Kinetics and degradation mechanisms of persistent organic pollutant by ozonation and advanced oxidation processes

Ozonation and advanced oxidation processes are effective in oxidising persistent organic pollutants into readily biodegradable compounds or even achieve complete mineralisation. Over the years, molecular ozone (O3) and hydroxyl radical (OH•) oxidation have been studied extensively. Recently, sulfate radical (SO4-•) oxidation is also of interest because SO4-• is a strong and more selective radical species compared to OH•. The main focus of this research was to assess the efficiencies, kinetics and mechanisms of ozonation, peroxone, and ozone/persulfate in oxidising persistent organic pollutant (p-cresol). The findings demonstrated that ozonation, peroxone, and ozone/persulfate were effective in oxidising 81.9% – 96.8% of 100 mg/L of p-cresol. Besides, significant improvement was observed in p-cresol degradation efficiencies and oxidative capacity of reaction medium when the pH medium increased to alkaline pH. This finding suggested that the predominant oxidant species in reaction medium converted to stronger oxidant species at elevated pH. In addition, the optimum operating conditions in ozonation, peroxone, and ozone/persulfate were as follows: pH 11, 30°C, 1.0 L O3/min, 1.0 mM of H2O2 or persulfate. Response surface methodology Box-Behnken design was also utilized to investigate the interaction between operating parameters. The findings revealed that the interaction between pH and operating temperature can enhance or hamper the process performance; whereas O3 gas flow rate has negligible influence to other parameters. Kinetic studies indicated that the degradation of p-cresol by ozonation, peroxone, and ozone/persulfate abided to pseudo first-order of kinetic reaction. Briefly, the kinetics of p-cresol degradation have positive linear relationship with pH and temperature, negative linear relationship with increasing initial concentration, and nonlinear relationship with O3 gas flow rate and oxidants dosage. Based on the gas chromatography mass spectrum analysis, the detected intermediate products of p-cresol degradation by ozonation, peroxone, and ozone/persulfate were all found less toxic compared to p-cresol. The detected aromatic intermediate products included 4-methylcatechol, 4-hydroxy-4-methylcyclohexadien-1-one, 4-methylbenzyl alcohol, 4-hydroxy-benzaldehyde, 5-methylfuran-2(3H)-one, 5-methyl-2-furaldehyde, 3,4-dihydroxy-benzaldehyde and 3,4-dihydroxy-benzoic acid. All of the intermediate products degraded to shorter chain of carboxylic acids with further oxidation. The findings also indicated that the major degradation pathway of p cresol was through benzene ring oxidation; whilst side-chain oxidation only occurred in peroxone and ozone/persulfate. The presence of the furanic compounds revealed that ring-opening products could undergo intramolecular nucleophilic substitution reaction, subsequently transforms the aliphatic compound to cyclic structure