Conditions for GFO to release PO4

[tonyespinoza]

What are the conditions which regulate GFO in phosphate release? (I understand it to be a two way street and obviously want to keeping going in the right direction.)

 

[lapin]

Most GFO will not release Po4 back into the water in our fish tanks. What happens it absorbs all it can then no more.

 

[tonyespinoza]

@lapin Thanks - that's what I had thought before I read this quote from @Randy Holmes-Farley:

GFO and aluminum oxide and calcium carbonate are all reversible . [emoji3]

For more context see this thread, Phosphate Absorption Rates in Aragonite, with this question posed by @jda.

 

[jda]

GFO will release phosphate when the water concentration is lower than it was when it bound the phosphate. A good example of this is that the GFO bound up a bunch of phosphate to equilibrium with the tank water... then there was a water change and the tank water concentration was lower... this will cause the GFO to release some phosphate back into the water.

The amount of P that GFO will bind is dependent on the concentration in the water. It binds exponentially more when the tank water level is higher. This is also true with Al Oxide and Aragonite/Dolomite/Calcite.

 

[tonyespinoza]

GFO will release phosphate when the water concentration is lower than it was when it bound the phosphate. A good example of this is that the GFO bound up a bunch of phosphate to equilibrium with the tank water... then there was a water change and the tank water concentration was lower... this will cause the GFO to release some phosphate back into the water.

The amount of P that GFO will bind is dependent on the concentration in the water. It binds exponentially more when the tank water level is higher. This is also true with Al Oxide and Aragonite/Dolomite/Calcite.

Thanks, but I'm sure I'm still missing something...

So let's say tank has PO4 at level X. New GFO is added via upflow reactor and absorbs some amount of PO4. As PO4 increases over time, GFO continues to absorb until it reaches capacity.

If PO4 drops to level Y due to a water change, is does GFO release PO4 proportionate to the delta between level X and Y?

Given this balancing act, what are the best practices with GFO? Remove old GFO before water changes sounds right... although I wonder what the version is for those of us who change small amounts daily.

What else is recommended to optimize the efficacy of GFO? I've heard some folks say run it 15 min every hour so it doesn't clump. I've heard others say never let it stop.

One idea I was playing with was to have the GFO in it's own sub sump always running, but vary the flow rate to that sump... (I have a lot of space and 500g water volume). But it sounds like that approach could have the real downside of having the GFO dump PO4 if the local concentration was reduced due to lower flow from the rest of the system?

 

[jda]

Yes, always change the GFO before a water change. No idea how to handle this on a system with continuous water change... seems like you will just lose a bit.

The best way to run GFO is small amounts, changed very often. The stuff will cover in organics and not be able to make contact with tank water - this can happen in as little as a few days. I don't use much of it, but when I do, it is just a few tablespoons changed daily - I just so little that the tank water never really moves down with much of a spike. I only really use it to treat live rock that I get from other systems, but LC has been working better for me lately as long as I have no corals in the system.

 

[Randy Holmes-Farley]

The binding of phosphate to GFO is an on/off equilibrium, and the more phosphate is in the water, the more binds to the GFO surfaces.

GFO readily releases phosphate if the amount bound to it is higher than the GFO can hold at that concentration in the water.

For example, if you let GFO soak in all the phosphate it wants at 0.1 ppm phosphate, then drop the concentration in the water to, say, 0.03 ppm, some phosphate will desorb from the surface.

This is the way phosphate binding to any mineral surface or polymer material works.

 

[Randy Holmes-Farley]

Thanks, but I'm sure I'm still missing something...

So let's say tank has PO4 at level X. New GFO is added via upflow reactor and absorbs some amount of PO4. As PO4 increases over time, GFO continues to absorb until it reaches capacity.

GFO aside, phosphate doesn't drop nearly much from a water change as you might expect for this exact reason, but relating to phosphate bound to all the calcium carbonate surfaces in the aquarium.

In any case, this is not a big concern, IMO. All of these factors tend to stabilize the phosphate concentration over time, not destabilize it. They make it harder to raise and lower phosphate levels in the water, no mater what you do, which presumably can be desirable.

