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 B-2(pin)(2) on alpha,beta-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.