Kinetics of H2-O2-H2O redox equilibria and formation of metastable H2O2 under low temperature hydrothermal conditions

Hydrothermal experiments were conducted to evaluate the kinetics of H-2(aq) oxidation in the homogeneous H-2-O-2-H2O system at conditions reflecting subsurface/near-seafloor hydrothermal environments (55-250 degrees C and 242-497 bar). 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 degrees 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 H2O2(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 H2O2 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 (center dot OH) through Fenton-type reactions, further broadening the implications of hydrogen peroxide in hydrothermal environments. (C) 2010 Elsevier Ltd. All rights reserved.