Determination of Trace Metal Distributions in the Iron Oxide Phases of Red Bed Sandstones by Chemometric Analysis of Whole Rock and Selective Leachate Data

Abstract

Hematite is known to be a sink for trace metals and study of the trace metal distributions within hematite-rich formations can provide evidence of past groundwater activity. The aim of this study is to develop a method specifically for determining the trace metal content of naturally occurring hematite in a sandstone formation. The elemental compositions of twelve samples of permo-triassic Red Bed sandstones from the St. Bees formation, Sellafield, Cumbria, were determined. An acid digestion procedure was used to produce solutions suitable for ICP-AES analysis for the major and trace elements (Al, Fe, Mg, Ca, Na, K, Ti, Mn, Ba, Sr). The same solution was used for ICP-MS analysis for lower abundance trace elements (Cr, Ni, Pb, Sn, Th, U, V, Zr). Fusion with LiBO₂, followed by an ion exchange preconcentration–separation procedure, provided solutions for rare earth elements, La, and Y determination by ICP-AES. Solutions for ICP-AES analysis of Si in the whole rock were prepared by a modified fusion procedure using 4 + 1 mixture of LiBO₂ and Li₂B₄O₇. A leaching procedure was developed to selectively extract the hematite phase of the rock samples. This procedure consisted of a preliminary extraction with 1 M acetic acid for 20 h at 20 °C to remove carbonates, followed by an extraction with 0.1 M oxalic acid–0.175 M ammonium oxalate (Tamm’s reagent) for 20 h at 70 °C to dissolve the iron oxide. The Tamm’s reagent leachate was analysed by ICP-AES and ICP-MS for the same suite of elements as the whole rock. Principal component analysis was applied to the whole rock and leachate data matrices. Qualitative assessment of the resulting principal components revealed elemental groupings which can be assigned to minerals known to exist in this rock type and showed the leaching procedure was not fully selective for the iron oxide phase. Chemometric mixture decomposition applied to the leachate data sets identified the extraction of four different mineral sources tentatively assigned to hematite, dolomite, chlorite and iron oxy-hydroxides. Quantitative estimates of the composition and proportion each of these components in each rock sample were calculated.

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