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

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.

Redox-Driven Formation of Mn(III) in Ice

Redox-driven reactions involving Mn(II) species adsorbed at Mn(IV) oxide surfaces can release Mn(III) in the form of dissolved Mn(III)-ligand species in natural waters. Using pyrophosphate (PP) as a model ligand, we show that freezing accelerates and enhances Mn(III) formation in the form of Mn(III)-PP complexes. This freeze-promoted reaction is explained by the concentration of Mn(IV) oxides and solutes (Mn(II), Na+, and Cl-) into the minute fractions of liquid water locked between ice (micro)crystals - the Liquid Intergrain Boundary (LIB). Time-resolved freezing experiments at -20 °C showed that Mn(III) yields were greatest at low salt (NaCl) content. In contrast, high salt content promoted Mn(III) formation through chloride complexation, although yields became lower as the cryosalt mineral hydrohalite (NaCl·2H2O) dehydrated the LIB by drawing water into its structure. Consecutive freeze-thaw cycles also showed that dissolved Mn(III) concentrations increased within the very first few minutes of each freezing event. Because each thaw event released unreacted PP previously locked in ice, each sequential freeze-thaw cycle increased Mn(III) yields, until ∼80% of the Mn was converted to Mn(III). This was achieved after only seven cycles. Finally, temperature-resolved freezing experiments down to -50 °C showed that the LIB produced the greatest quantities of Mn(III) at -10 °C, where the volumes were greater. Reactivity was however sustained in ice formed below the eutectic (-21.3 °C), down to -50 °C. We suspect that this sustained reactivity was driven by persistent forms of supercooled water, such as Mn(IV) oxide-bound thin water films. By demonstrating the freeze-driven production of Mn(III) by comproportionation of dissolved Mn(II) and Mn(IV) oxide, this study highlights the potentially important roles these reactions could play in the production of pools of Mn(III) in natural water and sediments of mid- and high-latitudes environments exposed to freeze-thaw episodes.

A review on the transformation of birnessite in the environment: Implication for the stabilization of heavy metals

Birnessite is ubiquitous in the natural environment where heavy metals are retained and easily transformed. The surface properties and structure of birnessite change with the changes in external environmental conditions, which also affects the fate of heavy metals. Clarifying the effect and mechanism of the birnessite phase transition process on heavy metals is the key to taking effective measures to prevent and control heavy metal pollution. Therefore, the four transformation pathways of birnessite are summarized first in this review. Second, the relationship between transformation pathways and environmental conditions is proposed. These relevant environmental conditions include abiotic (e.g., co-existing ions, pH, oxygen pressure, temperature, electric field, light, aging, pressure) and biotic factors (e.g., microorganisms, biomolecules). The phase transformation is achieved by the key intermediate of Mn(III) through interlayer-condensation, folding, neutralization-disproportionation, and dissolution-recrystallization mechanisms. The AOS (average oxidation state) of Mn and interlayer spacing are closely correlated with the phase transformation of birnessite. Last but not least, the mechanisms of heavy metals immobilization in the transformation process of birnessite are summed up. They involve isomorphous substitution, redox, complexation, hydration/dehydration, etc. The transformation of birnessite and its implication on heavy metals will be helpful for understanding and predicting the behavior of heavy metals and the crucial phase of manganese oxides/hydroxides in natural and engineered environments.

Synergetic effects of Mn(II) production and site availability on arsenite oxidation and arsenate adsorption on birnessite in the presence of low molecular weight organic acids

Manganese oxides and organic acids are key factors affecting arsenic mobility, but As(III) oxidation and adsorption in the coexistence of birnessite and low molecular weight organic acids (LMWOAs) are poorly understood. Herein, As(III) immobilization by birnessite was investigated with/without LMWOAs (including tartaric (TA), malate (MA), and succinic acids (SA) with two, one and zero hydroxyl groups, respectively). In the low-As(III) system with less Mn(II) production, LMWOAs generally inhibited As(III) oxidation. The slower decrease in As(III) concentration in TA-amended batches resulted from stronger bonding interaction between TA and edge sites, evidenced by higher removal of TA than MA and SA in solutions and the higher proportion of shifted C-OH component in solids. In high-As(III) systems with abundant Mn(II) production, higher concentrations of dissolved Mn and Mn(III) in LMWOA-amended batches than in LMWOA-free batches revealed that LMWOA-induced complexing dissolution caused the release of adsorbed Mn(II), which was conducive to As(III) oxidation and As(V) adsorption onto the edge sites. The lowest concentrations of dissolved Mn and Mn(III) in TA-amended batches indicated that the hydroxyl group constrained complexing dissolution. This study reveals that concentrations of produced Mn(II) determined the roles of LMWOAs in As(III) behavior and highlights the impacts of the hydroxyl group on arsenic mobility.

The pH-sensitive transformation of birnessite and its effect on the fate of norfloxacin

Birnessite plays a crucial role in regulating the fate of contaminants in soil, which is affected by the crystal structure of birnessite. In this study, the transformation of triclinic birnessite to hexagonal birnessite was examined at various pH values, and their reactivity towards norfloxacin was investigated. The findings indicate that the conversion from triclinic birnessite to hexagonal birnessite occurs under pH conditions lower than 7. The lower of the solution pH where the birnessite formed, the higher the surface reactivity. Throughout the transformation process, the migration of Mn3+ and the increased interlayer protons generated more reactive oxygen species, which enhanced the surface reactivity towards norfloxacin. Specifically, at a conversion pH of 1, the norfloxacin removal rate significantly increases from 14% to 97% compared to triclinic birnessite. The mechanism of norfloxacin removal by triclinic and hexagonal birnessite is illustrated. These findings provide valuable insights into the dynamic transformation of birnessites in aqueous environments with varying pH values and their impact on norfloxacin removal.

