Membrane Recycling: Exploring Ozone as a Viable Alternative to Chlorine for Polymeric Membrane Transformation

Ozone, a strong oxidant, induces oxidative degradation in various materials and is known as an effective chemical for polymer modification. This study assesses ozone as an alternative to chlorine oxidation for converting end-of-life reverse osmosis membranes into nanofiltration- and ultrafiltration-like membranes across various new and used reverse osmosis and nanofiltration membranes. Membranes were characterized in terms of permeability and salt rejection, as well as surface characterization. Experiments were conducted at high ozone exposure (20 ppm) and low ozone exposure (3 ppm). At high exposure, ozone was found to degrade both the polyamide (PA) and polysulfone (PSf) layers, opening new possibilities for polyester (backing layer) recycling. At low exposure, degradation was limited to the PA layer; ozone converted membranes more effectively than chlorine, achieving similar performance in less time and at lower doses75 and 225 L·m^-2·h^-1·bar^-1 for SW and BW membranes after 30 min at 3 ppm ozone, comparable to 6000 ppm chlorine over 50 h. Ozone significantly impacted NF90, raising the permeability to 150 L·m^-2·h^-1·bar^-1 in 15 min at 3 ppm, while NF270 remained more resistant at 35 L·m^-2·h^-1·bar^-1. Ozone caused patchy degradation due to bubble interactions, while chlorine led to uniform attack. These findings highlight ozone's potential as a viable and more sustainable alternative to chlorine for polymeric membrane transformation.