Heavy Metals: Oxidation and Metal Staining

September 8, 2017
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It's your one-way ticket to midnight. Call it heavy metal.
Oh wait, we're talking about dissolved metals in water. Just uhh...disregard that Sammy Hagar reference. Sorry.

In this article we will be discussing metals in swimming pools. How they interact with other things in water like chlorine, and also how to prevent issues like stains. We will cover oxidation, sequestration and chelation, and attempt to simplify these terms.

Metals are Oxidants

Water is the universal solvent, and can eventually dissolve just about anything. Yes, this includes metal minerals like calcium and magnesium, and it also includes heavy metals like copper, iron, manganese and silver. Unlike minerals (calcium and magnesium), heavy metals are oxidants, which means they get oxidized in the presence of chlorine. Calcium and magnesium do not.

Because heavy metals like copper and iron get oxidized, they are also reducing agents, because their electrons will reduce chlorine. So when chlorine gets "used up" in the beginning of the breakpoint chlorination process, it is because metals have reduced it. It takes more chlorine to overcome the metals in water and continue on with the breakpoint chlorination process. See the breakpoint chlorination chart with the metals highlighted:

heavy metals, pool metals, dissolved metal, metal stains, pool stains, remove stain in pool, iron stain, copper stain, green water, metal in pool waterHeavy metals themselves are virtually invisible when totally dissolved in water. That is, until those metals are oxidized. It is worth a quick recap of oxidation, in case you have not read about it already.

What is oxidation? 

Oxidation is when an atom loses electrons. Oxidation usually occurs in an oxidation-reduction (or “redox”) reaction. All that means is an oxidizer (HOCl in swimming pools) steals electrons from the atom. Reduction is actually the opposite of oxidation. When an oxidizer like chlorine steals electrons from the oxidant, those negatively-charged electrons (e-) reduce the charge of the oxidizer. It eventually breaks down HOCl into useless chloride ions that can no longer oxidize (Cl-). 

Metals are really easy to oxidize, and therefore they reduce chlorine rapidly. The good news is, once metals are oxidized, they tend to stay that way. Think about it, can you un-rust iron?

Example: Iron is oxidized by chlorine, water, and oxygen itself

Let’s take the easiest example: Iron (Fe). Iron is an easily oxidized metal because it does not hold its electrons with a very strong bond. When something like Oxygen (O2), Water (H2O) or Hypochlorous acid (HOCl) meets it, Iron tends to lose electrons rapidly. In this case, Iron is the reducing agent, because it loses electrons. Chlorine is the oxidizing agent, because it gains electrons. We know the byproduct of this reaction as rust. Rust is iron oxide—Fe(OH)2—and depending on factors like pH and salts, it comes in many forms. In fact, there are 16 different forms of iron oxide.

We are not chemists, so this is complex stuff that we will simplify for our audience. But before we do, the following formula is one example of what it looks like when chlorine oxidizes iron:

HOCI + 2Fe++ + 5H2O → 2Fe(OH)3↓ + Cl- + 5H

All you need to know is this: water, oxygen and the strong form of chlorine in a pool (HOCl) all oxidize iron, creating iron oxide (rust). In the water, that iron oxide can stay suspended in solution, or it can fall out of solution and stain. Naturally, that stain will be rust-colored. If you have rust stains in your pool, it’s probably iron oxide.

Chelating vs. Sequestering

SC1000, sc-1000, orenda, orenda startup, chelating agent, pool chelating agent, pool chelant, stain preventionA common question we get is the difference between chelation and sequestration. Most metal control products in the pool industry are sequestering agents, and most of those sequestering agents are phosphate-based (phosphonic or phosphoric acid). Orenda's metal control product, SC-1000, is a chelating agent that contains no phosphates at all.

Sequestration is kind of like a magnet. A sequest attracts electrons from metals and minerals (like calcium). When they bond with the sequestering agent, these metals cannot be oxidized, and calcium cannot bond to carbonate ions to form calcium carbonate scale or dust. The metals and minerals are held in suspension (not solution), and sometimes the clusters are large enough to be filtered out. These amazing benefits are why phosphate-based sequestering agents are added to drinking water...they help control scale and metal issues, which protects the infrastructure of the treatment plant and the pipes.

