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In Biochemistry of a sulfuric acid world, I assumed that autotrophs would strip hydrogen from H2SO4 (which is their biosolvent) to fix carbon, just like plants on Earth strip hydrogen from water (which is our biosolvent).

But, the one world we know of with large quantities of naturally occurring concentrated sulfuric acid (Venus) also has large quantities of hydrochloric acid. Now, it seems that, given the choice between water and HCl as hydrogen sources, water is probably energetically preferred. But what about the choice between HCl and H2SO4?

Is it more energetically favorable for plant-analogs on a sulfuric acid world, with HCl dissolved in the sulfuric acid, to split sulfuric acid and release oxygen and sulfur trioxide, or to split HCl and release chlorine gas?

Or, a third option--perhaps they produce liquid Peroxydisulfuric acid instead?

(There should also be large quantities of HF, but fluoride is unambiguously harder to oxidize than chloride, so generating free fluorine is not an option!)

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  • $\begingroup$ It's not necessarily down to energy differences in biochemistry. You can have an enzyme that selects whatever species you want, as long as the energetics are kinda, sorta feasible. (As an inorganic chemist, it drove me nuts). You won't get that peroxy species though, without looking at it or having encountered it before I will bet it will violently oxidise the crap out of pretty much any organic in its path. Cl2 is a hill, SO3 + O2 is a bigger hill (I think), that peroxy acid is Mt Everest, no creature short of an alien bombardier beetle will make it. $\endgroup$
    – user86462
    Commented Aug 16, 2022 at 0:46
  • $\begingroup$ @SeanOConnor I included the peroxy acid option because googling indicated it might be produced by H2SO4 electrolysis before SO3 and oxygen are. $\endgroup$
    – Logan R. Kearsley
    Commented Aug 16, 2022 at 1:51
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    $\begingroup$ Everything you've written is well measures and sensible, it's just that bit is implausible. Electrolysis is actually great at making high energy species; it's how we make chlorates, aluminum, and group 1 metals. $\endgroup$
    – user86462
    Commented Aug 16, 2022 at 6:44

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Sulfuric acid plants might produce hydrogen.

Iron can take the place of the hydrogen, which leaves as the gas. Or could certainly be captured to do chemistry!

Reaction of iron with acids Iron metal dissolves readily in dilute sulphuric acid in the absence of oxygen to form solutions containing the aquated Fe(II) ion together with hydrogen gas, H2. In practice, the Fe(II) is present as the complex ion [Fe(OH2)6]2+.

Fe(s) + H2SO4(aq) → Fe2+(aq) + SO42-(aq) + H2(g)

The waste product of this would be iron sulfate. I could imagine that autotrophs might use exogenous energy to regenerate the iron atom from the iron sulfate (or actually the [Fe(OH2)6]2+) and then use it to strip more hydrogen off of H2SO4. That seems like the kind of thing iron might get into.

I dont have a good feel for [Fe(OH2)6]2+. It must be stable in sulfuric acid. I thought if the OH2 got loose it would be water and that would be a very exothermic reaction in sulfuric acid. Maybe that energy could partially offset the energy required to strip the Fe away?

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  • $\begingroup$ That depends on having a reliable source of elemental iron, which seems unlikely, except for specialized chemotrophic microbes; or if it's just a catalyst, we'd need to know what the products of regerating Fe from FeSO4 are (I am very sceptical it would exist as a hexahydrate in anhydrous sulfuric acid)--that's what the ultimate waste product would be. $\endgroup$
    – Logan R. Kearsley
    Commented Aug 16, 2022 at 1:59

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