Kinetics of isotopic exchange of phosphate at the α-FeOOH-aqueous solution interface

Abstract

The kinetics of heterogeneous isotopic exchange of phosphate between ³²P-labelled dilute orthophosphate solutions and Fe(III) phosphate complexes on the surfaces of goethite (α-FeOOH) crystals in dilute suspension are described by the Elovich Equation. Concentration-independent rate constants are obtained at 25° for the pH range 2·6–12. The reference states for the rate constants are defined with respect to an approximation for the distribution of activation energies for exchange. Rates of exchange are first order with respect to the concentration of Fe(III) phosphate surface complexes, show little or no dependence on the concentration of phosphate in solution, and are subject to acid-base catalysis involving fractional powers of H⁺ and OH⁻ concentrations with a minimum rate at pH 9·0. The inert character of Fe(III) phosphate complexes is attributed to the formation of a briding ligand structure in binuclear complexes on the predominantly-exposed (100) crystal surfaces. The rate-determining step is postulated to be S_N1 dissociation or S_N2 bimolecular solvolysis of phosphate from Fe(III) co-ordination, in which the mechanism of catalysis is related to a hydrogen bonding stabilisation of the transition state.