pH effects on catalytic ozonation of carboxylic acids with metal on metal oxides catalysts

Summary

The pH influence on the catalytic ozonation of small carboxylic acids (succinic (SA), chloroacetic (CAA) and pyruvic (PA) acids) was studied and compared to ozonation alone. In the acidic systems, the conversion of these carboxylic acids and total organic carbon (TOC) during the catalytic ozonation is mainly due to the catalytic effect. Experiments on SA indicate that in the pH range 3.6–5.0, in contrast to the ozone behavior, the efficiency of the catalytic ozonation system is decreased with increasing pH due to the change of charges on the catalyst surface. In the basic pH range (7.2–10.0), the effect of the ozonation alone becomes important and the reaction in the presence of the catalyst is a combined effect of ozonation alone and catalytic oxidation. Although the apparent SA conversion by the catalytic ozonation in the basic systems is higher than in the acidic systems, the Total Organic Carbon conversion is not so much higher than in the latter. During the ozonation with the same catalyst, more than 80 of the CAA converted are mineralized. The oxidation of CAA leads to almost equimolar release of chloride ions. This ion has no detrimental influence on the catalytic effects under our experimental conditions. However, in the very dilute systems with pH value lower than the pKa of CAA, when the CAA concentration is decreased to about 0.6 mM, the catalytic oxidation is almost stopped. The low concentration of the dissociated CAA induces only weak adsorption and consequently small reaction rates. The total elimination of CAA with low concentration can be reached only if the pH is increased above the pKa. In addition to the distinct pH effect in the ozonation and catalytic ozonation system observed in this work, a further demonstration that separate mechanisms are involved in the two processes is provided by the analysis of oxidation by-products of pyruvic acid.