Mechanism investigation and optimisation of redox-flow desalination of highly saline source waters

Redox-flow desalination (RFD) is a promising alternative to reverse osmosis (RO) for treatment of highly saline waters though the desalination mechanism(s) and optimal operating conditions remain insufficiently understood. Here, we systematically optimize a four-chamber RFD cell for feed NaCl brines up to 36 g L −1 and develop a dynamic one-dimensional model to resolve coupled ion transport, redox kinetics, and water fluxes under constant-voltage operation. Increasing feed salinity enhanced electrical conductivity, reduced ohmic losses, and increased average salt separation rates (ASSRs), achieving a maximum of 1124.4 μg min −1 cm −2 at 36 g L −1 . However, ASSR increase became non-linear as the desalination transitioned from ohmic- to redox kinetics-limited regimes. Optimized redox electrolyte flow rates alleviated mass-transfer limitations and maintained high current efficiency under a low voltage (0.6 V), while brine flow rates and ion-exchange resins (IERs) offered limited benefits at high salinity. Comparative testing of K 3 [Fe(CN) 6 ]/K 4 [Fe(CN) 6 ], Fe 3+ / 2+ –DTPA, and BTMAP–Fc + /Fc identified BTMAP–Fc as a chemically stable, low-toxicity mediator which produced stable desalination with energy consumption (8.8 kWh m −3 ) comparable to that of ferri/ferrocyanide (8.3 kWh m −3 ) at seawater-level salinity. Osmotic and electro-osmotic water transport increased with salinity, leading to 15.3% water loss from the diluate, highlighting the need for low-permeability ion-exchange membranes and optimized operation control. Overall, this study optimizes RFD systems for treating highly saline waters and provides mechanistic insights with implications for redox mediator selection, mass transfer kinetics and water transport for sustainable concentrate management. • Optimized redox flow desalination (RFD) achieved high salt separation rates. • 1D model captured performance for different influent salinities and redox mediators. • Higher electrolyte flow reduced mass-transfer limits at low voltage (0.6 V). • RFD efficiency benefited from multi-stage process incorporating ion-exchange resin. • BTMAP–Fc enabled stable, low-toxicity desalination performance at ~8.8 kWh m −3 .