Rob:
First, I agree with you on several points, including the fact that people can have silica sand beds without diatom blooms. If you keep other nutrients low, diatoms may never be an issue, even in high silica tanks. I also agree that the benefits of aragonite are overblown, though I do use it.
I think, however, that the evidence that you supply is not convincing that people never get diatom blooms due to silica sand. Maybe they do, maybe they don’t. I’m not expressing an opinion on that question, just your evidence on whether it is possible or not.
Many tanks have high nitrate, phosphate, and organic levels. Some get green algae growth, others do not. Some get diatom growth, others do not. None of us really knows why in every case. I’m not convinced that such people won’t have diatom problems if silica is high, or if they have a lot of silica sand.
A line by line commentary:
“Quartz (SiO2) is considered “totally insoluble” in water according to the US MSDS,...”
Here are two references that give the silica solubility in pure water at 25 deg C and pH 8 to be 120 ppm.
http://osmonics.com/products/Page696.htm http://www.gc3.com/techdb/manual/depotext.htm
For purposes of an MSDS sheet, that’s probably close enough to insoluble. I’ve written MSDS sheets, and I might give that same description for use in a safety brochure. Does that mean that if I put 120 mg in a liter of water that it will immediately dissolve? No. It means that in the long term, it will, but that in the short term it may be kinetically limited. If I put a pound of finely divided SiO2 in a liter of water, it will approach the solubility much faster. How fast? I don’t know. It may be fast enough to be important, and it may not. Without kinetic data one way or the other, we cannot conclude whether dissolution of sand can be important to a reef tank or not. This point is also important in comparing sand to glass walls (below).
Further, the oceans are undersaturated everywhere with respect to silica according to “Chemical Oceanography” by Frank Millero (p. 300). Thus, there is the driving force, if not the kinetic pathway, for silica to dissolve in seawater.
“The fact is that quartz sand (and the walls of our aquarium and even the silicone rubber which is the most soluble of the lot) do not dissolve in seawater to be measurable”
First, silicone rubber itself contains no silica (maybe you know that; your sentence seems to imply that it contains silica, but that may be a misinterpretation on my part). Some silicones may have some added silica as a stiffener, but many do not. Second, silicone sealants are crosslinked polymers, and as such have zero solubility in any solvent (though trace impurities of other things in them may dissolve). Here’s a link that talks about silicones:
http://www.pbs.org/wgbh/pages/frontline/implants/corp/history.html
So, on to the point that the walls of the aquarium do not dissolve. How do you know that they do not? By measuring silica in the water? No. I’ve been making daily additions of silica to my tank (as highly soluble sodium silicate; equivalent to 0.02 ppm SiO2 every day) since October and none has ever become detectable with a Hach kit. Why? Presumably because tank creatures use it up as fast as I add it. The same could be true of silica from the walls, and from the sand. It is also true in the oceans, where diatoms use it up as fast as it is added. Silica effluent in rivers often doesn’t even make it to the ocean because it is consumed by diatoms in the estuary (again, according to Millero).
By not seeing the glass dissolve? No. To bring a 90 gallon tank from zero ppm silica to 0.2 ppm silica (which is the approximate NSW surface concentration), would only require 10 nanometers of the surface glass to dissolve. That’s only about 100 atoms. How can you tell that that hasn’t happened?
“you would expect to see big differences in the silica concentration around sandy beaches”
Why? The silica concentration in seawater is controlled by the growth of organisms. If diatoms drive the silica down to the level that they can no longer adequately draw it from solution (which they do, driving it down to less than 0.2 ppm), then the amount being added will not impact the concentration. It could impact the diatom concentration, if in fact dissolution of beaches were a significant addition to the ocean. I haven’t seen data on diatom concentrations comparing calcerous coasts to silica coasts. Perhaps you have that info.
“(roughly 2 ppm everywhere other than adjacent to the mouths of rivers...”
In Millero’s book, the ocean surface silica concentration is shown to be much lower in surface waters across most of the globe. More like 0.2 ppm. Do you have different information? He also shows that the silica concentration increases greatly as you go deeper, where diatoms are not using it up, and in fact dead, sinking diatoms are dissolving (giving it a nutrient type profile). Thus, the amount of silica that would have to dissolve to give NSW levels of silica in a tank is quite small.
“If it doesn’t make a difference on a global scale, how much difference do you think it can make in our tanks?”
Well, I’d argue that it could be huge. First, the amount of sand in a tank is vastly greater per amount of water than in the ocean. By a factor of what, a thousand? a million? More? So a little dissolution goes a long way in a tank, but nowhere in the ocean.
Second, the amount of sand per diatom may be vastly higher in our tanks than in the ocean (for the same reason). Third, since tanks don’t have river inputs or hydrothermal vents, other inputs, such as sand dissolution, may become the limiting factors.
“given that, it’s pretty hard to argue that using quartz sand is bad when the glass box that your putting it into is made of the same stuff.”
Well, ignoring Craig’s comments that it may not be the same, I don’t find it hard to argue at all. Based on the known solubility of silica, and the fact that it doesn’t immediately dissolve to reach that solubility, we must conclude that the dissolution is kinetically limited. Dissolution of solids, when slow, is typically limited by surface area (among other things). Rock candy and jaw breakers, for example dissolve a lot more slowly than a spoonful of sugar in your mouth.
The surface area in a sand bed is far, far higher than the glass walls of an aquarium (I calculated it once). Thus, the dissolution of a sand bed could be orders of magnitude greater than the walls of the aquarium.
So can we conclude that since a little of something is OK, that 10, or 100 or 1000 times as much is also necessarily OK? It might or might not. You likely would not say that about limewater, or iodine, or strontium, etc.. Why say it about silica?
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Randy Holmes-Farley
