T15 New Perspectives in Karst Geomicrobiology and Redox Geochemistry: Advances from 20 Years of Interdisciplinary Research and Exploring Our Future Frontiers

Sunday, 1 November 2015
8:00 AM - 12:00 PM
Room 340

THE ORIGIN OF H2S AND H2SO4 IN SULFURIC ACID CAVES: AN ENVIRONMENTAL CHEMISTRY PERSPECTIVE

Paper number: T15-13
Program: Technical Sessions
Time: 11:10 AM – 11:25 AM

Authors

  • Mark W. Joop Presenting Author
    New Mexico State University - Carlsbad, Carlsbad, NM

Abstract

Many karst scientists consider the source of H2S in the groundwater in active sulfuric acid caves to be deep, and even associate the source with petroleum reservoirs. However, the environmental fate of H2S is not being taken into account. H2S is a diprotic acid. Only in cold, acidic water will H2S not dissociate (H2S + H2O → H3O + HS) (Ka1 = 7 in pure water at 25°C, and this value decreases with increasing temperature and salinity). In most of the active sulfuric acid caves in the world, the spring water is thermal, saline, and close to neutral pH. H2S is not found in seawater because the pH range (7.5 – 8.4) and salinity prohibit its existence, as evidenced by the very short survival rate of H2S emitted from hydrothermal vents. Dissolved H2S cannot travel far from its source without dissociating or reacting with dissolved oxygen. The abiotic oxidation of H2S is very slow due to the lack of dissolved oxygen in groundwater and to kinetic constraints, but when it occurs, it produces sulfur according to the equations: 2H2S(aq) + O2(aq) → 2H2O + 2S, or H2S(aq) + O2(aq) → H2O2 + S. The reaction can also produce minor amounts of sulfite (SO3) and thiosulfate (S2O3), but not sulfuric acid.

Another misconception is that sulfuric acid can be produced by the abiotic subaqueous oxidation of H2S(aq) just below the water table, citing the equation H2S + 2O2 → H+ + HSO4 → 2H+ + SO4. However, sulfuric acid does not occur in nature in its anhydrous form because the hydration reaction is thermodynamically favorable (Ka1 = 2.4 x 106). The reaction is highly exothermic, and produces hydronium and sulfate according to the equations: H2SO4 + H2O → H3O + HSO4, and HSO4 + H2O → H3O + SO4 (Ka2 = 1.2 x 10‑2). Therefore, sulfuric acid would not form in a medium that strongly forces it to dissociate. All abiotic occurrences of sulfuric acid (e.g., acid rain) do not start with H2S, but with sulfur dioxide (SO2), which is oxidized in air to produce SO3, which in the presence of water vapor forms sulfuric acid. Even the manufacturing process starts with SO2, which is obtained by either burning sulfur, or as an alternative, incinerating H2S gas. Therefore, sulfuric acid cannot be produced directly by the oxidation of H2S, in the lab or in nature, without some intermediate steps involving oxidized sulfur