Mucilage extracted from wasted natto (fermented soybeans) as a low-cost poly-γ-glutamic acid based biosorbent: Removal of rare-earth metal Nd from aqueous solutions

Abstract

Since poly-γ-glutamic acid (γ-PGA) has been known as a potential biosorbent for removal of heavy metals from aqueous solutions, mucilage extracted from wasted natto (fermented soybeans) composed mainly of poly-glutamic acid and fructan can be expected as a low-cost γ-PGA based biosorbent. The removal capacity of natto mucilage for biosorption of toxic heavy or rare-earth metal Nd chosen as a model heavy metal from aqueous solutions was studied. The Nd removal efficiency with the natto mucilage dosage of 500mgL−1 was found to be comparable to that with the reagent-grade γ-PGA dosage of 100mgL−1. Although the maximum biosorption capacity of 51.3mg-Nd(g-natto mucilage)−1 is around a quarter of that for reagent-grade PGA or calcium alginate-poly glutamic acid hybrid gels reported in the literature, the estimated cost of the natto mucilage extracted from wasted natto using ethanol is less than one-tenth of the cost of the reagent-grade PGA. The spectra of FT-IR and XPS confirmed that the adsorption of Nd onto natto mucilage took place by electrostatic interaction with carboxylate anions and the Nd binding with amide and carboxylate anion groups of γ-PGA and functional groups on the surface of fructan also contributed to removal of Nd by natto mucilage. The present results confirmed that natto mucilage is a promising low-cost poly-γ-glutamic acid based biosorbent for removal of heavy metals from aqueous solutions.