Influence of anionic layer structure of Fe-oxyhydroxides on the structure of Cd surface complexes

The nature of crystallographic reactive sites on the lepidocrocite (gamma F eOOH) surface has been determined by atomic force microscopy (AFM) and exte nded X-ray absorption fine structure (EXAFS) spectroscopy and compared to t he surface bonding properties of goethite. To this end, the specific surfac e areas of lepidocrocite particles, and of their crystal faces, were calcul ated from the size and shape of individual particles determined by AFM, and the structure of Cd surface complexes was determined from Cd-Fe EXAFS dist ances. The combined results show that Cd forms solely mononuclear surface c omplexes, even at 100% surface coverage, and that hydrated Cd octahedra sor b on basal {010} and lateral {hk0}, {h0l} faces of lepidocrocite platelets by sharing edges with surface Fe octahedra. The absence, or scarcity, of co rner-sharing linkage between Fe and Cd octahedra on the surface of lepidocr ocite is in contrast to goethite (alpha FeOOH), where this type of complex is predominant. The explanation for the observed difference of Cd sorption mechanism on these two polymorphs lies not in the shape and relative surfac e area of their crystallographic faces, but in their different bulk structu res and, specifically, in the stacking mode of anion layers (O2-, OH-) whic h is hexagonal in alpha FeOOH and cubic in gamma FeOOH. This study demonstr ates that the stacking mode of anions in the sorbent solid is a key factor in determining the structure of surface complexes on mineral surfaces, (C) 2000 Academic Press.