Sulfidated zerovalent iron (S-ZVI) involves the controlled modification of the ZVI surface by reducing sulfur compounds to form a layer of iron(II) sulfide (FeS) on the surface of ZVI. Sulfidation increases the reactivity, selectivity, and longevity of ZVI for reductive dechlorination, particularly for colloidal or nanoscale ZVI, which has high reactivity but a short reactive lifespan in the subsurface. The improved selectivity and longevity result from the FeS surface layer effectively inhibiting corrosion of Fe(0) in water, the primary reaction that outcompetes abiotic dechlorination for reducing capacity of Fe(0). Increasing availability of Fe(0), coupled with the strong hydrophobicity and high conductivity of FeS, enhances the dechlorination rate. The overall objective of this project was to evaluate S-ZVI for treating residual chlorinated solvent sources.

Overall, S-ZVI represents an innovative in situ chemical reduction amendment whose benefits have been extensively demonstrated in laboratory research; yet rigorous field demonstration and performance verification were needed to advance the practical application of S-ZVI technology further. Two rounds of laboratory treatability studies were conducted to assess long-term performance of S-ZVI technology for treating site groundwater containing trichloroethene (TCE) and cis-1,2-dichloroethene (cDCE). An initial treatability study consisted of batch and column tests to identify the preferred product.

An initial treatability study consisted of batch and column tests, which identified Nanofer 25DS as the recommended product for the pilot test. However, Nanofer 25DS is supplied by NanoIron in the Czech Republic, and a disruption of Nanofer 25DS supply led to a supplemental treatability study column test to assess if S-MicroZVI supplied by Regenesis (San Clemente, California) can serve as an alternative to Nanofer 25DS. Results of both treatability studies showed that the S-ZVI products are highly effective and persistent in degrading TCE, but their reactivity with cDCE was lower. The degradation reactions primarily produced acetylene, ethene, and ethane, with a low level of vinyl chloride likely from a minor degradation pathway via hydrogenolysis. Post-column Fe(0) quantification and batch results indicated sulfidation markedly improved ZVI longevity. Ultimately, the field injection of S-ZVI and performance monitoring were not performed due to uncertainties related to the Nanofer 25DS supply and the proprietary nature of stabilizers contained in S-MicroZVI. 

Field injection and subsequent long-term performance monitoring of S-ZVI injection were not conducted due to the disruption of Nanofer 25DS supply and the proprietary nature of the stabilizers contained in S-MicroZVI. The disruption of Nanofer 25DS supply was temporary, and to our knowledge, the production has been restored. However, its production is in Europe, making its application in the U.S. market less practical compared to domestic S-ZVI alternatives, such as S-MicroZVI supplied by Regenesis. The proprietary nature of the stabilizers in S-MicroZVI was identified as an issue, and further consultation will be needed with the manufacturer (Regenesis, San Clemente, CA) and regulators on a site-specific basis to confirm they can be injected. These stabilizers are stated to be based on glycerol and readily biodegradable. (Project Completion - 2026)