Fe(II) Catalyzed Electron Transfer and Atom Exchange in Goethite

Fe^II-Fe^III electron transfer reactions of goethite (α-FeOOH) play a critical role in environmental biogeochemistry, for example influencing the fate and transport of contaminants in groundwater. We have measured Fe^II catalyzed recrystallization in goethite using a ⁵⁷Fe isotopic tracer in the presence of anions in order to determine the environmental relevance of this reaction. It was found that common groundwater anions including phosphate, silicate, bicarbonate and natural organic matter do not affect Fe electron transfer or atom exchange. This reaction was also proposed to drive trace metal cycling in the environment. Ni, Zn and Al were coprecipitated with goethite, and release kinetics were measured in parallel with Fe atom exchange. We found that Fe^II driven Fe atom exchange drives trace metal release, with increasing trace metal substitution slowing down recrystallization and release. This provides evidence that the kinetics of Fe^II driven recrystallization may vary greatly among environmental systems, as Al substitution in goethite can range from 0 to ~30%.  It has recently been proposed that these exchange reactions involve conduction of electrons through the goethite bulk, which has implications for developing mechanistic models of overall system behavior. We used ab initio and density functional theory methods to compute the conductivity of single-crystal goethite, which entails thermally-activated hopping of small polarons. The reorganization energy and electronic coupling matrix element were calculated using hybrid functionals (B3LYP and PBE0) as implemented in the CRYSTAL and NWChem quantum chemistry codes, respectively. These methods yield cell parameters within 1.5% of experimental values and the (+ - + -) magnetic structure relative to the b axis (space group Pbnm) to be the most stable, in agreement with experiment.