Interaction of natural and crosslinked chitosan membranes with Hg(II) ions

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Abstract

Fundamental investigation on adsorption and desorption of Hg(II) ions on chitosan membranes was performed. Batch experiments were used to study how these phenomena are affected by pH, concentration of Hg(II) ions, crosslinking agent (glutaraldehyde or epichlorohydrin) and desorbent solution. The adsorption depended on pH and speciation of mercury. Maximum capacity was reached near zero-charge-pH, the pHZPC. The amount of adsorbed Hg(II) ions increased when the adsorbent was crosslinked, mainly with glutaraldehyde. Since amino groups are usually unavailable in this situation, other groups play an important role in adsorption, such as imino bonds, unreacted aldehyde terminals and also unreacted hydroxyl groups from original chitosan. Hg(II) equilibrium concentrations were determined and the Langmuir model was fitted to the experimental data. The maximum adsorbed amounts, at pH 6.0, were 25.3, 30.3 and 75.5 mg/g, respectively, to natural, epichlorohydrin-crosslinked and glutaraldehyde-crosslinked chitosan membranes (wet weight). The pHZPC was in the range of 6–10 for the three different membranes. X-ray fluorescence analysis using synchrotron radiation demonstrated that natural chitosan presents more homogeneous adsorption than with crosslinked chitosan, within a 10 μm × 10 μm area. However, mappings of Hg in larger scale areas (1 mm × 1 mm), using energy dispersive X-ray spectroscopy, indicated a homogeneous distribution of Hg-adsorbent sites. The desorption process was carried out using NaCl (1 mol/L) and EDTA (10–4 mol/L) as eluent solutions. The best metal recovery was obtained when NaCl solution was used.

Introduction

Effective removal of toxic heavy metals from aqueous solution is fundamental to the environment conservation and public health [1]. Mercury, which is remarkably toxic and non-biodegradable, may reach the environment from a variety of sources, where it can be further converted into more toxic forms [2].

Several methods exist to remove toxic metals from aqueous solution such as ion exchange, reverse osmosis, adsorption, complexation and precipitation [3], [4], [5]. Adsorption is considered an effective and economical method for removal of pollutants from wastewater [6]. The adsorption capacity of several low-cost-adsorbents has been investigated, mainly using biopolymers, which are obtained from renewable sources and adsorb metallic ions selectively [7].

Among biopolymers, chitosan has been studied as a very promising material. Chitosan is a linear polysaccharide based on a glucosamine unit (Fig. 1). It is obtained from deacetylation of chitin, which is the major component of crustaceans shells. It is one of the most available biopolymers in nature. Chitosan has been described as a suitable biopolymer for removal of metal ions from wastewater [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18] since its amino and hydroxyl groups can act as chelation sites.

Several methods have been used to modify natural chitosan either physically or chemically. Crosslinking with glutaraldehyde (GLA) or epichlorohydrin (ECH) are examples of chemical modifications on chitosan. These reactions are usually carried out in order to prevent chitosan dissolution in acidic solutions or to improve metal adsorption properties, i.e., to increase capacity or to enhance selectivity. Since glutaraldehyde binds to amino groups and epichlorohydrin binds preferentially to hydroxyl groups, it is possible to use both crosslinking processes to better understand the adsorption mechanism [15], identifying which chemical functionality is responsible for metal-chitosan interaction. Fig. 2(A) and (B) show the possible structures formed by crosslinking using glutaraldehyde and epichlorohydrin, respectively.

This study compared the adsorption and desorption behaviour of Hg(II) on natural and crosslinked chitosan membranes, identifying which groups are responsible for the metal-chitosan interaction. Water content measurement, potentiometric titration, pHZPC analysis, scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), Fourier-transformed infrared spectroscopy with attenuated total reflectance device (FTIR-ATR) and X-ray fluorescence using synchrotron radiation (XRF) were used to characterize the adsorbents before and after Hg(II) adsorption. The study also investigated the effect of key parameters, such as pH, Hg(II) ion concentration, crosslinking agent, and type of desorbent solution on the adsorption and desorption processes.

Section snippets

Materials

Chitosan with molecular weight 9.90 × 105 g/gmoL was purchased from Sigma (USA). All other chemicals (mercuric chloride, glutaraldehyde, epichlorohydrin) were of analytical grade. Aqueous solutions were prepared using deionized water (Milli-Q ultrapure water).

Preparation and chemical modification of chitosan membranes

In order to obtain porous membranes, a chitosan solution 2.5% (w/w) was spread on a Petri dish. The dish was kept at 60 °C until a reduction of 50% in its initial weight. The membranes were then immersed in NaOH solution (1 mol/L) during 24 h

Porosity and potentiometric titration

Some properties of natural and crosslinked chitosan membranes are shown in Table 1. It is observed a porosity reduction after the crosslinking. This is associated to an increase in the hydrophobicity of the membranes as a result of the addition of the alkyl group by the crosslinking reactions. The morphology of the fractured surface was observed by scanning electron microscopy (Fig. 3); the microporous structure of the membranes were confirmed. These SEM images were in agreement with the

Conclusions

In this study, the viability of using natural and crosslinked chitosan membranes for removal of Hg(II) ions from aqueous solutions was confirmed. The amount of adsorbed and desorbed Hg(II) was influenced by some parameters such as the initial Hg(II) concentration, pH of solution, speciation of mercury, crosslinking agent and desorbent solution. The maximum adsorption amount was observed at pH 6.0, near pHZPC, and on glutaraldehyde-crosslinked chitosan, indicating that adsorption does not occur

Acknowledgements

The authors thank FAPESP for financial support, CNPq for the scholarship of Rodrigo S. Vieira and the Brazilian National Synchrotron Light Laboratory (LNLS) for XRF analyses.

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