Catalytic Ozonation For The Treatment Of Refractory Organic Wastewaters And Its Mechanism

The refractory organic wastewater is harmful to the human beings due to its poor biodegradability,complex composition and difficult treated by traditional methods.Advanced oxidation process?AOPs?are attractive methods to eliminate the color,to reduce the organic load and improve the biodegradability of recalcitrant contaminants.As a green and efficient AOPs,ozonation has been considered a promising method for wastewater treatment was studied widely as it has strong oxidation and without secondary pollution.However,several disadvantages which limits its application,such as low utilization ratio,high-cost and low mineralization efficiency.Many methods were used to promote the decomposition of ozone and formation of hydroxyl radical,increasing the mineralization efficiency and oxidation efficiency,it mainly including the catalyst added into the liquid phase to accelerate the ozone decomposition in the acid medium and the high pH value provided to initiate the ozone decomposition in the alkaline medium.However,the addition of catalysts has the disadvantages of low stability,difficult recovery of catalysts and the need for follow-up operations such as solid-liquid separation,and free radical scavengers can be produced under alkaline conditions,which not only capture a part of free radicals but also compete with free radicals,thus reducing the oxidation efficiency and hindering the application and development of this technology.In this study,the new catalytic ozonation systems were put forward for the treatment of organic wastewater,including O₃/Ca?OH?₂ system,ozone coupled with membrane treatment and catalytic ozonation by stainless steel mesh,the performance of the new systems was evaluated by studying the degradation and mineralization efficiency of p-nitrophenol and biologically-treated leachate,and the degradation mechanism of organic matter was researched,the important conclusions are as follows:?1?The degradation and mineralization of p-nitrophenol in aqueous solution was investigated in O₃-Ca?OH?₂ system.It is demonstrated that the degradation and TOC removal efficiency increased with increasing Ca?OH?₂ dosage before 2 g/L,increasing inlet ozone concentration and increasing gas flow rate,as well as decreasing initial phenol concentration.The reactor pressure and liquid phase temperature have little effects on the p-nitrophenol removal and TOC removal efficiency.When Ca?OH?₂ dosage exceeded 3g/L,the degradation and TOC removal almost reached 100%for phenol at 30 and 55 min,respectively.The mechanism for Ca?OH?₂ intensified mineralization of phenol solution is the simultaneous removal of CO₃^2-ions,as hydroxyl radical scavengers,due to the presence of Ca²⁺ions.Results indicated that O₃/Ca?OH?₂ system is a high efficient ozonation process for persistent organic wastewater treatment.?2?O₃/Ca?OH?₂ process was adopted to treat biologically-treated leachate in a microbubble gas-liquid reactor.The residual COD concentration is meeting the discharge standard after 120 min treatment.The effects of operating parameters such as Ca?OH?₂ dosage,reactor pressure,temperature,inlet ozone concentration and ozone flow rate on COD removal and mineralization?TOC removal?were studied systematically.This process was able to remove 89.86%of COD,65.35%of TOC and 92.12%of UV-254under the optimal conditions.And the mechanism for the intensification of Ca?OH?₂ was explored through analyzing the change of UV-254,3D-EEM and the organic matter present in the leachate.?3?The degradation of recalcitrant organic compounds in biologically-treated leachate was investigated using pre-ozonation in a micro bubble gas-liquid reactor.The effects of operating parameters such as reactor pressure,liquid phase temperature,inlet ozone concentration and ozone flow rate on micro filtration?MF?performance were studied systematically.It is demonstrated that the MF was obviously enhanced with ozonation time before 45 min and increasing inlet ozone concentration,as well as increasing ozone flow rate.Additionally,the reactor pressure and liquid phase temperature have little effects on improving MF performance.And the mechanism for the enhancement of MF performance was explored through analysis of the colloids present in the leachate by SEM and organic compounds by GC/MS.The micro flocculation effect and the reduction of hydrocarbon and heterocycle compounds in the leachate by ozonation treatment were responsible for improving the MF performance.?4?Stainless steel mesh was designed as three-dimensional packing structure as ozone catalyst.Ti-Mn loaded with stainless steel mesh was prepared by sol-gel method to treat the biologically-treated leachate.The surface properties and microstructures of catalyst was characterized by SEM,XRD and XPS.The effluent after treatment were analyzed by the3D-EEM and GC-MS,and the removal rate of organic matter was investigated.The stability,inactivation reason and regeneration method of the catalytic materials were studied.The results showed that Ti-Mn oxide supported on stainless steel mesh had promoting effect on ozone decomposition,and the COD removal rate reached 78.43%.The catalytic effect of different coating times showed that the two layers had the best promoting effect,and the COD removal rate reached 87.17%.However,its stability needs to be improved.The catalytic performance decreases with the increase of the number of times used as the catalyst adsorbs some organic pollutants on its surface,the intermediate products occupy the active site of the catalyst,which causing the deactivation of catalyst.The catalyst could be regenerated by in-situ alkali-acid-hydrogen peroxide-ethanol-water sequential cleaning,the COD removal rate after regeneration process could increase to80.55%.It provides an effective method for in-situ regeneration of catalyst.