Dissolved black carbon-FeOx precipitates as electron shuttles activate persulfate and facilitate Fe(III)/Fe(II) cycle for efficient iopamidol degradation: Non-radical pathway and outer-sphere mechanism

Iodinated X-ray contrast media, as a frequently used medical diagnosis in the imaging of internal organs, soft tissues, and blood vessels, are widely encountered in water environments because of their non-metabolic ability. Herein, a series of co-precipitates (DBC-FeO x ) of dissolved black carbon (DBC) and FeO x were successfully synthesized. The chelation of Fe species with COO – groups in DBC enhanced the excellent electron transfer capability and persulfate (PS) activation performance of the resulting DBC-FeO x , thus boosting iopamidol (IPM) degradation. The IPM removal rate constant in the DBC-FeO x /PS was 35 times and 4.6 times higher than those observed in the DBC/PS and FeO x /PS systems, respectively. There existed an appropriate C/Fe molar ratio in the DBC-FeO x co-precipitates for PS activation, and the IPM degradation was driven by both radical and non-radical pathways. Fe sites were the primary active sites in DBC-FeO x for PS activation, and C=O groups played an important role in accelerating the Fe(III)/Fe(II) cycle by promoting rapid electron transfer, thereby enhancing radical oxidation pathways. Concurrently, DBC-FeO x acted as an electron shuttle to promote the electron transfer from IPM to PS, creating non-radical pathways for IPM removal. IPM degradation processes involved amide hydrolysis oxidation, hydrogen extraction, and deiodination. The typical iodine species HOI was also converted to non-toxic IO 3 - after IPM decomposition, thus reducing potential risks.