Reef Chemistry Question of the Day #26 Hydrogen Sulfide 2

[Randy Holmes-Farley]

Reef Chemistry Question of the Day #26 Hydrogen Sulfide 2

As a follow up to yesterday's question on hydrogen sulfide...

Hydrogen sulfide is a colorless gas.

Why then are black sediments diagnostic of a hydrogen sulfide issue?

Good luck!











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[Shep]

I think its because when the sulfide binds with iron it becomes Fe(III, I think)S, which is a black precipitate.

 

[Randy Holmes-Farley]

Yes, that correct! Iron and other metals can precipitate black sulfides.

Even though most hydrogen sulfide in seawater is present as HS^- (94%) and H2S (6%), the tiny bit present as S^-- (0.016%) is enough to allow it to precipitate as metal sulfides.


This has more:


http://reefkeeping.com/issues/2005-12/rhf/index.phpf

from it:

Hydrogen sulfide produced in anoxic regions can produce several possible outcomes. One is to diffuse into more oxic (aerobic) regions, even into the water column itself, where it can be toxic to organisms. In the water column it can be oxidized to sulfate or other sulfur species (elemental sulfur (S), sulfite (SO3-) and others; Figure 1, right). 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). This blackness is the telltale sign of hydrogen sulfide formation that can be seen in anoxic seawater sediments, although similar appearing black precipitates may be formed from other materials.

and

Sulfide in seawater is also unstable toward precipitation with certain metals. The black deposits often seen in anoxic sediments are typically metal sulfides, especially ferrous sulfide (FeS) and pyrite (FeS2), with much smaller amounts of copper, manganese, zinc, nickel and cobalt sulfides. The exact processes whereby these metal sulfides form in marine sediments and elsewhere is complicated and still under study. In some areas, like the Orca basin in the Gulf of Mexico, deposited iron sulfides make up as much as 0.7% of the sediments' mass. So, iron sulfides are not necessarily a trace component.