Hydrogen sulfide reaction with water produces sulfuric acid

More on the Hydrogen Sulfide Reaction
by Penny Boston, Research Associate Professor of Cave and Karst Science

Hydrogen sulfide is pretty soluble in water, however the solution process is fairly slow. The reaction between oxygen dissolved in the water and the gas can go directly to elemental sulfur instead of producing sulfuric acid. In fact, in our caves it does and that is why downstream the water is milky from suspended sulfur colloids. The reaction of the hydrogen sulfide producing sulfuric acid seems to occur at lower oxygen concentrations and it has been suggested that the sulfide actually reacts with the dissociation products of water (HO- and H+) depending upon the pH of the water and possibly catalyzing metals suspended in solution. If this is true, then it is REALLY the water that is the reactant, not the dissolved oxygen. In the complexity of this cave stream, we do not know the precise partitioning of these possible reactions. But we do know that in some springs, the anoxic water coming out of the spring already contains sulfuric acid so one is forced to conclude that dissolved oxygen doesn't seem to be involved in that reaction.

On top of which, we do have clear evidence that a significant quantity of the sulfuric acid results from microbiological oxidation of the hydrogen sulfide (very different from the abiotic rxn). The whole cave is dripping with the gooey evidence of these little guys (Thiobacilli) at work.

Chemistry in the real world of complex environments often confuses us in light of the relatively neat and clean explanations of chemical phenomenon in the laboratory that we have all learned. Unexpected factors can operate in real systems that make their interpretation difficult, indeed. The overlap of microbially-mediated reactions with the convoluted interplay of geochemistry, mineralogy, and hydrology in the cave environment makes it an extremely challenging environment to investigate.