Oxide- and Silicate-Water Interfaces and Their Roles in Technology and the Environment

Interfacial reactions drive all elemental cycling on Earth and play pivotal roles in human activities such as agriculture, water purification, energy production and storage, environmental contaminant remediation, and nuclear waste repository management. The onset of the 21st century marked the beginning of a more detailed understanding of mineral aqueous interfaces enabled by advances in techniques that use tunable high-flux focused ultrafast laser and X-ray sources to provide near-atomic measurement resolution, as well as by nanofabrication approaches that enable transmission electron microscopy in a liquid cell. This leap into atomic- and nanometer-scale measurements has uncovered scale-dependent phenomena whose reaction thermodynamics, kinetics, and pathways deviate from previous observations made on larger systems. A second key advance is new experimental evidence for what scientists hypothesized but could not test previously, namely, interfacial chemical reactions are frequently driven by "anomalies" or "non-idealities" such as defects, nanoconfinement, and other nontypical chemical structures. Third, progress in computational chemistry has yielded new insights that allow a move beyond simple schematics, leading to a molecular model of these complex interfaces. In combination with surface-sensitive measurements, we have gained knowledge of the interfacial structure and dynamics, including the underlying solid surface and the immediately adjacent water and aqueous ions, enabling a better definition of what constitutes the oxide- and silicate-water interfaces. This critical review discusses how science progresses from understanding ideal solid-water interfaces to more realistic systems, focusing on accomplishments in the last 20 years and identifying challenges and future opportunities for the community to address. We anticipate that the next 20 years will focus on understanding and predicting dynamic transient and reactive structures over greater spatial and temporal ranges as well as systems of greater structural and chemical complexity. Closer collaborations of theoretical and experimental experts across disciplines will continue to be critical to achieving this great aspiration.

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.

Siderophore-Mediated Mobilization of Manganese Limits Iron Solubility in Mixed Mineral Systems

Recent laboratory and field studies show the need to consider the formation of aqueous Mn(III)-siderophore complexes in manganese (Mn) and iron (Fe) geochemical cycling, a shift from the historical view that aqueous Mn(III) species are unstable and thus unimportant. In this study, we quantified Mn and Fe mobilization by desferrioxamine B (DFOB), a terrestrial bacterial siderophore, in single (Mn or Fe) and mixed (Mn and Fe) mineral systems. We selected manganite (γ-MnOOH), δ-MnO₂, lepidocrocite (γ-FeOOH), and 2-line ferrihydrite (Fe₂O₃·0.5H₂O) as relevant mineral phases. We found that DFOB mobilized Mn(III) as Mn(III)-DFOB complexes to varying extents from both Mn(III,IV) oxyhydroxides but reduction of Mn(IV) to Mn(III) was required for the mobilization of Mn(III) from δ-MnO₂. The initial rates of Mn(III)-DFOB mobilization from manganite and δ-MnO₂ were not affected by the presence of lepidocrocite but decreased by a factor of 5 and 10 for manganite and δ-MnO₂, respectively, in the presence of 2-line ferrihydrite. Additionally, the decomposition of Mn(III)-DFOB complexes through Mn-for-Fe ligand exchange and/or ligand oxidation led to Mn(II) mobilization and Mn(III) precipitation in the mixed-mineral systems (∼10% (mol Mn/mol Fe)). As a result, the concentration of Fe(III) mobilized as Fe(III)-DFOB decreased by up to 50% and 80% in the presence of manganite and δ-MnO₂, respectively, compared to the single mineral systems. Our results demonstrate that siderophores, through their complexation of Mn(III), reduction of Mn(III,IV), and mobilization of Mn(II), can redistribute Mn to other soil minerals and limit the bioavailability of Fe in natural systems.

The important role of the interaction between manganese minerals and metals in environmental remediation: a review

With illegal discharge of wastewater containing inorganic and organic pollutants, combined pollution is common and needs urgent attention. Understanding the migration and transformation laws of pollutants in the environment has important guiding significance for environmental remediation. Due to the characteristics of adsorption, oxidation, and catalysis, manganese minerals play important role in the environment fate of pollutants. This review summarizes the forms of interaction between manganese minerals and metals, the environmental importance of the interaction between manganese minerals and metals, and the contribution of this interaction in improving performance of Mn-based composite for environmental remediation. The literatures have indicated that the interactions between manganese minerals and metals involve in surface adsorption, lattice replacement, and formation of association minerals. The interaction between manganese minerals and metals plays an important role in environmental behavior of element and environmental significance of manganese minerals. The synergistic or antagonistic effect resulted from the interaction influence the purification of heavy metal and organism pollutant. The synergistic effect benefited from the coordination of adsorption and oxidation, convenient electron transfer, abundant oxygen vacancies, and fast migration of lattice oxygen. Based on the synergy, Mn-based composites have been widely used for environmental remediation. The synthesize methods of Mn-based composites mainly include homogeneous coprecipitation, chemical etching route, hydrothermal, homogeneous chelating sol-gel, and ethylene glycol reduction strategy. This review is helpful to fully understand the migration and transformation process of pollutants in the environment, expand the resource utilization of manganese minerals for environmental remediation.

