Iron and PO4 in New Aragonite Sand

[Dan_P]

A recent conversation with @taricha and @Lasse on the subject of cyanobacteria mats causing phosphate release from the substrate stimulated my interest in measuring the iron and phosphate content of aquarium substrate. I also looked at new Caribsea aragonite sand and the fines washed out with RO/DI. Conversations with @flampton, @brandon429 and @taricha helped shape the design of my experiments. The analysis involved dissolving the new aragonite sand and then adjusting the pH to about 5. Blanks were run to measure any Fe and PO4 in the hydrochloric acid and sodium hydroxide used in the experiments. Measurements were made with Hanna PO4 and Fe Checkers. Data is summarized in the plot below.

The fines, or dust, removed by rinsing the sand with water contain a higher concentration of PO4 and iron than the pre-washed sand (“Sand”). The fines from the more vigorously mixed sand-water mixture (“Fines 2 (Vortex Mixed)”) seem to be somewhat more enriched. Because only a tiny fraction of the sand is made up of fines, their removal is inconsequential to the PO4 and iron content of the sand. Given that the error bars on these measurements are at least 10%, the ratio of PO4 to iron is probably near 1:1.

What do these measurement mean? They could be important if the PO4 and iron are accessible to microorganisms, something that needs looking into. Also, there seems to be a small amount of organic matter present in Caribsea aragonite sand that supports bacterial growth.

 

[NeonRabbit221B]

Nice! I love reef science experiments so this is a cool find.

 

[Randy Holmes-Farley]

I assume you measured a blank using the same total amount of acid, and subtracted any readings you got from it?

 

[Randy Holmes-Farley]

FWIW, Greg Hiller analyzed crushed coral for iron here and found about 10.5 ppm (0.001%):

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[brandon429]

linked to our rinse threads so we can learn the updates/discoveries nice work here as always Dan

 

[Dan_P]

I assume you measured a blank using the same total amount of acid, and subtracted any readings you got from it?

Exactly what I did

 

[Dan_P]

FWIW, Greg Hiller analyzed crushed coral for iron here and found about 10.5 ppm (0.001%):

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For me that’s in the same ballpark. Since I looked at exactly 1.0 batches of sand from 1.0 vendors, I have to wonder how the iron and PO4 contents varies across vendors and lots.

 

[Cory]

Could this be a reason to have a slow release of iron in the tank for our corals? I currently have a bare bottom tank but wonder if sand would increase coral growth.

 

[Randy Holmes-Farley]

Could this be a reason to have a slow release of iron in the tank for our corals? I currently have a bare bottom tank but wonder if sand would increase coral growth.

Maybe. But foods will likely be a much bigger, ongoing source of iron.

 

[jda]

I have long had chaeto fuges and have to add iron to my tank because of it, so I have no idea about sand and iron. However, IME, the long term amount of P bound by the aragonite (don't forget the rock here too), will be dependent on your husbandry, methods and techniques since any P bound in the sand can unbind too - the bound amount in your tank will adapt to you and you can have anywhere from a buffer (low) or reservoir (high). You can change this along the way, if you want.

Dr. Ron (or maybe it was Fenner) used to talk about matting bacteria using localized amounts of P and causing sand and rock to unbind underneath as a source of fuel with the lower concentration on top - perhaps a search on WWM might get you some more info? In any case, I have found it to be true.

Ocean based sand should be relatively P free if you work in into a larger water sample size. I let some aragonite (ARM large media back when it was available in aragonite - it is calcite now) soak in saltwater (heated, since it mattered) for a while and came up with an undetectable amount of P using Hannah Ultra Low. I had to get it stronger (less water) to get a measurement which was barely a trace and in like a 180 gallon tank had to go like 5 or 6 decimal spots to quantify it. Rock or sand from a beach, or some other terrestrial source usually is pretty high in bound phosphate.