Adsorption of toxic mercury(II) by an extracellular biopolymer poly(γ-glutamic acid)

Abstract

Adsorption of mercury(II) by an extracellular biopolymer, poly(γ-glutamic acid) (γ-PGA), was studied as a function of pH, temperature, agitation time, ionic strength, light and heavy metal ions. An appreciable adsorption occurred at pH > 3 and reached a maximum at pH 6. Isotherms were well predicted by Redlich–Peterson model with a dominating Freundlich behavior, implying the heterogeneous nature of mercury(II) adsorption. The adsorption followed an exothermic and spontaneous process with increased orderliness at solid/solution interface. The adsorption was rapid with 90% being attained within 5 min for a 80 mg/L mercury(II) solution, and the kinetic data were precisely described by pseudo second order model. Ionic strength due to added sodium salts reduced the mercury(II) binding with the coordinating ligands following the order: Cl − > SO 4 2 - ≫ NO 3 - . Both light and heavy metal ions decreased mercury(II) binding by γ-PGA, with calcium(II) ions showing a more pronounced effect than monovalent sodium and potassium ions, while the interfering heavy metal ions followed the order: Cu 2+ ≫ Cd 2+ > Zn 2+. Distilled water adjusted to pH 2 using hydrochloric acid recovered 98.8% of mercury(II), and γ-PGA reuse for five cycles of operation showed a loss of only 6.5%. IR spectra of γ-PGA and Hg(II)-γ-PGA revealed binding of mercury(II) with carboxylate and amide groups on γ-PGA.