I do not know the exact solubility of silica from dead diatoms, but even from quartz it is quite high. The issue in a reef tank is more of a kinetic one than an equilibrium solubility one, I think.
Feature Article: Silica In Reef Aquariums ? Advanced Aquarist | Aquarist Magazine and Blog
https://www.advancedaquarist.com/2003/1/aafeature1
The Dissolution of Quartz Sand
One of the issues that has been floating around the reef keeping hobby for a long time is the issue of whether “silica” sand actually releases soluble silica or not. It is remarkable that so many people have strong opinions on this issue, and yet so few people have ever bothered to do the easy experiment of measuring it. Many even fall for the trap of concluding that since their glass aquarium is not dissolving, then silica sand must not be either. All of the arguments against soluble silica being released from “silica” sand can be easily refuted, and I have done so in the past, but that is not the point of this article. Still, some background is worthwhile before getting to experimental results.
Silica sand is largely composed of quartz.
Quartz has a maximum solubility in pure freshwater of about 180 uM (11 ppm as SiO2)36, and is somewhat higher in seawater.37 That value is substantially in excess of the dissolved silica concentrations in any normal part of the ocean (excluding plumes from vents from hot springs and such). So why doesn’t quartz beach sand dissolve? It does, but it does so very slowly. The rate of dissolution of quartz has been studied, and it is very slow. 38 It is the slow dissolution of quartz, not the solubility itself, which allows it to remain on many ocean beaches.
A final comment on quartz sand is that it is known that organic acids can increase the rate of dissolution of quartz by at least a factor of ten.39 This may be especially applicable in reef tanks, where organic materials may be in abundance, particularly when organisms are living directly on the sand, potentially releasing such acids directly onto the sand surface.
The problem with extrapolating from the known very slow rate of dissolution of quartz to “silica sand” is that it simply is not pure quartz. The dissolution of soluble silica from “quartz sand” (98.5% SiO2) has long been known to exceed the solubility of quartz itself.40 Take a close look at some commercial “silica” sand. It isn’t even close to being white, which an absolutely pure quartz sand will be. There are all sorts of different colored particulates in it (some are even magnetic and can be picked out with a magnet). Without going into detail on mineralogy, suffice to say that there are many minerals that readily dissolve to release silicate into the water. Such dissolution is why freshwater rivers contain so much silica (typically 150 mM (9 ppm SiO2)).4 Your sand claims to be 98% quartz? What about that other 2%? Two percent of a 50-pound bag of sand is a pound of “other stuff”.
If you start with true beach sand, and don’t fracture it much, then it is very likely that you will detect little dissolution of silica from it in a few days (although I’ve not tried it), because most of the readily dissolved minerals would have disappeared long ago (or are trapped inside). But commercial play sands are not typically from beaches, and are not collected with any kind of gentleness. They are often mined from sand pits, crushed, screened, and generally treated rather roughly. This serves to break many of the grains, exposing new mineral inclusions that are then primed to dissolve. This source is, in my opinion, where most of the soluble silica comes from in “silica” sand.
So, on to some experiments. I bought some Quickcrete Play Sand from Home Depot and ran a number of tests on it. In all of the cases shown below the silica concentration was determined with a Hach low range silica kit after filtration through a 0.2 mm syringe filter. In cases where the concentration is above 1 ppm, the sample was diluted with RO/DI water prior to analysis. All experiments were carried out in the dark to reduce any effect due to diatom growth.
In the first experiment I took 3 cups of sand, and suspended it in 3 gallons of freshly made Instant Ocean salt mix that initially contained less than 0.8 mM of silica (0.05 ppm SiO2). After 48 hours of gentle stirring with a powerhead (the water was stirring, but not the sand), the silica concentration had risen to 17 mM (1.0 ppm SiO2).
I then rinsed the same sand 5 times with 1 gallon RO/DI water (1 minute each time), discarded the contents, and then ran the same stirring experiment with 2 new gallons of Instant Ocean salt mix. In 48 hours the silica concentration had again risen, this time to 15 mM (0.92 ppm SiO2). Then I let it sit unstirred for another 96 hours, and the concentration had risen more, to 23 mM (1.4 ppm SiO2).
In a different experiment, I took about 45 pounds of sand, and added 2 gallons of Instant Ocean salt mix. I let this mixture sit for 7 days, with once a day mixing with my hands for about 30 seconds. At then end of this test, the concentration was 90 mM (5.4 ppm SiO2).
It has been suggested that the amount of silica coming from calcerous sand might actually be as high or higher than that from silica sand. To test this hypothesis, I repeated the small-scale experiments above on a calcium carbonate sand from Home Depot (Southdown). In this case, there was some soluble silica released after the first 48 h, but only 1.6 mM (0.1 ppm SiO2), or about a factor of 10 lower than the silica sand. In a long-term test, the concentration had only risen to 5 mM (0.3 ppm SiO2) in 14 days with once a day stirring.
From these experiments, I conclude that:
- The “silica” play sand that I purchased from Home Depot can substantially raise the dissolved silica concentration in seawater.
- The dissolvable portion of the silica sand cannot be completely removed by several rinses with either fresh or salt water, although it may be decreased somewhat by that process.
- Southdown calcium carbonate sand (likely aragonite) can release soluble silica, but about ten fold less than the “silica” sand.
Is it OK to use silica sand? Probably. Many people do so. I also believe that not all “silica “ sands will be the same for the reasons described above relating to processing of the sand and the nature of the mineral inclusions present. So the fact that many people successfully use some (or many) types of silica sand does not necessarily imply that all people can use any type of “silica” sand without a problem.
In subsequent sections of this article I describe dosing recommendations for adding soluble silica. Is silica sand a good way to go from that perspective? I cannot really answer that. It probably provides some silica to reef tanks, but the amount is completely out of the control of the aquarist. For that reason alone, I believe that it would be a poor choice as the sole source of soluble silica for a reef tank. In a tank without any silica dosing, silica sand may, in fact, be more beneficial to the overall tank, at least from a silica delivery standpoint, than calcium carbonate sand. There are, of course, many other differences that might be the deciding factor on sand choice (color, texture, dissolution, particle size distribution, nutrient and metal binding properties of sands, etc). Many of these factors are more aesthetic than technical, and the technical ones are beyond the scope of this article.
