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RESEARCH PRODUCT

Speciation of Copper in Enriched Agricultural Lime

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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,H 2 O) 2 (Mn 4+ ,Mn 3+ ) 5 O 10 ) 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 Cu 2+ bound into lattice sites of the tectomanganate phase. The Mn-Cu, Cu-Mn, and Cu-Cu shell distances indicated a subsritution of the Mn 3+ tectomanganate tunnel edge sites by the Cu 2+ 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.