Microaerobic biodegradation of aromatic hydrocarbon mixtures: strategies for efficient nitrate and oxygen dosage

The biodegradation of organic aromatic compounds in subsurface environments is often hindered by limited dissolved oxygen. While oxygen supplementation can enhance in situ biodegradation, it poses financial and technical challenges. This study explores introducing low-oxygen concentrations in anaerobic environments for efficient contaminant removal, particularly in scenarios where coexisting pollutants are present. An innovative strategy of alternating nitrate-reducing and microaerobic conditions to stimulate biodegradation is proposed, utilizing nitrate initially to degrade easily-degradable compounds, and potentially reducing the need for additional oxygen. Batch experiments were conducted to assess the biodegradation of a BTEX, indene, indane, and naphthalene mixture using groundwater and sediments from an anaerobic contaminated aquifer. Two set-ups were incubated for 98 days to assess the redox transitions between microaerobic (oxygen concentrations < 0.5 mg O₂ L^-1) and nitrate-reducing conditions, aiming to minimize external electron acceptor usage while maximizing degradation. Comparative experiments under fully aerobic and fully anaerobic (nitrate-reducing) conditions were conducted, revealing that under microaerobic conditions, all compounds were completely degraded, achieving removal efficiencies comparable to fully aerobic conditions. A pre-treatment phase involving nitrate-reducing conditions followed by microaerobic conditions showed more effective utilization of oxygen specifically for contaminant degradation compared to fully aerobic conditions. Contrarily, under fully anaerobic conditions, without oxygen addition, partial degradation of ethylbenzene was observed after 400 days, while other compounds remained. The outcomes of this study can provide valuable insights for refining strategies involving oxygen and nitrate dosages, thereby enhancing the efficacy of in situ bioremediation approaches targeting complex hydrocarbon mixtures within anaerobic subsurface environments. KEY POINTS: • BTEX, indene, indane, and naphthalene mix biodegraded under microaerobic conditions • Subsurface microorganisms swiftly adapt from nitrate to microaerobic conditions • More oxygen directed to hydrocarbon biodegradation via a pre-anaerobic treatment.