Short-range and long-range order of phyllomanganate nanoparticles determined using high-energy X-ray scattering

Potential for high-grade recovery of rare earth elements and cobalt from acid mine drainage via adsorption to precipitated manganese (IV) oxides

High demand for rare earth elements (REEs) has increased interest in their recovery from unconventional sources, such as acid mine drainage (AMD). AMD contains elevated concentrations of Mn, Fe, and Al, which precipitate as (oxy)hydroxide minerals as pH is raised. These precipitates can remove cations including REEs and Co from solution via sorption and/or coprecipitation. In this study we developed a method to recover these critical minerals by sorption to MnO2, precipitated by oxidation of in situ Mn (II) with added KMnO4 at acidic pH. MnO2 solids were prepared with varying concentrations of KMnO4, SO42-, and Cl-, to elucidate the effects of excess KMnO4, SO42- concentration, and ionic strength on adsorption. When using a stoichiometric ratio of Mn (II) and KMnO4, 100% removal of REEs and Co occurred at approximately pH 3.5, nearly 2 pH units lower than was observed by sorption to Fe and Al hydroxysulfates. When using excess KMnO4 nearly 100% removal of REEs and Co was accomplished at approximately pH 2, although SO42- was found to inhibit REE sorption. From these results, we developed a two-stage process for recovery of REEs from AMD; a preliminary pH adjustment to remove Fe and Al hydroxy-sulfates, followed by adding KMnO4, precipitating MnO2, enabling recovery of REEs and Co. We tested this process in a representative synthetic AMD, achieving a grade of 6.16 mg REEs per g of solid, which is 65 % of the maximum possible grade based on solution composition. Fractionation of REEs was observed, with light REEs (LREEs) preferentially sorbed to MnO2 relative to both medium REEs (MREEs) and heavy REEs (HREEs). In contrast, preferential sorption of HREEs was observed for sorption to Fe and Al oxyhydroxides at all pH ranges. These results suggest the mechanisms of REE sorption differ among the solids and warrant further study.

Sequestration and oxidation of heavy metals mediated by Mn(II) oxidizing microorganisms in the aquatic environment

Microorganisms can oxidize Mn(II) to biogenic Mn oxides (BioMnOx), through enzyme-mediated processes and non-enzyme-mediated processes, which are generally considered as the source and sink of heavy metals due to highly reactive to sequestrate and oxidize heavy metals. Hence, the summary of interactions between Mn(II) oxidizing microorganisms (MnOM) and heavy metals is benefit for further work on microbial-mediated self-purification of water bodies. This review comprehensively summarizes the interactions between MnOM and heavy metals. The processes of BioMnOx production by MnOM has been firstly discussed. Moreover, the interactions between BioMnOx and various heavy metals are critically discussed. On the one hand, modes for heavy metals adsorbed on BioMnOx are summarized, such as electrostatic attraction, oxidative precipitation, ion exchange, surface complexation, and autocatalytic oxidation. On the other hand, adsorption and oxidation of representative heavy metals based on BioMnOx/Mn(II) are also discussed. Thirdly, the interactions between MnOM and heavy metals are also focused on. Finally, several perspectives which will contribute to future research are proposed. This review provides insight into the sequestration and oxidation of heavy metals mediated by Mn(II) oxidizing microorganisms. It might be helpful to understand the geochemical fate of heavy metals in the aquatic environment and the process of microbial-mediated water self-purification.

