PNNL

Abstract

The influence of the {001} basal plane of hematite on the composition of fast-frozen centrifuged wet pastes of hematite prepared at pH 4 and 9 and at ionic strengths of 0, 10 and 100 mM NaCl was investigated by x-ray photoelectron spectroscopy. Two hematite preparations consisted of micrometer-sized platelets with 42% (HEM-1) and 95% (HEM-8) of the surface terminated by the {001} basal plane. A third preparation of spherical shape with no recognizable crystal plane (HEM-control) was used as a control to these experiments. All hematite samples responded to changes in pH and ionic strength, showing that acid/base reactions of surface hydroxyl groups control the composition of the paste. The HEM-1 and HEM-8 sample exhibited divergent properties at the highest ionic strength (100 mM) with energy loss features in the Na 1s and Cl 2p spectra and an important water content. As the spectra were typical of hydrated Na+ and Cl- ions and that the surface concentrations were unusually large, the HEM-1 and HEM-8 samples are proposed to induce the formation of a three-dimensional distribution of these ions in the paste. The sodium, chloride and water content was also correlated to the fraction of the {001} basal plane present in the sample and provided evidence for an approximate stochiometric Na:Cl:H2O ratio of 1:1:2. The {001} basal plane of hematite is consequently proposeD to be the cause of this feature.