The survival of dissolved H 2 S is very pH, temperature, and salinity dependent: H 2 S + H 2 O → H 3 O + HS diprotic acidpKa 2 = 19 ± 2 Fig. 1

Chemistry of Spring Water of Active Sulfuric Acid Caves CAVE/COUNTRY CAVE SPRING WATER PARAMETERS Temperature °CpHSO 4 mg/LCl mg/LH 2 S mg/LDO mg/L Cueva de Villa Luz, Mexico ND-2.2 Frasassi Caves, Italy ≤ ≤ Acquasanta Terme Caves, Italy Montecchio Cave, Italy Parrano Cave, Italy Serpents Cave, France Aghia Paraskevi Caves, Greece , Lower Kane Cave, USA ND Fairy Cave, USA Fig. 2

H 2 S + 2O 2 H 2 SO 4 2H 2 S (aq) + O 2(aq) → 2S + 2H 2 O Further oxidation yields SO 3, S 2 O 3, and finally SO 4. Biotic oxidation enhances the rate by 3 or more orders of magnitude, but the outcome is the same. Sulfuric acid is the world’s number one industrial chemical, and over 230 million metric tons were produced in It is produced in a 4-step process: S (s) + O 2 → SO 2, 2SO 2 + O 2 → 2SO 3, SO 3 + H 2 O → H 2 SO 4 2H 2 S (g) + 3O 2(g) → 2SO 2 + 2H 2 O >200 ⁰C Fig. 3 Or as an alternative sulfur source:

gypsum rinds biofilms pipe A modern analog outside of speleogenesis: Biogenic Sulfide Corrosion Internal corrosion of concrete wastewater pipes and tanks (man-made caves) due to anaerobic sulfate-reducing bacteria generating H 2 S, which is oxidized to sulfur in the condensate, which aerobic sulfur-oxidizing bacteria metabolize into H 2 SO 4. Only occurs if the pipe or tank is partially full (air space above water is required) The acid converts CaCO 3 into gypsum A process studied since the 1940s Fig. 4