Carbon sequestration from emerging organic contaminants: Polymerization and stabilization of secondary amine-containing fluoroquinolones associated with complexed Fe(Ⅲ)

Organic carbon (OC) preservation is closely coupled with iron (Fe), but has not been strengthened for anthropogenic organic pollutants decontamination. Emerging organic contaminants (EOCs) degradation that associated with Fe phases have been well-documented. As an opposite-direction and carbon (C)-neutral pathway towards degradation, polymerization of 16 amine-containing EOCs mediated by Fe(Ⅲ)-catechol (CT) complex was investigated. It was found that in nucleophilic addition with CT, the pseudo-first-order rate constants (kobs, h−1) of norfloxacin (NOR), enoxacin (ENX), ciprofloxacin (CIP) and sarafloxacin (SAR) that belong to fluoroquinolones (FQs) were 1.9–8.0 times higher than the widely studied sulfonamides (SNs). Density functional theory (DFT) calculations and molecular dynamics (MD) simulations revealed that high dynamic accessibility, rather than nucleophilicity, of the secondary amine group without side chains in FQs was responsible for their high reactivities. Formation of high-humified products, aliphatic and condensed aromatic molecules, was facilitated in the complexed Fe(Ⅲ) system. Complexed Fe(Ⅲ) and the involved hydroxyl radical (HO) generation intensified hydroxylation and ring-opening of the transformation products (TPs), further increasing the molecular weight. Notably, the TPs co-precipitated with Fe(Ⅲ) to form an insoluble Fe(Ⅲ)-organics complex. Fe(Ⅲ)-TPs co-precipitate possessed a high spatial relation of Fe-C and high C/Fe ratio, contributing to stabilization of the polymerization products. This study provided new insights into the polymerization potential of amine-containing EOCs, and a decontamination pathway strengthened by Fe(Ⅲ)-polyphenols complexation.