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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 of57Fe 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.
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Recommendations for best practice for iron speciation by competitive ligand exchange adsorptive cathodic stripping voltammetry with salicylaldoxime
The method of competitive ligand exchange followed by adsorptive cathodic stripping voltammetry (CLEAdCSV) allows for the determination of dissolved iron (DFe) organic speciation parameters, i.e., ligand concentration (LFe) and conditional stability constant (log Kcond Fe′L ). Investigation of DFe organic speciation by CLEAdCSV has been conducted in a wide range of marine systems, but aspects of its application pose challenges that have yet to be explicitly addressed. Here, we present a set of observations and recommendations to work toward establishing best practice for DFe organic speciation measurements using the added ligand salicylaldoxime (SA). We detail conditioning procedures to ensure a stable AdCSV signal and discuss the processes at play during conditioning. We also present step-by-step guidelines to simplify CLE-AdCSV data treatment and interpretation using the softwares ECDSoft and ProMCC and a custom spreadsheet. We validate our application and interpretation methodology with the model siderophore deferoxamine B (DFO-B) in a natural seawater sample. The reproducibility of our application and interpretation methodology was evaluated by running duplicate titrations on 19 samples, many of which had been refrozen prior to the duplicate analysis. Nevertheless, 50% of the duplicate analyses agreed within 10% of their relative standard deviation (RSD), and up to 80% within 25% RSD, for both LFe and log Kcond Fe′L . Finally, we compared the sequential addition and equilibration of DFe and SA with overnight equilibration after simultaneous addition of DFe and SA on 24 samples. We found a rather good agreement between both procedures, with 60% of samples within 25% RSD for LFe (and 43% of samples for log KcondFe′L ), and it was not possible to predict differences in LFe or log KcondFe′L based on the method applied, suggesting specific association/dissociation kinetics for different ligand assemblages. Further investigation of the equilibration kinetics against SA may be helpful as a potential way to distinguish natural ligand assemblages.
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- PAR ID:
- 10514131
- Publisher / Repository:
- Elsevier
- Date Published:
- Journal Name:
- Marine Chemistry
- Volume:
- 259
- Issue:
- C
- ISSN:
- 0304-4203
- Page Range / eLocation ID:
- 104348
- Subject(s) / Keyword(s):
- iron ligands competitive ligand exchange-adsorptive cathodic stripping voltammetry (CLE-AdCSV) Salicylaldoxime (SA) Conditioning Interpretation Comparison
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
-
Accepted Manuscript
- Journal Article:
- https://doi.org/10.1016/j.marchem.2023.104348
