Metal removal and recovery from zinc refinery residue using microbiologically produced elemental chalcogen nanoparticles
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This research aimed at investigating the application of elemental chalcogen nanoparticles for the removal of metals from wastewaters. In the first part of the study, biogenic (Bio) selenium nanoparticles (SeNPs) were investigated for the removal of nickel (Ni) as a model divalent heavy metal ion in both batch and continuous mode of operation. The removal was instantaneous with an adsorption capacity of 29.411 mg Ni/g SeNPs. The continuous study was carried out in fixed bed adsorption columns with SeNPs immobilized in alginate as the supporting matrix and found a regeneration potency up to eight repeated sorption-desorption cycles with 59% of the original adsorption efficiency intact. Following this, other elemental chalcogens of sulfur and tellurium nanoparticles (TeNPs) were added to investigate the removal of the critical raw materials (CRMs) gallium (Ga) and germanium (Ge) from zinc (Zn) refinery residues. A comparative analysis of chalcogen based biosorption and sulfide based bioprecipitation revealed biosorption as the preferred technique for the removal of Ga and Zn. BioTeNPs were found as the best adsorbent for the removal of Ga with an adsorption capacity of 74 mg of Ga3+ per g BioTeNPs. The smaller size of BioTeNPs (5.30-86.1 nm) and higher negative zeta potential (-32 mV) of BioTeNPs imparted a superior adsorption efficiency. The second part of the study evaluated the reductive properties of extracellular polymeric substances (EPS) on selenite and tellurite to their respective NPs. The EPS was identified to control the shape, surface charge and hence the colloidal stability of the NPs. The predominant presence of proteins in the EPS component influenced the surface properties of NPs. The properties of NPs were checked both in individual and conjugated forms. The conjugated Se-Te NPs had a smaller size (24.7-123 nm) and more negative zeta-potential (- 33.5 mV) of Se-Te NPs compared to their individual forms (Se/Te NPs).