Deionization Resin (DI): How It Works - The Short Answer
Definition: Deionization (DI) is an ion‑exchange process that removes dissolved mineral ions using H⁺ cation and OH⁻ anion resins to produce very low‑conductivity, high‑resistivity water.
Deionization resin (DI resin) removes dissolved ions through ion exchange. A cation exchange resin in the H⁺ form swaps its hydrogen for cations such as calcium, magnesium, sodium, and iron. A anion exchange resin in the OH⁻ form swaps its hydroxide for anions such as chloride, nitrate, sulfate, bicarbonate, and silicate. The released H⁺ and OH⁻ combine to form H₂O, yielding very low conductivity, high‑resistivity water suitable for sensitive applications.
Key Takeaways at a Glance
Two‑stage ion exchange makes H₂O
Cation (H⁺) and anion (OH⁻) resins remove ions; H⁺ + OH⁻ combine to form water and drive resistivity up.
Performance hinges on capacity and EBCT
Selectivity, bed depth, and contact time control leakage and run length.
RO → DI with mixed‑bed polish
RO takes the load; DI polishes to ultra‑low conductivity. A mixed‑bed stage maximizes resistivity and purity.
Measure what matters
Monitor conductivity/resistivity (25 °C) and silica to time changeout; fix channeling and flow if quality drifts.
How Ion Exchange Works in Deionization
DI resins are tiny, porous polymer beads with fixed functional groups that carry charge. Water flows through a packed bed of these beads, and dissolved ions trade places with ions held on the resin. Two coordinated exchanges make the water itself the by‑product:
- Cation resin (H⁺ form): Negatively charged sulfonic sites (-SO₃⁻) hold H⁺. They exchange H⁺ for cations (Ca²⁺, Mg²⁺, Na⁺, Fe²⁺/Fe³⁺).
- Anion resin (OH⁻ form): Positively charged quaternary amine sites hold OH⁻. They exchange OH⁻ for anions (Cl⁻, NO₃⁻, SO₄²⁻, HCO₃⁻, silicate).
- Result: Released H⁺ and OH⁻ form H₂O, driving conductivity down and resistivity up (to ~18.2 MΩ·cm at 25 °C).
Exchange reactions (examples):
2R-SO₃H + Ca²⁺ ⇌ (R-SO₃)₂Ca + 2H⁺
R-N⁺(CH₃)₃OH⁻ + Cl⁻ ⇌ R-N⁺(CH₃)₃Cl⁻ + OH⁻
Capacity and selectivity: Each bead has a finite number of exchange sites (capacity, often in eq/L or kgr/ft³). Resins also prefer some ions over others (selectivity). The combination of capacity, selectivity, and contact time determines leakage and run length.
Selectivity, Leakage, and pH Behavior
- Silica and CO₂: Silica (as silicate) and dissolved CO₂ species are the toughest anions; SBA Type I resins minimize leakage. Degassing upstream reduces CO₂ load.
- Competitive ions: High concentrations of strongly held ions can displace weakly held ions, affecting breakthrough order.
- pH drift clues: If the cation stage exhausts first, effluent pH tends to drop; if the anion stage exhausts first, pH tends to rise. In mixed beds, this is less obvious—use meters.
Mass Transfer and Kinetics (Why EBCT Matters)
- Bed depth and EBCT: Deeper beds and longer empty bed contact time (EBCT) improve ion exchange efficiency and silica capture.
- Flow distribution: Proper distributors/screens prevent channeling so all resin sees equal flow.
- Temperature and viscosity: Colder water slows diffusion into beads; modestly warmer feeds (within spec) improve kinetics.
Cation Exchange: The First Half
Example (calcium removal):
2R-SO₃H + Ca²⁺ → (R-SO₃)₂Ca + 2H⁺
The resin captures Ca²⁺ and releases H⁺ into the water. The same principle applies to Mg²⁺, Na⁺, and other cations.
Anion Exchange: The Second Half
Example (chloride removal):
R-N⁺(CH₃)₃OH⁻ + Cl⁻ → R-N⁺(CH₃)₃Cl⁻ + OH⁻
The resin captures Cl⁻ and releases OH⁻, which pairs with H⁺ from the cation stage to form water.
Deionization Diagram: DI Process Visualized
Feed water containing NaCl passes cation resin (H⁺ form), exchanging Na⁺ for H⁺ to yield H⁺ + Cl⁻; then anion resin (OH⁻ form) exchanges Cl⁻ for OH⁻, and H⁺ + OH⁻ combine to form H₂O.
Resin Chemistry for DI (What Matters)
- Strong Acid Cation (SAC, H⁺ form): Sulfonic acid groups; removes hardness, sodium, and metals across a broad pH range.
- Strong Base Anion (SBA, OH⁻ form): Quaternary amines. Type I minimizes silica/CO₂ leakage; Type II offers higher capacity with more silica leakage.
- Weak resins (WAC/WBA): Useful in specialized trains (dealkalization/acid traps), but not typical for polishing to ultra‑low conductivity by themselves.
Design tip: For DI polishing, favor SAC + SBA Type I, adequate bed depth, and conservative flow.
