Ultra-efficient degradation of isoquinoline from shale gas wastewater with the diethylamine-ferrate(VI) system: The key role of Fe(IV)/Fe(V) active species

Abstract

Although isoquinoline (IQL) in shale gas wastewater contributes minimally to chemical oxygen demand, its potential high toxicity makes it an environmental risk factor that cannot be overlooked. This study introduces a synergistic diethylamine (Di)/ferrate (Fe(VI)) system for efficient degradation of IQL. Compared with Fe(VI) alone, the Di/Fe(VI) system demonstrated superior performance, achieving degradation efficiency of 80.5%. The degradation rate constant of the Di/Fe(VI) system was almost 3-fold larger than that measured with Fe(VI) alone in the degradation of IQL. Mechanistic studies, including radical quenching, electron paramagnetic resonance, pre-mixed experiments, Raman spectroscopy, and probe compounds tests suggested that high-valent iron intermediates (Fe(IV/V)) were responsible for IQL degradation in the Di/Fe(VI) system. The presence of Di promoted the generation of Fe(IV)/Fe(V) by donating electrons. Based on the intermediates identified with GC-MS measurements and density functional theory calculations, three reaction pathways for IQL degradation were proposed. ECOSAR prediction and Escherichia coli toxicity tests showed that the toxicity of IQL was significantly reduced after treatment with Di/Fe(VI) system. Optimal IQL degradation occurred at higher Fe(VI)/Di concentrations and lower pH, with minimal interference from common ions or matrix components. The system also effectively degraded pharmaceuticals (e.g., 2,4-di-tert-butylphenol, 6-methylquinoline, diclofenac, carbamazepine), demonstrating broad applicability for refractory pollutant treatment.