Anode Boundary Layer Extraction Strategy for H⁺-OH^- Separation in Undivided Electrolytic Cell: Modeling, Electrochemical Analysis, and Water Softening Application

To promote the application of the electrolysis processfor in situpH adjustment of wastewaters with complex matrices, a robust, industriallyscalable, and undivided electrolytic cell featuring the abstractionof H+ from the anode boundary layer was developed in thisstudy, where a tubular Ti porous membrane was used as the anode. Modeling,electrochemical analysis, and COMSOL simulation results show thatabstracting H+ from the anode boundary layer could effectivelyinhibit the transfer of H+ into the bulk solution withthe decrease in the anode double-layer thickness and electric double-layercapacitance. Increasing the current density from 6 to 22 mA cm(-2) and the influent rate from 330 to 11,100 mL min(-1) deteriorated the H+-OH- separation performance, owing to the enhanced electro-migrationof H+ into the bulk solution and turbulence state nearthe anode surface, respectively. The H+-OH- separation performance showed a volcanic type change trend withthe acid extraction rate with the optimal value at 50 mL min(-1), specifically the separation efficiency of 10%-72% and 16%-94%for H+ and OH- in the acidic and alkalineeffluents, respectively, at an influent rate of 500 mL min(-1) and a current density of 6-22 mA cm(-2).Importantly, the present undivided electrolytic cell showed betterH(+)-OH- separation performancethan that of the widely studied membrane-based divided system. Thefeasibility of this undivided electrolytic cell was also validatedby water softening experiments, where the Ca hardness removal efficiencywas 42%-92% with energy consumption of 1.2-4.6 kWh (kgCaCO(3))(-1) at a current density of 6-22mA cm(-2). In general, this new undivided electrolyticcell opened a new pathway for in situ electrochemically regulatingwastewater pH.