Sono-, Photo- and Sonophotocatalytic Decontamination of Organic Pollutants in Water:Studies on the Lack of Correlation between Pollutant Degradation and Concurrently formed H2O2

The degradation of trace amounts of phenol in water is studied under sono-, photo- and sonophotocatalytic conditions using ZnO as a catalyst. Sonophotocatalytic degradation is more than the sum of the respective sono- and photocatalytic degradation under otherwise identical conditions, indicating synergistic effect. The degradation proceeds through many intermediates and ultimately the parent compound is mineralized. The concentration of concurrently formed H₂O₂ increases and decreases periodically resulting in an oscillatory behaviour. The oscillation is more pronounced in sonocatalysis in which the degradation of phenol and corresponding formation of H₂O₂ are slower. In photocatalysis and sonophotocatalysis, where the degradation is faster, the amount of H₂O₂ is relatively more and the oscillation becomes weaker and tends towards stabilization. However, in all cases the degradation of phenol continues unabated until the mineralization is complete. The stabilized concentration of H₂O₂ is much less than the expected amount based on the degradation of the organic pollutant. Probable causes for the phenomena are discussed.