Nitrate Influences Trichloroethylene Dechlorination By Ball-milled Zero-Valent Iron Particles And The Mechanisms

Permeable reactive barrier（PRB）based on zero-valent iron（ZVI）has been well recognized as one of the most promosing and cost-effective technologies for trichloroethylene（TCE）contaminated groundwater remediation.For years,tremendous modifications for ZVI particles have emergied to improve the performance of ZVI particles.Recently,sulfidated ZVI prepared by ball mil ing has emerged as one of the most promising and cost-effective ZVI particles for groundwater remediation,which has a new FeS phase distributed on the surface of ZVI.Solution chemistry,such as water constituents,is a wel-recognized factor that influences ZVI performance by changing the constituents of ZVI surface in realistic conditions.NO₃^-has been frequently detected in groundwater because of it high mobility.The effect of NO₃^-,as a Fe⁰ reducible anion,on the performance of ZVI has been intensively investigated but contradictory results exist.So far,researches concerning impacts of NO₃^-on the performance of sulfidated ZVI particles are limited.Therefore,assessing the effects of NO₃^-on the performance of ball mil ed ZVI particles with and without FeS phases and clarifying their corresponding functional mechanisms are of great value.Mechanis tic roles of NO₃^-in TCE dechlorination by ball milled,micro-scale ZVI(mZVI^bm)and sulfidated micro-scale ZVI(S-mZVI^bm)were systematically explored through batch experiments and density functional theory（DFT）calculations within this paper.NO₃^-is an oxidant that induces electron competition.DFT calculations indicated that NO₃^-was mainly reduced on the Fe surfaces on both the mZVI^bm and S-mZVI^bm particles,which were also the dechlorination sites for mZVI^bm but not for S-mZVI^bm.At the same time,NO₃^-reduction produced Fe²⁺and contributed to the formation/crystallization of conductive iron（oxyhydro）oxides on Fe surfaces.In general,NO₃^-promoted TCE dechlorination by mZVI^bmm due to its decisive role in iron（oxyhydr）oxides transformation on the Fe surfaces;while NO₃^-deteriorated TCE dechlorination by S-mZVI^bm due to its predominant role as an electron competitor,which reduced the electrons available on dechlorination sites（i.e.,FeS surface）.Nonetheless,both the aged mZVI^bm and S-mZVI^bm particles in the presence of NO₃^-showed superior reductive reactivity than the aged counterparts without NO₃^-,resulting from the NO₃^--facilitated formation of crystalline Fe₃O₄.The results indicate that iron（oxyhydr）oxides transformation induced by NO₃^-on the surface of both mZVI^bm and S-ZVI^bm particles could be an alternative approach to sustain the reactivity of ZVI particles in TCE contaminated groundwater remediation.