Kinectics And Mechanisms Of Trichloroethylene Degradation Sulfidated Nano Zero Valent-iron In Groundwater

Trichloroethylene（TCE）is one of the persistent organic pollutants with the highest detection rate in soil and groundwater pollution.It possesses carcinogenic,teratogenic and mutagenic effects and thus seriously threatens ecological security and human health.The groundwater remediation technology with sulfidated nano zero valent iron（S-nZVI）as the core remediation material features high efficiency,low cost,and low potential environmental risk.It is considered to be one of the in-situ remediation technologies with promising application potential.At present,it has been proved that S-nZVI can greatly improve the degradation performance of TCE,and the dechlorination mechanism of TCE has been also investigated to some extent,but there are still the following scientific problems to be further clarified:1)Relationship between the surface characteristics and microstructure of S-nZVI and the degradation performance of TCE;2)Adsorption-reaction mechanism of TCE at S-nZVI interface;and 3)Identifying key influencing factors and rate-limiting steps in TCE degradation process.Therefore,seven kinds of S-nZVI with different nanostructures were synthesized by one step,two step,and ZIF-8 surface modification methods.The structure-activity relationship between S-nZVI surface characteristics and TCE degradation performance was also explored.The influence of typical environmental factors on TCE degradation was also investigated.Moreover,based on the theoretical calculation and intermediate product analysis,the adsorption-reaction steps of TCE degradation at S-nZVI interface was deeply explored.The kinetic characteristics of TCE degradation were analyzed.The main results are displayed as follows:1.Seven types of S-nZVI with different surface structure characteristics were designed and synthesized.The seven types of S-nZVI were characterized by SEM,TEM,BET,XRD and XPS.The morphology,structure and physical and chemical properties of the S-nZVI series materials were systematically analyzed and compared.The specific surface areas of S-nZVI with different Fe/S ratios（2.5,5,7.5,and 10）synthesized via one-step method were 95.66,53.52,57.60 and 27.11 m²g^-1,respectively,whereas,the specific surface area of S-nZVI synthesized via two-step method was low,indicating that the sulfidation can increase the specific surface area of the material.The specific surface area of ZIF-8@S-nZVI and ZIF-8@S/N-nZVI reached 54.19 and 60.21 m²g^-1,respectively,which were about twice that of S-nZVI under the same Fe/S ratio.With the increase of Fe/S ratio,the content of Fe⁰on the surface of S-nZVI synthesized by one-step method increased and the content of Fe（III）decreased,while the content of Fe（II）basically remained unchanged,and the sulfur element in the material mainly existed in the form of S^2-,S₂^2-and SO₄^2-.With the increase of sulfur content,the content of S^2-decreased and the content of S₂^2-increased.The content of Fe⁰was reduced by zif-8 coating,and the content of Fe⁰was further reduced after the introduction of N element into the S-nZVI.Furthermore,both ZIF-8 coating and the introduction of non-metallic element N reduced S^2-content and increased S₂^2-content.2.The effects of different types of S-nZVI and key factors on the TCE degradation performance were investigated.Degradation performance of TCE and hydrogen production were investigated with seven types of S-nZVIs,including the neat S-nZVI with four diferente Fe/S raio,ZIF-8@S-nZVI and ZIF-8@S/N-nZVI.The order of TCE degradation activity follows as S-nZVI_one-step>S-nZVI_two-step>ZIF-8@S-nZVI>ZIF-8@S/N-nZVI.In the degradation process witht the neat nZVI,the H₂evolution rate reached over 70%,which was not suitable for the degradation of TCE in groundwater.S-nZVI significantly improved the degradation performance of TCE.The performance with the S-nZVI synthesized by two-step method is weaker than that of one-step method,but the H₂evolution is the lowest.The reason is that the presence of Fe S_xlayer reduced the direct contact between Fe⁰and H⁺in water.Among the S-nZVI synthesized by one-step method,the best Fe/S ratio was 10,indicating that the higher