Improved methods for selective dissolution of Mn oxides: applications for studying trace element associations |
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| Institution: | 1. Unité de Recherche de Géochimie des Sols et des Eaux, Institut National de la Recherche Agronomique, BP 80, 13545 Aix-en-Provence cedex 04, France;2. Centre Européen de Recherche et d''Enseignement de Géosciences de l''Environnement, UMR 6635 CNRS-Université d''Aix-Marseille III, BP 80, 13545 Aix-en-Provence cedex 04, France;1. State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, 163, Xianlin Ave., Nanjing 210023, PR China;2. Jiangsu Key Laboratory of Environmental Change and Ecological Construction, School of Geographical Science, Nanjing Normal University, Wenyuan Road, Xianlin University District, Nanjing 210023, PR China;1. School of Earth and Ocean Sciences, University of Victoria, Victoria, BC V8W 2Y2, Canada;2. Department of Geoscience, University of Wisconsin – Madison, WI, 53706, USA;3. NASA Astrobiology Institute, Department of Geoscience, University of Wisconsin, Madison, WI, 53706, USA;4. Department of Earth Science, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan;5. Department of Geosciences, Princeton University, Guyot Hall, Princeton, NJ 08544, USA;1. Institute of Global Environmental Change, Xi''an Jiaotong University, Xi''an, China;2. Institute of Earth Sciences, Hebrew University of Jerusalem, Jerusalem, Israel;3. Department of Earth Sciences, University of Minnesota, Minneapolis, USA |
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| Abstract: | The association of rare earth and other trace elements with Fe and Mn oxides was studied in Fe-Mn-nodules from a lateritic soil from Serra do Navio (Northern Brazil). Two improved methods of selective dissolution by hydroxylamine hydrochloride and acidified hydrogen peroxide along with a classical Na–citrate–bicarbonate–dithionite method were used. The two former reagents were used to dissolve Mn oxides without significant dissolution of Fe oxides, and the latter reagent was used to dissolve both Mn and Fe oxides. Soil nodules and matrix were separated by hand. Inductively coupled plasma atomic emission spectrometry and inductively coupled plasma mass spectrometry after fusion with lithium metaborate, and X-ray diffraction were used to determine the elemental and mineralogical composition of the nodules and soil matrix. The latter was composed of kaolinite, gibbsite, goethite, hematite, and quartz. In the nodules, lithiophorite LiAl2(MnIV2MnIII)O6(OH)6 was detected in addition to the above-mentioned minerals. The presence of hollandite (BaMn8O16) and/or coronadite (PbMn8O16) in the nodules is also possible. In comparison to the matrix, the nodules were enriched in Mn, Fe, K, and P, and relatively poor in Si, Al, and Ti. The nodules were also enriched in all trace elements determined. Phosphorus, As and Cr were associated mainly with Fe oxides; Cu, Ni, and V were associated with both Fe and Mn oxides; and Ba, Co, and Pb were associated mainly with Mn oxides. Distribution of rare earth elements indicated a strong positive Ce-anomaly in the nodules, compared to the absence of any anomaly in the matrix. Some of Ce was associated with Mn oxides. The improved methods achieved almost complete release of Mn from the sample without decreasing the selectivity of dissolution, i.e., without dissolving significant amounts of Fe oxides and other minerals, and provided reliable information on associations of trace elements with Mn oxides. These methods are thus proposed to be included in sequential extraction schemes for fractionation of trace elements in soils and sediments. |
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