Elucidating an unrecognized iron leaching mechanism via sequential reduction-oxidation of passivation layer on...

Low-cost stainless-steel cathodes enable efficient electrochemical water purification by activating H₂O₂ into •OH radicals, yet the fundamental mechanisms governing the fate of the cathodes as well as their roles in contaminant removal remain unresolved. By modulating the applied current on the stainless-steel cathode, we achieve precise current-driven Fe leaching ranged from 0.63 to 2.65 mg/L, which catalytically activates H₂O₂ generated from an air diffusion cathode and achieves efficient homogeneous Electro-Fenton for organic contaminant removal. Contrary to the conventional view of Fe release as resulting solely from reductive dissolution, advanced analytical and computational techniques reveal that the Fe release originates from sequential cathodic reduction and H₂O₂-mediated oxidative dissolution of the passivation layer. Specifically, cathodic reduction converts surface Fe(III)-oxide/(oxy)hydroxide to soluble Fe(II) to disrupt the outer layer, while subsequently generated surface-bound •OH induces the oxidative dissolution of the inner Cr(III)-rich barrier, causing lattice collapse and exposing the Fe(0) substrate. This process enables oxidation of underlying Fe(0) to release Fe²⁺ that sustains Electro-Fenton reactions. Microstructural characterization further demonstrates that spatially heterogeneous Cr distribution within the passivation layer governs the corrosion dynamics. Potential application is further demonstrated through efficient contaminant removal in real-world surface water and groundwater, and in a flow-through reactor. Long-term operation (48 cycles) confirms that Fe leaching maintains efficient contaminant removal. Cr released into the solution predominantly exists as less toxic Cr(III), with potential risks effectively mitigated through post-treatment pH neutralization and intrinsic self-repair strategies. Collectively, the current-driven Fe-leaching mechanism fundamentally expands stainless-steel cathode applications beyond heterogeneous catalysis, establishing a highly efficient Electro-Fenton process.