Speciation of Copper in Enriched Agricultural Lime
- Michael Kersten *a,
- Tatiana Y. Reicha,
- Ka H. Luia,
- Gerald T. Schmidtb and
- Jörg Göttlicherc
- a Environmental Geochemistry Group, Institute of Geosciences, Johannes Gutenberg-Univ., Mainz 55099, Germany
b Environmental Geochemistry Group, Institute of Geosciences, Johannes Gutenberg-Univ., Mainz 55099, Germany, currently at, DECHEMA, Karl-Winnacker-Institut, Frankfurt am Main 60486, Germany
c Institute for Synchrotron Radiation, Karlsruhe Institute of Technology, Eggenstein-Leopoldshafen 76344, Germany
Agricultural liming materials are used to lower soil acidity and to improve microbial functionality and plant growth. A brownish colored agricultural lime was found to contain up to 180 mg kg−1 Cu, an amount well above any fertilizing materials code threshold. The dark color of the milled material was due to ample black and brown dendrites consisting of the tectomanganate mineral romanechite (ideally (Ba,H2O)2(Mn4+,Mn3+)5O10) and a mixture of the two nanocrystalline Fe oxides ferrihydrite and goethite. A four-step sequential extraction analysis of the lime powder was not sufficient to argue whether the Cu load was contained in the carbonate matrix or in the oxide dendrites, or, in the latter case, in which of the three oxide phases identified. Electron microprobe analysis confirmed that the manganate dendrites were responsible for elevated Cu concentrations in the limestone. Focused micro-x-ray absorption spectroscopy (μ-XAS) revealed the bonding environment of Cu in the romanechite dendrites to be octahedrally coordinated as the Jahn–Teller cation Cu2+ bound into lattice sites of the tectomanganate phase. The Mn–Cu, Cu–Mn, and Cu–Cu shell distances indicated a substitution of the Mn3+ tectomanganate tunnel edge sites by the Cu2+ cations. This structural intercalation of Cu is clearly more stable than Cu cations bound as surface adsorbate complexes. The Cu contained in the liming material is therefore not likely to be mobilized and bioavailable, and therefore the lime poses little threat to agricultural soils.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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