Phospate Removal - Odd and Cheap Discovery?

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smallpooldad

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Aug 6, 2008
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Honolulu
Warning to readers: Do not do this until Chem Geek has commented.

This one is definitely for Chem Geek,

Chem Geek as you are aware I just tried the "Hydrogen Peroxide Method" (see other post- "Used Hydrogen Peroxide for Removing Metal Stains - Results" adding 304 ozs of the 3% solution to my pool.

As you also know I add an HEDP sequestrant to the pool, Jack's Magic Purple Stuff, which contains phosphates. Many months ago I took a diluted phosphate reading and I stopped after 20,000 ppb. The phosphates as so often mentioned on this site are not an issue so long as the Free Chlorine and CyA are kept in check. Still a little annoying to know they are there. My guess is that they were in excess of 40,00 ppb when I started this method.

Being a little concerned that Hydrogen Peroxide and phosphates together might create an issue or problem in my pool I "Googled" this and found the following article at the "Journal of Hazardous Materials", titled "Removal of phosphate from secondary effluent with Fe2+ enhanced by H2O2 at nature pH/neutral pH":

http://219.217.227.105:8000/images/...+_enhanced_by_H2O2_at_nature_pHneutral_pH.pdf

If I read the article correctly too much Hydrogen Peroxide is not of benefit and can be detrimental to the process, but the presence of iron in the process is beneficial, but what would be the ideal quantity per 10,000 gals of 3% and 27.5% Hydrogen Peroxide solution.

Today, 6 days after the "Hydrogen Peroxide Method" and after adding 6 ozs of the Purple Stuff this morning I got my trusty Phosphate testing kit out and it seems that the reading is now 2,500 ppb. This with adding just 304 ozs of Hydrogen Peroxide in my SWG 10,000 gal pool, 6 or so days ago, a possible drop of around 37,000 ppm.

Very Important Warning: When you add Hydrogen Peroxide you remove all your chlorine and should only be done at night so there is no chance of an algae bloom see other post mentioned above for further details. You must add back chlorine before sunrise to get to your maintenance level in my case 5 or 6, 2 x 184 oz jugs of Clorox Non-Scented and acid to rebalance your pH

It seems from the article that the ideal pH to do this at is 7.2, which is where I started "Used Hydrogen Peroxide for Removing Metal Stains - Results".

This seems like a very inexpensive and less bothersome way of removing phosphates, with added benefit of removing metal stains. There was very little stuff in the filter, when I backwashed after 4 days, so where did the phosphates go to? Chem Geek please help.
 
Update and Correction:

I just doubled and tripled check my results and the result is a Phosphate level of around 10,000 ppb.

Not 2,500 ppb.

My apologies I only tested once this morning. It is still a great improvement over what I had but not quite as dramatic.

If I were to treat the pool again the same way I did before I might get it down to a lower level. If the time is available in the next few days, it might be done and results posted.

Chem Geek if you have time it would be great to know the optimal amount for a 10,000 gallon pool based on that paper. I come up with about 440 ozs based on a 3% solution, after the initial 126 ozs to neutralize the chlorine. This should reduce according to the paper the 20,000 ppmbby around 87% and the final result should be a measurement of around 2,600 ppb, if I did it one more time the number would be 338 ppb.

So if one had a 27.5% solution, the numbers would be divided by 9, therefore 126 ozs plus 440 ozs eguals 566 ozs divided by 9 gives 63 ozs of 27.5% solution to achieve a 87% reduction in phosphates.

Adding too much at once is ineffective as you mentioned in your response on the iron and is also mentioned in their paper regarding the phosphate.

However as this paper is way above my understanding I think it best to wait on your reply as to initial pH setting and amount of ounces of Hydrogen Peroxide required. It may also be that you say forget it as it will not make much or any difference, and that is OK as well, and that my phosphate number was 10,000 at the start and is still 10,000, although I am pretty sure they were not.

Thank you.
 
The paper is saying that phosphate can precipitate with both ferric iron (Fe3+) and iron oxides-hydroxides (i.e. rust). The study examined the combination of ferrous iron (Fe2+) with hydrogen peroxide (H2O2) for phosphate removal. They used phosphate concentrations of 2.52 mg Phosphorous per liter which is equivalent to 7.73 mg phosphate per liter or 7730 ppb phosphate. In their experiment, H2O2 concentrations up to around 5 ppm improved phosphate removal but higher concentrations did not though this is inconsistent with the optimal molar ratio they report later on. The mechanism appears to be where the hydrogen peroxide oxidizes the ferrous iron to ferric iron that then precipitates with phosphate. A molar ratio of Fe/P of 2.2 removed more than 96% of the phosphate so an excess of iron over phosphate is needed for this approach.

