copper

[J.Warrick]

I am curious to know at what pH and (probably more importantly) what alkalinity does copper resolubilize in the marine aquarium once it has been bound up in a calciferous substrate. I have read in many places that "if the pH drops..." it can come back into solution and kill your fish, but I can't seem to find anyone who can tell me the actual point. Is it an all or nothing type of situation?
I would appriciate any help you could supply.
J. Warrick

 

[randy holmes-farley]

Well, I don't have any data, but I expect it would become solubilized when the substrate dissolves. The circumstances are covered in the calcium article posted here, but in general it will be at pH in the mid 7's. A doubling of alkalinity will cause the pH where CaCO3 dissolves to drop by about 0.3 units.

[ April 19, 2002: Message edited by: Randy Holmes-Farley ]</p>

 

[CraigBingman]

NBS pH scale (the one that people in the hobby use)
TA = 2.3 meq/L
Temp = 20C
Pressure = 1 bar
Omega = 1, aragonite, pH 7.6
Omega = 1, calcite, pH 7.4

Calculation from CO2SYS, mentioned in one of my past articles. Omega = 1 means that the water is saturated with calcium carbonate at that pH value. Lowering the pH below that point will result in net dissolution of calcium carbonate.

As Randy mentioned, doubling the total alkalinity reduces the saturation pH by 0.3 pH units. So at a total alkalinity of 4.6 meq/L, pH(omega=1) for calcite is 7.1, and pH(omega=1) for aragonite is 7.3.

A fundamental problem with this is that we really aren't sure of the chemical activity or even the chemical identity of the material holding the copper. It is probably largely held in "detritial fines" in the system, and that material may be a really complicated mixture containing unknown amounts of calcium, magnesium, carbonate, phosphate and hydroxyl ions as major components (copper will probably be a relatively minor component of the mixture.)

Another problem is that the pH we really care about is the pH of the microenvironment where these detritial deposits are located. Much of it is probably in the substrate. The pH in the substrate can be much lower than the pH of the bulk tank water above it. So even if the calculation above is correct, the material might still be coming out of solution at tank pH values much higher than indicated, because the significant pH is the pH value of the pore water in the substrate.

In terms of husbandry-specific recommendations, if you have substrate that you suspect is copper-contaminated: get rid of it. Get rid of all of it, especially the fines. That will probably involve emptying the tank and cleaning out every bit of fine crud laying in the bottom of it.

If you are concerned about copper coming out of some piece of old coral skeleton that is sitting in your reef tank now, then that is probably less of a concern, especially if has been successfully overgrown with coralline algae.

If you have an old piece of presumably copper contaminated coral skeleton that has never been in a reef tank, don't ask for trouble: keep it out of the reef system.

Craig Bingman