Reaction mechanisms of CHCs and Cr(VI) with nZVI particles

Trichloroethene (TCE) and hexavalent chromium (Cr(VI)) represent frequent and highly toxic chlorinated hydrocarbon and heavy metal groundwater pollutants all over the world. Currently contaminated sites are often treated by digging the entire contaminated soil or aquifer and dumping it at disposal sites (Dig & Dump). Conventional remediation technologies as dig & dump are expensive and ineffective from the long-term perspective. Therefore novel in-situ technologies are developed to increase cost efficiency of remediation. Nanoscale zerovalent iron (nZVI) particles are increasingly used in laboratory and pilot-scale reductive technologies of water treatment. nZVI particles get in direct contact with the contaminant, where the particles release electrons that react with the contaminant and transform it into non-toxic substances. Numerous studies were conducted about nZVI for remediation of contaminated groundwater sites. But still little is known about the exact mechanisms taking place at nZVI surface and how the reaction is influenced by nanoparticle chemistry and surface atomic structure. We presume that the characteristics of the interaction between the particle surface and the contaminant are determined by molecular interactions at nanoscale. Observation of these fundamental nanoscale interactions is not possible by experimental studies. Therefore experimental studies will be combined with quantum chemical modeling. Through this way reaction properties that can be obtained by experimental studies can be linked to fundamental mechanisms that are derived from modeling. To determine these interactions, nZVI particles differing in chemical composition and morphology will be prepared and characterized in detail. Subsequently these particles will be used in laboratory experiments with two model contaminants, TCE and chromium (VI). In laboratory experiments degradation of TCE and transformation of Cr(VI) to less toxic Cr (III) will be investigated under well-defined experimental conditions. Laboratory experiments will provide different degradation properties for particles with different characteristics. These differences in obtained degradation will be linked to fundamental mechanisms that are derived from modeling. The aim is to discover fundamental iron nanoparticle surface reactant interaction mechanisms via combination of experimental results and quantum chemical modeling. A better understanding of basic mechanisms will help to improve particle synthesis in respect to practical applications as remediation technologies, wastewater treatment or applications that were not envisaged up to now. Ideally, the determination of fundamental mechanisms will enable to design optimal types of nZVI particles for real field-scale applications.

In the past the groundwater was often contaminated by chemicals leaking from industrial, or commercial sites. A prominent group of chemicals that still can be found at many contaminated sites are solvents like chlorinated hydrocarbons. These were used at dry cleaning sites and by the metal industry. These substances can be degraded directly in the ground by adding iron suspensions. The iron reacts with the solvent starting a process that ends up with complete degradation to water and carbon dioxide. In this project we could improve the reaction rate and specificity of this reaction by producing iron particles with sulphur or nitrogen. Both elements led to an increase of the speed of reaction compared to a pure iron suspension. These results are important for practical applications, since iron suspensions are very expensive compared to other substances that are used for degradation of contaminants, thus a more effective chemical reaction can save a lot of money. In the project batch experiments were performed in which degradation of trichloroethene - a chemical often used at dry cleaning sites and from metal industry - was investigated. In these batch experiments iron suspensions with containing different amounts of sulphur, nitrogen and carbon were compared. While amendment of carbon showed no difference to unamended iron particles, the degradation rate increased with increasing amounts of sulphur up to a mixing ratio of 10:100 (sulphur:iron). At a mixing ration of 25:100 the reaction rate decreased. Investigations of the particles showed that the sulphur was found at the surface of the iron particles forming a core-shell structure. A thin sulphur-shell appeared to improve the adsorption of solvent molecules to the iron surface. However, if the sulphur-shell becomes too thick, a direct contact between the solvent molecule and the iron surface is inhibited leading to the observed decrease in reaction rate. We also showed for the first time that a similar increase in reaction rate as was found with sulphur doped iron particles is possible with nitrogen amended iron particles. We expect that these findings have impact to further improve the practical application of iron suspensions for the remediation of sites contaminated with chlorinated solvents.