Critical evaluation of zinc speciation in geochemical reference materials by combining sequential extraction and XANES spectroscopy

抄録

We have identified and quantified Zn species in the Japanese geochemical reference materials (JLk-1, JSO-1, JSd-1-4, and JMs-1-2) by combining the sequential extraction procedure developed by the Community Bureau of Reference (BCR) and X-ray absorption near edge structure (XANES) spectroscopy. XANES spectroscopy revealed that: 1) target phases predicted by the BCR protocol, such as Zn-bearing Fe hydroxides and organic matter, are less abundant in the reference materials; and 2) Zn adsorbed to the phyllosilicates and the weathered surfaces of mafic minerals was selectively extracted by the BCR protocol. The weathering products of sphalerite, authigenic ZnS precipitate, Zn-bearing Fe hydroxides were identified in stream sediments influenced by mining activity and contaminated sediments. Phyllosilicates were the dominant phases of Zn extracted by the BCR protocol, even in these materials. The Zn²⁺ ion is hydrolytically-stable and easily dissolves in water. The speciation study revealed that Zn in nature would be selectively adsorbed by phyllosilicates, because their surfaces are charged negatively around neutral pH, and strongly bind Zn. The BCR extraction protocol does not provide the predicted results for Zn speciation, but is still helpful to the stability assessment, because Zn extracted at early-stages of extraction is interpreted to be a labile fraction that is easily-released to the environment.