Mechanism And Method Of Production Of Hydroxyl Radicals And Fast Degradation Of Sulfadiazine

The accumulation of antibiotics in river watersheds and lakes could induce the spread of antibiotic-resistance genes in drinking water,causing damage to marine ecological environment and human health.The conventional water treatment methods,such as chlorine,were insatiable for the goals of no antimicrobial activity and negative environmental effects.Hydroxyl radical(·OH),as the core of advanced oxidation technology(AOT),has the advantages of mineralizing organics non-selectively with no second pollution.However,the concentration and yield of·OH are very low in current chemical agents needing AOTs.Based on the establishment of an efficient·OH generation method by oxygen activated species(OAS)combined with water jet cavitation,this paper focuses on:the clarification of the chemical reaction mechanisms of high-efficient·OH formation by oxygen active species(especially O2+),and the chemical reaction mechanism of·OH mineralization of antibiotic sulfadiazine(SDZ).Following four research contents will be carried out:(1)The oxygen was introduced into the plasma reactor to form high concentration of oxygen active species(OAS),such as O2+,·O,and O3 by strong ionization discharge under synergetic discharge mode of micro-glow and micro-streamer in narrow discharge gap.A gas/liquid injector was used to facilitate·OH production through water jet cavitation.The electron spin resonance(ESR)method was used to verify the efficient formation of-OH,and the p-hydroxybenzoic acid high-performance liquid chromatograph method was esdablished to study the function relationship between total reactive oxygen(TRO)and·OH concentration.The maximum equilibrium concentration of·OH was 204.30 ?mol/L,and the generation time was only 1 s.The yield of· OH was 204.3?mol/L s,which was 2905 times higher than that of ozone method.(2)The·OH generation pathways through O2+,HO2-and·O2-reactions were investigated by using fluorescence and ESR methods.The formation of key reactive species in situ,such as H2O2 and·02-,were proved,and the maximum concentration of H2O2(6.22 mg/L)was 7.7 times higher than that of catalytic ozone system,and the concentration of·O2-(10.21 ?mol/L)was 1000 times higher than that of Fenton(Fe(?)-oxalic acid)system.The reaction rate constants for·OH generation equations were calculated by quantitative calculation methods.According to the experimental and theoretical calculation results,three reaction pathways of·OH generation are concluded as follows:?The hydrated clusters generated by O2+further dissociate into-OH.?In the presence of initiator HO2-,O3 is hydrolysed to generates-OH through radical chain reactions.?·O2-reacts with O3 to generate intermediate product·O3-,and the protonated HO3·dissociates to generate·OH.(3)The generated·OH was used to mineralize antibiotic sulfadiazine(SDZ)rapidly in ultrafine bubbles.The kinetics model of "dose-effect" functions between total reactive species(TRO)and SDZ was established.·OH was proved to be the key active species on the degradation of SDZ by analyzing the effect of different active specieses on the degradation of SDZ,and the reaction rate constant between·OH and SDZ was calculated as 1.96 × 109 L/mol·s.According to the evolution of total organic carbon and inorganic ions,the mineralization of SDZ by·OH was achieved when the mass ratio between TRO and SDZ was 7:1,and the treatment time(0.28 s)was much lower than that of electro-fenton system(>4 h).(4)The degradation position of SDZ were verified and the chemical reaction mechanisms were investigated.The chemical structures of SDZ such as dissociation equilibrium constant,net charge distribution,bond energy and activation energy were analyzed by experimental and quantitative calculation methods.The dissociation equilibrium constant on sulfonamide group(pK2)was 6.21,and the sulfonamide group,pyrimidine heterocycle and amino group on aniline of SDZ were the main·OH degradation reaction sites.It was proved that·OH was the main reactive active oxygen species to break pyrimidine heterocycles and aniline group of SDZ by using ESR method.Based on the analysis of high-performance liquid chromatograph-mass spectrometry/mass spectrometer and gas chromatograph-mass spectrometer,·OH mineralized the SDZ through the following three reaction pathways:1)oxidizing the amino group on aniline through hydrogen extraction substitution reaction;2)removing SO2 through breaking the S-C bond and S-N bond to break the pharmacophore group;3)breaking the N-C bond on pyrimidine heterocycle and opening the benzene ring to via dimerization dehydrogenation or hydroxylation steps.Finally,the·OH mineralizes these small molecules to CO2,H2O,and inorganic ions.(5)·OH degradation of antibiotics and·OH disinfection in the process of algae bloom water treatment in a drinking water treatment system of 12,000 m3 per day was completed during a period of algae blooms.In the process of conveying algae bloom water within only 20 s,the·OH after coagulation sedimentation at 1.0 mg/L and after sand filtration at 0.5 mg/L degraded norfloxacin(NFX)and SDZ to not detected.Based on the detected intermediates,·OH mineralizes SDZ and NFX into CO2 and H2O by breaking their pharmocophore group.The·OH disinfection inactivated the algae from 2.04 × 103 to 0 cells/mL,and disinfection byproducts,such as bromate,aldehydes,haloacetic acides and halomethanes were not detected.The tests of 106 drinking water quality indicators afterOH disinfection satisfied the Chinese Standards(GB5749-2006).Amining at the problem of accelerating the development of antibiotic-resistance bacteria caused by residual antibiotic in water bodies,this study established a new·OH production method to achieve the-OH advanced mineralization of antibiotic with high efficiency and safety,which provides a new idea for degradation of antibiotic in polluted water.