Note that if you look at Figure 4 you will see that the phosphate removal has to do with the presence of ferric iron, not hydrogen peroxide, and that the latter only slightly improves initial precipitation. The hydrogen peroxide just oxidizes ferrous to ferric. In the case of a pool with metal stains, the hydrogen peroxide is acting as both a reducing agent and an oxidizer so it first reduces the iron oxide-hydroxide (rust) stain to form ferrous ion in water and then oxidizes it to ferric ion where if phosphate is present it forms a precipitate thus removing both the iron and the phosphate from the water, assuming such precipitate is physically removed such as with coagulation/filtration.

As for quantities of hydrogen peroxide, it really depends on the amount of iron since you want enough to be able to oxidize it all to ferric form. The difficulty is that you want the resulting ferric ion to precipitate with phosphate and not to re-stain the plaster. Also, it's not clear how low one would be able to get both their iron and phosphate concentrations using this technique. The solubility product for iron phosphate dihydrate is 9.91x10-16 and for iron phosphate is 1.3x10-22. Using the former, at pH 7.5 this is an iron and phosphate concentration product of 711 ppm iron times ppb phosphate. The 2.2 molar excess they described of iron to phosphate would be for 1000 ppb phosphate an iron concentration of 1.3 ppm iron. From the iron phosphate dihydrate solubility this implies only around 50% or so removal, not 96%, so they must be presuming iron phosphate precipitation instead. If I use that number, then that is an iron and phosphate concentration product (at pH 7.5) of 0.09 ppb (not ppm) iron times ppb phosphate so much greater removal of both.

So the bottom line is that if you want to remove phosphate from the water, then one way to do that is with ferric ions. This doesn't require hydrogen peroxide, but if you've already got metal stains (i.e. rust), then the hydrogen peroxide can reduce them to ferrous ions that are water soluble, then oxidize them back to ferric ions that can then precipitate with phosphate. One gets a removal of both iron and phosphate as a result. Note however that what you are measuring as phosphate in the phosphate test may not be orthophosphate and you might be getting interference from the organic phosphate (HEDP) in the metal sequestrant. The precipitation of iron phosphate uses orthophosphate. The organic phosphate (HEDP) does not precipitate the iron but rather holds it in solution. So the "treatment" using this approach would be to not use metal sequestrant and instead to intentionally add orthophosphate (think fertilizer with just phosphorous and no nitrogen or potassium) unless you already had some to begin with.

Interestingly, when chlorine is used, then iron becomes ferric ion so in theory should have precipitated with the phosphate, but perhaps there is a greater tendency of chlorine oxidation of iron to form iron oxides-hydroxides as pool wall stains whereas hydrogen peroxide inhibits that because it is both a reducing agent as well as oxidizer so any staining gets undone. All quite complicated chemistry since a lot of things are going on at once.
 
chem geek,

Thank you the reply.

I must admit to be somewhat confused but I think I get the general gist. That being this might work (big might) but at 50% not 96% reduction in phosphates, and that hydrogen peroxide at 5 ppm would be the optimum amount to add, what would that be if one is using 3% hydrogen peroxide in fluid ounces?

When I do this again I will measure the phosphates prior to putting in the 3% solution, and then measure after to see if there is any change and will report back at a later date.

One last question, if one does and AA or H2O2 method the iron is held by the sequestrant in solution, if this is done say every 4 to 6 months, does the level of sequestrant need to increased say from 30 to 40, or from 40 to 50 as it is holding more iron?

Thank you.
 
Hydrogen peroxide at 3% by weight is 30 grams per liter since the density is nearly that of water in such a dilute solution. To get 5 milligrams per liter in 10,000 gallons (37,854 liters) requires 5*37854 = 189270 mg or 189.27 grams so that would be 189.27/30 = 6.3 liters (6.65 quarts) of 3% hydrogen peroxide solution. For comparison, this amount of hydrogen peroxide would neutralize around 10 ppm FC so it's more than you have been adding but not by a huge amount.