DI Configurations (Focused on Exchange Efficiency)
- Two‑bed DI (separate vessels): Cation (H⁺) → Anion (OH⁻). Efficient bulk removal; easy on‑site regeneration. Add a mixed‑bed polisher for ultra‑low conductivity and lower silica leakage.
- Mixed‑bed DI: Intimately mixed SAC + SBA; each droplet repeatedly encounters both resins, creating a micro‑polishing effect and very high resistivity.
- EDI (electrodeionization): Continuous ion removal with an electric field after RO. Think of it as membrane‑assisted ion exchange between two‑bed and mixed‑bed in purity.
DI Process: Steps That Influence Exchange
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1
Pretreatment: Sediment + carbon protect resin; RO upstream removes 95-99% TDS to extend DI run length.
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2
Cation exchange (H⁺ form): Swap H⁺ for Ca²⁺, Mg²⁺, Na⁺ and other cations; effluent carries H⁺ as the primary cation.
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3
Anion exchange (OH⁻ form): Swap OH⁻ for Cl⁻, NO₃⁻, SO₄²⁻, HCO₃⁻, silica; H⁺ + OH⁻ → H₂O.
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4
Mixed‑bed polish (common): Repeated cation/anion encounters drive ultra‑low conductivity and minimize silica leakage.
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5
Final filtration: Fine filtration (e.g., 0.2 µm) protects instruments and loops from particulates.
Measuring Exchange Results: Conductivity, Resistivity, TDS
- Conductivity (µS/cm) / Resistivity (MΩ·cm): Inversely related; higher resistivity means purer water. Ultrapure DI water is ~18.2 MΩ·cm (≈0.055 µS/cm) at 25 °C.
- TDS meters: Convenient proxy for ion content; for critical work, measure resistivity at 25 °C and track silica as needed.
- CO₂ caveat: Dissolved CO₂ converts to bicarbonate/carbonate and can depress resistivity. Degassing or strong anion Type I helps when CO₂ is significant.
Best Practices to Optimize Ion Exchange
- Use RO + DI: RO removes the heavy load; DI lasts 10-20× longer compared to DI‑only on tap water.
- Control flow/contact time: Slower flow and proper bed depth improve exchange efficiency and silica capture.
- Materials compatibility: Ultrapure water is chemically aggressive—use stainless steel (304/316), PVDF/PFA, or high‑grade PP where appropriate; avoid brass/copper downstream of DI.
- Monitor quality, not just volume: Track conductivity/resistivity, and use dual‑point monitoring (before/after DI) in critical applications.
- Plan replacement/regeneration: Rising conductivity after DI indicates exhaustion. Color‑change resin is helpful but confirm with a meter.
Common Misconceptions About DI
- “DI water is distilled.” Distillation and deionization are different processes; both can produce very pure water, often used together with RO/EDI in high‑purity trains.
- “Softened water is the same as deionized.” Softeners exchange hardness for sodium or potassium; TDS changes little. DI removes ions entirely.
- “DI water is unsafe to touch.” DI is process water, not a beverage, but incidental contact isn’t inherently hazardous. It’s simply low in dissolved ions.
Factors That Drive Ion Exchange Performance
DI Resin FAQs
What is deionization resin?
It’s an ion exchange media that replaces dissolved ions with H⁺ and OH⁻, which form H₂O—producing very low conductivity water.
How does DI resin work with RO?
RO removes 95-99% of dissolved solids; DI removes the remaining ions to reach ultra‑low conductivity. RO + DI is the most cost‑effective path to ultrapure water.
How long will DI resin last?
It depends on feedwater TDS/chemistry, flow rate, and whether RO is upstream. RO → DI commonly extends resin life by 10-20× versus DI‑only.
When should I replace DI resin?
Replace when product water conductivity rises (or resistivity drops) after DI. Use meters; treat color‑change indicators as a convenience, not a standard.
How do I control silica leakage?
Use Type I strong base anion resin and maintain adequate contact time. A mixed‑bed polisher after two‑bed DI improves silica removal further.
Is DI water safe to drink?
DI water is intended as process water. It’s not formulated for taste or dietary minerals; follow your application’s guidelines.
Do DI filters need to be replaced frequently?
Not if they’re sized correctly and fed with RO. Changeout frequency depends on feed TDS/CO₂ and usage. Small mixed‑bed cartridges on RO permeate often run hundreds to a few thousand gallons; DI on tap water exhausts much faster. Track resistivity/TDS and replace when product water rises above your spec. For planning, use our DI capacity calculator.
Is demineralized water low in TDS?
Yes. Properly demineralized (DI) water is very low in TDS—often 0-10 ppm when DI follows RO, and near‑zero on a fresh mixed‑bed polisher. Note that handheld TDS meters infer from conductivity and dissolved CO₂ can affect readings; resistivity (MΩ·cm at 25 °C) is the most reliable metric.
About the Author
Written by a Crystal Quest water treatment specialist. We design, build, and support DI and RO/DI systems for homes, labs, and industry—bringing hands-on field experience to every recommendation in this guide.
