High specific surface crown ether modified chitosan nanofiber membrane by low-temperature phase separation for efficient selective adsorption of lithium
Graphical abstract
Introduction
As the lightest and densest metal element in nature, lithium possesses many excellent chemical and physical properties, such as extremely high electrochemical activity, specific heat capacity, and redox potential, etc. [1]. Nowadays, lithium has been widely used in chemical, metallurgy, new energy, aerospace, medicine, nuclear power, and lithium battery industries [2], [3]. Lithium has become part and parcel of the global economic development. Therefore, the growing demand for lithium has asked for more research on its recovery.
Currently, various lithium extraction methods including precipitation [4], solvent extraction [5], membrane filtration and adsorption [6], [7] have been applied to extracting lithium. The traditional evaporation precipitation method can make a huge difference to the local ecosystems because of evaporating a large amount of water, especially in area with dry climates [8]. Additionally, solvent extraction is a method for extracting lithium by using organic solvents with special extraction properties for lithium. Although solvent extraction can effectively separate lithium, there are still problems in the selectivity and recovery of the extractant [9]. As a relatively new method, membrane technology is merely applicable to treating low concentration lithium solution because the membrane can be clogged due to a high concentration solution, resulting in low separation efficiency [10], [11]. Compared with the above methods, the adsorption process is not only simple, but also it is environmentally friendly without any secondary pollution [12]. Especially, this method is feasible for separating large volume liquid systems with high target metal ion concentrations. Therefore, adsorption has become the mainstream direction of extracting lithium from salt-lake brines with concentrations up to thousands of parts per million [13]. However, since the presence of multiple interfering ions such as K+, Na+, Mg2+ and Ca2+ in salt-lake brines, most of the existing lithium adsorbents have poor selectivity to Li+ [14]. Moreover, some adsorbents such as lithium ion sieves are in the form of powder with poor fluidity and permeability. The rate of corrosion damage of ion sieve adsorbents is large during the process of elution and regeneration [15]. These shortcomings limit the current applications of lithium adsorbents. Therefore, as the core of the lithium adsorption method, it is particularly important to search for an excellent adsorption material that not only has high selectivity for Li+, but also owns good stability including thermal and mechanical stability.
CEs exhibit high selectivity for lithium extraction because of its precise size selectivity to cations [16]. Further, the proximity between the cavity size of crown ethers ring and the diameter of metal ions decides whether CEs can catch a unique metal ion. In other words, the closer the size of their cavities and the diameter of target ions, the greater the probability that CEs can capture the metal ions [17], [18], [19]. However, small-molecular CEs have the disadvantages of high price, difficult recovery, and toxicity [20], [21], [22]. Therefore, several researchers have attempted to fix CEs which have been introduced into functional groups on various substrates such as fibers and porous materials to extract lithium through grafting or copolymerization. For example, Torrejos et al. [23] immobilized hydroxy-dibenzo-14-crown-4 ether on multi-walled carbon nanotubes to prepare solid-supported CEs as lithium adsorbents. Huang et al. [24] fabricated an effective CE functionalized adsorbent by immobilizing 2-methylol-12-crown-4 on the surface of polymeric glycidyl methacrylate. These studies have revealed the efficient selective adsorption of lithium by CE adsorbents which combined with macromolecular substances. However, the synthesis process of these CE modified adsorbents is usually complicated with a long cycle [25], [26]. Therefore, how to construct a novel CE adsorbent, which not only has the selective and high adsorption capacity of CEs but also easy to be prepared and recycled is an urgent problem we need to solve at present.
