Local Environments and Lithium Adsorption on the Iron Oxyhydroxides Lepidocrocite (γ-FeOOH) and Goethite (α-FeOOH): A 2H and 7Li Solid-State MAS NMR Study

Abstract

2H and 7Li MAS NMR spectroscopy techniques were applied to study the local surface and bulk environments of iron oxyhydroxide lepidocrocite (gamma-FeOOH). 2H variable-temperature (VT) MAS NMR experiments were performed, showing the presence of short-range, strong antiferromagnetic correlations, even at temperatures above the Néel temperature, T(N), 77 K. The formation of a Li+ inner-sphere complex on the surface of lepidocrocite was confirmed by the observation of a signal with a large 7Li hyperfine shift in the 7Li MAS NMR spectrum. The effect of pH and relative humidity (RH) on the concentrations of Li+ inner- and outer-sphere complexes was then explored, the concentration of the inner sphere complex increasing rapidly above the point of zero charge and with decreasing RH. Possible local environments of the adsorbed Li+ were identified by comparison with other layer-structured iron oxides such as gamma-LiFeO2 and o-LiFeO2. Li+ positions of Li+-sorbed and exchanged goethite were reanalyzed on the basis of the correlations between Li hyperfine shifts and Li local structures, and two different binding sites were proposed, the second binding site only becoming available at higher pH.