ABSTRACT
The stability of magnetite under oxidizing hydrothermal conditions was evaluated at temperatures of 120, 150, 180 and 275 °C. A well-characterized sample of commercially-available magnetite with a particle size of approximately 690 nm was oxidized by dissolved oxygen (DO) under alkaline hydrothermal conditions in titanium autoclaves. In these trials, the DO was always in equilibrium with the gas phase oxygen that was air-derived and was located above the hydrothermal solution, which contained ammonium hydroxide at a pH25 °C of approximately 9.5. Samples recovered by filtration were analysed by X-ray diffraction and scanning electron microscopy, while Fe(ii)/Fe ratios were determined by titration in conjunction with spectrophotometry. Oxidation between 120 and 180 °C was found to generate high concentrations of maghemite and hematite in the product, with the latter compound having either a hexagonal bipyramidal or rhombohedral morphology. The oxidation kinetics was consistent with a diffusion controlled process. The reaction probably proceeded via the outward diffusion of ferrous ions from the magnetite, forming a magnetite/maghemite core/shell structure in conjunction with the dissolution of maghemite and reprecipitation of hematite. Oxidation at 275 °C presented different characteristics from those observed at the lower temperatures. Negligible amounts of maghemite were found, and the primary oxidation product was hematite with no specific morphologies. Moreover, the kinetics was slower than at 180 °C. This unexpected temperature effect is attributed to the rapid growth, at 275 °C, of a dense layer of hematite on the surface of the magnetite that impeded the oxidation of magnetite.
ABSTRACT
In Renée C. Fox's 2014 book Doctors Without Borders: Humanitarian Quests, Impossible Dreams of Médecins San Frontières, the author explores the ethos of the medical organization, shedding a candid light on its internal culture and its struggles to define and defend its conception of humanitarian action. Through blogs from field workers, numerous testimonies of staff, and extracts from internal gatherings, Doctors Without Borders reveals a side largely unknown to the public and rarely presented in bibliographies of humanitarianism. Rather than praise the heroism of aid workers or denounce the cynicism of a system that feeds on people's suffering, Fox looks at the introspective nature of MSF's world.
