Synthesis and Characterization of Mechanochemically Sulfidated Microscale Zero Valent Iron for Dechlorination of Trichloroethylene in Groundwater

Author:Gu Ya Wei

Supervisor:he feng

Database:Doctor

Degree Year:2019

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Pages:126

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Chlorinated hydrocarbon,hardly degraded naturally,is a common industrial contaminant in groundwater.Its effective treatment is important for the safety of groundwater resources.Zero-valent iron(ZVI)is the most common and most likely applied material for chlorinated hydrocarbon remediation.In water treatment processes that involve contaminant reduction by ZVI,reduction of water to dihydrogen is a competing reaction that must be minimized to maximize the efficiency of electron utilization from the ZVI.Sulfidation has recently been shown to decrease H2 formation significantly,thus the electron efficiency/selectivity of contaminant reduction can be greatly increased.To date,the sulfidation focus on nanoscale ZVI(nZVI)to produce sulfidated nZVI(S-nZVI).However,there are many technical defects for the production and application of S-nZVI:(1)S-nZVI is prepared in aqueous phase system,during which the part of Fe0 will be consumed due to hydrogen evolution reaction;(2)A mass of high salinity wastewater is produced from solution-phase sulfidation agents(e.g.,bisulfide,dithionite or thiosulfate);(3)Reducing agent borohydride is relatively expensive,and could release a large amount of hydrogen(easy to explode);(4)The newly generated Fe0 and ferrous sulfide inevitably come into contact with air and are partially oxidized,reducing reactivity of S-nZVI for targeted contaminant;(5)The S-nZVI particles are prone to agglomeration,reducing its reactivity.To overcome these challenges,we developed a process for sulfidation of microscale ZVI by ball milling ZVI with elemental sulfur,called mechanochemically sulfidated microscale zero valent iron(S-mZVIbm).In this study,we systematically evaluate the activity,selectivity and pathway of S-mZVIbm for TCE dechloridation.The particle size of S-mZVIbm is 1/2 less than unsulfidated ball-milled control(mZVIbm)(6.3 vs.12.7μm).And S-mZVIbm exhibits reduced aggregation,relatively homogeneous distribution of Fe and S throughout the particle(not core-shell structure),enhanced reactivity with trichloroethylene(TCE),less H2 formation,and therefore greatly improved electron efficiency of TCE dechlorination(εe).Under ZVI-limited conditions(initial Fe0/TCE=1.6 mol/mol),S-mZVIbm gave surface-area normalized reduction rate constants(k′SA)andεe that were~2-and 10-fold greater than the unsulfidated ball-milled control(mZVIbm).Under TCE-limited conditions(initial Fe0/TCE=2000 mol/mol),sulfidation increased kSAA andεe≈5-and 50-fold,respectively.The major products from TCE degradation by S-mZVIbm were acetylene,ethene,and ethane,which is consistent with dechlorination byβ-elimination,as is typical of ZVI,iron oxides,and/or sulfides.However,electrochemical characterization shows that the sulfidated material has redox properties intermediate between ZVI and Fe3O4,mostly likely significant coverage of the surface with FeS.The TCE degradation performance of S-mZVIbm with different ball milling concitions including ball milling time,S/Fe molar ratio,S precursor and Fe precursor,was explored.It was found that the ball milling time and S/Fe molar ratio had a crucial impact for the performance of S-mZVIbm.The ZVI sulfidation-degree was increased with the prolonged ball milling time(TB).XRD pattern showed that ZVI sulfidation was thoroughly completed of the TB within 10 h.With the increased TB to 30 h,the peaks corresponding toα-Fe0 become broader,which is consistent with the decrease in crystallite size with the extended milling time.Meanwhile,kobs of S-mZVIbm for TCE degradation almost linearly increased with the increased TB.For the TB of 30 h,kobs of S-mZVIbm was 0.14 h-1,which is 1.6-fold greater than that of TB=2 h.However,kSAA firstly increased and then decreased with the continuously extended TB,and the kSAA reached the maximum value of 0.0256 L·h-1·m-22 when TB=5 h.This is mainly related to the change of specific surface area and crystallization of FeS with the increased TB.S/Fe molar ratio determined the distribution of FeS(the active site of TCE degradation)on the surface of particles.The larger S/Fe ratio brought to the more S-mZVIbm activity for TCE degradation and the less HER activity.In this study,when S/Fe=0.5,the kobs,εe,kobs,H2,n(0.163h-1,18.46%,1.3 mol·L·g-1·h-1,5.47)were all optimal.Generally,the S-mZVIbm had high-efficiency degradation of TCE when TB was greater than 10 h and S/Fe molar ratio was greater than 0.05.Additionally,sulfur and iron sources mainly affected HER,which was reflected in the form ofεe.Among all iron and sulfur sources,n and kobs,H2 of S-mZVIbm(S0/Aladdin)were the smallest(5.63 and 0.296μmol·L·g-1·h-1,respectively),thus theεe of S-mZVIbm was the highest(11.96%),which is also greater than S-mZVIbm obtained from other iron sources and sulfur sources.Groundwater pH is the most important geochemical parameter in controlling the interfacial reactions of ZVI with water and contaminants.In the study,mZVIbmm efficiently dechlorinated TCE at initial stage(<24 h)at pH 6-7,but got passivated at later stage due to pH rise caused by iron corrosion.At pH>9,mZVIbm almost completely lost its reactivity.In contrast,S-mZVIbm didn’t experience any reactivity loss during the whole reaction stage across pH 6-10 and could efficiently dechlorinate TCE at pH 10 with a reaction rate of 0.03 h-1.Increasing pH from 6 to 9 also enhanced electron utilization efficiency from 0.95%to 5.3%,and from 3.2%to 22%,for mZVIbm and S-mZVIbm,respectively.SEM images of the reacted particles showed that the corrosion product layer on S-mZVIbm had a puffy/porous structure while that on mZVIbm was dense,which may account for the mitigated passivation of S-mZVIbmm under alkaline pHs.Density functional theory calculations suggest that covered S atoms weaken the interactions of H2O molecules with Fe surfaces,which renders the S sites inefficient for H2O dissociation to cause passivation.In order to further improve the performance of S-mZVIbm,C-S-mZVIbm was prepared by ball milling mZVI together with elemental sulfur and carbon sources(e.g.,activated carbon,biomass carbon and graphite).For carbon content of 0.5%,1.0%,2%,kobs of C-S-mZVIbm with TCE degradation increased by 1.6%,40%and 113%,respectively,compared to that of S-mZVIbm.There are two possible explanations:(1)The carbon with hydrophobicity and/or porous structure allowed the adsorption of TCE(hydrophobic solvent),and thus increased the local concentration of TCE in the vicinity of active site and further facilitated to the rapid removal of the dissolved TCE.(2)The microbattery structure obtained from carbon and iron promoted the corrosion of zero-valent iron,and then accelerated the electron emission and degradation of TCE.However,the TCE degradation was seriously suppressed when the biochar content in C-S-mZVIbm reached 4%.This possible reason is that there was a mass of TCE adsorbed in pore of biochar,which was difficultly desorbed.It caused that TCE was unavailable for the active site.Moreover,the added carbon for C-S-mZVIbm resulted in the more obvious HER,which greatly reduced the electron efficiency.It is unfortunate that the final cis-DCE concentration increased from 9.3%to more than 20%during TCE degradation by C-S-mZVI.Therefore,how to accurately regulate interface of the C-S-mZVIbm for improving the reactivity and selectivity and reducing the generation of chlorine intermediate products is still challenge.