Silica polymerizes. Boron does not.

In IX and EDI systems, silica can become a fouling and scaling problem due to polymerization. Boron, on the other hand, remains soluble and non-polymeric—but is harder to remove because it stays neutral in typical operating conditions.

Silica vs. Boron: Two Very Different Behaviors

Silica: Highly Polymerizable

Silica enters water mainly as silicic acid (Si(OH)₄).
At higher concentrations or at neutral-to-high pH, silica begins to polymerize:

• Si–O–Si bonds form between molecules

• Small dimers become long-chain polymers

• Polymers become colloidal silica

• Eventually a gel-like solid forms, creating silica scale

This polymerization is why silica is one of the most notorious foulants in RO, IX, and EDI systems.

Boron: Does Not Polymerize

Boron enters water primarily as boric acid (B(OH)₃), which:

• Is nonionic at neutral pH

• Does not self-condense

• Does not form colloids or gels

• Stays fully dissolved, even at high concentrations

At higher pH, boric acid converts to borate (B(OH)₄⁻), but even then it only forms small, well-defined species. These are not polymers and do not lead to scaling.

What Happens in IX and EDI Systems?

Silica in IX/EDI

• Strong-base anion resins remove silica effectively

• But once silica polymerizes, removal efficiency drops sharply

• Polymeric silica can foul IX resins and EDI stacks

• In EDI, polymeric silica can deposit on membranes and spacers

Boron in IX/EDI

• Boron does not polymerize, so it does not form fouling colloids

• The challenge is not scaling—it’s removal efficiency

• Boric acid is neutral, so conventional IX resins do not capture it well

• In EDI, boron removal is limited unless pH is elevated to form borate (ionic)

Thus, boron tends to pass through IX and EDI, not because it polymerizes, but because its chemistry makes it hard for resins and membranes to capture.

Why Do Engineers Think Boron Behaves Like Silica?

Because both species:

• Are present in natural waters

• Are weak acids

• Are poorly rejected by RO at neutral pH

But chemically, they are entirely different:

Practical Takeaways for Ultrapure Water System Designers

Good news:

You don’t need to worry about boron forming colloids or plugging IX or EDI systems.

But:

Boron is difficult to remove unless the pH is raised. This is why:

• High-pH RO (two-pass RO with pH adjustment) is standard for boron removal

• Specialized boron-selective resins exist, but have limited capacity

• EDI will not significantly remove boron at typical feed pH values

Conclusion

Silica polymerizes. Boron does not.

In IX and EDI systems, silica can become a fouling and scaling problem due to polymerization. Boron, on the other hand, remains soluble and non-polymeric—but is harder to remove because it stays neutral in typical operating conditions.

If your system struggles with either silica or boron, the required solutions are completely different. Understanding these chemical differences is the key to long-term, trouble-free operation.