Preference of Co over Al for substitution of Fe in goethite (α-FeOOH) structure: Mechanism revealed from EXAFS, XPS, DFT and linear free energy correlation model

Abstract

Goethite is the most stable iron hydroxide mineral in geological environments, and it is usually crystallized with more than one type of substituent. However, the synergetic or antagonistic effects of coexisting cations on substitution for lattice Fe in gothite are unclear. In this study, a series of Co-, Al-, and Co + Al co-substituted goethite samples were synthesized at room temperature and then investigated by powder XRD, X-ray photoelectron spectroscopy (XPS), X-ray absorption fine structure spectroscopy (XAFS), density function theory (DFT) calculations, and acid dissolution experiments. All the obtained samples were pure goethite, except the one with a high total content of Co + Al that contained a portion of 2-line ferrihydrite. Lattice parameter a of Co + Al co-substituted goethite was linearly decreased with the final Co content, lattice parameter b and calculated crystal density had the best negatively linear relationships with Al content while lattice parameter c, cell volume, average edge-sharing Fe − Me (Me = Fe, Al, and Co) distances along c⁎ and b⁎ axis (E′ and E) and average corner-sharing Fe − Me (DC) distances had the best linear correlations with the total contents of Co + Al. Aluminum and Co were found to be evenly distributed in goethite structure, and both reduced the mineral particle size. Co coexisted in a mixed valance of +2 and +3, and the proportion of Co3+ increased with the increase of Co content in Co-substituted goethite owing to relatively high electric potential energy and decreased with the increase of coexisting Al3+ content in Co + Al co-substituted goethites because of relatively low alkalinity in local environments around Co2+. Incorporation of Co greatly promoted the mineral dissolution in 2 M HCl solution at 298 K. Fe K-edge EXAFS analysis indicated almost constant local order around Fe in these samples. Coexisting Al had almost no effect on Co incorporation into goethite structure but Co suppressed Al substitution for lattice Fe. This preference of Co over Al to substitute for Fe was explained by the predicted partition coefficients (Kd) of common trivalent substituents in goethite by using the linear free energy relationship. It highlights the necessity to take into consideration not only the substituent physical properties (size and charge) but also their chemical properties (chemical bonding energy and chemical potential). These results enhanced our understanding of the incorporation of exotic cations into the iron oxide structures and the mutual effects of coexisting cations on substitution for lattice Fe.