Metals in the water and the use of flocculants

[SingingSailor]

Hi,
This is my first post in this wonderful forum. I am interested in pool chemistry, and I have learned a lot from all the experts on this forum.
I live in Sweden, and the common pool advice in Sweden sometimes differs a bit from your advice here. I have a few questions related to this, here is my first. I hope you have the time to answer (I know you have the knowledge!).

In Sweden, the advice for metals in the water is to oxidize through superchlorination, and then filter away the metals, maybe using a flocculant to improver the filtration.
See for example this page, about discoloured water:
http://www.pahlen.com/eng/poolguiden.php?page=problem

Flocculants are recommended to use to precipitate metals and clump them together to bee able to filter them out.
See for example this link:
http://www.folkpool.se/pool-produkter/p ... kemi-ovrig (Swedish!)
The flocculant here is based on aluminumsulphate.

Intex has a similar advice in a pool manual:
Colored water:
(Description): Water turns blue, brown, or black when first treated with chlorine.
(Cause): Copper, iron or manganese in water being oxidized by the added chlorine. Common.
(Remedy): Adjust pH to recommended level. Run filter until water is clear. Clean cartridge frequently.

In your pool school, you state “there is no practical way to remove metals from the water short of replacing the water…” category/pool-school/metal%20stains

Here’s another site, claiming that staining can result from low pH, low alkalinity and low calcium hardness:
http://www.havuz.org/pool-problems/pool-staining.htm
I agree that this can maybe cause metal corrosion, but isn’t the opposite that would cause staining?

Here´s my questions:
1. Can you or can you not remove metals from the water by chlorination, flocculants and filtration? (Are we maybe talking about metals in different forms?)
2. What are your experience on using flocculants? Are they (only) effective on metals?
3. What chemical reactions are taking place regarding metals in the water?

I don´t know if "the deep end" is the right place for this post, feel free to move it if necessary.

Regards,
SingingSailor

 

[JasonLion]

The Deep End is a wonderful place for this post

There have been a couple of cases where metal levels appeared to fall but we have yet to come upon a procedure that will remove metals repeatably for everyone who tries. So long as the metals are dissolved in the water you can not remove them. Raising PH and adding chlorine will both tend to cause the metals to come out of solution, however this nearly always results in the metals being deposited on the surface of the pool, rather than forming particles that could be filtered out.

Flocculants are occasionally useful, causing particle to clump and fall to the bottom of the pool where they can be vacuumed up. This is hardly ever required, since the filter will do the same thing though more slowly. Flocculants have no particular interaction with metals. If you can get the metals to form particles suspended in the water you can remove them, either with flocculants or otherwise. The trick is to get the metals to form particles instead of depositing on the pool surfaces.

 

[SingingSailor]

I assemble and fill my pool with tap water in spring. At first, the water is a bit browny. This clears up the first week, and a brown dust falls to the bottom that I vacuum up to waste. The brown dust continues to form for a few weeks, less and less as time passes. The guy from the water plant told me this was because of iron that is released from the pipes because I have a much higher flow rate than usual when I fill the pool. This seems reasonable, and consistent with the links I posted above, i.e. chlorination makes the iron precipitate and fall to the bottom. I don´t have any problems with stains.

On the other hand, here att TFP you say that metals that precipitate almost always deposit on the surface of the pool instead of forming particles.

Are we talking about completely different chemical processes here, or why do we have different experiences?

 

[JasonLion]

It has a lot to do with the pool surface. Metals don't like to deposit on vinyl, metals are very likely to deposit on fiberglass, and plaster is in between. What you describe can happen in a vinyl pool, but would never happen in a fiberglass pool.

 

[solarboy]

Sorry to bump an old thread but I use the Swedish technique all the time very successfully and as long as the flocculated oxides don't hang around in the pool too long it doesn't seem to cause staining (in glass tiles pools anyway).
The main tricks are to overfill the pool so when you vacuum to waste you do not need to add any more water (and thus metals). Another trick is to maintain the pool at shock levels (using dichlor to add CYA for later on) until you have no overnight FC loss (indicating that all the metals have been oxidised) and then turning the pumps off for 36-48 hours I usually set up the cleaning equipment before I let it settle so as not to disturb the sediment later on.
It usually takes 2 session like this to fully clean the oxides out. I then backwash and add DE to the filter to get any residuals and leave the pumps running 27/7 for a week or so during which time I add CYA and let the FC drop down to normal levels.
I've never seen metal sequestering agents for sale here so this is the only method as we have no RO service here either, and to be honest I'd rather just not have the metals in the water to appear later.
Another tip is to not use the pool auto fill at first as the water running down the wall will stain, so use a hosepipe and move it around a bit (as well as the jet eyeball direction) at least until the water level is above the jets/autofill inlet.

 

[teapot]

The danger of superclorination is that it raises the Ph to 8+ and certainly in the case of copper it will drop out of suspension at Ph 8.3 and above.
As Jason said staining doesn't seem a problem with vinyl pool, probably because the vinyl contains heavy metals from it's manufacture and metals have a + charge so will repel other metals. In Europe vinyl liners are no longer manufacured with heavy metals in them so whether staining could become an issue in the future we will have to wait and see.

When people mention coagulants and flocs it's the usual pool store stuff for removing fine organics missed by the filter, these are generally cationic so have a positive charge that causes negative particles to be attracted to them forming larger flocs that can be removed. Anionic on the other hand have a negative charge that attracts positive particles (metals) so causing flocs that can also be removed by filtration. Anionic flocs are lighter than Cationic (which sink to the bottom) so the skimmer should be able to catch them.

