Recovery of rare-earth metal neodymium from aqueous solutions by poly-γ-glutamic acid and its sodium salt as biosorbents: Effects of solution pH on neodymium recovery mechanisms

Abstract

For recovery of metals from low-concentration sources, biosorption is one of promising technologies and poly-γ-glutamic acid (γ-PGA) has been known as a potential biosorbent for recovery of heavy metals from aqueous solutions. Effects of solution pH on recovery of rare-earth metal Nd are systematically examined to clarify mechanisms of Nd recovery by γ-PGA and its sodium salt (γ-PGANa). The recovery efficiency of Nd by γ-PGA increases from 2.4 to 19.6% as pH increases from 2 to 4. Subsequently the Nd recovery efficiencies for γ-PGA and γ-PGANa remain almost constant in the range of pH from 4 to 7. For pH > 7 the increase in Nd recovery is significant and 100% recovery of Nd is achieved at pH 9. The pH dependency on Nd recovery by γ-PGANa is similar to that by γ-PGA. Contributions of adsorption and precipitation/coagulation to Nd recovery process are quantified. Whereas the adsorption dominates Nd recovery at lower pH (<∼4), the precipitation/coagulation controls Nd recovery process for pH > 7. At higher pH, purple gel precipitates are observed. The maximum adsorption capacities for γ-PGA and γ-PGANa are 215 mg-Nd/(g-γ-PGA) at pH 4 and 305 mg-Nd/(g-γ-PGANa) at pH 3, respectively. From the spectra of FT-IR and XPS, the biosorption of Nd onto γ-PGA and γ-PGANa via electrostatic interaction with carboxylate anions at pH 3 is verified. The Nd complexation with amide and carboxylate anion groups on γ-PGA and γ-PGANa may also contribute to the Nd recovery. The biosorption isotherms for Nd recovery by γ-PGA and γ-PGANa can be satisfactorily fitted by the Langmuir model. The thermodynamic studies suggest that the biosorptions of Nd by γ-PGA and γ-PGANa are endothermic. The utilization of γ-PGA and γ-PGANa as potential and eco-friendly biosorbents for the highly effective recovery of Nd from aqueous solution is confirmed.