Sea-Buckthorn-Like MnO₂ Decorated Titanate Nanotubes with Oxidation Property and Photocatalytic Activity for Enhanced Degradation of 17β-Estradiol under Solar Light

A new class of sea-buckthorn-like composite material, MnO2/TNTs (TNT = titanate nanotube), was developed and used for 17β-estradiol (E2) degradation in this study. The new material was characterized as amorphous MnO2 nanoparticles decorated on titanate nanotubes, with chemical formula 0.4MnO2·Na1.1H0.9Ti3O7. The MnO2 fraction can preactivate E2 through one-electron oxidation, while the TNT skeleton is the primary photocatalysis center. Unlike the traditional weak oxidation and photocatalytic degradation, the synergetic effect of these two processes leads to efficient E2 removal by MnO2/TNTs under simulated solar light. The apparent first-order rate constant (k1) was determined to be 0.198 min–1, which was ∼28 times that for MnO2/TNTs direct oxidation without light and ∼15 times of that for calcined TNTs photocatalysis with light. Moreover, the higher TOC elimination rate (82.6% at 1 h) was also obtained compared to that in the pure MnO2 system. Dual-enhanced mechanisms are proposed to interpret the high E2 degradation efficiency: (1) heterojunction structure of MnO2 and titanate results in inhibited electron–hole recombination and promoted visible-light-driven photocatalytic activity, and (2) synergy of preoxidation and photocatalysis leads to high reactivity on activated E2 radical and •OH coupling. Product identification and density functional theory (DFT) calculation further confirm the reaction pathway of the radical coupling, which is a key linkage between preoxidation and photocatalysis. The developed MnO2/TNTs materials appear promising for the degradation of emerging phenolic pollutants under solar light.