Electromembrane separation coupled with urea electrolysis for energy-saving recovery of quaternary ammonium hydroxide from industrial wastewater

The electrolytic recovery of quaternary ammonium hydroxide (R₄NOH) from the wastewater produced in the process of template-synthesizing molecular sieves has considerable economic benefits. The current practices are challenged by the anode corrosion and high operation costs due to harmful chlorine chemicals in wastewater and high energy consumption. This work proposes a promising energy-saving strategy by combining the recovery of quaternary ammonium hydroxides with urea electrolysis technology. This strategy reduces the anodic oxidation potential and achieves low-energy consumption while avoiding the formation of hypochlorite ions. Through the laboratory-scale closed membrane electrolysis system (CMES) cycle test, it was found that when the cathode output concentration of 0.5 M was reached at a current density of 50 mA cm⁻², the energy consumption was saved by 64.7% compared with the traditional technology. Along with the achievement of the expected recovery rate, the high value-added R₄NOH were recycled. More specifically, the ultra-low voltage of the electrolyze protected the ecosystem by avoiding the corrosion of harmful chlorine chemicals and demonstrated the potential for large-scale applications at the reduced process costs.