Organic buffers act as reductants of abiotic and biogenic manganese oxides

Proton activity is the master variable in many biogeochemical reactions. To control pH, laboratory studies involving redox-sensitive minerals like manganese (Mn) oxides frequently use organic buffers (typically Good's buffers); however, two Good's buffers, HEPES and MES, have been shown to reduce Mn(IV) to Mn(III). Because Mn(III) strongly controls mineral reactivity, avoiding experimental artefacts that increase Mn(III) content is critical to avoid confounding results. Here, we quantified the extent of Mn reduction upon reaction between Mn oxides and several Good's buffers (MES, pK_a = 6.10; PIPES, pK_a = 6.76; MOPS, pK_a = 7.28; HEPES, pK_a = 7.48) and TRIS (pK_a = 8.1) buffer. For δ-MnO₂, Mn reduction was rapid, with up to 35% solid-phase Mn(III) generated within 1 h of reaction with Good's buffers; aqueous Mn was minimal in all Good's buffers experiments except those where pH was one unit below the buffer pK_a and the reaction proceeded for 24 h. Additionally, the extent of Mn reduction after 24 h increased in the order MES < MOPS < PIPES < HEPES << TRIS. Of the variables tested, the initial Mn(II,III) content had the greatest effect on susceptibility to reduction, such that Mn reduction scaled inversely with the initial average oxidation number (AMON) of the oxide. For biogenic Mn oxides, which consist of a mixture of Mn oxides, bacterial cells and extracelluar polymeric substances, the extent of Mn reduction was lower than predicted from experiments using abiotic analogs and may result from biotic re-oxidation of reduced Mn or a difference in the reducibility of abiotic versus biogenic oxides. The results from this study show that organic buffers, including morpholinic and piperazinic Good's buffers and TRIS, should be avoided for pH control in Mn oxide systems due to their ability to transfer electrons to Mn, which modifies the composition and reactivity of these redox-active minerals.

New insight of Mn(III) in δ-MnO2 for peroxymonosulfate activation reaction: Via direct electron transfer or via free radical reactions

Due to the increasing of industrial plastic waste and its refractory characteristics, it is extremely urgent to develop new degradation technology and environmentally friendly catalyst for industrial plastic waste. Manganese oxides are one of the most promising candidates for the catalytic degradation of plastic wastes. However, an improved understanding of the structural properties affecting their catalytic activity is required for high-efficient wastewater treatment. We herein report the surface reactivity effects of δ-MnO₂ structural defects with regards to Bisphenol A (BPA) degradation/probe in the presence of peroxymonosulfate (PMS). Four δ-MnO_x samples with different Mn(III) contents (different Mn(III)-deficient sample) were prepared and their structural properties as well as surface reactivity were characterized by batch test, ESR and XAFS analysis. For the Mn(III)-deficient sample, BPA removal was principally affected by direct electron transfer, with the adsorbed BPA degraded following hydroxylation. In contrast, a small fraction of Mn(III) substitution in δ-MnO₂ could significantly encouraged the activation of PMS to produce SO₄^-☐and ☐OH, and a BPA degradation via beta scission. Moreover, the Mn(III)-rich δ-MnO₂ demonstrate a high BPA removal rate even with a low sample load, which performed well following a reuse of five times. Our results provide a new way for the improvement of δ-MnO₂ activity for the use of industrial plastic wastes treatment.

Abiotic transformation of atrazine in aqueous phase by biogenic bixbyite-type Mn2O3 produced by a soil-derived Mn(II)-oxidizing bacterium of Providencia sp

Recently, biogenic Mn oxides (BioMnO_x) are considered as the promising degradation agents for environmental organic contaminants. However, little information is available for the degradation of atrazine by BioMnO_x. In this work, BioMnO_x, generated by a soil-derived Mn(II)-oxidizing bacterium, Providencia sp. LLDRA6, was explored to degrade atrazine. To begin with, collective results from mineral characterization analyses demonstrated that this BioMnO_x was biogenic bixbyite-type Mn₂O₃. After that, purified biogenic Mn₂O₃ was found to exhibit a much higher removal efficiency for atrazine in aqueous phase, as compared to unpurified biogenic Mn₂O₃ and LLDRA6 biomass. During the atrazine removal by biogenic Mn₂O₃, six intermediate degradation products were discovered, comprising deethylatrazine (DEA), hydroxylatrazine (HA), deethylhydroxyatrazine (DEHA), ammeline, cyanuric acid, and 5-methylhexahydro-1,3,5-triazine-2-thione (MTT). Particularly, the intermediate, MTT, was considered as a new degradation product of atrazine, which was not described previously. Meanwhile, Mn(II) ions were released from biogenic Mn₂O₃, and on the surface of biogenic Mn₂O₃, the content of hydroxyl O species increased at the expense of that of lattice and water O species, but the fundamental crystalline structure of this Mn oxide remained unchanged. Additionally, no dissociative Mn(III) was found to involve in atrazine degradation. In summary, these results demonstrated that both the non-oxidative and oxidative reactions underlay the degradation of atrazine by biogenic Mn₂O₃.

