Full Length ArticleConstruction of horizontal stratum landform-like composite foams and their methyl orange adsorption capacity
Graphical abstract
Introduction
During the past decade, porous materials have drawn much attention for their extensive applications such as tissue engineering [1], drug delivery [2], environmental protection [3], [4] and so on. Lots of methods have been performed to construct porous materials, among those methods, unidirectional freeze-casting technique (UDFC) stands out because it is friendly to environment, simple to operators, and controllable to designers [5]. As a promising and versatile technique, UDFC introduces neither chemical reactions nor any other compounds during the preparation process [6], [7]. Actually, it is the simplest way to prepare oriented foams so far. During UDFC, a slurry is poured into a mold and then frozen with a uniaxial thermal gradient created by liquid nitrogen or other thermostat. Ice crystals grow along the uniaxial thermal gradient, push the particles or polymers to the gap of grown ice crystals. After the sublimation of the ice crystals, the pores are left as the replica of the ice crystals [8]. Water, which is environment-friendly and economically feasible, is often chose as the solvent. The removal of it can be easily realized by freeze-drying, so that the potential complications (including byproducts or the chemical processes used to remove template) can be avoided [9].
As we know, porous materials with high surface area to volume and well-ordered architectures are good candidates for adsorbents [3], [10]. However, the majority of the foams prepared by UDFC were reported to apply to tissue engineering [1] and biological field [11], [12], only a few studies focused on the adsorption of contaminant. In general, azo dye wastewater is hard to degrade because of its high chroma, high content of organic compounds, and complexity of the components [13], [14]. In addition, azo dyes are known to be toxic, mutagenic and carcinogenic [15]. Hence, it is necessary and important to remove them before discharging. Nowadays, several methods have been reported, but most of conventional methods are either technically complicated or economically unfavorable [16], and the adsorption treatments are still dominant.
For environmental pollutants adsorption, the candidates derived from nature are more preferable. In many studies, organic, inorganic and composite aligned porous materials have been exploited by UDFC, such as alumina [17], [18], silicate cement [19], hydroxyapatite [1], poly(vinyl alcohol) [2], gelatin [11], chitin [20], chitosan-gelatin/graphene oxide [21] and so on.
Chitosan (CS), famous for its non-poisonous, biocompatibility, biodegradability, and adsorption properties, has been employed in UDFC [22]. CS-based composites composed of two or more distinct components will produce new properties in structure or function that pure CS does not have, or boost the excellent properties of pure CS material to obtain unprecedented performances [9], [23]. Rectorite (REC), a kind of layered silicate, has been reported with remarkable intercalation/exfoliation properties. The polymer chains could be intercalated into its interlayer to enhance the interlayer spacing [24]. As a result, REC was often used to mix with polymer, such as lysozyme [25], poly(lactic acid) [26] and CS [27]. Carbon nanotubes (CNTs) with the hollow structures as well as their large specific surface area are used as ideal absorbent. Their adsorption of inorganic and organic contaminants has been proved by experimental studies [28], [29]. Hence, it will be meaningful to prepare ordered porous CS foams with incorporation of REC and CNTs.
Based on above considerations, a series of CS/REC/CNTs foams were fabricated via UDFC in this study. The morphology, composition, structure, surface area, and adsorption capacity of the foams on methyl orange (MO) were examined. Furthermore, the effect factors such as initial pH, adsorbent dose, adsorption time and initial concentration of MO were discussed.
Section snippets
Materials
Chitosan (CS, medium molecular weight) was supplied by Sigma Aldrich Chemical Reagent Co. Ltd. Rectorite (Ca2+-REC) was purchased from Hubei Mingliu Inc., China. Carbon nanotubes (CNTs) were provided by Nanotech Port Co. Ltd., Shenzhen, China. Analytical reagent acetic acid and methyl orange (MO) were purchased from Sinopharm Chemical Reagent Co. Ltd., Shanghai, China. All aqueous solutions were prepared using purified water with a resistance of 18.2 MΩ cm.
Preparation of CS/REC/CNTs blend suspensions
2% CS solution was prepared by
Morphology investigation of the foam
The FE-SEM images were showed in Fig. 1. Obviously, the porous materials with a well-patterned and multilayered films were developed successfully. The beautiful continuous parallel layers could be observed in Fig. 1a and the edge of the layer was similar with a sharp knife, which was analogous to the structure of the horizontal stratum landform. The completely horizontally straight-lined layers remarkably testified that the layers’ construction process via UDFC occurred expeditiously and
Conclusion
To investigate whether the incorporation of CNTs and REC could improve the properties of CS foams, CS/REC/CNTs composite foams were fabricated via UDFC. The predesigned structure was constructed, the horizontal stratum landform-like layers were successful built up and the cross section of lamellas of the foam looked like the wings of a butterfly. The interesting structure might be useful for their further applications.
Besides, the adsorption of the composite foams on MO was systemically
Acknowledgments
The project was funded by National Natural Science Foundation of China (No. 51473125), and partially supported by Natural Science Foundation of Hubei Province of China (Team Project, No. 2015CFA017).
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