by Sally Bouorm | December 1, 2011 2:25 pm
By Karen Rigsby and Zach Hansen
Maintaining appropriate levels of calcium hardness is a key balance factor in basic recreational water care. Typically, water in a vinyl-lined or fibreglass swimming pool should contain a calcium hardness level of 175 to 225 parts per million (ppm), while the recommended range for plaster-surfaced pools is a little higher at 200 to 275 ppm. In addition, the overall balance of calcium hardness is directly related to other key balancing factors, total alkalinity (TA) and pH. Managing calcium hardness is extremely important due to the problems that can occur if it is not properly controlled.
If the calcium level in swimming pool water is too low, it can cause plaster and grout to deteriorate and metal to corrode. The composition of plaster and grout includes calcium-based minerals such as silicates (Si), hydroxides (OH−), carbonates (CO32−), and sulfates (SO42−4). If the water in contact with these surfaces does not have sufficient calcium hardness, these calcium-based minerals can slowly dissolve, leaving voids in the surface. This is a result of material erosion called surface etch. Etching can be uniform or irregular and can cause a roughened outward appearance. Calcium (Ca) can also leach from areas below the surface by diffusion, which can result in loss of structural strength and, possibly, breaking away of significant amounts of underlying material.
Alternatively, although the metal corrosion process can be complex, it is known that low levels of calcium hardness can accelerate corrosion of certain metal alloys—particularly on hot surfaces such as heat exchangers. Thus, insufficient levels of calcium hardness can reduce the service life of components made from such metal alloys and contaminate the water with dissolved metals that can ultimately cause staining.
It is important for a pool operator to understand the cost of operating a pool with low calcium hardness far outweighs the cost to treat it, as low calcium hardness can be easily corrected. For example, a calcium hardness increaser can be used as a cost-effective method to boost calcium levels and appropriately balance pool water. However, an effective method of water testing is also critical for an operator to measure calcium hardness levels in a pool, and then adjust accordingly.
On the other hand, if calcium hardness is too high, precipitation (the formation of a suspension of an insoluble compound when mixing two solutions) can occur, which can cause cloudy water or scale on surfaces. The upper limit recommendations for calcium hardness exist to prevent the pool from becoming oversaturated with calcium-based minerals, leading to mineral precipitating from solution. The most commonly formed calcium-based mineral is calcium carbonate (CaCO3), which is how management of calcium hardness directly describes the concentration of bicarbonate (HCO3−) and carbonate (CO3−2) ions in the water, where pH determines their subsequent ion ratio: bicarbonate to carbonate (HCO−:CO3−2). At higher pH, a greater concentration of carbonate ions will exist in the water.
It is simple to understand how calcium reacts with carbonate to form insoluble calcium carbonate by looking at the following reaction depicted in Figure 1.
It is important that pH is maintained to minimize the concentration of carbonate. However, calcium carbonate can precipitate even though most of the alkalinity (AT) is in the form of bicarbonate, if either calcium hardness or alkalinity levels are too high. This process is illustrated in the reactions below. First calcium combines with bicarbonate to produce soluble calcium bicarbonate. Then, calcium bicarbonate reacts to create precipitated calcium carbonate, carbon dioxide and water (see Figure 2).
When calcium carbonate precipitates it can cause cloudy water and sediment to settle on the pool or spa floor. This places a demand on the filtration system and can create a need for more frequent vacuuming. In more severe cases, calcium carbonate can attach to surfaces, called scale. It is important to prevent scaling on heaters as it impedes the efficiency of heat transfer, wastes energy, promotes localized conditions of under-deposit corrosion and shortens heater life. On other surfaces such as pool walls, scale can create a dull unattractive appearance and cause the surface to feel rough. It could also clog pipes and filters. Ideally, calcium levels should never reach more than 400 ppm. However, in hard source water areas, where calcium hardness could be higher than 400 ppm straight from the tap, scale formation can be managed through the use of scale-inhibiting products.
Shown below (Figure 3) is the calcium carbonate equilibrium in pool water. Note: this reaction is reversible, which means it can proceed either way based on a change in common water conditions (i.e. temperature, pH and ion concentration).
|FIGURE 1: Ca2+ (calcium in solution) + CO32− (carbonate in solution) → CaCO3 (precipitated solid-scale)
Ca2+ (calcium) + 2HCO3− (bicarbonate) → Ca(HCO3)2 (calcium bicarbonate)
Ca(HCO3)2 (calcium bicarbonate) → CaCO3 (calcium carbonate-scale) + CO2 (carbon dioxide) + H2O (water)
Ca2+ (calcium) + 2HCO3− (bicarbonate) ←→ CaCO3 (scale) + H2O (water) + CO2 (carbon dioxide)
HCO3− (bicarbonate) → H+ (hydrogen) + CO3−2 (carbonate)
An increase in water temperature causes increased molecular motion. This simply means all the calcium and carbonate molecules move around much faster. As a result, they are more likely to bump into each other, causing them to form a bond. Another side-effect of increased water temperature is a decrease in carbon dioxide (CO2) concentration. This causes the forward reaction to proceed so that balance can be achieved, which creates scale formation.
