Solid-state transformation of nanocrystalline phyllomanganate into tectomanganate: influence of initial layer and interlayer structure

Effect of Mn2+ concentration on the growth of δ-MnO2 crystals under acidic conditions

δ-MnO2 is an important component of environmental minerals and is among the strongest sorbents and oxidants. The crystalline morphology of δ-MnO2 is one of the key factors affecting its reactivity. In this work, δ-MnO2 was initially synthesized and placed in an acidic environment to react with Mn2+ and undergo a crystalline transformation. During the transformation of crystalline δ-MnO2, kinetic sampling was conducted, followed by analyses of the structures and morphologies of the samples. The results showed that at pH 2.5 and 4, δ-MnO2 nanoflakes spontaneously self-assembled into nanoribbons via edge-to-edge assembly in the initial stage. Subsequently, these nanoribbons attached to each other to form primary nanorods through a face-to-face assembly along the c-axis. These primary nanorods then assembled along the (001) planes and lateral surfaces, achieving further growth and thickening. Since a lower pH is more favorable for the formation of vacancies in δ-MnO2, δ-MnO2 can rapidly adsorb Mn2+ directly onto the vacancies to form tunnel walls. At the same time, the rapid formation of the tunnel walls leads to a quick establishment of hydrogen bonding between adjacent nanoribbons, enabling the assembly of these nanoribbons into primary nanorods. Therefore, in a solution with the same concentration of Mn2+, the structure transformation and morphology evolution of δ-MnO2 to α-MnO2 occur faster at pH 2.5 than at pH 4. These findings provide insights into the mechanism for crystal growth from layer-based to tunnel-based nanorods and methods for efficient and controlled syntheses of nanomaterials.

Structure-Property Interplay Within Microporous Manganese Dioxide Tunnels For Sustainable Energy Storage

Tunnel-structured manganese dioxides (MnO2 ), also known as octahedral molecule sieves (OMS), are widely studied in geochemistry, deionization, energy storage and (electro)catalysis. These functionalities originate from their characteristic sub-nanoscale tunnel framework, which, with a high degree of structural polymorphism and rich surface chemistry, can reversibly absorb and transport various ions. An intensive understanding of their structure-property relationship is prerequisite for functionality optimization, which has been recently approached by implementation of advanced (in situ) characterizations providing significant atomistic sciences. This review will thus timely cover recent advancements related to OMS and their energy storage applications, with a focus on the atomistic insights pioneered by researchers including our group: the origins of structural polymorphism and heterogeneity, the evolution of faceted OMS crystals and its effect on electrocatalysis, the ion transport/storage properties and their implication for processing OMS. These studies represent a clear rational behind recent endeavors investigating the historically applied OMS materials, the summary of which is expected to deepen the scientific understandings and guide material engineering for functionality control.

Aqueous Co removal by mycogenic Mn oxides from simulated mining wastewaters

Naturally occurring manganese (Mn) oxide minerals often form by microbial Mn(II) oxidation, resulting in nanocrystalline Mn(III/IV) oxide phases with high reactivity that can influence the uptake and release of many metals (e.g., Ni, Cu, Co, and Zn). During formation, the structure and composition of biogenic Mn oxides can be altered in the presence of other metals, which in turn affects the minerals' ability to bind these metals. These processes are further influenced by the chemistry of the aqueous environment and the type and physiology of microorganisms involved. Conditions extending to environments that typify mining and industrial wastewaters (e.g., increased salt content, low nutrient, and high metal concentrations) have not been well investigated thus limiting the understanding of metal interactions with biogenic Mn oxides. By integrating geochemistry, microscopic, and spectroscopic techniques, we examined the capacity of Mn oxides produced by the Mn(II)-oxidizing Ascomycete fungus Periconia sp. SMF1 isolated from the Minnesota Soudan Mine to remove the metal co-contaminant Co(II) from synthetic waters that are representative of mining wastewaters currently undergoing remediation efforts. We compared two different applied remediation strategies under the same conditions: coprecipitation of Co with mycogenic Mn oxides versus adsorption of Co with pre-formed fungal Mn oxides. Co(II) was effectively removed from solution by fungal Mn oxides through two different mechanisms: incorporation into, and adsorption onto, Mn oxides. These mechanisms were similar for both remediation strategies, indicating the general effectiveness of Co(II) removal by these oxides. The mycogenic Mn oxides were primarily a nanoparticulate, poorly-crystalline birnessite-like phases with slight differences depending on the chemical conditions during formation. The relatively fast and complete removal of aqueous Co(II) during biomineralization as well as the subsequent structural incorporation of Co into the Mn oxide structure illustrated a sustainable cycle capable of continuously remediating Co(II) from metal-polluted environments.

