Help! GFO becoming a Calcium magnet

[robbyg]

Since getting the Trident I have become more obsessed with numbers than I should be.
My Alk has been bouncing between 8.5 and 8.8 and behaves in a very predictable manor. Eg. I dose more ALK I get the reading I would expect. My Magnesium goes between 1380 and 1400 with almost an inverse of logic but that is not really important. The big problem is my calcium which I had at a very steady 410-415 for the last 2 months but then it slowly started to drop to 390. I found the problem which was two lumps of precipitation in my tanks sand and when removed things started to get near 400 again and then suddenly plummeted over the last week to 375. I dosed more Ca (EV 2 part) and the numbers get even lower and the Mg goes up slightly. I get why the Mg is going up, I assume its because it's not being used.

Today I looked at my GFO and it seems like in the last week it has become winter in there! I see lots of white specs moving in my GFO media. So now I know where all my calcium is going. The last batch of GFO was starting to do the same thing before i changed it. The media is 3 weeks old, I thought it lasted longer than that since my phosphates are pretty low. The dosing is done just before the return pump and the GFO pump is before that, so it is the furthest it can be from the dosing lines.

My question is as follows. Is GFO a substance that naturally creates precipitation or is this something that only happens when it is exhausted? What are the main underlying factor that triggers precipitation when the ALK is not high. I have been playing with dosing intervals 3x per day 8 hours apart of 20ml of ALK that is dosed over a 30 minute period then a 30 minute wait and I dose 20ml of Ca. I recently decreased the wait time to 15 minutes between Alk and Ca and now I am wondering if that is the problem. How long are you guys waiting before adding the Part B. My tank turn over is about 8x

BTW my sand is not new, it's at least 5 years old. I am using Phosban in the reactor. pH is 8.3

Thanks for any help.

 

[Randy Holmes-Farley]

GFO (and the iron ions it releases) can induce the precipitaiton of calcium carbonate on it and downstream of it. That can increase the demand for calcium and alkalinity.

I discuss it here:

Iron Oxide Hydroxide (GFO) Phosphate Binders by Randy Holmes-Farley - Reefkeeping.com

from it:


What else might iron oxide hydroxide do? Precipitation of CaCO3

Many aquarists 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.

 

[robbyg]

Randy Thanks for taking the time to post the reply. I was wondering do you have a published book that contains all of your Aquarium Chemistry articles?

That is Excellent GFO Info Randy I am still trying to read and digest it. I am assuming that as the GFO gets used up the surface Iron levels dip and your saying that is a possible reason why it's more likely to start precipitation?

Also any suggestions on the time Interval between adding Part A and then adding Part B?

Thanks

 

[Randy Holmes-Farley]

Randy Thanks for taking the time to post the reply. I was wondering do you have a published book that contains all of your Aquarium Chemistry articles?

That is Excellent GFO Info Randy I am still trying to read and digest it. I am assuming that as the GFO gets used up the surface Iron levels dip and your saying that is a possible reason why it's more likely to start precipitation?

Also any suggestions on the time Interval between adding Part A and then adding Part B?

Thanks

Thanks. The sticky at the top of the chemistry forum lists all of my articles. No book, Sorry. Try putting links that don't work into the web archive. The magazines periodically change the URL, messing up my links.

The precipitation of calcium carbonate has nothing to do with it being used up. I think it is most likely on fresh GFO.