H2S formation in substrates

I had recently made a statement that I didn't think it was likely
to get H2S in substrates which are primarily mineral ie. sand/gravel/clay
but in reviewing some data I think I'll disagree with myself now.
If the substrate has sufficient organic material (ie fish poop)
that is mixed in deep enough (about 2" deep give or take) and very little
iron in mineral form then yes, H2S could form along with methane.
In oxygen rich water, H2S doesn't exist long enough to be a problem
since it would be rapidly oxidized.

Here is an article I've posted on the internet news group
rec.aquaria.freshwater.plants which I'll include here because I know
not everyone here reads the newsgroup religiously. Apologies to George
who always seems to read everything since you'll see it twice. :-)

Summary: substrates with laterite should not produce H2S.

Subject: Re: H2S kill?(was: laterite kill?)
Newsgroups: rec.aquaria.freshwater.plants

George Booth (booth at lvld_hp.com) wrote:
: A. Inniss (andrewi at u_washington.edu) wrote:
: > 	Experience aside, my understanding is that under certain
: > conditions, the bacteria that in effect produce the H2S outcompete
: > denitrifying bacteria, 

: My understanding is that the bacteria prefer the nitrogenous compounds
: and will consume them first, performing useful denitrification in the
: process.  When nitrogen runs out (as in a "stagnant" substrate), they 
: will start consuming sulfur bearing compounds and begin producing H2S. 

I'm not sure that the bacteria are necessarily the same ones; there are 
thousands if not millions of types and each is adapted in many ways to 
survive in various environments. In a low redox environment, two groups of
bacteria may actually be active, one metabolizing nitrates and producing
ammonia, methane etc. and others metabolizing sulphates and sulpher
containing compounds to produce sulphide ions or hydrogen sulphide.
If there is still nitrate present (ie. the redox potential is
above +100 mV) then the sulphide will be oxidized by the nitrate
or anything else that's handy. If there is iron present, then
hydrogen sulphide will react with that to produce FeS. The net result
is that at redox of about 300 (higher or lower depending on which
of the reports you look at) you'll get production of nitrogen dioxide
and ammonia with any sulphides being produced being oxidized by
the higher(?) potential ions.

There are many kinds of bacteria that can metabolize more than
one kind of compound for energy. The ones that can use both
oxygen and nitrate or other compounds are called facultative anaerobes.
Those that cannot use oxygen at all (and die in the presence of
oxygen) are called obligate anaerobes.

Here are some interesting data which Paul Krombholz and I traded:

> >... the Sikora & Keeny paper
> >"Further aspects of soil chemistry under anaerobic conditions" 1983
> >in Mires: swamp, bog, fen and moor. Elsevier, Amsterdam, The Netherlands.
> >Fe2+ formation would occur at higher redox than sulfide formation.
> >Here's table 6.1 which might be useful:
> >
> >Possible systems operating in flooded environments as related to
> >redox potential (Takai & Kamura 1966 etc...)
> >
> >System                 Redox (mV)**2     Micro-organisms involved
> >O2 disappearance      +500 - +350        aerobes
> >Nitrate disappearance +350 - +100        }
> >Mn2+ formation        below +400         } facultative anaerobes
> >Fe2+ formation        below +400         }
> >Sulfide formation     0 - -150
> >Hydrogen, methane form. below -150       obligate anaerobes
> I found some notes I took from an older article: MORTIMER, C.H., 1941-42.
> The exchange of dissolved substances between mud and water in lakes.  J.
> Ecol. 29: 280-329.30: 147-201.
> Mortimer made a graph of redox potential versus substrate depth in mud from
> an eutrophic lake and also in mud from an oligotrophic lake.  In the first
> 2 cm. of the eutrophic mud the redox potential went from 600 mv to about 0
> mv.  It reached a negative 100 or so mv. at about 5 cm. and then gradually
> increased a little with increasing depth to about 0 again.  The redox
> potential in the oligotrophic mud dropped to about 150 mv. at  5 cm. and
> then stayed the same thereafter.  He gives ranges for verious reductions of
> plant nutrients that differe a little from those you cite above:
> NO3-----> NO2        0.45 to 0.40 volt
> NO2-----> NH4        0.40 to 0.35 volt
> Fe+++ ------> Fe++   0.3 to 0.2 volt
> SO4 ------> S        0.1 to 0.06 volt.
> Note that the sulfur reduction is to S, not S--.

This suggests that nitrate is only stable in complex organic compounds
not yet decomposed or in the top 1 centimeter of rich organic substrates.
It is considerably more stable in less rich substrates (oligotrophic)
where it can exist in the top 2 cm or so and would be reduced much less
quickly to ammonia.

Its also interesting to note that any environment fertile enough to
reach -150 mV and produce methane can also produce H2S if there is
insufficient Fe. If we're getting bubbles from the substrate,
the question is are they nitrogen, nitrogen dioxide or are they
methane possibly tinged with H2S?

 Steve Pushak - spush at hcsd_hac.com - Vancouver, BC, Canada

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