Chitosan-coated mesoporous microspheres of calcium silicate hydrate: Environmentally friendly synthesis and application as a highly efficient adsorbent for heavy metal ions

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Highlights

Abstract

Chitosan-coated calcium silicate hydrate (CSH/chitosan) mesoporous microspheres formed by self-assembly of nanosheets have been synthesized in aqueous solution under ambient conditions without using any toxic surfactant or organic solvent. The method reported herein has advantages of simplicity, low cost and being environmentally friendly. The BET specific surface area of CSH/chitosan mesoporous microspheres is measured to be as high as ∼356 m2 g–1, which is considerably high among calcium silicate materials. The as-prepared CSH/chitosan mesoporous microspheres are promising adsorbent and exhibit a quick and highly efficient adsorption behavior toward heavy metal ions of Ni2+, Zn2+, Cr3+, Pb2+ Cu2+ and Cd2+ in aqueous solution. The adsorption kinetics can be well fitted by the pseudo second-order model. The maximum adsorption amounts of Ni2+, Zn2+, Pb2+, Cu2+ and Cd2+ on CSH/chitosan mesoporous microspheres are extremely high, which are 406.6, 400, 796, 425 and 578 mg/g, respectively. The CSH/chitosan adsorbent exhibits the highest affinity for Pb2+ ions among five heavy metal ions. The adsorption capacities of the CSH/chitosan adsorbent toward heavy metal ions are relatively high compared with those reported in the literature.

Introduction

It is well known that the waste water containing heavy metal ions as pollutants is a serious threat to plants, animals and human beings owing to their bioaccumulation, nonbiodegradability and toxicity even at low concentrations. Therefore, efficient and rapid removal of toxic heavy metal ions from the waste water by appropriate technologies has been a crucial issue [1]. The conventional technologies for the removal of heavy metal ions from aqueous solution include chemical precipitation, ion exchange, electrochemical treatment, and adsorption and so on. Among these methods, adsorption is attractive due to its advantages of low cost, high efficiency and simple operation. The common adsorbents primarily include activated carbon, zeolites, biomacromolecules, and polymeric materials and so on. However, some adsorbents suffer from low adsorption capacities and long operation time. Therefore, research is urgently needed to explore new promising adsorbents for heavy metal ions in waste water.

Chitosan is a linear polysaccharide composed of randomly distributed β-(1-4)-linked D-glucosamine (deacetylated unit) and N-acetyl-D-glucosamine (acetylated unit). Chitosan is usually obtained by alkaline deacetylation of chitin such as shrimp and other crustacean shells. Chitosan has received considerable attention in the removal of heavy metal ions owing to its low cost, easily obtainable sources and excellent adsorption capacity. Functionalized chitosan materials show a superior adsorption capacity for the treatment of waste water containing heavy metal ions and organic pollutants [2]. Several examples have been reported for the application of chitosan materials in waste water treatment [3], [4], [5]. These studies show that chitosan and its derivatives can be used as adsorbents for waste water treatment. However, the solubility and low stability are the main disadvantages for the applications of chitosan materials. Hence, various chemical and physical methods have been developed to improve the solubility and stability of chitosan. For example, the composites by coating chitosan on supporting inorganic materials like silica gel, and chitosan-based composite nanofibers have been successfully prepared and investigated for the application in adsorption of metal ions [6], [7].

Calcium silicates are one kind of the biomaterials which have received considerable attention owing to their excellent bioactivity and biocompatibility. Some studies have been carried out to explore their applications in drug delivery [8], [9], [10], bone tissue engineering [11] and adsorption for heavy metal ions [9]. It has been reported that the Al-substituted tobermorite is a promising sorbent for Co2+, Cd2+ and Zn2+ ions in acidic aqueous media [12]. Chen et al. [13] reported that the adsorption capacities of carbonated tricalcium silicate for Pb2+, Cr3+, Zn2+ and Cu2+ ions were measured to be 94.4, 83, 52.1 and 31.4 mg g–1, respectively. In this paper, calcium silicate hydrate (CSH) mesoporous microspheres are selected as the supporting material for chitosan coating owing to their large specific surface area and high adsorption capacity [8].

Mesoporous materials have been extensively studied due to their outstanding performance in adsorption, drug delivery, catalysis and medical treatment [14], [15]. Generally, surfactants are used as templates to obtain the mesoporous materials, but the residual surfactants may result in cytotoxicity for subsequent applications [16], [17]. In this study, we have developed a simple, low-cost and environmentally friendly aqueous solution method for the synthesis of chitosan-coated calcium silicate hydrate (CSH/chitosan) mesoporous microspheres constructed by self-assembly of nanosheets with a very high specific surface area of 356 m2 g–1 without using any toxic surfactant or organic solvent, and the product has been explored as the highly efficient adsorbent for heavy metal ions in waste water treatment. The as-prepared CSH/chitosan mesoporous microspheres show very high adsorption capacities for the removal of heavy metal ions in aqueous solution. The CSH/chitosan adsorbent exhibits the highest affinity for Pb2+ ions among five heavy metal ions investigated in this work. To the best of our knowledge, this is the first report on the highly efficient removal of heavy metal ions from aqueous solution using chitosan-coated CSH mesoporous microspheres with a high specific surface area.

Section snippets

Preparation of CSH/chitosan nanocomposite

Calcium silicate hydrate (CSH) mesoporous microspheres were prepared according to a method previously reported by this research group [8] with a modification. Briefly, Ca(NO3)2·4H2O (3.540 g) was dissolved into 500 mL deionized water, then, 5 mL 4 M NaOH aqueous solution and 2 mL tetraethyl orthosilicate (TEOS) were separately added into the above solution. The resulting solution was magnetically stirred and ultrasonically irradiated for 1 h under ambient conditions with a high-intensity ultrasonic

Characterization of CSH/chitosan mesoporous microspheres

Both SEM and TEM micrographs of the CSH/chitosan product in Fig. 1 exhibit the morphology of relatively uniform mesoporous microspheres with sizes of about 1 μm. The high-magnification SEM micrograph (Fig. 1b) shows that each CSH/chitosan mesoporous microsphere is constructed by self-assembly of nanosheets with a mesoporous structure.

Fig. 2 shows the N2 adsorption–desorption isotherms (Fig. 2a) and pore size distribution curve (Fig. 2b) of the as-prepared CSH/chitosan mesoporous microspheres.

Conclusions

In summary, chitosan-coated CSH mesoporous microspheres with a high specific surface area have been prepared by a fast, low-cost and environmentally friendly method in aqueous solution in the absence of any toxic surfactant or organic solvent. The as-prepared CSH/chitosan mesoporous microspheres are constructed by self-assembly of nanosheets, forming a mesoporous structure with a high specific surface area of 356 m2 g−1 and an average pore size of 12 nm. The as-prepared chitosan-coated CSH

Acknowledgments

The financial support from the National Natural Science Foundation of China (51172260, 51102258, 51121064), the National Basic Research Program of China (973 Program, No. 2012CB933600), and the Science and Technology Commission of Shanghai (11nm0506600, 12ZR1452100) is gratefully acknowledged.

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