Study On The Aging Of Sulfidated Micro-sized Zero Valent Iron In Air And Its Subsequent Reactivity For Chloramphenicol Degradation In Different Acid Systems

With the development of economy and society,the problem of water pollution has been paid more and more attention.Antibiotics,as a new type of organic pollutants,have attracted more and more attention due to environmental pollution and ecological toxicity.It has become a research hotspot in the field of water treatment to seek an efficient method for antibiotic removal.Zero-valent iron(ZVI)has been widely used in the degradation of various organic pollutants including chloramphenicol due to its cheap and easy availability,strong reductive property and environmental friendliness.However,ZVI had some defects such as poor electron selectivity and easy passivation.Sulfidated zero valent iron(S-ZVI)is a modified material prepared by modifying sulfur compounds on the surface of ZVI.S-ZVI could enhance the reactivity of ZVI and improve the selectivity towards pollutants.Chloramphenicol(CAP)is a nitroaromatic chloride antibiotics,which has been widely used in the treatment of various sensitive bacterial infections.Since it has serious side effects on the hematopoietic system,its production and use have been restricted in many countries.In China,the residual problem of CAP in water and organisms is severe,and the pollution is still very serious."Aging" refers to the change in the chemical composition,structure and physical properties of materials due to the combined action of heat,oxygen,water,light,microorganisms,chemical media and other environmental factors.Aging can change the composition of iron oxide shell of ZVI and directly affect the reactivity of ZVI.In previous studies,S-ZVI had different degradation of CAP under the same experimental conditions due to different storage days.The oxidation layer formed during the aging process of ZVI in the air also had impacts on the ZVI reactivity to degrade pollutants.In practical applications of ZVI-based materials,it is worth studying whether the material is still reactive after being placed in the air for several days,weeks or even months.So far,most of the aging studies on ZVI materials only consider the reactivity of ZVI in aqueous solution,ignoring whether the time of its placement in the air has an effect on the reactivity of ZVI in aqueous solution.In this work,we investigated the degradation of chloramphenicol(CAP)in sulfuric acid(SA)and citric acid(CA)systems by wet-synthesized sulfidated micro-sized zero valent iron(S-mZVI)in dry air and the mechanism of degradation of chloramphenicol(CAP)in materials with different aging days.The results show that with the increase of the aging days in the air,the degradation effect of S-mZVI on CAP in different acid systems presents a parabolic trend of first increasing and then decreasing,and the degradation effect of S-mZVI on CAP reaches the best within 5-9 days.The degradation pathway of CAP can be divided into oxidation pathway and reduction pathway.The analysis of ultra-performance liquid chromatography-tandem mass spectrometry(UPLC-MS/MS)showed that the degradation products of CAP in SA and CA systems were similar.The reduction products of CAP mainly include CAP-O,CAP-20,CAP-H2O,CAP-2O-Cl,etc.The results of X-ray photoelectron spectroscopy(XPS)and X-ray diffraction(XRD)showed that the characteristic peak of Fe3O4 was detected on the surface of the material with the increase of aging days.When S-mZVI aging in the air for 5-9 days,the peak of Fe3O4 was relatively large,and the characteristic peak of Fe3O4 gradually disappeared with the increase of subsequent aging days.The generation of Fe3O4 may be the reason why S-mZVI with different aging days has different degradation effects on CAP.In addition,in different acid systems of sulfuric acid and citric acid,the degradation curves of S-mZVI to CAP in CA and SA systems were different.The reasons for the difference include two aspects.On the one hand,S-mZVI reacted in the CA system adsorbed CAP in the early stage of reaction,so the concentration of CAP in the CA system decreased rapidly in the early stage of reaction.At the same time,CA,as a weak organic acid,could be adsorbed on the surface of S-mZVI at the later stage of the reaction,blocking the active site of S-mZVI to CAP,while SA did not adsorb on S-mZVI(or the adsorption was weak),which made the degradation curves of S-mZVI to CAP in CA and SA systems were different.On the other hand,under the condition that CA regulated different initial pH,the degradation curves of S-mZVI to CAP were significantly different,while no significant difference was found in the SA system with different initial pH.Combined with FTIR analysis,it was speculated that CA would adsorb on the surface of S-mZVI when the system pH was>3.7.As a result,the degradation efficiency of S-mZVI to CAP in the system with the initial pH of 3.5 and 4.0 regulated by CA was significantly lower than that in the system with the initial pH of 3.0 regulated by CA.Which was also the reason for the obvious difference in the degradation curves of different initial pH values in CA systems.