Sequential reduction of bivalent copper and nickel in electroplating wastewater using bioelectrochemical systems inoculated with novel Enterococcus species.

Electroplating wastewater, characterized by high concentrations of bivalent copper (Cu²⁺) and nickel (Ni²⁺), poses significant environmental and health risks. This study explores the potential of novel Enterococcus species AMZ3, AMZ8, and AMZ5 as biocatalysts in bioelectrochemical systems (BES) for the dual purpose of electricity generation and heavy metal recovery. The strains were isolated from microbial fuel cell (MFC) biofilms and evaluated in single-chamber MFCs and dual-compartment systems. A mixed culture of the strains outperformed individual species, achieving a peak power and current densities of 439.78 mW/m² and 5.31 A/m², respectively. In addition, the system achieved a remarkable chemical oxygen demand removal efficiency of 94.6 ± 11.23% and a Coulombic efficiency of 33.7 ± 7.11%. Enhanced electrocatalytic activity in mixed-culture systems was attributed to synergistic microbial interactions, superior biofilm formation, and elevated extracellular polymeric substance protein content. Cyclic voltammetry and electrochemical impedance spectroscopy revealed reduced internal resistance and robust electron transfer pathways in the reactor containing the biofilms of the mixed Enterococcus species. Furthermore, BES with the mixed Enterococcus biofilms achieved copper and nickel removal efficiencies of 99.99 ± 0.01 and 99.96 ± 0.02%, respectively. The reduction and recovery of these metals occurred at the cathode, where copper was predominantly recovered as Cu⁰ through bioelectrochemical reduction, while nickel was recovered as metallic Ni⁰ through bioelectrochemical reduction, with surface-bound Ni²⁺ also detected, likely formed post-deposition due to oxidative surface processes, as revealed by SEM-EDX, XRD, and XPS analyses. These findings establish the feasibility of mixed Enterococcus cultures in sustainable wastewater treatment, paving the way for scalable BES applications.