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Mechanism of Silica Removal in Electrodeionization (Updated 2026)

ABSTRACT

Electrodeionization (EDI) is a continuous separation process that uses a DC electric field to remove ionic species from aqueous streams, most commonly high purity water. Silica removal is critically important in both power generation (boiler makeup water) and semiconductor manufacturing. However, EDI operates through charge transport, while silica (SiO₂) exists primarily as an uncharged species in neutral water. The removal mechanism is therefore not immediately intuitive.

This tech brief introduces the Electropure EDI technical model, and how to improve silica and boron removal.

An operating EDI module can be described as having three functional regions. The Working Zone (Zone 1) removes the majority of charged ions. The Intermediate Zone (Zone 2) primarily removes bicarbonate species derived from dissolved carbon dioxide. The Polishing Zone (Zone 3) is where silica and boron become ionized and can be transported out of the product stream.

This paper clarifies the physical-chemical mechanism responsible for silica removal and proposes operational strategies to improve performance in applications requiring extremely low silica concentrations. Recommendations are also provided for regions with naturally high-silica feedwaters such as Mexico and Japan.

INTRODUCTION

Natural waters typically contain 5–110 ppm (mg/L) silica. At low concentrations, conventional treatment processes — softening, filtration, reverse osmosis (RO), and EDI — are effective. At elevated concentrations (>30 mg/L), more careful optimization is required to both prevent scaling and maximize removal efficiency.

High-pressure boilers generally require <10 ppb (µg/L) silica, while semiconductor manufacturing may require concentrations as low as 0.1 µg/L (100 ppt). Achieving these specifications requires greater than 99.99% silica removal from feedwater.

SILICA CHEMISTRY

Silica in industrial waters exists in two principal forms:

  • Reactive (monomeric) silica

  • Unreactive (colloidal or polymerized) silica

At neutral pH, monomeric silica exists primarily as silicic acid: H4SiO4

Ionization occurs according to:

H4SiO4 ⇌ H++H3SiO4−

Because the pKa of silicic acid ≈ 9.7, only a very small fraction is ionized at neutral pH.

Colloidal silica and mixed silicate species are effectively removed by filtration and RO through size exclusion. However, dissolved monomeric silica may pass through RO membranes and must be removed downstream via ion exchange or EDI or specific adsorbents. Since it is uncharged, the EDI removal mechanism is not obvious.

RO REJECTION MECHANISM

RO rejects silica through two mechanisms:

  1. Size exclusion — nearly complete rejection of colloidal silica

  2. Charge and molecular transport — partial rejection of dissolved monomeric silica

Commercial RO membranes typically are 95% to 99.5% silica rejection, although the silica speciation is rarely defined.

EDI MECHANISM

EDI continuously removes ions using ion exchange resins positioned between ion-selective membranes under a DC potential. Ions transfer from the aqueous phase to resin sites, migrate across membranes, and are removed in a concentrate stream.

Anions migrate toward the anode, cations toward the cathode, producing purified water.

This Electropure™ EDI model divides the module into three functional zones with distinct chemistry.

ZONE 1 — WORKING BED

Most conductivity is removed here (Na⁺, Cl⁻ equivalents from RO permeate).

Anion resins are primarily in the chloride form and transport chloride ions across the membrane. Cation resins similarly transport sodium ions.

Carbon dioxide and bicarbonate are weakly retained because:

  • Bicarbonate selectivity is ~¼ that of chloride

  • CO₂ exists partly as uncharged carbonic acid

Therefore CO₂/HCO₃⁻ and molecular silica pass through Zone 1.

ZONE 2 — CO₂ REMOVAL BED

With primary ions removed, resin surfaces shift to H⁺ and OH⁻ forms.

Bicarbonate now adsorbs strongly (up to 24× higher selectivity) onto hydroxide-form anion resin. Removing bicarbonate shifts equilibrium and pulls dissolved CO₂ out of solution.

Silica and boron remain neutral and pass into Zone 3.

ZONE 3 — POLISHING BED

Here a different mechanism dominates: water splitting.

WATER SPLITTING

Under high electric field gradients (≈1–2 V/mm), water dissociates at resin junctions:

H2O → H+ + OH−

These ions are continuously removed, sustaining localized extreme pH conditions (≈1 and ≈13).

Effects:

  1. Continuous regeneration of resin surfaces

  2. Bacteriostatic environment

  3. Creation of conditions necessary for silica and boron ionization

SILICA REMOVAL MECHANISM

At hydroxide-form resin surfaces:

H4SiO4 + OH− → H3SiO4− + H2O

The resin extracts a proton, converting neutral silicic acid into the anionic species.

The newly formed silicate ion adsorbs to the resin and migrates toward the EDI anode where it exits via the concentrate stream.

Efficient removal therefore requires:

  1. Water splitting

  2. High OH⁻ surface concentration

  3. Balanced adsorption strength allowing migration

SILICA FOULING

Silica naturally polymerizes at neutral pH: (SiO2)n

If voltage is insufficient, removal slows, surface silica concentration rises, and polysilica form deposits on resins — commonly observed in high-silica regions of the world.

Adequate voltage maintains high pH surfaces and keeps silica in the mobile ionic form.

SUMMARY

The Electropure™ 3-Zone model describes EDI as:

  • Zone 1: Removal of primary ionic conductivity

  • Zone 2: Removal of CO₂/bicarbonate

  • Zone 3: Ionization and removal of silica and boron

Silica becomes removable only after conversion to an anion through hydroxide-mediated proton abstraction, driven by water splitting.

Optimizing silica removal requires:

Silica fouling is prevented by minimizing feed silica concentration, and maintaining sufficient electric field strength.

Boron removal follows the same mechanism due to parallel aqueous chemistry.

  1. Bates, Wayne (2001) “RO Water Chemistry”, Hydranautics Membrane Corp. technical paper posted on website, http://www.hydranautics.com/docs/papers/04_ro_water_chemistry.pdf
  2. Meyers, Peter (1999) “Behavior of Silica in ion Exchange and Other Systems” ResinTech, Inc., IWC-99-64
  3. Balazs Analytical (2007), “Ultrapure Water Monitoring Guidelines” Revision 2.0
  4. Ning, Robert Y. (2002), “Discussion of silica speciation, fouling, control, and maximum reduction”, Elsevier Science B.V.