Polyelectrolyte nanofiltration membranes for base separation and recovery

Area of Science:

• Materials Science
• Chemical Engineering
• Environmental Science

Background:

• Nanofiltration (NF) membranes are crucial for resource recovery, particularly in separating monovalent/divalent ions.
• Conventional polyamide NF membranes lack stability in extreme alkaline conditions, hindering applications like base separation.
• Base separation is vital for extracting multivalent anions (carbonates, phosphates) from hydroxide-rich streams.

Purpose of the Study:

• To develop and evaluate alkaline-resistant polyelectrolyte membranes for effective hydroxide/carbonate and hydroxide/phosphate separation.
• To assess the stability of these novel membranes under prolonged exposure to highly alkaline conditions.
• To compare the performance and stability against commercial polyamide NF membranes.

Main Methods:

• Fabrication of polyelectrolyte membranes using alternating layers of poly(diallyl dimethylammonium chloride) (PDADMAC) and poly(sodium 4-styrenesulfonate) (PSS) on a polyethersulfone substrate.
• Testing membrane performance for hydroxide/carbonate and hydroxide/phosphate separation at varying pH and concentrations.
• Conducting long-term stability tests in pH 13 KOH solutions.

Main Results:

• Higher feed solution pH enhanced carbonate and phosphate rejection by promoting ion deprotonation and electrostatic repulsion.
• Increased carbonate and phosphate concentrations reduced rejection due to charge screening.
• A six-bilayer PDADMAC/PSS membrane achieved >99% removal of carbonates and phosphates with significant hydroxide passage at pH 13.
• PDADMAC/PSS membranes maintained excellent ion selectivity for four weeks in pH 13 KOH, unlike commercial polyamide membranes that degraded within one week.

Conclusions:

• PDADMAC/PSS polyelectrolyte membranes demonstrate exceptional stability and selectivity in highly alkaline environments.
• These membranes offer a durable and effective solution for critical resource recovery applications involving base separation.
• The findings pave the way for advanced separation technologies in challenging industrial conditions.