Development of an adsorbent via chitosan nano-organoclay assembly to remove hexavalent chromium from wastewater

https://doi.org/10.1016/j.ijbiomac.2016.09.111Get rights and content

Abstract

This study involves nano-biocomposite (chitosan-nanoclay composite, CNC) preparation using solvent casting method with chitosan and organoclay (Cloisite 20A) and investigation of Cr6+ ion adsorption. The influence of contact time, solution pH, initial metal concentration, concentration of the adsorbents and temperature on the removal of Cr6+ ions were examined. Langmuir, Freundlich and Scatchard adsorption isotherms were tested. Maximum adsorption capacity of 128.43 mg of Cr6+/g for CNC and 21.83 mg of Cr6+/g for chitosan were obtained according to the Scatchard adsorption isotherm at pH of 2 and 3, respectively. Obtained results showed that the adsorption data for Cr6+ ions on CNC correlated well with the Langmuir isotherm model.

Introduction

In recent years, nano-biocomposites with enhanced properties have been considered as promising alternatives for industrial applications. Nano-biocomposites consist of a biopolymer matrix incorporated with particles (nanoparticles) with at least one dimension in the nanometer range. The polymer matrices used for nano-biocomposite synthesis are biodegradable polymers such as starch, cellulose, poly(lactic acid), poly(hydroxy alkanoates), pectin, chitosan, etc. These enable a wider range of disposal options with minor environmental effects possible [1]. However, unlike synthetic plastic-based nanocomposites, in the relevant literature, the number of reports about generation and application of nano-biocomposites in adsorption process is quite scarce. Hence, the aim of the study is the preparation of chitosan–nanoclay composite (CNC) to be used in water treatment technologies.

Chitosan, a cationic biopolymer, is commercially produced by alkaline N-deacetylation of chitin, the main component of the shells of crab, shrimp, and krill [2], [3], [4], [5]. It is characterized by its acetylation degree and molecular weight. Industrial chitosan has molecular weights ranging from 5000 to 1,000,000 gmol−1. The acetylation degrees can change from 2 to 60% according to the producer. Its chemical structure, represented in Fig. 1, is a random linear chaining of N-acetyl-d-glucosamine units (acetylated unit) and d-glucosamine (deacetylated unit) linked by β (1  4) linkages. Since it contains amino groups, chitosan has some particular properties. It is protonated in acidic to neutral solutions generating a pH-dependent charge density [1], [6]. The main advantages of chitosan are its low cost [7], environmental friendly feature, [8] and apparent natural abundance. Hence, chitosan-based materials have been studied in the literature for its chelating properties and consequently for the detoxification of effluents [9], [10]. The high hydrophilicity of chitosan and presence of a large number of hydroxyl groups of glucose units and functional groups (acetamido, primary amino and/or hydroxyl groups) in its structure make this material suitable for heavy metal adsorption and thus water treatment [6]. The reactive amino group selectively binds transition metal ions but not alkali and alkaline earth metal ions [11].

Clays are cost-effective for immobilizing toxic contaminants as they are low-cost, readily available and stable, and have high adsorptive and ion exchange properties [12]. Using various chemical/physical methods, clay can be modified as desired to achieve the surface properties for effective immobilization of specific compounds labeled as organoclay. Organoclays have been studied for their unique sorption behavior of both hydrophobic organic and ionic contaminants [12], [13]. The presence of heavy metals, ionic contaminants in the environment is a major problem due to their toxicity and threat to environment, human life, and other living beings. In order to protect, human beings and other living cells, toxic heavy metals are to be removed from the wastewater. Various methods such as ion-exchange, membrane filtration, chemical precipitation, adsorption, electrochemical treatment technologies are used to remove heavy metal ions [14]. Chromium is the most common heavy metal as a groundwater contaminant and it occurs in two oxidation states; Cr6+ such as chromate and bichromate (CrO4 2−, HCrO4) and trivalent chromium (Cr3+). Cr6+ is more carcinogenic and toxic to humans than Cr3+ [15]. In order to make a contribution to the remediation of wastewater, several studies are realized for the removal of hexavalent chromium by chitosan and organoclay; yet, separately [6], [16], [17], [18].

In water treatment research, most natural aluminosilicate clays are highly hydrophilic and consequently show very low adsorption for hydrophobic organic contaminants [19]. However, organoclays used in this study, can function as a sorption sink for toxic metals. Organoclays are not common in this field due to the current limited understanding of the interaction between clay and metal ions.

In this respect, the main aim of this study is to produce a new nano-biocomposite based on organic-inorganic hybrid of chitosan-nanoclay and evaluate its adsorption capacity. For this purpose, organoclay (Cloisite 20A, quaternary ammonium modified natural montmorillonite) is selected to prepare a new nano-biocomposite as an adsorbent.

Section snippets

Experimental materials

All chemicals were of analytical grade and aqueous solutions of Cr6+ were prepared by dissolving K2Cr2O7 powder with deionized water. 0.1 M HCl and 0.1 M NaOH solutions were used to adjust the pH of the solution phase. Batch experiments were carried out to determine the adsorption in a plastic container made of polypropylene.

Average particles size of chitosan flakes were around 1300 μm. The experiments were carried out without further pretreatment of chitosan flakes. Chitosan with high molecular

X-ray diffraction analysis (XRD)

Fig. 2 shows XRD patterns of chitosan flakes, chitosan-nanoclay composite (CNC), Cr6+ loaded chitosan and Cr6+ loaded CNC.

Chitosan (Fig. 2a) has characteristic peak at 2θ = 20.42°. The XRD pattern of the Cr6+ loaded chitosan (Fig. 2c) is similar to chitosan. This indicates that no chemical reaction takes place during the process. On the other hand, the XRD pattern of composite (Fig. 2b), CNC, shows two obvious diffraction peaks around 2θ = 19.87° and 22.09°. Obviously, the intensity of the peak

Conclusions

This work showed that the adsorption of Cr6+ ions from aqueous solutions using chitosan and nano-biocomposite, CNC. The adsorption process has been interpreted in terms of the Freundlich, Langmuir and Scardthard isotherm models. The research of adsorption isotherm demonstrated that, adsorption reactions of nano-biocomposite belonged to Langmuir model and Freundlich model was suitable for neat chitosan. The maximum adsorption capacity of 128.43 mg of Cr6+/g for CNC and 21.83 mg of Cr6+/g for

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