Reaction mechanism of CHCs with S-nZVI particles

Materials based on zero-valent iron (ZVI), especially those in nano-scale form (nZVI), are promising agents for contaminant removal due to their high specific surface area, high reducing ability, easy in situ applicability and environmentally benign transformation products. It has been shown that nZVI are able to remove efficiently a wide variety of common groundwater contaminants, such as chlorinated hydrocarbons (CHCs). The full-scale application of nZVI has been, however, not fully exploited, mainly due to fast particle aggregation and low electron selectivity of nZVI that readily reacts with natural reducible species such as water, resulting in a decrease of reducing capacity. In recent years, a great interest has been devoted to modifying ZVI-based materials with reducing sulfur compounds such as sulfides. Sulfidation was found to remarkably increase the reactivity of ZVI with target contaminants, and, at the same time, substantially limit undesirable reactions with water. The factors controlling the increased reactivity and selectivity of sulfidated ZVI (S-ZVI) materials are, however, not well understood. Several possible mechanisms describing the role of an iron sulfide surface layer, some of them contradictory, have been postulated, including a more efficient charge- transfer, higher surface roughness and surface area, increased hydrophobicity and inhibition of hydrogen recombination. The overall objective of the current project is to elucidate the reaction mechanisms and pathways responsible for the increased selectivity of S-ZVI surfaces towards CHCs dechlorination using molecular modeling methods. The theoretical results will be complemented by experimental data on the reactivity of trichloroethylene (TCE) on various ZVI-based materials, which were acquired in recent experimental studies. The specific aims of the current project are to: i) build representative structural models encompassing surfaces of zero-valent iron, iron sulfides, and (hydr- )oxides, including surface defects; ii) characterize and compare the affinity of different reaction sites with reactants and intermediates; iii) characterize reaction mechanisms, including transition states and reaction rates of TCE and water reactions on studied mineral surfaces; and iv) investigate the influence of solvent (water) on reaction mechanisms. Computer simulations will be based of quantum chemical methods for periodic models representing surfaces of various ZVI, FeS, and iron oxyhydroxide phases. The better understanding of the particle surfacereactant interactions will facilitate the development of more selective ZVI-based materials with improved performance in real applications.

Chlorinated hydrocarbons (CHCs) are pervasive groundwater and soil contaminants found at many polluted sites worldwide, stemming from their use as chemical intermediates, degreasing solvents, and dry-cleaning agents. Zero-valent iron (ZVI), particularly in nanoscale form (nZVI), offers a promising strategy to effectively degrade CHCs directly in the subsurface into inorganic chloride and hydrocarbons, which can then be readily mineralized by bacteria. However, exploiting the full potential of this promising remediation technology has been hindered by the rapid reaction of (n)ZVI with water and other natural reducible species, leading to its fast passivation and increased treatment costs. In recent years, sulfidation has emerged as an efficient approach to suppress (n)ZVI corrosion, thereby enhancing its selectivity and reactive lifetime for environmental applications. This breakthrough discovery has substantially improved the cost-effectiveness and applicability of these materials. In this project, we investigated the mechanisms responsible for the improved performance of sulfidated (n)ZVI (S-(n)ZVI) by using molecular modeling methods based on quantum chemistry. Employing various models representing iron, iron sulfide, and iron (oxyhydr)oxide surfaces, we explored the role of sulfur in the surface interactions with water and chlorinated ethenes, such as trichloroethene, which we used as model CHC contaminants in our calculations. Our findings revealed that while the S-(n)ZVI surface is less reactive with contaminants than the pristine (n)ZVI surface, it remains more reactive than the oxidized (n)ZVI surface commonly present on particles in aqueous environments. Furthermore, sulfidation was found to decrease surface interactions with water molecules, rendering the (n)ZVI surface more hydrophobic and protecting it from oxidation, thereby maintaining reduced Fe sites that are highly reactive with contaminants. Our investigations also shed light on the selectivity of S-(n)ZVI surfaces for the degradation of specific CHCs. The two primary factors controlling the CHC degradation rates at S-(n)ZVI surfaces were the feasibility of the electron transfer from iron to the contaminant molecule and the size of the contaminant molecule. In addition to our work on S-(n)ZVI, we focused on the description of reaction mechanisms of CHC degradation reactions at the surface of iron nitride (FexN) minerals, which were recently found to exhibit even higher CHC degradation performance compared to S-(n)ZVI. The better understanding of the particle surface-reactant interactions for various (n)ZVI modifications achieved through this project represents a basis for the rational design of new, highly efficient, and environmentally sustainable treatments for sites polluted with CHCs and similar contaminants.