Study On The Mechanisms Of Citrateenhanced Nano-zero-valent Iron To Degrade β-estradiol In Water

Every year,large amounts of steroid estrogen(SEs)are produced from the sewage treatment plants and livestock breeding industries.The SEs discharged from the sewage treatment plants can be effectively controlled by the promotion of the treatment process.However,the SEs discharged with non-point source pollution which caused by livestock and poultry breeding,are extremely difficult to control.Up to now,most major rivers and lakes in China have been polluted by SEs,and SEs in the environment can infiltrate into the soil and the surrounding water bodies under the effects of rainfall leaching and surface runoff.They show a worried impact on the ecological environment and human health.Now,nano zero valent iron(nZVI)is regarded as an ideal environmental restoration material and have been widely used in in-situ restoration of water and soil environment due to its superior performance.In this research,a new type of nZVI that could efficiently degrade SEs in the aquatic environment was designed based on the needs of the water body restoration,physicochemical properties of SEs,and the defects of nZVI.The degradation mechanisms of SEs were also explored.In this study,17β-estradiol(17β-E2),which has the strongest estrogenic activity,was used as the target.The citrates which were environment-friendly and easily biodegradable were used as the organic ligands to strengthen the degradation performance of nZVI.The performance of nZVI,nZVI-Citrate,and stably citrate ligand loaded nZVI(CLs-nZVI)were discussed respectively.Combined with the quantum chemical calculations based on the density functional function theory(DFT),the degradation mechanisms and degradation pathways of 17β-E2 were analyzed.nZVI could effectively degrade 17β-E2 under acidic conditions,but the degradation rate was about 10% under neutral conditions.The characterization of nZVI before and after the reaction showed that they had obvious oxidation,passivation,and agglomeration after the reaction.In the nZVI-Citrate system,the degradation rate of 17β-E2 under neutral conditions has been significantly improved,and the oxidation and agglomeration of nZVI were significantly inhibited.Furthermore,the micro-pressure hydrothermal method was used to load citrate ligands on the surface of nZVI,and the optimal parameters for the synthesis were obtained by the factor screening and response surface.The characterization of the optimal CLs-nZVI found that its surface was loaded with a spatial network structure formed by the coupling of citrates and their carboxyl-rich fragments,which made CLs-nZVI had better dispersion performance,oxidation resistance and acid corrosion resistance.The analysis of degradation mechanisms showed that the nZVI-Citrate system could significantly increase the degradation efficiency of 17β-E2 under neutral condition,and the degradation rates under all conditions were also significantly increased.However,the degradation efficiencies of 17β-E2 were inhibited under acidic conditions due to the competition between free citrates and 17β-E2.CLs-nZVI retained the degradation performance of the nZVI-Citrate system,while the acid inhibition was lifted.Scavenging experiments showed that the enhancement under acidic conditions was dominated by hydroxyl radicals(·OH),while the enhancement of degradation efficiencies in the nZVICitrate and CLs-nZVI systems under neutral conditions basically did not depend on the generation of free radicals.The singlet oxygen(1O2)tended to play a role in the later stage of the reaction.Other tests showed that the degradation efficiencies,degradation rates,and the contribution of free radicals were closely related to dissolved oxygen,p H,ferrous and iron ions.The analysis of degradation pathways combined with computational chemistry showed that 17β-E2 could convert to estrone(E1)under the action of nZVI,its benzene ring could be broken,and the laccase-like reactions were found in this system.With the participation of citrates,in addition to the similar conversion pathways,17β-E2 could also convert to estriol(E3).There was a specific cross-coupling between 17β-E2,its degradation products,and ligands in the system.This research has provided a new strategy for SEs restorations contaminated water,which could adapt to the neutral water environment and effectively degrade SEs.