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Schwertmannite, a poorly crystalline iron oxyhydroxysulfate, is an iron-bearing mineral that plays a pivotal role in various environmental processes, particularly in the treatment of acidic mine drainage [1]. Due to its ability to adsorb metal ions, anions, and its high surface area-to-volume-ratio, Schwertmannite has drawn significant attention as a potential medium for mitigating environmental contamination [2]. However, its poorly crystalline structure presents challenges in characterizing its composition, making it difficult to detect and quantify trace impurities. One such impurity is goethite, another iron mineral that can form under similar conditions due to higher thermodynamical stability [3]. Differentiating between schwertmannite and goethite in environmental samples or synthetic preparations is critical, as the presence of goethite influences the chemical reactivity and stability of schwertmannite, altering its efficiency in ecological applications.
The aim of this study is to explore the use of magnetic measurements obtained from a Vibrating Sample Magnetometer (VSM), to detect goethite impurities in schwertmannite. Goethite typically has a higher magnetization and a different temperature dependence of its magnetic properties compared to schwertmannite. The presence of trace amounts of goethite can, therefore, be detected through careful analysis of the magnetic response of the sample. In this study, Schwertmannite samples with different concentrations of goethite were synthesized with an environmentally friendly and fast method, and the effect of the impurities on the properties of the resulting materials was investigated with VSM, XRD, FT-IR, $^{57}$Fe Mössbauer spectroscopy, and TEM analyses. The VSM allows for the precise measurement of the magnetic moment of the sample as a function of an applied magnetic field, providing detailed insights into the magnetic behavior of the mineral phases present. This sensible and accurate detection was the only technique capable of detecting the presence of goethite even in the purest sample.
These findings demonstrate that VSM-based magnetic characterization can serve as an effective tool for identifying goethite impurities in schwertmannite and contribute to the knowledge about the characterization of poorly crystalline iron materials, and highlight the potential of magnetic techniques for improving our understanding of these materials in natural and engineered systems.
References
[1] J. M. Blgham et al., “A poorly crystallized oxyhydroxysulfate of iron formed by bacterial oxidation of Fe(II) in acid mine waters,” Geochimica et Cosmochimica Acta, vol. 54, no. 10. Elsevier BV, pp. 2743–2758, Oct. 1990. https://doi.org/10.1016/0016-7037(90)90009-a
[2] B. Marouane et al., “The potential of granulated schwertmannite adsorbents to remove oxyanions (SeO32−, SeO42−, MoO42−, PO43−, Sb(OH)6−) from contaminated water,” Journal of Geochemical Exploration, vol. 223. Elsevier BV, p. 106708, Apr. 2021. https://doi.org/10.1016/j.gexplo.2020.106708
[3] P. Acero et al., “The behavior of trace elements during schwertmannite precipitation and subsequent transformation into goethite and jarosite,” Geochimica et Cosmochimica Acta, vol. 70, no. 16. Elsevier BV, pp. 4130–4139, Aug. 2006. https://doi.org/10.1016/j.gca.2006.06.1367
