Decoupled two-stage electrolysis for copper and persulfate recovery from micro-etching wastewater: Process...

Simultaneous copper recovery and sodium persulfate (SPS) regeneration from spent printed circuit board (PCB) micro-etching wastewater present significant challenges due to the intrinsic incompatibility of their operational windows. To break this physical bottleneck, a two-stage integrated electrolysis system was developed to functionally decouple bulk copper deposition from SPS regeneration. Specifically, Stage I removes the bulk of dissolved Cu²⁺ before the electrolyte enters Stage Ⅱ, thereby mitigating late-stage dendritic growth and reducing Cu²⁺-related transport constraints during BDD-mediated SPS regeneration. EPR and radical-quenching experiments supported a radical-mediated sulfate-oxidation pathway involving BDD-generated ·OH. Unconstrained optimization via response surface methodology (RSM) yielded an SPS production of 99.2 ± 1.82 g/L with a current efficiency of 68.2 ± 1.23%. To reconcile theoretical kinetics with practical semi-continuous mass-balance requirements, a constrained optimization strategy was implemented, securing a net SPS increment >35 g/L with 79.20% current efficiency. In situ electrochemical impedance spectroscopy (EIS) revealed a V-shaped evolution in apparent interfacial resistance, consistent with the late-stage energy penalty associated with concentration polarization. Ultimately, compared with standalone membrane electrolysis (37.22 kWh/m³), the integrated strategy reduced SEC_V to 29.85 kWh/m³, corresponding to a 19.8% reduction in electrolysis-cell electrical energy consumption (auxiliary circulation energy excluded). This work provides an engineering framework toward closed-loop wastewater recycling, linking interfacial electrochemical behavior to process-level energy optimization.