Transformation Of δ-MnO₂ In Substructure And Pathway Of Chemical Formation Of Todorokite

Birnessite, one of the most common Mn minerals in soils, is an important precursor for other manganese oxide minerals, and can be formed in alkaline or acid soils. Vernadite is a natural occurring Mn oxide phase and widespread in the nature environment with layer structure. Vernadite is a variant of birnessite, pseudo-hexagonal symmetry. Acid birnessite is a poorly crystalline hexagonal birnessite formed in acid media, but different from the well-crystallized triclinic birnessite, formed in alkaline media. As a result, the two birnessite varieties show various chemical compositions and different substructures. The Mn layers in hexagonal birnessite have varying amounts of vacant sites up to 16.7%. However, the ideal triclinic birnessite consists of vacancy-free Mn-bearing octahedral layers arranged alternately by a Mn(Ⅲ)O6 octahedral chain and two Mn(Ⅳ)O6 octahedral chain and its symmetry type departure from the hexagonal layer symmetry to triclinic symmetry that originated from Jahn-Teller distortion due to partial Mn3+ substitution for Mn4+. Generally, it is thought that the formation of natural birnessite and vernadite which with hexagonal symmetry is bacterially-mediated processes. Studies have shown that microbial Mn(Ⅱ) oxidation products were amorphous, hexagonal, layer type Mn oxides with nanoparticle size similar to vernadite. Furthermore, todorokite is also ubiquitous in natural settings, and can be formed via a hydrothermal chemical route at relatively high temperature and pressure. In addition, todorokite can also be found in a large number of supergene environments, such as soil and sediments at relatively low temperature and pressure. But its formation pathway and mechanism in natural systems is not fully understood. In soil, todorokite only exists in calcareous alkaline environment, less stable in acid condition. Therefore, the formation pathway of todorokite, and its relationship with vernadite and hexagonal birnessite in the environment are worthy of further investigation.In this master’s thesis, the transformation of vernadite and hexagonal birnessite to triclinic birnessite, and then into todorokite were investigated by using XRD, TEM/ED, SEM, BET, micropore and chemical composition analyses. The factors governing transformation of these Mn oxides and their relationships with the formation environments in soils were also discussed. The main results are as follows.1. The pH, concentration of Mn(Ⅱ) and coexisting cations obviously impact layer structure of birnessite at room temperature, and also determine the final products and the transformations rate. Mn(Ⅱ) was found to initiate transformation of the crystal structure of hexagonal birnessite to other crystalline forms, while no transformation of hexagonal birnessite occurred in water suspensions after addition of KOH or NaOH. Sorption of Mn2+ by hexagonal birnessite at room temperature results in formation of new Mn phases, nsutite or an intergrowth of ramsdellite. Structural change of hexagonal birnessite to triclinic birnessite is allowed with appropriate Mn (Ⅱ) addition in alkaline condition. About 1 g hexagonal birnessite can be converted into triclinic birnessite completely with 12-15 mM Mn(Ⅱ) at pH 13. The ratio and rate of hexagonal birnessite transformation decrease gradually with the drop of pH at the same Mn(Ⅱ) concentration. The conversion reaction cannot occur when the pH decreases to 7. As the concentration of Mn(Ⅱ) in the solution increased, maintaining a high pH (pH> 9) at the same time, hausmannite were observed; the conversion reaction cannot occur when the concentration of Mn(Ⅱ) is decreased. Feitknechtite is the main transformation product when the concentration of Mn (Ⅱ) in the solution exceeds 10 mM with a neutral or weak alkali pH (pH 9). When Na+ is the mainly coexisting cation in the solution, transformation of hexagonal birnessite to triclinic birnessite is favourable at room temperature, whereas K+ exist the opposite effect.2. The converted birnessites from hexagonal birnessite had the same morphologies and the similar chemical compositions with those of the reported triclinic birnessite. It is indicated that the formation of triclinic birnessite is faciliated from the precursor hexagonal birnessite with low AOS. When the reaction system was protected from interference of dissolved O2 by bubbling with N2, the transformation from hexagonal birnessite to triclinic birnessite was not apparently influenced.3. The influence of pH, concentration of Mn(Ⅱ) and coexisting cations on the transformation of vernadite to triclinic birnessite had the consistent results with the transformation from hexagonal birnessite to triclinic birnessite. However, vernadite is much easier to be converted into triclinic birnessite. About 1 g vernadite can convert into triclinic birnessite completely with 5 mM Mn(Ⅱ) at pH≥8.4. The vernadites with different type of interlayer cations possess different AOS, and the crystallinity of vernadite is lower than hexagonal birnessite, but the conversion rate is much faster than it. The transformation rate and level of K-vernadite are slightly lower than those of Na-vernadite, and the former have higher AOS and relatively better crystallinity and stability. When the reaction system was treated with N2, it has a little effect on the transformation of vernadite to triclinic birnessite. While N2 treatment can avoid Mn(Ⅱ) oxidation and promote vernadite full adsorption of Mn(Ⅱ) and thus the concentration of Mn(Ⅱ) that required to generated triclinic birnessite from vernadite is slightly decreased. TEM images reveal that converted products from vernadite has a flake crystal with the average AOS 3.62-3.73 and the chemical composions are close to those of the reported triclinic birnessite samples.5. Feitknechtite can be formed through oxidation of the adsorbed Mn(Ⅱ) by hexagonal birnessite and vernadite under a certain concentration of Mn(Ⅱ) at pH 7-9. The formation conditions for conversion of hexagonal birnessite into feitknechtite were:the hexagonal birnessite weight of 1.0 g, the Mn(Ⅱ) concentration of 12 mM and the pH of 7-8. The product was a well-crystallized crystal with slat and rod shaped morphologies. Under the same condition, vernadite can convert into better purity feitknechtite with the average AOS 3.34-3.44, and the range of BET surface area was 113.5-205.8 m2/g, the crystal morphology was slat-shaped and fibrous.6. The triclinic birnessite, obtained from the transformation of hexagonal birnessite and vernadite, can convert into todorokite completely by heating refluxing process after ion exchange for 24 h in Mg system. The product has the similar morphologies and structural characteristics with the natural todorokites. It consisted of long fibers in morphology with the thickness of 10-25 nm and the length of 0.5-6μm. The BET surface area was found to be 166.5-274.6m2/g, and a major micropore size distribution peak centered at 0.49 nm for the product by the Horvath-Kawazoe (HK) method. The average chemical compositions of the todorokite from hexagonal birnessite and vernadite as the precursors are Mg0.192MnO1.962·1.30H2O and Mg0.195MnO1.910·1.92H2O, respectively. Their AOSs are 3.54 and 3.43, respectively.7. The type of interlayer cations in the precursors, hexagonal birnessite and vernadite, slightly influenced the morphology of the formed todorokite. Compared with Na+, K+ was able to maintain the original morphology of the parent. The final produced todorokite from the different precursors have the different characteristics of the diffraction peaks. The strongest peak for the todorokites from hexagonal birnessite transformation appears at 0.47 nm, but the strongest peak for the todorokites from vernadite transformation appears at 0.96 nm. The diffraction peaks of the product were observed broadening, shifting and collapsing, and the strongest diffraction peaks changed after the samples heated at 120℃. However, the morphologies of the heated samples remain unchanged.