Effect Of PH And Co-existing Anions On The Removal Of Chloramphenicol By Sulfidated Of Zerovalent Iron/Ferrous Ion System

Chloramphenicol（CAP）is an effective broad-spectrum antibiotic.Its side effects and toxicities are harmful to human health.Zero-valent iron（ZVI）is widely used to remove organic pollutants from soil,groundwater and wastewater because of its environment-friendly,cost-effective and abundant,and stronger reduction ability.However,ZVI technology has the disadvantages of easy passivation and low electron selectivity to pollutants.Compared with ZVI,the reactivity and selectivity of sulfidated of zero-valent iron（S-ZVI）towards pollutants are significantly improved.Therefore,S-ZVI has become a research hotspot in this field.Water chemistry parameters,such as pH,co-existing anions and co-existing cations,have great influence on the removal of pollutants by ZVI/S-ZVI.As a cation,ferrous ion(Fe²⁺)is commonly used to enhance the ability of ZVI/S-ZVI to remove contaminants because Fe²⁺ promotes the formation of semiconductor Fe3O4.However,most studies only consider the effect of Fe²⁺ itself on the reaction system,ignoring the role of coexisting anions introduced by adding Fe²⁺ in the reaction process.Therefore,this study focused on the influencing mechanism of pH and co-existing anions on the removal of CAP by S-ZVI/Fe²⁺.In this study,the Fe²⁺ concentration,the presence of oxygen,the adding time of Fe²⁺ and two kinds of coexisting anions（Cl-and SO42-）were selected as independent variables,and the removal effect of CAP was taken as dependent variable to explore the effects of Fe²⁺ on the removal of CAP by S-ZVI under different experimental conditions.The results showed that the addition of FeCl2 could buffer the pH of the solution to the condition of weak acid and promote the removal of CAP by S-ZVI.With the increase of Fe²⁺ dosage,the promoting effect increased first and then decreased and the best effect occurred at Fe²⁺ dosage of 2 mM.Oxygen significantly inhibited CAP removal by S-ZVI/Fe²⁺.Adding Fe²⁺ at different times points did not change the CAP removing ability by S-ZVI,but only changed the instantaneous removal rate of CAP.Different from FeCl2,FeSO4 could hardly promote the removal of CAP by S-ZVI.Based on the above research results,the influence of buffer pH and coexisting ions on the reaction was studied,to explore the influencing mechanism of Fe²⁺ on the removal of CAP by S-ZVI.The results showed that pH buffers of 2-morpholine ethanesulfonic acid（MES）,4-hydroxyethyl piperazine ethanesulfonic acid（HEPES）and trimethylaminomethane（TRIS）could buffer the pH of solution to 6,7 and 8,respectively.All of them could promote the removal of CAP by S-ZVI,and the promotion effect decreased in turn.Similar to Fe²⁺,the addition of Na+,Ca²⁺and Mg²⁺ could accelerate the removal of CAP by S-ZVI.In addition,the removal effect of CAP was strongly correlated with the concentration of added NaCl,and almost had no correlation with the concentration of added Na2SO4.Therefore,FeCl2 promoted the removal of CAP by S-ZVI was mainly because of the pH buffering effect of Fe²⁺and the ion effect of Cl-.In addition,a series of characterization methods were used in this study to explore the reasons why FeSO4 could not promote CAP removal by S-ZVI.The results of High Performance Liquid Chromatography-Mass Spectrometry（HPLC-MS）showed that only the denitration reaction of CAP occurred,and Na+,Fe²⁺,Cl-and SO42-did not change the degradation pathway of CAP.The scanning electron microscopy（SEM）showed that the morphology of S-ZVI before and after the reaction with FeSO4 as the background electrolyte was a flaky structure containing FeSx,which indicated that FeSO4 might inhibit the participation of FeSx in the reaction.Correspondingly,the results of X-ray diffraction（XRD）showed that,under the condition of FeSO4 as the background electrolyte,only Fe0 characteristic peak existed on the surface of S-ZVI after the reaction,and no characteristic peaks of Fe2O3 or Fe3O4 corrosion products existed.In addition,X-ray photoelectron spectroscopy（XPS）analysis showed that SO42-would partially adsorb on the surface of S-ZVI,which might lead to the occupation of active sites on the surface of S-ZVI.Therefore,the main reason why FeSO4 cannot promote the removal of CAP by S-ZVI is that FeSO4 can inhibit the corrosion of S-ZVI.The specific mechanism may be that FeSO4 inhibits the FeSx layer from participating in the reaction or SO42-adsorbed on the surface of S-ZVI occupied the reaction site of CAP.This study can provide a scientific basis for the engineering application of Fe²⁺/sulfidated zero-valent iron in the treatment of chloramphenicol,and provide technical support for the treatment of chloramphenicol in antibiotic polluted water.