New insights into the transformation of effluent organic matter during Fe(II)-assisted advanced oxidation processes:...

The negative effects of effluent organic matter (EfOM) on receiving aquatic environments and advanced treatment facilities pose significant concerns. However, the effective removal of EfOM is challenging due to its chemically complex nature and its refractory characteristics. In this study, two Fe(II)-assisted oxidation processes including UV/Fe(II)/H₂O₂ and UV/Fe(II)/persulfate (UV/Fe(II)/PS) were investigated to promote EfOM reduction. Fe(II) was essential for promoting EfOM degradation. The mineralization rate of EfOM increased from 7 to 29% with 2 mM Fe(II) addition in the UV/H₂O₂ process and to 23% with 0.8 mM Fe(II) addition in the UV/PS process. A preliminary experiment was conducted to obtain the optimal molar ratio of oxidant to Fe(II) for practical applications based on different indicators. The form of Fe(III) prevalent at different pH values strongly affected Fe(II)/Fe(III) cycling, thus determining the progress of EfOM degradation. A machine learning approach consisting of parallel factor analysis coupled with self-organizing maps (PARAFAC-SOM) was employed with fluorescence spectra to visualize the degradation behavior of EfOM in the different reaction systems. Four components (i.e., two humic-like substances, one fulvic acid, and one tryptophan-like substance) were eventually identified, and their reductions reached more than 62% during the Fe(II)-assisted oxidation processes. The degradation orders for each component in the different oxidation processes were initially evaluated by SOM analysis with F_max percentage data. The degradation behavior of EfOM in the UV/Fe(II)/H₂O₂ and UV/Fe(II)/PS systems exhibited different trends based on the best matching unit map and component planes. The humic-like component was more refractory than the other three components in both oxidation processes. The microbial humic-like and high-molecular-weight fulvic acid substances showed higher reactivity with SO·₄^- than with ·OH, while the tryptophan-like substance was more reactive in the UV/Fe(II)/H₂O₂ system than in the UV/Fe(II)/PS system. The outcomes of this study provide new insights into the degradation behavior of EfOM, promoting the development of advanced wastewater treatments.