Empowering electroactive microorganisms for soil remediation: Challenges in the bioelectrochemical removal of petroleum hydrocarbons

Microbial electrochemical technologies (MET) are increasingly being considered for environmental remediation applications, mainly for their unique capability to enhance microbial degradation processes in an environmentally sustainable manner (e.g., without requiring addition of chemicals and with little or even no energy consumption). To date, however, the application of MET for the remediation of saturated and unsaturated soils contaminated by petroleum hydrocarbons (PH) remains challenged by a number of environmental and operational factors which have, so far, hampered a more rapid deployment of the technology. In this context, this critical review has comprehensively analyzed the recent scientific literature dealing with electrobioremediation of PH-contaminated soils, in order to disentangle the impact of key process parameters (e.g., type of electrodes, system configurations, design criteria) and environmental conditions (e.g., soil characteristics and strategies to manipulate thereof, type of contaminants, composition of PH-degrading communities) on the overall remediation performance. Interestingly, the analysis revealed that MET-based soil electrobioremediation has been successfully applied to remove a variety of PH (from alkanes to polycyclic aromatic hydrocarbons and mixtures thereof) from soils displaying a broad range of electric conductivities (0.2-6 mS/cm), using different system configurations (from simple graphite rod buried within soils to more complex tubular electrode assemblies). To date, the limited radius-of-influence of electrodes buried in contaminated soils, which is typically lower than 50 cm, appears to be a main limiting factor which requires specific strategies (e.g., amendment of soil with conductive materials/minerals and/or surfactants) to be properly addressed. Finally, the study highlights the urgent need for pilot-scale testing to confirm the promising results obtained at the laboratory-scale under more controlled, yet often far-less representative, conditions as well as to catalyze the commercial and societal interest towards this novel technology.