Elucidating the impacts of cobalt (II) ions on extracellular electron transfer and pollutant degradation by anodic biofilms in bioelectrochemical systems during industrial wastewater treatment

Abstract

Electrogenic biofilms in bioelectrochemical systems (BES) are critical in wastewater treatment. Industrial effluents often contain cobalt (Co²⁺); however, its impact on biofilms is unknown. This study investigated how increasing Co²⁺ concentrations (0-30 mg/L) affect BES biofilm community dynamics, extracellular polymeric substances, microbial metabolism, electron transfer gene expression, and electrochemical performance. The research revealed that as Co²⁺ concentrations increased, power generation progressively declined, from 345.43 ± 4.07 mW/m² at 0 mg/L to 160.51 ± 0.86 mW/m² at 30 mg/L Co²⁺. However, 5 mg/L Co²⁺ had less effect. The Co²⁺ removal efficiency in the reactors fed with 5 and 10 mg/L concentrations exceeded 99% and 94%, respectively. However, at 20 and 30 mg/L, the removal efficiency decreased substantially, likely because of reduced biofilm viability. FTIR indicated the participation of biofilm functional groups in Co²⁺ uptake. XPS revealed Co²⁺ presence in biofilms as CoO and Co(OH)₂, indicating precipitation also aided removal. Cyclic voltammetry and electrochemical impedance spectroscopy tests revealed that 5 mg/L Co²⁺ had little impact on the electrocatalytic activity, while higher concentrations impaired it. Furthermore, at a concentration of 5 mg/L Co²⁺, there was an increase in the proportion of the genus Anaeromusa-Anaeroarcus, while the genus Geobacter declined at all tested Co²⁺ concentrations. Additionally, higher concentrations of Co²⁺ suppressed the expression of extracellular electron transfer genes but increased the expression of Co²⁺-resistance genes. Overall, this study establishes how Co²⁺ impacts electrogenic biofilm composition, function, and treatment efficacy, laying the groundwork for the optimized application of BES in remediating Co²⁺-contaminated wastewater.

Keywords: Bioelectrochemical system; Cobalt (II); Electrogenic biofilms; Extracellular electron transfer.