage
Fig. 2 shows the changes of concentration and removal efficiency under different applied voltages. The Co ions concentration rapidly decreased in the first 30 min, after which it began to slow down. The Co ions concentration decreased by 0.75, 1.25, 1.71 and 1.89 mg L−1 after running for 240 min at corresponding voltage 0 V, 0.6 V, 0.9 V and 1.2 V (Fig. 2a), the removal efficiency was 15.11%, 25.64%, 32.69% and 36.54% (Fig. 2b), respectively, which increased with increase of applied voltage (
Conclusions
Electro-enhanced removal of Co ions from aqueous solution by CDI was investigated. The removal efficiency of Co ions had positive correlation with applied voltage (R2 = 0.9991), which increased from 15.11% to 36.54% when the applied voltage increased from 0 V to 1.2 V. However, the increasing initial Co ions concentration and coexisted ions (Sr and Cs) largely inhibited the removal efficiency of Co ions. After fitting the adsorption data, PSO model was better than PFO for each applied voltage
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
The research was supported by the National Key Research and Development Program (2016YFC1402507) and the Program for Changjiang Scholars and Innovative Research Team in University (IRT-13026).
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