Am I developing metal sulfides? (And does this increasing Alk?)

[rmurken]

Below is a photo of the dark underside of my tank, which I occasionally glance at more out of curiosity than anything.

In the little more than a year the tank has been set up, the underside has always just looked like normal-colored sugar-fine aragonite sand. Until I checked yesterday to find that a gray crescent had formed. Weird! Behold:

The streak or arc is on the order of 10” or 25cm and roughly correlates with the rear quarter of the perimeter of a piece of live rock on the surface of the sand. Photo:

Looks normal!

First, on reviewing @Randy Holmes-Farley ’s article on hydrogen sulfide, I strongly suspect that the gray arc coincides with a region that has become anoxic and is generating iron sulfide or some combo of metal sulfides.

I also recently experienced a round of rising alkalinity with no supplementation. It’s back to my normal FOWLR+zoa .2 dKh/day consumption. (N.b.: “FZWLR” anyone?)

But I am curious if folks would agree that:

1. The gray arc is metal sulfides resulting from anoxic generation of H2S by microbes.

2. The generation of H2S could explain the upward drift in alk. Specifically, Randy’s article has the following general equation for production of H2S, which appears to throw off some bicarbonate:
(CH2O)106(NH3)16(H3PO4) + 53 SO4-- —-> 56 CO2 + 50 HCO3- + 53 HS- + 16 NH3 + 53 H2O + PO4---

I’d expect the tank to blow off the CO2 pretty quickly, although while my Alk was rising I did have a drop in pH that resolved after I opened a window.

Parameters have otherwise returned to normal.

The underside receives very little light, and there are no signs of algae growth or critters burrowing down there, which is consistent with anoxia.

Thanks!

 

[X-37B]

Interesting! Following

 

[rmurken]

Ugh, I typo’d the subject line! Still clear though. But can’t change it. :/

 

[Randy Holmes-Farley Verified Community Expert]

It does look like metal sulfide to me. Not necessarily a problem if it stays confined, but I’d be careful to not stir up that sand.

 

[Randy Holmes-Farley Verified Community Expert]

In the alk part, I don’t think it causes rising alkalinity. If you convert that HS- to S- - to precipitate an iron sulfide, H+ is released in roughly the same amount to offset the HCO3- noted in the equation.

 

[rmurken]

In the alk part, I don’t think it causes rising alkalinity. If you convert that HS- to S- - to precipitate an iron sulfide, H+ is released in roughly the same amount to offset the HCO3- noted in the equation.

I see. I had assumed that the HS was out of play because it was unlikely to dissociate at any significant rate at tank pH’s but look, I was an English Major and my last chem class was in 10th grade...a different kind of HS entirely.

Thanks, Randy.

 

[rmurken]

In the alk part, I don’t think it causes rising alkalinity. If you convert that HS- to S- - to precipitate an iron sulfide, H+ is released in roughly the same amount to offset the HCO3- noted in the equation.

I see you made that point here: “On the other hand, even though the S-- form is only a small fraction of the total, it is also very important as it is this form that precipitates with metals to form the black deposits characteristic of hydrogen sulfide formation in sediments.”

...and how that could be a strategy for management of waste H2S. Got it.

 

[Randy Holmes-Farley Verified Community Expert]

To make it clearer:

HS- —> H+ + S- -

S- - + Fe++ —> FeS ( black solid )

that leaves the H+ which offsets the HCO3- in terms of alk

 

[rmurken]

It does look like metal sulfide to me. Not necessarily a problem if it stays confined, but I’d be careful to not stir up that sand.

Follow-up question: What do you think of using a syringe to pump some FeDTPA into the sand in that area, and then stirring it up?

Would think that it would be a plausible way to drive all the H2S to react with Fe++. Then you could stir up the bed in that area and disrupt further H2S formation by giving the sand a stir now and then, and possibly thinning the layer there.

Would think that excess DTPA would largely be caught with carbon and otherwise mineralize and not be a big deal, so long as not too much is needed. I guess it could grab other metal ions on its way out but not thinking of using all that much....

 

[Randy Holmes-Farley Verified Community Expert]

I think stirring it up is more likely to cause harm than good. Having iron in the water and GFO in the system could both be useful.