Adsorption and desorption of Cu(II), Cd(II) and Pb(II) ions using chitosan crosslinked with epichlorohydrin-triphosphate as the adsorbent

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Abstract

In this study, chitosan (CTS) was crosslinked with both epichlorohydrin (ECH) and triphosphate (TPP), by covalent and ionic crosslinking, respectively. The resulting new CTS–ECH–TPP adsorbent was characterized by CHN analysis, EDS, FTIR spectroscopy, TGA and DSC, and the adsorption and desorption of Cu(II), Cd(II) and Pb(II) ions in aqueous solution were investigated. Potentiometric studies were also performed and revealed three titratable protons for each pKa value of 5.14, 6.76 and 9.08. The results obtained showed that the optimum pH values for adsorption were 6.0 for Cu(II), 7.0 for Cd(II) and 5.0 for Pb(II). The kinetics study demonstrated that the adsorption process proceeded according to the pseudo-second-order model. Three isotherm models (Langmuir, Freundlich and Dubinin-Radushkevich) were employed in the analysis of the adsorption equilibrium data. The Langmuir model resulted in the best fit and the new adsorbent had maximum adsorption capacities for Cu(II), Cd(II) and Pb(II) ions of 130.72, 83.75 and 166.94 mg g−1, respectively. Desorption studies revealed that HNO3 and HCl were the best eluents for desorption of Cu(II), Cd(II) and Pb(II) ions from the crosslinked chitosan.

Introduction

The increasing levels of toxic heavy metal ions discharged to the environment have received considerable attention due to the adverse effects on receiving waters. The potential sources of heavy metal ions in wastewaters include fertilizers, fungicides, metals used in manufacturing, paints, pigments, and batteries. These ions are a hazard to public health and the environment when discharged inappropriately. Many methods such as ion exchange, precipitation, adsorption, and membrane processes have been used for the removal of toxic metal ions. In particular, adsorption is recognized as an effective and economic method for the removal of pollutants from wastewaters. However, because this process is expensive, low-cost biosorbents have been given increasing attention as they can significantly reduce the cost of an adsorption system [1], [2], [3].

In particular, chitosan (CTS) has been studied due to its high capacity to adsorb heavy metal ions, dyes, and proteins. Other useful features of CTS include its abundance, non-toxicity, hydrophilicity, biocompatibility, biodegradability, and antibacterial and antimicrobial properties [4], [5], [6], [7], [8]. It has several potential applications, for instance, in biomedical products, cosmetics, pharmaceutical processes, food processing, agricultural chemicals, wastewater treatment, and metal chelating agents [9], [10], [11].

Chitosan is obtained from the deacetylation of the natural biopolymer chitin, found in crustaceous shells, insects, and fungal cell walls. CTS consists of β-(1,4)-2-acetamido-2-deoxy-β-d-glucose and β-(1,4)-2-amino-2-deoxy-β-d-glucose units and contains high contents of amino and hydroxyl groups, which favors the modification of this biopolymer and the introduction of new functional groups [6], [12], [13].

CTS is soluble in most dilute mineral acids, except in sulfuric acid solutions and dilute organic acids, such as acetic, propionic, formic and lactic acids. Consequently, its chemical stability needs to be reinforced through treatments using crosslinking agents for application in acidic media. This treatment induces new linkages between the chitosan chains allowing the polymer to be highly resistant to dissolution, even in harsh solutions, such as hydrochloric acid [14], [15]. The crosslinking procedure may be performed by reaction of CTS with different agents such as glutaraldehyde (GLA), ethylene glycol diglycidyl ether (EGDE) and epichlorohydrin (ECH). Triphosphate (TPP) has also been proposed as a possible crosslinking agent [3], [4], [16].

ECH is a crosslinking mono-functional agent used to form covalent bonds with the carbon atoms of the hydroxyl groups of chitosan, resulting in the rupturing of the epoxide ring and the removal of a chlorine atom [17].

TPP is a non-toxic multivalent anion that can form cross-links by ionic interaction between the positively charged amino groups of chitosan and the negatively charged counterion of the TPP molecules. This interaction can be controlled by the charge density of TPP and chitosan, which is dependent on the pH of the solution [4], [5], [18], [19].

Pentasodium TPP can be dissolved in water to dissociate into OH and triphosphoric ions, as shown in the following dissociation reactions [5], [20].Na5P3O10+5H2O5Na++H5P3O10+5OHH5P3O10+OHH4P3O10+H2O(pKa1=)H4P3O10+OHH3P3O102+H2O(pKa2=1.1)H3P3O102+OHH2P3O103+H2O(pKa3=2.3)H2P3O103+OHHP3O104+H2O(pKa4=6.3)HP3O104+OHP3O105+H2O(pKa5=8.9)

In this study, we prepared a novel type of modified chitosan by incorporating epichlorohydrin (via a covalent crosslinking reaction) and triphosphate (via an ionic crosslinking reaction), and evaluated its potential in terms of the adsorption and desorption of Cu(II), Cd(II) and Pb(II) ions.

Section snippets

Instrumentation

The chitosan–epichlorohydrin–triphosphate (CTS–ECH–TPP) adsorbent was prepared using a Dispersor Extratur® Quimis Q252 M. The CHN elemental analysis was performed with a Carlo Erba CHNS-O-E1110 analyzer. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) were performed using a JEOL JSM-6390LV instrument. Infrared (IR) spectra were obtained with KBr pellets in the range of 2000–450 cm−1, using a PerkinElmer 2000 FT-IR spectrometer. Thermogravimetric analysis (TGA)

Elemental (CHN) analysis and EDS

The C, H and N values for the CTS composition were 39.43%, 8.41% and 7.30%, respectively, and for the CTS–ECH–TPP adsorbent they were 29.00%, 7.44% and 4.78%, respectively. The semi-quantitative results obtained from the EDS revealed an atomic percentage of phosphorus in the new adsorbent of 4.46%. It was observed that there was a decrease in the percentage of C, H and N atoms and the presence of P atoms after the chitosan modification.

FTIR spectroscopy

The FTIR spectra for the CTS, TPP and CTS–ECH–TPP samples

Conclusions

The modification of chitosan was confirmed by CHN analysis, EDS, FTIR spectroscopy, TGA and DSC.

The adsorption of Cu(II), Cd(II) and Pb(II) ions was shown to be dependent on the solution pH, and the optimum pH values for the adsorption were 6.0, 7.0 and 5.0, respectively.

The kinetics study demonstrated that the kinetic mechanism for the adsorption of metal ions followed a pseudo-second-order model, which provided the best experimental data correlation.

The Langmuir isotherm model provided the

Acknowledgements

The authors wish to thank Conselho Nacional de Pesquisa e Desenvolvimento–CNPq, Brazil for financial support.

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