CS is the only natural alkaline polysaccharide prepared from the deacetylation of chitin. CS is a light yellow powder. It is insoluble in water and ethanol, but can be dissolved in weak acid solutions [27], [28]. The source of CS is abundant and itself is green and non-toxic. In addition, CS is easy to complex with heavy metal ions because of a large number of active groups such as amino and hydroxyl groups on the surface of CS [29], [30]. Furthermore, whether in the form of film or particle, CS can complex with metal ions to generate stable complex compounds, so as to capture metal ions. Thus, CS has been the primary choice for the adsorption of metal ions [31], [32]. Especially, CS nanofibers have been widely applied as a metal ion adsorbent in many studies because of its high specific surface area [33]. Electrospinning technology is a common method for preparing nanofiber materials, while the fabrication process usually connected with high concentration organic solvents such as polyvinyl alcohol and dichloromethane, which are corrosive and toxic [34]. Therefore, seeking for a simpler and greener preparation method of chitosan nanofibers to obtain a high specific surface area is the key to improving the adsorption capacity of the adsorbents.
In this study, we developed a simple route to produce a novel CE modified CS nanofiber membrane. Firstly, we immobilized 2H12C4 on the surface of CS by the ring opening reaction of epichlorohydrin (ECH). Then, a green low-temperature phase separation method was performed to make CE modified CS nanofiber composite membranes. As far as we know, this research on CE modified CS nanofiber membranes for the recovery of lithium is reported for the first time. The characteristics of CS-CE were analyzed via Scanning Electron Microscopy (SEM), Fourier Transform Infrared Spectrometry (FT-IR), X-ray photoelectron spectroscopy (XPS), Brunauer-Emmett-Teller (BET) and Thermogravimetric Analysis (TGA). A chain of experiments on static adsorption performance were carried out to explore the adsorption behaviors, including pH effects, adsorption kinetics, adsorption isotherm, adsorption thermodynamics, selectivity, and regeneration, which were discussed in detail in the results and discussion section.
Section snippets
Sample materials and reagents
CS was supplied by Saan Chemical Technology CO., LTD (Shanghai, China) with a degree of deacetylation more than 95%. The molecular formula of CS is (C6H11NO4)n and the molecular weight of CS unit is about 161.2 [35]. 2H12C4 was purchased from TCI Chemical Industry Development CO., LTD (Shanghai, China). ECH was purchased from Shanghai LingFeng Chemical Reagent CO., LTD. Sodium hydride (NaH) (60% dispersion in mineral oil), potassium bromide and lithium chloride (anhydrous grade, 98%) were
Analysis of the reaction mechanism
As a strong base, the negatively charged hydrogen ion (H−) on NaH could capture the positively charged hydrogen ion (H+) of the active hydroxyl group (OH) on 2H12C4 to form oxygen negative ions (O−). At the same time, under alkaline conditions, the nucleophilic reagent, OH and NH2 directly attacked the two carbon atoms adjacent to the oxygen atom in the epoxy group. It is generally preferred to attack the side with fewer substituents because of the small steric hindrance. This behavior induced
Conclusions
In summary, the characterization of CS-CE showed that we had successfully prepared a novel CE modified CS nanofiber membrane with a high specific surface area through a simple and environmentally friendly low-temperature phase separation method. As a result, the specific surface area of this film was 111.55 m2 g−1, which was higher than many other lithium sorbents. In static adsorption experiments for Li+, CS-CE reached the maximum adsorption capacity with 297 mg g−1 at pH 7.0 higher than
CRediT authorship contribution statement
Qian Cheng: Conceptualization, Methodology, Formal analysis, Writing - original draft, Writing - review & editing. Yuzhe Zhang: Validation, Resources, Supervision, Funding acquisition. Xudong Zheng: Investigation, Writing - review & editing, Supervision, Funding acquisition. Wen Sun: Project administration. BoTao Li: Project administration. Dandan Wang: Investigation, Project administration. Zhongyu Li: Resources, Funding acquisition.
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
This work was financially supported by National Natural Science Foundation of China (No. 21876015, No. 21808018, No. 21822807, and No. 22008014), Science and Technology Support Program of Changzhou (No. CE20185015), Natural Science Research Project of Jiangsu Province (No.18KJB610002), Postgraduate Research & Practice Innovation Program of Jiangsu Province (No. KYCX20_2579, No. KYCX20_2592 and No. KYCX20_2595). The authors would like to thank Wang Liang from Shiyanjia Lab (www.shiyanjia.com)
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