Novel applications of strain Cupriavidus metallidurans CH34 for toluene removal in bioelectrochemical systems

Bioelectrochemical systems (BESs) have proven to be a useful tool for bioremediation and have been applied to achieve the oxidation of hydrocarbons at the anode. Microbial metabolism can be stimulated in a BES when overpotential is applied, increasing the rate of pollutant degradation. The aim of this work was to test the exoelectrogenic capacity of Cupriavidus metallidurans CH34 and to determine if the application of overpotential stimulates bioremediation of toluene-polluted water. Exoelectrogenic activity was studied with succinate as sole carbon source. C. metallidurans CH34 showed exoelectrogenic activity (maximum current density obtained was 0.65 mA/m2), reaching a 92% of succinate removal after 42 h. Although the upper pathway of toluene degradation from C. metallidurans CH34 has not been reported, C. metallidurans CH34 significantly reduced the concentration of toluene (from 100 to 42 ppm in 27 days) under denitrifying conditions. The concentration of toluene in the abiotic control remained stable. A Microbial Fuel Cell (MFC) was set with a pure culture of C. metallidurans CH34 using toluene as sole carbon source. When C. metallidurans CH34 was inoculated in a MFC containing toluene as sole carbon source, current densities reached 0.80 mA/m2. Conversely, in the abiotic control no current was detected. Toluene concentration in the control remained stable (38 ppm) while in inoculated MFC toluene decreased by 37% after 10 days. A Microbial Electrolysis Cell (MEC) was set with a pure culture of C. metallidurans CH34 using toluene as sole carbon source. In the MEC, overpotential (+800 mV) was applied between the anode and the cathode. In the biotic MEC, seven respikes were performed. Current density reached a maximum of 45 mA/m2 after the 7th respike, while in the control, current densities remained constant at 10 mA/m2. Current density increased when toluene was further spiked in the reactors. After the initial adsorption of toluene on the anodes of both inoculated and abiotic MECs, above 90% of the initial toluene in the biotic MEC was removed, while this compound remained stable in the abiotic reactor. Coulombic efficiencies of the toluene mineralization process increased with time. The coulombic efficiency between the second and the third respike was 18.9%, while the coulombic efficiency of the last period increased to 76.9%. These results showed that C. metallidurans CH34 is an exoelectrogen able to degrade toluene under anaerobic conditions and that the application of an external overpotential increased the rate of toluene removal and current production.