Reaktionsmechanismus von CHCs mit S-nZVI Partikeln

Improper handling and disposal of chlorinated hydrocarbons (CHCs), widely used industrial solvents, metal degreasers and dry-cleaning agents, have led to widespread soil and groundwater contamination on a global scale. In-situ remediation by zero-valent iron nanoparticles (nZVI) has been suggested as easy, cost-effective and environmentally-benign strategy to degrade CHCs. The full-scale application of nZVI has been, however, limited, essentially due to low selectivity leading to rapid oxidation of nZVI by natural reducible species and fast particle aggregation. In recent years, a great interest has been devoted to modification of nZVI by sulfur compounds (S-nZVI). Sulfidation was found to remarkably increase the reactivity of nZVI with various contaminants including CHCs, and, at the same time, significantly minimize unfavorable reaction with water. The factors controlling this advantageous reactivity profile of S-nZVI are not well understood. The aim of the proposed project is to shed more light on mechanisms responsible for the increased selectivity of S-nZVI towards CHC using molecular modeling techniques. Following hypotheses will be investigated: i) iron sulfide (FeS) shell is less hydrated than oxidized particle shell and thus can adsorb better hydrophobic substances, ii) FeS coating may decrease iron corrosion due to blocking direct access of Fe0 to water, iii) FeS shell can better facilitate electron transfer from the nZVI core to adsorbed contaminants than the oxidized shell and iv) the rate of hydrogenation reactions depends on the availability of reactive hydrogen species at the particle surface. Molecular modeling techniques based on quantum mechanics will be used. Cluster and periodic slab models will be constructed for nZVI, S-nZVI and oxidized Fe surfaces. Trichloroethene (TCE) will be used as a representative substance for CHCs. The adsorption, dechlorination and hydrogenation steps of various species (i.e., TCE, water, reaction intermediates) will be modeled to describe the fundamental mechanism of TCE surface-mediated reactions. No computational studies to date have elaborated on the mechanism controlling the increased reactivity and selectivity of S-nZVI. The knowledge of mechanisms governing the unique reactivity profile of S-nZVI could facilitate the development of more selective ZVI-based materials with an extended lifetime that could enable a widespread use of this highly promising remediation technology. The applicant Dr. M. Brumovský will be responsible for performing and interpreting all the molecular modeling calculations required to reach the defined aims under the supervision of the co-applicant Univ.-Doz. Dipl.-Ing. Dr. D. Tunega. The investigations will be performed in collaboration with two external partners with long-term experiences in modeling ZVI surfaces and material synthesis and characterization.