Interface mechanisms of the catalytic ozonation of humic acids over siliceous ferrihydrite: Morphology, stability, and the catalytic process

Ferrihydrite (Fh), a precursor of more crystalline Fe (hydr)oxides, exhibits decent catalytic behavior; however, the instability of its amorphous structure limits its engineering applications. Siliceous ferrihydrite (FhSi) was readily synthesized in this study by co-precipitation. The formation of Fe-O-Si linkages did not alter the amorphous state of pure Fh, but increased the surface area (SBET), reduced the point of zero charge (pHZPC), and prevented the leaching of more iron. X-ray diffraction, Mo.ssbauer and pyridine-Fourier transform infrared (FTIR) spectroscopies, and potentiometric titration revealed the presence of silicon-occupied portions of growth sites on the Fh surface, which increased the coordination symmetry around the Fe atom and inhibited the transition of Fh to more stable crystalline Fe (hydr)oxides during repeated use. Meanwhile, the density of surface hydroxyl groups (Ds) and the total acid content of the catalytic system after five cycles of catalytic ozonation were 56.75 % and 63.58 % higher than those of freshly prepared system, thereby benefiting the catalysis of ozone for generating .OH. In addition, the lower pHZPC of the FhSi/O3 system compared to that of the Fh/O3 system promoted the generation of neutral surface-hydroxyl species on the surface of FhSi, which enabled a decent catalytic performance in alkaline solutions, regardless of the catalytic cycle. Moreover, the removal of humic acids (HA) followed a hydroxy radical reaction, which involved self-decomposition (14.15 %), catalytic ozonation (21.58 %), and peroxone and Fenton-like reactions (64.27 %).

Automated summary

The study synthesized siliceous ferrihydrite (FhSi) through co-precipitation, which increased surface area, reduced the point of zero charge, and prevented iron leaching. The presence of silicon-occupied growth sites enhanced coordination symmetry, inhibited transition to more stable crystalline Fe (hydr)oxides, and improved catalytic performance in alkaline solutions. After five cycles of catalytic ozonation, the FhSi system exhibited higher surface hydroxyl group density and total acid content, leading to enhanced catalytic activity for ozone generation.