If both of the following hydrogen sulfide and oxygen chemical equations can occur, under what circumstances is the harmless pure sulfur and the toxic sulfur dioxide produced?

$\ce{2H2S + O2 -> 2H2O + 2S}$

$\ce{2H2S + 3O2 -> 2H2O + 2SO2}$

Specifically I have a fresh water aquarium with sand, plants, and small creatures I collected from a creek. I want to prevent hydrogen sulfide building up by oxygenation with an air pump, but I want to make sure the environment does not result in the harmful sulfur dioxide in the tank.

  • $\begingroup$ IMHO, it would end as sulphate $\ce{SO4^2-}$ $\endgroup$ – Poutnik Sep 27 at 16:46
  • $\begingroup$ Okay, so how do I get rid of $\ce{SO4^2-}$ in the aquarium? $\endgroup$ – HaoQi Li Sep 27 at 18:10
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    $\begingroup$ Sulphates are natural ions in water. Anything that comes by sulphan oxidation is harmless, compared to the equivalent amount of original sulphan. $\endgroup$ – Poutnik Sep 27 at 18:12

Per my review of conceivable chemistry, harsh conditions are not required, but likely acidic conditions could increasingly be problematic! Per the context of the question, to quote:

Specifically I have a freshwater aquarium with sand, plants, and small creatures I collected from a creek. I want to prevent hydrogen sulfide building up by oxygenation with an air pump, but I want to make sure the environment does not result in the harmful sulfur dioxide in the tank.

So, consider the scenarios if one passes air/O2, in the presence of say a photocatalyst (B-12, dye,..) or perhaps sonolysis (sound vibrations), into:

$\ce{H2S (aq) = H+ (aq) + HS- (aq)}$

possible issues in time depending on pH.

For example, with sonolysis:

$\ce{H2O + Sonolysis-> .H + .OH}$

albeit, any vibrations (say, from air-pump) are not that likely effective here.

However, continuing with the illustrative chemistry (source of sulfur related radical chemistry, see Page 7, this source):

$\ce{.OH + H2S -> .HS + H2O }$

$\ce{.HS + O2 -> .SO2- + H+ }$

$\ce{.SO2- + O2 -> SO2 + .O2- }$

which is one path to SO2 not contingent on pH, but elevated pH would remove formed sulfur dioxide!

$\ce{.H + O2 -> .HO2 }$

$\ce{.HO2 <=> H+ + .O2- \text{pK}_a 4.88}$

Now, if water in the fish tanks gets too acidic (pH < 4.88), then possible further aqueous reactions include:

$\ce{ .HO2 + .HO2 -> H2O2 + O2 (Slow) }$

$\ce{ .HO2 + HS- -> HO2- + .HS }$

And importantly:

$\ce{ H+ + HO2- = H2O2 }$

which readily converts H2S into H2O + S. Expect with time some aqueous SO2 and perhaps also sulfate as well.

More likely, however, is strong sunlight exposure acting on natural coloration present in the fish tank serving as a weak photocatalyst, generating some electron holes (h+) and also solvated electrons (e-(aq)):

$\ce{Dye + hv -> [Dye]{*} -> Dye + h+ + e- }$

$\ce{H2O + h+ -> H+ + .OH }$

$\ce{.OH + H2S -> .HS + H2O }$

where the reaction could continue (per the sonolysis scheme above with one path to SO2 not pH dependent) where pH < 4.88 further promotes sulfur species, $\ce{ S, SO2 {and} SO4(2-)}$.

So, my review of possible chemistry suggests keep the pH near neutral to be more safe from SO2 (but not entirely), but interestingly if H2S concentrations become dangerous, more acidic pH could be a solution with sunlight, accomodating glass, and some dye.

[EDIT] Confirming source on the scenario of employing light (and also ferrous) in the removal of hydrogen sulfide, to quote:

Such oxidation reactions are catalyzed both by soluble metals such as iron and by light. Hydrogen sulfide also can combine with metals such as iron (Fe++) to precipitate as black iron sulfide (Figure 1 bottom; FeS and FeS2).

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  • $\begingroup$ I am curious on why the only answer that actually answers the question and has a confirming source on the action of light on H2S (with advanced content on how this could, in effect occur) would not be viewed/accepted as a correct/acceptable answer. $\endgroup$ – AJKOER Oct 1 at 14:35

Sulfur dioxide is not a problem ; in an aquarium it will make very dilute sulfurous acid which will likely neutralize with some carbonate mineral/gravel. H2S is lethal , extended exposure at 50 ppm for a day , or short exposure at 500 ppm. On the other hand , what you smell in aquarium sand is less then 1 ppm, the sense of smell is very sensitive to H2S. There are several answers on the internet but I expect most people can smell H2S at 0.1 ppm. An aerator will have no significant affect on H2S in an aquarium. When I clean my pond , which is well aerated ,the smell of H2s is strong from decomposing leaves, etc.

Regarding the first part of your question ; Hundreds of tons a day of elemental sulfur are made ,world wide, every day by burning H2S with limited air at high temperature. By recycling and control of gas compositions the only products are elemental sulfur and water,

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  • $\begingroup$ So what can I do in my aquarium to reduce $\ce{H2S}$ without changing water? There are many micro-organisms in the aquarium which is from the creek water that I prefer not to be taken out in a water change. $\endgroup$ – HaoQi Li Sep 27 at 18:14
  • $\begingroup$ I don't think you need to do anything. It is normal for small amounts of H2S to be present in the debris on the bottom of natural bodies of water. And even aquifers have sulfate reducing bacteria that put H2 S into well water. $\endgroup$ – blacksmith37 Sep 27 at 20:15
  • $\begingroup$ Please note, this answer has not technically answered the question: "Do hydrogen sulfide and oxygen produce pure sulfur or sulfur dioxide? If both, under which circumstances does each scenario occur?" That would require an exploration of the chemistry which is not provided. But, I enjoyed reading your reply (no rating action taken). $\endgroup$ – AJKOER Sep 29 at 22:12

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