Cadmium sorption on chitosan sorbents: kinetic and equilibrium studies
Introduction
Cadmium is widely used in several industries including metallurgy, surface treatment, dye synthesis or battery production. Due to its toxicity, cadmium removal from aqueous effluents has been classified as a priority in the last decade. Industrial wastewaters are usually treated through chemical precipitation. However, these processes involve the production of highly toxic sludges, which must be further treated before being environmentally safe for disposal [1]. Thus, other treatments have been developed including ion exchange, adsorption, electrodeposition and membrane systems [1]. Though these treatments are adequate for the treatment of medium to high concentration solutions, there is still a need for the development of new materials/processes for the treatment of dilute solutions, including the use of low cost materials of biological origin for the sorption of metals from dilute solutions [2], [3].
Chitin is an aminopolysaccharide which is readily found in nature: fungal biomass, crustacean shells, insect cuticles. It is commercially extracted from crustacea shells [4]. This polymer is constituted by acetylglucosamine units. An alkaline treatment at boiling temperature allows the acetylated groups to be removed and the deacetylation degree to be increased. This chemical treatment leads to the formation of chitosan, a partially deacetylated form of chitin. The amine groups are able to sorb metals through several mechanisms including chemical interactions, such as chelation, electrostatic interactions, such as ion exchange, or the formation of ion pairs [5], [6], [7]. The kind of interaction depends on the metal, its chemistry and the pH of the solution [6], [7], [8], [9], [10], [11]. Chitosan was also studied for cadmium removal using raw material [12], [13] and derivatives produced by physical and/or chemical modifications [14], [15], [16], [17].
Owing to the low porosity of chitosan, sorption performances are frequently controlled by mass transfer resistance. The influence of diffusion mechanisms is also controlled by the chemistry of the metal and by the conditioning of the biopolymer. Since chitosan is soluble in acidic solutions, it is necessary to increase its chemical stability in acidic solutions for the sorption of several metal anions whose recovery is only effective at pH lower than 4–5 [6], [10], [18]. The crosslinking treatment can be carried out using several chemical reagents such as glutaraldehyde [14], [19]. Glutaraldehyde crosslinking treatment may affect the sorption efficiency, involving a decrease in the number of free amine groups and a decrease in the accessibility to internal sites. This restricting effect may be decreased using chitosan whose structure has been physically modified: the gel bead formation procedure has been proposed and investigated by several groups [10], [14]. This physical modification allows: (i) the polymer network to be expanded: the increase in the polymer network opening enhances the diffusion of large size molecules; and (ii) the crystalline state of the polymer to be reduced. Piron et al. [20] have shown that the dissolving of chitosan followed by a freeze-drying of the resulting solution allows a strong decrease in the crystalline structure of the polymer which can be, in turn correlated to the improvement of sorption kinetics.
This work concentrates on the study of cadmium sorption using gel beads and flakes. The influence of several experimental parameters on sorption kinetics is investigated with both raw and crosslinked materials: particle size, agitation velocity, temperature. The diffusion of cadmium in the gel beads is also studied using SEM-EDAX analysis (scanning electron microscope coupled with an energy-dispersed analysis of X ray apparatus) for several points of a typical kinetic curve obtained on raw chitosan gel beads put in contact with a cadmium solution.
Section snippets
Materials
Chitosan was used as received from ABER Technologie (Plouvien, France). Previous characterization performed on the raw material had shown that the deacetylation degree was about 87% and the molecular weight was approximately 125.000 g mol−1 [10]. Flakes were ground and sieved in four fractions. G1<125 μm<G2<250 μm<G3<500 μm<G4<710 μm. Chitosan gel beads were prepared according to a method which was previously described [10], [14]. A chitosan solution prepared by dissolving chitosan flakes in an
Influence of pH
Fig. 1 shows the influence of pH on the sorption of cadmium using chitosan gel beads for pH ranging between 4 and 7. Sorption efficiency increased with increasing the pH of the solution. In acidic solutions, a strong competition existed between cadmium ions and protons for sorption sites and the sorption efficiency was decreased. Moreover, the protonation of amine groups induced an electrostatic repulsion of cadmium cations. The degree of protonation of the amine groups depends on both the
Conclusion
Chitosan is very effective at removing cadmium with sorption capacity exceeding 150 mg g−1 (about 1.5 mmol g−1). Sorption equilibrium and kinetics are hardly affected by temperature in the range 10–40°C or agitation speed (60–340 rpm; NRe: 2400–13,600). Sorption capacities decrease as particle size increases. Although decreasing the particle size of the sorbent makes it possible to reduce the time required to reach equilibrium, it changes the external and intraparticle diffusion coefficients by
Acknowledgments
The authors would like to thank the University of Guanajuato (Fomento a la Investigación 1998–1999) for its financial contribution, the CONCYTEG (Grant 99-16-203-021) and the Franco–Mexican Programme PCP (99/4). The authors would like to thank M.S. Dolores Elena Alvarez Gasca (from the Microscopy Laboratory of the Architecture Faculty, University of Guanajuato) and M.S. Armando Obregón (from the Microscopy Laboratory of IIBE, University of Guanajuato) for their helpful contribution in the
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