Formatted Title
Sulfidated Zero-Valent Iron: Theory, Mechanisms, and Performance Review
Background/Objectives
Background/Objectives. Zero-valent iron (ZVI) is a powerful reductant that can accomplish the in situ remediation of chlorinated hydrocarbons and other toxic groundwater contaminants. Small particle size ZVI can enable injections into porous lithologies at low pressures that result in a uniform product distribution. However, small particle size ZVI products have higher surface areas and are susceptible to hydrolysis reactions that consume ZVI. Hydrolysis is unproductive and has the effect of shortening the material’s effective lifetime. Using sulfidated zero-valent iron (SZVI) can help overcome some of the shortcomings of bare ZVI. A key feature of SZVI is the particle’s core shell configuration with a thin surface layer of reduced iron sulfide and a ZVI interior. Iron sulfide is hydrophobic in nature and is thought to effectively inhibit unwanted hydrolysis reactions and lead to an extended reactive lifetime. It is also believed that the hydrophobic particle surface enhances the sorption of chlorinated hydrocarbons onto the particle surface where the electrochemical reduction reactions occur, accelerating reaction kinetics in relation to bare ZVI. The experimental results were used to develop theories and mechanisms that explain the unique and beneficial properties of the material.
Approach/Activities
Approach/Activities. Secondary mass ion spectrometry (SIMS) was used to verify the core-shell configuration of sulfidated iron particles by conducting a series of 20 ablative surface scans. Batch treatability studies were used to calculate the reaction kinetics for SZVI with both parent (trichloroethene, carbon tetrachloride) and daughter (cis-1,2 dichloroethene) compounds with reactivity compared to bare unsulfidated ZVI. Product longevity was calculated by adding two- to three-micrometer diameter SZVI to a sand column into which a solution of TCE and dissolved oxygen was continuously pumped for a four-year period. Treatability studies were also performed with mixtures of SZVI and organic bioremediation amendments to quantify the ability of SZVI to promote accelerated biological degradation. Concurrent biodegradation is important for eliminating partially reduced groundwater contaminants such as vinyl chloride and dichloromethane that exhibit slower reactivity with SZVI.
Results/Lessons Learned
Results/Lessons Learned. The results of treatability studies showed that SZVI increases the reaction kinetics with trichloroethene by about 30 times compared to bare iron. SZVI reactivity with cis-dichloroethene and chloroform was significantly slower. These results can be explained by the presence of the hydrophobic surface of SZVI that will attract and concentrate hydrophobic contaminants such as TCE on the particle surface where the degradation reactions occur. The column study showed that a representative in situ dose of SZVI maintained efficacy for over four years even when subjected to 20 mg/L of TCE in oxic water. This confirmed the hypothesis that SZVI limits the unwanted hydrolysis reactions that would have exhausted the reactive capacity of bare ZVI. The experiments using mixtures of SZVI and bioremediation amendments showed positive synergies with fewer daughter products produced and those that were generated were present for shorter durations. This suggests that SZVI promotes a better environment for biodegradation that promotes the accelerated degradation of daughter products.