Effect of Cd and Al Coincorporation on the Structures and Properties of Goethite

Goethite is one of the most common and active Fe oxyhydroxide minerals in soils. Although single metal substitution for Fe in goethite is extensively studied, investigation is rare on simultaneous incorporation by Al and Cd, which is more common in some metal-polluted ecosystems. In this study, a series of Al and Cd cosubstituted goethite crystals were synthesized at room temperature and characterized by powder X-ray diffraction, Rietveld structure refinement, wet chemical analysis, transmission electron microscopy, and infrared spectroscopy. The highest substitution levels of Al and Cd for Fe are ∼7.8 and ∼9.9 mol %, respectively. Increasing Cd concentration in goethite is accompanied by decreasing crystal size and crystallinity. Incorporation of Cd into the goethite structure is hardly affected by Al, but the incorporation of Al is greatly inhibited by Cd. Rietveld structure refinement analysis shows a significantly positive relationship among unit-cell dimensions, crystal density, and Cd concentration....