Preparation and characterization of magnetic chelating resin based on chitosan for adsorption of Cu(II), Co(II), and Ni(II) ions

Abstract

Cross-linked magnetic chitosan–diacetylmonoxime Schiff’s base resin (CSMO) was prepared for adsorption of metal ions. CSMO obtained was investigated by means of scanning electron microscope (SEM), FTIR, ¹H NMR, wide-angle X-ray diffraction (WAXRD), magnetic properties and thermogravimetric analysis (TGA). The adsorption properties of cross-linked magnetic CSMO resin toward Cu²⁺, Co²⁺ and Ni²⁺ ions were evaluated. Various factors affecting the uptake behavior such as contact time, temperature, pH and initial concentration of the metal ions were investigated. The kinetics was evaluated utilizing the pseudo-first-order and pseudo-second-order. The equilibrium data were analyzed using the Langmuir, Freundlich, and Tempkin isotherm models. The adsorption kinetics followed the mechanism of the pseudo-second-order equation for all systems studied, evidencing chemical sorption as the rate-limiting step of adsorption mechanism and not involving a mass transfer in solution. The best interpretation for the equilibrium data was given by Langmuir isotherm, and the maximum adsorption capacities were 95 ± 4, 60 ± 1.5, and 47 ± 1.5 mg/g for Cu²⁺, Co²⁺ and Ni²⁺ ions, respectively. Cross-linked magnetic CSMO displayed higher adsorption capacity for Cu²⁺ in all pH ranges studied. The adsorption capacity of the metal ions decreased with increasing temperature. The metal ion-loaded cross-linked magnetic CSMO were regenerated with an efficiency of greater than 84% using 0.01–0.1 M ethylendiamine tetraacetic acid (EDTA).

Introduction

Discharges of effluents containing toxic compounds, even in low concentrations, have caused the pollution of soils, phreatic mantles, and water bearing stratum, deep aquifers, as well as continental and coastal waters. Wastewaters from mining, galvanoplastic, and foundry industries mainly contain heavy metals such as cadmium, cobalt, chromium, copper, mercury, nickel, zinc, and lead [1]. Heavy metals are highly toxic at low concentrations and can accumulate in living organisms, causing several disorders and diseases [2]. The main techniques that have been used on metal content reduction from industrial waste are chemical precipitation, ion exchange, membrane filtration, electrolytic methods, reverse osmosis, solvent extraction, and adsorption [3], [4], [5]. However, these methods are limited by high operational cost and/or may also be inefficient in the removal of some toxic metal ions, mainly at trace level concentrations [1], [6].

One of the promising methods is the use of chelating resins. Chelating resins are easily regenerated from metal ions and they differ from activated carbon and ion exchange resins in their high selectivity in sorption processes [7]. Many articles that cover a vast number of different chelating resins were reported [8], [9], [10], [11], [12]. Recently, it was reported on the use of magnetic resins in removal of some metals from aqueous solutions [13], [14], [15], [16]. These magnetic resins are easily collected from aqueous media using an external magnetic field and displayed higher uptake capacity compared to the magnetic particles-free resin [17], [18]. These methods are also cheap and often highly scalable. Moreover, techniques employing magnetism are more amenable to automation [19]. Attention has recently been focused on chitosan and its derivative as bioadsorbents. Chitosan is a major component of crustacean shells and one of the most abundant biopolymers in nature [20]. It is characterized by its ability to uptake several metal ions through different mechanisms, depending on the type of metal ion and the pH of the solution. The dissolution of chitosan was decreased through cross-linking treatments. The cross-linking procedure may be performed by reaction of chitosan with different cross-linking agents such as glutaraldehyde [21], [22], glyoxal [23], oxidized β-cyclodextrin (β-cyclodextrin polyaldehyde) [24], ethyleneglycol diglycidyl ether [25] or glycerolpolyglycidylether [26]. Tri-polyphosphate has also been selected as a possible cross-linking agent, which can be used for the preparation of chitosan gel beads by the coagulation/neutralization effect [27]. The cross-linking step may cause a significant decrease in metal uptake efficiency especially in the case of chemical reactions involving amine groups [28]. However, this limiting effect of chemical cross-linking significantly depends on the procedure used [16], [22], [29], [30]. So the efforts have been directed towards modification with chelating functionalities before cross-linking to improve the adsorption capacity.

In the present work, the cross-linked magnetic chitosan–diacetylmonoxime Schiff’s base (CSMO) resin was prepared with the purpose of improving its features as an adsorbent, such as selectivity and adsorption capacity. The structure of the modified chitosan was confirmed using SEM, FTIR, ¹H NMR spectroscopy and X-ray diffraction. The adsorption equilibrium and the kinetics of Cu²⁺, Co²⁺ and Ni²⁺ ions in aqueous solutions with magnetic cross-linked CSMO resin were also investigated.