Dissolving aragonite utilizing sand-bed gradients

[brandon4291]

Randy, from reading your articles I have gathered it is relatively hard to use an aragonite sand bed as calcium/alkalinity support in marine aquaria--due to the factors of saturation, pH etc as preventative mechanisms...

My question is, does the amount of carbonic acid generated by aerobic bacteria metabolism in a normal sand bed create enough of an acidic region to break down aragonite substrate? although its probably not enough to provide the sole alkalinity/Ca+ support needed, it is interesting if there is degradation of the substrate because of this occurence. It is also interesting if there is enough degradation of the substrate that would require regular additions to your sand bed to keep it from dissolving away eventually...

For clarity, we are talking about a sand bed of 3-4 inches in a smaller aquarium, maybe ten-twenty gallons. This bed would not likely have anoxic conditions associated with true nitrate reduction if that makes any difference (the only +/- gradients would be areas surrounding bacterial colonies{carbonic acid}, other than that the sandbed would be oxic and roughly equal to the pH of the water column)

Its fun to look into any changes a Nano-DSB might endure over a period of time.

Thanks in advance

Brandon M

 

[Anonymous]

Not exactly a "Randy" here, but if I may, I'd like to offer what I understand the situation to be.

From what I understand, the answer is yes, certain calcareous substrata, such as Aragonite sand, do dissolve over time (sometimes not much, sometimes incredibly quickly). The apparent conclusion one might make, when concurrently observing growth of calcareous flora and fauna, is that there is utilization of the available minerals. I know that I have had to replace the sand (Aragonite) quite often in the bamboo shark brooder, but that system is linked to the entire coral lab, and there is good growth in those other tanks.

I have no definitive publication to link or lead you towards (at this moment), so unfortunately, I can't say look here for growth and uptake ratios. As I said, this "as I understand it", which really means that much of this is what I've gleaned through reading and (much more) brain picking.

However, it does not appear one should assume that this release/uptake of minerals via substrata is sufficient for completely healthy growth conditions. I believe this may also be true even when one is using pre-filtered real oceanwater.

Though surely not as informative as what Mr. Farley-Holmes has stored brainside, I hope my input has helped a bit.

 

[brandon4291]

Seamaiden I do think that helps--your observation of dissolving substrate in that tank is right where Im looking. Not so much for support of corals or the like, but whether aragonite substrate would dissolve in a reef aquarium at all. I had stated somewhat of an absolute in an earlier post, that aragonite substrates won't dissolve (in a nano reef) *to the point of needing replacement* but apparently it may be time to edit yet another post!

One factor that probably plays a critical role in this dissolving substrate is the type of substrate in question: sand vs gravel chips for example. That opens up a host of factors such as surface area exposure which affects solubility. In my mind I was thinking coarse aragonite chips, surely they wouldnt just dissolve away in time---but never considered aragonite sand substrate. Even at that I wouldnt think it would readily dissolve unless some of the saturation is reduced or unless there are variances in pH that allow for this action. I come here to check my solubility schemes and change 'em up from time to time if necessary!

Thanks for your reply, see ya in the sump shortly.

Brandon

 

[randy holmes-farley]

It is hard to get enough to dissolve that way to support all but minimal demand for calcium and alkalinity, but it clearly does dissolve to some extent in many tanks.

Here is an article by Julian that describes one of the premier folks (Jaubert) that initially got aquarists interested in such systems:

http://www.advancedaquarist.com/issues/ ... eature.htm

 

[brandon4291]

Thanks for the article referral Randy.

Do you think there may be any variances in solubility between a primarily aerobic nano reef sand bed (where there is considerable exchange of water between the bed and the above water column) and a deep plenum bed that has areas of reduced oxygenation/circulation?

In a nano reef with chipped aragonite substrate and good coral growth, one would maintain the alkalinity and calcium measures on the high side and it just seems like this would not readily cause the chips to slowly go into solution... surely some degree of this occurence is found in sand type setups and not in setups using the coarser material?

Brandon

 

[Anonymous]

If I may again...

As I have come to understand the situation, it appears that there are more than one variables that affect the dissolution of calcareous substrata--whether in the form of sand, gravel, chips (I think sand would have the greatest surface area), or l/r.
For example, if you've got vigorous coralline or stony coral growth happening, the biological processes may acidify the water, exacerbating the mineral uptake already "caused" by these organisms in the first place. I have read that, in some sys's, all of these materials, including big chunks of l/r, may be reduced rather significantly be these same processes. (Assuming you would choose to use it in the first place.) You couldn't really dedicate too much space to it, could you? Unless you got really "fresh" stuff and q/t'd it yourself to grow out what survived.

There is one question in my mind, and that is: what is the discrepancy between what someone, such as Sprung, is saying re: his experiences from which he is deriving certain information, and what is actually achievable in a nano. I think that some things change once you go beyond a certain scale. However, I also think that you have much more experience with sys's of this kind than I.

For instance, if the recommended substrate depth is (I think) 8-10mm, then, can you create that depth? What about the effect(s) of the width, if there are any, eh? I haven't kept anything as small as what you do, so I couldn't offer up any personal experience--that's where yours come in.

 

[randy holmes-farley]

Aragonite will not dissolve at any appreciable rate if the pH is not reduced below that of the tank itself, because the water is always supersaturated with respect to calcium carbonate.

If the pH is lowered for any reason inside a sand bed, then the calcium carbonate can begin to dissolve. The pH down inside my sand bed is substantially below that of the water above the sand (mid to upper 7's compared to 8.4 above it).

The pH in the sand can be lower do to either aerobic or anaerobic respiration where organic materials are broken down. Both produce about the same net amounts of acid.

That said, if the sand is aerobic and organics are breaking down, that necessarily means that there is significant O2 exchange between the water column and the sand.

If there is substantial O2 exchange, then there is likely to be a lot of H+ and OH- exchange (and other acids and bases), and that will limit the pH drop that is experienced in the sand.

So in that sense, low O2 regions may be more likely to experience sand dissolution, but that has little to do with the O2 itself.

One problem is that even if the pH drops low enough for some CaCO3 to dissolve, the calcium and carbonate/bicarbonate are going to be slow to get out, for the same reasons that the acids and bases in the water column are slow to get in.

So it is not an especially efficient process for dissolving sand. It is also dependent on the degradation of organics in the sand, which is likely to be dependent on the nature of the organsims living and moving about in the sand.

 

[Anonymous]

In other words, there are a few variables?