Oxidative Tuning of a Commercial SWRO Membrane toward Low-Salt-Rejection RO for Brine Concentration: Pristine-Coupon Experiments, Practical Limitations, and System-Scale Modeling

Motivated by circular-economy reuse of end-of-life (EoL) SWRO elements for brine concentration, this study investigates oxidative tuning of a commercial seawater reverse osmosis (SWRO) polyamide membrane toward low-salt-rejection reverse osmosis (LSRRO) operation. To isolate intrinsic oxidation–transport relationships from uncontrolled field variability, the modification and transport characterization were intentionally conducted on pristine flat-sheet coupons cut from a commercial spiral-wound element. Accordingly, bulk oxidation doses derived here should not be interpreted as direct prescriptions for EoL elements, where foulant oxidant demand and polymer aging can strongly alter chlorine consumption and reaction kinetics. Two oxidation protocols were evaluated: static soaking and pressurized dead-end filtration oxidation. For filtration oxidation, hypochlorite rejection and concentration polarization (CP) can elevate the oxidant concentration at the membrane surface; therefore, CT bulk and CV bulk represent nominal bulk exposures rather than directly measured surface doses. Oxidation increased water permeance modestly but increased salt permeability by orders of magnitude, enabling tunable low-rejection behavior. However, anomalous cases of high salt leakage with limited water flux suggest that porous-support integrity (compaction/pore blockage) can become rate-limiting under harsh oxidation and high-salinity operation. A system-scale LSRRO model was updated to compute osmotic pressure using a non-ideal, water-activity formulation (including Pitzer-type electrolyte activity corrections for high-salinity brines) and to represent salinity-dependent effective salt transport (Donnan screening). Specific energy consumption (SEC) was compared only under identical separation constraints (seawater range permeate target for SWRO recycle), and the tuned-RO window was benchmarked against representative commercial nanofiltration (NF) membranes. The results define practical operating windows and key validation steps required for translating oxidative downcycling to real EoL elements.