Unveiling the dual role of ferrous ion in nitro‑antibiotic removal by zero‑valent iron: Governing parent compound removal and driving toxic intermediate accumulation.

Ferrous ion (Fe2+) can effectively promote the removal of nitro-antibiotics by zero-valent iron (Fe0), but the applicability, limitations, and underlying mechanisms of this enhancement remain unclear. This study systematically investigated the effects of two ferrous salts (FeCl2 and FeSO4) on both the removal efficiency and degradation pathways of chloramphenicol (CAP), nitrofurazone (NFZ), and metronidazole (MNZ) by Fe0. The batch experiments and in situ FTIR spectrum revealed that FeCl2 effectively enhanced the removal of three antibiotics, whereas FeSO4 selectively inhibited the reduction of CAP and MNZ by forming inner-sphere SO42- complexes that competed for reactive sites. The LC-MS/MS demonstrated the dual role of Fe²⁺ by both affecting the removal of parent compounds and promoting the accumulation of toxic intermediates (e.g., hydroxylamine and azoxy intermediates). Density functional theory (DFT) calculations confirmed that the nitro-associated molecular structure fundamentally dictated the different degradation pathways among the three antibiotics, and Fe2+ could amplify this inherent difference by altering the interfacial microenvironment and reactive species. This work provides a theoretical basis for predicting the performance of the Fe0/Fe2+ system and for developing precise treatment technologies for wastewater contaminated with nitro‑antibiotics.