As for needing more sequestrant as you get more iron, the answer would be yes except that hopefully you are removing some of that iron with water dilution. You add more sequestrant, but that's just to get back to the same concentration where you started. At some point, you should get to a steady-state where the amount of iron getting added to your pool and held by sequestrant equals the amount removed by water dilution. You would then be adding enough sequestrant to make up for that lost by water dilution and by breakdown from chlorine -- that is, you'd be maintaining your sequestrant concentration.
 
chem geek,

Thank you for the numbers that was very clear and yes I see your point about equilibrium with the sequestrant which I think is higher than 40 ppm, possibly 50 or 60.

When I do the "Hydrogen Peroxide Treatment" again, it might be some time hopefully, I will post the results as to whether or not it lowers phosphates. In the meanwhile someone else who does this, and has a phosphate and sequestrant test kits, may be able to supply an answer sooner than me.

No more questions. Your constant replies have made this very clear and thanks to you and your many posts me and to others the pool looks very good.
 
Chem Geek,

As you know I did another Hydrogen Peroxide method with the AA method for iron. Yesterday I tested for Phosphates and the level was still 10,000 ppb, the same level as it was after the first treatment, although it had dropped when I first did it from what I thought was in excess of 20,000.

I had added about 40 ppm of the Purple Stuff some days before doing the aforementioned treatments to up the ppm level to around 100, after the treatment the sequestrant level had fallen to around 70 -75.

I asked Jack's Magic how much phosphate was added if 1 ppm of the Purple Stuff is added, they stated 2 ppm. Does this seem correct to you? If so did I actually remove any phosphates bearing in mind that I added 40 ppm of the Purple Stuff and the test kit measures phosphates in ppb?

I promised to follow up if I added HP again and this therefore is the follow up.

Thank you.
 
HEDP (C2H8O7P2) is 206.3 g/mole and contains two phosphorous that after HEDP breaks down will form 2 phosphate (PO43-) groups where each is 94.9714 g/mole so though one mole of HEDP will yield 2 moles of phosphate, 1 ppm of HEDP will yield 2*94.9714/206.3 = 0.92 ppm phosphate. So it's much closer to 1-to-1 where 1 ppm HEDP yields 920 ppb so close to 1000 ppb.
 
chem geek,

Thank you for the reply.

It seems, if I understand you correctly and I might not, that when the sequestrant level was raised by 40 ppm from 60 to 100 ppm, the phosphate reading should have now read the initial 10,000 ppb plus 920 ppb times 40 (36,800), or 46,800. The actual reading came in at 10,000 after the Hydogen Peroxide treatment, so the HP treatment seems to have reduced the phosphate level by approximately 36,800. To re-verify this result the next time I do an HP treatment for calcium staining I will check the phosphate reading and report back as to whether or not it had and effect on the phosphate level.

This might happen quite soon as I am intrigued.

Thank you.
 
Well, this assumes that the phosphate test is measuring all the phosphate in HEDP. The real way for you to tell is to measure the phosphate level before you add hydrogen peroxide and again after you've gotten rid of the hydrogen peroxide by adding chlorine again, but before you add any more HEDP.
 

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chem geek,

Last night checked phosphate level it measured about 10,000 ppb as before, no sequestrant had been added. Then added 336 ozs of Hydrogen Peroxide, after lowering pH to 7.0. Ran pump all night, this morning phosphates measured about 2,500 . I do think the quantity of phosphate removed is pH dependant, the lower the pH the more is removed.

Please also see updated post on "Used Hydrogen Peroxide for Removing Metal Stains - Results", for additional information here:

http://www.troublefreepool.com/used-hydrogen-peroxide-for-removing-metal-stains-results-t56485.html

Thank you.
 
Well, that's fascinating. The only mechanism we know of is what I wrote earlier in this thread where phosphate precipitates with ferric iron and with iron oxides-hydroxides. So the only purpose of the hydrogen peroxide is to dislodge or breakup (via reduction) some of the iron that might be found in stains or in precipitate elsewhere such as in the filter. Then this iron can then get oxidized and re-precipitate with phosphate.

So it would be interesting to see if water with high phosphate has this get lowered at all if one adds hydrogen peroxide and doesn't have any iron in the water. Obviously, that's not something you could test in your pool since you've got so much iron in it (including iron oxides-hydroxides as well as iron bound to HEDP).
 
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