Fe-speciation in kaolins : a diffuse reflectance study

Abstract--Diffuse r flectance spectra of kaolins have been recorded in samples from different environ-ments. They show the systematic presence of Fe-oxides, even in bleached kaolins, with no contribution from the Fe 3+ ions substituted in kaolinite. Second derivative spectra of various Fe-phases (hematite, goethite, lepidocrocite, maghemite, akaganeite, f rrihydrite and Fe-polymer) may be differentiated by the position of a diagnostic band corresponding tothe 2QA0 ~ 2(4T~(4G)) transition. The systematic com-parison of diffuse reflectance spectra of unbleached and bleached kaolins has demonstrated the differences between the Fe-oxides occurring as coatings and as occluded phases. The features observed in second derivative spectral curves are consistent with assignments ofcrystal field transitions togoethite, hematite, akaganeite, and aged hydrous ferric oxides. The optical determination f the Fe-phases associated to kaolins assists in the interpretation f the formation conditions of these minerals. Key Words--Diffuse r flectance spectroscopy, Fe-speciation, Kaolin, Second erivative spectra

Grands bassins fluviaux périatlantiques : Congo, Niger, Amazone

Les différents projets d'aide au développement élaborés et réalisés dans le nord de la Côte d'Ivoire nécessitent une bonne maîtrise de la gestion des ressources naturelles en général, et des ressources en eau en particulier. C'est dans cette optique qu'ont été réalisé des travaux de modélisation de la relation pluie-débit sur 16 bassins versants (100 à 4 500 km2) des hauts bassins du Niger, du Bandama et du Sassandra. L'objectif était de caractériser les valeurs des paramètres après calage, afin de pouvoir utiliser ces algorithmes en simulation sur des bassins versants, non jaugés ou mal connus, dans le cadre de projets d'aménagement. Ces modèles pluie-débit ont été calés année par année sur les bassins versants retenus. On a ensuite, procédé, sur les jeux de paramètres calés, à une analyse de données (ACP, régression linéaire multiple) destinée à mettre en évidence les relations entre les valeurs prises par les paramètres des modèles et différentes variables climatiques, physiques ou caractéristiques de l'occuption des sols. Ces relations ont été validées et testées. Elles présentent un niveau d'adéquation tout à fait satisfaisant entre hydrogrammes annuels observés et calculés, ce qui autorise leur utilisation dans le cadre de projets d'aménagements, à vocation agronomique en particulier. (Résumé d'auteur

Sr isotopic evidence for ion-exchange buffering in tropical laterites from the Parana, Brazil

International audienceSr isotopes have been measured in laterite sequences and in associated waters from the region of Ribeirao Preto (Parana basin, Brasil), in order to characterize the water-rock interactions corresponding to the successive stages of weathering of these continental flood basalts, We analyzed detailed sections of concentric rims of alteration in ball-shaped spheroids developed around cores of fresh basalts. Waters were also analyzed on the different types of facies in this region: unaltered basalts, thick weathered profiles of lateritic material, and surrounding sandstones, The results show that a major part of the initial Sr is removed from the basalt during the earliest stages of weathering, within the narrow grey rims surrounding the unweathered core. Radiogenic Sr is also leached from hydrothermal minerals located in basalt fractures or disseminated in the basalt groundmass at this early stage. Very high water/rock ratios (of at least 100,000) are calculated for this process, from the Sr-87/Sr-86 in the related waters. A general trend of increasing Sr-87/Sr-86 ratios is observed in the increasingly weathered facies (inner yellow cortexes, red kaolinitic soils and matrices) and in their related waters. The very high Sr-87/Sr-86 ratios of the red kaolinitic horizons probably result from isotopic exchange with radiogenic rainwaters or sandstone waters, and also from the presence of residual or neoformed radiogenic minerals. Budget calculations, as well as experimental leaching of kaolinite samples, suggest that significant amounts of radiogenic Sr can be leached from these facies during weathering, Thus, the kaolinitic facies of tropical laterite profiles are able first to trap strontium from surficial waters, then to provide radiogenic Sr to the waters that weather the fresh basalts at the weathering front. Therefore, the thick weathered horizons must be considered as a natural cation-exchange layer, that play an active role in the weathering process

Facteurs prédictifs de la mortalité des patients admis aux urgences médicales des cliniques universitaires de Kinshasa

Background: The management of medical emergencies is poorly organized in the Democratic Republic of Congo. In addition, the mortality of patients attending the medical emergency unit of Kinshasa University Hospital is relatively high, with death of patients occurring rather early. To date, factors associated with this mortality have been poorly elucidated. This study aimed to identify predictive factors of all-cause mortality in patients admitted to the medical emergency unit of the Kinshasa University Hospital. Methods: Analytical prospective study of all patients admitted from 15th January to 15th February 2011 in the emergency unit of the internal medicine department of Kinshasa University Hospital (427 patients). Among these patients, 13 were dead at arrival and were excluded from this study. The 414 patients included were followed until discharge from the hospital. Demographic, clinical, biological, diagnostic, therapeutical and evolutive data were collected. Four multivariate logistic regression models were used to identify risk factors associated with mortality. Results: Patients' median age was 40. years (interquartile range, 28-58 years), 54.5% were male, and 15.9% had a life-threatening pathological condition on admission. The overall mortality was 12.3%. According to multivariate analyses, transfer from other health care structures (OR: 3.5; 95% CI: 1.7-7.1), Glasgow Coma Scale score less than 14 on admission (OR: 11.1; 95% CI: 4.7-26.3), high creatinine level (OR: 4.2; 95% CI: 1.8-9.7), presence of cardiovascular (OR: 2.9; 95% CI: 1.5-5.7), renal (OR: 7.4; 95% CI: 3.2-17.3), hematologic and/or respiratory (OR: 6.1; 95% CI: 1.7-21.4) diseases, presence of sepsis and/or meningitis and encephalitis (OR: 5.2; 95% CI: 1.6-17.0) were significantly associated with a high risk of death. However, the Glasgow Coma Scale score less than 14 on admission and renal disease were the only predictive factors of mortality remaining after including demographic, clinical, diagnostic and therapeutical variables in the logistic regression model. Conclusion: Our study showed that transfer from another health care structure, low Glasgow Coma Scale score on admission, high creatinine level, cardiovascular, renal, hematologic and/or respiratory diseases, sepsis and/or meningitis and encephalitis were associated with an increased risk of death in Kinshasa University Hospital patients admitted in the medical emergency unit. © 2013 Elsevier Masson SAS.SCOPUS: ar.jinfo:eu-repo/semantics/publishe