Kinetics of H_2-O_2-H_2O redox equilibria and formation of metastable H2O2 under low temperature hydrothermal conditions

摘要

Hydrothermal experiments were conducted to evaluate the kinetics of H_(2(aq)) oxidation in the homogeneous H_2-O_2-H_2O system at conditions reflecting subsurfaceear-seafloor hydrothermal environments (55-250°C and 242-497bar). The kinetics of the water-forming reaction that controls the fundamental equilibrium between dissolved H_(2(aq)) and O_(2(aq)), are expected to impose significant constraints on the redox gradients that develop when mixing occurs between oxygenated seawater and high-temperature anoxic vent fluid at near-seafloor conditions. Experimental data indicate that, indeed, the kinetics of H_(2(aq))-O_(2(aq)) equilibrium become slower with decreasing temperature, allowing excess H_(2(aq)) to remain in solution. Sluggish reaction rates of H_(2(aq)) oxidation suggest that active microbial populations in near-seafloor and subsurface environments could potentially utilize both H_(2(aq)) and O_(2(aq)), even at temperatures lower than 40°C due to H_(2(aq)) persistence in the seawater/vent fluid mixtures. For these H_2-O_2 disequilibrium conditions, redox gradients along the seawater/hydrothermal fluid mixing interface are not sharp and microbially-mediated H_(2(aq)) oxidation coupled with a lack of other electron acceptors (e.g. nitrate) could provide an important energy source available at low-temperature diffuse flow vent sites.More importantly, when H_(2(aq))-O_(2(aq)) disequilibrium conditions apply, formation of metastable hydrogen peroxide is observed. The yield of H_2O_(2(aq)) synthesis appears to be enhanced under conditions of elevated H_(2(aq))/O_(2(aq)) molar ratios that correspond to abundant H_(2(aq)) concentrations. Formation of metastable H_2O_2 is expected to affect the distribution of dissolved organic carbon (DOC) owing to the existence of an additional strong oxidizing agent. Oxidation of magnetite and/or Fe~(++) by hydrogen peroxide could also induce formation of metastable hydroxyl radicals (?OH) through Fenton-type reactions, further broadening the implications of hydrogen peroxide in hydrothermal environments.