Study Of Combined Ultrasound With Fenton Treatment For The Degradation Of Polycyclic Aromatic Hydrocarbons In Textile Dying Sludge

Textile dyeing sludge is the primary solid waste created during textile dyeing wastewater treatment.In 2014, the total discharge of textile dyeing sludge in China was 4.91 million tons (80% moisture content). And the main disposal of the sludge was landfill and incineration. In addition, our previous study showed that polycyclic aromatic hydrocarbons (PAHs), known to be toxic, mutative, carcinogenic and teratogenic to humans, were a type of recalcitrant organic pollutants in textile dyeing sludge. At some textile dyeing plants, the contamination level of PAHs in sludge has been reported to be as high as 16.7 mg/kg dry sludge.Therefore, it is necessary to remove the PAHs in textile dyeing sludge.In the present study, the PAHs in textile dyeing sludge were treated using a combined Fenton and ultrasound approach. The specific objectives were as follows:(1) to evaluate the kinetics coefficients and the synergistic effect between ultrasonic irradiation and the Fenton process for the degradation of PAHs in textile dyeing sludge. (2) To investigate the effects of various operating parameters on the degradation of PAHs during the US/Fenton process, such as ultrasonic density, reaction time, molar ratio of H2O2 to Fe2+, pH value and H2O2 dosage.And (3)to determine the interaction effects of different variables and obtain the optimal conditions using the central composite design (CCD) model for the treatment of PAHsin textile dyeing sludge. The main results of these studies could be summarized as follows.US combined with Fenton treatment could be more effective for the degradation of PAHs in textile dyeing sludge, which presents an enhancement in TOC removal and the degradation of PAHs in comparison with either of the individual methods. It is observed that the degradation of ∑PAHs was 22.7%,55.4% and 65.9% after 40 min using US,Fenton and US/Fenton process,respectively. And the removed of TOC in sludge was 5.6%,11.4%,19.5%.The degrees of synergy of the AHs and TOC removals were 25.8 and 35.0, respectively,which suggested a significant synergistic effect. In addition, compared to the peaked degradation of PAHs, the removed of TOC was rising during 30 to 40 min, which indicated that the degradation of PAHs would be affected by the other organic compounds in the sludge.The degradation of PAHs was significant affected by ultrasonic density, reaction time, molar ratio of H2O2 to Fe2+, pH value and H2O2 dosage. The conditions were optimized by the single factor experiments as followed:a H2O2 concentration of 140 mmol/L, an ultrasonic density of 360 W/L, a pH value of 3.0, a H2O2 to Fe2+ molar ratio of 1 and a reaction time of 40 min. A low ultrasonic density had an advantage in degrading the ∑PAHs during the US/Fenton process. However, less ·OH radicals would be generated due to the production of large cavityunder a high ultrasonic density (excessed 1080W/L), which inhibited the degradation. Moreover, a high ultrasonic density could lead to a higher degradation of high molecular weightPAHs than low molecular weightPAHs. The conditions were optimized as followed:a H2O2 concentration of 152 mmol/L, an ultrasonic density of 408 W/L, a pH value of 3.7, a H2O2 to Fe2+ molar ratio of 1.3 and a reaction time of 43 min by CCD. The highest degradation of PAHs in textile dyeing sludge was 83.9% under the optimal conditions of the US/Fenton process, which was similar to the predicted value (85.0%) of the CCD.