Carboxymethyl-β-cyclodextrin conjugated magnetic nanoparticles as nano-adsorbents for removal of copper ions: Synthesis and adsorption studies

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Abstract

A novel nano-adsorbent, carboxymethyl-β-cyclodextrin modified Fe3O4 nanoparticles (CMCD-MNPs) is fabricated for removal of copper ions from aqueous solution by grafting CM-β-CD onto the magnetite surface via carbodiimide method. The characteristics results of FTIR, TEM, TGA and XPS show that CM-β-CD is grafted onto Fe3O4 nanoparticles. The grafted CM-β-CD on the Fe3O4 nanoparticles contributes to an enhancement of the adsorption capacity because of the strong abilities of the multiple hydroxyl and carboxyl groups in CM-β-CD to adsorb metal ions. The adsorption of Cu2+ onto CMCD-MNPs is found to be dependent on pH and temperature. Adsorption equilibrium is achieved in 30 min and the adsorption kinetics of Cu2+ is found to follow a pseudo-second-order kinetic model. Equilibrium data for Cu2+ adsorption are fitted well by Langmuir isotherm model. The maximum adsorption capacity for Cu2+ ions is estimated to be 47.2 mg/g at 25 °C. Furthermore, thermodynamic parameters reveal the feasibility, spontaneity and exothermic nature of the adsorption process. FTIR and XPS reveal that Cu2+ adsorption onto CMCD-MNPs mainly involves the oxygen atoms in CM-β-CD to form surface-complexes. In addition, the copper ions can be desorbed from CMCD-MNPs by citric acid solution with 96.2% desorption efficiency and the CMCD-MNPs exhibit good recyclability.

Introduction

Heavy metal pollution in industrial wastewater has attracted global attention because of its adverse effects on the environment and human health [1], [2]. It is, therefore, crucial to remove these heavy metals from wastewaters effectively before their discharge into the environment. Currently heavy metal removal methods include chemical precipitation, ion exchange, liquid–liquid extraction, membrane filtration, adsorption and biosorption [3]. Among these methods, adsorption has increasingly received more attention as it is simple and cost effective [4], [5].

Recently, adsorption using magnetic nano-sized adsorbents has attracted significant attentions due to the specific characteristics [2], [5], [6], [7], [8], [9]. Their basic properties, extremely small size, high surface-area-to volume ratio and the absence of internal diffusion resistance provides better kinetics for adsorption of metal ions from aqueous solution. Magnetic nano-adsorbents have the advantages of both magnetic separation techniques and nano-sized materials, which can be easily recovered or manipulated with an external magnetic field. Moreover, the incorporation of magnetic particles with other functionalized materials such as multi-walled carbon nanotubes (MWCNT) [10], [11], [12], zeolites [13], activated carbon [14] etc., which are effective for the removal of both organic and inorganic pollutants is potentially a promising method to facilitate the separation and recovery of the adsorbents. Various types of magnetic nano-adsorbents with tailored surface reactivity by using natural or synthetic polymers such as, chitosan, gum arabic, alginate, poly(acrylic acid) etc., have been used recently for heavy metal removal from wastewater [2], [5], [6], [7], [8], [10], [15], [16], [17]. However, the effectiveness of magnetic nano-adsorbents modified with cyclodextrin – an important class of polysaccharides has not been discussed yet.

β-cyclodextrin (β-CD) is a cyclic oligosaccharide consist of seven α-d-glucose units connected through α-(1,4) linkages. The structure of these molecules is toroidal, truncated cones containing an apolar cavity with primary hydroxyl groups lying on the outside and the secondary hydroxyl groups inside [18], [19]. Cyclodextrins can form inclusion complexes with a wide variety of organic and inorganic compounds in its hydrophobic cavity [18]. Although there is less report on environmental applications of cyclodextrins for heavy metal removal, there is evidence that metal can be complexed by cyclodextrins through hydroxyl groups. Metal ion complexes with cyclodextrins could have a wide range of applications in catalysis and molecular recognition [20]. Some properties such as aqueous solubility and metal complexation potential can be altered by substituting functional groups to the outside of the cyclodextrin [21]. For example, carboxymethyl-β-cyclodextrin (CM-β-CD) has the ability to complex heavy metals such as cadmium, nickel, strontium and mercury through the interactions between the metal ions and –COOH functional groups [22], [23]. Insoluble cyclodextrin polymers and cyclodextrin immobilized on various supports including inorganic carriers are currently being investigated for adsorptive specificity towards heavy metal ions [24], [25]. Very recently, Hu et al. showed that the grafted β-CD on the MWCNT/iron oxides contributes to an enhancement of the adsorption capacity because of the strong complexation abilities of the multiple hydroxyl groups in β-CD with the lead ions [12]. However, to our knowledge, no adsorptive study of heavy metals on magnetic nanoparticles covalently bonded with CM-β-CD has been reported so far. The high surface area to volume ratio of the Fe3O4 nanoparticles and magnetic properties together with the adsorption capabilities of CM-β-CD through complex formation would facilitate the removal of heavy metals from wastewater.

In this study, a novel magnetic nano-adsorbent was synthesized for the adsorption of metal ions by surface modification of Fe3O4 nanoparticles with CM-β-CD. The adsorption behavior of these sorbents was investigated using Cu2+ as the target metal contaminant because of its extensive environmental impacts. The effects of several factors such as, pH, initial Cu2+ concentration and temperature were also studied. The adsorption mechanisms of Cu2+ onto CMCD-MNPs were investigated through FTIR and XPS analyses. In addition, the regeneration and reusability of the adsorbents were also evaluated.

Section snippets

Materials

In this work, the following chemicals were used for fabricating CMCD-MNPs: iron (II) chloride tetrahydrate (99%) [Alfa Aesar], iron (III) chloride hexahydrate (98%) [Alfa Aesar], ammonium hydroxide (25%) [Merck], β-cyclodextrin (β-CD, 99%) [Tokyo Kasie Kogyo (Japan)], carbodiimide (cyanamide, CH2N2, 98%) [Sigma], copper (II) nitrate [Sigma], chloroacetic acid (99%) [Alfa Aesar]. All other chemicals were of analytical grade and used as received without further purification.

Preparation of carboxymethyl-β-cyclodextrin (CM-β-CD)

CM-β-CD was prepared

Synthesis and characterization of magnetic nanoparticles

CMCD-MNPs (BET surface area ∼110.9 m2/g) were synthesized using a two-steps procedure. The first step involves the formation of carboxyl group onto β-CD by reacting chloroacetic acid with β-CD in the alkaline condition. The average number of carboxylate groups (3.1) per molecule of CM-β-CD is determined using 1H NMR [29] as well as by titrating the carboxylate groups with NaOH solution [21]. Before titrating CM-β-CD with NaOH, the CM-β-CD -Na salt was converted to its acidic form by passing 50 ml

Conclusions

In this study, we have successfully synthesized a magnetic novel nano-adsorbent comprising Fe3O4 nanoparticles modified with CM-β-CD having magnetic properties exhibited by magnetite and adsorption properties of CM-β-CD. Grafting of CM-β-CD onto the magnetic nano-adsorbents is confirmed by FTIR, TGA, and XPS analyses. These magnetic nano-adsorbents can effectively be used to remove Cu2+ from aqueous solution. The solution pH greatly affects the adsorption of Cu2+ onto CMCD-MNPs. Adsorption of Cu

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