Wet Air Oxidation Of High Concentration Of Nitrogen Heterocyclic Compounds:Reaction Mechanism And Persulfate Promoted Technology

Wet air oxidation(WAO)is very suitable for treatment of high COD concentration of stubborn organic wastewater,due to its clean process.The organics are decomposed into small carboxylic acids which can be treated by biodegradation or mineralized into CO₂,H₂O and inorganic acids.However,most reseaches are focused on WAO of phenolics,and few reports on the degradation of heterocyclic compounds can be retrieved.Further more,the main reactive radicals in WAO process are controversial.Several kinds of typical nitrogen heterocyclic compounds(NHCs),namely indole(ID),benzopyrazole(BP),benzotriazole(BT),benzimidazole(BM)and benzothiazole(BTH)which are widely presented in pharmaceutical wastewater were degraded via WAO.The degradation efficiencies and reaction rules of these NHCs were investigated.Based on WAO process,a novel technology of persulfate promoted WAO process(PWAO)was proposed and succeeded in achieving high COD removal at relative lower temperature with a low dosage of persulfate addition into WAO process.At the same time,functional microsphere polymers were also obtained in PWAO process and can be resued in wastewater treament.The detail results of this thesis are listed as follow:(1)The degradation efficiencies and reaction rules of several typical heterocyclic compounds were studied.Reaction temperatue and time were both significant factors in WAO of NHCs.The reaction rates of NHCs were higher at acidic solution of p H4.0 than those at higher p H value of 6.5 and 9.0.The effects of anions showed that CO₃^2-,PO₄^3-,and SO₄^2-anions acted as inhibitors in WAO of NHCs.On the contrary,Cl^-and NO₃^-promoted the degradation of these NHCs.The active energies of ID,BP and BT increase as nitrogen contents in the hetercyclic ring increase,and the reactive order of the three compounds was ID>BP>BT.Under the optimal reaction condition,all the NHCs could be completely decomposed,and the COD removal rates of ID,BP and BT were 76.7%,75.2%and 71.4%,respectively.N₂ was mainly produced via an extrusion reaction in WAO of BP and BT,while the denitrification of ID was based on the reaction between NH₄⁺and NO₃^-after heterocyclic ring broken,which led to much higher nitrogen removal of BP and BT than ID.The biologic acute toxicity test indicated that WAO process can effectively detoxify the NHCs.(2)Electron paramagnetic resonance(EPR)test verified that·OH was the main reactive oxidative species in the WAO process via oxygen activated at high temperature.Quantum chemistry was introduced to determine the reactive sites of NHCs reaction with·OH and the stability of possible products,combined with the ESI-MS detection,reasonable reaction routes of NHCs in WAO processes were proposed.(3)A novel technology of persulfate promoted WAO process(PWAO)was proposed to improve the COD removal at moderate reaction condtion.EPR test revealed that reactive oxidative species can be generated in a wide temperature range,which made PWAO process had a better performance in COD removal.About 94.7%of COD can be eliminated from BTH solution(initioal COD 2300 mg/L)at 160?in120 min,and 65.0%of BTH was converted into functinal polymers and oxidation of residual organics in water happened simultaneously.Other NHCs with high COD concentration including ID,BM and BT can also be partially converted to polymers and partially oxidized via PWAO.(4)Reaction mechanism and the effects of operation parameters on the formation of microspheric polymers in PWAO of NHCs were investigated.The polymers obtained in PWAO of NHCs were microsphper polymers,with size distribution ranging from 0.1?m?10?m for different NHCs.At the optimal condtion,about37.2%of ID,21.5%of BM and 23.1%of BT could be converted into polymers.After carbonization,these polymers transferred into doped carbon materials,and the nitrogen&sulfur co-doping carbon material had the highest specific surface area(778.3 m²/g),with good absorption capacity and catalytic effect in removal of organics.The absorption capacity of each gram of nitrogen&sulfur co-doping carbon material was 600 mg for BTH or 300 mg for BM.These doped carbon materials made from BTH,BM,BT and carbon materials retrieved from ID also showed good catalytic effects in persulfate oxidation.