Regular Articleβ-Cyclodextrin functionalized SBA-15 via amide linkage as a super adsorbent for rapid removal of methyl blue
Graphical abstract
Introduction
Methyl blue (MB) is a typical bulky anionic dye, which consists of a triamino-triphenylmethane group and sodium aromatic sulfonate groups (Scheme 1a). Due to its xenobiotic and antiseptic properties, MB is widely used in a number of biological and industrial applications, such as microscopic staining, textile and leather production [1], [2]. The widespread use of MB inevitably leads to emission of wastewater containing MB, and the discharge of MB contained wastewater would be undoubtedly escalated worldwide especially in China while its economy has been experiencing rapid growth. MB present in the industrial effluent, like most other synthetic dyes, is hardly-biodegradable and recalcitrant, posing potential threat to human beings. Therefore, it is urgently needed to remove MB from the related wastewater.
In the last decade, several technologies have been used and evaluated for remediation of MB contaminated water, including chemical degradation,[3] photocatalytic degradation, [4] and adsorption [5], [6], [7]. Among them, adsorption treatment is of special concern due to the merits of high efficiency, simple operation and low cost, etc. There are many existing reports concerning removal of MB from aqueous solution with numerous adsorbents, such as graphene based sorbents, [8], [9], [10] modified clays, [11], [12] inorganic porous microspheres, [13], [14] magnetic particles, [15], [16] metal organic frameworks, [17], [18] and polymeric adsorbents [19], [20]. However, compared to their excellent ability for adsorbing the low-molecular-weight dyes, the above adsorbents mostly exhibited moderate adsorption properties to MB with the molecular weight of 799.8 g·mol−1. The current limitation observed in these most extensively studied microporous adsorbents for removal of common toxic dye can be assigned to a mass transport limitation that the small-sized pores are too narrow to accommodate the bulky MB molecules and thus prone to be completely blocked. Given active site accessibility and easy elution of MB during adsorption/desorption processes, it is desired to fabricate efficient adsorbent using ordered mesoporous materials as matrix.
As one of the most prominent mesoporous silicas, hexagonally ordered silica SBA-15 exhibits many advantages in adsorption-related applications (especially for the large-molecule involved processes).[21] The highly ordered mesoporous structure with uniform pore size distribution facilitates the transfer of bulky molecules from the surface of SBA-15 into the pores, making high occupation of the pore.[22] Thus, SBA-15 has become a promising candidate for the fabrication of high-performance adsorbents. To improve the adsorption capacity of SBA-15, surface modifications have been used successfully in tailoring the surface chemistry. Many organic-functional groups have been applied to improve the adsorption capacity of SBA-15 for organic molecules [23], [24], [25], [26], [27]. Typically, a SBA-15/polyamidoamine dendrimer hybrid which was terminated with amine groups has been used for adsorption of anionic dyes [28]. However, to our best knowledge, few SBA-15 based adsorbent has been applied for MB adsorption.
It is well known that β-cyclodextrin (β-CD), a cyclic oligosaccharide with hydrophobic interior cavity, possesses remarkable ability to form inclusion complexes with a wide range of organic dyes (including MB) through host–guest interaction [29], [30], [31]. In particular, owing to the distinctive advantages such as low cost, easy access and abundance of hydroxyl groups on the external rim, β-CD has been widely explored to synthesize high-performance adsorbents. There have been numerous reports to immobilize β-CD on the matrix of various supports with differing strategies [32], [33]. For example, Kim et al. fabricated a hierarchically porous composite scaffold composed of SBA-15 and reduced graphene oxide functionalized with β-CD to remove bisphenol A from wastewater [34]. However, the tedious fabrication procedure in the above study decreased the synthetic efficiency to some extent. Hence, it is desirable to develop a simple and straightforward approach for constructing β-CD functionalized SBA-15 adsorbent.
