The overall objective of the project is to evaluate sulfidated zerovalent iron (S-ZVI), an innovative in situ chemical reduction (ISCR) amendment, for treating residual chlorinated solvent sources. The advantages of S-ZVI have been extensively demonstrated in recent laboratory studies, including rapid reduction rates, enhanced selectivity, and extended reactive longevity for dechlorination. But S-ZVI has not been rigorously evaluated in the field. The specific technical objectives of this project include:

1. Identify an effective S-ZVI formulation specifically tailored to site conditions;
2. Demonstrate effective distribution of S-ZVI in the subsurface; and
3. Assess the long-term performance of S-ZVI for source treatment.

S-ZVI involves controlled modification of the ZVI surface by reducing sulfur compounds to form a layer of iron sulfide (FeS) on the surface of ZVI. Sulfidation increases the reactivity, selectivity, and longevity of ZVI for reductive dechlorination. This enhancement is particularly significant for small scale/high surface area ZVI material, such as nano ZVI (nZVI), which has faced major challenges during field applications due to its short reactive lifespan in the subsurface. The improved longevity is a result of 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). Inhibited corrosion, however, does not inhibit the overall dechlorination reactivity because the reaction is no longer mediated by atomic hydrogen (produced from corrosion); instead, the reaction occurs via direct electron transfer from Fe(0). Increasing availability of Fe(0), coupled with the strong hydrophobicity and high conductivity of FeS, all enhance the dechlorination rate. Despite all these changes, the primary abiotic dechlorination pathway for S-ZVI is beta elimination that largely bypasses the production of toxic intermediates, such as cis-1,2-dichlorethene (cDCE) and vinyl chloride (VC). Overall, S-ZVI represents an innovative ISCR amendment, whose benefits have been extensively demonstrated in laboratory research, but rigorous field demonstration and performance evaluation is required to further advance the practical application of S-ZVI technology.

The outcome of this project will provide DoD site managers with a novel ISCR alternative to address residual chlorinated solvent contamination. S-ZVI is the first kind of chemical amendment that possesses high selectivity characteristics for dechlorination. The high selectivity will result in significantly enhanced longevity and efficiency of the amendment. This efficiency will decrease the overall time to remediation and lower the cost of remediation by minimizing repeated injections and reducing performance monitoring timeframes that contribute to the cost and uncertainty for conventional amendment injection-based remedies. (Anticipated Project Completion - 2024)