A review on adsorption characteristics and influencing mechanism of heavy metals in farmland soil

The accumulation of heavy metals in soil and crops is considered to be a severe environmental problem due to its various harmful effects on animals and plants. Soil adsorption is an essential characteristic of mud, which is the fundamental reason for soil to have a specific self-purification capacity and environmental capacity for heavy metals. The adsorption of heavy metals by soil reduces the uptake of these pollutants by crops, thereby limiting food contamination. Therefore, the adsorption of heavy metals in crop soils was taken as the primary research object. Based on the entire reading of the literature, the previous research results were compared and discussed from the four aspects of heterogeneity, physical and chemical properties, competitive adsorption, and external factors. The influencing mechanism of heavy metal adsorption characteristics in soil was reviewed. Finally, suggestions and prospects for future research on heavy metal adsorption were put forward.

Chemostratigraphic and Textural Indicators of Nucleation and Growth of Polymetallic Nodules from the Clarion-Clipperton Fracture Zone (IOM Claim Area)

The detailed mineralogical and microgeochemical characteristics of polymetallic nodules collected from the Interoceanmetal Joint Organization (IOM, Szczecin, Poland) claim area, Eastern Clarion-Clipperton Fracture Zone (CCFZ, Eastern Pacific) were described in this study. The obtained data were applied for the delimitation of nodule growth generations and estimation of the growth ratios (back-stripping using the Co-chronometer method). The applied methods included bulk X-ray powder diffraction (XRD) and electron probe microanalysis (EPMA), providing information about Mn-Fe minerals and clays composing nodules, as well as the geochemical zonation of the growth generations. The analyzed nodules were mostly diagenetic (Mn/Fe > 5), with less influence on the hydrogenous processes, dominated by the presence of 10-Å phyllomanganates represented by todorokite/buserite, additionally mixed with birnessite and vernadite. The specific lithotype (intranodulith), being an integral part of polymetallic nodules, developed as a result of the secondary diagenetic processes of lithification and the cementation of Fe-rich clays (potentially nontronite and Fe-rich smectite), barite, zeolites (Na-phillipsite), bioapatite, biogenic remnants, and detrital material, occurs in holes, microcaverns, and open fractures in between ore colloforms. The contents of ∑(Ni, Cu, and Co) varied from 1.54 to 3.06 wt %. Several remnants of siliceous microorganisms (radiolarians and diatoms) were found to form pseudomorphs. The applied Co-chronometer method indicated that the nodules’ age is mainly Middle Pliocene to Middle Pleistocene, and the growth rates are typical of diagenetic and mixed hydrogenetic–diagenetic (HD) processes. Additionally, few nodules showed suboxic conditions of nucleation. Growth processes in the eastern part of the CCFZ deposit might have been induced with the Plio-Pleistocene changes in the paleooceanographic conditions related to the deglaciation of the Northern Hemisphere.

