Dottorato in PROCESSI CHIMICI PER L'INDUSTRIA E PER
L'AMBIENTE

Titolo della tesi: Harnessing Electrobioremediation for Sustainable Groundwater Decontamination: Mechanisms, Applications and Efficacy

This doctoral thesis presents a comprehensive investigation into the application of microbial electrochemical technologies (MET) for the bioremediation of groundwater contaminated by diverse pollutants, including petroleum hydrocarbons, chlorinated solvents, and heavy metals. MET are recently attracting considerable attention in the field of soil and groundwater remediation since, unlike conventional methods, allow for a more sustainable and effective treatment and a greater control over biodegradation processes, making it possible to target specific contaminants and optimize the required environmental conditions in situ. The research includes five distinct studies, each focusing on the characterization and development of MET systems designed to enhance the removal efficiency of multiple contaminants through integrated oxidative and reductive processes. The first study examines the degradation pathways of toluene, a representative petroleum hydrocarbon, within a tubular bioelectrochemical reactor. Through continuous-flow operations at varying influent concentrations, we elucidated a syntrophic degradation mechanism involving aromatic ring breaking and subsequent conversion of metabolic intermediates into volatile fatty acids (VFAs). The anode was continuously polarized at +0.2 V vs. SHE. Under these conditions, the highest achieved toluene removal rate reached 71 ± 13 mg L⁻¹ d⁻¹, with an average degradation of the influent contaminant load of about 70%. The microbial community analysis revealed a cooperative interaction among hydrocarbon degraders, fermentative bacteria, and electroactive microorganisms, highlighting the synergy essential for optimal contaminant breakdown. In the second study, we demonstrated the simultaneous treatment of a synthetic groundwater mixture containing toluene and trichloroethene (TCE) using the previously developed tubular bioelectrochemical reactor. The anode was continuously polarized at +0.2 V vs. SHE, which enabled maximum degradation rates of 150 µmol L⁻¹ d⁻¹ for toluene and 500 µeq L⁻¹ d⁻¹ for TCE. This polarized graphite anode effectively promoted toluene oxidation, generating a significant electric current that further supported the H₂-driven biological reduction of TCE to less-chlorinated intermediates, such as cis-DCE and VC. A phylogenetic analysis of the reactor's microbial community highlighted the functional potential for both anaerobic toluene oxidation and TCE reductive dechlorination, emphasizing the system’s capacity for multi-contaminant treatment. The third study explored the concurrent degradation of toluene and chloroform in an the anaerobic bioelectrochemical reactor. Our results demonstrated the effective removal of both contaminants, with maximum degradation rates of 47 µmol L⁻¹ d⁻¹ for toluene and 60 µmol L⁻¹ d⁻¹ for chloroform when the anode was polarized at +0.4 V vs. (SHE). However, the presence of acetate as a co-substrate exhibited competitive inhibition effects on toluene degradation, highlighting the complexities inherent in multi-pollutant bioremediation strategies. The fourth study focused on the integration of copper removal with toluene degradation in a single-chamber bioelectrochemical cell. We achieved near-complete removal of both contaminants by exploiting the electric current generated from toluene oxidation at the anode to drive the abiotic reduction and precipitation of copper at the cathode. Detailed chemical and microbiological characterizations revealed a robust anodic biofilm capable of effective biodegradation, alongside uniform deposition of Cu₂O nanoparticles on the cathode. Finally, the fifth study investigated the novel combination of bioelectrochemical systems with conductive carbon nanotube membranes for the treatment of nitrate-contaminated waters. This approach facilitated simultaneous filtration and biodegradation, leading to efficient nitrate reduction without nitrite accumulation. The electroactive properties of the membranes enhanced microbial attachment and activity, resulting in stable nitrate removal rates exceeding 900 mg N/m²·d. Collectively, these studies contribute significant insights into the mechanisms and optimization of microbial electrochemical technologies for groundwater bioremediation, underscoring their potential for addressing complex contamination scenarios. Future research directions include refining reactor design, exploring alternative electrode materials, and scaling up the systems for practical applications.