Boron vs silica in EDI
Two important species for semiconductor ultrapure water—boron and silica—behave very differently in EDI systems due to their underlying acid–base chemistry.
Ionization Behavior of Boron and Silica
Both boric acid and silicic acid are only weakly ionized at neutral pH:
- pKa of boric acid (B(OH)₃): ~9.2
- pKa of silicic acid (H₄SiO₄): ~9.8
In ultrapure water (pH =7.0), the predominant species are uncharged:
- Silica: Si(OH)₄ / H₄SiO₄
- Boron: B(OH)₃
However, the way these two acids become ionized at ion-exchange resin surfaces is fundamentally different.
Why Silica Ionizes at SBA Resin Surfaces
Silicic acid is a Brønsted–Lowry acid, meaning it can donate a proton. When uncharged silica encounters a strong-base anion (SBA) resin surface loaded with OH⁻ ions:
- Silicic acid donates a proton to the OH⁻ on the resin surface.
- It becomes negatively charged as H₃SiO₄⁻.
- The newly ionized silica is attracted to the quaternary ammonium sites on the SBA resin surface.
- Because H₃SiO₄⁻ remains charged, the EDI electric field can drive it toward the anode, enabling removal.
This mechanism explains why silica can be efficiently removed by EDI even though it is neutral in bulk solution.
Why Boron Does Not Behave the Same Way
Boric acid is not a Brønsted-Lowry acid—it cannot donate a proton. Instead, it behaves as a Lewis acid, meaning it must accept an OH⁻ to form its charged species:
- Uncharged boric acid must pull an OH⁻ ion into the molecule to form B(OH)₄⁻.
- This process is energetically and kinetically less favorable at resin surfaces compared with silica’s proton-donation mechanism.
- As a result, boron ionizes poorly at strong base anion resin surfaces, and adsorption is minimal.
- Boron can be chelated using boron-specific adsorbants.
Boron-specific resins, when used, rely on chelating, uncharged tertiary amine groups and are typically acrylic macroporous resins. The tertiary amine does not cause B(OH)₃ to become ionic. Further, because boron adsorbs deep inside the resin pores they cannot migrate effectively under the EDI voltage gradient. This prevents efficient transport toward the anode, limiting EDI removal.
Implications for Boron Removal
Due to these chemical differences, EDI has inherently higher silica removal potential but poor boron removal unless conditions are modified.
To improve boron removal, several approaches can be used:
Ultrapure water system optimization for boron removal
2-Pass RO with interstage pH > 9.5, EDI, boron-polisher
Increase pH to ionize B(OH)₃ before the EDI
Raising pH above ~9.2 shifts boron into its charged form B(OH)₄⁻, enabling SBA adsorption and electric-field–driven migration.
Develop improved resins
A next-generation resin combining quaternary ammonium (strong-base) and tertiary amine (chelating) functionality in a gel matrix could allow surface-level boron adsorption while still providing a mechanism for B(OH)₃ ionization and sufficient mobility for EDI transport.
Summary:
Boron and silica are chemically different, so behave differently with EDI and ion exchange.