Well that's the theory.

 

[carlscan26]

What heavy metals are we talking about with vinyl pools?

 

[JasonLion]

Copper and iron mostly. Zinc, silver, magnesium, and a few others can also be issues sometimes.

 

[chem geek]

When people mention coagulants and flocs it's the usual pool store stuff for removing fine organics missed by the filter, these are generally cationic so have a positive charge that causes negative particles to be attracted to them forming larger flocs that can be removed. Anionic on the other hand have a negative charge that attracts positive particles (metals) so causing flocs that can also be removed by filtration. Anionic flocs are lighter than Cationic (which sink to the bottom) so the skimmer should be able to catch them.

Actually, when one is talking about using coagulants, clarifiers or flocculants to remove metals from the water, it's about removing metal oxides and related suspended precipitates, not metal ions themselves. These precipitates are neutral in charge, but are generally polar molecules so some of the better clarifiers and flocculants can sweep them up so can in theory remove such metals. Of course, raising the pH enough to precipitate such metals also risks staining them on pool surfaces as others have noted.

It is true, however, that using an anionic floc can attract metal ions themselves so would be more useful at removing metals without the risk of staining that comes from higher pH to produce precipitates.

Another approach to removing metal ions might be use of the CuLator product, though it is quite expensive. I've got a bag in my pool skimmer right now so we'll see if it takes care of the yellowing of plaster I get when the pH rises. I don't measure any significant metal (<= 0.2 ppm) in the metal tests (iron, copper) so it might not work, but we'll see.

Then there's Ben Powell's technique of pouring a pH Up or Borax product into the skimmer to produce a cloud of calcium carbonate that gets caught in the filter and has the higher pH in the pool capture metal oxide precipitate. The idea is to raise the pH enough to have the metals caught in the filter, but not so much as to stain. Tricky, at best.

 

[teapot]

What heavy metals are we talking about with vinyl pools?

In the vinyl itself, cadmium and zinc were the most common but manufacturing processes around the world would mean differences.

We are talking very small amounts though.

 

[solarboy]

With the amount of iron and manganese in the fill water here, I only have to raise FC to shock levels while leaving pH in the normal ranges and the metals still oxidise out. The floc we use here is Aluminium PolychloroSulphate and this does cause the precipitate to sink to the bottom.

 

[chem geek]

Raising the FC to shock levels using any hypochorite source of chlorine will also raise the pH upon addition which is the primary cause of precipitating the metals. Having the metals oxidize (say from ferrous ion to ferric ion) happens even at regular chlorine levels.

 

[solarboy]

Raising the FC to shock levels using any hypochorite source of chlorine will also raise the pH upon addition which is the primary cause of precipitating the metals. Having the metals oxidize (say from ferrous ion to ferric ion) happens even at regular chlorine levels.

I normally use dichlor to get a nice dose of CYA in the water early on. I'm ignorant enough to not know if dichlor is hypochlorite or not. I have noticed this happening at regular chlorine levels too. I didn't realise it was the high pH which was actually causing the precipitation.

 

[chem geek]

Dichlor isn't hypochlorite so won't change the pH very much so if you see this happening with Dichlor, then that's more likely to be the chlorine as you surmised. If you are talking about fresh fill water that hasn't been exposed to chlorine before, then it's the chlorine that is oxidizing the ferrous iron to ferric that then forms precipitate so you were right about it being the chlorine. I was referring to already having chlorinated water in a pool and then raising the pH to find metal staining starting to occur or precipitate (cloudiness or coloring of the water) occurring.

 

[DrF]

Copper and iron mostly. Zinc, silver, magnesium, and a few others can also be issues sometimes.

Copper has a minimum solubility at a pH of approximately 8-9 while (ferric/oxidized) iron has a minimum solubility around 6.5-7.5, so both species should already be mostly in particulate form in your pool. Minimum solubilities for zinc, magnesium, etc are quite a bit higher so more will be in soluble form. The issue is even though a large percent of the metals are in particulate form, the particles are very small so pass through the filter and charged so do not settle, or the metals are bound up with a chelating agent and unable to come out of solution. A common chelating agent is EDTA, which is found in most personal care products and in fertilizers. Addition of a coagulant (what is referred to as a flocculant in pool talk) like aluminum sulfate (alum) or other metal salt will improve metal removal efficiency primarily by 2 mechanisms:
1. The very fine metal particles are negatively charged so repel each other. Addition of a coagulant can neutralize the charge, which allows the particles to stick together and can then be removed by the filter or by settling (or in my case, by dissolved air flotation). The pH is key because changes in pH will result in changes in charges of constituents in the water, so keep the pH in the normal recommended range for pools. Aluminum works best at a slightly lower pH, and since most coagulants are highly acidic, you can get better results if you let the pH drop down to around 6-6.5.
2. Coagulants also will tie up with EDTA and other chelating agents, which will release the other bound up metal, allowing it to precipitate, and the coagulant-EDTA complex can be removed (unlike the soluble metal-EDTA complex).

I typically prefer to use sodium aluminate or aluminum chlorohydrate instead of traditional alum or aluminum chloride as it will not crash the pH like the latter. Unfortunately, I am unsure if either are available at a pool supply place. I typically get mine from water treatment chemical suppliers since I work in the business. After coagulation, the residual metal concentrations should be well below problem level. Unfortunately, if zinc, magnesium, or another metal that is insoluble at high pH is still at too high of levels, it can be difficult to remove (at least in a pool) since the pH will need to be increased significantly or you will need a sulfide-based metal scavenger to remove.