Secondary amine-activated ferrate(VI) for isoquinoline degradation: Relationship between molecular structure and reactive performance

This study elucidates the electronic structure-activity relationship between secondary amines and ferrate(VI) (Fe(VI)) activation. Comparative experiments demonstrated that pyrrolidine (Py) significantly outperformed diethylamine (Di) in enhancing Fe(VI)’s oxidation capability across various dosage conditions, pollutants, and water matrices. Specifically, for isoquinoline (IQL) degradation, Py-Fe(VI) achieved an approximately 7-fold higher rate constant than Fe(VI) alone, versus a 2-fold improvement observed with Di-Fe(VI). Mechanistic studies combining quenching experiments and EPR characterization corroborated Fe(IV)/Fe(V) as dominant reactive species for IQL degradation, with kinetic modeling revealing that Fe(IV) contributes > 80 % to IQL degradation in all processes. Electrochemical analysis via cyclic voltammetry and electrochemical impedance spectroscopy studies suggested that Fe(VI) activation by Di and Py might involve the formation of iron-secondary amine complexes. Density functional theory calculations highlighted Py's lower energy barrier for Fe(VI) complexation (27.7 vs. Di's 29.1 kcal/mol), accelerating activation. Secondary amines were shown to stabilize Fe(IV) via coordination, extending its reactive lifetime. Systematic evaluation of various secondary amines revealed a significant negative correlation between the highest occupied molecular orbital energy levels of amines and Fe(VI) activation performance. Moderate electron-donating capacity promotes iron complexation and pollutant degradation. This work establishes a molecular design framework for Fe(VI) activators while providing new insights into high-valent iron-mediated oxidation mechanisms, advancing sustainable water treatment strategies.