Why does GFO decrease the alkalinity ?

[Charles Silva]

Hello all

Why does GFO decrease the alkalinity ?
Does the reaction produce H+ , reduces the pH and drop alk ?

Thanks in Advance
Charles Silva
Rio de Janeiro - Brazil

 

[Dan_P]

Hello all

Why does GFO decrease the alkalinity ?
Does the reaction produce H+ , reduces the pH and drop alk ?

Thanks in Advance
Charles Silva
Rio de Janeiro - Brazil

Precipitation of calcium carbonate.

 

[LC8Sumi]

Watch this:

 

[Randy Holmes-Farley]

That can happen on or downstream of the GFO. The iron can induce precipitation.

 

[Undertaker]

That can happen on or downstream of the GFO. The iron can induce precipitation.

I use two table spoons and change it when my phosphates start raising. So would changing it out early stop the precipitation?

 

[Daniel@R2R]

I actually didn't know this about GFO

 

[Randy Holmes-Farley]

I use two table spoons and change it when my phosphates start raising. So would changing it out early stop the precipitation?

I don’t think so.

 

[Randy Holmes-Farley]

I discuss it here

http://reefkeeping.com/issues/2004-11/rhf/index.htm

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.

 

[RevMH]

I discuss it here

http://reefkeeping.com/issues/2004-11/rhf/index.htm

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.

Wow, thank you!

 

[Daniel@R2R]

Randy always has the answer.

 

[Nick30G]

I have experienced this sometimes as well. But can I get Randy's answer dumbed down for me? LOL

 

[Randy Holmes-Farley]

I have experienced this sometimes as well. But can I get Randy's answer dumbed down for me? LOL

Lol. Sure

The chemicals in GFO induce calcium to combine with carbonate to form solid calcium carbonate, which reduces alk and calcium in the water. The solid material That forms attaches to the GFO, and to surfaces just downstream of it.

 

[SudzFD]

Watch this:

Thank you for posting this. It was extremely helpful!

 

[NowGlazeIT]

I actually didn't know this about GFO

I think many don’t. I was lucky enough to stumble upon one of Randy’s responses About GFO and ALK just a month ago. Prior to that I never heard about it. So many reefers well monitoring their tanks nowadays!

 

[ReeferGuyAlexK]

Lol. Sure

The chemicals in GFO induce calcium to combine with carbonate to form solid calcium carbonate, which reduces alk and calcium in the water. The solid material That forms attaches to the GFO, and to surfaces just downstream of it.

Does it do this constantly or will the GFO stop reducing the kh after a week of use or so?

 

[Randy Holmes-Farley]

Does it do this constantly or will the GFO stop reducing the kh after a week of use or so?

Are you noticing a big effect? Usually the effect is hardly noticeable.

I expect it happens fastest when initially added, and then eventually the GFO surfaces can get covered in things that reduce iron release and CaCO3 precipitation.

 

[ReeferGuyAlexK]

Are you noticing a big effect? Usually the effect is hardly noticeable.

I expect it happens fastest when initially added, and then eventually the GFO surfaces can get covered in things that reduce iron release and CaCO3 precipitation.

I did add GFO a week or so back but took it out that evening due to a lot of reefers advising me not too use recommended dosage especially since my tank has been running perfectly good for a year and sps growing but i honestly believe i have a little too much phosphates. So when i added it and removed it the evening i tested my kh and it dropped quite a lot. I had to up my daily dosing to get it back to where i normally run my kh (7.7-8) after returned to normal daily dosage and kh didnt drop again that low.

 

[Oregon Grown Reef]

Are you noticing a big effect? Usually the effect is hardly noticeable.

This has been my experience as well. Someone told me to test my alkalinity after adding gfo and it dropped around 0.2-0.3. I then tested nitrates and found they had risen a bit since I had done my last alk and nitrate test, so a decrease in alk is likely explained there moreso than from gfo. It hasn't depreciated any noticeable amount when nitrates were kept stable.

 

[ReeferGuyAlexK]

I did add GFO a week or so back but took it out that evening due to a lot of reefers advising me not too use recommended dosage especially since my tank has been running perfectly good for a year and sps growing but i honestly believe i have a little too much phosphates. So when i added it and removed it the evening i tested my kh and it dropped quite a lot. I had to up my daily dosing to get it back to where i normally run my kh (7.7-8) after returned to normal daily dosage and kh didnt drop again that low.

It dropped more than 0.8dkh even after my completed daily dose, which is way more than kh consumption for sure.

I want to add some GFO again but like 1/4 dose and slowly work my way into it, but dont want something constantly consuming my kh that ill have to dose a lot more than i actually need to.

 

[Oregon Grown Reef]

It dropped more than 0.8dkh even after my completed daily dose, which is way more than kh consumption for sure.

I want to add some GFO again but like 1/4 dose and slowly work my way into it, but dont want something constantly consuming my kh that ill have to dose a lot more than i actually need to.

0.8 dKh is a large drop. Did you test more than once? What alk test were you using?