Adsorption of chromium from aqueous solutions using crosslinked chitosan–diethylenetriaminepentaacetic acid
Introduction
Chromium is one of the major contaminants in wastewater as it is widely used in industries, the most important sources being pigments, electroplating, leather and mining industries. Chromium is present in the aqueous environment as Cr3+ and Cr6+. Hexavalent chromium exists in water as oxyanions such as chromate (HCrO4−) and dichromate (Cr2O72−) [1]. While Cr3+ is essential to human metabolism at low concentrations, Cr6+ is toxic and carcinogenic [2]. Due to its severe toxicity, US EPA and ISS India IS:10500 set the tolerance limit for the discharge of Cr6+ ions into surface water as 0.1 mg/L and in potable water as 0.05 mg/L [3], [4].
Cr6+ is mainly removed from wastewaters by its reduction to Cr3+ with the use of various chemical reductants and microorganisms which require the use of expensive and toxic chemical reductants/nutrients [5], [6]. Furthermore, cell death also occurs due to the toxicity of Cr6+. Bio-adsorption using agro-waste/biomass and biopolymers has been adopted as an effective method to remove chromium and other toxic metals from aqueous environments [7], [8], [9]. During the past three decades, the use of CH as a bio-adsorbent has been gaining increased attention [10]. The adsorption and chelating properties of chitosan were further improved by functionalization or derivatization [11]. For instance, CH has been modified by different ligands such as barbituric acid, oxine, glycine, iminodiaceticacid (IDA), ethyleneglycol-bis(2-aminoethylether)-N,N,N′,N′-tetraacetic acid (EGTA), ethylenediaminetetraacetic acid (EDTA) and diethylenetriaminepentaacetic acid (DTPA) [5], [12], [13], [14], [15], [16], [17], [18], [19]. Evelina Repo et al. have used EDTA-modified CH for the removal of Co(II) in the presence of EDTA and other interfering species from aqueous solutions [20]. They also had modified CH-silica hybrid with EDTA and used them as adsorbent for the removal of cobalt, nickel, cadmium and lead ions from aqueous solutions [21]. They further have reviewed the use of amino polycarboxylic acid functionalized adsorbents for the removal of heavy metals from water [22]. Chitosan grafted with poly(ethylene imine) and cross-linked with epichlorohydrin has been used by Kyzas et al. for the simultaneous removal of a reactive dye and Cr6+ [23]. They also had investigated the potential of N-(2-carboxybenzyl)-grafted CH for the removal of both positively charged (Cu(II), Ni(II)) and negatively charged (Cr(VI), As(V)) metal ions from aqueous solutions as well as succinyl grafted chitosan for the removal of zinc and cationic dye from binary mixtures [24], [25]. Zarebsiki first used DTPA for the determination of trace levels of Cr3+ using several polarographic techniques [26]. This method has been improved using a hanging mercury drop electrode (HMDE) for the speciation analysis of chromium in different media. The complexation of Cr3+ and Cr6+ with diethylenetriaminepentaacetic acid (DTPA), the redox behavior of these complexes and their adsorption on the mercury electrode surface were investigated by Sander et al. using electrochemical techniques and UV–vis spectroscopy [27].
DTPA-modified chitosan as well as silica gel have been used by Evelina Repo et al. for the removal of cobalt as its EDTA complex [28]. However, the adsorption potential of CD for chromium has not been studied earlier. Bearing these factors in mind it was felt that chemical modification of CH with diethylentriaminepentaacetic acid (DTPA) may result in an adsorbent for chromium with interesting binding properties. Our objective was thus to synthesize chitosan DTPA complex and study its potential for adsorption of Cr6+ as well its reduction to Cr3+. The potential of CD in the treatment of chrome plating effluent with minimum manipulation in original pH of the effluent has also been investigated.
Section snippets
Materials
All the reagents used were of AR grade. CH from Crab shells (Sigma Aldrich, USA) and DTPA (National Chemicals, India) were used as such without further grinding and sieving.
Synthesis of adsorbents (chitosan crosslinked with DTPA)
A 100 ml of 1% CH in 1% acetic acid solution was added to 100 ml of 0.5 M solution of DTPA (DTPA dissolved in 0.41 M NaOH) and stirred for 24 h for the formation of cross-linked CH with DTPA (CD). A white gel material was formed which was then filtered, washed with double distilled water to neutral and dried overnight in vacuum
IR spectra analysis
The FT-IR spectrum (Fig. 1) of CH showed characteristic bands corresponding to OH and NH stretching vibrations at 3619 and 3420 cm−1, respectively. The CH and the CO stretching vibrations were obtained at 2857 and 1051 cm−1, respectively. The NH bending vibration at 1650 cm−1 is shifted to 1653 cm−1 after grafting with DTPA. Similarly, the CN bending vibration observed at 1393 cm−1 resulted in a shift to 1398 cm−1. These changes could be attributed to the effective grafting of DTPA onto chitosan. The
Conclusion
The potential of chitosan crosslinked with DTPA as adsorbent for Cr6+ was investigated. The optimum pH for removal of Cr6+ was found to be 3. Among the kinetic models tested, the adsorption kinetics was best described by the pseudo-second-order equation. The data fitted well with the Langmuir adsorption isotherm model. Adsorption thermodynamics indicate the adsorption of the metal onto CD to be exothermic process. XPS and ESR studies indicated the reduction of Cr6+ adsorbed onto CD to Cr3+. The
Acknowledgement
The authors are thankful to Head, Department of Chemistry, for providing necessary facilities required to carry out this work. Ronak Bhatt is thankful to GNFC for giving support.
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