Reef Chemistry Question of the Day #42 GFO

Randy Holmes-Farley

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[h=3]Reef Chemistry Question of the Day #42[/h]
Granular ferric oxide (GFO; ferric oxide hydroxide; the iron based phosphate binders) might be expected to usefully reduce all of the following in a reef aquarium except:


A. Nitrate
B. Phosphate
C. Organics
D. Hydrogen sulfide
E. Silicate

Good luck!























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Randy Holmes-Farley

Randy Holmes-Farley

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And the answer is... A. Nitrate.................... Very high rate of correct answers on this one!

Here are some article sections with more explanation:

How iron oxide hydroxide binds phosphate

Phosphate is believed to bind to iron oxide hydroxide through a direct ionic interaction between one or two negatively charged oxygen ions on the phosphate with the ferric ions (Fe+++) in the solid. Figure 1 shows phosphate in solution above a GFO solid. Figure 2 shows it bound via two ionic bonds, with the displacement of hydroxide. These figures are included so that aquarists can understand what is happening, but are not intended to claim that the surfaces are not also covered with other ions that are known to bind to GFO surfaces, including sulfate, chloride, calcium, magnesium, trace metals, and organics.

Since phosphate binding takes place at the surface of the GFO (and not deep down inside it), the amount of surface area is a very important attribute in determining how much phosphate such a mineral can bind. Even though the commercial materials appear to be reasonably large particles (Salifert claims 0.2 - 2 mm on their product label), they actually have a high internal surface area, somewhat similar to activated carbon. Consequently, particle size is an unreliable means by which to gauge available surface area (though it is reliable for nonporous solids such as table salt). I have not seen any measures of accessible surface area for the commercial GFO sold to aquarists, but one research group1 analyzed its own material and found a surface area of 40-50 m2/g, while another group reported 20 m2/g for a sample of goethite and 250 m2/g for a sample of amorphous iron hydroxide.2 Those values are much lower than activated carbon (typically with hundreds to thousands of m2/g), but much larger than 1 mm cubes of a nonporous solid would provide (0.002 m2/g).

For these reasons (crystallinity, extent of hydration with water, and surface area), aquarists cannot simply substitute different commercial brands, or other industrial materials, and expect to attain identical results in aquaria.

Interestingly, the concentration of phosphate, in marine sediment pore waters whose sediment is iron oxide hydroxide, appears to be controlled to a great extent by phosphate bound to the iron.3 Even more importantly, this bound phosphate is still available to the water column by exchange, so the sequestering is temporary rather than permanent.3 This fact may be unimportant in an application where the GFO is presented and then removed with its bound phosphate, but in other applications, such as mud substrates or GFO accidentally released into the aquarium, it may become more of an issue.

In aquaria, there is no doubt that GFO is effective at rapidly and efficiently reducing orthophosphate concentrations. It may also be at least partially effective at reducing organic phosphate levels, but fewer data are available on such removal since few aquarists measure organic phosphate.

What else does iron oxide hydroxide bind? Metals

These materials are known to bind a wide range of other compounds from water, including trace metals, arsenic, selenium,2 silicate, and organics. Metals such as manganese, cobalt, nickel, and zinc are known to bind to iron oxide hydroxide in simulated seawater solutions.4,5 It has also been claimed that the binding of copper and zinc by natural iron oxide hydroxide sediments exerts a powerful control on the concentration of copper and zinc in polluted rivers and estuaries.6 Although not studied in seawater, it has also been observed that phosphate binding by iron oxide hydroxide actually increases its binding of copper, cadmium, and nickel in freshwater.7

Whether the binding of any of these ions is important in aquaria, and whether it should be considered a benefit or a detriment, remains to be established for each trace metal. Nevertheless, it is something that aquarists should keep in mind, and it may also be important in suggesting potential explanations for some of the biological effects of using these materials that are discussed later in this article.

