What Is Ion Exchange?

So you’ve heard that ion exchange is a good option for treating water, but what exactly does it do? Is it worth the money? How can you benefit from the process?

We’ll be answering all your questions in this guide, but first: a quick overview of ion exchange.

Ion exchange involves exchanging undesirable dissolved ions (like calcium and magnesium ions, which cause water hardness), with desirable ions (like sodium).

Read on to learn about the function of ion exchangers, how they work, their uses, and their benefits.

🤔How Does Ion Exchange Work?

During the ion exchange process, unwanted ions – atoms or molecules that contain unbalanced amounts of electrons and protons – are replaced with more desirable ions inside an ion exchange resin.

Ions come in two different forms: anions and cations. Anions are negatively charged ions, while cations are – you guessed it – positively charged ions.

The IE process requires a liquid or a solid to work. Water is most commonly used as the liquid, while a resin material is the most common solid. Zeolite can also be used as a solid. 

ion exchange resin
Ion Exchange Resin

Anion vs Cation Exchange

In the anion exchange process, the solid material is positively charged. The positive charge attracts desirable negatively charged cations.

These cations stick to the surface of the resin. When water interacts with the solid resin, the desirable cations are released into the water, while the undesirable cations (the contaminants that need to be removed) are attracted to the resin. The result is that the undesirable cations are trapped in the resin, while equal amounts of desirable cations are released into the water.

The cation exchange process is almost exactly the same, except the solid material is negatively charged, and the desirable and undesirable cations are positively charged. Cation exchange is commonly used in water softeners, with sodium chloride or hydrochloric acid as the desirable cations, and hardness minerals being the undesirable cations.

➰ The Ion Exchange Regeneration Cycle

Ion exchange resins can only hold so many desirable and undesirable cations or anions. The resin begins by being saturated with desirable ions, and over time, these cations are released into the water, to be replaced with undesirable ions.

Eventually, the resin is saturated with undesirable ions, and is depleted of desirable ions. When this happens, the system needs to perform regeneration, flushing away the unwanted ions and replenishing the resin with desirable ions.

During the regeneration process, a liquid (usually water) is used to wash out the resin, sending its contents down the drain. Following this, the desirable ions fill the resin and stick to the surface. This continues until the resin is once again saturated with desirable ions.

The frequency that the system will need to regenerate depends largely on two factors:

  • Water usage. The higher the demand for water, the faster the desirable ions will be used, and the quicker the resin will become depleted.
  • Water quality. The higher the level of unwanted ions in the source water, the more desirable ions will be needed to replace these ions. This causes the resin to deplete at a faster rate.

📌 Uses of Ion Exchange

There are a number of uses of ion exchange – and some of them are too scientific to cover in this guide! We’ve stuck to the uses that you’re most likely to want to know about.

Ion Exchange for Water Treatment

The ion exchange process is a popular drinking water treatment method. A number of common contaminants can be removed from water using specialized resins. Some of the contaminants targeted are nitrate, arsenic, calcium, magnesium, boron, perchlorate, and uranium.

Ion exchange for water treatment takes place in water softeners or water filters, depending on the desired outcome of the treatment. 

ion exchange water treatment application
Industrial ion exchange water treatment application

Ion Exchange Water Softening

The most common use of ion exchange for water treatment is water softening.

What is ion exchange in a water softener? Let’s take a look at water softeners in more detail.

Traditional salt-based water softeners use ion exchange to replace calcium and magnesium (positively charged) ions with sodium (also positively charged) ions. The process takes place in a negatively charged resin bed.

There are two tanks in a water softening ion exchange system: the sodium tank and the resin tank. Sodium ions are drawn into the resin tank in a salt brine solution, where sodium is attracted to the opposite charge of the resin beads.

Ion Exchange Process

Sodium remains in the resin beads until water flows through the resin tank, and the ion exchange softening process occurs. During water softening, all calcium and magnesium ions are removed from water, resulting in salt-softened water.

When the ion exchange resin is completely saturated with magnesium and calcium ions, the system performs a regeneration cycle, flushing the minerals away and replacing them with more sodium ions.

regeneration process

Ion Exchange Water Filtration

Ion exchange filter media is a small-scale version of an ion exchange resin in a water softener. The ion exchange reaction is similar, swapping unwanted ions for preferred ions in drinking water.

Several manufacturers use cation exchange resins in their filters, such as Brita, which uses cation exchange resins in its pitcher filters. A cation exchange resin replaces positively charged ions (like calcium ions, magnesium ions, lead ions, and copper ions) with positively charged hydrogen ions.

Ion exchangers are used to replace negatively charged ions (like sulfate, phosphate, and nitrate) with negatively charged hydroxide or chloride ions. Ion exchangers are most commonly used to target nitrate – a difficult-to-remove contaminant – in drinking water.

Water filters using an ion or cation resin can’t regenerate like water softeners. The disadvantage of this is that once the resin is saturated with exchangeable ions, it can’t be flushed out, and the filter will need to be replaced.

