Regular Article
Selective electrosorption of Ca2+ by MXene cathodes coupled with NiAl-LMO anodes through ion intercalation

https://doi.org/10.1016/j.jcis.2021.01.058Get rights and content

Highlights

  • MXene electrodes with selective electro-sorption of Ca2+have been fabricated.

  • Low hydration energy and high valence of Ca2+ contributed to Ca2+/Na+ selectivity.

  • Electro-sorption was facilitated by NiAl-LMO cathode due to valence compensation.

  • MXene electrode has great stability and can be cyclically used in Ca2+ removal.

Abstract

Capacitive deionization (CDI), or electrosorption, is a desalination technology that exhibits significant potential; however, its major technical requirement of selective ion separation poses a challenge for its further practical application. Herein, a titanium carbide (MXene)–layered electrosorption electrode with high selectivity for Ca2+ was fabricated. The prepared MXene electrode had many surface hydroxyl functional groups that serve as adsorption sites for Ca2+. Ca2+ was successfully inserted into the interlayers of the MXene cathode and formed a strong interaction with [Ti-O] bonds during the capacitive deionization process. When a Ni-Al layered metal oxide anion intercalation electrode was employed as the counter electrode, Ca2+ adsorption by the MXene electrode was significantly enhanced due to the valence compensation balance effect. The maximum Ca2+ electrosorption capacity of the MXene electrode reached 1011.82 mg per gram effective MXene material, which is 6.3 times higher than that of Na+ based on the Langmuir adsorption isotherm model. The MXene electrode exhibited prominent selectivity for Ca2+ ions in the presence of Na+ and Mg2+. The Ca2+/Mg2+ selectivity factor for electrosorption reached 2.63, and Ca2+/Na+ selectivity factor could achieve 9.84, respectively. After five electrosorption/desorption cycles, the Ca2+ removal rate only decreased from 46.96% to 45.34%, suggesting that the MXene electrode has excellent stability. Our study demonstrated a novel CDI electrode and technical approach for softening water.

Introduction

Calcium ions (Ca2+) are both greatly beneficial and persistently challenging for water treatment engineers because they are essential for human health but lead to scaling issues in excess. Ca2+ scaling may result in sensory discomfort when drinking boiled water [1] and scale-fouling on heat exchangers, thereby increasing energy consumption [2], [3]. Currently known Ca2+ removal methods include ion exchange, chemical precipitation, and membrane separation [4], [5], [6], [7], but these techniques generally require high chemical requirements, energy consumption, or operating costs in the application process [8]. It is necessary to develop an easily operated, energy-saving, and environmentally friendly method for Ca2+ removal.

Capacitive deionization (CDI), or electrosorption, is a promising rapid and facile desalination approach [9], [10], [11]. Electrodes adsorb oppositely charged ions in water to achieve desalination and desorb into the solution when the applied bias is terminated or reversed to regenerate the electrode [12], [13]. The activated carbon electrode can achieve a maximum adsorption capacity of 9.98 mg Ca2+/g [14], while the maximum adsorption capacity of the carbon nanotube composite electrode can reach 20.80 mg Ca2+/g [15]. However, reported electrodes, which are the key factor for CDI performance, did not exhibit any selectivity for Ca2+. Ca2+, Mg2+, Na+, and K+ in aqueous solutions compete for adsorption sites on CDI electrodes, resulting in low electrosorption capacities of Ca2+ and the removal of coexisting non-target ions. Selectivity for certain ions is highly demanded and difficult to facilitate in electrosorption processes [16].

It has been reported that the removal of bivalent cations is higher than that of monovalent cations in CDI [17], [18]. In addition, for ions with the same valence, ions with a smaller hydration radius achieved higher removal rates due to size compatibility [19], [20], [21]. In addition to the distinct electrosorption behavior caused by the physicochemical properties of ions, electrode materials are influential in CDI selectivity [22], [23], [24]. MXenes, as typical two-dimensional carbon/nitride nano-layered materials, are potential CDI electrode materials because of their large specific surface area, high conductivity, and good electrochemical stability in salt solutions [25], [26], [27]. MXene is etched to form a layered structure with enormous surface hydroxyl functional groups, which can provide abundant adsorption sites and intercalation space for Ca2+ [28]. In the coexistence of Ca2+, Mg2+, and Na+, MXene may exhibit selectivity for Ca2+ and Mg2+ due to the higher valences compared with Na+.

In this study, an MXene CDI electrode with excellent Ca2+ selectivity was fabricated, and its performance on Ca2+ and coexisting cations (Mg2+, Na+, and Pb2+) was examined. The electrosorption kinetics and capacity under different electrode combinations and solution chemical conditions were investigated to optimize Ca2+ removal by CDI. The electrode structure in the CDI process was characterized to elucidate the mechanisms of Ca2+ interaction into MXene coupled with NiAl layered metal oxide (NiAl-LMO) electrode. Furthermore, adsorption–desorption circle tests were performed to verify the stability of the prepared MXene electrode. This study provides a novel method using CDI to selectively remove Ca2+ from water.

Section snippets

Electrode fabrication

Sodium chloride, calcium chloride, magnesium chloride, and lead nitrate were chemical pure, which were purchased from Sinopharm Chemical Reagent Co., Ltd. (China). Ti3C2Tx nanosheets were obtained using a liquid etching method [29]. Briefly, 5 g of Ti3AlC2 powders (Forsman, China) were poured into 100 mL hydrofluoric acid (HF, Sinopharm Chemical Reagent Co., Ltd, China) and etched in a water bath for 20 h at 60 °C. The solution was then transferred to a centrifuge tube for centrifugation for

Morphology and structure of MXene electrodes

As shown in Fig. 1a, MXene after etching exhibit an obvious multilayer structure, indicating the aluminum atomic layer has been successfully exfoliated. Thin MXene layers were observed in the TEM images (Fig. 1b). In addition, it can be inferred from the XPS results (Fig. 1c) that the surface of MXene contains abundant F, Ti, C, O, and Al elements. The XRD patterns of the precursor Ti3AlC2 and etched MXene are shown in Fig. 1d. The disappearance of the characteristic peak of Al at 39° indicates

Conclusion

The MXene electrode exhibited excellent selectivity for Ca2+ ions in the presence of Na+ and Mg2+. When CF was selected as the counter electrode, the Ca2+/Mg2+ selectivity factor for electrosorption reached 1.97, and Ca2+/Na+ selectivity factor could achieve 5.85, respectively. In addition, the couple of cation selective electrode and anion sorption electrode was employed to promote the performance of CDI system. When the NiAl-LMO cathode (anion sorption electrode) was used as the counter

CRediT authorship contribution statement

Jingqiu Sun: Writing - original draft, Visualization, Writing - review & editing. Qi Mu: Investigation, Methodology, Formal analysis, Writing - original draft. Tianyu Wang: Methodology, Validation. Jing Qi: Validation. Chengzhi Hu: Conceptualization, Funding acquisition, Project administration, 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

The authors are grateful for the National Natural Science Foundation of China (No. 51978646), Young Scientists Fund of the National Natural Science Foundation of China (No. 52000174), and the Chinese Academy of Sciences (QYZDY-SSW-DQC004).

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