Trace Element Adsorption By Aragonite Sand

Dan_P

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I am planning an experiment to look at algae growth in Instant Ocean dosed with trace elements. ICP testing is likely in my future, but to begin with I wanted to shake the tree of knowledge on the topic of “trace element adsorption to aragonite sand”. A brief look at internet resources indicate that trace elements can be incorporated in calcium carbonate crystals. There is much less information on whether I can expect aragonite sand to adsorb trace elements like it does phosphate. All facts and viewpoints welcome.
 

Randy Holmes-Farley

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It’s a very complicated question since it will depend on whether the sand and the trace metal is itself bound or coated by organics, but certainly many bare metal ions will bind to aragonite while strongly chelated ones will not.

See the black bars of Figure 2 in this paper, for example. It’s not aragonite, but it is calcium carbonate.

 

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I am planning an experiment to look at algae growth in Instant Ocean dosed with trace elements. ICP testing is likely in my future, but to begin with I wanted to shake the tree of knowledge on the topic of “trace element adsorption to aragonite sand”. A brief look at internet resources indicate that trace elements can be incorporated in calcium carbonate crystals. There is much less information on whether I can expect aragonite sand to adsorb trace elements like it does phosphate. All facts and viewpoints welcome.
Given my unique sand filter, this is going to be really interesting. Unfortunately the branches on my tree of knowledge are barren, except that when I soak my sand (which maybe calcite) in tap water for 5 minutes, with detectable copper, nothing appears to leach back out ( in lanthanum and RODI ), obviously just using a hobby kit.
 
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Dan_P

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It’s a very complicated question since it will depend on whether the sand and the trace metal is itself bound or coated by organics, but certainly many bare metal ions will bind to aragonite while strongly chelated ones will not.

See the black bars of Figure 2 in this paper, for example. It’s not aragonite, but it is calcium carbonate.

Thank you for the reference. This gives me a good start on thinking this through. Here are some thoughts I had from reading the paper.

Tossing powdered calcium carbonate, e.g., coral snow, into an aquarium might adsorb trace elements to varying degrees, possibly proportional to the solubility constant of their carbonate salt. The total amount of each element adsorbed varies. It seems the surface can become saturated like in the case of phosphate. Adsorbed trace elements might not be bioavailable.

Organic material, i.e., biosorbents, can bind trace elements via ion exchange. Biosorbent efficacy can be improved when deprotonated. This would seem likely to happen in seawater. The total amount of bound metal depends on the number of binding sites on the biosorbent. Surface area and porosity play a role. Biosorbent bound trace elements might still be bioavailable if not removed by filtration, adsorption onto GAC or removed by foam fractionation.

I am tempted to measure the removal of trace elements by fine aragonite sand.
 
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Dan_P

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Given my unique sand filter, this is going to be really interesting. Unfortunately the branches on my tree of knowledge are barren, except that when I soak my sand (which maybe calcite) in tap water for 5 minutes, with detectable copper, nothing appears to leach back out ( in lanthanum and RODI ), obviously just using a hobby kit.
I would make an educated guess that the copper will not readily desorb if it sticks to aragonite because it turned into copper carbonate. Maybe a weak acid wash would release it.
 

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