Dan, a great question! First, on the solubility of copper ions at pH8+. This problem seems simple but actually isn't. Asking for Cu++ solubilities at any pH, is not a valid question. Cu++ solubilities can only be evaluated in the presense of an associated anion (cu++ is a cation or positively charged ion, whereas an anion is a negatively charged ion). For instance, the solubilities of Copper Sulfate and Copper Nitrate will be different, potentially quite different. To make things more complicated, when you have a mixture of copper sulfate and copper nitrate, something called the common ion effect must be taken into account. In a complex mixture of salts such as sea water, the problem can only be solved easily by making some assumptions and/or determining it experimentally.
However, to what I think is the heart of your question, which regards the binding properties of metal ions to the substrate. The short answers are yes and probably! Let me explain. In chemistry, there is no such thing as an absolute. In other words, it is invalid to ever say that something would adsorb irreversibly to any substrate. A couple concepts. First that of dynamic equilibrium. Any molecule that is capable of adsorbing into some substrate, however arbitrary, does so with what is called a binding constant. The ratio of bound to unbound copper is the binding constant and it stays a constant, all other things being held equally. Also...the higher the binding constant, the tighter the molecule will stick to the adsorbate. Another way of saying that is that the lifetime of the adsorbing molecule on the adorbate will be longer than something with a lesser binding constant. Dynanic equilibrium means that at any time, some molecules are adsorbed on the substrate and some are dissolved in solution AND THEY ARE EXCHANGING ALL THE TIME. That is the adsorbed molecules are breaking off and floating away, and new molecules float up and become adsorbed and the rates of these two processes are equal. The average time it takes a molecule to become adsorbed and then to break free and float away is determined by the binding constant.
Second concept is limiting or comparative rates. Lets imagine that cu++ binds only very weakly with the substrate. If this is the case, then any given molecule will spend most of its time in the water, and very little time on the substrate. In this case, your cu in water is in dynamic equilibrium with the cu on the substrate. When you change your water, the cu concentration in the water is lessened and some cu in the substrate will immediately leech off to reach its new equilibrium level. If you do water changes at a rate faster than the binding constant, you will eventually wash all the cu off of the substrate. Now lets imaging the binding constant is very high and the cu ions spend about a year on the substrate before breaking loose. In this case, if you do a water change, you will still reduce the concentration of cu++ in the water, but the cu's on the substrate won't come off for another year. In this case the approach to equilibrium is so slow that even changing water every day won't get you anywhere very fast. In this case, substrate changes or relatively frequent, partial substrate changes would be necessary to reduce cu on the substrate.
Ok...enough of the rhetoric. Basically, I don't know the binding constants of cu onto argonite. If they are low, then water changes will keep substrate free of heavy metals. If they are high, you need to do partial substrate changes on a regular basis. This is the same question I am interested in regarding phosphates or any buildup in the sandbed. I will be looking further into this in the coming weeks. However, I need to find some data to put things into perspective...hope that helps...luckily I type fast...sincerely...Collin
That should help. If not, let me know specifics and I can try to break it down more
