Decomposition of Oxalate in Dilute Aqueous Solutions: Evidence for the Strong Synergism of Ozonolysis Combined with Ultrasonic Irradiation

The simultaneous application of ultrasonic irradiation with ozonation was demonstrated to be effective for the oxidation of oxalic acid (H2C2O4/HC2O4/C2O4), a particularly recalcitrant pollutant, to CO2 and H2O. Degradation rates obtained using sonolytic ozonation were found to be more than 16-times faster than predicted by the linear addition of the independent systems. Model calculations and batch ozone decomposition experiments in the presence of oxalate and/or ultrasound suggest that ozone efficiently reacts with carboxyl anion radical, CO2⋅ produced by the OH-radical attack on HC2O4. We propose that this reaction propagates a free-radical chain mechanism which increases OH-radicals concentrations within the bulk solution. We believe that this mechanism is responsible for the rapid degradation of oxalate, perhaps magnified by the additional ⋅OH-radicals that results from the thermal decomposition of O3 within acoustic cavitation bubbles. Oxalate was also oxidized by the mixture of H2O2 and O3. However, addition of H2O2 to systems undergoing sonolytic ozonation did not appreciably affect the observed degradation kinetics, indicating that the O3/ultrasound/HC2O4 system is predominantly driven by free-radical chemistry.