Thallium in aquatic environments and the factors controlling Tl behavior

Although thallium (Tl) usually exists in a very low level in the natural environment, it is highly toxic. With the development of mining and metallurgical industry and the wide application of Tl in the field of high technologies, Tl poses an increasing threat to the ecological environment and human health. This paper summarizes the research results of the toxicity of Tl as well as the distribution, occurrence forms, migration, and transformation mechanism of Tl in rivers, lakes, mining areas, estuaries, coastal waters, and oceans. It also discusses the influence mechanisms of pH, redox potential, suspended particulate matters, photochemical reaction, natural minerals, cation/anion, organic matters, and microorganisms on the environmental behavior of Tl. This paper points out the shortcomings of Tl research methods in water environment, and looks forward to the future development directions: First, the technology for separating Tl(III) and Tl(I) is still immature, especially it is difficult to effectively separate Tl(III) and Tl(I) in seawater. Second, the development of many advanced in situ detection technologies will bring great convenience to the studies of the dynamic mechanisms of Tl migration and transformation in the environments. Third, adsorption is the most effective mechanism to remove Tl from water, in which modified metal oxides or macrocyclic organic compounds have high application potential.

Transformation of Hexagonal Birnessite upon Reaction with Thallium(I): Effects of Birnessite Crystallinity, pH, and Thallium Concentration

We examined the uptake of Tl(I) by two hexagonal birnessites and related phase transformations in laboratory experiments over 12 sequential additions of 0.01 M Tl(I)/Mn at pH 4.0, 6.0, and 8.0. The Tl-reacted Mn oxides were characterized for their structure, Tl binding, and morphology using X-ray diffraction, X-ray photoelectron and X-ray absorption spectroscopies, and transmission electron microscopy. Very limited Tl oxidation was observed in contrast to previous works, where equal Tl(I)/Mn was added in a single step. Instead, both birnessites transformed into a 2 × 2 tunneled phase with dehydrated Tl(I) in its tunnels at pH 4, but only partially at pH 6, and at pH 8.0 they remained layered. The first four to nine sequential Tl(I)/Mn additions resulted in lower residual dissolved Tl⁺ concentrations than when the same amounts of Tl(I)/Mn were added in single steps. This study thus shows that the repeated reaction of hexagonal birnessites with smaller Tl(I)/Mn at ambient temperature triggers a complete phase conversion with Tl(I) as the sole reacting cation. The novel pathway found may be more relevant for contaminated environments and may help explain the formation of minerals like thalliomelane [Tl⁺(Mn_7.5⁴⁺Cu_0.5²⁺)O₁₆]; it also points to the possibility that other reducing species trigger similar Mn oxide transformation reactions.

Inhibition of Oxyanions on Redox-driven Transformation of Layered Manganese Oxides

Layered manganese (Mn) oxides, such as birnessite, can reductively transform into other phases and thereby affect the environmental behavior of Mn oxides. Solution chemistry strongly influences the transformation, but the effects of oxyanions remain unknown. We determined the products and rates of Mn(II)-driven reductive transformation of δ-MnO₂, a nanoparticulate hexagonal birnessite, in the presence of phosphate or silicate at pH 6-8 and a wide range of Mn(II)/MnO₂ molar ratios. Without the oxyanions, δ-MnO₂ transforms into triclinic birnessite (T-bir) and 4 × 4 tunneled Mn oxide (TMO) at low Mn(II)/MnO₂ ratios (0.09 and 0.13) and into δ-MnOOH and Mn₃O₄ with minor poorly crystallized α- and γ-MnOOH at high Mn(II)/MnO₂ ratios (0.5 and 1). The presence of phosphate or silicate substantially decreases the rate and extent of the above transformation, probably due to adsorption of the oxyanions on layer edges or the formation of Mn(II,III)-oxyanion ternary complexes on vacancies of δ-MnO₂, adversely interfering with electron transfer, Mn(III) distribution, and structural rearrangements. The oxyanions also reduce the crystallinity and particle sizes of the transformation products, ascribed to adsorption of the oxyanions on the products, preventing their further particle growth. This study enriches our understanding of the solution chemistry control on redox-driven transformation of Mn oxides.

