Adsorption capacities of poly-γ-glutamic acid and its sodium salt for cesium removal from radioactive wastewaters

Cesium removal from radioactive wastewaters was examined using water-insoluble poly-gamma-glutamic acid (gamma-PGA) and water-soluble sodium salt form poly-gamma-L-glutamic acid (gamma-PGANa) as biosorbents. The maximum adsorption capacities at equilibrium of gamma-PGA and gamma-PGANa for Cs were 345 mg-Cs(g-gamma-PGA)(-1) at pH 6.0 and 290 mg-Cs(g-gamma-PGANa)(-1) at pH 9.0, respectively. At lower pH < pK(a), the carboxyl groups of gamma-PGA primarily remained in the protonated form and adsorption of Cs only slightly occurred. At higher pH > pK(a), the adsorption of Cs was significantly facilitated due to ionization of carboxyl groups to carboxylate ion. Adsorption of Cs at pH > 9.0 was inhibited due to the hydrolysis of Cs. The Langmuir model could successfully describe the isotherm data. For gamma-PGA and gamma-PGANa, the maximum adsorption capacities at equilibrium in the Langmuir model were 446 and 333 mg-Cs(g-adsorbent)-1, respectively. The high adsorption capacities confirmed a potential utilization of gamma-PGA and gamma-PGANa for Cs removal. The adsorption of Cs by both gamma-PGA and gamma-PGANa attained the equilibrium within 0.5 min. The very quick equilibration is a benefit from the viewpoint of practical application. The spectra of FT-IR and XPS before and after adsorption confirmed the adsorption of Cs onto gamma-PGA and gamma-PGANa via electrostatic interaction with carboxylate anions. (C) 2016 Elsevier Ltd. All rights reserved.