Direct regeneration of hydrogels based on lemon peel and its isolated microcrystalline cellulose: Characterization and application for methylene blue adsorption
Graphical abstract
Introduction
Recently, the biopolymer-based materials (e.g., cellulose, chitosan, and starch) have captured increasing interests due to their low cost, biodegradability, easy availability, sustainability, nontoxicity, and renewability [1], [2]. Among these biopolymers, cellulose is the most abundant natural polymer on earth that can be sustainably produced from lignocellulosic biomass, showing a total annual biomass production of approximately 1.5 × 1012 tons in the world [3]. In addition to processing into common products such as paper and textiles, some high value-added cellulose-based products such as hydrogels, emulsions, adsorbents, nanocomposites, and carriers have also been applied in food [4], energy [5], water purification [6], pharmaceutical and biomedical applications [1], due to advantageous properties of cellulose (e.g., excellent biocompatibility, nontoxicity, biodegradability and great mechanical strength) [7], [8]. Besides cotton and wood pulp as main sources for the industrial processing of cellulose [9], some low-cost agricultural by-products such as straw, leaves, corn cob, and fruit peels can also be utilized as cellulosic resource for isolating cellulose with different morphological and structural properties [7], [10], whose efficient utilization rather than being discarded as waste is conducive to solving shortages in global resources and eco-sustainability issues.
Cellulose is a linear homopolymer consisting of β-1,4 linked glucopyranose units and contains numerous intra- and inter-molecular hydrogen bonds forming highly crystalline domains, thus hampering its dissolution in water and common organic solvents [11]. Recently, tremendous efforts have been devoted to broadening the range of applications of cellulose by increasing its processability and functionality, such as dissolution [12], modification (e.g., esterification, etherification, phosphorylation, and silylation) [13], and micro/nano processing (i.e., microcrystalline cellulose and nanocellulose) [4], [9]. Among these approaches, direct dissolution of cellulose and then fabricating cellulose-based materials have attracted considerable attention, especially in constructing the eco-friendly cellulose-based materials (e.g., hydrogel and film) by using a facile dissolution-regeneration method [14]. Ionic liquids (ILs) are the most promising green solvents for dissolving and processing cellulose currently, whose popularity is mainly attributed to their advantages of high solubility for cellulose, low melting points, negligible vapor pressure, recyclability, low toxicity, and high chemical and thermal stability [15]. The dissolution mechanism of cellulose in ILs can be summarized as the formation of electron donor-electron acceptor (EDA) complexes that involves the interactions between oxygen and hydrogen atoms of cellulose-OH with the loosely bounded anions of ILs, consequently resulting in cellulose dissolution by opening of the hydrogen bonds between molecular chains of the cellulose [16]. After subsequent coagulation in anti-solvent (e.g., water, ethanol, and methanol), the hydrogen bonds between cellulose and ILs are disrupted and then reformed hydrogel by cellulose-cellulose hydrogen bonds, eliminating the demands of chemical cross linkers such as epichlorohydrin, glyoxal, and glutaraldehyde [17].
Compared with pure cellulose, developing hydrogel materials directly from lignocellulosic biomass must be a more promising strategy since the isolation of cellulose generally involves a series of harsh separation and purification processes and generates undesirable side products [18], [19]. The consumption of bleaching chemicals can be effectively limited when using lignocellulosic biomass as a raw material, which is more environment-friendly and less expensive than using pure cellulose. The main macromolecular components (i.e., cellulose, hemicelluloses, and lignin) in lignocellulosic biomass contain abundant hydroxyl functional groups that endows it with hydrogel formulation potential [20]. The dissolution, deconstruction and reorganization of these components can be easily achieved by constructing hydrogels in ILs. However, attempts on this approach are scarce and have only recently been reported in the literature. Our previous work found that lignin and hemicellulose can affect the structure of pineapple peel cellulose hydrogels regenerated in BmimCl. Similarly, Kalinoski and Shi [21] found that the presence of lignin and xylan improved the mechanical strength and antimicrobial properties of cellulosic hydrogel. Shen et al. [22] reported that the presence of lignin in lignocellulosic hydrogel increased its adsorption ability. However, it should be noted that dissolving lignocellulose biomass (especially fruit or vegetable waste) in ILs and then forming regeneration precipitates are not always coagulated into hydrogels or just form a collapsible hydrogel structure due to relatively low cellulose content [23]. Currently, high content of cellulose is necessary for constructing lignocellulosic hydrogels through dissolution-regeneration method, in which microcrystalline cellulose (MCC) has great potential as a structural skeleton due to its high mechanical strength and crystal structure.
