How Do Zeolites Remove Impurities in Water Filters?

by Stephen Bernal on August 2, 2010

There are many different methods for removing impurities from our drinking water or the water that feeds out entire house. One method is the use of a zeolite bed to remove ions that contribute to water hardness like calcium and magnesium through ion exchange. Zeolites are minerals that are structured such that there are many pores and surfaces for positive ions to attach. In chemistry, surface area is very important to how minerals interact. Because of their porous nature, there is a lot of surface area in zeolites leaving more spaces for the ions to deposit.

A fresh zeolite bed in a water filtration system will have a zeolite resin with sodium ions attached in as many of the pores and crevices as possible. Hard water contains positive ions such as calcium or magnesium. These elements carry a positive charge of two while sodium has a positive charge of only one. As hard water passes over and through the zeolite bed, the more positively charged ions in the water are attracted to the zeolite bed and the negatively charged mineral is more attracted to them than the sodium ions. The sodium ions are dropped and the calcium and magnesium start to fill in the pores and crevices. The sodium ions are exchanged for calcium or magnesium ions, hence the name ion exchange.

When all of the pores and crevices are full of calcium and magnesium, then the zeolite bed needs to be regenerated so you can continue to use it. We need to get the zeolite to drop the calcium and magnesium in favor of more sodium. The way this is accomplished is by passing a heavily brined water solution through the bed. The chlorine ions are much more negatively charged than the zeolite so the calcium and magnesium are attracted to them. The sodium leaves the chlorine and attaches itself back on the zeolite.

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