Another way metals contribute to staining is when a pool’s water balance is adjusted. As water balances, it reaches a point of equilibrium. Essentially, the water becomes full and will no longer dissolve any additional metals.
When balancing water, it is done according to a calcium (Ca)/bicarbonate (HCO3−) saturation equilibrium. This means the water is nearly saturated with the optimal amount of calcium and bicarbonate based on the water conditions (i.e. temperature, pH, total dissolved solids [TDS], etc.) to prevent further dissolution of undesirable metals, such as copper from a heat exchanger or calcium from a plaster surface. Water condition adjustments bringing saturation above the current balance will force metals out of solution and cause staining.
Staining is driven by metal hydroxide precipitation; therefore, the most significant parameter to consider is pH. If pH is lower, the hydroxide concentration is lower and dissolved metals remain more soluble. However, a pH too low will contribute to staining, as it will dissolve more metal ions into the water. Water with a low pH is more aggressive, making components more susceptible to oxidation.
Alternatively, a higher pH will have an increased hydroxide concentration, which leaves more hydroxide ions available to react and precipitate with the metals in solution. This is why maintaining proper water balance is important.
Inorganic stains are treated differently, depending on the type of metal precipitate. The type of stain present should be identified first before attempting to treat it. A stain identification kit will not only determine if metals were the cause of the stain, but more importantly, which specific metal (i.e. copper or iron) was the culprit. Once identified, the user can select the appropriate stain remover for effective treatment.
Prior to performing the stain identification test, the stained area should be inspected to make sure it does not have a raised texture. If it does, then it is likely scale and a product designed for scale prevention should be used.
Ascorbic acid (C6H8O6) is the most common compound used to remove iron stains. It is a strong reducing agent, with a low oxidation potential that donates electrons to species with a higher oxidation potential. When applied to an iron hydroxide stain, the reducing mechanism is as follows:
In the case of iron staining, the iron is taking an electron from the ascorbic acid. As a result, its oxidation state is reduced from Fe+3 to Fe+2, reversing prior oxidation and returning it to a soluble form. As divalent iron, it dissolves back into solution and the stain is removed.
It is important to note, ascorbic acid is easily oxidized by free chlorine where a product of that reaction is hydrochloric acid. Therefore, it is best practice to reduce the free available chlorine (FAC) level in the pool to less than one part per million (ppm). Once the treatment is completed and the stain is removed, pH and total alkalinity (TA) will need to be balanced and a shock application will be required to re-establish an effective chlorine residual.
Typically, copper exists only as an ion in its divalent state; therefore, a reducing agent, such as ascorbic acid is largely ineffective in stain removal. As such, sulfamic acid ([NH2]HSO3) is normally used to remove copper stains. Although it is extremely effective at lifting stains, sulfamic acid drastically affects water chemistry, as seen below:
(1) (NH2) HSO3 + H2O → (NH4)HSO4
(2) (NH4) HSO4 + HOCl → NH2Cl + H2SO4 + H2O
In water, sulfamic acid hydrolyzes to ammonium bisulfate (NH4HSO4), which then reacts with free chlorine to form monochloramine, sulfuric acid (H2SO4) and water. It is the generation of sulfuric acid that removes the stain:
H2SO4 + Cu(OH)2 → Cu+2 + SO4-2 + 2H2O
When sulfamic acid is applied, the low pH at the stain surface will react with the hydroxide from the copper hydroxide precipitate to form water, allowing the copper to be re-dissolved into solution.
In addition to lowering the pH, this will also have a detrimental effect on the water’s total alkalinity, which can deteriorate heating elements. Therefore, this treatment should only be used if the heater can be bypassed. Due to the significant changes in overall water balance, it is important to rebalance after completing the full treatment process.
It should also be noted that a byproduct of the reaction is monochloramine. A combined chlorine residual will also be present for several weeks after the copper treatment is finished. To ensure sanitized water, it is important to establish a free chlorine residual prior to re-entering the pool even though a combined chlorine level will be present.
Brushing the stained areas prior to and during the treatment process will help facilitate the stain removal process.
Pool operators should note that lifting a metal stain from a surface is only the first step. In order to prevent re-staining, the metals have to be physically removed from the water once they are re-dissolved.