Enhanced degradation of organic contaminants by Fe(III)/peroxymonosulfate process with l-cysteine

The difficulty in Fe(III)/Fe(II) conversion in the Fe(III)/peroxymonosulfate (PMS) process limits its efficiency and application. Herein, l-cysteine (Cys), a green natural organic ligand with reducing capability, was innovatively introduced into Fe(III)/PMS to construct an excellent Cys/Fe(III)/PMS process. The Cys/Fe(III)/PMS process, at room temperature, can degrade a variety of organic contaminants, including dyes, phenolic compounds, and pharmaceuticals. In subsequent experiments with acid orange 7 (AO7), the AO7 degradation efficiency followed pseudo-first-order kinetic which exhibited an initial “fast stage” and a second “slow stage”. The rate constant values ranged depending on the initial Cys, Fe(III), PMS, and AO7 concentrations, reaction temperature, and pH values. In addition, the presence of Cl−, NO3−, and SO42− had negligible impact while HCO3− and humic acid inhibited the degradation of AO7. Furthermore, radical scavenger experiments and methyl phenyl sulfoxide (PMSO) transformation assay indicated that sulfate radical, hydroxyl radical, and ferryl ion (Fe(IV)) were the dominant reactive species involved in the Cys/Fe(III)/PMS process. Finally, based on the results of gas chromatography-mass spectrometry, several AO7 degradation pathways, including N=N cleavage, hydroxylation, and ring opening were proposed. This study provided a new insight to improve the efficiency of Fe(III)/PMS process by accelerating Fe(III)/Fe(II) cycle with Cys.