Fe-Mn Oxides Based Multifunctional Adsorptive/Electrosensing Nanoplatforms: Dynamic Site Rearrangement for Metal Ion Selectivity

Achieving structural requirements for the exclusive selectivity of adsorbent to a specific metal remains challenging, as certain metal ions show similar adsorptive behaviors and preference toward a given site. We reported the morphology and oxidation state-dependent selectivity manipulating of layered oxides by controlling the dynamic evolution of different adsorptive sites. The computational investigation predicted the site-specific partitioning trends of metal ions at two sites of manganese oxide (MnO₂) layers: the lateral edge sites (LESs) and octahedral vacancy sites (OVSs). In contrast to the predominant occupation of the OVSs for other metal ions, the binding of lead (Pb) ions was energetically favored at both the sites. We assembled ultrathin MnO₂ nanosheets on the magnetic iron oxides to first enhance the accessibility of the LESs. A sequential ligand-promoted partial reduction of the atomic MnO₂ layers induced the edge-to-interlayer migration of Mn atoms to block the nonspecific OVSs and activate the LESs, enabling a superior selectivity to Pb. In addition, the iron oxides helped construct a multifunctional adsorptive/electrosensing platform for Pb regarding their facile magnetic separation and electrochemical activity. Simultaneous selective adsorption and on-site monitoring of Pb(II) were achieved on this nanoplatform, owing to its satisfactory stability and sensitivity without an obvious matrix effect.

A Fungal-Mediated Cryptic Selenium Cycle Linked to Manganese Biogeochemistry

Selenium (Se) redox chemistry is a determining factor for its environmental toxicity and mobility. Currently, millions of people are impacted by Se deficiency or toxicity, and in geologic history, several mass extinctions have been linked to extreme Se deficiency. Importantly, microbial activity and interactions with other biogeochemically active elements can drastically alter Se oxidation state and form, impacting its bioavailability. Here, we use wet geochemistry, spectroscopy, and electron microscopy to identify a cryptic, or hidden, Se cycle involving the reoxidation of biogenic volatile Se compounds in the presence of biogenic manganese [Mn(III, IV)] oxides and oxyhydroxides (hereafter referred to as "Mn oxides"). Using two common environmental Ascomycete fungi, Paraconiothyrium sporulosum and Stagonospora sp., we observed that aerobic Se(IV and VI) bioreduction to Se(0) and Se(-II) occurs simultaneously alongside the opposite redox biomineralization process of mycogenic Mn(II) oxidation to Mn oxides. Selenium bioreduction produced stable Se(0) nanoparticles and organoselenium compounds. However, mycogenic Mn oxides rapidly oxidized volatile Se products, recycling these compounds back to soluble forms. Given their abundance in natural systems, biogenic Mn oxides likely play an important role mediating Se biogeochemistry. Elucidating this cryptic Se cycle is essential for understanding and predicting Se behavior in diverse environmental systems.