Hierarchical porosity via layer-tunnel conversion of macroporous δ-MnO2 nanosheet assemblies

This work reports the layer-tunnel conversion of porous dehydrated synthetic alkali-free δ-MnO₂ analogs prepared by exfoliation, flocculation, and heat treatment of nanosheets derived from highly crystalline potassium birnessite. High surface area porous solids result, with specific surface areas of 90–130 m² g⁻¹ and isotherms characteristic of both micro and macropores. The microstructures of the re-assembled floccules are reminiscent of crumpled paper where single and re-stacked nanosheets form the walls of interconnected macropores. The atomic and local structures of the floccules heat treated from 60–400 °C are tracked by Raman spectroscopy and synchrotron X-ray total scattering measurements. During heating, the nanosheets comprising the pore walls condense to form tunnel-structured fragments beginning at temperatures below 100 °C, while the microstructure with high surface area remains intact. The flocc microstructure remains largely unchanged in samples heated up to 400 °C while an increasing fraction of the sample is transformed, at least locally, to possess 1D tunnels characteristic of α-MnO₂. Cyclic voltammetry in Na₂SO₄ aqueous electrolyte reflects the nanoscale structural evolution, where intercalative pseudocapacitance diminishes with the degree of transformation. Collectively, these results demonstrate that it is feasible to tailor the materials for applications incorporating nanoporous solids and nanofluidics, and specifically imply strategies to maintain a kinetically accessible interlayer contribute to Na intercalative pseudocapacitance.

This work reports the layer-tunnel conversion of porous dehydrated synthetic alkali-free δ-MnO₂ analogs prepared by exfoliation, flocculation, and heat treatment of nanosheets derived from highly crystalline potassium birnessite.

The structure and crystal chemistry of vernadite in ferromanganese crusts

The structure and crystal chemistry of vernadite in ferromanganese crusts from the Magellan Seamount in the north-west Pacific Ocean have been investigated using synchrotron X-ray diffraction (XRD), X-ray pair distribution function (PDF) and high-resolution transmission electron microscopy (TEM). XRD patterns of vernadite mainly show two strong diffraction peaks at 2.42-2.43 Å and 1.41 Å without or with a broad (001) diffraction peak, indicating thin layer nanophases along the c-direction. TEM images show flat and curved sheet-like nanocrystals with (001) layer thickness of ∼7.2 Å and ∼9.6 Å, and their interstratified structure. PDF patterns of the vernadite are similar to those from synthetic δ-MnO₂ and defective birnessite, suggesting a phyllomanganate framework. Combined XRD/PDF patterns suggest that vernadite in the outer part is associated with a higher density interlayer species at triple-edge sharing sites. The proportion of the 10 Å phase increases from the outer (young) part to the inner (old) part of the Mn crusts due to aging and sorption of Mn, Co and Ni from ambient seawater. This study suggests that this combined method of synchrotron radiation XRD/PDF and high-resolution TEM is a powerful tool to determine atomic structures of poorly crystallized nano-minerals. The mixture model of vernadite structure will help to understand the partitioning and distribution of trace elements in the ferromanganese crusts.

Oxidation of reduced daughter products from 2,4-dinitroanisole (DNAN) by Mn(IV) and Fe(III) oxides

Abiotic transformation of anthropogenic compounds by redox-active metal oxides affects contaminant fate in soil. The capacity of birnessite and ferrihydrite to oxidize the insensitive munitions compound, 2,4-dinitroanisol (DNAN), and its amine-containing daughter products, 2-methoxy-5-nitro aniline (MENA) and 2,4-diaminoanisole (DAAN), was studied in stirred reactors at controlled pH (7.0). Aqueous suspensions were reacted at metal oxide solid to solution mass ratios (SSR) of 0.15, 1.5 and 15 g kg^-1 and solutions were analyzed after 0-3 h by high performance liquid chromatography coupled with photodiode array or mass spectrometry detection. Results indicate that DNAN was resistant to oxidation by birnessite and ferrihydrite. Ferrihydrite did not oxidize MENA, but MENA was susceptible to rapid oxidation by birnessite, with nitrogen largely mineralized to nitrite. This is the first report on mineralization of nonphenolic aromatics and the release of mineralized N from aromatic amines following reaction with birnessite. DAAN was oxidized by both solids, but ca. ten times higher rate was observed with birnessite as compared to ferrihydrite at an SSR of 1.5 g kg^-1. At 15 g kg^-1 SSR, DAAN was removed from solution within 5 min of reaction with birnessite. CO_2(g) evolution experiments indicate mineralization of 15 and 12% of the carbon associated with MENA and DAAN, respectively, under oxic conditions with birnessite at SSR of 15 g kg^-1. The results taken as a whole indicate that initial reductive (bio)transformation products of DNAN are readily oxidized by birnessite. The oxidizability of the reduced DNAN products was increased with progressive (bio)reduction as reflected by impacts on the oxidation rate.

