Ion Exchange (engineering)
Ion exchange is a reversible chemical process in which ions of the same charge are exchanged between a solution and an insoluble solid ion-exchange resin, allowing selective removal, concentration, or substitution of specific ions. Cation exchange resins remove positively charged ions (e.g., Ca²⁺, Mg²⁺, heavy metals) while anion exchange resins target negatively charged species (e.g., NO₃⁻, SO₄²⁻). Ion exchange is fundamental to water softening, demineralization, pharmaceutical purification, and nuclear waste treatment.
Key Formula
K_A^B = ([B̄][A]) / ([Ā][B])
LaTeX: K_{A}^{B} = \frac{[\bar{B}][A]}{[\bar{A}][B]}
| Symbol | Meaning | Unit |
|---|---|---|
| K_A^B | Selectivity coefficient of ion B relative to A | dimensionless |
| [B̄] | Concentration of ion B in resin phase | meq/g |
| [A] | Concentration of ion A in solution phase | meq/L |
| [Ā] | Concentration of ion A in resin phase | meq/g |
| [B] | Concentration of ion B in solution phase | meq/L |
Worked Example
Problem
A cation exchange resin is used to soften water by replacing Ca²⁺ with Na⁺. The selectivity coefficient K_Na^Ca = 2.5. If the resin contains 0.8 meq/g Ca²⁺ and 0.2 meq/g Na⁺, and the solution has 4 meq/L Na⁺, calculate the equilibrium Ca²⁺ concentration in solution.
Solution
Step 1: Rearrange the selectivity equation: [Ca²⁺]solution = ([Ca²⁺]resin × [Na⁺]solution) / (K × [Na⁺]resin). Step 2: Substitute: [Ca²⁺] = (0.8 × 4) / (2.5 × 0.2) = 3.2 / 0.5 = 6.4 meq/L.
Answer
Equilibrium Ca²⁺ concentration in solution = 6.4 meq/L, showing the resin preferentially holds calcium.
Types of Ion Exchange Resins and Their Properties
| Resin Type | Functional Group | Target Ions | Regenerant |
|---|---|---|---|
| Strong acid cation (SAC) | –SO₃H | Ca²⁺, Mg²⁺, Na⁺, K⁺ | HCl or H₂SO₄ |
| Weak acid cation (WAC) | –COOH | Hardness ions, heavy metals | HCl or H₂SO₄ |
| Strong base anion (SBA) | –N(CH₃)₃⁺ | Cl⁻, SO₄²⁻, NO₃⁻, SiO₂ | NaOH |
| Weak base anion (WBA) | –NHR₂ | Strong acid anions | NH₃ or Na₂CO₃ |
| Chelating resin | –Iminodiacetic acid | Heavy metals (Pb²⁺, Cu²⁺) | Acid or base |
Interactive Tools
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Related Terms
Engineering Adsorption
Engineering adsorption is a surface-based separation process in which molecules (adsorbate) from a fluid phase adhere to the surface of a solid material (adsorbent) via physical or chemical interactions, enabling removal or recovery of target species from gases or liquids. It is used industrially for air purification, solvent recovery, water treatment, and chromatographic separation. The process is characterized by equilibrium isotherms and mass transfer kinetics, with the adsorbent regenerated by temperature or pressure changes to allow repeated cycles.
Membrane Separation
Membrane separation is a process in which a semi-permeable membrane selectively allows certain molecules or ions to pass through while retaining others, driven by a concentration, pressure, or electrical potential gradient. Common forms include reverse osmosis, nanofiltration, ultrafiltration, microfiltration, and pervaporation, each distinguished by the size range of species separated. Membrane processes are highly energy-efficient alternatives to thermal separation methods and are critical in water purification, food processing, and pharmaceutical applications.
Industrial Filtration
Industrial filtration is a mechanical separation process that removes solid particles from a liquid or gas stream by passing the mixture through a porous medium that retains the solids (filter cake) while allowing the fluid (filtrate) to pass through. It is fundamental to chemical processing, wastewater treatment, food and beverage production, and pharmaceutical manufacturing. The efficiency of filtration depends on particle size, filter medium properties, applied pressure, and the characteristics of the slurry.
From Greek "ion" (going), the present participle of "ienai" (to go), coined by Michael Faraday in 1834 for electrically charged particles. "Exchange" from Latin "excambiare" (to barter). Ion exchange resins were first synthesized by Adams and Holmes in 1935.