Synthesis and Aerobic Oxidation Catalysis of Mesoporous Todorokite-Type Manganese Oxide Nanoparticles by Crystallization of Precursors

The pursuit of a high surface area while maintaining high catalytic performance remains a challenge due to a trade-off relationship between these two features in some cases. In this study, mesoporous todorokite-type manganese oxide (OMS-1) nanoparticles with high specific surface areas were synthesized in one step by a new synthesis approach involving crystallization (i.e., solid-state transformation) of a precursor produced by a redox reaction between MnO₄^- and Mn²⁺ reagents. The use of a low-crystallinity precursor with small particles is essential to achieve this solid-state transformation into OMS-1 nanoparticles. The specific surface area reached up to ca. 250 m² g^-1, which is much larger than those (13-185 m² g^-1) for Mg-OMS-1 synthesized by previously reported methods including multistep synthesis or dissolution/precipitation processes. Despite ultrasmall nanoparticles, a linear correlation between the catalytic reaction rates of OMS-1 and the surface areas was observed without a trade-off relationship between particle size and catalytic performance. These OMS-1 nanoparticles exhibited the highest catalytic activity among the Mn-based catalysts tested for the oxidation of benzyl alcohol and thioanisole with molecular oxygen (O₂) as the sole oxidant, including highly active β-MnO₂ nanoparticles. The present OMS-1 nanomaterial could also act as a recyclable heterogeneous catalyst for the aerobic oxidation of various aromatic alcohols and sulfides under mild reaction conditions. The mechanistic studies showed that alcohol oxidation proceeds with oxygen species caused by the solid, and the high surface area of OMS-1 significantly contributes to an enhancement of the catalytic activity for aerobic oxidation.

Alteration of birnessite reactivity in dynamic anoxic/oxic environments

Although the oxidative capacity of manganese oxides has been widely investigated, potential changes of the surface reactivity in dynamic anoxic/oxic environments have been often overlooked. In this study, we showed that the reactivity of layer structured manganese oxide (birnessite) was highly sensitive to variable redox conditions within environmentally relevant ranges of pH (4.0 - 8.0), ionic strength (0-100 mM NaCl) and Mn(II)/MnO₂ molar ratio (0-0.58) using ofloxacine (OFL), a typical antibiotic, as a target contaminant. In oxic conditions, OFL removal was enhanced relative to anoxic environments under alkaline conditions. Surface-catalyzed oxidation of Mn(II) enabled the formation of more reactive Mn(III) sites for OFL oxidation. However, an increase in Mn(II)/MnO₂ molar ratio suppressed MnO₂ reactivity, probably because of competitive binding between Mn(II) and OFL and/or modification in MnO₂ surface charge. Monovalent cations (e.g., Na⁺) may compensate the charge deficiency caused by the presence of Mn(III), and affect the aggregation of MnO₂ particles, particularly under oxic conditions. An enhancement in the removal efficiency of OFL was then confirmed in the dynamic two-step anoxic/oxic process, which emulates oscillating redox conditions in environmental settings. These findings call for a thorough examination of the reactivity changes at environmental mineral surfaces (e.g., MnO₂) in natural systems that may be subjected to alternation between anaerobic and oxygenated conditions.

Oxygen Limitation Accelerates Regeneration of Active Sites on a MnO2 Surface: Promoting Transformation of Organic Matter and Carbon Preservation

Birnessite (δ-MnO₂) is a layered manganese oxide widely present in the environment and actively participates in the transformation of natural organic matter (NOM) in biogeochemical processes. However, the effect of oxygen on the dynamic interface processes of NOM and δ-MnO₂ remains unclear. This study systematically investigated the interactions between δ-MnO₂ and fulvic acid (FA) under both aerobic and anaerobic conditions. FA was transformed by δ-MnO₂ via direct electron transfer and the generated reactive oxygen species (ROS). During the 32-day reaction, 79.8% of total organic carbon (TOC) in solution was removed under anaerobic conditions, unexpectedly higher than that under aerobic conditions (69.8%), suggesting that oxygen limitation was more conducive to the oxidative transformation of FA by δ-MnO₂. The oxygen vacancies (O_V) on the surface of δ-MnO₂ were more exposed under anaerobic conditions, thus promoting the adsorption and transformation of FA as well as regeneration of the active sites. Additionally, the reaction of FA with δ-MnO₂ weakened the strongly bonded lattice oxygen (O_latt), and the released O_latt was an important source of ROS. Interestingly, a part of organic carbon (OC) was preserved by forming MnCO₃, which might be a novel mechanism for carbon preservation. These findings contribute to an improved understanding of the dynamic interface processes between MnO₂ and NOM and provide new insights into the effects of oxygen limitation on the cycling and preservation of OC.

Impacts of environmental levels of hydrogen peroxide and oxyanions on the redox activity of MnO2 particles

Despite the widespread presence of hydrogen peroxide (H₂O₂) in surface water and groundwater systems, little is known about the impact of environmental levels of H₂O₂ on the redox activity of minerals. Here we demonstrate that environmental concentrations of H₂O₂ can alter the reactivity of birnessite-type manganese oxide, an earth-abundant functional material, and decrease its oxidative activity in natural systems across a wide range of pH values (4-8). The H₂O₂-induced reductive dissolution generates Mn(II) that will re-bind to MnO₂ surfaces, thereby affecting the surface charge of MnO₂. Competition of Bisphenol A (BPA), used as a target compound here, and Mn(II) to interact with reactive surface sites may cause suppression of the oxidative ability of MnO₂. This suppressive effect becomes more effective in the presence of oxyanions such as phosphate or silicate at concentrations comparable to those encountered in natural waters. Unlike nitrate, adsorption of phosphate or silicate onto birnessite increased in the presence of Mn(II) added or generated through H₂O₂-induced reduction of MnO₂. This suggests that naturally occurring anions and H₂O₂ may have synergetic effects on the reactivity of birnessite-type manganese oxide at a range of environmentally relevant H₂O₂ amounts. As layered structure manganese oxides play a key role in the global carbon cycle as well as pollutant dynamics, the impact of environmental levels of hydrogen peroxide (H₂O₂/MnO₂ molar ratio ≤ 0.3) should be considered in environmental fate and transport models.