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

The development of manganese-based water oxidation electrocatalysts is desirable for the production of solar fuels, as manganese is earth-abundant, inexpensive, non-toxic, and has been employed by the Photosystem II in nature for a billion years. Herein, we directly constructed a 3 D nanoarchitectured turbostratic δ-MnO_x on carbon nanotube-modified nickel foam (MnO_x /CNT/NF) by electrodeposition and a subsequent annealing process. The MnO_x /CNT/NF electrode gives a benchmark catalytic current density (10 mA cm^-2 ) at an overpotential (η) of 270 mV under alkaline conditions. A steady current density of 19 mA cm^-2 is obtained during electrolysis at 1.53 V for 1.0 h. To the best of our knowledge, this work represents the most efficient manganese-oxide-based water oxidation electrode and demonstrates that manganese oxides, as a structural and functional model of oxygen-evolving complex (OEC) in Photosystem II, can also become comparable to those of most Ni- and Co-based catalysts.

(Na,□)5[MnO2]13 nanorods: a new tunnel structure for electrode materials determined ab initio and refined through a combination of electron and synchrotron diffraction data

(Na_x□_1 - x)₅[MnO₂]₁₃ has been synthesized with x = 0.80 (4), corresponding to Na_0.31[MnO₂]. This well known material is usually cited as Na_0.4[MnO₂] and is believed to have a romanèchite-like framework. Here, its true structure is determined, ab initio, by single-crystal electron diffraction tomography (EDT) and refined both by EDT data applying dynamical scattering theory and by the Rietveld method based on synchrotron powder diffraction data (χ² = 0.690, R_wp = 0.051, R_p = 0.037, R_F2 = 0.035). The unit cell is monoclinic C2/m, a = 22.5199 (6), b = 2.83987 (6), c = 14.8815 (4) Å, β = 105.0925 (16)°, V = 918.90 (4) ų, Z = 2. A hitherto unknown [MnO₂] framework is found, which is mainly based on edge- and corner-sharing octahedra and comprises three types of tunnels: per unit cell, two are defined by S-shaped 10-rings, four by egg-shaped 8-rings, and two by slightly oval 6-rings of Mn polyhedra. Na occupies all tunnels. The so-determined structure excellently explains previous reports on the electrochemistry of (Na,□)₅[MnO₂]₁₃. The trivalent Mn³⁺ ions concentrate at two of the seven Mn sites where larger Mn-O distances and Jahn-Teller distortion are observed. One of the Mn³⁺ sites is five-coordinated in a square pyramid which, on oxidation to Mn⁴⁺, may easily undergo topotactic transformation to an octahedron suggesting a possible pathway for the transition among different tunnel structures.

Sorption selectivity of birnessite particle edges: a d-PDF analysis of Cd(ii) and Pb(ii) sorption by δ-MnO2 and ferrihydrite

