Efficiency And Mechanisms Of Trichloroethylene Removal By Modified Nanoscale Zero-valent Iron In Aqueous Solution

Nanoscale zero-valent iron(NZVI)has been widely used for the degradation of trichloroethylene(TCE)in contaminated water,however,it is suffering from being easy to aggregate,having low adsorption capacity and relatively lower dechlorination efficiency for TCE.In order to overcome the shortcomings of NZVI,modified NZVI particles with biochar(NZVI/BC)and sulfide-modified nanoscale zero-valent iron(S/NZVI)were synthesized and applied for the degradation of TCE in this thesis.The results are as follows:Part one: NZVI/BC composites were applied in this study for removal and degradation of TCE.The effects of pyrolysis temperature of BC,mass r atio of NZVI/BC and solution pH on the removal and degradation efficiency of TCE were studied.The different pyrolysis temperatures of BC resulted in the differences on the surface areas,aromaticity and noncarbonized fractions,which determined the sorption capacity of TCE.Compared with pure NZVI,the NZVI/BC at different mass ratios could increase the removal efficiency of TCE to 99%,which was attributed to the higher adsorption capacity of BC for TCE.Besides,the yield of final products(ethane,ethylene and acetylene)differed at different mass ratios of NZVI/BC.Generally,the main product was ethylene in all reactions and the yield of acetylene and ethane were relatively low.Solution pH had little effect on the total removal of TCE but significantly influenced the yield of final products.The yield of ethylene decreased with the increasing pH.The results indicate that solution pH could not affect the sorption of TCE but influenced the degradation rate of TCE by NZVI/BC.Part two: S/NZVI has been considered as an efficient material to degrade trichloroethylene(TCE)in groundwater.However,some critical factors influencing the dechlorination of TCE by S/NZVI have not been investigated clearly.In this study,the effects of Fe/S molar ratio,initial pH,dissolved oxygen and particle aging on TCE dechlorination by S/NZVI(using dithionite as sulfidation reagent)were studied.Besides,the feasibility of reactivation of the aged-NZVI by sulfidation treatment was looked into.The results show that the Fe/S molar ratio and initial pH significantly influenced the TCE dechlorination,and a higher TCE dechlorination was observed at Fe/S molar ratio of ~60 under alkaline condition.Spectroscopic analyses demonstrate that the enhanced TCE dechlorination was associated with the presence of FeS on the surface of S/NZVI.Dissolved oxygen had little effect on TCE dechlorination by S/NZVI,revealing that the FeS layer could be able to alleviate the surface passivation of NZVI caused by oxidation.Aging of S/NZVI up to 10~20 d only slightly decreased the dechlorination efficiency of TCE.Although an obvious drop in dechorination efficiency was observed for the S/NZVI aged for 30 d,it still exhibited a higher reactivity than the bare NZVI.This indicates that sulfidation of NZVI did prolong its lifetime.Additionally,sulfidation treatment was used to reactivate the aged NZVI,and the results show that the reactivated NZVI even had higher reactivity than the fresh NZVI,suggesting that sulfidation treatment would be a p romising method to reactivate the aged NZVI.