Ion Exchange in the Food Industry

The food industry also requires the ion exchange process for several manufacturing purposes.

Wine and fruit juice processing, whey demineralization, and cane sugar decolorization all need to exchange certain ions with more desirable or valuable ions to produce the desired result.

🔔Pros and Cons of Ion Exchange

Wondering whether an ion exchange system is the right choice for you? Let’s take a look at the pros and cons of the process.

Ion Exchange Pros:

The biggest benefits of ion exchange are: 

  • Powerful method of water softening. You won’t find a better treatment for hard water than a traditional IE water softener.
  • High success rate. Ion exchange is incredibly effective at removing charged ions from liquids. As long as the right water parameters are set, the treatment is guaranteed to work.
  • Long lifespan. The average lifespan of an ion exchange resin is 8-12 years, so you don’t need to perform frequent maintenance.
  • Targets a range of contaminants. You can choose between different types of ion exchange resins, removing hardness minerals and contaminants that make water unsafe to drink.
  • Cheap to maintain. Most ion exchange systems only need to be topped up with desirable ions every few weeks, costing less than $100 per year for household applications.

Ion Exchange Cons:

Some of the most notable setbacks of ion exchange are: 

  • Can be affected by poor water quality. Certain contaminants in water can scale, clog, or foul the ion exchange resin. You may need to install a pre-treatment system to prevent resin damage.
  • Increases the acidity of water. Because ion exchange typically removes alkaline minerals, it reduces the pH of water, increasing the likelihood of metal leaching.
  • Regeneration is needed. If the resin isn’t replenished frequently, the ion exchange process simply can’t work. Regeneration can only occur if the system has constant access to desirable ions.
  • Not 100% maintenance-free. Although ion exchange requires less maintenance than some water treatment options, maintenance is still needed, whether in the form of topping up the desirable ions or replacing a filter with a new one.
  • Can cause water waste. If you’re using an IE salt-based water softener, the system will waste several gallons of water during regeneration. This is unavoidable.

💬Ion Exchange FAQ

What is the role of ion exchange?

There are several roles of ion and anion exchange systems. In water treatment, ion exchangers can be used for water softening, demineralization, dealkalization, denitrification, deionization, and disinfection. Aside from being used in residential applications, the systems can also be used for industrial and agricultural water treatment.

Can drinking water be purified by ion exchange?

No, ion exchange processes can’t purify drinking water. Ion exchange systems can be used to remove select contaminants from water, but ion exchangers aren’t capable of removing all contaminants from water. So, you can selectively filter water with ion exchange resins, but you can’t remove all total dissolved solids from water. High levels of TDS can actually damage an ion exchange system, and pre-treatment filtration may be required before ion exchange to prevent resin fouling.

Is ion exchange better for filtration or softening?

It depends on what you want to remove! Ion exchange systems can be really effective in removing select ionic contaminants and impurities. Most commonly, ion exchange materials are used in water softeners because of their effectiveness in removing the dissolved ionic contaminants that cause water hardness, replacing them with sodium on the resin surface. Those on restricted sodium diets can use potassium chloride instead of sodium chloride to achieve the same results.

Because of their ability to remove hardness ions in a whole-home application, we’d say that ion exchangers are a better choice for use in water softening equipment.

Are ion exchangers expensive?

Again, it depends on what you’re buying. A small ion or cation exchanger in a water filter is much more affordable than a hefty water softening system. Industrial ion exchangers are even more expensive.

The average cost of a point-of-use IE filter is $25-$75, while the average cost of an IE water softener is $1,000-$1,500. IE softening resins last for 8-10 years, so the upfront investment is worthwhile.

What’s the difference between ion and anion exchange?

The key difference is the charge of the ions that are exchanged. An ion exchange system exchanges ions that are positively charged, while anion exchange resins exchange negatively charged ions. Both systems perform a complete regeneration when the resin is saturated with exchangeable ions.

What are the common compositions for cation exchange?

There are two common compositions: weak acid cation resins and strong acid cation resins. If you’re up for a science lesson, here’s how each of these resins are made:

A weak acid resin is made of an acrylic polymer that has been hydrolyzed with caustic soda or sulfuric acid, producing functional groups of carboxylic acid. Strong acid cation resins are made from a polystyrene matrix that’s charged by hydrogen or sodium ions, depending on whether it is being used for demineralization or water softening.

What’s the difference between using a strong-base anion and a weak-base anion?

Whether an anion is strong-base or weak-base determines what it can be used for. Systems using weak-base anions focus on water demineralization, while strong-base anions are highly ionized and react with anions to convert an acidic solution into pure water.

🎓Takeaway

Ion exchange is a highly reliable, effective, affordable process that is most commonly used in the water and food treatment industries. Ion exchangers can be used in residential and industrial applications. Water softeners are the most popular household systems using ion exchange to produce soft, scale-free water.