Rejection Retention as a Measure of Chlorine Resistance: Initial-Rejection Bias in Filler-Modified Poly(vinyl alcohol) Reverse Osmosis Membranes

Chlorine resistance of reverse osmosis membranes is usually reported as the fraction of salt rejection retained after chlorination, a metric that depends on the initial rejection. This secondary reanalysis quantifies the dependence for published data on substrate-free poly(vinyl alcohol) (PVA) membranes containing multi-walled carbon nanotubes (MWCNT), Gum Arabic (GA) or graphene oxide (GO). Within the solution–diffusion model, the change in salt passage was expressed as an observed ratio, kR, which also includes any change in flux or swelling and is not a direct measure of chemical degradation. Judged by rejection retention, all three fillers reduced the relative rejection loss about tenfold at their best-performing loadings. Relative to each study’s control, however, GO produced no consistent reduction in kR across loadings (3.11 ± 0.48 versus 3.20); MWCNT produced a moderate reduction (1.16–1.49 versus 1.76) and GA a reduction only at 0.1–0.5 wt%; these differences persisted, marginally for MWCNT, at rejection uncertainties of two percentage points. For GO and GA, most or all of the apparent improvement (97–111% in the more robust decomposition order) reflected the higher initial rejection. Rejection retention therefore overstates the benefit of fillers that raise selectivity; the salt-passage ratio offers a simple correction when post-exposure flux is unavailable.