Current Practices for Analyzing Soils and Sediments via Mössbauer Spectroscopy

Abstract

Environmental scientists are increasingly returning to Mössbauer spectroscopy (MBS) to reveal details about iron (Fe)-bearing phases in soils and sediments. MBS is particularly powerful at distinguishing between Fe(II) and Fe(III) and, given appropriate background information, can offer exceptionally precise information on Fe speciation in compositionally complex environmental samples. However, there are relatively few accessible guides for analyzing environmental samples by MBS. In this review, we seek to distill the essential understanding of MBS for earth scientists and provide guidance on analysis, spectral fitting, and interpretation for new practitioners and a consolidation of approaches for experienced users. As a rule, Fe phases in soils and sediments are more disordered and complex than synthetic or geogenic Fe minerals. We cover the most successful ways MBS can be applied to soils, including the determination of Fe(II)/Fe(III) ratios, characterization of Fe (oxyhydr)oxide crystallinity, and the use of 57Fe isotope spikes, as well as highlighting how to avoid common pitfalls and arrive at Fe phase identification and quantification by leveraging complimentary data and environment context. We outline procedures for sample preparation, analysis, and spectral fitting using decision trees based on the analytical goals and sample conditions. The fitting and interpretation of magnetically ordered ferrous phases at low temperature is lacking in the literature and so we offer an expanded discussion of approaches to these challenging spectra. We provide a discussion and fitting guidance for the most common Fe phases in soils and sediments organized around environmental contexts: young soils (and sediments derived from them) dominated by aluminosilicates, highly weathered soils rich in Fe oxides, organic-rich soils, soils in sulfur-rich environments, and soils exposed to anoxia. For each context, we describe expected Fe phases and their characteristic spectral features while emphasizing the importance of complementary analyses for reliable interpretation. Finally, we identify two critical needs in the field: improved theoretical frameworks for fitting low-temperature ferrous octets and Fe–sulfur phases and a need for standardization of parameter reporting and data sharing within the environmental MBS community. This review aims to both facilitate broader adoption of MBS in the environmental sciences and advance the technique's application to complex natural samples.