Effects of dissolved oxygen and carbon dioxide under oxygen-rich conditions on the biooxidation process of refractory gold concentrate and the microbial community

• The biooxidation of the concentrate under different DO and CO2 levels was studied.
• Microbial community structure was detected under different DO and CO2 levels.
• The cellular ROS level under oxygen-rich condition was measured.
• Bacterial growth and energy utilization were found to be decoupled.

Biomining microorganisms obtain energy from the oxidation of reduced sulfur and iron (II) by using dissolved oxygen (DO) as the electron acceptor. Carbon dioxide, the carbon source for biomining microorganisms, is essential for biooxidation. However, to date, no published reports exist regarding the effect of reactive oxygen species (ROS) and the CO2 content in an oxygen-rich condition (when oxygen is sufficient) on the biooxidation process. In this study, a microbial community was used to oxidize refractory sulfide gold concentrate in a 1.5 L experimental stirred tank reactor. The effects of DO in a slurry and the CO2 content in the intake gas on the biooxidation process, bacterial growth and microbial community were investigated. It was found that the biooxidation efficiency increased at first and then decreased as the DO level elevated, while the content of ROS significantly increased within the bacteria cells. Under an oxygen-rich condition, the biomass increased as the CO2 content increased, while the biooxidation efficiency first increased and then decreased. These changes revealed that the oxidation activity of biomining microorganisms was inhibited by a high CO2 content and that bacterial growth and energy utilization were decoupled. Leptospirillum ferriphilum-like bacteria and Sulfobacillus thermosulfidooxidans were the dominate strains in the experiment. As the DO increased, the relative proportion of L. ferriphilum-like bacteria in the bacteria community first increased and then decreased, while S. thermosulfidooxidans showed the opposite trend. With an increasing CO2 content in the intake gas, the relative proportion of S. thermosulfidooxidans increased, while that of L. ferriphilum-like bacteria decreased.