What else does iron oxide hydroxide bind? Organics

Granular ferric oxide is also known to bind organic materials.8-10 In addition to many studies on the binding of man-made chemicals8 and natural organics from freshwater (such as humic acids),9 it has also been demonstrated that dissolved organic phosphate is readily removed from seawater by binding it to iron hydroxide.10

The binding of organics, especially those containing phosphorus, is likely to be beneficial in most aquarium circumstances. One possible exception is during treatments of the aquarium with organic medications. How effective this binding is compared to activated carbon, or whether they even bind the same materials, is unknown. It would be expected, however, that certain very polar organic materials might well be bound by GFO, and not by carbon. These would include certain natural biochemicals that might readily provide phosphate to algae, but that are too polar to be absorbed by activated carbon.

It should also be noted that GFO would not be expected to be very effective at binding purely hydrophobic molecules that will bind well to activated carbon. Consequently, GFO and carbon are in some ways complimentary in their ability to bind organic materials. If I had to select between the two for removing dissolved organics from aquaria, I'd select activated carbon.

Preventing and Dealing with Hydrogen Sulfide
Aquarists sometimes have to deal with situations where hydrogen sulfide exposure is a possibility. The suggestions below may be useful in preventing or correcting such problems.

...


8. If an anoxic sand bed needs to be removed from a reef aquarium, and there are organisms that cannot be relocated out of harms way, the following precautions may be useful based on the principles detailed in previous sections, although I've not tested any to see how effective they are:

A. Remove delicate organisms from the tank system, if possible.
B. Perform the change when the lights are as bright as possible, preferably near the end of the light cycle. The lights drive the O2 concentration higher, speeding the oxidative removal of hydrogen sulfide, and the light itself will catalyze the oxidation of H2S.
C. Maximize aeration. A high oxygen level drives hydrogen sulfide oxidation, and high aeration will drive some off as volatile H2S gas.
D. Add an iron supplement to help catalyze oxidation of hydrogen sulfide and the precipitation of ferrous and/or ferric sulfide. Use one chelated to an organic; either ferrous or ferric iron will work.
E. Pass the water over iron oxide/hydroxide (GFO) to convert hydrogen sulfide to elemental sulfur.
F. Pass the water over activated carbon, which may bind some sulfide, and may also catalyze the oxidation. If forced to choose between carbon and GFO, I'd pick the GFO media.

These have more:

Hydrogen Sulfide and the Reef Aquarium
http://reefkeeping.com/issues/2005-12/rhf/index.php

Iron Oxide Hydroxide (GFO) Phosphate Binders
http://reefkeeping.com/issues/2004-11/rhf/index.htm





 
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Electrobes

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Randy, out of curiosity what are you thoughts on GFO use? There is a discussion elsewhere I am having (RC) about the Marine Pure blocks and it somehow delved into what he claims as your reason for not using GFO.

"But in summary the last thread that I read was that Randy hasn't used GFO in at least a year plus due to unfavorable effects leading to potential insoluable toxicities in the material. "

Could you tell me about the validity of the statement? I haven't found it in your articles and it's something I'd like to know more about if true. The author of the above quote couldn't point me to the source of that statement after I asked for it.
 
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Randy Holmes-Farley

Randy Holmes-Farley

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Randy, out of curiosity what are you thoughts on GFO use? There is a discussion elsewhere I am having (RC) about the Marine Pure blocks and it somehow delved into what he claims as your reason for not using GFO.

"But in summary the last thread that I read was that Randy hasn't used GFO in at least a year plus due to unfavorable effects leading to potential insoluable toxicities in the material. "

Could you tell me about the validity of the statement? I haven't found it in your articles and it's something I'd like to know more about if true. The author of the above quote couldn't point me to the source of that statement after I asked for it.

I currently use GFO, so I'm not sure what that is referring to. :)

Some people are concerned about released iron, but I am not (and there is no evidence that it happens, but it may) and I dose iron anyway.

The only concern I'd have with iron dosing of GFO is the potential for the iron to allow cyanobacteria to thrive when it might otherwise be iron limited. That is just a hypothesis, but is the one potential concern. :)
 

Electrobes

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Argh I wrote an explanation but Tapatalk had a fit. Oh well I'll explain later.
 

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