Degradation And Mechanisms Of Polycyclic Aromatic Hydrocarbons（PAHS） In Textile Dyeing Sludge By O₃/H₂O₂ Treatment

Though textile dyeing wastewater after treatment by A/O process can meet the emission standard, large quantities of toxic pollutants, such as heavy metal, benzene series, poycyclic aromatic hydrocarbons (PAHs), etc., still remain in dyeing sludge. PAHs which accumulated in textile dyeing sludge may pose a potential threat to human and organisms. It has become an urgent issue to cut down the PAHs from the source and reduce the risks to the organisms.A combined O3/H2O2 treatment has been proposed to remove PAHs from textile dyeing sludge in this study due to the source reduction principle. Fluorene (Fl), phenanthrene (Ph), anthracene (An) and dibenz[a,h]anthracene (DBA) were selected to represent typical PAHs. The effects of O3 flow rate, pH, reaction time, H2O2, radical inhibitors and O2 concentration on the degradation of four types of PAHs were examined for the response surface methodology to determine the optimal conditions for degradation. The degradation kinetics of the four PAHs were calculated based on the dynamics equations, and the synergistic mechanisms for PAH degradation were determined through particle size analysis, which could provide theoretical foundation to the O3/H2O2 process for the removal of PAHs from sludge. The main conclusions could be summarized as follows.The degradation of PAHs in textile dyeing sludge was dramatically influenced by O3 flow rate, pH, reaction time, H2O2, radical inhibitors. The addition of H2O2 to the O3 process accelerated the degradation of the four PAHs. The removal rate increased by approximately 6% for Fl,27% for Ph,21% for An and 5% for DBA. The optimum degradation conditions were determined by the response surface method with an O3 flow rate of 0.40 L/min, an H2O2 doses of 9 mL, a reaction time of 15 min, and pH 7.0 to achieve a high removal efficiency (88% for Fl,85% for Ph,71% for An and 81% for DBA). The response surface indicated the high O3 flow rate and the long reaction time were conducive to the degradation of Fl, but it is detrimental to the degradation of Ph, An and DBA. The removal rate of Fl, Ph and An reach the peak under anaerobic conditions and the highest removal rate was 72%,81%,82%, respectively, when the reaction time was 30 min, but removal rate of Fl, Ph and An under aerobic conditions were only 57%s 69%、67%, respectively at 30 min, while the removal rate of DBA reaches the peak of 68% when the reaction time was 30 min under aerobic conditions and it was only 52% under anaerobic conditions, indicating that purging gas (O2 and N2) into the O3/H2O2 process can significantly increase PAHs degradation rate.The degradation of the PAHs can be well fitted by pseudo-first-order kinetics (R2=0.9715). There existed different mechanisms in the degradation of four target PAHs by O3/H2O2 process. The addition of isopropanol resulted in a decrease of the oxidation rate of the PAHs, which indicated that Ph, An and Fl were not only degraded by O3, but partially oxidized by hydroxyl radicals, whereas DBA was oxidized by O3The effect is not simply a summation of the individual processes of O3, H2O2 in the complex textile dyeing sludge matrix, but there exists a mutual synergy between the processes. The addition of H2O2 was favorable to improve the volumetric mass transfer coefficient and the-OH concentration. H2O2 can break sludge into small particles and effectively release the PAHs into the sludge liquid phase so that O3/H2O2 can extremely degrade PAHs not only in the textile dyeing sludge surface but also inside the sludge.