Identifiability Limits of Forward-Osmosis Membrane Transport from Water and Reverse-Solute Flux Observations

Forward osmosis (FO) membrane performance is commonly described through water flux Jw, reverse-solute flux Js, and the specific reverse-solute ratio RJ = Js/Jw. These observables provide a low-dimensional projection of a coupled transport system where selective-layer permeability, support resistance, concentration polarization, and defect-mediated leakage interact, producing non-unique links between membrane organization and measured performance. The objective of this study is to determine to what extent the commonly reported FO observables Jw, Js, and RJ can constrain the underlying transport mechanisms, and to quantify the residual ambiguity when different combinations of selective-layer permeability, support resistance, concentration polarization, and defect-mediated leakage remain compatible with the same measured response. To address this objective, the study develops an admissible-inference framework in the experimentally accessible (Jw, RJ) plane, where published FO measurements are analyzed through observable-space mapping, concentration-polarization amplification, covariance-rank analysis, Fisher/SVD sensitivity diagnostics, and admissible-volume mapping. The resulting framework determines which transport interpretations remain compatible with the observations and which mechanistic directions require additional independent measurements to be resolved.