 

[tonyespinoza]

GFO aside, phosphate doesn't drop nearly much from a water change as you might expect for this exact reason, but relating to phosphate bound to all the calcium carbonate surfaces in the aquarium.

this is all making a lot more sense to me now. thanks for the detailed responses.

is there any way to calculate, model or monitor the capacity of our systems to absorb Phosphate? seems like sampling our water and measuring PO4 at a given moment is like focusing on pH rather than dKH... (i guess this is somewhat circular as it relates to the thread referenced above.)

 

[lapin]

@lapin Thanks - that's what I had thought before I read this quote from @Randy Holmes-Farley:

For more context see this thread, Phosphate Absorption Rates in Aragonite, with this question posed by @jda.

I never imagined that this would happen in a normal reef tank. I guess it could.
Most times when my Po4 starts to rise its time to change the GFO. I think it has absorbed all it can.
Maybe the GFO is releasing Po4 back into the water. Time to change GFO in this case also.
Thanks
Learn somethin new every day

 

[Randy Holmes-Farley]

The amount of phosphate that GFO readily releases phosphate if the amount bound to it is higher than t

I never imagined that this would happen in a normal reef tank. I guess it could.
Most times when my Po4 starts to rise its time to change the GFO. I think it has absorbed all it can.
Maybe the GFO is releasing Po4 back into the water. Time to change GFO in this case also.
Thanks
Learn somethin new every day

I would just slightly alter your statement:

"Most times when my Po4 starts to rise its time to change the GFO. I think it has absorbed all it can at the phosphate level I want to maintain."

 

[Randy Holmes-Farley]

this is all making a lot more sense to me now. thanks for the detailed responses.

is there any way to calculate, model or monitor the capacity of our systems to absorb Phosphate? seems like sampling our water and measuring PO4 at a given moment is like focusing on pH rather than dKH... (i guess this is somewhat circular as it relates to the thread referenced above.)

There's no way to readily measure bound phosphate in an operating reef tank except by how phosphate in the water changes when you add or remove it. That determination is confounded by organisms using it or releasing it at the same time. I'm also not sure what you would do with the value.

FWIW, this is not related to reef tanks only. It also happens in the ocean:

Effect of Temperature and Salinity on Phosphate Sorption on Marine Sediments

https://pubs.acs.org/doi/10.1021/es200867p

 

[tonyespinoza]

I'm also not sure what you would do with the value.

One prevalent mindset out there is "don't chase numbers" and yet there's the inverse movement around dosing and ICP, etc. Perhaps there's no going back -- it seems we will increasingly have more information about our tanks than knowledge as to how to use it. My interpretation of the "don't chase" mindset is that the information does not dictate the response in all cases. It also implies that we relate to our systems on the whole and use our ability to observe and interpret the system's well being above and beyond the numbers.

It seems easiest for most folks to initially grok reef keeping in terms of import and export (e.g., my bioload needs to correspond to filtration, i need to import calcium and bicarbonate to match consumption, i need to export nitrate, etc.) And drawing a 2 dimensional curve on the computer screen reinforces that interpretation of the first order derivative rather than the second.

But I've always liked to think of the tank as an organism onto itself (and the more we learn about our own gut biome the more it seems to match) and so designing the system and understanding it's carrying capacity for life has to happen either in concrete terms or as some "sense" we develop over time (whereby we don't have to chase numbers). From an organism mindset, I like to think of "homeostasis" (more in the bio sense than chem) as the real indication -- something closer to a sense of well-being and therefore thriving.

Accumulation of time always reveals the fragility (the truth) of our approach. We can have a good run for a few years and then face new challenges as we find out what we did or didn't do.

So back to the comment above. Knowing the instantaneous PO4 is a bit like going to the doctor and finding out that my temperature is high. It suggests an infection. But whether I should intervene and drop the temp or it would be iatrogenic to do so is always the next question we should ask.

I understand ORP is not something we can treat as a system-level "homeostatic" measure for a number of reasons. I'm sure this is just a layman's fantasy but the idea of measuring the PO4 buffering capacity of our system feels desire-able to me and I wonder if we aren't inherently building/intuiting that model through our husbandry and superstitions (wisdom?) about our aquaria all the time.

 

[Randy Holmes-Farley]

IMO, people who say don’t chase numbers are expecting people will do it incorrectly. I cannot see any problem when doing a proper chase.