Retention of 226^{226}Ra by barite: The role of internal porosity

The role of internal macropores and nano-scale pores for the uptake of 226Ra into barite was studied via scanning and transmission electron microscopy as well as focused ion beam methods. A temporal evolution of the internal microstructure and the Ra distribution was observed on samples taken from long-term Ra uptake experiments. The results of this study clearly show a significant impact of the presence of Ra leading to a complete reconstruction of the internal barite microstructure, whereas the microstructure of Ra-free reference samples remained unchanged. The initial internal barite microstructure contains a connected network of macropores and a layered structure of nano-scale pores which, in the presence of Ra, coalesced in favor of larger pores during the experiment. A clear relationship between the Ra uptake and the internal porosity was observed by high-resolution STEM-EDX mappings. Starting from strongly enhanced Ra concentrations in the solid in the vicinity of the pores, Ra is temporarily inhomogeneously distributed within the barite particles. At later stages of the long-term experiment the Ra distribution becomes homogenous while nano-scale and macro-scale pores disappear. In conclusion, the uptake of Ra into barite takes place by a special case of dissolution/reprecipitation from the inside of the particle to the outside

La impostura desarrollista y los tres momentos de la reforma agraria en Colombia: liberalismo, proteccionismo y neoliberalismo. Cronología de una muerte anunciada

El discurso del desarrollo generalizado durante el siglo XX es un engaño con apariencia de verdad; esto posibilita hablar de una impostura desarrollista. En este sentido, los procesos de reforma agraria en Colombia se encontraban –y se encuentran– dictaminados por la idea mesiánica del desarrollo (por esa impostura desarrollista). Esto llevó a que durante el siglo XX las políticas de reforma agraria, impartidas por el gobierno nacional, hayan obedecido a tres estrategias globales, siempre bajo el discurso generalizado del desarrollo. La reforma agraria en Colombia inició en un momento liberal, pasó por el proteccionismo y finalmente llegó al momento actual denominado neoliberalismo. Así las cosas, este trabajo pretende analizar la influencia de la impostura desarrollista en los tres momentos de reforma agraria en Colombia a partir de una metodología histórico-hermenéutica, la cual propiciará comprender los procesos sociales desde una lectura interpretativa, reflexiva y crítica que nos conduzca a una reinterpretación del sentido teórico–conceptual de la impostura desarrollista y su relación con la reforma agraria en Colombia

Nano-structural features of barite crystals observed by electron microscopy and atom probe tomography

For the first time a barite samplewas investigated by applying electronmicroscopy and atom probe tomographyin a complementary approach to characterize its inner microstructure. Pores covering the size range from a fewnanometers to a few micrometers were identified inside particles of a commercially available high purity barite.The baritematrix appeared to be a single crystal. A particular feature of the observed porous structure is a systemof layers containing a high number of homogeneously distributed nano-scale pores, alternating with layers containinga limited number of larger pores. High-resolution chemical analyses indicate that the pores contain a solutionof water and sodium chloride. The filling of these nano-scale inclusions was interpreted as residua of theinitial supersaturated solution used for particle growth by precipitation. These findings explain the high reactivityobserved in previous Ra uptake studies on the same type of barite and provide further implications for the interpretationof these results. Our results offer a new analytical approach and necessary reference data for future investigationson changes of the internal barite structure during these Ra uptake experiments

The solid solution–aqueous solution system (Sr,Ba,Ra)SO4 + H2O: A combined experimental and theoretical study of phase equilibria at Sr-rich compositions

Solid solution formation between RaSO4 and other isostructural sulfates has been known for a long time as a process potentially controlling Ra concentrations in aquifers. Here, we measured the Ra uptake in the Sr-rich corner of the ternary (Sr,Ba,Ra)SO4 solid solution - aqueous solution (SS-AS) system by equilibrating SrSO4, BaSO4 and mixed (Sr,Ba)SO4 solids with Ra-bearing aqueous solutions for up to 1302 days at 90 °C at a solid/liquid ratio of 5 g/L. The recrystallization experiments were combined with electron microscopy studies of the solids. The evolution of the solid and aqueous phases was interpreted based on thermodynamic modelling applying a recently revised thermodynamic dataset for the (Sr,Ba,Ra)SO4 + H2O system.The recrystallization process involved several metastable phases, starting from the least soluble, Ba- and Ra-rich precipitates and ending with Sr-rich solids, whose compositions approached the predicted equilibrium states. The composition and the time evolution of metastable phases were correlated with changes in the computed supersaturation functions. Particularly, the formation of Ra- Ba- and Sr-rich rims on primary barite grains in the experiment with the mechanical mixture of 99% of celestine and 1% of barite has been explained by combining calculated supersaturation conditions with considerations of structural misfit.A key result was the observed final Raaq concentration which is about one order of magnitude lower compared to the initial concentration, implying an efficient uptake into the newly formed solid phases. These concentrations appeared to be just slightly lower than those predicted by the thermodynamic calculations, implying that the complete SS-AS equilibrium was close to be reached in Sr-rich systems in the recrystallization experiments lasting for about 3.5 years