| Trichloroethylene(TCE)is a representative of persistent toxic chlorinated organic compounds,which has attracted global attention because of its harm to environment and human health.The nano zero valent iron(NZVI)is considered as a good pollutant removal material because of its high reduction performance,especially for the persistent chlorinated organic pollutants such as trichloroethylene.However,the NZVI is easy to be oxidized in the natural environment,resulting in the deposition of surface iron oxide,passivation of materials,reduction of surface active sites,which is not conducive to the release of internal Fe0,and ultimately reduces the reduction and removal efficiency of TCE by NZVI.Therefore,how to modify NZVI,improve its reaction activity and antioxidant capacity has become a research hotspot.In this study,NZVI was modified by sulfurization.The sulfurized nanometer zero valent iron(S-NZVI)was prepared by one-step synthesis using calcium polysulfide,and the NZVI without adding calcium polysulfide were prepared as the control.It was revealed by scanning electron microscope(SEM)that the morphology of NZVI was mainly spherical,while that of S-NZVI was mainly spherical and flaky.Meanwhile,with the increase of S/Fe ratio,the content of spherical material decreased,while that of flaky material increased.At the same time,the surface area and roughness of the modified S-NZVI increased,and it had the iron sulfide shell structure which was beneficial to inhibit zero valent iron core releasing.In addition,the X-ray diffraction(XRD)analysis showed that the crystallinity of the shell was good,including the microcrystalline structure of ferrous sulfide(Fe S)and elemental iron(Fe0),and the amorphous structure of ferrous disulfide(Fe S2).In this paper,the removal of TCE by S-NZVI and the influencing factors were studied.The results showed that the nano zero valent iron modified by calcium polysulfide had a high removal efficiency of TCE,and the removal rate can reach 99.8% within 48 h.It revealed that the main reduction product of TCE was acetylene,followed by a small amount of cis-1,2-dichloroethylene.The degradation process of TCE was almost p H-independent.Although the degradation rate decreased under alkaline condition slightly,the efficiency could still reach more than 90%.The reduction performance of S-NZVI was significantly affected by the S/Fe ratio.The nanoparticles with the S/Fe ratio of 0.5 has the best reduction effect.In addition,the S-NZVI has good recycling performance that the removal efficiency of TCE still remained above 95% after four times of recycling.It has been confirmed by experiments that it contained a variety of sulfur forms of iron sulfide including Fe S2,Fe S and elemental iron,which were all involved in the removal process.It is proved by carbon stable isotope analysis that Fe(?)and S22-played leading roles in acid and alkaline conditions,respectively.At the same time,S-NZVI has a good co-degradation effect in the binary pollution system of TCE and carbon tetrachloride,showing that the nano zero valent iron sulfide modified by calcium polysulfide has a broad application prospect in the removal of various chlorinated saturated aliphatic hydrocarbons.In this study,we synthesized SNZVI by polysulfide for the first time that contained various sulfur forms,which was used to remove the wastewater polluted by TCE with wide range of p H value.It can also be used as an effective reducing agent to remove a variety of chlorinated aliphatic hydrocarbons to achieve complete reductive dechlorination.It can provide a theoretical basis for the wide application of S-NZVI in wastewater and groundwater in-situ remediation. |
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