GFO and ALK drop

[JR's Reef]

What causes Alk to drop if you use to much GFO?
Thanks in advance

[Randy Holmes-Farley]

A few things may contribute, including precipitation of calcium carbonate that is induced by dissolved iron,a and increased calcification by organisms as phosphate drops.

This article discusses these possibilities:

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


from it:

What else might iron oxide hydroxide do? Precipitation of CaCO3

Many Aquarius using GFO have reported unusually extensive precipitation of carbonates on the solid GFO, and elsewhere in the system. Such precipitation can, for example, be a contributing factor in the caking of such materials, and can coat other surfaces in the aquarium. This precipitation can also contribute to a drop in alkalinity and possibly pH as it removes carbonate from the water column. The effect of calcium will be similar, but smaller on a percentage basis, with a drop of only 20 ppm calcium for every 1 meq/L (2.8 dkH) drop in alkalinity. Increased calcification by corals and coralline algae (possibly spurred by reduced phosphate) can also cause similar drops in calcium, alkalinity, and pH.

Dissolution of these precipitates with acid, accompanied by bubbling, indicates that these deposits are carbonates, and are most likely calcium carbonate since it is supersaturated in most reef aquaria (and in the ocean). Several factors may contribute to this precipitation. Many of these are rather straightforward. It is known, for example, that phosphate inhibits the precipitation of calcium carbonate. Much like the role that magnesium plays in seawater, phosphate binds to the growing calcium carbonate crystals, poisoning their surface against further precipitation of calcium carbonate. Many organic materials are also known to inhibit this precipitation. Near the surface of the GFO, and downstream from it, the organics and phosphate are expected to be lower in concentration than upstream from it. The reduction in concentration of these inhibitors may well permit increased abiotic precipitation of calcium carbonate on such surfaces.

Two more esoteric events may, however, be equally important. The first is that the local pH near the GFO surfaces may be higher than in the bulk solution. This effect arises as phosphate and other inorganic and organic ions displace hydroxide from the surface. Figure 2, for example, shows phosphate displacing two hydroxide ions. The net swap of HPO4-- for 2 OH- will raise the local pH. The supersaturation of calcium carbonate increases as the pH rises, driving the precipitation of calcium carbonate.

Another possible role may be played by the iron itself. GFO is not completely insoluble. The solubility of iron hydroxide in natural seawater is small, but still significant (0.02 - 2 ppb), although it is largely controlled by the availability of organic ligands.11-13 One interesting possibility lies in the way that soluble iron actually impacts the precipitation of calcium carbonate.

At high concentrations, iron inhibits the precipitation of calcium carbonate. While different researchers find different threshold concentrations for this inhibition (>25 ppm in one case,14>7ppm in another case15), it is a well established and studied phenomenon. The mechanism is believed to be the same as for magnesium, phosphate, and organics, which all poison the growing calcium carbonate surface.

At much lower concentrations, however, iron actually increases the precipitation of calcium carbonate by acting as a site for nucleation of new crystals. In one case this happened at 100 ppb dissolved iron, increasing the rate of scaling (the precipitation of calcium carbonate on surfaces) by about 60%.14 In another case, the induction time for precipitation (that is, the time it takes for precipitation to begin once the water becomes supersaturated) was reduced by 40% at 1.4 ppm iron and the overall precipitation rate was increased by 32% at 560 ppb (lower iron levels were not tested).15 These studies were carried out in freshwater, and I have not seen similar studies in seawater.

Is the natural dissolution of GFO important in the nucleation of calcium carbonate precipitation? I am not sure. But it is clearly one possible explanation that fits the observations of aquarists as well as known phenomena involving iron.

[cmoresps]

Randy, I had the same blasted problem, couldn't figure it out for months, read your article and boom! Thanks a million for all of your hard work and dedication to the hobby!

One question though, when using GFO, it left iron stains (a good amount) on the inside of my sump. I tried to remove it but it doesnt look like its going to come off without using a nasty cleaner such as CLR. Should I be concerned over the stains, compared to the system size I cant believe it could do much damage.

Finally Kent came out with a phosphate remover made of ceramic, claims there is no aluminum in it, but it doenst look as effective as GFO. Have you heard anything about the biproduct of using it? Personally Im going to stick with skimming and ozone for a while but wanted to check.

Thanks in advance

[Randy Holmes-Farley]

I don't think the stains are a concern .

I think Kent's product is aluminum oxide, which is correctly labeled as a ceramic. I think they claim it doesn't release aluminum, right, not that doesn't contain aluminum? I would not believe that it doesn't release any. Seachem made the same claim until I measured it (actually, they did so afterward as well).