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

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) (K_a1 = 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) + O_2(aq) → 2H₂O + 2S, or H₂S_(aq) + O_2(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 (K_a1 = 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 (K_a2 = 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