A biogenerated polymetallic catalyst from society's wastes

Aims: Preparation of the new metals-polymeric composite, Metx-EPS (I), to be used as a green catalyst in water or in two-phase aqueous conditions. Study design: Recovery and valorization of polymetallic wastes to obtain directly new catalysts using a microorganism to explore their application in removal of difficult and dangerous chemical pollutants present in aqueous environment. Place and Duration of Study: Dipartimento di Scienze Molecolari e Nanosistemi, Università Ca’ Foscari Venezia, Venezia Mestre, Italy; University of Nova Gorica, Nova Gorica, Slovenia, Institut Jozef Stefan, Ljubljana, Slovenia and Department of Biology, Biotechnical Faculty, University of Ljubljana, Ljubljana, Slovenia, between February 2018 and January 2019. Methodology: For the preparation of Metx-EPS (I), the metals source was an exhausted catalytic converter that was grinded and treated with an acidic solution at 20-25°C. After filtration, the solution was concentrated, neutralized and added to a broth of Klebsiella oxytoca DSM 29614 to produce (I) where metals are embedded in a peculiar polysaccharide structure. The composite was easily recovered from the fermentation broth and purified. The process protocol was verified many times and was shown to be reproducible satisfactorily. The % recovery of metals, originally present in the converter, was good as determined by atomic absorption. The morphology and the chemical state of main metals in (I) were investigated by X-ray absorption spectroscopy methods (XANES and EXAFS). No metallic alloy seems to be evident. Results: As first application of (I) as catalyst, the hydrodechlorination treatment of polychlorinated biphenyls (PCBs) was studied in water/methanol. A significant removal of higher chlorinated congeners was observed working at 1MPA of hydrogen and 60°C. This result improves significantly and surprisingly the methodology, previously studied by us using mono- or bi-metals embedded in the same polysaccharide moiety, indicating that positive synergies among the different metals were operating. Conclusion: The preparation of this new polymetallic species embedded in a polysaccharide moiety starting from spent catalytic converters represents an alternative valorisation of metallic wastes. Moreover, a synergic effect was exerted by the different metals when the catalyst Metx-EPS (I) was used in the hydrodechlorination treatment of polychlorinated biphenyls (PCBs) in water/methanol. Finally, a promising preliminary proof of concept for the removal of polychlorinated aromatic pollutants even in contaminated aqueous sites was carried out.