Does reducing phosphate reduce silica?

[Pete Hammersley]

I’ve heard a few youtubers (not always the most reliable source of info) say in a flash that GFO reduces silica, but with no explanation how? Is this true? And if so, I’d love to understand it better if someone could please explaine? I don’t like to just have the answer given to me, I need to get my head around the how and the why.

I have what is some form of Diatoms and wanted to reduce the problem.

Thanks in advance.

 

[reefluvrr]

I do not know if GFO removes silicates, but I have used Phosguard and it removes silicates.
I have tested my water with Triton. Sure enough silicates were reduced. Be aware though, aluminum levels was riased though. I did not see any adverse effect in my tank though. HTH

 

[Randy Holmes-Farley Verified Community Expert]

Yes, GFO can bind silicate. Silicate binds to the iron surface forming iron silicate structures, just like phosphate forms iron phosphate.

https://pubs.acs.org/doi/10.1021/es010996t
Modeling Silica Sorption to Iron Hydroxide

Abstract
Experiments were conducted to investigate the fundamentals of silica sorption onto preformed ferric hydroxide at pH 5.0−9.5 and silica concentrations of 0−200 mg/L as SiO2. At all pHs studied, sorption densities exceeding monolayer sorption were observed at silica levels typical of natural waters. Under some circumstances, sorption equaled or exceeded a monolayer while the particle zeta potential remained positive, a phenomenon that is completely inconsistent with available surface complexation models. To address this deficiency, an extended surface complexation model was formulated in which soluble dimeric silica (i.e., Si2O2(OH)5-) sorbs directly to iron surface sites. This new model fit sorption density data up to 0.40 mol SiO2/mol Fe and accurately predicted trends in zeta potential and observed H+ release during silica sorption to ferric hydroxide.

 

[Miami Reef]

Yes, GFO can bind silicate. Silicate binds to the iron surface forming iron silicate structures, just like phosphate forms iron phosphate.

https://pubs.acs.org/doi/10.1021/es010996t
Modeling Silica Sorption to Iron Hydroxide

Abstract
Experiments were conducted to investigate the fundamentals of silica sorption onto preformed ferric hydroxide at pH 5.0−9.5 and silica concentrations of 0−200 mg/L as SiO2. At all pHs studied, sorption densities exceeding monolayer sorption were observed at silica levels typical of natural waters. Under some circumstances, sorption equaled or exceeded a monolayer while the particle zeta potential remained positive, a phenomenon that is completely inconsistent with available surface complexation models. To address this deficiency, an extended surface complexation model was formulated in which soluble dimeric silica (i.e., Si2O2(OH)5-) sorbs directly to iron surface sites. This new model fit sorption density data up to 0.40 mol SiO2/mol Fe and accurately predicted trends in zeta potential and observed H+ release during silica sorption to ferric hydroxide.

How can I know if GFO has a higher affinity toward phosphate over silicate? I dose silicate, but I don’t know if the GFO will just remove silica and leave PO₄ in the water.

Is there a test of some sort one can do?

 

[Randy Holmes-Farley Verified Community Expert]

How can I know if GFO has a higher affinity toward phosphate over silicate? I dose silicate, but I don’t know if the GFO will just remove silica and leave PO₄ in the water.

Is there a test of some sort one can do?

You can put GFO in a large water volume containing 1 ppm silica, let it stabilize, then see if it can still drop phosphate added to the water. Or you can add them both at the same time to seawater plus GFO.

 

[Miami Reef]

You can put GFO in a large water volume containing 1 ppm silica, let it stabilize, then see if it can still drop phosphate added to the water. Or you can add them both at the same time to seawater plus GFO.

I like the idea of adding both phosphate and silicate into the water. It will simulate what happens in my reef tank by determining which ion has more affinity to GFO.

I think presaturating GFO with silica prior would mainly check if phosphate can displace silicate. Do you think that line of reasoning is somewhat accurate or does displacement and affinity occur simultaneously to GFO?

How would you apply GFO to the water? In an old test, I just put GFO into a larger beaker with a magnetic stirrer. If I remember correctly, the water had a slight tea color from the GFO granules. I have a 0.22 um syringe filter, but how would you do it?

How big of a volume were you thinking? Can a 2,000 mL beaker work or should I use my 5 gallon buckets? I just want accurate results. I don’t care about the extra work.

 

[Randy Holmes-Farley Verified Community Expert]

An “easy” experiment would be to take two buckets, a way to gently mix each (say, an airstone), spread a fixed amount of GFO on the bottom of each (say, 0.5 g), then add phosphate to one and phosphate plus silicate to the other. Measure phosphate in each after 48 h or longer.