Using Complementary Methods of Synchrotron Radiation Powder Diffraction and Pair Distribution Function to Refine Crystal Structures with High Quality Parameters—A Review

Determination of the atomic-scale structures of certain fine-grained minerals using single-crystal X-ray diffraction (XRD) has been challenging because they commonly occur as submicron and nanocrystals in the geological environment. Synchrotron powder diffraction and scattering techniques are useful complementary methods for studying this type of minerals. In this review, we discussed three example studies investigated by combined methods of synchrotron radiation XRD and pair distribution function (PDF) techniques: (1) low-temperature cristobalite; (2) kaolinite; and (3) vernadite. Powder XRD is useful to determine the average structure including unit-cell parameters, fractional atomic coordinates, occupancies and isotropic atomic displacement parameters. X-ray/Neutron PDF methods are sensitive to study the local structure with anisotropic atomic displacement parameters (ADP). The results and case studies suggest that the crystal structure and high-quality ADP values can be obtained using the combined methods. The method can be useful to characterize crystals and minerals that are not suitable for single-crystal XRD.

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.

Effects of Co(II) ion exchange, Ni(II)- and V(V)-doping on the transformation behaviors of Cr(III) on hexagonal turbostratic birnessite-water interfaces

Natural birnessite-like minerals are commonly enriched in various transitional metals (TMs), which greatly modify the mineral structure and properties. However few studies are yet conducted systematically on the effects of TM doping on birnessite reactivity towards Cr(III) oxidation. In the present study, the transformation behaviors of Cr(III) on Co-, Ni-, V-containing birnessites were investigated. Co and Ni doping generally decrease the mineral crystalline sizes and hydrodynamic sizes (D_H) while V-doping greatly decreases the crystalline sizes but not the D_H, owing to particle aggregation. Co and Ni firstly decrease and then increase the mineral zeta potentials (ζ) at pH4 while V decreases ζ. Electrochemical specific capacitances for Co-containing birnessites are gradually reduced, while those for Ni-doped birnessites are slightly reduced and for V-doped birnessites increased, which have a positively linear relationship with the amounts of Cr(III) oxidized by these samples. Cr(III) removal efficiencies from solution by these Co-, Ni- and V-containing birnessites are 26-51%, ∼62-72% and ∼96-100%, respectively, compared to ∼92% by pure birnessite. Cr(III) oxidation kinetics analysis demonstrates the gradual decrease of Mn(IV) and concurrent increase of Mn(III) and the adsorption of mainly Cr(III) on mineral surfaces. A negatively linear relationship exists between birnessite lateral sizes and the proportions of Mn(IV/III) consumed to oxidize Cr(III). Apparent initial Cr(III) oxidation rate (k_obs) for Co-containing birnessites are greatly reduced, while those for Ni-doped samples moderately decreased and for V-doped samples first increased and then decreased. A positively or negatively linear relationship exists between k_obs or the amount of Mn(II) released and the mineral Mn(IV) content respectively. Cr(III) oxidation probably initiates from layer edge sites of Ni-doped birnessites but the vacancies of Co- and V-containing birnessites. These results provide insights into the reaction mechanisms of Cr(III) with natural birnessite-like minerals.

Thallium Sorption onto Manganese Oxides

The sorption of thallium (Tl) onto manganese (Mn) oxides critically influences its environmental fate and geochemical cycling and is also of interest in water treatment. Combined quantitative and mechanistic understanding of Tl sorption onto Mn oxides, however, is limited. We investigated the uptake of dissolved Tl(I) by environmentally relevant phyllo- and tectomanganates and used X-ray absorption spectroscopy to determine the oxidation state and local coordination of sorbed Tl. We show that extremely strong sorption of Tl onto vacancy-containing layered δ-MnO₂ at low dissolved Tl(I) concentrations (log K_d ≥ 7.4 for ≤10^-8 M Tl(I); K_d in (L/kg)) is due to oxidative uptake of Tl and that less specific nonoxidative Tl uptake only becomes dominant at very high Tl(I) concentrations (>10^-6 M). Partial reduction of δ-MnO₂ induces phase changes that result in inhibited oxidative Tl uptake and lower Tl sorption affinity (log K_d 6.2-6.4 at 10^-8 M Tl(I)) and capacity. Triclinic birnessite, which features no vacancy sites, and todorokite, a 3 × 3 tectomanganate, bind Tl with lower sorption affinity than δ-MnO₂, mainly as hydrated Tl⁺ in interlayers (triclinic birnessite; log K_d 5.5 at 10^-8 M Tl(I)) or tunnels (todorokite). In cryptomelane, a 2 × 2 tectomanganate, dehydrated Tl⁺ replaces structural K⁺. The new quantitative and mechanistic insights from this study contribute to an improved understanding of the uptake of Tl by key Mn oxides and its relevance in natural and engineered systems.

