Sulfidated zero-valent iron bimetals for passive remediation of chlorinated vapors in the subsurface

This study explores a novel application of sulfidated zero-valent iron (S-ZVI) bimetals for the treatment of chlorinated solvents in the vapor phase. The potential of these reactive materials was investigated through batch, column, and modeling tests. The materials were produced by disc milling of ZVI, sulfur (S), copper (Cu), and nickel (Ni) with molar ratios of 0.05 and 0.2. The reactivity of the materials was assessed through vapor degradation batch tests conducted under partially saturated conditions using trichloroethylene (TCE) as a model compound. Sulfidated materials with a 0.05 S/ZVI molar ratio were the most reactive, achieving up to 99% degradation of TCE vapors within 18 hours and first-order degradation constants of 5-5.7 d^-1. Compared to the non-sulfidated materials, sulfidated ones remained reactive even after aging by exposure to air for 30 days. In all tests, C₃-C₆ hydrocarbons were detected as main byproducts, indicating β-elimination as the dominant TCE degradation pathway, with minor dichloroethylene and vinyl chloride amounts from the hydrogenolysis pathway. To evaluate the use of sulfidated bimetals as Horizontal Permeable Reactive Barriers (HPRBs) for treating chlorinated vapors in the subsurface, TCE diffusion column tests were performed using a 5 cm thick reactive layer of S-ZVI-Ni. These tests demonstrated up to 70% degradation over 25 days. By integrating the column test results into an analytical model, it was estimated that an 18 cm HPRB could ensure up to 99% degradation of TCE vapors. These findings highlight the potential of S-ZVI bimetals as an effective passive mitigation system for reducing chlorinated solvent vapor emissions from the subsurface.