Introduction: Applications and Fundamental Principles

Applications of Ion Exchange:

Today, there are many different industrial applications of ion exchangers, mostly for purification. Some examples are:

Water and Waste Treatment

Softening (removal of hardness), demineralizing, silica removal, and alkalinity reduction

Removal of cations and anions from boiler feeds

Deionizing water

Treatment of trade effluents. Standards for ten states

Purification

Recovery of organic and inorganic substances

Separation of ion mixtures

Fundamental principles

First, the exchanger must have a balance of charges and interaction of ions. In addition, the selectivity of each ion is the basis for development of an exchanger. The Donnan Exclusion allows for acceptance of a predominating ion. This comes from the ability of the resin to exclude one ion over another. Also, there is a screening affect that takes place that rejects large ions or polymers. Moreover, the transport of a substance through a column is a reflection of the affinity of the ions present. Further fundamental principles involve other influences on mobility such as swelling and surface area.

While there are numerous functional groups that have charge, only a few are commonly used for man-made ion exchange resins. These are:

-COOH which is weakly ionized to -COO¯
-SO₃H which is strongly ionized to -SO₃¯
weak anionic - primary amines: -NH₂ => -NH₃⁺, secondary amines: -RNH => -RNH⁺, and tertiary amines: -R₂N => -R₂N⁺
-NR₃⁺ that has a strong, permanent charge. (R stands for some organic group)

These groups are sufficient to allow selection of a resin with either weak or strong positive or negative charge.

These functional groups are on a polymeric backbone. Making ion exchange resins with styrene chemistry

Relative affinities of various ions

The affinity of an ion for a charged site on the resin depends on the hydrated radius of the ion. As this radius is smaller for ions with greater charge, the relative affinities depend on the charge or valence as shown:

single-charged ion < ion with 2 charges < ion with 3 charges < multi-charged ion

This is ideal for most practical situations because we can prepare a resin in the form of a monovalent ion such as Na⁺ or H⁺ and exchange for an ion such as Ca⁺⁺, Mg⁺⁺, or Fe⁺⁺⁺ and have the relative affinity in our favor.