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

Abstract

To promote the application of the electrolysis process for in situ pH adjustment of wastewaters with complex matrices, a robust, industrially scalable, and undivided electrolytic cell featuring the abstraction of H + from the anode boundary layer was developed in this study, where a tubular Ti porous membrane was used as the anode. Modeling, electrochemical analysis, and COMSOL simulation results show that abstracting H + from the anode boundary layer could effectively inhibit the transfer of H + into the bulk solution with the decrease in the anode double-layer thickness and electric double-layer capacitance. 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-migration of H + into the bulk solution and turbulence state near the anode surface, respectively. The H + –OH – separation performance showed a volcanic type change trend with the 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 alkaline effluents, 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 better H + –OH – separation performance than that of the widely studied membrane-based divided system. The feasibility of this undivided electrolytic cell was also validated by water softening experiments, where the Ca hardness removal efficiency was 42%–92% with energy consumption of 1.2–4.6 kWh (kgCaCO 3 ) −1 at a current density of 6–22 mA cm –2 . In general, this new undivided electrolytic cell opened a new pathway for in situ electrochemically regulating wastewater pH.

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