 

[Randy Holmes-Farley Verified Community Expert]

You could use new salt water or old tank water from a water change.

 

[Miami Reef]

An “easy” experiment would be to take two buckets, a way to gently mix each (say, an airstone), spread a fixed amount of GFO on the bottom of each (say, 0.5 g), then add phosphate to one and phosphate plus silicate to the other. Measure phosphate in each after 48 h or longer.

Wow. This is excellent! I didn’t think it could be this simple. It would be very informative for me. I’ll let you know the results if/when I do it.

 

[Miami Reef]

An “easy” experiment would be to take two buckets, a way to gently mix each (say, an airstone), spread a fixed amount of GFO on the bottom of each (say, 0.5 g), then add phosphate to one and phosphate plus silicate to the other. Measure phosphate in each after 48 h or longer.

I have two 5 L jugs. One has 2 ppm silcate. They both have 0.25 ppm phosphate. I spiked them equally and tested them separately.

Now is the part where I add GFO. It needs to be a balancing act because if I add too much GFO to drop both solutions to 0.00 ppm PO₄, it won’t be a useful test.

How much GFO would you add to each? Would you rinse the GFO prior?

 

[Dan_P]

I have two 5 L jugs. One has 2 ppm silcate. They both have 0.25 ppm phosphate. I spiked them equally and tested them separately.

Now is the part where I add GFO. It needs to be a balancing act because if I add too much GFO to drop both solutions to 0.00 ppm PO₄, it won’t be a useful test.

How much GFO would you add to each? Would you rinse the GFO prior?

Doing the study in two separate vessels won’t tell you whether PO4 will displace silicate or vice versa. i would observe GFO binding of several mixtures of both species. Or for more fun, bind PO4 to GFO and then see how large a silicate concentration is needed to knock od PO4 from GFO. Do the reverse for even more fun.

 

[Miami Reef]

I’m keeping it simple.

2 vessels:
1 with 2 ppm silica and 0.25 ppm phosphate
1 with 0.25 ppm phosphate

I added 0.6 g of pre rinsed GFO. I want to know if the presence of silica decreases the effectiveness of GFO binding phosphate.

 

[Randy Holmes-Farley Verified Community Expert]

I have two 5 L jugs. One has 2 ppm silcate. They both have 0.25 ppm phosphate. I spiked them equally and tested them separately.

Now is the part where I add GFO. It needs to be a balancing act because if I add too much GFO to drop both solutions to 0.00 ppm PO₄, it won’t be a useful test.

How much GFO would you add to each? Would you rinse the GFO prior?

Yes, like the three bears and the porridge, but I’m not sure what amount is best. Start with less than you think because you can always add more.

 

[Miami Reef]

I had to add 0.5 tsp to each because the phosphate in the control didn’t budge after half a day of aerating and occasionally stirring the GFO. It wasn’t enough. Phosphate binding to GFO should be a relatively quick process, with at least some measurable effect after several hours, especially since there aren’t any calcareous substrates.

 

[Randy Holmes-Farley Verified Community Expert]

I had to add 0.5 tsp to each because the phosphate in the control didn’t budge after half a day of aerating and occasionally stirring the GFO. It wasn’t enough. Phosphate binding to GFO should be a relatively quick process, with at least some measurable effect after several hours, especially since there aren’t any calcareous substrates.

Yes, I agree.

 

[Miami Reef]

Okay, okay. Not bad stats!

Control: 0.07 ppm
Silicate: 0.11 ppm

I was expecting the silicate container to be much, much higher. I now added even more GFO to both. I want to see what happens when pushing for <0.03 ppm. Will silicate start displacing phosphate as we add more GFO and go lower?

 

[Randy Holmes-Farley Verified Community Expert]

Nice work. :)

 

[Miami Reef]

Results are in!

I added more GFO and let them both sit for a day. Both tested at exactly 0.01 ppm. Exactly the same!

My conclusion: GFO prefers phosphate. Even high doses of silicate (2 ppm) didn’t seem to decrease the GFO’s effectiveness.

I’ve wanted to know this answer for so long! I’m very happy, and this works perfectly in my favor because I like to add both GFO and silicate to my tank.

 

[Randy Holmes-Farley Verified Community Expert]

Nice.

If you still have the containers, you might add more phosphate to see if any competition kicks in as more phosphate is trying to get onto the gfo. :)