Oxidation of V(IV) by Birnessite: Kinetics and Surface Complexation

Vanadium is a redox-active metal that has been added to the EPA's Contaminant Candidate List with a notification level of 50 μg L-1 due to mounting evidence that VV exposure can lead to adverse health outcomes. Groundwater V concentration exceeds the notification level in many locations, yet geochemical controls on its mobility are poorly understood. Here, we examined the redox interaction between VIV and birnessite (MnO2), a well-characterized oxidant and a scavenger of many trace metals. In our findings, birnessite quickly oxidized sparingly soluble VIV species such as häggite [V2O3(OH)2] into highly mobile and toxic vanadate (HnVO4(3-n)-) in continuously stirred batch reactors under neutral pH conditions. Synchrotron X-ray absorption spectroscopic (XAS) analysis of in situ and ex situ experiments showed that oxidation of VIV occurs in two stages, which are both rapid relative to the measured dissolution rate of the VIV solid. Concomitantly, the reduction of birnessite during VIV oxidation generated soluble MnII, which led to the formation of the MnIII oxyhydroxide feitknechtite (β-MnOOH) upon back-reaction with birnessite. XAS analysis confirmed a bidentate-mononuclear edge-sharing complex formed between VV and birnessite, although retention of VV was minimal relative to the aqueous quantities generated. In summary, we demonstrate that Mn oxides are effective oxidants of VIV in the environment with the potential to increase dissolved V concentrations in aquifers subject to redox oscillations.

Accelerated catalytic oxidation of dissolved manganese(II) by chlorine in the presence of in situ-growing 3D manganese(III)/(IV) oxide nanosheet assembly in zeolite filter

Manganese contamination is ubiquitous in ground water. Water eutrophication also exaggerates manganese release and contamination in surface water. However, conventional manganese(II) removal process through sand filter is low-efficiency and long-term ripening. Manganese exceeding standard is still a bottleneck issue for drinking water plants. To provide a quick-setup and low-cost means, we invented an accelerated catalytic oxidation filtration process through porous zeolite filter with dynamically coating of manganese oxide nanocatalysts. In dynamic filtration process, the addition of chlorine less than redox stoichiometric consumption can efficiently remove dissolved manganese(II) from contaminated tap water, ground water and Songhua river water. Characterization results showed that a continuous manganese(III)/(IV) oxide nanosheet catalyst was dynamically in situ-growing and assembled into 3D porous superstructure in the reactive Zeolite@MnO_x(s) filter. Active Mn(III) species on the edges of MnO_x(s) nanosheets were dynamically generated and transferred into stable Mn(IV) species on the layer-structured surface. The cycling transformation of manganese(III)/(IV) species was responsible for the accelerated catalytic oxidation of dissolved manganese(II) by chlorine. Without process changes in drinking water plant, the porous Zeolite@MnO_x(s) media could be feasibly integrated onto the existing sand filtration tanks for emergence handling of manganese(II) contamination. This novel reactive Zeolite@MnO_x(s) filter with higher hydraulic conductivity provides a high-efficiency, scalable and low-cost technique for the manganese(II) removal from various of water environments.

Antimonate Controls Manganese(II)-Induced Transformation of Birnessite at a Circumneutral pH

Manganese (Mn) oxides, such as birnessite (δ-MnO₂), are ubiquitous mineral phases in soils and sediments that can interact strongly with antimony (Sb). The reaction between birnessite and aqueous Mn(II) can induce the formation of secondary Mn oxides. Here, we studied to what extent different loadings of antimonate (herein termed Sb(V)) sorbed to birnessite determine the products formed during Mn(II)-induced transformation (at pH 7.5) and corresponding changes in Sb behavior. In the presence of 10 mM Mn(II)_aq, low Sb(V)_aq (10 μmol L^-1) triggered the transformation of birnessite to a feitknechtite (β-Mn(III)OOH) intermediary phase within 1 day, which further transformed into manganite (γ-Mn(III)OOH) over 30 days. Medium and high concentrations of Sb(V)_aq (200 and 600 μmol L^-1, respectively) led to the formation of manganite, hausmannite (Mn(II)Mn(III)₂O₄), and groutite (αMn(III)OOH). The reaction of Mn(II) with birnessite enhanced Sb(V)_aq removal compared to Mn(II)-free treatments. Antimony K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy revealed that heterovalent substitution of Sb(V) for Mn(III) occurred within the secondary Mn oxides, which formed via the Mn(II)-induced transformation of Sb(V)-sorbed birnessite. Overall, Sb(V) strongly influenced the products of the Mn(II)-induced transformation of birnessite, which in turn attenuated Sb mobility via incorporation of Sb(V) within the secondary Mn oxide phases.

The Energetic Potential for Undiscovered Manganese Metabolisms in Nature

Microorganisms are found in nearly every surface and near-surface environment, where they gain energy by catalyzing reactions among a wide variety of chemical compounds. The discovery of new catabolic strategies and microbial habitats can therefore be guided by determining which redox reactions can supply energy under environmentally-relevant conditions. In this study, we have explored the thermodynamic potential of redox reactions involving manganese, one of the most abundant transition metals in the Earth's crust. In particular, we have assessed the Gibbs energies of comproportionation and disproportionation reactions involving Mn²⁺ and several Mn-bearing oxide and oxyhydroxide minerals containing Mn in the +II, +III, and +IV oxidation states as a function of temperature (0-100°C) and pH (1-13). In addition, we also calculated the energetic potential of Mn²⁺ oxidation coupled to O₂, NO₂ ^-, NO₃ ^-, and FeOOH. Results show that these reactions-none of which, except O₂ + Mn²⁺, are known catabolisms-can provide energy to microorganisms, particularly at higher pH values and temperatures. Comproportionation between Mn²⁺ and pyrolusite, for example, can yield 10 s of kJ (mol Mn)^-1. Disproportionation of Mn³⁺ can yield more than 100 kJ (mol Mn)^-1 at conditions relevant to natural settings such as sediments, ferromanganese nodules and crusts, bioreactors and suboxic portions of the water column. Of the Mn²⁺ oxidation reactions, the one with nitrite as the electron acceptor is most energy yielding under most combinations of pH and temperature. We posit that several Mn redox reactions represent heretofore unknown microbial metabolisms.

