Catalytic Ozonation of Humic Acids by Ce–Ti Composite Catalysts

Ce-doped TiO2 nanoparticles with different Ce contents were used for the ozonation of humic acids (HA). The catalysts were synthesized by the sol–gel technique at molar ratios of Ce to Ti between 0.2 and 1.0 and characterized by X-ray diffraction and values of the point of zero charge (pHPZC). Results suggest that the average crystallite size of the Ce–Ti composites decreases from 23.39 to 12.22 nm with increasing Ce amount, which increases the specific surface area of crystallite. The pHPZC of the Ce–Ti composites decreases from 5.3 to 3.9 with increasing Ce content and shifts to the acidic range. The results of HA ozonation suggest that the Ti–Ce(1/0.8) composite has the highest catalytic efficiency due to its high surface area as well as low pHPZC. The Ce(0.8)–Ti composite increases the ozone utilization efficiency by 62.0% compared to the ozonation alone, and a significant decrease in the high apparent molecular weight components toward smaller molecules was observed. In the presence of Ce(0.8)–Ti composite catalyst, the apparent firstorder rate constants of the homogeneous reaction (0.054 g L–1 min−1) is lower than that of the heterogeneous reaction (0.067 g L−1 min−1), moreover, the kinetic contribution of the heterogeneous reactions to HA degradation (δhet) was less than 50% at all pH conditions, implying that the catalytic degradation of HA mainly occurred in the solution.