Sodium alginate/graphene oxide aerogel with enhanced strength–toughness and its heavy metal adsorption study

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Abstract

Ordered porous sodium alginate/graphene oxide (SAGO) aerogel was fabricated by in situ crosslinking and freeze-drying method. GO, as reinforcing filler, can be easily incorporated with SA matrix by self-assembly via hydrogen bonding interaction. Compared with pure SA aerogel, the as-prepared SAGO exhibited excellent mechanical strength and elasticity, and the compression strength of SAGO can reach up to 324 kPa and remain 249 kPa after five compression cycles when 4 wt% GO was added, which were considered significant improvements. SEM result presents that the addition of GO obviously improves the porous structures of aerogel, which is beneficial for the enhancement of strength–toughness and adsorbability. As a consequence, the adsorption process of SAGO is better described by pseudo-second-order kinetic model and Langmuir isotherm, with maximum monolayer adsorption capacities of 98.0 mg/g for Cu2+ and 267.4 mg/g for Pb2+, which are extremely high adsorption capacities for metal ions and show far more promise for application in sewage treatment.

Introduction

Generally, for most ionic polysaccharides, the ability to bind divalent cations and form gels is the key to their biological functions and technological applications [1]. As natural polysaccharide derived from brown sea algae, sodium alginate (SA) is a linear polyanionic copolymer composed of (1–4)-linked β-d-mannuronic acid (M) and α-l-guluronic acid (G) residues [2], [3]. The gelling of SA is mainly achieved by the exchange of Na+ from the G residues with the divalent cations (such as Ca2+). The divalent cations bind to different chains of G blocks to form a structure like an “egg box” [4], [5], resulting in a three-dimensional network between the cross-linking of different chains. Because of its reversible solubility, SA can be fabricated in various forms such as films, fibers, beads and aerogels [6], [7], [8], [9], [10], [11].

Recently, there is a growing interest in using biopolymers for aerogel production, which dues to that the resulting aerogels reveal both specific inheritable functions of the initial biopolymer and distinctive features of aerogels (open porous structure with high specific surface and pore volume) [12]. The synergy of properties has prompted to view biopolymer aerogels as promising candidates for versatile applications. SA aerogel has a great many of favorable properties such as hydrophilicity, biocompatibility, biodegradability, strong ion-exchange and gel-forming abilities, holding great promises for tissue engineering [13], [14], [15], drug delivery [16], [17], [18], sewage treatment [19], [20], thermal insulation [12] and as starting materials for carbon aerogels [21]. Apart from above advantages, pure SA aerogels still display some structural unsatisfactory properties in weak mechanical strength, structural nonuniformity and fragile collapse [22], which will limit their applications in many fields.

To handle this problem, an innovative technology that has gained attention is the addition of reinforcing fillers, which has been considered to be an effective method for improving the mechanical performance and toughness of aerogels. Among popular fillers, graphene oxide (GO) exhibits great potential due to its outstanding mechanical properties, high binding potential, high aspect ratio, excellent flexibility and superior processability. GO sheets can be easily produced by thermal oxidation as suggested by Hummers. This procedure introduces abundant oxygen-containing functional groups (hydroxyl, carboxyl, epoxy and ketone groups) [23], [24], which facilitate the interfacial interaction between GO sheets and hydrophilic matrix via hydrogen bond, ionic bond and covalent bond. So GO sheets have been increasingly proved to be ideal reinforcing fillers for composites [25], [26], [27]. However, another challenge still exists, namely that the gelation process of SA is so quick that the formation time of several seconds often results in brittle structure and heterogeneity of pore size [6], [22].

Based on the consideration, the SA/GO (SAGO) aerogel possessing homogeneous porous structure with high mechanical strength and favorable resilience was obtained via in situ crosslinking induced by d-glucono-δ-lactone (GDL)-driving-Ca2+-release process in the presence of GO, and the unidirectional homogeneous porous structure was achieved by freeze-drying treatment. And its surface morphology, mechanical property and adsorption capacity for metal ions were investigated.

Section snippets

Materials

SA (medium viscosity, M = 250,000 g/mol) was purchased from Sigma–Aldrich Shanghai Trading Co., Ltd (Shanghai, China) and it was used without further purification. Graphite, CaCO3 and d-glucono-δ-lactone (GDL) were obtained from Sinopharm Chemical Reagent Co., Ltd (Suzhou, China). Cu(NO3)2·3H2O and Pb(NO3)2 served as the sources of Cu2+ and Pb2+ and were acquired from Meryer Chemical Technology Co., Ltd (Shanghai, China).

Preparation of GO

GO was prepared from purified natural graphite using a modified Hummers’

Characterization of GO

Fig. 1 shows the photos and AFM image of GO suspensions that were used to prepare the monolithic SAGO aerogel. The exfoliated GO can be readily dispersed in deionized water with mild ultrasonic treatment and formed transparent suspensions that can maintain for several months. AFM image demonstrates that the GO sheets consist of one or several layers with each thickness of ca. 1.2 nm. These well dispersed GO suspensions are very useful for fabricating monolithic aerogels with enhanced mechanical

Conclusions

In conclusion, GO sheets were used as reinforcing fillers for improving the mechanical performance including elasticity of neat SA aerogel. A series of three-dimensional mold-shaped SAGO aerogels were prepared via in situ crosslinking induced by GDL as a gelation promoter. With 4 wt% GO incorporation, the compressive strength can achieve 324 kPa and still remain 249 kPa at the fifth compression cycle. The outstanding mechanical strength and satisfactory elasticity can be attributed to the strong

Acknowledgements

The authors are grateful for the financial support by the National Natural Science Foundation of China (No. 51403141) and Natural Science Foundation of Jiangsu Province, China (No. BK20140347).

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