As pH is a measure of the hydrogen ion (H+) concentration in the water, and is calculated on a negative logarithmic scale, an increase in pH indicates a lower hydrogen concentration. As a result, the hydrogen component of bicarbonate jumps off and dissociates into hydrogen and carbonate as illustrated in Figure 4:
Intuitively, if there is more carbonate ion, there is a greater probability that it will bump into calcium, in the right configuration, to force a bond and precipitate out calcium carbonate scale.
As described above, greater concentrations of calcium and carbonate in the water lead to greater incidences where they come into contact with each other and form scale.
Scale is formed through a process called nucleation. Nucleation is a physical reaction that occurs when components in a solution start to precipitate out, forming nuclei that attract more precipitate. In a simple example of nucleation, supersaturated sugar water is used to make rock candy, with the sugar crystals nucleating and growing into larger crystals.
There are essentially two mechanisms of nucleation, homogeneous and heterogeneous.
This happens when the solution is fairly uniform and the process occurs somewhat spontaneously and randomly. As the ions come in contact with each other they begin to form bonds and align with one another. At some point, enough ions will become aligned to begin the process of crystal growth. The action of crystal growth causes a solid to form in a highly organized structure where the molecules are closely packed with fixed positions.
This happens when crystals grow on an activation site. An activation site is any type of irregular surface. A common example would be carbon dioxide bubbling that occurs from an etched glass filled with a carbonated beverage. Some examples in pools are plaster surfaces, corroded surfaces (e.g. heat exchangers) and even dirt and other matter in the pool. Any type of irregular surface can serve as an activation site for crystal growth. Heterogeneous nucleation is more common than homogeneous nucleation.
Scale prevention/inhibition is the disruption of the scale formation process. This can be accomplished through various means such as sequestering calcium, binding to newly formed scale clusters (poisoning), or dispersing the scale clusters back into bulk solution. Some products offer one type of inhibition while others are more multi-functional.
A commonly used scale inhibitor is 1-Hydroxyethylidene-1,1-Diphosphonic Acid (HEDP). This particular compound is a great inhibitor as it binds to divalent (+2) cations such as iron and calcium.
Note the end of the molecule has essentially the same structure as carbonate, so the calcium molecule is likely to bind there. Once calcium ions begin to bind to the HEDP it makes it very difficult for the scale clusters’ crystal structure to continue growing. HEDP will prevent scale by sequestering calcium and inhibiting scale cluster growth.
Other non-phosphonated products such as polyacrylic acids (PAAs) or polycarboxylic acids, which act as dispersants, are also available. They do not provide inhibition by sequestering calcium or by poisoning. Rather, they work by dispersing scale clusters back into solution to prevent settling.
Also, compounds exist which distort the surface of the crystal structure of formed scale. By ‘rounding off’ the surface activation sites needed for further scale formation, the heterogeneous nucleation process described above is inhibited. These products can also work as a dispersant to prevent settling of scale clusters.
The best protection will come from using a multi-functional product, which employs a variety of methods (sequestration, poisoning and dispersion) to assist with scale treatment and prevention.
Karen Rigsby is the leader of technical services for BioLab, a Chemtura Company. She has been involved with the recreational side of water treatment since 2001, focusing on education, problem resolution and new product development. She began her career in the water treatment industry at BioLab as an analytical chemist in the research and development group. Prior to recreational water, Rigsby was employed by the Georgia Bureau of Investigation as a forensic chemist. Rigsby received her bachelor of science in chemistry from Georgia Tech and is a member of the Association of Pool & Spa Professionals (APSP) Recreational Water Quality Committee and a National Swimming Pool Foundation (NSPF) certified instructor.
Zach Hansen is a new product specialist for BioLab, where he started his career working in automated controller and feeder equipment development. Over the last four years he has focused on new product commercialization and development for the company. Hansen received his bachelor of science degree in chemical engineering from Auburn University in 2004. He can be reached via e-mail at email@example.com.
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