Abstract
Hydrothermal experiments were conducted to evaluate the kinetics of H2(aq) oxidation in the homogeneous H2-O2-H2O system at conditions reflecting subsurface/near-seafloor hydrothermal environments (55-250°C and 242-497bar). The kinetics of the water-forming reaction that controls the fundamental equilibrium between dissolved H2(aq) and O2(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 H2(aq)-O2(aq) equilibrium become slower with decreasing temperature, allowing excess H2(aq) to remain in solution. Sluggish reaction rates of H2(aq) oxidation suggest that active microbial populations in near-seafloor and subsurface environments could potentially utilize both H2(aq) and O2(aq), even at temperatures lower than 40°C due to H2(aq) persistence in the seawater/vent fluid mixtures. For these H2-O2 disequilibrium conditions, redox gradients along the seawater/hydrothermal fluid mixing interface are not sharp and microbially-mediated H2(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 H2(aq)-O2(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 H2(aq)/O2(aq) molar ratios that correspond to abundant H2(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 (•OH) through Fenton-type reactions, further broadening the implications of hydrogen peroxide in hydrothermal environments.
Original language | English (US) |
---|---|
Pages (from-to) | 1594-1607 |
Number of pages | 14 |
Journal | Geochimica et Cosmochimica Acta |
Volume | 75 |
Issue number | 6 |
DOIs | |
State | Published - Mar 15 2011 |
Bibliographical note
Funding Information:This research was conducted with partial support from the NSF OCE-0752221 and the Geophysical Laboratory Postdoctoral Fellowship. We would also like to acknowledge contributions by the W.M. Keck Foundation and Shell towards supporting the hydrothermal lab at the Geophysical Lab. SMS acknowledges support from NSF OCE-0452333 and the Alfried-Krupp Wissenschaftskolleg Greifswald (Germany), while W.E.S. acknowledges support from NSF Grants OCE-0549457 and OCE-0813861 . Finally, we would like to thank the contributions and comments from the Associate Editor, Dr. J. Alt and three anonymous reviewers.