Staining is a problem many pool and spa owners will contend with at some point. It is a nuisance that detracts from a pool or spa’s overall aesthetic value and can contribute to surface deterioration. Staining can be prevented; however, the conditions, which ultimately cause it to happen must be recognized first.
Once an operator understands the mechanisms causing surface stains, common sense methods can usually be employed to avoid problems. However, there are also times when prevention fails. In these cases, treatments are available to remove stains and this is where identification becomes important. The ability to differentiate stains will help determine the specific treatment required, as well as shed light on the source of the contaminants causing the problem. If a pool operator knows the source, further preventive action can be taken.
Types of stain contaminants
The key to preventing pool and spa staining is understanding how and why they form. Stains can originate from either organic or inorganic contaminants.
These contaminants are introduced to the pool as organic debris from the surrounding environment. Plant matter, such as leaves, sticks, flowers, seeds or other materials drift into the pool and ultimately sink to the bottom and decomposes. As the debris decays, it leaches tannins and other pigments that generally cause yellow or brownish stains on the pool’s surface.
These contaminants are introduced into the pool from minerals that dissolve into water and dissociate into their metal components. These metals come from a variety of sources and cause coloured water or stains. The most common path of introduction is through pool source water.
Whether it is municipal or well water, all natural water sources contain dissolved metals. The amount and type, however, varies greatly depending on the region. Metals can also be introduced into the pool via products, such as copper algaecides or ionizers, or as a result of component corrosion. For example, metal parts, such as pump impellers, heat exchangers and filter components can be a source of metal contaminants causing stains. Thus, proper water balance is a contributing factor in controlling and preventing components from corroding and causing stains.
The best solution available for treating organic stains is to shock the water and brush the pool walls frequently. Always maintain the appropriate sanitizer residual and perform proper maintenance, including circulation, filtration and brushing or vacuuming surfaces. Stains that are organic in nature will fade over time with the appropriate care.
To better understand how metals can stain a pool one must understand their underlying chemistry. When dissolved in water, metals are present as positively charged particles called cations. Under certain conditions, the metal cations can chemically bind to negative anions, which cause the metals to drop out of solution (e.g. as a metal salt or mineral) and leave coloured stains on pool or spa surfaces. The oxidation process or alterations in water balance dictate when metals will precipitate and cause staining.
Corrosion of metal components is an oxidation process. The most important corrosion related staining is from system components containing iron (Fe) or copper (Cu). When the metals in these components are in their elemental state (e.g. refined copper in a heat exchanger or processed iron in steel) they have an oxidation number of zero (Fe0 or Cu0).
Corrosion occurs when these metals are oxidized to a divalent state by an oxidizing agent present in the water, such as hypochlorous acid (HClO) or oxygen (O), yielding Fe2+ (ferrous ion) and Cu2+ (cupric ion):
Fe0 + oxidizer → Fe+2
Cu0 + oxidizer → Cu+2
These divalent ions are water soluble, meaning they will dissolve and dissociate into the pool upon oxidation. It is important to note that oxidation of system components is largely driven by oxidant concentration. Whether it is dissolved oxygen or hypochlorous acid, typical concentrations found in pools are very slow to oxidize copper from a heat exchanger or iron from a steel impeller. Normally, oxidation of such components is caused by improper water balance, which is discussed in the following section.
Alternatively, metals can enter a pool through source water or from products applied for treatments, such as copper-based algaecides. When dissolved in water, copper and iron are in a divalent state, as shown above. Incidentally, ferrous iron has the capacity to oxidize further to form trivalent ferric iron:
Fe+2 + oxidizer → Fe+3
The trivalent product of the reaction (Fe+3) is insoluble. As pools operate in slightly alkaline (AT) conditions, trivalent iron will react with excess hydroxides (OH−) in the water to form iron hydroxide precipitate, as shown below:
Fe+3 + 3 OH− → Fe(OH)3
Though divalent copper will not oxidize further, it too can undergo a similar reaction in alkaline conditions to form copper hydroxide (Cu[OH]2) precipitation:
Cu+2 + 2 OH− → Cu(OH)2
This is the reason why applications of alkaline products, such as calcium hypochlorite (Ca[ClO]2), bleach or even the byproducts of electrolytic chlorine generation tend to exhibit a greater oxidizing effect to create staining. Due to the high pH, such products create localized areas in the water that are high in hydroxide concentration. This relative increase in hydroxide accelerates the reaction rate that forms the metal hydroxide precipitate, which then creates surface staining.
Another mechanism where oxidation can contribute to staining is through the interaction with metal containing products (e.g. copper-based algaecides). It has been established that elemental copper dissolved in water can react in alkaline conditions to cause surface stains. To help prevent this, some copper-based algaecides incorporate a chelating complex to inhibit the precipitation reaction. However, many chelants are themselves susceptible to hypochlorous acid oxidation. Once oxidized, they will degrade and eliminate the chelation effect on the copper. Once free, the copper is available to react to form staining. Common chelants, such as triethanolamine (C6H15NO3) and ethylenediaminetetraacetic acid (EDTA) contain nitrogenous (N) groups that will react with chlorine to form chloramines (NH2Cl).
Therefore, products that use these chelants are less effective at preventing copper staining. Alternatively, chlorine resistant chelants, such as polyacrylate are much more stable in a chlorine environment. They will not degrade under normal pool conditions and are more effective at preventing copper staining.
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.
Once the metal ions are re-dissolved back into the water, they should be sequestered to prevent further staining. A common sequestering agent used in pools is 1-Hydroxy Ethylidene-1,1-Diphosphonic Acid (HEDP). In a slightly alkaline solution, such as pool water, it will form complexes with metal ions, as shown below:
The complex is essentially a weak bond between the ions, and because of the two negative oxygen atoms, it has a particular affinity for divalent ions, such as soluble iron, copper or calcium. The formation of the complex will prevent the ions from reacting to produce precipitates or scale. It is even effective at lifting fresh stains; however, it may not remove old, existing stains.
Ethylenediaminetetraacetic acid (EDTA) is another common product and is a very strong metal chelator. It should not be used in chlorinated pools, as it is very unstable in the presence of chlorine. In fact, it can even lead to the formation of chloramines due to the nitrogen in the structure. It is a great option for pools using biguanide (C2H7N5) as a sanitizer. Citric acid (C6H8O7) is also a good sequestrant, but much like EDTA, its performance is significantly reduced in a chlorinated environment.
Once sequestered, the metals must be physically removed through filtration. Many times, a filter aid can assist with this process. Cellulose filter aids are a good option, especially those that have been specifically processed to exert a charge affinity on the surface of the fibre. As such, it is able to adsorb metal ions onto the fibre surface and effectively remove them from solution.
Once collected in the filter, the metals can be physically removed from the system through backwash. In order to ensure all metals are removed, the final step is to chemically clean the filter media with an acidic cleaner.
This article was written by Karen Rigsby & Zach Hansen, and originally appeared on Pool & Spa Marketing [link].