Phenanthrene degradation and concomitant electricity generation using a bioelectrochemical process

Environmental pollution by petroleum hydrocarbons resulting from oil spillages, leaks or indiscriminate disposal is a serious problem. The hydrocarbons are toxic, posing serious threats to human health from contaminated groundwater and soils. While a number of approaches (e.g. biostimulation) are available to deal with the problem, many of them are expensive, ineffective, too slow or environmentally unfriendly. Recently, the use of bioelectrochemical systems (BES) for promoting the enhanced anaerobic degradation of petroleum hydrocarbons has been suggested. In this study, five different inocula (S. oneidensis MR1, P. aeruginosa, undefined mixed culture, co-culture of S. oneidensis and P.aeruginosa; and the co-culture + the undefined mixed culture) were evaluated for their potential in co-metabolic degradation of phenanthrene (a model petroleum hydrocarbon) in a microbial fuel cell (MFC). All the five inocula showed very high potential for phenanthrene degradation with a minimum degradation efficiency of 95%. P.aeruginosa gave the highest degradation rate of 0.055 mM/d with the S.oneidensis giving the lowest degradation rate of 0.027 mM/d. These degradation rates are considerably higher than those observed for anaerobic degradation in a non-MFC reactor. The maximum power density of the MFCs was 1.1 mW/m2 obtained using a co-culture; the lowest, 0.19 mW/m2, was obtained using P.aeruginosa. Coulombic efficiencies for all MFCs was very low (range 0.4 -1.0%) perhaps due to electron diversion to methane formation or reaction with oxygen from the cathode to form water. Adsorption studies revealed that the anode (carbon) electrode has a high affinity for phenanthrene with 90% of the solute adsorbed in less than 3h.This suggests the carbon electrode could be used in co-localising petroleum contaminants in the environment while they are being degraded by electrochemically active bacteria iin the anode of a MFC. This work highlights the possibility of using microbial fuel cells to achieve high degradation rates of phenanthrene through co-metabolism with concomitant electricity production and could potentially be utilised as an independent system or be integrated with other bioremediation technologies (e.g. pump and treat or permeable reactive barriers) to remediate petroleum hydrocarbons from contaminated subsurface environments or industrial effluents.