Optimized sequential extraction for Organic-Bound manganese in Soils: Enhancing the characterization of Mn(III) species

Le résumé fourni par la source

Redox cycling among multiple valence states of manganese in soils exerts a fundamental control over the biogeochemical cycling of virtually all elements. The redox reactive intermediate Mn(III) species complexed by organic matter has increasingly emerged as a dominant factor in these processes, and there is a need for a simple and reliable analytical method for the determination of the reactive Mn(III) species. However, the specificity of existing chemical extraction approaches remains uncertain, particularly with respect to whether the extraction process substantially dissolves other Mn(III)-bearing oxide phases and thereby biases the quantification of organically complexed Mn(III). To address this limitation, this study proposes an improved sequential extraction protocol refined for discerning manganese (Mn) fractions in soils, particularly incorporating an alkaline pyrophosphate (PP) extraction step for organic-bound reactive Mn(III) species. The method was validated using a set of Mn mineral standards and then applied to three groups of soil samples that span low, intermediate, and high Mn(III) contents based on linear combination fitting (LCF) analysis of X-ray absorption near edge structure (XANES) spectroscopy, including one group characterized by a high proportion of organically bound Mn(III). Alkaline PP extraction at pH 10.5 for 12 h caused negligible dissolution of common Mn(III)-bearing minerals in soils. Furthermore, correlation analysis between sequentially extracted Mn fractions and XANES-LCF fitting results revealed a positive relationship between PP-extractable Mn and organically bound Mn(III), suggesting that PP extraction may provide an effective approach for selectively targeting organically bound Mn(III). In addition, acidification prior to inductively coupled plasma mass spectrometry (ICP-MS) analysis may cause precipitation of organically complexed Mn in PP extracts. Validation experiments involving digestion and reduction treatments indicated that immediate analysis after acidification is essential, and that digestion is required when precipitates are present. The sequential extraction method developed in this study can supplement and refine the interpretation of spectroscopic analyses, offering a comprehensive and straightforward view of soil Mn speciation, especially for organically bound Mn(III) species.

Ce résumé expose les affirmations des auteurs. BNTIC ne l’interprète pas comme une validation indépendante des résultats.