Adsorption of acid dyes from aqueous solutions by the ethylenediamine-modified magnetic chitosan nanoparticles

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Abstract

The adsorption characteristics of Acid Orange 7(AO7) and Acid Orange 10 (AO10) from aqueous solutions onto the ethylenediamine-modified magnetic chitosan nanoparticles (EMCN) have been investigated. The EMCN were essentially monodispersed and had a main particle size distribution of 15–40 nm and saturated magnetization of 25.6 emu/g. The adsorption experiments indicated that the maximum adsorption capacity occurred at pH 4.0 for AO7 and pH 3.0 for AO10, respectively. Due to the small diameter and the high surface reactivity, the adsorption equilibrium of AO7 and AO10 onto the EMCN reached very quickly. Equilibrium experiments fitted well with the Langmuir isotherm model, and the maximum adsorption capacity of the EMCN at 298 K was determined to be 3.47 mmol/g for AO7 and 2.25 mmol/g for AO10, respectively. Thermodynamic parameters such as enthalpy change (ΔH°), free energy change (ΔG°) and entropy change (ΔS°) were estimated and the results indicated that the adsorption process was spontaneous and exothermic. Furthermore, the EMCN could be regenerated through the desorption of the dyes in NH4OH/NH4Cl solution (pH 10.0) and could be reused to adsorb the dyes again.

Introduction

Most of dyes released during textiles, clothing, printing, and dyeing processes are considered as hazardous and toxic to some organisms and may cause allergic dermatitis, skin irritation, carcinogenic and mutagenic to human and aquatic organisms [1]. However, these dyes in wastewaters are difficult to remove, because they have recalcitrant molecules (particularly azo dyes with an aromatic structure), are resistant to aerobic digestion, and are stable to oxidizing agents. Several techniques are available for the treatment of the dyes such as an electrochemical technique destroying the colour groups [2], a bio-degradation process mineralising the colourless organic intermediates [3], chemical oxidation including homogeneous and heterogeneous photocatalytic oxidation [4], [5]. Among the many techniques of dye removal, adsorption is the procedure of choice and gives the best results as it can be used to remove different types of dyes [6], [7].

Adsorption of different dyes has been studied using low cost adsorbents such as activated carbon [8], kaolin [9], montmorillonite clay [10], waste red mud [11], Fullers earth and fired clay [12]. Currently chitosan is being greatly exploited because it is relatively cheap and exhibit higher adsorption capacities [13], [14]. Chitosan is a cationic biopolymer obtained from alkaline N-deacetylation of chitin, the second most abundant biopolymer in nature. Chitosan exhibits a higher adsorption capacity and faster adsorption rate of anionic dye pollutants than many conventional adsorbents due to the presence of large amounts of amino (−NH2) groups [13]. In acidic solution, the amino groups of chitosan are easily protonated and can bind anionic dye anions. Chitosan resins are highly selective, efficient and easily regenerable relative to other adsorbent materials. The use of chitosan resins for the removal of dyes from aqueous solutions was recently reported by several authors [6], [14], [15], [16], [17].

Chitosan is usually needed to be cross-linked to improve its chemical stability in acid media. Although the crosslinking method may enhance the resistance of chitosan against acids, the process may reduce its adsorption capacity of dyes, especially when the crosslinking procedure involves in the reaction of amino groups, which are expected to play a great part in the adsorption process. In order to improve the adsorption capacity and selectivity of dyes, a number of chitosan derivates have been obtained by grafting functional groups such as acrylic and acrylamide [14], poly(methylmethacrylate) [15], poly(alkyl methacrylate) [16], and vinyl acetate [17] through a crosslinked chitosan back bone.

Most of the chitosan-based adsorbents were submicron to micron-sized and need large internal porosities to ensure adequate surface area for adsorption. However, the diffusion limitation within the particles led to the decreases in the adsorption rate and available capacity. Compared to the traditional micron-sized supports used in separation process, nano-sized adsorbents possess quite good performance due to high specific surface area and the absence of internal diffusion resistance [18], [19]. However, the nano-adsorbents could not be separated easily from aqueous solution by filtration or centrifugation. Magnetic nano-adsorbents can be manipulated by an external magnetic field and hence facilitate phase separation.

Several studies have indicated that –NH2 groups in chitosan are the main sites for the adsorption of dyes containing D-SO3 groups through ionic interactions of the colored dye ions with the protonated amino groups on the chitosan [6], [13], [14]. In this work, the magnetic chitosan nanoparticles (MCN) were prepared and then modified with ethylenediamine (EMCN) to increase the –NH2 active groups and thus enhance the adsorption capacity for acid dyes. The adsorption behaviour of the EMCN toward Acid Orange 7 and Acid Orange 10 was studied. The equilibrium isotherms and thermodynamic parameters were determined and discussed.

Section snippets

Chemicals and reagents

Chitosan with 40 mesh, 90% degree of deacetylation and molecular weight of 1.3 × 105 was purchased from Yuhuan Ocean Biology Company (Zhejiang, China). Glutaraldehyde, epichlorohydrine, ethylenediamine, Acid Orange 7 and Acid Orange 10 were supplied by Aldrich and Sigma Chemical, and were used without any further purification. All the other reagents used in this work were of analytical grade. The structures of dye Acid Orange 7 and Acid Orange 10 are presented in Fig. 1, and the chemical

Characterization of EMCN

The TEM micrograph (Fig. 2) showed that the EMCN were essentially monodispersed and had a main particle size distribution of 15–40 nm. The magnetization measurement performed with VSM (Fig. 3) indicated that the saturation magnetization of the EMCN was 25.6 emu/g. As mentioned in a previous report, this magnetic susceptibility value is sufficient for this resin to be used in wastewater treatment [21]. Meanwhile, the insert figure in Fig. 3 indicated that nanoparticles had the characteristics of

Conclusion

This research have demonstrated that the EMCN can be used for the effective adsorption of AO7 and AO10 from aqueous solution. The interest in using the EMCN for removal or recovery of dyes from aqueous solutions is related to their unique characteristics, such as very large surface area and high surface reactivity. In addition, the EMCN is easily seperated by the external magnetic field, by this way the problem of phase seperation using the traditional adsorbents can be resolved. The EMCN

Acknowledgments

This work was supported by the Science & technology Pillar Program of Jiangxi, China (No. 2009BSB08600), and the scientific research fund from the Education Bureau of Jiangxi, China (No. GJJ10494).

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