Equilibrium and kinetic studies on sorption of basic dyes by a natural biopolymer poly(γ-glutamic acid)

A bacteria-derived biodegradable and non-toxic biopolymer poly(gamma-glutamic acid) (gamma-PGA) was evaluated as an adsorbent for removal of basic dyes from aqueous solution. Sorption experiments were carried out with three basic dyes Auramine O (Au-O), Rhodamine B (Rh-B) and Safranin O (Sa-O) by a batch mode at different initial dye concentrations, agitation times, gamma-PGA doses, temperatures, pH and added salts. Sorption isotherms were well described by the Redlich-Peterson equation, closely followed by Sips, when compared to Freundich and Langmuir models. The maximum sorption capacity derived from Langmuir model at 301 K was 277.29, 390.25 and 502.83 mg/g for Au-O, Rh-B and Sa-O dyes, respectively. The kinetics of dyes sorption on gamma-PGA followed a pseudo second order equation and the rate was controlled by chemical sorption. An ion-exchange model assuming adsorption to be a chemical phenomenon also predicted the kinetic data precisely. Thermodynamic parameters (Delta H degrees, Delta S degrees and Delta G degrees) obtained revealed dye sorption to be an exothermic and spontaneous process with decreasing randomness of dyes at the solid/solution interface. Dye sorption increased with increase in solution pH and reached a plateau at pH 5, while desorption of about 98% of dyes from spent gamma-PGA occurred at pH 1. The incorporation of salts decreased the dye sorption, suggesting the binding of dyes on gamma-PGA may involve ion-exchange mechanism. (c) 2006 Elsevier B.V. All rights reserved.