The Behavior Mechanism Of Metal Coagulants In The Hybrid Ozonation-coagulation System

Because of the limited dissolved organic matters removal efficiency in conventional advanced wastewater treatment process,the hybrid ozonation-coagulation process was developed for the purpose of effectively removing the residual organic matters of wastewater treatment plant effluent（WWTP）and ensuring the safety of water quality.The removal characteristics of the effluent dissolved organic matter and Ibuprofen,as well as the reaction properties of the hydroxyl radical were analyzed in this study.The results would provide theoretical basis for improvement of wastewater reclamation.Compared with the conventional pre-ozonation-coagulation process,the hybrid ozonation-coagulation process was put forwarded in which coagulation and ozonation could synchronize in one unit,to improve the removal efficiency of dissolved organic matters by the interactions between ozone and coagulants.In order to obtain the mechanism of removal of dissolved organic matters by the hybrid ozonation-coagulation process,the influence of Al-based coagulants on ozone decomposition and Para-chlorobenzoic acid（pCBA）removal was monitored.The results revealed that the hybrid ozonation-coagulation process was involved in the·OH reaction and coagulant could promote ozone decomposition due to its enhancing effect on·OH production during the ozonation process.In the hybrid ozonation-coagulation process,Al-based coagulants and hydrolyzed products could act as catalyst to enhance ozone decomposition into·OH which is a powerful,low selectivity oxidant to improve the organics removal efficiency.Based on the promoting mechanism between ozone and coagulants,the tert-butanol（tBuOH）assay and competition method was established to evaluate the·OH yields in ozonation process and clear the mechanism of Al-based coagulants in the role of the hybrid ozonation-coagulation process.Due to the addition of excess tBuOH in the tBuOH assay,the generated·OH was scavenged by t BuOH,which subsequently inhibited the propagation reactions.Therefore,tBuOH assay can reveal the·OH yield during the initiation step of the chain reactions.The average·OH yield in the hybrid ozonation-coagulation process was 31.3%,which was slightly higher than that in ozonation（Ф=25.47%）in pure water,similarly,the·OH yield was 17.89%and12.84%respectively in WWTP effluent.Therefore,tBuOH assay can reveal AlCl₃·6H₂O had little effect on the initiation step of chain reactions.As a consequence,the·OH yield in the initiation step had little difference for the hybrid ozonation-coagulation and ozonation processes.In order to truly reflect the·OH yield in the hybrid ozonation-coagulation process,competition method was established to overcome the limitations of t BuOH assay.The average·OH yield（Ф=79.8%）was much higher than that obtained by the tBuOH assay in pure water.Additionally,the hybrid ozonation-coagulation process gave a higher·OH yield（Ф=70.07%）than that of ozonation（Ф=26.20%）,which determined AlCl₃·6H₂O was involved in the propagation reactions rather than the initiation steps of·OH generation in the hybrid ozonation-coagulation process.According to tranisent steady-state·OH model,R_ct pattern and the pseudo-first-order kinetics equation,the initiation,promotion,and inhibition rate constants of Al-based coagulants was derived to explore the specific role of coagulants in the hybrid ozonation-coagulation process.The results showed that the reaction rate constant of AlCl₃·6H₂O as initiator,promoter and inhibitor was 0.239 L·（mol·s）^-1,7.24×10⁸ L·（mol·s）^-1 and 113 L·（mol·s）^-1,respectively.Compared with the traditional initiation,promotion,and inhibition rate constants,the results indicated that AlCl₃·6H₂O was most likely to stimulate·OH formation as a promotor during promotion steps of the chain reactions in the hybrid ozonation-coagulation process.