Microbial electrochemical strategies for monitoring and remediating organic pollution in groundwater and sediments

Abstract

Microbial electrochemistry is a biotechnological field that explores interaction between microorganisms and electrically conductive materials. Such studies have evolved to develop a plethora of environmental applications, known as Microbial Electrochemical Technologies (MET). MET can be used to clean up polluted environments by utilizing electrodes as terminal electron acceptors or donors, which enables microbial metabolism to occur beyond natural conditions. This technology is highly versatile and can be applied to a range of matrices, including wastewater, groundwater, sediment and soil. However, the implementation of MET in real-field applications requires overcoming microbial, technological, and economic challenges. Despite these challenges, MET exhibit great potential as a strategy to enhance environmental remediation. In this thesis, we have explored the capability of MET in natural environments, for both i) to detect groundwater-contaminants like petroleum derived compounds and agrochemical compounds like lindane (Chapter 2) and ii) to remediate lindanepolluted environments (Chapter 3 and Chapter 4). Through our research, we have demonstrated the ability of these technologies to both detect the presence of contaminants but also to stimulate their degradation, leading to the restoration of natural environments. Chapter 1 represents an updated state of the art regarding the thesis topic: electrobioremediation and bioelectrochemical detection of contaminants in polluted soil. We indeed provided an overview about environmental pollution specially devoted to aromatic (BTEX) and chlorinated hydrocarbons including their impact in the environment and human health. Furthermore, we reviewed various methods for detecting and removing pollutants from the environment, specially those relevant to our research. In the final section of the chapter, we introduce MET, including their fundamental principles and applications, with an emphasis on how to enhance microbial metabolism of electroactive bacteria for detecting and remediating polluted environments. Under the following statement one of the best ways to prevent contamination is to monitor risky locations, we have explored innovative methods for developing in-situ early detection of pollutants in groundwater. Indeed, in Chapter 2, we used microbial electrochemical strategies to detect contaminants, such as petroleum hydrocarbons or agrochemicals, in groundwater at microcosm and mesocosm scales. The biosensor consisted of a 3-electrode configuration with a working electrode polarized at anodic potential (0.6 V vs. Ag/AgCl), inserted inside a piezometer. A microbial community of uncontaminated groundwater was used to colonize the electrode, then we observed a response (<2 hours) to a pulse containing a mixture of pollutants such as BTEX and ETBE. Additionally, we also tested the response to complex mixtures using a kerosene spike. We used a biocathode-based sensor strategy (- 0.6 V vs. Ag/AgCl) to monitor electrical current consumption, associated with dehalogenation, in the presence of the insecticide lindane (gamma-hexachlorocyclohexane). Electrobioremediation is a strategy to clean-up pollution using a combination of electrochemical tools and microbiology. In Chapter 3, we designed and validated at different configurations for removing a widely used pesticide, lindane, from a synthetic and a real polluted soil. Cathodic configuration resulted to show the higher remediation efficiency. In fact, electrode acted as an electron donor and removed lindane ca. 10 times faster than natural attenuation. Moreover, different isomers of lindane were removed using different configurations. Finally, we could demonstrate that even nonflooded polluted soil could be electrobioremediated. For several years lindane production residues were discharged in Sabiñanigo (Huesca, Spain) and, eventually, landscape was vastly polluted. In this context, we explored strategies ways to clean-up contaminated real soil using in-situ electrobioremediation (Chapter 4). Over a period of 20 weeks, different electrobioremediation configurations were tested. The results revealed a cathode-based configuration as the most effective to remove HCH contaminants. Different isomers throw up different removal efficiencies. The majority isomer, α-HCH, was almost completely removed; however, the most persistent isomer, β-HCH, was only partially removed. Furthermore, phytoxicity analysis showed that a cathode-based configuration was effective for promoting plant growth. Regarding the composition of microbial community, cathode-based configurations selected cathodophilic bacteria, while anode-based configurations selected anodophilic and aromatic degrading bacteria. Finally, in Chapter 5 we have included a general discussion, a series of conclusions after results from the current thesis, and future strategies for optimizing the electrobioremediation actions. The general discussion is presented as a questionanswer format, highlighting the favorable impact of electromicrobiology for remediating polluted environments at physical, chemical, and biological level.