Colloids and Surfaces A: Physicochemical and Engineering Aspects
Interaction of natural and crosslinked chitosan membranes with Hg(II) ions
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.
References (35)
- et al.
Dithiocarbamate-incorporated monosize polystyrene microspheres for selective removal of mercury ions
React. Funct. Polym.
(2000) - et al.
The removal of mercury(II) from dilute aqueous solution by activated carbon
Water Res.
(1984) - et al.
A simplified equilibrium model for sorption of heavy metal ions from aqueous solution on chitosan
Water Res.
(2002) - et al.
Equilibrium studies of the sorption of Cu(II) ions onto chitosan
J. Colloid Interface Sci.
(2002) Removal of copper from aqueous solution by chitosan in prawn shell: adsorption equilibrium and kinetics
J. Hazard. Mater.
(2002)Interactions of metal ions with chitosan-based sorbents: a review
Sep. Purif. Technol.
(2004)- et al.
Uptake of mercury by thiol-grafted chitosan gel beads
Water Res.
(2004) - et al.
Lead sorption from aqueous solutions on chitosan nanoparticles
Colloids Surf. A
(2004) - et al.
Arsenic(V) sorption on molybdate-impregnated chitosan beads
Colloids Surf. A
(2000) - et al.
Degree of deacetilation of chitosan using conductimetric titration and solid-state NMR
Carbohydr. Res.
(1993)