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.

An Electrolytic Zn-MnO2 Battery for High-Voltage and Scalable Energy Storage

Zinc-based electrochemistry is attracting significant attention for practical energy storage owing to its uniqueness in terms of low cost and high safety. However, the grid-scale application is plagued by limited output voltage and inadequate energy density when compared with more conventional Li-ion batteries. Herein, we propose a latent high-voltage MnO₂ electrolysis process in a conventional Zn-ion battery, and report a new electrolytic Zn-MnO₂ system, via enabled proton and electron dynamics, that maximizes the electrolysis process. Compared with other Zn-based electrochemical devices, this new electrolytic Zn-MnO₂ battery has a record-high output voltage of 1.95 V and an imposing gravimetric capacity of about 570 mAh g^-1 , together with a record energy density of approximately 409 Wh kg^-1 when both anode and cathode active materials are taken into consideration. The cost was conservatively estimated at <US$ 10 per kWh. This result opens a new opportunity for the development of Zn-based batteries, and should be of immediate benefit for low-cost practical energy storage and grid-scale applications.

Coordination geometry of Zn2+ on hexagonal turbostratic birnessites with different Mn average oxidation states and its stability under acid dissolution

Hexagonal turbostratic birnessite, with the characteristics of high contents of vacancies, varying amounts of structural and adsorbed Mn³⁺, and small particle size, undergoes strong adsorption reactions with trace metal (TM) contaminants. While the interactions of TM, i.e., Zn²⁺, with birnessite are well understood, the effect of birnessite structural characteristics on the coordination and stability of Zn²⁺ on the mineral surfaces under proton attack is as yet unclear. In the present study, the effects of a series of synthesized hexagonal turbostratic birnessites with different Mn average oxide states (AOSs) on the coordination geometry of adsorbed Zn²⁺ and its stability under acidic conditions were investigated. With decreasing Mn AOS, birnessite exhibits smaller particle sizes and thus larger specific surface area, higher amounts of layer Mn³⁺ and thus longer distances for the first MnO and MnMn shells, but a low quantity of available vacancies and thus low adsorption capacity for Zn²⁺. Zn K-edge EXAFS spectroscopy demonstrates that birnessite with low Mn AOS has smaller adsorption capacity but more tetrahedral Zn (^IVZn) complexes on vacancies than octahedral (^VIZn) complexes, and Zn²⁺ is more unstable under acidic conditions than that adsorbed on birnessite with high Mn AOS. High Zn²⁺ loading favors the formation of ^VIZn complexes over ^IVZn complexes, and the release of Zn²⁺ is faster than at low loading. These results will deepen our understanding of the interaction mechanisms of various TMs with natural birnessites, and the stability and thus the potential toxicity of heavy metal pollutants sequestered by engineered nano-sized metal oxide materials.

