Degradation Of Oxalic Acid By Heterogeneous Catalytic Ozonation

Ozone as a strong oxidant has been widely adopted in advanced oxidation processible (AOPs) for treating the water is from industry and agriculture containing high concentration of organics and hard-degradation pollutants. However, oxalic acid which is intermediate or final product of a variety of complex organic acid in the process of ozone oxidation reaction is a problem, because of reacting with ozone very slowly. Heterogeneous ozonation catalytic can enhance the ozone oxidation. In this study tow types of modified silicon nitride catalyst (Si-N:Fe and Si-N:Fe/Al2O3) were used and catalytic degradation of oxalic acid experiments have been done, which were included the characterization of modified silicon nitride, the optimal reaction condition, oxidation mechanism and the the effect of degrading oxalic acid. Meanwhile preliminary study about the continuous use of the catalyst and the reaction kinetics model has started.Experimental results are as follows:①The crystal structure of Si-N:Fe and Si-N:Fe/Al2O3 is mainlyα-Si3N4 andβ-Si3N4, containing FeSi, Fe, Al2O3, Si2N2O.The surface of Si-N:Fe is smooth, close multi-faceted prismatic crystal, content of iron is 82090mg/kg, the specific area is 0.7693m2/g and pHpzc is 4.2. The surface of Si-N:Fe/Al2O3 is flake crystal like fish scale, content of iron is 82090mg/kg, the specific area is 0.4695m2/g and pHpzc is 3.1. Two types of catalysts'FTIR characteristics abosorption mainly in the 424cm-1～580cm-1.②The optimal reaction yield was reached when the ozone concentration was 40mg/L, the flux is 200mL/min, the dosage of Si-N:Fe was 0.1g (Si-N:Fe/Al2O3was 0.25g), and the pH was 3.0. The catalysts will be deactivated when the pH of reaction system was higher or lower than the catalysts'pHpzc.③It was concluded that-OH is the principle species responsible for the degradation of oxalic acid. The production rate of -OH was controlled by desorption of the ferric ion and the number of surface-active position of catalyst. Of these the complex balancing system of high-low iron is key factor for the degradation rate.Compare the tow catalysts, Si-N:Fe/Al2O3 is more active, fit for wider range concentration of ozone and can be used in more systems which are different concentration of oxalic acid with one dosage than Si-N:Fe.④The result of heterogeneous ozonation catalytic reaction is fitting with the second-order degradation, and the reaction rate constant incresase 1 to 2 orders of magnitude than ozone only under acidic conditions. The degradation rate of oxalic acid was the highest with ozone/Si-N:Fe (74.6%) and ozone/Si-N:Fe/Al2O3 (85.7%), but it dropped rapidly with ozone (57% and 68.17%) respectively. Degradation and mineralization of catalysts/ozone is better than honeycomb ceramics and Fe3+.The result of research on heterogeneous ozonation catalytic reaction indicates that the modified silicon nitride catalysits can improve the reaction rate of degrading oxalic acid. The result provides a scientific basis for further medium-sized enperiments and engineering projects, as well as a theoretical basis for further updating the silicon nitride catalyst.