| Advanced treatment technologies for drinking water purification are becoming more important since micropollutants are widely distributed in the drinking water sources of China and the standard for drinking water quality is gradually improved. Investigation and exploitation of drinking water treatment technologies seem crucially urgent nowadays. Ozonation technology has been widely used in water treatment plants, whereas, its application was restrained due to the incomplete oxidation and the low ozone use efficiency. Catalytic ozonation technology can avoid the limitation of ozonation technology; therefore, it has got much more attention and developed rapidly in recent years.Activated carbon (AC) is a promising catalyst except for its instability in heterogeneous catalytic ozonation process. In this study, taking graphitic carbons (carbon nanotubes (CNTs) and graphite) as catalysts for ozonation process and oxalic acid as refractory target pollutant, the effect and mechanism of oxalic acid removal were investigated in the presence of ozone and graphitic carbons.The physico-chemical properties, catalytic activity and stability of graphitic carbons in heterogeneous catalytic ozonation process were investigated, and were compared with AC. The results of BET and selective neutralization analysis showed that graphitic carbons have lower surface area and less functional groups than that of AC. In heterogeneous catalytic ozonation experiment on oxalic acid degradation, CNTs and AC showed similar performance and both of the two above catalysts have higher activity than that of graphite. However, graphitic carbons were more stable than AC, especially for graphite.The influence of different operation conditions on CNTs catalyzed ozonation process was investigated, including the oxalic acid initial concentration, ozone gas concentrations, catalyst dosage, temperature, and initial pH of solution. Experimental results indicated that the variable of ozone gas concentration has little effect on the removal efficiency of oxalic acid; the variables of oxalic acid initial concentration, catalyst dosage, temperature, and initial pH of solution have strong effect on oxalic acid removal, especially for the initial pH of solution. A high efficiency on oxalic acid removal was obtained when the initial pH of solution was 3.0, higher or lower pH value around this point would result in a decrease in removal efficiency.Surface modification on CNTs was carried out using two methods, preozonation treatment and heat treatment. The relationship between surface property and catalytic performance of CNTs was investigated based on the results obtained with modification of CNTs. The exposure time to ozone gas increased the number of acid groups and decreased the number of basic groups on CNTs; and the catalytic activity of CNTs went down as well as its pHPZC value. As for heat treatment in nitrogen or hydrogen atmosphere, the number of acid groups decreased while the number of basic groups increased on CNTs. Besides, the higher pHPZC value was obtained in heat treatment with a higher temperature and the catalytic activity of corresponding catalyst was enhanced. The above results clearly indicated that the catalytic performance of CNTs strongly depends on the number of basic groups and the pHPZC value. CNTs with a large number of basic groups and a high pHPZC value should have a good performance in catalytic ozonation process.Pt/graphite and Pt/CNTs catalysts were prepared by incipient wetness impregnation using H2PtCl6·6H2O as precursor, and the Pt/AC catalyst was prepared with the same procedure for comparison. The micro-morphology and structure of catalysts were characterized by SEM, TEM, XRD and XPS. The active phase on all the catalysts surface is in the form of Pt0. However, with a same loading amount, the effect of Pt loading on catalytic activity is totally different for graphitic carbons and AC. Pt loading on graphite and CNTs enhanced their catalytic ability, whereas, no obvious effect was found on the performance of AC. This distinction should be caused by the different pore structures of the above carbon materials.Taking oxalic acid degradation efficiency as an index, the preparation conditions of Pt/graphite and Pt/CNTs were optimized. It was proved that pretreatment of supports and impregnation time had no effect on the activity of catalysts. Pt loading amount, reduction temperature and pyrolysis atmosphere significantly influenced the activity of catalysts. Solvent had no effect on the performance of Pt/graphite catalyst but obviously influenced the activity of Pt/CNTs catalyst. ICP analysis indicated that the active component, Pt, on both of the two catalysts surface were stable in heterogeneous catalytic ozonation process.The mechanisms of oxalic acid degradation by graphitic carbons or graphitic carbons supported Pt catalyzed ozonation processes were studied. Using the tert-butanol as radical scavenger, the role of hydroxyl radicals in CNTs catalyzed ozonation process was investigated. It was found that the catalytic activity of CNTs was inhibited to some extent after tert-butanol addition, which means that the hydroxyl radicals have participated in oxalic acid degradation process. Based on the effect of pH value on the performance of CNTs, the adsorption of oxalic acid on CNTs at different pH was analyzed. It could be concluded that surface reactions play an important role in catalytic ozonation process for oxalic acid degradation. However, the catalytic activity of graphitic carbons supported Pt was not remarkably influenced by tert-butanol addition. According to structure analysis, the dispersion and the chemical form of Pt have significant influence on the catalysts performance. It was pointed out that oxalic acid degradation mainly occurred on catalyst surface, and the redox couple of Ptred/Ptox was supposed to play a key role in this catalytic ozonation process.
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