Impact of Mn(II)-Manganese Oxide Reactions on Ni and Zn Speciation

Layered Mn oxide minerals (phyllomanganates) often control trace metal fate in natural systems. The strong uptake of metals such as Ni and Zn by phyllomanganates results from adsorption on or incorporation into vacancy sites. Mn(II) also binds to vacancies and subsequent comproportionation with structural Mn(IV) may alter sheet structures by forming larger and distorted Mn(III)O₆ octahedra. Such Mn(II)-phyllomanganate reactions may thus alter metal uptake by blocking key reactive sites. Here we investigate the effect of Mn(II) on Ni and Zn binding to phyllomanganates of varying initial vacancy content (δ-MnO₂, hexagonal birnessite, and triclinic birnessite) at pH 4 and 7 under anaerobic conditions. Dissolved Mn(II) decreases macroscopic Ni and Zn uptake at pH 4 but not pH 7. Extended X-ray absorption fine structure spectroscopy demonstrates that decreased uptake at pH 4 corresponds with altered Ni and Zn adsorption mechanisms. These metals transition from binding in the interlayer to sheet edges, with Zn increasing its tetrahedrally coordinated fraction. These effects on metal uptake and binding correlate with Mn(II)-induced structural changes, which are more substantial at pH 4 than 7. Through these structural effects and the pH-dependence of Mn(II)-metal competitive adsorption, system pH largely controls metal binding to phyllomanganates in the presence of dissolved Mn(II).

The critical role of point defects in improving the specific capacitance of δ-MnO2 nanosheets

3D porous nanostructures built from 2D δ-MnO₂ nanosheets are an environmentally friendly and industrially scalable class of supercapacitor electrode material. While both the electrochemistry and defects of this material have been studied, the role of defects in improving the energy storage density of these materials has not been addressed. In this work, δ-MnO₂ nanosheet assemblies with 150 m² g⁻¹ specific surface area are prepared by exfoliation of crystalline K_xMnO₂ and subsequent reassembly. Equilibration at different pH introduces intentional Mn vacancies into the nanosheets, increasing pseudocapacitance to over 300 F g⁻¹, reducing charge transfer resistance as low as 3 Ω, and providing a 50% improvement in cycling stability. X-ray absorption spectroscopy and high-energy X-ray scattering demonstrate a correlation between the defect content and the improved electrochemical performance. The results show that Mn vacancies provide ion intercalation sites which concurrently improve specific capacitance, charge transfer resistance and cycling stability.

Two-dimensional solids are of interest for energy storage due to their large accessible surface area, enabling rapid charge/discharge. Here, the authors quantify the point defects in oxide nanosheets, demonstrating that intentional introduction of charged point defects improves the charge storage behaviour.

Quantitative X-ray pair distribution function analysis of nanocrystalline calcium silicate hydrates: a contribution to the understanding of cement chemistry

Quantitative analysis of the X-ray pair distribution function collected on calcium silicate hydrates having Ca/Si ratios ranging between 0.57 and 1.47 was applied. With increasing Ca/Si ratio, Si bridging tetrahedra are omitted and Ca(OH)₂ is detected at the highest ratios.

The structural evolution of nanocrystalline calcium silicate hydrate (C–S–H) as a function of its calcium to silicon (Ca/Si) ratio has been probed using qualitative and quantitative X-ray atomic pair distribution function analysis of synchrotron X-ray scattering data. Whatever the Ca/Si ratio, the C–S–H structure is similar to that of tobermorite. When the Ca/Si ratio increases from ∼0.6 to ∼1.2, Si wollastonite-like chains progressively depolymerize through preferential omission of Si bridging tetrahedra. When the Ca/Si ratio approaches ∼1.5, nanosheets of portlandite are detected in samples aged for 1 d, while microcrystalline portlandite is detected in samples aged for 1 year. High-resolution transmission electron microscopy imaging shows that the tobermorite-like structure is maintained to Ca/Si > 3.

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.

