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 H2O2 decomposition. However, the rate-limiting step of proton transfer through the passivation layer of ZVI restricted the Fe(II) release via Fe0 core corrosion. Herein we modified the shell of ZVI with highly proton-conductive FeC2O4·2H2O by ball-milling (OA-ZVIbm), and demonstrated its high heterogeneous Fenton performance of thiamphenicol (TAP) removal, with 500 times enhancement of the rate constant. More importantly, the OA-ZVIbm/H2O2 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-ZVIbm/H2O2 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-ZVIbm (45.54% vs. 27.52% in ZVIbm, according to Fe 2p XPS profiles) was oxidized by H2O2 and hydrolyzed to generate protons, and the FeC2O4·2H2O shell favored the fast transfer of protons to inner Fe0, therefore, the consumption-regeneration cycle of protons were accelerated to drove the production of Fe(II) for Fenton reactions, demonstrated by the more prominent H2 evolution and nearly 100% H2O2 decomposition by OA-ZVIbm. Furthermore, the FeC2O4·2H2O 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.