Basic Research Needs for Geosciences - Energetics Meetings and ...

PRIORITY RESEARCH DIRECTION:MINERAL-WATER INTERFACE COMPLEXITY AND DYNAMICSSolid-state charge transport at mineral-water interfacesHeterogeneous electron transfer is the basis for much of interfacial geochemistry,controlling processes such as metal cycling, contaminant transport, and microbialactivity. For example, reductive transformation of iron oxide minerals is a key part of theiron biogeochemical cycle. Reduction of lattice Fe 3+ to Fe 2+ leads to dissolution of thesolid mineral phase, but the electronic structure of many iron oxides allows for electrontransport within the lattice. Thus iron surface sites undergoing reduction do notnecessarily correspond to the locations of iron release. Using state-of-the-art molecularmodeling tools such as Density Functional Theory coupled with molecular dynamicssimulations, the mobility of electrons at various low-index hematite surfaces has beenevaluated, with some surprisingly high electron hopping rates predicted (Kerisit andRosso 2006). These findings point to the possibility of lattice self-diffusion of electronsas an explanation for, for example, the transfer of electrons from the points of microbialcell attachment to reactive sites through the hematite lattice. Thus electron transfer atmineral-water interfaces is complicated in many cases by the electronic structure of thesolid phase.Combined quantum mechanical and molecular dynamics simulations reveal anintrinsically high self-diffusion mobility for electrons donated to iron atoms in themineral hematite (Kerisit and Rosso 2006). (a) A simulation cell for the hydroxylatedhematite (001)-water interface (red—oxygen ions, blue—iron ions, white—hydrogenions) showing two possible electron migration pathways (green—iron pair and lightblue—iron pair). (b) Free energy calculations show that the mobility is sensitive to thecrystallographic termination and the structure at the interface, and reveal iron sites inthe uppermost few Ångstroms of hematite (001) that preferentially trap mobile electrons.(c) New mineral-water interface models must incorporate a link between interfacialredox reactions and solid-state electron migration to describe processes such as theautocatalytic growth of hematite (001) by adsorption of Fe 2+ and subsequent electrontransfer into the solid.110 Basic Research Needs for Geosciences: Facilitating 21 st Century Energy Systems