calcium carbonate heavy metals binding

[charles matthews]

Randy:

I have been interested in ways to goose natural dissolution of aragonite. I was thinking of doing the experiment of putting a ten gallon tank inside a reef, filling it with aragonite to 12 inch depth, and placing a plenum underneath with an undergravel filter. I would leave one airlift tube OPEN so the plenum would actually be open to the aquarium water. At the other end the plenum would place a small heater set to 84 degrees, causing thermal convection upward. Turtle grass planted on top. So you have a settling filter on the bottom of the tank, an upside-down aerobic bed, and turtle grass roots reoxygenating the upflow.

It occurred to me that most of the heavy metal binding might be to the lower layer of this setup, not only to calcium carbonate but also to organics. The advantage would be that the reverse circulation would prevent resolubilization, unlike a traditional sand bed, which has metal precipitates on the top surface, and may be resolubilized by algae and bacterial action. All of this is just a crazy thought, of course.

And I wondered what you thought of its potential as a calcium reactor, denitrator, and general capacity to hold heavy metals before perhaps having to be replaced a la Dr. Shimek (assuming such measures are needed in terms of binding by organic chelators and rendering free heavy metals less toxic)? I mean, could you take a shot at the potential for long term storage of heavy metals in such a device?

 

[Randy Holmes-Farley]

As a denitrator, the flow will be critical. Too fast, and you won't go low enough in O2.

On the metals binding to organics, as the organics are degraded, they will be released. That may limit the effectiveness.

IME, copper in tank water doesn't bind well enough to calcium carbonate to be removed in this fashion (I put a fairly large amount of Southdown sand into a container of tank water. The copper went down, but not by much). IMO, the problem is that metals bound to organics (as copper will be in tank water) aren't especially attracted to calcium carbonate.

 

[charles matthews]

heavy metals, photoreduction, skimmers

heavy metals, photoreduction, skimmers

Randy, thanks for your time with my questions. I really want to ask one more! I was thinking of Diana Walsted's book, THe Ecology of the Planted Aquarium, in which she discusses photoreduction of iron, and a possible protective effect of detritus for lead, cadmium and the like. She uses soil as a substrate in her planted tanks, and some window light. She points out that sometimes greenwater occurs, and speculates that this occurs due to photoreduction of iron bound to organics in the water, as well as photoreduction of iron on the surface of the substrate. She cures the greenwater by taping black paper over the bottom of the back of the tank, shading the substrate.

And I was thinking about the possible role in photoreduction in producing the somewhat similar levels of toxic metals in marine tanks which are skimmed. There are free and bound species related to organic particulates, interacting with reef lighting- and those organic particulates, but not the free forms, are removed by skimming. Could the lighting and skimming of organics be the dominating factors in the steady state concentrations of heavy metals in reef tanks? Am I being stupid to think in some way that the photoreduced free species are being left behind by the skimmer and accumulate? Thanks for the chemistry lesson!

 

[Randy Holmes-Farley]

Could the lighting and skimming of organics be the dominating factors in the steady state concentrations of heavy metals in reef tanks

Sure.

For folks who add EDTA iron to their tanks (I have, but prefer iron citrate), the mechanism for bioavailability that is discussed in detail in "Captive Seawater Fishes" by Stephen Spotte, is degradation of the EDTA by UV.

FWIW, I think that iron citrrate may be bioavailable even without any UV.

For metals attached to general organic chelators in the tank, UV may play a very important role.

FWIW, I expect that the reduce iron (Fe++) will also be chelated (though differently perhaps), but the bioavailability of the metals is what we are concerned with and that changes with both chelation, concentration, and ionization state.