Oxalated zero valent iron enables highly efficient heterogeneous Fenton reaction by self-adapting pH and accelerating proton cycle.

Heterogeneous Fenton reactions of zero-valent iron (ZVI) requires the sufficient release of Fe(II) to catalyze the HO decomposition. However, the rate-limiting step of proton transfer through the passivation layer of ZVI restricted the Fe(II) release via Fe core corrosion. Herein we modified the shell of ZVI with highly proton-conductive FeCO·2HO by ball-milling (OA-ZVI), and demonstrated its high heterogeneous Fenton performance of thiamphenicol (TAP) removal, with 500 times enhancement of the rate constant. More importantly, the OA-ZVI/HO showed little attenuation of the Fenton activity during 13 successive cycles, and was applicable across a wide pH range of 3.5-9.5. Interestingly, the OA-ZVI/HO reaction showed pH self-adapting ability, which initially reduced and then sustained the solution pH in the range of 3.5-5.2. The abundant intrinsic surface Fe(II) of OA-ZVI (45.54% vs. 27.52% in ZVI, according to Fe 2p XPS profiles) was oxidized by HO and hydrolyzed to generate protons, and the FeCO·2HO shell favored the fast transfer of protons to inner Fe, therefore, the consumption-regeneration cycle of protons were accelerated to drove the production of Fe(II) for Fenton reactions, demonstrated by the more prominent H evolution and nearly 100% HO decomposition by OA-ZVI. Furthermore, the FeCO·2HO shell was stable and slightly decreased from 1.9% to 1.7% after the Fenton reaction. This study clarified the significance of proton transfer on the reactivity of ZVI, and provided an efficient strategy to achieve the highly efficient and robust heterogeneous Fenton reaction of ZVI for pollution control.