Abstract
In this research, the capabilities of culture supernatants generated by the oxalate-producing fungus Aspergillus niger for the bioprecipitation and biorecovery of cobalt and nickel were investigated, as was the influence of extracellular polymeric substances (EPS) on these processes. The removal of cobalt from solution was >90% for all tested Co concentrations: maximal nickel recovery was >80%. Energy-dispersive X-ray analysis (EDXA) and X-ray diffraction (XRD) confirmed the formation of cobalt and nickel oxalate. In a mixture of cobalt and nickel, cobalt oxalate appeared to predominate precipitation and was dependent on the mixture ratios of the two metals. The presence of EPS together with oxalate in solution decreased the recovery of nickel but did not influence the recovery of cobalt. Concentrations of extracellular protein showed a significant decrease after precipitation while no significant difference was found for extracellular polysaccharide concentrations before and after oxalate precipitation. These results showed that extracellular protein rather than extracellular polysaccharide played a more important role in influencing the biorecovery of metal oxalates from solution. Excitation–emission matrix (EEM) fluorescence spectroscopy showed that aromatic protein-like and hydrophobic acid-like substances from the EPS complexed with cobalt but did not for nickel. The humic acid-like substances from the EPS showed a higher affinity for cobalt than for nickel.
Highlights
Microorganisms can play an important role in both the remediation and biorecovery of metals (Gadd 2010; Liang and Gadd 2017)
The experimental fungus used in this study was Aspergillus niger (ATCC 1015), which was incubated on malt extract agar slants (Lab M Limited, Heywood, Lancashire, UK) at 25 °C in the dark for 7 days to prepare spore suspensions for inoculation of liquid media according to a previous study (Kang et al 2019)
The 7-day-old supernatants collected from A. niger cultures exhibited significantly higher recovery of nickel than supernatants collected after 14 days (p < 0.05)
Summary
Microorganisms can play an important role in both the remediation and biorecovery of metals (Gadd 2010; Liang and Gadd 2017). Metal immobilization or recovery from solution can be achieved by bioprecipitation where metals are transformed from soluble species to insoluble compounds, such as oxides, carbonates, phosphates, oxalates, and sulfides (Haferburg and Kothe 2007; Tsezos 2009; Gadd et al 2014; Liang and Gadd 2017). Calcium recovery of more than 90% was achieved from a calcium-rich industrial wastewater using bacterial MICP
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