Nature of Activated Manganese Oxide for Oxygen Evolution

Electrodeposited manganese oxide films (MnOx) are promising stable oxygen evolution catalysts. They are able to catalyze the oxygen evolution reaction in acidic solutions but with only modest activity when prepared by constant anodic potential deposition. We now show that the performance of these catalysts is improved when they are "activated" by potential cycling protocols, as measured by Tafel analysis (where lower slope is better): upon activation the Tafel slope decreases from ∼120 to ∼70 mV/decade in neutral conditions and from ∼650 to ∼90 mV/decade in acidic solutions. Electrochemical, spectroscopic, and structural methods were employed to study the activation process and support a mechanism where the original birnessite-like MnOx (δ-MnO2) undergoes a phase change, induced by comproportionation with cathodically generated Mn(OH)2, to a hausmannite-like intermediate (α-Mn3O4). Subsequent anodic conditioning from voltage cycling or water oxidation produces a disordered birnessite-like phase, which is highly active for oxygen evolution. At pH 2.5, the current density of activated MnOx (at an overpotential of 600 mV) is 2 orders of magnitude higher than that of the original MnOx and begins to approach that of Ru and Ir oxides in acid.

Time-Resolved Investigation of Cobalt Oxidation by Mn(III)-Rich δ-MnO2 Using Quick X-ray Absorption Spectroscopy

Manganese oxides are important environmental oxidants that control the fate of many organic and inorganic species including cobalt. We applied ex situ quick X-ray absorption spectroscopy (QXAS) to determine the time evolution of Co(II) and Co(III) surface loadings and their respective average surface speciation in Mn(III)-rich δ-MnO2 samples at pH 6.5 and loadings of 0.01-0.20 mol Co mol(-1) Mn. In this Mn oxide, which contained few unoccupied vacancies but abundant Mn(III) at edge and interlayer sites, Co(II) sorption and oxidation started at the particle edges. We found no evidence for Co(II) oxidation by interlayer Mn(III) or Mn(III, IV) adjacent to vacancy sites at <10 min. After 10 min, basal surface sites were implicated due to slow Co oxidation by interlayer Mn(III) and reactive sites formed upon removal of interlayer Mn(III), such that 50-60% of the sorbed Co was incorporated into the MnO2 sheets or adsorbed at vacancy sites by 12 h. Our findings indicate that the redox reactivity of surface sites depends on Mn valence and crystallographic location, with Mn(III) at the edges being the most effective oxidant at short reaction times and Mn(III,IV) in the MnO2 sheet contributing at longer reaction times.

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.

Rate and mechanism of the photoreduction of birnessite (MnO2) nanosheets

The photoreductive dissolution of Mn(IV) oxide minerals in sunlit aquatic environments couples the Mn cycle to the oxidation of organic matter and fate of trace elements associated with Mn oxides, but the intrinsic rate and mechanism of mineral dissolution in the absence of organic electron donors is unknown. We investigated the photoreduction of δ-MnO2 nanosheets at pH 6.5 with Na or Ca as the interlayer cation under 400-nm light irradiation and quantified the yield and timescales of Mn(III) production. Our study of transient intermediate states using time-resolved optical and X-ray absorption spectroscopy showed key roles for chemically distinct Mn(III) species. The reaction pathway involves (i) formation of Jahn-Teller distorted Mn(III) sites in the octahedral sheet within 0.6 ps of photoexcitation; (ii) Mn(III) migration into the interlayer within 600 ps; and (iii) increased nanosheet stacking. We propose that irreversible Mn reduction is coupled to hole-scavenging by surface water molecules or hydroxyl groups, with associated radical formation. This work demonstrates the importance of direct MnO2 photoreduction in environmental processes and provides a framework to test new hypotheses regarding the role of organic molecules and metal species in photochemical reactions with Mn oxide phases. The timescales for the production and evolution of Mn(III) species and a catalytic role for interlayer Ca(2+) identified here from spectroscopic measurements can also guide the design of efficient Mn-based catalysts for water oxidation.

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.

