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MnO2 nanozyme-driven polymerization and decomposition mechanisms of 17β-estradiol: Influence of humic acid

Sun, Kai, Liu, Qingzhu, Li, Shunyao, Qi, Yongbo, Si, Youbin
Journal of hazardous materials 2020 v.393 pp. 122393
active sites, carbon cycle, catalytic activity, enzymes, estradiol, estrone, humic acids, hydroxylation, manganese dioxide, nanomaterials, oxidation, pH, polymerization, polymers
Nanozymes, which display the bifunctional properties of nanomaterials and natural enzymes, are useful tools for environmental remediation. In this research, nano-MnO₂ was selected for its intrinsic enzyme-like activity to remove 17β-estradiol (E2). Results indicated that nano-MnO₂ exhibited laccase-like activity (7.22 U·mg⁻¹) and removed 97.3 % of E2 at pH 6. Humic acid (HA) impeded E2 removal (only 72.4 %) by competing with E2 for the catalytic sites of the MnO₂ nanozyme surface, and there was a good linear correlation between the kinetic constants and HA concentrations (R² = 0.9489). Notably, the phenolic −OH of E2 interacted with HA to yield various polymeric products via radical-driven covalent coupling, resulting in ablation of phenolic −OH but increase of ether groups in the polymeric structure. Intermediate products, including estrone, E2 homo-/hetero-oligomers, E2 hydroxylated and quinone-like products, as well as aromatic ring-opening species, were identified. Interestingly, HA hindered the extent of E2 oxidation, homo-coupling, and decomposition but accelerated E2 and HA hetero-coupling. A reasonable catalytic pathway of E2 and HA involving MnO₂ nanozyme was proposed. These findings provide novel insights into the influence of HA on MnO₂ nanozyme-driven E2 radical polymerization and decomposition, consequently favoring the ecological water restoration and the global carbon cycle.