Establishing the Kinetics of Aqueous Reactions at Fe(III) Molecules and Minerals

University Of California-Davis, Davis CA

Investigators

Abstract

Investigators propose to establish a linear-free-energy relation (LFER) to predict rates of ligand substitution on Fe(III)-oxide minerals from the <FeIII-OH2> bond lengths, which can either be calculated or measured. In the last funding period we showed experimentally that such a LFER probably exists between <FeIII-OH2> bond lengths and rates of solvent exchanges. They also used similar data on nanometer-size Al(III) ions to establish a LFER for aluminous surface structures. They coupled 'rare event' simulation methods to experimental data on rates of water-exchanges from Al(III) metals in large nanometersize clusters. This work showed, for the first time, that the rates at the surfaces are extraordinarily and surprisingly rapid. Investigators now want to extend the work to Fe(III) (hydr)oxide materials. They use aqueous complexes with particularly useful <FeIII-OH2> bond lengths and aqueous stabilities to establish an experimental scale of rates and bond lengths. In the last funding period, they mastered a 17O-NMR line-broadening method for paramagnetic metals like Fe(III). This method is new to geochemistry, but otherwise well understood and dependable. Once the experimental scale is established, they repeat their 'rare-event' simulations to estimate rates for Fe(III)-(hydr)oxide surface structures that are experimentally inaccessible. For bridging oxygens, and to complement the 17O-NMR measurements, they employ a new ElectroSpray-Ionization Mass-Spectrometry (ESI-MS) method that allows to follow the rates of oxygen-isotope exchanges in the dissolved molecules. With ESI-MS, we follow reactions at oxo bridges and the carboxylate oxygens in a bonded organic ligand. With this method, geochemists can study an enormous range of compounds that were previously impossible, such as the Mn(IV,III)-oxo clusters. In this sense, and in others, this research is both pioneering and transformative to geochemistry. This research is also Transformative because water-exchange rates are the most fundamental timescale for describing aqueous reactions---slow loss of these bound waters precedes most ligand substitutions ('adsorptions') and many electron exchanges. In soils, these reactions are usually part of a complex network, but these elementary reactions control the essential step. This research is Transformative because reactions are also at the appropriate scale to improve methods of simulation, which will be heavily employed by geochemists for cases where experiments are impossible. They are trying to make aqueous geochemistry quantitative at the molecule scale. The Intellectual Merit is fundamental research in aqueous reactivity where they answer questions such as: 'What are the most promising types of computationally inexpensive methods that can yield hidden reaction pathways in systems that are sufficiently large to be geochemically relevant?' The research also ties to the increased awareness about nanometer-size clusters in natural water chemistry, in transporting toxicants or as toxicants themselves, and as the fundamental building block for amorphous materials. The Broader Impacts extend well beyond Earth science because this research is used by many disciplines, including colloid chemistry, nanoscience, materials science and medicine (metalloproteins such as ferritin and enzymes such as the purple-acid phosphatases resemble these clusters). Also, enrollment at UCD draws disproportionately from the high population of new immigrants to the United States and lures them into Chemistry and Geochemistry. These funds bring nontraditional but highly talented students into the Earth Sciences, which comprise the population of UCD.