Chelation divides and conquers. Rather than attracting a whole bunch of metals and clumping them together, chelating agents divide up and attract individual ions of metals and minerals. Chelation occurs ion by ion, and holds metals and minerals in solution (not suspension). This is preferred for minerals like calcium, but is a disadvantage for trying to filter out and remove metals. Chelated metals tend to pass right through filters, because they are not sequestered together.

Here is our marvelously over-simplified diagram that attempts to illustrate the difference between sequestration and chelation:

Sequestration vs. Chelation, metal sequestering, chelated metals, chelated iron, SC-1000, orenda chelant, pool chemistry, chelant, sequest

Metal Pool Sanitizers and Algaecides

Some metals can both sanitize and be oxidized. Let's take copper as one example. Copper is a popular product in the pool business because it can be a strong algaecide. This can occur because copper can both be oxidized AND sanitize. It's great for a quick kill against algae, but copper algaecides leave behind [you guessed it] copper. The EPA limits for copper levels in drinking water, and because of that eventual toxicity, Orenda recommends against regular use of copper products. Yes, even if the copper algaecide is chelated. It stays in the water long-term, and if you remember, Orenda's philosophy is Proactive pool care without chemical conflicts or long-term byproducts.

There are popular metal-based systems on the market in the swimming pool business, and most of them are very effective at keeping pools clean and sanitized. These systems are usually referred to as mineral systems, and they work by allowing the metal to be dissolved into water, intentionally. Normally these metals are already chelated before they are introduced into the water.

Copper and Silver are two of the most popular mineral systems available. There are certainly benefits to using mineral systems, like how long they last, and how easy they are to use. It's not that mineral systems are bad, it's just that eventually there is enough metal in the water that it can become problematic. This problem can be accounted for by partially draining and diluting with fresh water each season.

How to prevent metal stains in a pool

Preventative, proactive pool care is what we at Orenda teach every day. We are fundamental believers in minimalist, meaningful pool care, which includes thinking ahead. We advocate for testing fill water and filtering metals out of the tap water that fills a pool. Also, we encourage plaster crews to filter out metals from the water they use to mix the plaster itself, lest those metals continue to oxidize in the pool’s surface and cause chronic staining problems.

To prevent metal stains (rust stains, for example), it’s about keeping metal levels in the water low, and managing whatever metals you may have in the tap water. This can be difficult when the tap water has high metal content in it. It’s even more difficult when using metal-based products by choice (the good news is, you can choose to stop using them).

For the metals that still get into the water, chelation or sequestration are both very effective. 

To recap stain prevention: 1) reduce metals in the water through filtration and removal, as low as possible, and 2) control metals through chelation and sequestration with a product like SC-1000.

How to remove metal stains in a pool

Removing existing stains is a different issue, but similar steps to remedy.

  1. If you have stains, you should add a chelant or sequestering agent to the water. This will grab metals released during step 2.
  2. The second step is to try and lift the stain from the affected areas. You can accelerate the stain removal with ascorbic acid. Monitor the pH of the water when doing this step. Pool operators may want to vacuum or brush the stained area too. Try to apply the ascorbic acid (also called Vitamin C, or citric acid) evenly across the stain. If it's a small stain, use a PVC pipe that reaches the stain so that the granular acid lands on the stain only.
  3. Once the acid has had a chance to lift the stain, brush thoroughly.

Pool stain removal can take minutes or it can take weeks. There are too many variables that impact the process to discuss here. Moving forward from stain removal, follow the steps above for prevention, which should help eliminate the problem long term.

In full disclosure, stain removal is not Orenda's specialty. We are far better at preventing them than removing them. If you already have stubborn stains, consider more specialized products and techniques.

Conclusion

Pool stains are a nuisance, but an understanding of why they occur can help you manage them. Oxidation of these metals is inevitable in the presence of an oxidizer like chlorine. Rather than trying to prevent oxidation from occurring, we say minimize the amount of metals to begin with, and chelate what is left over.

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