Recovery and removal of rare earth elements from mine wastewater using synthesized bio-nanoparticles derived from Bacillus cereus

The efficient removal and recovery of rare earth elements (REEs) from mine wastewater is highly desirable due to the increasing public demands to both protect the environment and recover valuable REEs resources. In this study, bio-nanoparticles (BC@FeNPs-EPS), which combined extracellular polymeric substances (EPS) and iron nanoparticles (FeNPs) were synthesized via Bacillus cereus and used to remove REEs such as Eu(III) from mine wastewaters. The removal efficiency of BC@FeNPs-EPS (90.2%) was consistently higher than the removal efficiencies of the three constituent components alone, FeNPs (85%), Free cell (67%) and EPS (10%) exhibited. SEM and XPS analysis indicated the existence of substantial organic capping agents derived from microorganisms on the FeNPs, where comparison of FTIR spectra before and after exposure to Eu(III) showed changes in both wavenumber and peak intensity of these various organic functional groups suggesting the formation of surface complexes with Eu(III), whereas Zeta potential measurements of the material also supported electrostatic interactions between Eu(III) and BC@FeNPs-EPS. Adsorption kinetics almost perfectly correlated with the non-linear pseudo-second-order model. Collectively, these results indicated that the removal mechanism for Eu(III) by BC@FeNPs-EPS involved both surface complexation and ion exchange and the additive effect removal of Eu(III) by FeNPs, Free cell and EPS. In addition, the practical removal efficiency of a range of REEs from real mine wastewaters by BC@FeNPs-EPS was consistently high being 88% for Eu(III), 89% for Tb(III) and 89% for Ce(III). The material produced was also highly reusable, maintaining a high Eu(III) removal rate (69%) even after 5 consecutive removal cycles.