Removal kinetics of cationic azo-dye from aqueous solution by poly-γ-glutamic acid biosorbent: Contributions of adsorption and complexation/precipitation to Basic Orange 2 removal

Mechanisms of cationic azo-dye Basic Orange 2 (BO2) removal by poly-gamma-glutamic acid (PGA) biosorbent were quantitatively examined. The contributions of adsorption and complexation/precipitation on BO2 removal by PGA were quantified at pH3-9 and the maximum overall BO2 removal of 99.5% was obtained at pH5. At pH less than 4((similar to)pKa of PGA), the positively charged protonated PGA did not favor the ion-exchange adsorption of positively charged protonated BO2 due to electrostatic repulsion. In the range of 4 <= pH <= 5((similar to)pKa of BO2), the ion-exchange adsorption of positively charged BO2 onto negatively charged deprotonated PGA via electrostatic attraction and the complexation/precipitation of BO2 with PGA via the formation of ionic hydrogen-bonded organic ammonium carboxylate salt complexes were stimulated and as a result the overall removal of BO2 by PGA was maximized at pH5. For pH >= 6 the contribution of complexation/precipitation was significantly suppressed due to neutral nature of BO2 and the removal of BO2 by PGA was dominated by the van der Waals adsorption and hydrophobic interactions rather than the ion-exchange adsorption. A novel phenomenological kinetic model developed by considering complexation/precipitation of BO2 with PGA besides adsorption of BO2 onto PGA could satisfactorily simulate the transitional profiles of BO2 removal and quantify the contributions of adsorption and complexation/precipitation.