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

In surficial environments, the fate of many elements is influenced by their interactions with the phyllomanganate vernadite, a nano-sized and turbostratic variety of birnessite. To advance our understanding of the surface reactivity of vernadite as a function of pH, synthetic vernadite (δ-MnO2) was equilibrated at pH ranging from 3 to 10 and characterized structurally using chemical methods, thermogravimetry and modelling of powder X-ray diffraction (XRD) patterns. With decreasing pH, the number of vacant layer sites increases in the octahedral layers of δ-MnO2(from 0.14 per layer octahedron at pH 10 to 0.17 at pH 3), whereas the number of layer Mn3+is, within errors, equal to 0.12 per layer octahedron over the whole pH range. Vacant layer sites are capped by interlayer Mn3+sorbed as triple corner-sharing surface complexes (TC sites). The increasing number of interlayer Mn3+with decreasing pH (from 0.075 per layer octahedron at pH 10 to 0.175 at pH 3) results in the decrease of the average Mn oxidation degree (from 3.80 ± 0.01 at pH 10 to 3.70 ± 0.01 at pH 3) and in the lowering of the Na/Mn ratio (from 27.66 ± 0.20 at pH 10 to 6.99 ± 0.16 at pH 3). In addition, in-plane unit-cell parameters are negatively correlated to the number of interlayer Mn at TC sites and decrease with decreasing pH (fromb= 2.842 Å at pH 10 tob= 2.834 Å at pH 3), layer symmetry being systematically hexagonal witha=b× 31/2. Finally, modelling of X-ray diffraction (XRD) patterns indicates that crystallite size in theabplane and along thec* axis decreases with decreasing pH, ranging respectively from 7 nm to 6 nm, and from 1.2 nm to 1.0 nm (pH 10 and 3, respectively). Following their characterization, dry samples were sealed in polystyrene vials, kept in the dark, and re-analysed 4 and 8 years later. With ageing time and despite the dry state, layer Mn3+extensively migrates to the interlayer most likely to minimize steric strains resulting from the Jahn–Teller distortion of Mn3+octahedra. When the number of interlayer Mn3+at TC sites resulting from this migration reaches the maximum value of ∼ 1/3 per layer octahedron, interlayer species from adjacent layers share their coordination sphere, resulting in cryptomelane-like tunnel structure fragments (with a 2  × 2 tunnel size) with a significantly improved layer stacking order.

A critical review of the reactivity of manganese oxides with organic contaminants

Naturally occurring manganese (Mn(iii/iv)) oxides are ubiquitous in a wide range of environmental settings and play a key role in numerous biogeochemical cycles. In addition, Mn(iii/iv) oxides are powerful oxidants that are capable of oxidizing a wide range of compounds. This review critically assesses the reactivity of Mn oxides with organic contaminants. Initial work with organic reductants employed high concentrations of model compounds (e.g., substituted phenols and anilines) and emphasized the reductive dissolution of the Mn oxides. Studies with lower concentrations of organic contaminants demonstrate that Mn oxides are capable of oxidizing a wide range of compounds (e.g., antibacterial agents, endocrine disruptors, and pesticides). Both model compounds and organic contaminants undergo similar reaction mechanisms on the oxide surface. The oxidation rates of organic compounds by manganese oxides are dependent upon solution conditions, such as pH and the presence of cations, anions, or dissolved organic matter. Similarly, physicochemical properties of the minerals used affect the rates of organic compound oxidation, which increase with the average oxidation state, redox potential, and specific surface area of the Mn oxides. Due to their reactivity with contaminants under environmentally relevant conditions, Mn oxides may oxidize contaminants in soils and/or be applied in water treatment applications.

Energetic basis of catalytic activity of layered nanophase calcium manganese oxides for water oxidation

Previous measurements show that calcium manganese oxide nanoparticles are better water oxidation catalysts than binary manganese oxides (Mn3O4, Mn2O3, and MnO2). The probable reasons for such enhancement involve a combination of factors: The calcium manganese oxide materials have a layered structure with considerable thermodynamic stability and a high surface area, their low surface energy suggests relatively loose binding of H2O on the internal and external surfaces, and they possess mixed-valent manganese with internal oxidation enthalpy independent of the Mn(3+)/Mn(4+) ratio and much smaller in magnitude than the Mn2O3-MnO2 couple. These factors enhance catalytic ability by providing easy access for solutes and water to active sites and facile electron transfer between manganese in different oxidation states.