Regeneration of GFO question

BradB · Mar 8, 2016

I have read the steps at:
http://www.advancedaquarist.com/2010/10/chemistry

What I don't understand is, doesn't some of the phosphate rebind to the GFO during the rinse process? Suppose I rinse away 99% of the sodium hydroxide and phosphate solution. Now I have a solution with 1% of my sodium hydroxide and 1% of phosphate. Is 1% of my sodium hydroxide enough to do anything to prevent the GFO from binding the phosphate or free bound phosphate?

I used 1% as an example. If 10% of the sodium hydroxide was not enough to have an unbinding effect, then 10% of my phosphate would rebind, which is significant. Whatever threshold actually fits, wouldn't it be better to do multiple cycles (add sodium hydroxide, rinse with RO water, add sodium hydroxide, rinse, repeat)?

Randy Holmes-Farley · Mar 8, 2016

I think the simple answer is that it works, but it doesn't remove every phosphate ion.

Multiple cycles might be better, as it is with any sort of ion exchange mechanism, but is it worth it to get slightly more capacity?

I would say that the last point isn't correct.

"I used 1% as an example. If 10% of the sodium hydroxide was not enough to have an unbinding effect, then 10% of my phosphate would rebind, which is significant. "
You are not "just" rinsing with RO water (or at least should not be). You are removing most of the high pH water and phosphate, then rinsing. If the water contains 95% of the phosphate from the GFO, and you remove 95% of this fluid, you have discarded 90% of the phosphate before you even begin to add any rinse water.

I'd also point out that DI water at any pH will remove the phosphate given adequate volumes. The high pH just makes it more efficient.

BradB · Mar 8, 2016

Randy Holmes-Farley said:
I think the simple answer is that it works, but it doesn't remove every phosphate ion.

Multiple cycles might be better, as it is with any sort of ion exchange mechanism, but is it worth it to get slightly more capacity?

If I use half as much sodium hydroxide, remove most of the high pH water, and more water with the other half of the phosphate, that takes an extra 10 minutes in a multi-day process. If it gets me more slightly more capacity, it is worth it. Also, I wonder if I can do better with two 500 gram sodium hydroxide rinses than a single 1500 gram rinse.

What exactly causes the phosphate to unbind? Is it just high pH water? Because a very very small amount of hydroxide (a small fraction of what the article recommends) would do that.

Randy Holmes-Farley · Mar 9, 2016

Using the same total amount of hydroxide split into two different wash steps may work better (should work better), but I would not assume that less hydroxide overall will work just as well. If the effect comes from hydroxide and phosphate competing for binding sites on the iron, more hydroxide means less phosphate will bind. Less might be needed, but I presume folks would have tried using less.

While the situation is not identical (its not seawater, which is important), here's a study of phosphate binding as a function of pH to a material like GFO, which may give a sense of the pH effect on binding as well as why it changes.

see page 38, Figure 11.
http://www.switchurbanwater.eu/outputs/pdfs/w3-2_gen_phd_d3.2.2_phd_thesis_-_sperlich.pdf