Role of natural nanoparticles, imogolite and allophane, on the mobility of trace metals in soils from La Reunion Island

This research adresses the sensitive field of waste management in an insular context. The case of the Reunion Island bears some aggravating factors making the waste management an even more sensitive subject: 1) it is a small island with a fast growing population and associated activities 2) the agricultural soils (mostly andosols) have an elevated natural concentration of trace metals. An inexpensive form of waste management is to reuse agricultural- waste (especially pig manure) as fertilizer. However, reuse of waste, may modify physico-chemical conditions and trace elements can be released from the soils. The accuracy of the prediction of metal fluxes in soils will be greatly enhanced by a detailed knowledge of the pollutants' speciation, which controls their mobility, toxicity and ultimately their bioavailability. Although the trace metals Cu, Ni, Zn selected for this study because of their occurrence in the soils and wastes, may bind to numerous components of the soils (clays, Fe and Al oxides, organics), we chose to focus on 2 types of nanosized alumino-silicates: the tubular imogolites (2nm diameter) and the spherical allophane (3-5nm). The choice of these phases is justified by their high abundance in the andosols of the Reunion, and their suspected high reactivity arising from the presence of -SiOH and -AlOH at the surface of structures whose specific surface area can reach 700m2/g. Here we detail using a molecular approach the types of the binding mechanisms between pollutants and natural occurring nanoparticles. (Texte integral

Mechanisms of formation and reactivity of imogolite types material

Reactivity of nanopar8cles represents a central issue for many laboratories around the world. Among many supported efforts the control of the morphology of nanopar8cles is mo8vated by the fact that morphology strongly influence the proper8es of the final products. Among the vast family of available nanopar8cles, imogolite is a clay nanotube for which perfect control of the diameter is possible. Imogolites were first observed in volcanic soils[1]. They are natural aluminosilicate nanotubes having the general formula (OH)3Al2O3SiOH with a 2 nm external diameter and up to micrometers in length. The impressive monodispersity in imogolite nanotube diameter has mo8vated research on their forma8on mechanism. Synthesis protocols to produce imogolite were quickly developed. Farmer et al. were the first to obtain synthe8c imogolite using low concentra8ons of AlCl3 and SiO 2 monomers as star8ng materials (millimolar concentra8ons of the reagents) [2]. However, the produc8on of large amount of imogolite or imogolite type materials remained challenging for long 8me. We will present our most recent results concerning the possibility to produce imogolite type materials from highly concentrated stock solu8ons. We will also detail the possibility to form double wall Al- Ge nanotubes and the different stages of their forma8on [3-7]. We will then detail the surface reac8vity of these nanotubes toward metals at he lab scale as well as in natural soil. (Résumé d'auteur

Gestion de l'instabilité des prix agricoles en Afrique. Quatre conditions d'efficacité des politiques

La flambée des prix des produits agricoles de 2007/2008, suivie d'une baisse en 2009/2010, puis d'une nouvelle hausse en 2010/2011, a placé la gestion de l'instabilité des prix agricoles au coeur des débats politiques. De nombreux pays en développement ont mis en oeuvre des politiques pour limiter l'instabilité des prix agricoles et ses effets préjudiciables, sans toujours atteindre les résultats escomptés. L'analyse d'expériences récentes en Afrique montre que, pour être efficace, chaque mesure politique suppose de satisfaire quatre conditions : elle doit s'appuyer sur des connaissances solides ; elle doit être prévisible ; son financement doit être assuré ; et sa mise en oeuvre contrôlée

New findings on the structure of natural and synthetic aluminosilicates nanoparticles

Abundance within andosols of highly reactive aluminosilicate nanoparticles makes of these an important factor affecting soil dynamics (carbon sequestration1, trapping pollutants2 ...). Gaining knowledge of the structural characteristics of such nanoparticles is of fundamental importance to understand their interactions with the different soil compartments. Aluminosilicate nanoparticles can adopt two main structures. Imogolites (Al2SiO3(OH)4), natural aluminosilicate nanotubes that have been well characterized since their discovery in 19723; And allophanes, aluminosilicates with identical chemical composition but with a different structure. allophanes have been described as hollow nanospheres with a diameter ranging from 3 to 5 nm and their structure depends on the Al/Si ratio: (i) Al-rich allophanes (Al/Si=2, Imogolite type local structure); (ii) Si-rich allophanes (Al/Si<2). Nonetheless, the only evidence for allophanes spherical nature up to date has solely come from TEM observations. The actual morphological structure of allophanes still needs to be further investigated. In the present work, Aluminosilicate samples obtained from soils collected in La Reunion (a French volcanic island in the Indian Ocean region) are studied using an array of diverse characterization techniques. While XRD and FTIR results are consistent with the characteristic allophane fingerprint, NMR analysis reveals an imogolite-type local environment of silicon and aluminium, pointing to a type i Al-rich structure. However, no spherical objects could be observed using TEM. In view of such observations, we propose that the structure of these type i Allophane is not consistent with that of a hollow sphere geometry. To obtain further insight into this matter, we synthesised aluminosilicate nanoparticles (both allophane and imogolite), and thoroughly characterized them using a wide variety of high specificity techniques ranging from the macro crystalline structure (TEM, XRD) to the atomic scale (XAFS, PDF). Our findings point to a structure consistent with that of a short imogolite nanotube type structure, rather than a hollow sphere. (Résumé d'auteur

Managing agricultural price volatility in Africa. Context matters for policy effectiveness

Soaring agricultural prices in 2007/2008, followed by decreasing prices in 2009/2010 then a new surge in late 2010/2011, have placed the management of agricultural price volatility at the heart of policy debates. Many developing countries have implemented policies to limit agricultural price volatility and its adverse effects, without always achieving the expected results. Analysis of recent experiences in Africa shows that in order to be effective, a policy measure must meet four conditions: it must be based on robust knowledge; it must be predictable; its funding must be secured; and its enforcement must be monitored

Structure of short-range ordered aluminosilicates in andic horizons of volcanic soils

Very high levels of C content characterize Andic horizons of volcanic soils. Stabilization of organic matter is due to the presence of short-range ordered aluminosilicates (imogolites, allophanes or proto-imogolites). These phases are often characterized through selective chemical extractions from which the "allophane" content is calculated. However, chemical dissolutions preclude the characterization of the structure of the short-range ordered aluminosilicates. Imogolite is easily distinguishable because of its tubular structure, whereas allophane compounds-usually described as spheres-are harder to identify, especially because of their variable structure and occurrence patterns. In addition, the local structure of allophanes can be very similar to that of proto-imogolite (imogolite precursor). Strangely, this similarity is seldom considered in most characterization studies. In this context, our study focuses on the structure of two short range-ordered aluminosilicates of two different origins, from: (i) an Andosol B horizon (Andosol sample); and (ii) a weathered pumice grain (pumice sample). These natural samples were compared to a synthetic proto-imogolite. The three samples were analyzed using experimental tools that are commonly used for the identification of these nanophases (chemical composition, X-ray diffraction, nuclear magnetic resonance, Fourier transform infrared spectroscopy and transmission electron microscopy). The three samples exhibited the same local structure, but significant differences were observed at a larger scale. The pumice sample clearly showed ring-shaped particles, while the Andosol sample and the synthetic proto-imogolite were amorphous. Our results suggest that poorly ordered proto-imogolite, rather than allophanes, is present in Andosol horizons. (Résumé d'auteur