Birnessite minerals (layer-type MnO2), which bear both internal (cation vacancies) and external (particle edges) metal sorption sites, are important sinks of contaminants in soils and sediments. Although the particle edges of birnessite minerals often dominate the total reactive surface area, especially in the case of nanoscale crystallites, the metal sorption reactivity of birnessite particle edges remains elusive. In this study, we investigated the sorption selectivity of birnessite particle edges by combining Cd(ii) and Pb(ii) adsorption isotherms at pH 5.5 with surface structural characterization by differential pair distribution function (d-PDF) analysis. We compared the sorption reactivity of δ-MnO2 to that of the nanomineral, 2-line ferrihydrite, which exhibits only external surface sites. Our results show that, whereas Cd(ii) and Pb(ii) both bind to birnessite layer vacancies, only Pb(ii) binds extensively to birnessite particle edges. For ferrihydrite, significant Pb(ii) adsorption to external sites was observed (roughly 20 mol%), whereas Cd(ii) sorption was negligible. These results are supported by bond valence calculations that show comparable degrees of saturation of oxygen atoms on birnessite and ferrihydrite particle edges. Therefore, we propose that the sorption selectivity of birnessite edges follows the same order of that reported previously for ferrihydrite: Ca(ii) < Cd(ii) < Ni(ii) < Zn(ii) < Cu(ii) < Pb(ii).

Structure of nanocrystalline calcium silicate hydrates: insights from X-ray diffraction, synchrotron X-ray absorption and nuclear magnetic resonance

The structure of nanocrystalline calcium silicate hydrates (C-S-H) having Ca/Si ratios ranging between 0.57 ± 0.05 and 1.47 ± 0.04 was studied using an electron probe micro-analyser, powder X-ray diffraction, ²⁹Si magic angle spinning NMR, and Fourier-transform infrared and synchrotron X-ray absorption spectroscopies. All samples can be described as nanocrystalline and defective tobermorite. At low Ca/Si ratio, the Si chains are defect free and the Si Q³ and Q² environments account, respectively, for up to 40.2 ± 1.5% and 55.6 ± 3.0% of the total Si, with part of the Q³ Si being attributable to remnants of the synthesis reactant. As the Ca/Si ratio increases up to 0.87 ± 0.02, the Si Q³ environment decreases down to 0 and is preferentially replaced by the Q² environment, which reaches 87.9 ± 2.0%. At higher ratios, Q² decreases down to 32.0 ± 7.6% for Ca/Si = 1.38 ± 0.03 and is replaced by the Q¹ environment, which peaks at 68.1 ± 3.8%. The combination of X-ray diffraction and NMR allowed capturing the depolymerization of Si chains as well as a two-step variation in the layer-to-layer distance. This latter first increases from ∼11.3 Å (for samples having a Ca/Si ratio <∼0.6) up to 12.25 Å at Ca/Si = 0.87 ± 0.02, probably as a result of a weaker layer-to-layer connectivity, and then decreases down to 11 Å when the Ca/Si ratio reaches 1.38 ± 0.03. The decrease in layer-to-layer distance results from the incorporation of interlayer Ca that may form a Ca(OH)₂-like structure, nanocrystalline and intermixed with C-S-H layers, at high Ca/Si ratios.

Redox Reactions between Mn(II) and Hexagonal Birnessite Change Its Layer Symmetry

Birnessite, a phyllomanganate and the most common type of Mn oxide, affects the fate and transport of numerous contaminants and nutrients in nature. Birnessite exhibits hexagonal (HexLayBir) or orthogonal (OrthLayBir) layer symmetry. The two types of birnessite contain contrasting content of layer vacancies and Mn(III), and accordingly have different sorption and oxidation abilities. OrthLayBir can transform to HexLayBir, but it is still vaguely understood if and how the reverse transformation occurs. Here, we show that HexLayBir (e.g., δ-MnO2 and acid birnessite) transforms to OrthLayBir after reaction with aqueous Mn(II) at low Mn(II)/Mn (in HexLayBir) molar ratios (5-24%) and pH ≥ 8. The transformation is promoted by higher pH values, as well as smaller particle size, and/or greater stacking disorder of HexLayBir. The transformation is ascribed to Mn(III) formation via the comproportionation reaction between Mn(II) adsorbed on vacant sites and the surrounding layer Mn(IV), and the subsequent migration of the Mn(III) into the vacancies with an ordered distribution in the birnessite layers. This study indicates that aqueous Mn(II) and pH are critical environmental factors controlling birnessite layer structure and reactivity in the environment.

