Effects of bicarbonate on electro-bioremediation of phenanthrene-contaminated groundwater

Electro-bioremediation under anaerobic conditions is an effective approach for refractory organic matter removal in groundwater. Bicarbonate (HCO₃^-) is an inorganic carbon source and electron acceptor in groundwater, however, the influencing mechanism of HCO₃^- on pollutant removal of electro-bioremediation remains unclear. Herein, the effects of HCO₃^- concentration on electro-bioremediation of phenanthrene (PHE)-contaminated groundwater were investigated. HCO₃^- could facilitate the PHE degradation while an HCO₃^- concentration of higher than 1000 mg L^-1 had a significant inhibition effect. Among the HCO₃^- concentration of 100-5000 mg L^-1, the highest PHE degradation efficiency of 75.04-80.18 % was achieved in the electro-biochemical reactor with 500 mg L^-1 HCO₃^-. The PHE removal efficiency was negatively correlated with the current density during the electro-bioremediation process, due to the effect of HCO₃^- concentrations on the electrolyte conductivity in the reactors. The electro-bioremediation process could increase the richness of diversity of microbes. Methanomethylovorans and the PHE-degrading bacteria including Pelolinea, Clostridium sensu stricto 5, Diaphorobacter, Methyloversatilis and Flavobacterium were the main microbes involved in PHE degradation. Of them, Methanomethylovorans was significantly positively correlated with the PHE removal efficiency. The potential metabolic function analysis revealed that the bacterial chemotaxis, flagellar assembly, carbohydrate metabolism and ABC transporters were prompted with the addition of HCO₃^-, while they were inhibited with the increasing HCO₃^- concentration. These findings suggested that electro-bioremediation technology was suitable for the remediation of polycyclic aromatic hydrocarbons such as PHE-contaminated groundwater in low bicarbonate areas.