sand bed binding

[charles matthews]

Randy,

I'm sure you have treated this subject before but I can't find it. I constructed an experimental substrate in which the flow was directed under an 8 inch deep Southdown bed, flowing along the tank bottom and then up to the refugium water above the sand. There was a slow upward drift through the sand grains as documented by food coloring injections- more than through a DSB.

When the refugium declined I tore it down. There was extreme binding of the substrate- concrete that took me days to break out.

Placed in acetic acid, a clump retained the same size but seemed to dissolve the grains- it felt crumbly. I think this left the glycoprotein matrix which caused the binding.

I dose pickling lime as a slurry in another remote sump twice daily, KH 10 calcium about 450, nutrients undetectable. Tank is fed heavily.

Is my interpretation correct? And have you seen clumping on normally constructed DSBs without extreme values of ca/alk?

Thanks!

Charles

[Randy Holmes-Farley]

Very interesting experiment.

The deposited material might also have been a magnesium/calcium cabonate that is harder to dissolve than is the original substrate, unless it is totally resistant to acid.

With such a flow you might well have kept the pH higher in the sand bed (compared to lower or no flow), leading to more cementing in some fashion.

[bertoni]

Have you tried dissolving some of the substrate in bleach? Would that help determine the type of binding that was occurring. Not my area of expertise, but if the sand was bound together by bacteria, for example, it's been suggested that chlorine would break down the clumps.

[Randy Holmes-Farley]

Yes, that is an interesting suggestion. I don't believe that bleach will dissolve CaCO3/MgCO3 at all.

[charles matthews]

The flow was very slow through this 8 inch southdown bed, but greataeer than diffusion, as determined by dye movement. It took about fifteen minutes to partially clear an injected dye two inches down. given that thickness, and that fine sand, I assume any upward movement was essentially deoxygenated and acidic. There was a normal precipitation line a few inches above the bottom, and one about an inch below the surface. I assume the bed was acidic- and more so as the binding progressed.

The bleach is a fantastic idea. I will do it and report back.

[Randy Holmes-Farley]

I assume the bed was acidic- and more so as the binding progressed.


Maybe. I'm not sure how to know if the flow was high enough to prevent a pH drop in the interstitial water.

[charles matthews]

I have one piece in bleach and one in white vinegar. The bleach hasn't dissolved the piece at all. The vinegar has dissolved aout half the other sample (about 1.5 inch cube piece divided in half). I haven't tried to crumble the pieces.

Does this mean we are dealing with calcium carbonate and not glycoprotein bacterial products? Am I using too much calk, in spite of the bulk water measurements? Surely you don't think the sand bed becomes alkalotic adn precipitates calcium carbonate? Perhaps the bed is hotter due to decomposition, and that causes calcium carbonate to precipitate out?

[Randy Holmes-Farley]

If you take fresh CaCO3 sand and add it to reef tank water, you will precipitate some CaCO3/MgCO3 onto it. I've done that myself. Once it gets coated with magnesium and organics and such, these processes usually stop. However, it could be that in a sand bed, some process (such as bacterial action) changes the dynamic in some way that allows ongoing precipitation under some conditions.

I'd be surprised if the sand bed ends up being hotter.

It may also be that the ppt is a mixture of organics and CaCO3 that is resistant to bleach. Was it in straight bleach?

[charles matthews]

Straight bleach, now I believe for 48 hours. I have lots more if you can suggest anything else to identify the process.