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.

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.

Reductive transformation of birnessite and the mobility of co-associated antimony

Manganese (Mn) oxide minerals, such as birnessite, are thought to play an important role in affecting the mobility and fate of antimony (Sb) in the environment. In this study, we investigate Sb partitioning and speciation during anoxic incubation of Sb(V)-coprecipitated birnessite in the presence and absence of Mn(II)_aq at pH 5.5 and 7.5. Antimony K-edge XANES spectroscopy revealed that Sb(V) persisted as the only solid-phase Sb species for all experimental treatments. Manganese K-edge EXAFS and XRD results showed that, in the absence of Mn(II), the Sb(V)-bearing birnessite underwent no detectable mineralogical transformation during 7 days. In contrast, the addition of 10 mM Mn(II) at pH 7.5 induced relatively rapid (within 24 h) transformation of birnessite to manganite (~93%) and hausmannite (~7%). Importantly, no detectable Sb was measured in the aqueous phase for this treatment (compared with up to ∼90 μmol L^-1 Sb in the corresponding Mn(II)-free treatment). At pH 5.5 , birnessite reacted with 10 mM Mn(II)_aq displayed no detectable mineralogical transformation, yet had substantially increased Sb retention in the solid phase, relative to the corresponding Mn(II)-free treatment. These findings suggest that the Mn(II)-induced transformation and recrystallization of birnessite can exert an important control on the mobility of co-associated Sb.

Removal of lincomycin from aqueous solution by birnessite: kinetics, mechanism, and effect of common ions

The removal of lincomycin (LIN) from aqueous solution by birnessite was investigated by batch experiments. When the dosage of birnessite is 500 mg L^-1 and the initial concentration of LIN is 15.5 μmol L^-1, more than 90% of LIN was removed within 240 min at pH 4.90. Under different conditions, the reactions were well fitted with the second-order model (R² > 0.95). The removal kinetics and the reaction mechanism were described. The presence of cations (e.g., K⁺, Ca²⁺, Mg²⁺, Fe²⁺, and Mn²⁺) inhibited the removal of LIN by birnessite, following the order: Mn²⁺ > Fe²⁺ > Ca²⁺ > Mg²⁺ > K⁺ ≈ Na⁺, due to the sorption of cations on birnessite, companying with the electron transfer and precipitation of oxides (for Mn²⁺ and Fe²⁺). The addition of Cu²⁺, SO₄^2-, or NO₃^- improved the reactions. The presence of Cu²⁺ could oxidize antibiotics, and the repulsion between SO₄^2-or NO₃^- and birnessite might disperse the birnessite suspensions during the reactions. Mn(IV) and Mn(III) were the core Mn species that play an important role in LIN removal. These findings will help to understand the removal process of LIN and illustrate the influence of cations and anions on the removal of similar pollutants by birnessite.

Mn2+ effect on manganese oxides (MnOx) nanoparticles aggregation in solution: Chemical adsorption and cation bridging

Manganese oxides (MnO_x) and Mn²⁺ usually co-exist in the natural environment, as well as in water treatments for Mn²⁺ removal. Therefore, it is necessary to investigate the influence of Mn²⁺ on the stability of MnO_x nanoparticles, as it is vital to their fate and reactivity. In this study, we used the time-resolved dynamic light scattering technique to study the influence of Mn²⁺ on the initial aggregation kinetics of MnO_x nanoparticles. The results show that Mn²⁺ was highly efficient in destabilizing MnO_x nanoparticles. The critical coagulation concentration ratio of Mn²⁺ (0.3 mM) to Na⁺ (30 mM) was 2^-6.64, which is beyond the ratio range indicated by the Schulze-Hardy rule. This is due to the coordination bond formed between Mn²⁺ and the surface O of MnO_x, which could efficiently decrease the negative surface charge of MnO_x. As a result, in the co-presence of Mn²⁺ and Na⁺, a small amount of Mn²⁺ (5 μM) could efficiently neutralize the negative charge of MnO_x, thereby decreasing the amount of Na⁺, which mainly destabilized nanoparticles through electric double-layer compression, required to initiate aggregation. Further, Mn²⁺ behaved as a cation bridge linking both the negatively charged MnO_x and humic acid, thereby increasing the stability of the MnO_x nanoparticles as a result of the steric repulsion of the adsorbed humic acid. The results of this study enhance the understanding of the stability of the MnO_x nanoparticles in the natural environment, as well as in water treatments.

Arsenite oxidation and arsenic adsorption on birnessite in the absence and the presence of citrate or EDTA

Birnessite not only oxidizes arsenite into arsenate but also interacts with organic matter in various ways. However, effects of organic matter on interaction between As and birnessite remain unclear. This study investigated effects of citrate and EDTA (3.12 and 2.05 mM, respectively) on oxidation of As(III) (1.07 mM) and adsorption of As(V) (0.67 mM) on birnessite (5.19 mM as Mn) at near-neutral pH. We found that As(V) adsorption on birnessite was enhanced by citrate and EDTA, which resulted from the increase in active adsorption sites via dissolution of birnessite. In comparison with citrate batches, more As was adsorbed on birnessite in EDTA batches, where dissolved Mn was mainly presented as Mn(III)-EDTA complex. Citrate or EDTA-induced dissolution of birnessite did not decrease the As(III) oxidation rate in the initial stage where As(III) oxidation rate was rapid. Afterwards, As(III) oxidation was conspicuously suppressed in citrate-amended batches, which was mainly attributed to the decrease in adsorption sites by adsorption of citrate/Mn(II)-citrate complex. This suppression was enhanced by the increase in concentrations of dissolved Mn(II). Citrate inhibited As adsorption after As(III) oxidation due to the strong competitive adsorption of citrate/Mn(II)-citrate complex. However, the As(III) oxidation rate was increased in EDTA-amended batches in the late stage, which mainly derived from the increase in the active sites via birnessite dissolution. The strong complexation ability of EDTA led to formation of Mn(III)-EDTA complex. Arsenic adsorption was not affected due to the limited competitive adsorption of the complex on the solid. This work reveals the critical role of low molecular weight organic acids in geochemical behaviors of As and Mn in aqueous environment.

