Molecular Structure and Sulfur Content Affect Reductive Dechlorination of Chlorinated Ethenes by Sulfidized Nanoscale Zerovalent Iron

Sulfidized nanoscale zerovalent iron (SNZVI) with desirable proper-ties and reactivity has recently emerged as a promising groundwater remediationagent. However, little information is available on how the molecular structure ofchlorinated ethenes (CEs) affects their dechlorination by SNZVI or whether thesulfur content of SNZVI can alter their dechlorination pathway and reactivity. Here,we show that the reactivity (up to 30-fold) and selectivity (up to 70-fold)improvements of SNZVI (compared to NZVI) toward CEs depended on the chlorinenumber, chlorine position, and sulfur content. Low CEs (i.e., vinyl chloride andcis-1,2-dichloroethene) and high CEs (perchloroethene) tended to be dechlorinated bySNZVI primarily via atomic H and direct electron transfer, respectively, while SNZVIcould efficiently and selectively dechlorinate trichloroethene andtrans-1,2-dichloroethene via both pathways. Increasing the sulfidation degree of SNZVIsuppressed its ability to produce atomic H but promoted electron transfer and thusaltered the relative contributions of atomic H and electron transfer to the CE dechlorination, resulting in different reactivities andselectivities. These were indicated by the correlations of CE dechlorination rates and improvements with CE molecular descriptors,H2evolution rates, and electron transfer indicators of SNZVI. These mechanistic insights indicate the importance of determining thestructure-specific properties and reactivity of both SNZVI materials and their target contaminants and can lead to a more rationaldesign of SNZVI forin situgroundwater remediation of various CEs

Automated summary

This study reveals the influence of sulfidized nanoscale zerovalent iron (SNZVI) on the dechlorination reactivity and selectivity of chlorinated ethenes (CEs). SNZVI exhibits significantly improved dechlorination reactivity and selectivity compared to zerovalent iron, depending on the chlorine number, chlorine position, and sulfur content of the CEs. By controlling the sulfidation degree of SNZVI, the dechlorination pathway and reactivity of CEs can be altered. These findings are important for the rational design of SNZVI for in situ groundwater remediation of various CEs.