Metals and Sequestrants

[TreeFiter]

I have read a few posts about sequestrants and how they work, and I've come up with some questions. If I understand correctly sequestrants prevent metals from forming precipitates. So if I were to encounter a pool where the water was clouded with rust, the iron has already oxidized creating tiny rust particles that are floating in suspension. Am I correct to say that a sequestrant would not help here? I encountered such a pool this morning. I've never seen such rusty looking water. There was only about 1 ft of visibility. My thoughts are that if it has already precipitated, filtration would be the best way to take care of it because it would be removed from the pool when you backwash eliminating the iron rather than just controlling it. What would happen if I did add sequestrant to a pool like this? Would it reverse the oxidation and return the iron into solution?

For that matter, does dissolved Iron show its presence visibly? I've always been told that clear green water usually meant there was iron. I'm just not sure if that means dissolved, or if it is a precipitate starting to form, or if I was just given bad information. We usually use the green water as an indicator of metals, and add sequestrant. That usually gets rid of the color. I have noticed that it also results in a rusty backwash afterwards. So I'm wondering if the sequestrant got rid of the green color, or if the iron oxidized and was filtered out, or was it a little bit of both? Would the iron have oxidized and filtered out even if I hadn't added the sequestrant.

What about other metals? Does it work pretty much the same as Iron (oxidized products are insoluble and will precipitate)? What color water would indicate other metals?

 

[JasonLion]

Disolved iron is invisible, and does not color or cloud the water. When it forms really tiny crystals it colors the water, which don't usually filter out, though can sometimes if they get larger. Larger particles can cloud the water, though that is fairly unusual without sequestrant being involved, and they will usually filter out.

Sequestrant binds to the iron and prevents it from doing much of anything (as long as it remains bound). Sequestrant will also make it more likely that iron micro crystals will redissolve, though what actually happens depends mostly on the PH. Larger particles usually only occur when you have sequestrant in the water, as sequestrant bound to iron will sometimes clump into particles large enough to cloud the water. If it is large enough to cloud the water it is usually large enough to be filtered out, thought there can be exceptions. It is also possible for it to redissolve inside the filter if you don't clean the filter often enough.

There is always some balance between particles and dissolved iron. The iron is usually dissolving or precipitating at any given moment, but there will always be some iron in each form, though nearly all of it can be in one form or the other, again depending mostly on the PH.

Iron crystals color the water yellow, which often looks green against a blue liner. Copper colors the pool a bright emerald green. Other metals cause other colors, but that is far less common.

 

[chem geek]

Think of it as a good thing if you can see water clouded with rust because you've got a decent chance of filtering it out to actually remove it from the water. In fact, raising the pH causes metal ions to precipitate out of solution and would be a great way to remove them if one could ensure that they don't stain pool surfaces. The problem is that there isn't a reliable way to do that. In some cases, one can try adding sodium carbonate / soda ash / washing soda to the skimmer to force metal precipitation into the filter and then backwash/clean it, but again one can risk staining pool surfaces if one is not careful.

For finer filtration, there are options such as Adding DE to a Sand Filter or using The Slime Bag. After you've physically removed whatever bulk metal precipitates you can, then you can lower the pH which can dissolve the finer precipitate and then use a metal sequestrant to bind to it. To remove the last of the metal in the pool is hard. There's a product called CuLator® that is expensive and has gotten mixed reviews so we don't recommend it yet, but if you do use it be sure to let us know if it works for you so we can get some better statistics on this product.

 

[TreeFiter]

For the pool I was at today, staining isn't really an issue (its an ancient concrete pool that should have been bulldozed years ago), but the vast majority of my pools have vinyl liners and staining is often an issue.

Unfortunately this pool has a bad multiport, and will only run on recirculate. So until we replace the multiport i'm hoping a little aeration helps to keep the iron oxidized. I may get lucky and be able to vac it out on waste.

 

[chem geek]

You can also take a look at this post for another way to filter out iron particles if you don't want to use the regular filter for that purpose. This technique of using fiber-fill from a pillow was mentioned on this forum as well, but the post I linked to gives more details about the procedure. Home-grown for sure, but sometimes these techniques work the best.

 

[crokett]

That's an interesting idea. There seems to be some disagreement whether you can filter iron out of a pool. Maybe you can with the floc agent. Jack's Magic staff says you can with their sequestering agents. For my sand filter, it seems the filter was catching something. The first couple times I backflushed after opening the pool, the pool water was perfect but I had a lot of rusty colored water come out of the filter. I no longer get that water out of the filter during back-flushing.

 

[JasonLion]

It is easy to filter iron particles out, which is what chem geek was talking about. Iron particles are fairly common in well water.

Getting dissolved iron out is an entirely different story. Sequestrant will bind to the iron, and will occasionally clump into clumps large enough to get filtered out, but the clumping process is not reliable enough to depend on.

 

[TreeFiter]

I've seen pools with that "green" water color, and typically I've added sequestrant to "get rid" of the iron. The following week, the pool is clear, and the backwash is very rusty. Was the rusty backwash a result of the sequestrant, or was it despite the sequestrant? It sounds like sequestrant prevents the metals from oxidizing and becoming an insoluble precipitate, so it should prevent it from filtering out. Is that correct? I'm thinking that the filter would have picked up the iron either way, and possibly would have picked up more of the iron if I didn't add sequestrant.

 

[chem geek]

More likely it's that both happened -- the filter filtered out iron oxide-hydroxides and the metal sequestrant bound to iron ions. Metal sequestrants have a limited capacity to remove already oxidized particles. Usually it's low pH and a reducing agent that are needed to remove existing stains by essentially redissolving them into metal ions that will then bind to a metal sequestrant. You can see the combination of these procedures in Metals in the Water and Metal Stains.

So if you've got mostly visible metal in the water itself, especially visible particles (i.e. not just a "clear green"), then it's usually best to filter this out first. After that, then one can tackle any metal staining on pool surfaces using the "remove and bind" approach described above.

 

[crokett]

Thanks Jason. Sometimes I think I need a chemistry degree to understand this stuff.