Sulfur denitrifier and H2S

[UK_Pete]

Hi Randy,

Is it possible to say what happens if a sulfur denitrifier has too little flow (apart from ORP dropping)? I've read aquarium lore that says it generates hydrogen sulfide but I can't find literature which explains this. Theres quite a bit out there relating to sulfur denitrifiers but nothing on that topic. Does the bacteria switch from removing oxygen from the nitrate, to removing hydrogen from the water, and 'reacting' that with the sulfur? Is there a limit to how much H2S it will produce or will this reaction just proceed until it runs out of sulfur or hydrogen, if that is the reaction (surely not though)?

 

[Randy Holmes-Farley Verified Community Expert]

The sulfur part itself likely won't produce hydrogen sulfide (I've not seen that this happens, but it might; if it did, the elemental sulfur is playing the same role that sulfate does), but even an empty reactor flowing slow enough can produce H2S as organics in the water are metabolized by bacteria.

 

[UK_Pete]

Thanks Randy. I think I'm getting confused because there are 2 different processes possible in the anerobic sulfur nitrate reactor.

On the one hand, bacteria can take the oxygen off the nitrate molecule, attach it to the sulfur molecule, and gain energy in the process, turning nitrate and sulfur into nitrogen gas and sulfate. And on the other hand, bacterial can take organics and sulfate (and the other electron acceptors you mention), use the oxygen from the sulfate to oxidise the organic molecule, leaving sulfide, and gain energy in the process, but this process is limited by available organics. Once the organics are all used up, theres nothing left to oxidise in ASW (no more electron donors?).

Does that mean that H2S production in a sulfur denitrifier would be limited by organic content of the influent water, and that there would be a fixed relationship between influent organic content and effluent H2S under very low flow conditions? And that even in fresh mixed organic free ASW spiked with nitrate, a sulfur denitrifier will still remove nitrate, but with no possibility of H2S production?

 

[Randy Holmes-Farley Verified Community Expert]

Yes, H2S production is limited by organics present (and by the presence of O2 or nitrate).

The conversion of elemental S to H2S is a reduction, not an oxidation. I'm not sure what reducing agent would be present to drive that.

 

[UK_Pete]

So in the sulfur reactor with low flow (ignoring the preceding aerobic reactions), and plenty of sulfur, first (1) the bacteria will use organics and nitrate and produce nitrogen and other harmless products until one of either nitrate or organics have been used up fully. From there it can go one of two ways:

2) If there is remaining nitrate present (no suitable organics left), bacteria will use the remaining nitrate and the sulfur granules to make nitrogen and sulfate, and there will be no H2S production (but alk will be consumed).

Or

3) If there are remaining organics present, bacteria will use sulfate and the remaining organics to create H2S, after which point theres no energy sources left so H2S production will stop?

The conversion of elemental S to H2S is a reduction, not an oxidation.

Bit confused by that. Is that a potential different process unrelated to the above?

 

[Randy Holmes-Farley Verified Community Expert]

I don't know if 1 happens faster or slower than 2 in a typical sulfur denitrator.

In either case, 3 would only happen later, and may not be a concern until the flow gets really slow.

By the reduction stuff, I just meant that going from S to H2S is a reduction of the sulfur (it moves to a higher negative charge, 0 to -2) and to have that happen, you must have a reducing agent to accomplish that. Organics cannot. Oxygen and nitrate cannot. Something like H2 could, but that is not likely present.

 

[Duffs]

What is the potential damage of h2s the reef tank and can you plumb the effluent line to a skimmer to help decrease it

 

[Randy Holmes-Farley Verified Community Expert]

Many people run these without H2S issues, so I don't think it is a big risk, but hydrogen sulfide is quite toxic. Not sure running it through a skimmer will help much as it will be mostly present as HS- in seawater at tank pH, so won't blow off that readily. GFO can get rid of it, however.

 

[UK_Pete]

Re H2, is there not enough H2 in tank water to do that? Not sure if my understanding of pH is right in this context but I guess at pH 8, there is 10^-8 moles of H2 in a litre of water - 10 ppb I think that is. Sounds like quite a small amount, but is this H2 available for the S -> H2S reaction or am I confusing different things?

From looking at the equations in the literature[1] for the various types of denitrification, it looks to me like process 1 and 3 add alk. Is that right? They have an OH or a HCO3 on the right of the equation. Also, the sulfur equation has an NH4 on the left, which I read as meaning the sulfur reaction requires ammonium. Is that ammonium created from the nitrate during the reduction of nitrate? Otherwise I don't see how this can be seen as a purely NO3 reaction if it needs ammonia.

