Formation of todorokite from “c-disordered” H+-birnessites: the roles of average manganese oxidation state and interlayer cations

Abstract

BackgroundTodorokite, a 3 × 3 tectomanganate, is one of three main manganese oxide minerals in marine nodules and can be used as an active MnO6 octahedral molecular sieve. The formation of todorokite is closely associated with the poorly crystalline phyllomanganates in nature. However, the effect of the preparative parameters on the transformation of “c-disordered” H+-birnessites, analogue to natural phyllomanganates, into todorokite has not yet been explored.ResultsSynthesis of “c-disordered” H+-birnessites with different average manganese oxidation states (AOS) was performed by controlling the MnO4−/Mn2+ ratio in low-concentrated NaOH or KOH media. Further transformation to todorokite, using “c-disordered” H+-birnessites pre-exchanged with Na+ or K+ or not before exchange with Mg2+, was conducted under reflux conditions to investigate the effects of Mn AOS and interlayer cations. The results show that all of these “c-disordered” H+-birnessites exhibit hexagonal layer symmetry and can be transformed into todorokite to different extents. “c-disordered” H+-birnessite without pre-exchange treatment contains lower levels of Na/K and is preferably transformed into ramsdellite with a smaller 1 × 2 tunnel structure rather than todorokite. Na+ pre-exchange, i.e. to form Na-H-birnessite, greatly enhances transformation into todorokite, whereas K+ pre-exchange, i.e. to form K-H-birnessite, inhibits the transformation. This is because the interlayer K+ of birnessite cannot be completely exchanged with Mg2+, which restrains the formation of tunnel “walls” with 1 nm in length. When the Mn AOS values of Na-H-birnessite increase from 3.58 to 3.74, the rate and extent of the transformation sharply decrease, indicating that a key process is Mn(III) species migration from layer into interlayer to form the tunnel structure during todorokite formation.ConclusionsStructural Mn(III), together with the content and type of interlayer metal ions, plays a crucial role in the transformation of “c-disordered” H+-birnessites with hexagonal symmetry into todorokite. This provides further explanation for the common occurrence of todorokite in the hydrothermal ocean environment, where is usually enriched in large metal ions such as Mg, Ca, Ni, Co and etc. These results have significant implications for exploring the origin and formation process of todorokite in various geochemical settings and promoting the practical application of todorokite in many fields.Graphical abstractXRD patterns of Mg2+-exchanged and reflux treatment products for the synthetic “c-disordered” H+-birnessites.

Transformation of Co-containing birnessite to todorokite: Effect of Co on the transformation and implications for Co mobility

“…In agreement with XRD data, FTIR results show that all reflux products contain todorokite-like structures as evidenced by the presence of this peak at ~761 cm -1 ( Fig. 5) (Julien et al, 2004;Atkins et al, 2014;Zhao et al, 2015). The intensity of this peak decreases with Co loading, however, suggesting that the proportion of todorokite-like structures in the reflux products decreases with increasing Co content.…”

“…Prior to spectra acquisition, samples were mixed with KBr in a 1:80 ratio and then pressed into transparent slices. Spectra were recorded over 4000-400 cm -1 with a resolution of 1 cm -1 and with an average of 64 scans per sample (Zhao et al, 2015). Because it can be difficult to distinguish tectomanganate from buserite, lithiophorite-or asbolane-like phases using XRD (e.g., Burns et al, 1983Burns et al, , 1985Bodeï et al, 2007;Saratovsky et al, 2009;Feng et al, 2010) The specific surface area of the reflux products was determined by the Brunauer−Emmett−Teller (BET) method using an Autosorb-1 standard physical adsorption analyzer (Quantachrome Autosorb-1, JEDL-6390/LV).…”

“…Both Co(II) oxidation and the subsequent migration of 144 Co(III) into the mineral structure appear to be key to the high capacity of layered Mn 145 oxides for Co scavenging (Yin et al, 2014) and their resulting Co-enrichment 146 (Loganathan and Burau, 1973;Davies and Morgan, 1989).As Co(III)O 6 octahedra are 147 not Jahn-Teller distorted, Co-enrichment in birnessite layers may modify layer 148 symmetry from orthogonal to hexagonal (Yin et al, 2015). This enrichment may also 149 impact the ability of Co-containing phyllomanganates to transform into 150 tectomanganates as the abundance of distorted Mn(III) octahedra within the 151 octahedral layers appears to be a key factor for the formation of tunnel structures 152 (Burns et al, 1985;Bodeï et al, 2007;Cui et al, 2008;Atkins et al, 2014Atkins et al, , 2016Grangeon et al, 2014Grangeon et al, , 2015Zhao et al, 2015).…”

Electrocatalytic Water Oxidation Promoted by 3 D Nanoarchitectured Turbostratic δ‐MnOx on Carbon Nanotubes

“…Differently from the typical XRD diffractions of the typical δ‐MnO 2 , the absence of 12.0° (001) and 24.2° (002) reflections indicates a disordered stacking of the MnO 6 octahedral sheets . Combining the results of the XRD and Raman analyses, the MnO x coated on the CNT/NF is identified as a turbostratic δ‐MnO x phase, which generally contains abundant vacancies and lattice strains in the structure that are the potentially active sites for water oxidation …”

“…Herein, the electrodeposition of MnO x on CNT/NF was performed in a 5.0 m m Mn(CH 3 COO) 2 solution with 50 m m sodium acetate electrolyte at 1.4 V versus normal hydrogen electrode (NHE). Electrodeposition under the weakly basic and high‐potential conditions can produce turbostratic δ‐phase MnO x on the CNT/NF . A subsequent annealing of the electrodeposited MnO x was done at 300 °C to remove mainly the adsorbed and interlayer water.…”