Ion Exchange

Figure 1: Typical ion exchange configuration.

Included Processes: Greensand Filtration

Ion exchange (IX) processes are reversible chemical reactions for removing dissolved ions from solution and replacing them with other similarly charged ions. In water treatment, it is primarily used for softening where calcium and magnesium ions are removed from water; however, it is being used more frequently for the removal of other dissolved ionic species.

In a cation exchange process, positively charged ions on the surface of the IX resin are exchanged with positively charged ions available on the resin surface - typically sodium. Water softening is the most widely used cation exchange process. Similarly, in anion exchange negatively charged ions are exchanged with negatively charged ions on the resin surface - typically chloride. Contaminants such as nitrate, fluoride, sulfate, and arsenic, as well as others, can all be removed by anion exchange.

The exchange medium consists of a solid phase of naturally occurring materials (zeolites) or a synthetic resin having a mobile ion attached to an immobile functional acid or base group. Both anion and cation resins are produced from the same basic organic polymers but they differ in the functional group attached to the resin. The mobile ions are exchanged with solute ions having a stronger affinity to the functional group (e.g. calcium ion replaces sodium ion or sulfate ion replaces chloride ion).

When the capacity of the resin is exhausted, it is necessary to regenerate the resin using a saturated solution to restore the capacity of the resin and return the resin to its initial condition. Brine, or sodium chloride solution, is most the commonly used regenerant, although others, such as strong acids (hydrochloric acid, sulfuric acid) or strong bases (sodium hydroxide) may also be used.

Manganese greensand is also used for IX processes.  Greensand is processed to form a thin layer of manganese dioxide on the outer surface of the sand.  This layer adsorbs and catalyzes the oxidation of iron, manganese, and other dissolved ionic species such as arsenic and radium.  In order for the media to retain its oxidation/adsorption properties, it much be regenerated with permanganate or chlorine.

IX processes can be operated in a batch or continuous mode. Most water treatment IX processes operate in a continuous mode. Continuous ion exchange processes are usually of the down-flow and packed-bed column type. When the resin capacity is exhausted, it is regenerated.

Competition for ion exchange sites on a resin can greatly impact a given system¿s efficiency in removing contaminants. Generally, ions with higher valence, greater atomic weights and smaller radii are said to have a greater affinity for (be preferred by) IX resins. Relative affinities of common ions are:

Ag⁺>Cs⁺>K⁺>Na⁺>Li⁺

Ba⁺²>Sr⁺²>Ca⁺²>Mg⁺²

I^->NO₃^->CN^-->HSO₄^->NO₂^->Cl^->HCO₃^-

Those substances with high affinities can continue to load to higher concentrations on the resins by displacing other previously exchanged potentially regulated ions with lower relative affinities. This is referred as chromatographic peaking.

The effectiveness of IX processes can also be affected by scaling of minerals, chemical precipitants, and surface clogging all of which leads to resin fouling. In order to reduce such occurrences, appropriate pretreatment measures such as filtration of suspended solids, or addition of chemicals to reduce scaling may be practiced.

The primary residual generated by IX processes is the spent regenerant. The spent regenerant will have very high total dissolved solids (TDS) concentrations, as it will include all of the ions removed by the resin as well as the excess regenerant ions (e.g., Na⁺, Cl^-). Disposal of the spent regenerant most frequently will require discharge to a wastewater treatment facility. Depending on the TDS and other contaminant concentrations in the spent regenerant, it may be necessary to evaluate the impacts on wastewater treatment plant discharges and National Pollutant Discharge Elimination System (NPDES) requirements.