Sequestering metals
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.
Filtration
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.
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 technical services engineer 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 zach.hansen@chemtura.com.