Process-based modeling of arsenic(III) oxidation by manganese oxides under circumneutral pH conditions

Numerous experimental studies have identified a multi-step reaction mechanism to control arsenite (As(III)) oxidation by manganese (Mn) oxides. The studies highlighted the importance of edge sites and intermediate processes, e.g., surface passivation by reaction products. However, the identified reaction mechanism and controlling factors have rarely been evaluated in a quantitative context. In this study, a process-based modeling framework was developed to delineate and quantify the relative contributions and rates of the different processes affecting As(III) oxidation by Mn oxides. The model development and parameterization were constrained by experimental observations from literature studies involving environmentally relevant Mn oxides at circumneutral pH using both batch and stirred-flow reactors. Our modeling results highlight the importance of a transitional phase, solely evident in the stirred-flow experiments, where As(III) oxidation gradually shifts from fast reacting Mn(IV) to slowly reacting Mn(III) edge sites. The relative abundance of these edge sites was the most important factor controlling the oxidation rate, whereas surface passivation restricted oxidation only in the stirred-flow experiment. The Mn(III) edge sites were demonstrated to play a crucial role in the oxidation and therefore in controlling the long-term fate of As. This study provided an improved understanding of Mn oxide reactivity and the significance in the cycling of redox-sensitive metal(loid)s in the environment.

Microbially Induced Mineralization of Layered Mn Oxides Electroactive in Li Batteries

Nanoparticles produced by bacteria, fungi, or plants generally have physicochemical properties such as size, shape, crystalline structure, magnetic properties, and stability which are difficult to obtain by chemical synthesis. For instance, Mn(II)-oxidizing organisms promote the biomineralization of manganese oxides with specific textures under ambient conditions. Controlling their crystallinity and texture may offer environmentally relevant routes of Mn oxide synthesis with potential technological applications, e.g., for energy storage. However, whereas the electrochemical activity of synthetic (abiotic) Mn oxides has been extensively studied, the electroactivity of Mn biominerals has been seldom investigated yet. Here we evaluated the electroactivity of biologically induced biominerals produced by the Mn(II)-oxidizer bacteria Pseudomonas putida strain MnB1. For this purpose, we explored the mechanisms of Mn biomineralization, including the kinetics of Mn(II) oxidation, under different conditions. Manganese speciation, biomineral structure, and texture as well as organic matter content were determined by a combination of X-ray diffraction, electron and X-ray microscopies, and thermogravimetric analyses coupled to mass spectrometry. Our results evidence the formation of an organic–inorganic composite material and a competition between the enzymatic (biotic) oxidation of Mn(II) to Mn(IV) yielding MnO₂ birnessite and the abiotic formation of Mn(III), of which the ratio depends on oxygenation levels and activity of the bacteria. We reveal that a subtle control over the conditions of the microbial environment orients the birnessite to Mn(III)-phases ratio and the porosity of the assembly, which both strongly impact the bulk electroactivity of the composite biomineral. The electrochemical properties were tested in lithium battery configuration and exhibit very appealing performances (voltage, capacity, reversibility, and power capability), thanks to the specific texture resulting from the microbially driven synthesis route. Given that such electroactive Mn biominerals are widespread in the environment, our study opens an alternative route for the synthesis of performing electrode materials under environment-friendly conditions.

Tunable Mn Oxidation State and Redox Potential of Birnessite Coexisting with Aqueous Mn(II) in Mildly Acidic Environments

As the dominant manganese oxide mineral phase in terrestrial and aquatic environments, birnessite plays an important role in many biogeochemical processes. The coexistence of birnessite with aqueous Mn2+ is commonly found in the subsurface environments undergoing Mn redox cycling. This study investigates the change in Mn average oxidation state (AOS) of birnessite after reaction with 0.1–0.4 mM Mn2+ at pH 4.5–6.5, under conditions in which phase transformation of birnessite by Mn2+ was not detectable. The amount of Mn2+ uptake by birnessite and the equilibrium concentration of Mn(III) proportionally increased with the initial concentration of Mn2+. The Mn AOS of birnessite particles became 3.87, 3.75, 3.64, and 3.53, respectively, after reaction with 0.1, 0.2, 0.3, and 0.4 mM Mn2+ at pH 5.5. Oxidation potentials (Eh) of birnessite with different AOS values were estimated using the equilibrium concentrations of hydroquinone oxidized by the birnessite samples, indicating that Eh was linearly proportional to AOS. The oxidation kinetics of bisphenol A (BPA), a model organic pollutant, by birnessite suggest that the logarithms of surface area-normalized pseudo-first-order initial rate constants (log kSA) for BPA degradation by birnessite were linearly correlated with the Eh or AOS values of birnessite with AOS greater than 3.64.