Herein, β-CD functionalized SBA-15 via amide linkage (hereafter abbreviated as SBA15-A-CD) comprising multiple functional components was synthesized for MB removal from water according to Scheme 1, including: (1) SBA15: the ordered channels of mesoporous silica matrix that allows post-synthetic functionalization by amino-terminated silane coupling agent and accommodates adsorbate molecules; (2) A: the primary amine functions which were uniformly distributed in the channels of SBA-15 and partially transformed into amide groups in further amination; (3) CD: the pendent β-CD moieties which were chemically tethered to the amino-terminal by amidation. Clearly, SBA15-A-CD can be obtained by a facile, two-step and post-functionalization approach using SBA-15 as the matrix. Specifically, the channel of SBA-15 was amino-terminated through coupling reaction with 3-aminopropyltriethoxysilane, followed by amidation between the amine functions with the carboxyl groups of carboxymethyl-β-CD (CM-β-CD), affording the hybrid composites adsorbent SBA15-A-CD. To verify the successful functionalization of SBA-15 through the post-modification process, these samples were thoroughly characterized by various methods. Besides, in order to confirm the potential applications of the as-prepared hybrid, a series of adsorption experiments were conducted systematically. The adsorption enhancement mechanisms of SBA15-A-CD for removal of MB were also probed through theoretical calculation and 2D NMR technique. Our experiments show that SBA15-A-CD has excellent adsorption ability toward MB and other analogues of the bulky anion dye.
Section snippets
Chemicals
Pluronic P123 (EO20PO70EO20) triblock copolymer and methyl blue (MB) were provided by Sigma-Aldrich Reagent Co., USA. N-(3-Dimethylaminopropyl)-N′-ethyl carbodiimide hydrochloride (EDC⋅HCl, 97%), 4-dimethyl aminopyridine (DMAP, 99%), β-cyclodextrin (β-CD, >99%), Congo red (CR, 98%) and Rhodamine B (RB, 98%) were purchased from Adamas-beta Chemical Reagent Co., China. (3-Aminopropyl) triethoxysilane (APTES, 99%) was obtained from Macklin Chemical Reagent Co., China. Tetraethyl orthosilicate
Surface chemical properties
Firstly, the amination of SBA-15 and the subsequent immobilization of β-CD through amidation was monitored by FTIR. As shown in Fig. 1a, the stretching and bending vibration bands of Si-O-Si appear at 1086 and 799 cm−1, indicating the existence of Si-O-Si framework in the as-synthesized materials.[21] The spectrum on SBA15-A also registers the aliphatic CH stretching vibration (2932 cm−1) and the bending vibration of NH (1547 cm−1). Moreover, compared with SBA-15, the apparent weakening of the
Conclusion
SBA15-A-CD was obtained by a facile two-step procedure involving amino-functionalization of SBA-15, followed by the amidation of CM-β-CD with amine functions. The as-synthesized hybrid adsorbents preserved the ordered mesoscopic structure after sequential modifications including the amine coupling procedure and further immobilization of β-CD, as confirmed by the characterization results. Systematical investigation on the adsorption properties of SBA15-A-CD2 toward MB was conducted. These
CRediT authorship contribution statement
Dongli Li: Investigation, Formal analysis, Writing - review & editing. Kungang Chai: Conceptualization, Supervision, Funding acquisition, Writing - review & editing. Xingdong Yao: Writing - review & editing. Liqin Zhou: Software. Kongyou Wu: Formal analysis. Zhenghui Huang: Investigation. Juntao Yan: Investigation. Xingzhen Qin: Formal analysis. Wei Wei: Formal analysis. Hongbing Ji: Conceptualization, Supervision, Funding acquisition, Writing - review & editing.
Declaration of Competing Interest
The authors declared that there is no conflict of interest.
Acknowledgments
This work was supported by National Natural Science Foundations of China (21868002 and 21961160741), the Natural Science Foundation of Guangxi Province (No. 2018GXNSFAA281206), Specific research project of Guangxi for research bases and talents (AD18126005), and special funding for ‘Guangxi Bagui Scholars’. Theoretical calculations were done using the computing facilities of the High-Performance Computing Center in Guangxi University.
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