Inverse electrochemical probing of thermodynamic activity in hypersaline brines using reverse electrodialysis

Hypersaline electrolytes are difficult to characterize because strong non-ideality decouples concentration from chemical potential, while conventional electrochemical sensors often fail at high ionic strength. Here we introduce a model-based framework that treats reverse electrodialysis (RED) as an electrochemical transducer for inverse thermodynamic probing: deviations of stack open-circuit voltage (OCV) from an ideal-solution Nernst baseline encode the non-ideal chemical-potential state of complex brines. We couple a permselectivity-corrected RED electrochemical model with multi-ion Pitzer thermodynamics and perform seasonal simulations for hypersaline brine–treated wastewater systems representative of inland saline environments. Systematic OCV deviations define a measurable residual voltage, ΔEγ, that isolates activity-coefficient contributions and constrains mean ionic activity-coefficient contrasts between the concentrated and dilute streams. Using the resulting mapping between ΔEγ and γ±, we show that OCV measurements can, in principle, be inverted to infer effective NaCl-equivalent activity (thermodynamic salinity) in real time. Despite large seasonal changes in composition and total salt load, hypersaline brines exhibit constrained effective activities of 1.00–1.11 mol kg ⁻¹ , substantially lower than implied by concentration-only assumptions. This proof-of-concept establishes, for the first time, a direct mapping between electrochemical voltage observables (ΔEγ) and reduced thermodynamic descriptors governing activity-coefficient contrasts (γ _± ), positioning RED/ED stacks as electrochemical transducers for activity-based sensing and diagnostics in non-ideal, multi-ionic electrolytes, with implications for hypersaline brine geochemistry and management.