Water Flow and Dissolved MnII Alter Transformation of Pipemidic Acid by Manganese Oxide

Manganese oxides have been proposed as promising geomedia to remove trace organic contaminants in both natural soils and artificial infiltration systems. Although MnO_x-based redox processes have been largely investigated, little is known on the effects of water flow and dissolved Mn^II on manganese-mediated redox reactions in saturated porous media. Here, we have demonstrated that the reactive transport of a widely used quinolone antibiotic, pipemidic acid (PIP), in MnO₂-coated sand (MCS) columns is altered by the presence of dissolved Mn^II, generated in situ as reduced ions or present in inflow solution. Decreasing the flow rate or flow interruption facilitated oxidation reactions and generated redox byproducts (Mn^II and PIP_ox). However, preloading of MCS columns with dissolved Mn^II led to suppressed reactivity with PIP. When PIP and Mn^II are simultaneously injected, competition between PIP and Mn^II for binding at the edge sites takes place during the initial kinetic phase of reaction, while at a later breakthrough time Mn^II will occupy both edge and vacancy sites due to the continuous supply of Mn^II. We also developed a reactive transport model that accounts for adsorption kinetics to predict changes in transport behavior of antibiotics in the presence of different doses of dissolved Mn^II. This work has strong implications for an accurate assessment of the reactivity of manganese oxides used as engineered geomedia for quinolone remediation and in developing transport models of antibiotics in natural systems.

Influence of Fe(II) on Sb(III) oxidation and adsorption by MnO2 under acidic conditions

The transformation and transport of Sb are significantly influenced by strong oxides (e.g. MnO₂) in the natural environment. Furthermore, Fe(II) can coexist with Sb(III) and MnO₂ in waters contaminated by acidic mine drainage. However, role of Fe(II) in Sb(III) oxidation and adsorption by MnO₂ remains unclear. Therefore, in the present study, the effects of Fe(II) on the oxidation and adsorption of Sb(III) by MnO₂ under acidic conditions (pH 3) and the mechanism thereof were comprehensively investigated. The results of kinetic experiments showed that, in the presence of soluble Fe(II), Sb(III) oxidation is inhibited, but adsorption is promoted. Further characterization confirmed that Fe(III) compounds are formed around MnO₂ particles and that these inhibit Sb(III) oxidation. However, two different Fe(III) compounds are formed around MnO₂ particles depending on how the Fe(II) is introduced into the experimental system. In the simultaneous oxidation system, poorly crystallized or amorphous FeSb precipitates are formed (probably FeSbO₄) around MnO₂ particles, while in the Fe(II) pretreated oxidation system, schwertmannite is formed. Thus, the present study revealed that Fe(II) is critical to Sb(III) oxidation and adsorption by MnO₂ and that the mechanism of its action is depend upon how it is introduced into the reaction system. This information is of relevance to predicting the fate of Sb.

The antimony sorption and transport mechanisms in removal experiment by Mn-coated biochar

A method of Mn-coated biochar production was developed, which showed great removal ability of trivalent antimony (Sb(III)) (0.94 mg g^-1) and pentavalent antimony (Sb(V)) (0.73 mg g^-1), and the adsorption capacity was stable under different pH. According to the adsorption kinetics and isotherm, the adsorption process of both Sb(III) and Sb(V) was chemisorption, which was both monolayer and poly layers heterogeneous chemisorption process. X-ray photoelectron spectroscopy (XPS) and attenuated total reflectance Fourier-transform infrared (ATR-FTIR) spectroscopy analyses indicated that the main oxides and functional groups involved in the adsorption were manganese oxides (MnO_x), carboxyl and hydroxyl groups and Sb(V) was combined with Mn-coated biochar by inner-sphere complexation. Sb(III) was oxidized by oxygen and MnO_x which was both on the surface of biochar and dissolved in solution. Furthermore, X-ray absorption near-edge structure (XANES) showed that Sb(V) was the main species after Sb(III) and Sb(V) adsorbed on the Mn-coated biochar. Extended X-ray absorption fine structure (EXAFS) analysis indicated that Sb(III) and MnO_x formed the monodentate mononuclear and corner-sharing complexes with a structure of Mn-O-Sb on Mn-coated biochar. While Sb(V) and MnO_x formed inner-sphere complexes including edge-sharing and corner-sharing complexes. The new synthetic material can contribute to develop new remediation strategies for treating Sb-contaminated water.

Electrochemical adsorption of cadmium and arsenic by natural Fe-Mn nodules

Fe-Mn nodules are widely distributed and regarded as excellent adsorbents for heavy metals. Their adsorption-desorption reactions with heavy metal ions are usually accompanied by redox processes. Herein, Fe-Mn nodules were used as adsorbents for Cd(II) and As(III,V) at a constant cell voltage under electrochemically controlled reduction and oxidation, respectively. The results showed that the adsorption performance for Cd(II) and As(III,V) was enhanced respectively due to the decrease and increase of Mn average oxidation state (Mn AOS) in Fe-Mn nodules. High birnessite content and Mn average oxidation state (Mn AOS) improved the adsorption of Cd(II) and As(III,V). The adsorption capacity for Cd(II) and total As increased with increasing voltage. With increasing pH, the adsorption capacity for Cd(II) increased first and then reached equilibrium, and that of total As decreased and then increased. The Cd(II) electrochemical adsorption capacity (129.9 mg g^-1) and the removal efficiency for total As at 1.2 V (83.6 %) in As-containing wastewater at an initial concentration of 4.068 mg L^-1 were remarkably higher than the corresponding inorganic adsorption performance (9.46 mg g^-1 and 70.5 %, respectively). This work may further promote the application of natural Fe-Mn nodules in the adsorption of heavy metals from wastewaters.

