Efficiently catalytic ozonation of 2,4-dichlorophenoxyacetic acid by natural ferrihydrite: A pH dependent and surface -OH group involved reaction mechanism.

The heterogeneous catalytic ozonation with natural iron oxides has been proven to be a powerful technology for the removal of recalcitrant organics in water due to the involvement of reactive oxygen species. However, little information can be obtained about the performance of Ferrihydrite in catalytic ozonation especially the relavant reaction mechanism. In this study, Ferrihydrite was synthesized via a simple precipitation method and 2,4-Dichlorophenoxyacetic acid (2,4-D) degradation was used to evaluate its catalytic ozonation performance. Compared with sole ozonation, Ferrihydrite had an excellent activity in catalytic ozonation and 2,4-D was always efficiently degraded (>90%) at a wide pH range (3.0-8.0). Electron spin resonance (ESR) and radical scavenging tests proved that •OH and O2•- were the dominant reactive oxygen species (ROS) in 2,4-D degradation (92.33% vs. 77.4% in ozone alone) and mineralization (63% vs. 16.2% in ozone alone). Based on a series of characterizations, Ferrihydrite processed a higher BET area and surface -OH groups than other iron oxides such as FeOOH, Fe2O3 and Fe3O4. The efficiently exposed surface -OH group with a high density was the reactive centers for the generation of ROS. Importantly, pHPZC of Ferrihydrite (6.3) and pKa of 2,4-D (2.73) induced a pH-dependent 2,4-D removal patterns (surface reaction at pH < 6.3 and reaction in bulk solution at pH > 6.3) were proposed via the electrostatic attraction or repulsion between the hydrogenated/hydroxylated surface of Ferrihydrite and negative charged 2,4-D.