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(γ-glutamic acid) (γ-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, γ-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 γ-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 (ΔH°, ΔS° and ΔG°) 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 γ-PGA occurred at pH 1. The incorporation of salts decreased the dye sorption, suggesting the binding of dyes on γ-PGA may involve ion-exchange mechanism.