Cryptomelane formation from nanocrystalline vernadite precursor: a high energy X-ray scattering and transmission electron microscopy perspective on reaction mechanisms

BACKGROUND

Vernadite is a nanocrystalline and turbostratic phyllomanganate which is ubiquitous in the environment. Its layers are built of (MnO6)(8-) octahedra connected through their edges and frequently contain vacancies and  (or) isomorphic substitutions. Both create a layer charge deficit that can exceed 1 valence unit per layer octahedron and thus induces a strong chemical reactivity. In addition, vernadite has a high affinity for many trace elements (e.g., Co, Ni, and Zn) and possesses a redox potential that allows for the oxidation of redox-sensitive elements (e.g., As, Cr, Tl). As a result, vernadite acts as a sink for many trace metal elements. In the environment, vernadite is often found associated with tectomanganates (e.g., todorokite and cryptomelane) of which it is thought to be the precursor. The transformation mechanism is not yet fully understood however and the fate of metals initially contained in vernadite structure during this transformation is still debated. In the present work, the transformation of synthetic vernadite (δ-MnO2) to synthetic cryptomelane under conditions analogous to those prevailing in soils (dry state, room temperature and ambient pressure, in the dark) and over a time scale of ~10 years was monitored using high-energy X-ray scattering (with both Bragg-rod and pair distribution function formalisms) and transmission electron microscopy.

RESULTS

Migration of Mn(3+) from layer to interlayer to release strains and their subsequent sorption above newly formed vacancy in a triple-corner sharing configuration initiate the reaction. Reaction proceeds with preferential growth to form needle-like crystals that subsequently aggregate. Finally, the resulting lath-shaped crystals stack, with n × 120° (n = 1 or 2) rotations between crystals. Resulting cryptomelane crystal sizes are ~50-150 nm in the ab plane and ~10-50 nm along c*, that is a tenfold increase compared to fresh samples.

CONCLUSION

The presently observed transformation mechanism is analogous to that observed in other studies that used higher temperatures and (or) pressure, and resulting tectomanganate crystals have a number of morphological characteristics similar to natural ones. This pleads for the relevance of the proposed mechanism to environmental conditions.

Impacts of aqueous Mn(II) on the sorption of Zn(II) by hexagonal birnessite

We used a combination of batch studies and spectroscopic analyses to assess the impacts of aqueous Mn(II) on the solubility and speciation of Zn(II) in anoxic suspensions of hexagonal birnessite at pH 6.5 and 7.5. Introduction of aqueous Mn(II) into pre-equilibrated Zn(II)-birnessite suspensions leads to desorption of Zn(II) at pH 6.5, but enhances Zn(II) sorption at pH 7.5. XAS results show that Zn(II) adsorbs as tetrahedral and octahedral triple-corner-sharing complexes at layer vacancy sites when reacted with birnessite in the absence of Mn(II). Addition of aqueous Mn(II) causes no discernible change in Zn(II) surface speciation at pH 6.5, but triggers conversion of adsorbed Zn(II) into spinel Zn(II)1-xMn(II)xMn(III)2O4 precipitates at pH 7.5. This conversion is driven by electron transfer from adsorbed Mn(II) to structural Mn(IV) generating Mn(III) surface species that coprecipitate with Zn(II) and Mn(II). Our results demonstrate substantial production of these reactive Mn(III) surface species within 30 min of contact of the birnessite substrate with aqueous Mn(II). Their importance as a control on the sorption and redox reactivity of Mn-oxides toward Zn(II) and other trace metal(loid)s in environments undergoing biogeochemical manganese redox cycling requires further study.

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

BACKGROUND

Todorokite, 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.

RESULTS

Synthesis of "c-disordered" H(+)-birnessites with different average manganese oxidation states (AOS) was performed by controlling the MnO4 (-)/Mn(2+) 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 Mg(2+), 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 Mg(2+), 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.

CONCLUSIONS

Structural 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 Mg(2+)-exchanged and reflux treatment products for the synthetic "c-disordered" H(+)-birnessites.