Abstract
The synthesis methods used to produce todorokite (10 Å manganate, OMS-1) and birnessite (7 Å manganate), which are abundant in marine manganese nodules, have been studied to confirm whether pure mineral phases can be obtained and to compare their physicochemical characteristics. The physicochemical characteristics of todorokite and its precursor Na–birnessite can vary widely based on the precursors used during their synthesis. Birnessite can be synthesized via three mechanisms, i.e., the oxidation of Mn2+, a redox reaction between Mn2+ and MnO4−, or the reduction of MnO4−. Herein, four precursors are used to synthesize birnessite using different methods before being transformed into todorokite. The characteristics of the birnessite and todorokite synthesized using different methods are investigated via X-ray diffraction (XRD), chemical analysis, Brunauer–Emmett–Teller (BET), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), and field emission scanning electron microscopy (SEM). Based on the method used, birnessite and todorokite exhibit distinct physicochemical features, including crystallinity, crystal structure, specific surface area, oxidation state of manganese, thermal stability, and morphology. Thus, the characteristics of birnessite and todorokite are closely correlated, indicating the importance of designing suitable methods to synthesize them for specific applications.
Highlights
Manganese oxides can be observed in different natural forms, including manganese nodules, layers in sediments [1,2,3], hydrothermal alteration products [4], or rock varnish forms [5,6]
Our study showed that the physicochemical properties of birnessite and todorokite were greatly affected by the synthesis methods
X-ray diffraction (XRD) results indicated that birnessite exhibited different crystal characteristics, which can be related to its chemical composition, depending on the synthesis method used
Summary
Manganese oxides can be observed in different natural forms, including manganese nodules, layers in sediments [1,2,3], hydrothermal alteration products [4], or rock varnish forms [5,6]. Buserite is a layered manganese oxide with hydrated Mg2+ between the layers; this is obtained by an ion exchange reaction that increases the d-spacing to 10 Å. The various methods used to synthesize single-phase high-purity manganese oxide have been studied to understand the natural formation process, crystal structure, and chemical composition of todorokite and its suitability for various applications [49,50,51,64,65,66,67,68,69,70,71,72,73]. Feng et al [68,75] synthesized todorokite under refluxing conditions at atmospheric pressure, resulting in a material with characteristics considerably similar to those of the natural samples formed by hydrothermal processes. The two sets of results are correlated to obtain design principles for various mineral applications
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