Study On Oxidative Degradation Of Typical Organic Pollutants By Persulfate Activated With Iron Oxide

Sulfate radical-based advanced oxidation processes（SR-AOPs）have become one of the most potential methods for the degradation of organic pollutants in water due to its advantages of high degradation rate,no secondary pollution and low cost.Magnetite with the advantages of strong magnetism,good environmental affinity and large natural reserves has been widely used in the treatment and remediation of environmental pollution.In this paper,magnetite nanoparticles（MNPs）with different exposed crystal facets were prepared by hydrothermal method and used as catalyst to activate persulfate（PS）for tetracycline degradation.The effect of exposed crystal plane on the catalytic degradation of magnetite and its reaction mechanism were studied.The effect of natural organic matter humic acid（HA）and chelating agent L-Cys on the catalytic performance of magnetite was also discussed.Thus,the specific research contents of this dissertation are as follows:（1）Three kinds of magnetite nanoparticles with different exposed crystal facets were synthesized by hydrothermal methods to investigate the effects of exposed facets on the catalytic performance.It was found that the catalytic efficiency followed the order of MNPs{111}>MNPs{100}>MNPs{110}.Hydroxide,physically adsorbed H₂O on the MNPs surface and the transformations of Fe（II）/Fe（III）were the main factors affecting the high catalytic performance of MNPs{111},as illustrated by the X-ray photoelectron spectroscopy（XPS）analysis.Density functional theory（DFT）analysis showed that the exposed crystal facet of magnetite determine the electron density and density of states on the catalyst surface,thus affecting the catalytic performance of the catalysts.Attenuated total reflection Fourier transform infrared spectroscopy（ATR-FTIR）,electron paramagnetic resonance（EPR）spectroscopy and quenching experiment were used to propose the corresponding catalytic oxidation mechanism,and confirmed that SO₄^·-and·OH were the main free radicals involved in the catalytic degradation of tetracycline.The reusability and stability of MNPS were evaluated by cyclic experiments.（2）Three modified humic acids were obtained by selective blocking of specific functional groups（phenolic hydroxyl and carboxylic acid groups）,and characterized by Fourier transform infrared spectroscopy（FTIR）and three-dimensional fluorescence spectroscopy（3DEEM）.The results show that lower concentration of HA can promote the degradation of organic pollutants in PS/MNPs reaction system,while higher concentration of HA can inhibit the degradation of organic pollutants.By comparing the effects of different modified humic acids on the degradation of PS/MNPs system,it was found that both carboxylic acid groups and phenolic hydroxyl groups in HA participated in the degradation process of pollutants,and the carboxylic acid groups had a more significant impact on the degradation efficiency.Combined with ATR-FTIR spectroscopy and two-dimensional correlation（2DCOS）analysis,it can be confirmed that the carboxylic acid group of HA was complexed with MNPs preferentially,while the complexation of phenolic hydroxyl group with MNPs lags behind.Quenching experiment and EPR spectrum analysis,it is confirmed that SO₄^·-and·OH are the main free radical species in the system after adding humic acid.（3）The effect of chelating agent L-Cys on the removal efficiency of pollutants in PS/MNPs system was investigated.It was found that L-Cys could effectively promote the removal of tetracycline in PS/MNPs system compared with ethylenediaminetetraacetic acid,butyrate chelators.XPS analysis and determination of iron ions in the reaction system indicated that L-Cys can enhance the ability of Fe（III）reduction to Fe（II）.In addition,the effects of metal ions and inorganic ions on L-Cys/PS/MNPs system were further investigated.Low concentration of Cl^-has little effect on the degradation efficiency of tetracycline in L-Cys/MNPs system.When the concentration of Cl^-reached 50 m M,the degradation efficiency of tetracycline was significantly inhibited.The presence of HCO₃^-also inhibits the removal effect of pollutants,while the effect of NO₃^-and Mg²⁺on tetracycline removal efficiency was not significant.