Lemon (Citrus limon) is the third most important cultivated citrus species after orange and mandarin, with a production of 7.3 million tons around the world annually [24]. The consummation of lemon mainly includes fresh eating and processing into juice, lemonade and dried lemon slices, which not only brings nutrition but also simultaneously generates enormous solid wastes accounting for 57% of the total lemon weight [25]. The discarded lemon wastes (i.e., peels, seeds, and pulps) are generally left for natural biodegradation and ultimately result in environmental pollutions. The peel and seed of lemon contains more than 20% of cellulose. Our previous studies have focused on the nanocellulose derived from lemon seed and its stabilized Pickering emulsions, showing a good potential as cellulose-based products [26], [27]. However, there is little information available about the utilization of lemon wastes as a potential lignocellulose for constructing eco-friendly hydrogels.
Herein, we aimed to fabricate lignocellulose hydrogels based on lemon peel (LP) and its isolated microcrystalline cellulose (LPMCC) by a facile and green dissolution-regeneration method, using BmimCl as a solvent and distilled water as a regenerated solvent. The LPMCC in hydrogels can act as a structural backbone to enhance the hydrogel network and the main components in LP (i.e., cellulose, hemicellulose, and lignin) can act as a structure modifier. The hydrogels designed as low-cost biosorbents were applied to the removal of methylene blue (MB), which is selected herein because of its high solubility, health hazards and widespread application in textile and paper industries [28]. The influence of various LP addition on the structure and MB adsorption of the hydrogels was comparatively investigated. Up to now, the efficient fabrication of hydrogels directly from the combination of agricultural wastes and its isolated MCC has been rarely reported. Accordingly, such simple combination and dissolution- regeneration to design eco-friendly hydrogel would be an attractive and meaningful approach for high utilization of agricultural wastes.
Section snippets
Materials and reagents
Lemon (Citrus limon) peel as a major waste of lemon industry was kindly provided by Chongqing Huida Lemon Processing Group Co., Ltd. (Chongqing, China). BmimCl was purchased from Lanzhou Institute of Chemistry Physics, Chinese Academy of Sciences (Lanzhou, China). MB was supplied by Guangzhou Chemical Reagent Co., Ltd. (Guangzhou, China). All other chemicals and solvents used in this study were of analytical grade.
Pretreatment of LP and its chemical composition
The freshly collected LP was washed with distilled water and then crushed with a
FTIR analysis
FTIR spectra of LP, LPMCC, LPMCCH and LPMCC/LPH hydrogels are displayed in Fig. 1. Compared with LP and LPMCC, all the hydrogels maintained the characteristic bands of their initial components without showing new peaks, indicating a physical dissolution process and cross-linking of LPMCC and LP in BmimCl [21]. For LP and LPMCC/LPH hydrogels, there are characteristic bands at 1740 cm−1 (acetyl ester/carbonyl aldehyde groups of hemicellulose/lignin), 1600–1450 cm−1 (CC aromatic skeletal
Conclusion
This work developed fabrication of hydrogels based on lemon peel (LP) and its isolated MCC (LPMCC) using a green and facile dissolution-regeneration method. The hydrogel retained the non-cellulosic component upon solidification in the hydrogels due to hydrogen bonding interactions. Various LP addition resulted in the changes of structure and properties of the hydrogels, including the improvement of thermal stability and the formation of more uniform sheet-like skeletons with porous morphology.
CRediT authorship contribution statement
Hongjie Dai: Methodology, Investigation, Visualization, Writing-Original Draft, Funding acquisition. Yuan Chen: Investigation, Visualization. Liang Ma: Investigation, Resources, Formal analysis. Yuhao Zhang: Conceptualization, Funding acquisition. Bo Cui: Methodology, Writing-Review & Editing.
Declaration of competing interest
The authors declare that they have no conflict of interest.
Acknowledgments
This study was supported by the Foundation of State Key Laboratory of Biobased Material and Green Papermaking, Qilu University of Technology, Shandong Academy of Sciences (No. KF201923), National Natural Science Foundation of China (No. 31901683), and Natural Science Foundation of Chongqing, China (No. cstc2020jcyj-msxmX0087), and Chongqing Young Eagle Project of China (CY210202).
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