The Structure of Sulfidized Zero-Valent Iron by One-Pot Synthesis: Impact on Contaminant Selectivity and Long-Term Performance.

Sulfidized zerovalent iron (sZVI) is widely studied because of its remarkable reactivity with a number of groundwater contaminants. Nonetheless, its nanoscale structure is not well understood. As such, there is an uncertainty on how sZVI structure controls its reactivity and fate in the subsurface environment. Using pair distribution function analyses, we show that sZVI made from one-pot synthesis using dithionite as sulfur precursor consists of an Fe core with a shell composed dominantly of short-range ordered Fe(OH) and FeS having coherent scattering domains of less than 8 Å. Reactivity experiments show that increasing shell material significantly decreases rate for cis-dichloroethene (cis-DCE) reduction, whereas the opposite is observed for trichloroethene (TCE). The results are consistent with a conceptual model wherein cis-DCE reduction requires active Fe sites, which become largely inaccessible when shell material is abundant. Conversely, an increase in FeS shell volume led to faster TCE reduction via direct electron transfer. Aging experiments showed that sZVI retained >50% of its TCE removal efficiency after 30-day exposure to artificial groundwaters. The decline in sZVI reactivity due to long-term exposure to groundwater, is attributed to Fe oxidation from water reduction coupled by reorganization and recrystallization of the poorly ordered shell material, which in turn reduced access to reactive FeS sites.