Nickel and copper biosorption from electroplating wastewater by EPS-producing bacteria

The rapid growth of heavy industries and the unregulated discharge of metal contaminated materials cause significant adverse effects on both human health and environment. Biological systems and engineering have been proved to be efficient for preventing and controlling heavy metal pollution, as well as to recover some precious metals present in contaminated wastewater. Biotechnological applications reside on the exploitation of heavy metal resistant bacteria that uptake and block metals inside the cell or bind them to outer structures such as cell wall or extracellular polymeric substance (EPS) layers. Taking into consideration the heavy metal binding activity of bacterial EPS, the ultimate scope of the present work was to select efficient metal binding strains to be exploited in a process of metal removal from electroplating wastewater. Ninety five strains were tested for their ability to resist to nickel, copper and chromium VI in liquid mineral medium and to produce EPS in sucrose-rich medium. Nickel and copper bioaccumulation was monitored by ICP-MS analysis in experiments were conducted in electroplating wastewater with non proliferating cell suspensions, previously grown in EPS-inducing and –not inducing conditions. Diverse species of Bacillus, Proteobacteria and Actinobacteria were able to resist up to 2 mM nickel, 5 mM copper and chromium VI. Genes coding for heavy metals efflux pumps were identified in two Stenotrophomonas maltophilia strains (98% identity to CzcA of Stenotrophomonas maltophilia D457, Acc. Num. YP006182928). EPS producing phenotype was rare and only nine strains were able to produce EPS in sucrose-rich medium. Among the most promising strains, Stenotrophomonas maltophilia and Serratia plymuthica strains accumulated nickel up to 300.37 and 422.63 mg g-1 dried cells. Metal removal was negligible in the presence of cell suspensions grown in EPS-not inducing conditions. These results show that the EPS produced by some of the tested strains are involved in metal biding processes and they are agents for heavy metal removal. The selected strains will be used in experimentations at pilot scale and further exploited, in order to provide adequate solutions to persisting environmental problem. Acknowledgements: The financial supports by MIUR-Regione Lombardia contract No. 30066464 and by Fondazione CARIPLO contract No. 2014-1301 were greatly appreciated.