Ion exchange is a process that swaps undesirable ions for healthy ones. The solid material is usually a resin or a type of hardened mineral called zeolite.
The ions have an electric charge, and the more positive their charge, the more they stick to the resin. The desirable ions are called cations and the undesirable ions are anions.
Water Softening
When the main goal is to reduce the amount of calcium and magnesium in your water, you can use an ion exchange system. The tall, narrow tank where this occurs contains several cubic feet of porous resin material (usually a styrene-divinylbenzene copolymer or zeolite) that is negatively charged. As hard water flows through this, it grabs onto the beads and leaves the undesirable cations behind.
The beads trade the cations for sodium ions that are naturally found in the resin and that are positively charged. The resulting soft water is quite effective. Over time, the resin will lose its ability to exchange ions and needs to be recharged. This can be done by flushing the resin with a concentrated solution of common salt.
A common concern with this type of system is that it will add too much sodium to your diet. In reality, however, the system only releases about 37 milligrams of sodium per quart of softened water. This is less than a typical serving of cheddar cheese, and far less than the amount of sodium typically found in processed foods. The softened water also has more potassium, which is important for proper health.
Water Purification
Using ion exchange in water treatment is an effective way to improve the quality of surface water. Ion exchange is a process whereby unwanted ions in water and wastewater are exchanged for other ions with the same charge, often in the form of salts (cations) or acids (anions). This process is used for both drinking and industrial water purification applications, such as deionisation, dealkalization and softening.
For example, a home water softening system uses ion exchange to remove the cations associated with hardness in water. As the water passes over the resin, these cations are replaced with desirable sodium ions. The result is softened, scale-free water suitable for household use and laundry. The resin must be regenerated regularly, with the undesirable ions flushed away and the desirable ions added back into the system.
Water softening is typically cheaper to maintain than other water purification technologies such as filtration, sedimentation, oxidation-reduction, sorption-desorption and biodegradation. It is also more reliable than other methods that rely on microbial degradation of contaminants, which may not be complete.
Ion exchange systems are not ideal for removing some contaminants, especially heavy metals. This can be due to the poor selectivity of ion exchange resins and the difficulties in separating ions of different types from one another. This can be overcome by specialised resins or other chemical treatments, which are more costly and complicated to implement.
Drinking Water Treatment
From the lake to your tap, water goes through many steps to become safe for us to consume. Water treatment is an ancient practice — coagulation via alum was used in 4000 BCE, and the first public water treatment plant was built in 1804. Yet improving drinking water is only recently moving beyond cosmetic concerns to pathogen removal.
Ion exchange can be a useful process for improving drinking water quality. When used for water softening, it can achieve high removal efficiencies for positively charged contaminants like calcium and magnesium that cause hardness. It can also remove the positively charged ions in barium, radium and strontium that are present in some water sources.
UVAOP is effective for reducing concentrations of organic micropollutants, such as 1,4-dioxane, N-nitrosodimethylamine (NDMA) and methyl tert-butyl ether (MTBE). It can also be useful for removing trace levels of disinfection byproduct precursors.
Storage is an effective method for reducing bacterial contamination in drinking water. The combination of increased temperature and several biological factors, including anaerobic conditions, reduces bacteria to varying extents, depending on the contaminant and season. However, it should be noted that the reduction of E. coli during storage does not always meet the regulatory standards of the Drinking Water Act. It is also important to note that bacterial counts can rebound after storage, especially if the water is reused for irrigation or for other purposes.
Industrial Applications
Ion exchange systems can be used in a number of industrial applications, including water purification, removing dissolved metals from aqueous solutions, and demineralization. They can also be used to remove contaminants from wastewater and for the separation of charged molecules from uncharged ones, such as proteins or nucleotides.
Ion exchange resins are insoluble, synthetic polymeric materials with ionic functional groups that can be exchanged for other ions. They are typically produced in the form of beads or membranes, with microporous compartments that provide a large surface area for ion exchange reactions to take place. They are available in a wide variety of sizes, shapes and pore structures. The resins are generally based on polystyrene and polyacrylate, although they may also be based on inorganic and natural organic materials.
Typically, the resins will be loaded with a specific set of ions, and when a sample of water is passed through them, the undesirable ions will be washed away by the resin while the desired ions are bound to it. The undesirable ions will then be released when the resin is regenerated.
Eventually, the resin beds will become depleted of desirable ions and need to be regenerated for continued use. This is usually done by flushing the resin bed with a salt brine solution that contains sodium ions. Regeneration of the resins will create wastewater that will need to be treated and disposed of.