Ind. Eng. Chem. Res. 1999, 38, 1411-1414
1411
Adsorption of Gold(III) Ions onto Chitosan and N-Carboxymethyl Chitosan: Equilibrium Studies W. S. Wan Ngah* and K. H. Liang School of Chemical Sciences, Universiti Sains Malaysia, 11800 Pulau Pinang, Malaysia
The adsorption of gold(III) ions onto chitosan and N-carboxymethyl chitosan (NCMC) has been investigated. Experiments were carried out as a function of pH, agitation period, and concentration of Au3+ ions. It was shown that the adsorption of Au3+ ions occurred only at the optimum pH. Kinetic studies showed a rapid adsorption of Au3+ from aqueous solution. Adsorption isothermal data could be interpreted by the Langmuir equation. Langmuir constants have been determined. The experimental data of the adsorption equilibrium from Au3+ solutions correlated well with the Langmuir isotherm equation. The uptake of Au3+ on chitosan and NCMC were 30.95 mg/g of chitosan and 33.90 mg/g of NCMC. The Au3+ ions were readily removed from chitosan and NCMC by treatment with an aqueous EDTA solution. Introduction Rare, precious, or strategic metals, especially gold, are very important. Commercial value and toxicity are the two properties of these metals which warrant their recovery from solution. Gold is introduced into the water from leach mining and waste solution from metalfinishing industries. Gold can be recovered by the gravity concentration, amalgamation, retorting, and smelting procedures, and the tailings from this process can be treated with alkali cyanide.1 As well as providing an alternative recovery processes for strategic metals, the naturally second most abundant biopolymer, in nature next to cellulose,2 chitosan, might be used where more expensive ion-exchange resins are not feasible. Chitin, (1f4)-2-acetamido-2-deoxy-β-D-glucan is a natural occurring polymer extracted from crustacean shells, such as prawns, crabs, krill, and shrimp. This polymer contained both glucosamine and N-acetylglucosamine monomers. Chitosan, (1f4)-2-amino-2-acetamido-2-deoxy-β-D-glucan is obtained from partial Ndeacetylation of chitin with a strong alkali solution such as sodium hydroxide. Chitosan is considered to be a copolymer of N-acetyl-D-glucosamine and D-glucosamine. Chitosan has already been described as a suitable natural polymer for the collection of metal ions,3-12 since the amine group on the chitosan chain can act as chelation sites for metal ions. The amine and two hydroxyl groups on each glucosamine in the repeating unit of chitosan can act a reactive site for chemical modification. A number of reviews deal with chemical modifications of chitosan and its applications.13-15 A special interest was paid to the use of NCMC (also called glycine glucan), to adsorb metal ions. The carboxymethylation of chitosan is approach to the introduction of carboxyl groups via a Schiff reaction with glyoxylic acid and reducing it with sodium cyanoborohydride or sodium borohydride.16,17 NCMC was reported to enhance Co, Cu, Ni, Cd, and Pb ions adsorption capacity or exhibit higher chelating ability compared to chitosan18-20 due to the high local concentration of the chelating groups (carboxyl groups in the glycine of NCMC). * To whom correspondence should be addressed.
In the present study, the adsorption of Au3+ onto chitosan and NCMC was investigated in detail. Some of the physicochemical parameters of the adsorption were evaluated at the dynamic and equilibrium conditions. The isotherm equation used to describe experimental equilibrium data was also described. Experimental Section Chitosan and Reagents. Samples of chitosan with average molecular weights between 105 and 106 and with 87% degree of N-deacetylation21 prepared from shells of prawns were kindly donated from ChitinChitosan Research Centre, Universiti Kebangsaan Malaysia, Bangi, Malaysia. They were ground by using a laboratory jar mill to a consistent particle size (
