Ni²⁺ and Cu²⁺ Biosorption by EPS-Producing Serratia plymuthica Strains and Potential Bio-Catalysis of the Organo–Metal Complexes

In this study, a biosorption system for nickel (Ni2+) and copper (Cu2+) removal by selected exopolymeric substance-producing bacterial strains was evaluated from the perspective of water remediation. A preliminary screening in a biofilm-based filtration system allowed the selection of two best-performing Serratia plymuthica strains for specific Ni2+ and Cu2+ removal from synthetic solutions, as well as the definition of the optimal growth conditions. Further tests were conducted in a planktonic cell system in order to evaluate: (i) the effect of contact time, (ii) the effect of initial metal concentration, and (iii) the effect of biomass dose. S. plymuthica strain SC3I(2) was able to remove 89.4% of Ni2+ from a 50 mg L−1 solution, and showed maximum biosorption capacity of 33.5 mg g−1, while S. plymuthica strain As3-5a(5) removed up to 91.5% of Cu2+ from a 200 mg L−1 solution, yielding maximum biosorption capacity of 80.5 mg g−1. Adsorption equilibria of both metals were reached within 30 min, most of the process occurring in the first 2–4 min. Only Ni2+ biosorption data were adequately described by Langmuir and Freundlich isothermal models, as Cu2+ was in part subjected to complexation on the exopolymeric substances. The capability of the exopolymeric substances to stably coordinate a transition metal as Cu2+ offers the possibility of the eco-friendly re-use of these new hybrid systems as catalysts for application in addition reaction of B2(pin)2 on α,β-unsaturated chalcones with good results. The systems formed by biomass and Ni2+ were instead evaluated in transfer hydrogenation of imines. The biosorption performances of both strains indicate that they have the potential to be exploited in bioremediation technologies and the obtained organo–metal complexes might be valorized for biocatalytic purposes.