Optimization for silver remediation from aqueous solution by novel bacterial isolates using response surface methodology: Recovery and characterization of biogenic AgNPs

Highlights: • Three highly efficient bacterial strains capable of silver removal were isolated from silver mining site and characterized. • The RSM is a powerful tool for optimizing the operational conditions for silver removal by bacterial isolates. • Among three strains, Enterobacter cloacae alone can remove up to 100% of added silver and synthesized AgNPs. • The synthesized AgNPs were recovered through centrifugation and by applying external voltage directly from the crude mixture. • UV-vis spectroscopy, DLS, SEM, FTIR, TEM, EDS mapping were used to characterize biogenic AgNPs. -- Abstract: Silver is a toxic but precious heavy metal that has been implemented in diverse biomedical and environmental sectors. Extensive use of this metal has provoked severe environmental concerns. Hence there is an increasing demand for the development of a simple, inexpensive and eco-friendly approach for the remediation and recovery of silver. In this study, novel bacterial strains Enterobacter cloacae SMP1, Cupriavidus necator SMP2, and Bacillus megaterium SMP3 were isolated from silver mining site for the sake of silver remediation. Various experimental factors including temperature, pH and inoculum size (I_S) were optimized for silver remediation by SMP1 using central composite design (CCD) based on response surface methodology (RSM). For maximum 100% removal of silver the optimized values of temperature, pH and I_S were 23.5 °C, 7.5 and 2% (v/v) respectively in less than 10 h of incubation. Simultaneously, silver nanoparticles (AgNPs) were harvested through centrifugation (M1) and by applying voltage (M2) to the crude remediation mixture. The AgNPs, characterized by UV–vis spectroscopy, dynamic light scattering (DLS), and cryo-scanning transmission electron microscopy (Cryo-SETM), were spherical shaped and 1.75–8.7 nm in diameter. The average zeta potentials (ZP) of AgNPs isolated by M1, and M2 were -35.8 mV and -45.2 mV respectively.