Pool metals and metal staining

Metals are naturally existing elements found in every place; in water, soil and stones. Metals in soil and stones dissolve in acidic rainwater, which is then carried to surface water and groundwater. Through this, the dissolved metals find their way to pools and tubs when filled. Exposure of metals to water cause staining on pool surfaces and equipment. This article discusses the techniques to control the presence of pool metals. It also discusses the different kinds of metal staining and the relevant chemicals to eliminate and prevent stains.

Metals in pools and hot tub water

There are 91 existing metals, but only 6 are problematic to hot tubs and pools. The 6 metals include magnesium, copper, cobalt, iron, manganese and calcium. Here we will only discuss 4 that are cobalt, copper, magnesium and iron.

Fill water, including almost all municipal and well water supplies that fill hot tubs and pools, contains a few metals. It is advisable to test for metal concentration in source water before filling the hot tubs and pools.

Tests for iron and copper are easily accessible, though the majority only measures the free form of the dissolved metals. They also do not test the sequestered and complexed forms of dissolved metals. A two-phase metal test containing adequate reducing and releasing agents give better results on free, complexed and some oxidized metals.

Sources of metal contamination in the pool and hot tub water are plaster and pebble surfaces, water features, decking materials, metal equipment, galvanic corrosion, chemicals and runoff.

In addition, flagstone, marble, and stone contain iron that leaches into the pool after every rainfall. Imbalanced water may cause the introduction of copper from heat exchangers found in gas-fired water heaters. Moreover, copper-based algaecides and mineral-based water treatment may be a source of pool metals. Plant fertilizers can also introduce metals when incorrectly applied or due to uncontrolled flooding. More sources of metal include irrigation methods with well water.

Preventing stains

water treatment

Avoid using untreated water with high metal concentration (higher than 1ppm) in hot tubs or pools. Treat the water when the metal concentration is higher than 0.2 ppm. Sediment filters (10 microns or less) eliminate the majority of the larger oxidized particles. Use filters that have metal adsorbents to treat water, to minimize dissolved metals concentration.

Primary Sanitizers and Stabilizers

Maintain proper concentration of primary sanitizers and other similar stabilizers to prevent organic contaminants. These include; pathogens, non-hazardous bacteria and algae minimizing the need for a copper-based algaecide.

Bonding of Metals and Water

The majority of the pool and hot tub equipment are metal-made. For example, electrolytic halogen generators, the heating core of electric heat pump, gas-fired pool and hot tub heaters, ladders, light rings and handrails.

To enhance chemical and electrical safety, install a bonding system in the hot tub or pool. The bonding system involves joining the copper wire with all the metal elements and water combined. This minimizes electric potential difference. Lack of this system enables dissolution of pool metals, causing staining that damages the equipment. This is the process known as galvanic corrosion and can heighten due to several factors such as salt water, stray direct current as well as its power supplies. Anodes (zinc metal) reduce the damage on the equipment but don’t deal with the cause of corrosion. Test the bonding system every year, or any other time there is corrosion.

Pool & hot tub pH

pH is the most vital yet most ignored factor in preventing metal release in water. Low pH water (below 7) is acidic and corrosive. Acidic water dissolves metals from the pool equipment.

Improper water chemistry harms pool surfaces and machinery. Always ensure to maintain an accurate pH, alkalinity (carbonate) and calcium (hardness). If inadequate, water draws carbonate and calcium from cementitious surfaces, discharging metals into the water.

Sequestering Agents

Sequestering agents join with the dissolved pool of metals to reduce oxidation hence reduces staining. They chelate the metals, not eliminate them. They require a complex modern ultra filtration system to remove sequestered metal particles as the particles are too small for the pool filtration system. Add this agent on a regular basis, as a secondary disinfection and primary sanitizer systems keep on destroying them.

Ways to reduce metal concentration in hot tubs and pool water

  • Bind the metals using polymer and water-insoluble natural adsorbents
  • Filter the water through a reverse osmosis system
  • Remove and replace a part of the water with fresh metal-free water

Removing stains from the pool and hot tub surfaces

1. Metal stain removal from the surface

2. Chelation of the dissolved metals in the water with a sequestering agent

3. Elimination of the metals from the water.

To get rid of metal pool stains, add oxalic acid, ascorbic acid and citric acid or a similar product into the pool water. An experienced pool service expert should only execute the use of muriatic acid in the removal of metal stains as it can lead to permanent damage to the pool surface. Chelation of dissolved metals and minimizing concentrations of dissolved metal have been tackled earlier.

Figuring out the difference between metal stains and those by organic contamination can be difficult. For evaluation, establish the origin of the stain, assesses the water chemistry and test the stain with either citric acid or ascorbic acid.

Note: Some organic stains, similar to those brought about by iron bacteria, vanish after treatment with ascorbic acid but return with the introduction of residual chlorine.

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Mike Hunter

Mike is the owner of the local pool shop. He's been in the business for over 20 years and knows everything there is to know about pools. He's always happy to help his customers with whatever they need, whether it's advice on pool maintenance or choosing the right chemicals. He's also a bit of a pool expert, and is always happy to share his knowledge with anyone who's interested.

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