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Al speciation in tropical podzols of the upper Amazon Basin: A solid-state Al MAS and MQMAS NMR study
Authors:Marion Bardy  Christian Bonhomme  Jocelyne Maquet  Thierry Allard  Georges Calas
Institution:a Institut de Minéralogie et de Physique des Milieux Condensés (IMPMC), UMR 7590, CNRS, Universités Paris 6 et 7, et IPGP, Case 115, 4 place Jussieu, 75252 Paris cedex 05, France
b Laboratoire de Chimie Bioorganique et Organique Physique (LCBOP), UMR CNRS 7618 BioEMCo, ENSCP, 11 rue Pierre et Marie Curie, 75231 Paris cedex 05, France
c Laboratoire de Chimie de la Matière Condensée, UMR 7574, CNRS, Université Paris 6, Case 174, 4 place Jussieu, 75252 Paris Cedex, France
d Institut de Recherche pour le Développement (IRD), UMR 161 CEREGE, Europôle Méditerranéen de l’Arbois BP 80, 13545 Aix en Provence cedex, France
e Laboratoire des Systèmes Interfaciaux à l’Echelle Nanométrique (SIEN), UMR 7142, CNRS, Université Paris 6, Case 196, 4 place Jussieu, 75252 Paris cedex 05, France
Abstract:In the upper Amazon Basin, aluminum previously accumulated in lateritic formations is massively remobilised in soils by podzolization and exported in waters. We have investigated the speciation of aluminum in the clay-size fractions of eight horizons of waterlogged podzols lying in a depression of a plateau. The horizons illustrate the main steps involved in the podzolization of laterites. They belong to eluviated topsoil A horizons and illuviated subsoil Bhs, Bh and 2BCs horizons of weakly and better-expressed podzols located at the margin and centre of the depression. For the first time, aluminum speciation is quantitatively assessed in soils by spectroscopic methods, namely FTIR, 27Al magic angle spinning (MAS) and multiple-quantum magic angle spinning (MQMAS), nuclear magnetic resonance (NMR). The results thus obtained are compared to chemical extraction data.Solid-state 27Al MAS NMR spectra enable to distinguish Al bound to organic compounds from that incorporated in secondary mineral phases detected by FTIR. MQMAS experiments additionally show that both chemical shifts and quadrupolar constants are distributed for Al nuclei linked with organic compounds. Similar amounts of chelated Al are obtained from NMR spectra and chemical extractions. The study enables to highlight three major steps in the fate of aluminum. (i) Aluminum is first released by mineral weathering, feeds complexing sites of organic matter and accumulates in subsurface Bhs horizons of weakly expressed podzols (acidocomplexolysis). (ii) Complexes of aluminum with organic matter (Al-OM) then migrate downwards in sandy horizons of better-expressed podzols and accumulate at depth in less permeable 2BCs horizons. (iii) The minor amounts of aluminum present in the 2BCs horizon of the downslope podzol show that aluminum is eventually exported towards the river network, either complexed with organic matter or as Al3+ ions after desorption from organic compounds, due to decreasing pH or biodegradation of organic ligands. The direct spectroscopic determination of Al-speciation during the formation of podzolic soils opens new perspectives to trace metal loads in the rivers of the upper Amazon Basin.
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