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Please Welcome Collin Cross

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mojoreef

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I would like to announce that Dr. Collin Cross has joined up with Reef Frontiers to share his knowledge and insight with our members. Collin is a Physical Chemist and has worked in diverse areas such as materials science, structural biophysics, analytical chemistry and process/fuel additives. Collin has authored 12 publications in scientific journals, holds 4 US patents and currently works for GE Water & Process Technologies as a Research Project Leader. Collin has had a lifelong interest in marine animals and has been interested in Marine aquariums since 1982. He has had two tanks during that timeframe, his last tank being set up only earlier this year after a break of many years.

Collin will be taking care of our Chemistry forum, so make sure you drop in to say hello and welcome him aboard, and invite others that Collin may be able to help.
 

Witfull

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Jan 27, 2004
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New Jersey
greetings Dr. CW. glad to have you here! i look forward you you making my brain hurt as it stretches as it attempts to gain just a fraction of your wealth of knowledge.

..."give it to me straight Doctor, i can take it"
Dr. Johnny Fever, WKRP.
 

aquariumdebacle

electrolyte addict
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Jul 4, 2003
Messages
613
Location
Seattle
First, welcome to reeffrontiers!
I would like some information on the solubility of copper ions at pH of 8+. Specifically do ions that have adsorbed onto a calcarious substrate ever come out of solution if the pH stays above eight? Do we need to periodically remove substrates that have accumlated heavy metals?
 

cwcross

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Aug 18, 2004
Messages
250
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
 

cwcross

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Joined
Aug 18, 2004
Messages
250
In light of the previous discussion, I will simplify somewhat. We can break this process into three regimes.

1.) Binding constant is small, or adsorbed lifetime is short with respect to the frequency of water changes. In this case, a given metal (or other bad actor) will not build up on substrate and water changes only will work great.

2.) Binding constant is very large, or adsorbed lifetime is very long compared to water change frequency. Bad actors will build up on substrate, but probably won't present a problem because they will stay on substrate and water changes will keep concentrations in the water very low.

3.) Binding constant or adsorbed lifetime on the same order of water change frequency. In this case, metals could build up in the substrate but could also leech out again significantly between water changes.

Case three is the one we should worry about. One way to solve this problem would be to perform substrate changes similar to water changes. Maybe replace 10% of substrate montly??? This would solve the problem! Question of the day is... is it really necessary??? This is what we need to determine. Another important factor, which I did not discuss but is vitally important, is the rate of toxins entering the system and how it compares to the water change rate and adsorbed lifetime. For all these arguments, I assumed toxins enter fast, or in one large lump, compared to water changes. My thinking/feeling is that this is NOT the case and that heavy metals, phosphates etc. WILL NOT build up and that this is folklore only. I will use this as my starting point and try to prove myself wrong. Please help..Sincerely...Collin
 

cwcross

Well-known member
Joined
Aug 18, 2004
Messages
250
Sorry Big t,

take some advil and read 10 more times! :) That should help. If not, let me know specifics and I can try to break it down more
 

mojoreef

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Collin it gets deeper then than that. Remember Cu is a bioactive/nutrient metal, that changes the whole equation. Now you need to think about stuff like humic acids and alike. Then you have to take into account that approximately 99.97 % of all Cu in seawater ends up being organically bound up. A large amount by bacteria, legands, chelators and so on. I would guesstimate that virtually no Cu get bound to sand or rock, and if a minute amount did, it would be for long.

Mike
 
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