A citrate-loaded nano-zero-valent iron heterogeneous Fenton system for steroid estrogens degradation under different acidity levels: The effects and mechanisms

Abstract

• An excellent performance nZVI heterogeneous catalyst was developed and optimized. • The enhancement mechanisms of citrate ligands to nZVI were comprehensively explored. • Citrates and their hydroxyl-rich fragments formed a special spatial network structure on nZVI surface. • A degradation mechanism different from the traditional Fenton-like reaction was proposed. Today, the steroid estrogens (SEs) discharged with non-point source pollutions have brought potential risks to the environment. It is necessary to develop new restoration strategies for SEs. In this study, a heterogeneous nano-zero-valent iron Fenton system loaded with citrate ligands (C-nZVI) was proposed. It obviously enhanced the degradation rates of 17β-estradiol (17β-E2), and the degradation efficiency under neutral condition was also enhanced by 4.59 times. The characterization of C-nZVI showed that citrates and their hydroxyl-rich fragments formed a spatial network structure and covered on its surface. This allowed C-nZVI to overcome most of the inherent defects of nZVI and had ideal abilities to resist agglomeration, acid corrosion, and oxidation. Same as the traditional Fenton-like reaction, the generation of hydroxyl radicals (·OH) and superoxide radicals (·O 2 – ) was the main way for C-nZVI to degrade 17β-E2. The ligands as the ions transport channel further stimulated the excitation of ·OH and ·O 2 – , and the singlet oxygen ( 1 O 2 ) was also found to play a role in the latter part of the reaction. However, C-nZVI had a special mechanism of first coupling between citrate ligands and 17β-E2, as well as subsequent degradation by ROSs, which might be the reason why C-nZVI has a much faster degradation efficiency than the general nZVI. And the degradation process of 17β-E2 showed its four rings might be destroyed one by one starting from the benzene ring. Therefore, this heterogeneous Fenton system assisted by chelating agent ligands has a certain value for the research of water emergency restoration.