Do you have metals in your tap water? Have you ever filled a pool and had the water get discolored when you added chlorine for the first time? If so, do you know why that happened? And more importantly, do you know how it can be prevented?
Dissolved metals are commonly found in tap water
Source water is different everywhere you go, which is why we insist on testing tap water before filling a pool, or even servicing the pool. If you don’t know what is coming out of the tap, you will be at a severe disadvantage when it comes to managing pool chemistry. Since most pool operators test only a few factors–pH, total alkalinity, chlorine, and sometimes calcium hardness–otherwise predictable problems occur. To really grasp what will happen to the pool, one must know what is coming out of the tap.
If you have ever seen discolored water, or pools with metal stains, that’s an obvious sign of metal content in your water. Iron, when oxidized, turns brownish or orange in color. Stains will be ugly and visible to anyone looking. Copper has more of a light green, or even turquoise look to it when it gets oxidized. Manganese has a dark color, between black and purple, depending on the severity. It is also not uncommon to see a blend of multiple metals for unique colors and stains.
What causes metal stains and discoloration?
Metals are usually the first contaminants in water that chlorine oxidizes. This has to do with chemistry and the ease of oxidation. Here is a chart that shows the breakpoint chlorination curve, and it starts with the first thing chlorine attacks: metals.
The chart says in the first section “Destruction of chlorine residual by reducing compounds.” In this case, metals are the reducing compounds, because they get oxidized. Being oxidized means metals lose an electron and take on an oxygen from the oxidizer (chlorine, either HOCl or OCl-). After metals have been oxidized, at the expense of some of the free available chlorine, chlorine then begins to oxidize non-living organics and nitrogen compounds that form combined chlorine.
But let’s get back to metals. When metals are oxidized, they become a new substance altogether. It is usually with oxidation that metals become visible because of their color. Purely dissolved metals that have not been oxidized are normally invisible in water. In other words, oxidation of metals brings out their color in water.
Can metal stains be prevented?
Metal stains in pools can be prevented. The way to prevent metal stains is to control metals. There are a few ways to control metals, but for the purposes of this article, let’s stay with what is practical in the pool business: removal, sequestration, and chelation.
How to remove metals from water
With certain types of filter media, metals can actually be filtered out. Such products do exist, but they can be costly. One such example is reverse osmosis, which takes just about everything out of the water…metals included. There are specific metal trapping filters that can be attached to hoses and plumbing that target heavy metals specifically as well. Some pool filters can also trap metals if they have a small enough filter media. Even so, with the help of certain sequestering agents, filters can capture metals to be backwashed or removed from the water.
If your metals are very high, removing them is a good idea. The less heavy metals you have in your water, the easier to manage.
Sequestering agents are common in the pool industry. Sequestering agents work kind of like a ‘metal magnet’, which attract metal ions into a cluster. When the electrons bind to the sequestering agent, oxidation of those metals can no longer occur, which is great! Sequestering prevents metal stains and discoloration of water. That being said, the metals are in suspension, and still in the water. The good news is, sequestered metals can more easily be caught by a filter and removed.
The challenge with sequestering metals is that most of the sequestering products are some kind of phosphate-based acid. Phosphonic acid is the most common type, and it is used even in municipal water treatment systems. Sequestering metals protects infrastructure and can also prevent scale in water treatment pipes. Normally this would be no big deal, except that phosphonic acid is basically liquid phosphates, and food for microorganisms like algae. So pool operators now have elevated phosphate levels to deal with.
What do you think happens when the pool operator uses a phosphate remover? The sequestering agent is wiped out, and the metals it was holding are now released back in the water, and available to be oxidized once again. The use of phosphate-based sequestering agents, therefore, should be used either temporarily for metal removal, or at least with the knowledge of their impact on phosphate levels.
Chelation is similar to sequestration, in that it binds with metals and prevents oxidation (and metal stains). Unlike a sequest, which binds many metal ions together into a cluster of sorts, a chelant grabs individual metal ions. No clustering, just individual metal ions that have been chelated separately.
In our far-from-scientific lingo, we think of it like this: sequestering agents cluster metals and hold them in suspension; chelating agents grab individual metal ions separately and hold them in solution.
Our Metal and Scale Inhibitor (MSI) is an NSF-50 Certified chelating agent. It is not phosphate-based, and therefore compatible with phosphate removers. MSI is great at preventing stains and discoloration because it holds metals in solution and protects them from oxidation. That being said, MSI is not meant to be used to remove metals. When you hold metals in solution, they may pass through filters, unlike clustered metals bound to a sequestering agent.
Prevention is easier than Correction
“An ounce of prevention is worth a pound of cure.” – Benjamin Franklin
You can remove, sequester or chelate metals before they are oxidized. If you do, metal stains should not occur. Be sure to test tap water to know the metal content, so you know how much is being introduced on a weekly basis when the pool is replenished with water. Once stains begin to form, it may take citric acid to lift them off the surface, and that process is aggressive on plaster.
We hope this article helps you better understand metals, and gives you hope that yes, you can prevent metal stains and discolorations in your pool. If you have specific questions, contact us or your local NextGeneration Dealer.