Degradation Performance And Mechanism Of Pharmaceutical Pollutants In Electro-Peroxone System Enhancing By Micro-Nano Bubbles

Pharmaceutical pollutants in the water environment with trace concentration levels(e.g.,ng L^-1～μg L^-1)will cause irreversible impact on human and ecosystem.Currently the Electro-peroxone（EP）process has been used to treat pharmaceutical pollutants from water effectively.The EP oxidation system could react hydrogen peroxide（H₂O₂）generated by in-situ electrochemical two-electron process of oxygen reduction reaction（2e^-ORR）with ozone to produce hydroxyl radicals（·OH）,thereby effectively removing pharmaceutical pollutants.However,in the EP process with conventional aeration,there are mass transfer limitation of oxygen and ozone.How to improve H₂O₂ production and ozone utilization are still worth exploring.Compared with millimeter sized large bubbles under conventional aeration,micro-nano bubbles（MNBs,diameter range of<50μm）could greatly improve oxygen and ozone transfer efficiency.MNBs has shown good technical advantages and broad application prospects.However,in the EP system,the study of the impact of MNBs on gas mass transfer performance and the ability to remove pharmaceutical pollutants in the EP system is inadequate.Therefore,this paper focused on combining MNBs aeration technology with EP technology.Compared with conventional aeration,the impact of MNBs aeration on the mass transfer process of oxygen and ozone were studied.And the effect of MNBs on electrochemical H₂O₂production via 2e^-ORR and pharmaceutical pollutants removal under EP system were discussed.Furthermore,the removal effect of pharmaceutical pollutants from actual wastewater was investigated.The following results provided technical and theoretical basis for efficient control of pharmaceutical pollutants in the water environment.（1）Electrochemical H₂O₂ generation via oxygen reduction has to face dissolved oxygen transfer limitation in conventional aeration system.To mitigate this problem,boosting oxygen diffusion by micro-nano bubbles（MNBs）for highly-efficient H₂O₂ generation on air-calcining graphite felt（GF600）has been studied.The oxygen transfer coefficient and rate of MNBs were0.160 min^-1 and 0.382 kg m^-3 h^-1,about 4 times and 5 times greater than those of conventional aeration,respectively.As a result,the GF600 electrode under MNBs aeration could produce140.4±1.6 mg L^-1 H₂O₂ with an ultrahigh coulombic efficiency of 97.8±0.9%,but only47.9±2.5 mg L^-1 H₂O₂ with 65.9±3.7%under conventional aeration.By increasing the potential on the GF600 electrode,the H₂O₂ production showed the first rising then descending trend and the optimized potential of-0.6 V vs Ag/Ag Cl.Besides,during 5 cycles the H₂O₂ production of GF600 was stably ranged from 124.8 mg L^-1 to 145.2 mg L^-1 under MNBs aeration.Thus,MNBs could effectively solve O₂ mass transfer limitation,paving a promising way for in-situ synthesis of H₂O₂.（2）Ozone mass transfer plays a dominationg roles in ozone utilization rate in EP process,in which the transportation of ozone was limitaed by conventional aeration.To overcome this limitation,the EP system with MNBs aeration（MNBs-O₃-E）was prepared.Thus,the mass transfer of ozone,the removal rate of single pharmaceutical pollutants,and the generation performance of·OH were investigated.When the ozone concentrations introduced were 5,20,and 45 mg L^-1,the ozone mass transfer coefficients under MNBs aeration were 0.536 min^-1,0.490 min^-1,and 0.265 min^-1,respectively,which were higher than those under conventional aeration(0.220 min^-1,0.115 min^-1,and 0.09 min^-1).With the same ozone dosage,the ozone overflow rate under MNBs aeration conditions was lower（about 30%of that under conventiona aeration）,which could significantly reduce the waste of ozone.It was found that,in the MNBs-O₃-E process,the ibuprofen removal rate was 98.4±1.5%,·OH exposure was 0.97×10^-9 M s,and the ozone utilization rate was 262.38±4.04 mg_（IBU）mg^-1_（O3）.The better·OH exposure of MNBs-O₃-E system was mainly attributed to the following points:（1）MNBs could generate·OH at the gas-liquid interface during the blasting process,and stimulate oxygen,ozone,and H₂O₂ to produce·OH;（2）MNBs aeration improved the mass transfer of oxygen and ozone,increaseing the generation of H₂O₂ by 2e^-ORR and the solubility of ozone.And the interaction of H₂O₂ and ozone could generate more·OH for effective single pharmaceutical pollutants removel.In addition to the removal rate of ibuprofen reaching 98.4±1.5%,the removal rate of sulfamethoxazole,ribavirin,norfloxacin,tetracycline and ampillin exceeded 99%under MNBs-O₃-E process.（3）Pharmaceutical pollutants usually exist in water environment at very low concentrations.And in the degradation process of combined pharmaceutical pollutants,other pollutants would interfer this process.MNBs-O₃-E process was developed for the removal of combined pharmaceutical contaminants at trace concentrations.When the initial concentration of pollutants was 2500μg L^-1,250μg L^-1 and 50μg L^-1,the common degradation rate of the six model pollutants exceeds 95%,96%and 94%,respectively.The most easily degradable pollutants were ribavirin and ampillin.The more easily degradable pollutants were sulfamethoxazole and tetracycline.And ibuprofen and norfloxacin were difficult to be removed than others.Based on the theoretical calculation method,the reasons for the different degradation priorities of the six pollutants attacked by·OH were analyzed.The atoms at the location of electron-donating group had a large Fukui function value,such as amino（-NH₂）and hydroxyl（-OH）.Especially,the electron-donating groups would promote degradation of pharmaceutical pollutants,whereas electron-withdrawing groups had the opposite effect on degradation of compounds（such as-F and carboxyl（-COOH））.With the interference of humic acid,the removal rate of compound pharmaceutical pollutants exceeded 80%by MNBs-O₃-E system.The compounds susceptible to interference from humic acids were norfloxacin and tetracycline,while the other four pollutants were less affected.After treatment with MNBs-O₃-E,the removal rates of pharmaceutical pollutants of hospital A wastewater,hospital B wastewater,effluent from secondary sedimentation tank of sewage plant and aquaculture wastewater were 99.10%,98.59%,94.14%and 94.26%,respectively.The MNBs-O₃-E system had excellent removal efficiency for combined pharmaceutical pollutants with low concentration.