[1] Biological nitrate removal processes from drinking water supply-a review

 

[Randy Holmes-Farley Verified Community Expert]

There should not be appreciable H2 except possibly deep in anoxic sediments. The pH relationship is for H+ and OH-.

 

[Randy Holmes-Farley Verified Community Expert]

Process 1 adds alkalinity while process 2 lowers it.

 

[UK_Pete]

Thanks Randy.

So regarding removing H2S (or HS- if thats the form it takes) with GFO, I guess from your article its not a catalytic reaction but will result in the GFO becoming iron sulfide on the outside of the grains, and that would limit its life if it was exposed to H2S continually?

I'm just wondering about the nanomolar N & P thing we were discussing recently, that having a very large (like 20% tank volume) sulfur denitrifier with 'insufficient flow' would keep nitrate to these levels, as well as removing organics. Seems quite an interesting thing to try as an experiment. H2S would need to be dealt with, but a post filter with GFO would keep phosphates and H2S at bay perhaps. But would be GFO become overloaded in a short time I wonder. Ideally H2S would be oxidised back to sulfate somehow so it didnt deplete sulfate.

Taking the typical organic molecule mol weight from your H2S article, which generates 53 HS (the 4th equation under hydrogen sulfide production), thats a ratio of 67:1 for organic : HS. So with influent of 1 ppm organics, effluent would be 15 ppb HS. Is it too difficult to hazard a guess as to how long GFO would last in that environment with a flow of say 1 litre per minute?

 

[Randy Holmes-Farley Verified Community Expert]

There can be both catalytic and noncatalytic processes. The first section below is a catalytic process in that the Fe++ formed will quickly be reoxidized by air:

Hydrogen Sulfide and the Reef Aquarium by Randy Holmes-Farley - Reefkeeping.com

Stability of Hydrogen Sulfide in Water: Iron Hydroxide Oxidation
It has been shown that particles of iron oxide/hydroxide can react with hydrogen sulfide fairly rapidly to produce elemental sulfur and reduced metals. For example, Fe+++ (ferric iron) is reduced to Fe++ (ferrous iron) as the sulfide is oxidized to elemental sulfur. The process involves sulfide coming to the surface and giving up electrons to the metal ions that take them up.3 The rate of this process is decreased by sulfate, and less so by magnesium and calcium ions. It is strongly decreased by phosphate and silicate that bind to the particles' surfaces. Certain organic chelators (EDTA and TRIS, for example) accelerate this process by speeding the cycling between Fe++ and Fe+++.4 It has been claimed that this process is rapid enough to purify seawater polluted with hydrogen sulfide, as in mariculture systems.5


Since many reef aquarists have sufficient amounts of iron oxide/hydroxides in their systems to bind iron, this may be an important mechanism for hydrogen sulfide detoxification in some reef aquaria.


Stability of Hydrogen Sulfide in Water: Metal Sulfide Precipitation
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.6 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.7 So, iron sulfides are not necessarily a trace component.

 

[UK_Pete]

So I guess if the ferric iron is in an oxygenated environment, once its reduced by the H2S it can then gain its lost oxygen molecule back from the water, and its ready to oxidise another H2S molecule then. So in a GFO H2S stripper you would imagine that a recirculating design with good aeration of the water at some point in the circulating flow would be the obvious design to try?

Do you think the poisoning by phosphate / silicate or fouling with elemental sulfur would be a problem with using GFO? Assuming you change your GFO to keep it working properly with phosphate that is, so like every 2 to 4 weeks.

 

[Randy Holmes-Farley Verified Community Expert]

Yes to the first paragraph.

Too much other stuff on the GFO probably deters its effectiveness, but I'm not sure how much.

 

[UK_Pete]

Thanks Randy. If you just had slow flow, like a drop a second, without a GFO or other filter after the denitrifier, do you think either of these would work?

1) A residual in the tank of 10 ppb chelated ferrous iron as you used to use (or gluconate as you now do) - the oxidation state is wrong I think but I'm pretty sure I remember you saying you thought much of the iron quickly changes to ferric in the aquarium. Since it would not be used up I imaging it might be effective, but would the low concentration mean H2S levels would not be satisfactorily low?

2) And a residual of hydrogen peroxide, like 1 ppm H2O2 : tank water, dosed throughout the day?

Or maybe the potential H2S level at that flow rate would be so low anyway, that it would not carry significant H2S into the tank (ie if the water is 15 ppb H2S, and the flow is only a litre an hour, the quantity of H2S per day is low).