the Fe⁰content is,the more conducive to TCE degradation.In addition,with the increase of temperature,the reaction activity of S-nZVI to TCE increased,which enhanced the TCE degradation.The 24-h degradation efficiency of S-nZVI with Fe/S ratio of 10 reached 92.6%.When the p H is in the range of 6-9,the maximum value of TCE removal efficiency was 93.2%when the p H was 7.Moreover,SO₄^2-promoted the TCE degradation by S-nZVI,while Cl^-and NO₃^-inhibited it.3.The pathway and mechanism of TCE degradation by S-nZVI were clarified.Based on quantum mechanics-molecular dynamics simulation,it is found that low sulfur nZVI is conducive to the degradation of TCE,the presence of Fe S on the surface of S-nZVI is benefit for the TCE adsorption and degredation.Moreover,it is clear that the preferred dechlorination site of S-nZVI is Cl2,which determines that the dechlorination pathway of TCE by S-nZVI mainly followsβ-elimination reaction pathway.Furthermore,the results of product analysis demonstrated the theoretical calculation results,and all the types of S-nZVI followsβ-elimination reaction pathway.The main gaseous products were acetylene,ethylene and ethane,and no products such as DCEs and VC were detected.It is found that the accumulation of acetylene,ethylene,and ethane during TCE degradation by S-nZVI synthesized by one-step method was different with the various sulfur contents.Under the condition of high TCE concentration,the lower the sulfur content,the greater the accumulation of acetylene and ethylene,indicating that the electron utilization efficiency of iron atom in the TCE degradation was high.It is worth noting that under the condition of low TCE concentration,the product of TCE degradation by S-nZVI with low sulfur content is mainly ethylene,and the accumulation of acetylene was small.This is mainly due to the excessive iron content as an electron donor and excellent electron transfer performance under the condition of low TCE concentration,which resulted in the rapid transformation of the intermediate product acetylene to ethylene.4.The kinetic characteristics of TCE degradation by S-nZVI were analyzed.The effects of temperature,p H and anion on the kinetic parameters were investigated.It was found that that the degradation order of TCE by S-nZVI is affected by the relative content of Fe⁰.When the relative content of Fe⁰is low,it follows zero-order reaction.When Fe0 is abundant,it follows first-order reaction.When Fe/S ratio was 7.5 and 10,the relative content of Fe⁰is abundant,and S-nZVI shows the characteristics of first-order reaction kinetics under the tested TCE concentrations.Moreover,with the increase of initial TCE concentration,the rate constant showed a downward trend.Appropriate reaction temperature can not only accelerate the reaction rate,but also reduce the occurrence of side reactions.At the same temperature,the larger the Fe/S ratio in S-nZVI,the larger the K_sa,indicating that a small amount of sulfur can increase the reaction rate.Overall,the values p H did not noticeably vary the k_s.However,the neutral environment was conducive to the degradation of TCE.Whereas,the weak acidic environment was conducive to the side reaction of hydrogen evolution.Under the weak alkaline environment,the iron oxide layer on the surface of S-nZVI was passivated,reducing the contact between Fe⁰and TCE,resulting in the decrease of reaction rate.Appropriate concentration of SO₄^2-（1 m M）can promote the TCE degradation with a maximum k_saof0.22 L h^-1m^-2.The presence of Cl^-and NO₃^-inhibited the degradation rate constant.In summary,S-nZVI degraded TCE viaβ-elimination reaction pathway with acetylene,ethylene and ethane as the main gaseous products.Kinetic analysis showed that TCE degradation accorded with zero order or first-order reaction kinetics,which was dependant upon the concentrations of Fe⁰in S-nZVI and TCE.The sulfur element on the surface of S-nZVI exists in the form of Fe S,which is conducive to TCE adsorption and electron transfer on its surface.Therefore,S-nZVI with low sulfur content（Fe/S=10）exhibited better degradation performance.Compared with the neat nZVI,the S-nZVI_one-step（Fe/S=10）exhibilited 2-8 times higher TCE removal efficiency within the same raction time.