Synthesis And Characterization Of Mechanochemically Sulfidated Microscale Zero Valent Iron For Dechlorination Of Trichloroethylene In Groundwater

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-mZVI^bm).In this study,we systematically evaluate the activity,selectivity and pathway of S-mZVI^bm for TCE dechloridation.The particle size of S-mZVI^bm is 1/2 less than unsulfidated ball-milled control(mZVI^bm)?6.3 vs.12.7?m?.And S-mZVI^bm 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??_e?.Under ZVI-limited conditions?initial Fe⁰/TCE=1.6 mol/mol?,S-mZVI^bm gave surface-area normalized reduction rate constants(k?_SA)and?_e that were?2-and 10-fold greater than the unsulfidated ball-milled control(mZVI^bm).Under TCE-limited conditions?initial Fe⁰/TCE=2000 mol/mol?,sulfidation increased k_SAA and?_e?5-and 50-fold,respectively.The major products from TCE degradation by S-mZVI^bm were acetylene,ethene,and ethane,which is consistent with dechlorination by?-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-mZVI^bm 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-mZVI^bm.The ZVI sulfidation-degree was increased with the prolonged ball milling time?T_B?.XRD pattern showed that ZVI sulfidation was thoroughly completed of the T_B within 10 h.With the increased T_B to 30 h,the peaks corresponding to?-Fe⁰ become broader,which is consistent with the decrease in crystallite size with the extended milling time.Meanwhile,k_obs of S-mZVI^bm for TCE degradation almost linearly increased with the increased T_B.For the T_B of 30 h,k_obs of S-mZVI^bm was 0.14 h^-1,which is 1.6-fold greater than that of TB=2 h.However,k_SAA firstly increased and then decreased with the continuously extended T_B,and the k_SAA 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-mZVI^bm activity for TCE degradation and the less HER activity.In this study,when S/Fe=0.5,the k_obs,?_e,k_obs,H2,n(0.163h^-1,18.46%,1.3 mol·L·g^-1·h^-1,5.47)were all optimal.Generally,the S-mZVI^bm had high-efficiency degradation of TCE when T_B 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 of?_e.Among all iron and sulfur sources,n and k_obs,H2 of S-mZVI^bm?S⁰/Aladdin?were the smallest(5.63 and 0.296?mol·L·g^-1·h^-1,respectively),thus the?_e of S-mZVI^bm was the highest?11.96%?,which is also greater than S-mZVI^bm 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,mZVI^bmm 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,mZVI^bm almost completely lost its reactivity.In contrast,S-mZVI^bm 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 mZVI^bm and S-mZVI^bm,respectively.SEM images of the reacted particles showed that the corrosion product layer on S-mZVI^bm had a puffy/porous structure while that on mZVI^bm was dense,which may account for the mitigated passivation of S-mZVI^bmm 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-mZVI^bm,C-S-mZVI^bm 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%,k_obs of C-S-mZVI^bm with TCE degradation increased by 1.6%,40%and 113%,respectively,compared to that of S-mZVI^bm.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-mZVI^bm 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-mZVI^bm 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-mZVI^bm for improving the reactivity and selectivity and reducing the generation of chlorine intermediate products is still challenge.