Structure-Reactivity Relationships in the Adsorption and Degradation of Substituted Phenylarsonic Acids on Birnessite (δ-MnO2)

Phenylarsonic acid compounds could be oxidized by manganese oxides in surface soils, resulting in quick release of inorganic arsenic. This study investigated the structure-reactivity relationships in the adsorption and oxidative degradation of six substituted phenylarsonic acids on the surface of a major type of manganese oxides, birnessite (δ-MnO₂), using batch experiments conducted under acidic to neutral conditions. The initial adsorption rates of the substituted phenylarsonic acids on δ-MnO₂ decreased in the order of phenylarsonic acid (PAA) > 4-aminophenylarsonic acid (p-ASA) ≈ 2-aminophenylarsonic acid (2-APAA) > 4-hydroxyphenylarsonic acid (4-HPAA) > 2-nitrophenylarsonic acid (2-NPAA) > 4-hydroxy-3-nitrophenylarsonic acid (ROX), which could be attributed to steric hindrance of the substituents and the hydrophobicity of these compounds. The oxidation rates of these structural analogues by δ-MnO₂ decreased in the order of p-ASA ≈ 2-APAA > 4-HPAA > ROX, while 2-NPAA and PAA were nonreactive because of the lack of electron-donating substituents on their aromatic rings. The redox reactivity of these compounds agrees well with the electron density at C₁, which is determined by the types and position of the substituents on the aromatic ring. Although cleavage of the arsonic acid group from the aromatic ring was the predominant transformation pathway, a range of adduct products also formed through cross-coupling of the radicals and radical substitution. The contribution of radical coupling and substitution in overall degradation decreased in the order of p-ASA > 2-APAA > 4-HPAA > ROX, which results from the varying reactivity and steric hindrance of the substituents. These insights could help better understand and predict the fate of substituted phenylarsonic acids in manganese oxide-rich surface soils and the associated environmental risk of arsenic pollution.

Thallium(I) sequestration by jarosite and birnessite: Structural incorporation vs surface adsorption

Jarosite and birnessite secondary minerals play a pivotal role in the mobility, transport and fate of trace elements in the environment, although geochemical interactions of these compounds with extremely toxic thallium (Tl) remain poorly known. In this study, we investigated the sorption behavior of Tl(I) onto synthetic jarosite and birnessite, two minerals commonly found in soils and sediments as well as in mining-impacted areas where harsh conditions are involved. To achieve this, sorption and desorption experiments were carried out under two different acidic conditions and various Tl(I) concentrations to mimic natural scenarios. In addition, X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and inductively coupled plasma (ICP) analyses were conducted to determine the performance of both minerals for Tl(I) sequestration. Our results indicate that both phases can effectively remove aqueous Tl by different sorption mechanisms. Jarosite preferentially incorporates Tl(I) into the structure to form Tl(I)-jarosite and eventually the mineral dorallcharite (TlFe₃(SO₄)₂(OH)₆) as increasing amounts of Tl are employed. Birnessite, however, favorably uptakes Tl(I) through an irreversible surface adsorption mechanism, underlining the affinity of Tl for this mineral in the entire concentration range studied (0.5-5 mmol L^-1). Lastly, the presence of Tl(I) in conditions where aqueous molar ratio Tl/Mn is ∼0.25 inhibits the formation of birnessite since oxidation of Tl(I) to Tl(III) followed by precipitation of avicennite (Tl₂O₃) take place. Thus, the present research may provide useful insights on the role of both jarosite and birnessite minerals in Tl environmental cycles.

Mineralization Enhancement of Pharmaceutical Contaminants by Radical-Based Oxidation Promoted by Oxide-Bound Metal Ions

While the use of transition metal oxides in catalyzing advanced oxidation reactions has been widely investigated, very few reports have focused on how the preliminary contact of oxides with target compounds may affect the succession of reaction. In this study, we examined the adsorption and electron transfer reactions of two fluoroquinolones, flumequine (FLU), and norfloxacin (NOR), with goethite (α-FeOOH) or manganese (Mn) oxide, and their impact on the subsequent mineralization of target compounds using H₂O₂ or S₂O₈^2- under UVA irradiation. Intriguingly, higher total organic carbon (TOC) removal was achieved when antibiotics and metal oxides were allowed for preequilibration before starting the oxidation reaction. The rate and extent of TOC removal are strongly dependent on the molecule structure and the redox-active mineral used, and much less on the preequilibration time. This high efficiency can be ascribed to the presence of reduced metal ions, chemically or photochemically generated during the first stage, onto oxide minerals. Oxide-bound Mn^II plays a crucial role in catalyzing oxidant decomposition and then producing greater amounts of radical species through a photoassisted redox cycle, regardless of the underlying surface, Mn^IVO₂ or Mn^IIIOOH. This finding will be of fundamental and practical significance to Mn-based oxidation reactions and wastewater treatment processes.

Impact of dissolved O2 on phenol oxidation by δ-MnO2

Although redox reactions of organic contaminants with manganese oxides have been extensively studied, the role of dissolved O2 in these processes has largely been overlooked. In this study, the oxidative degradation of phenol by δ-MnO2 was investigated under both oxic and anoxic conditions. Dissolved O2 inhibited phenol degradation due to its promoting role in the reoxidation and precipitation of reduced Mn(ii) to Mn(iii) on the δ-MnO2 surface, resulting in partial transformation of δ-MnO2 to "c-disordered" H+-birnessite at pH 5.5 and feitknechtite, manganite, and hausmannite at pH 7.0 and 8.5. The reformed Mn(iii) phases could reduce phenol oxidation by blocking reactive sites of δ-MnO2. In addition, dissolved O2 caused a higher degree of particle agglomeration and a more severe specific surface area decrease, and hence lower reactivity of δ-MnO2. These findings revealed that after reductive dissolution by phenol and reoxidation by dissolved O2 throughout continuous redox cycling, δ-MnO2 became less reactive rather than being regenerated. These results can provide new insights into the understanding of the oxidation of organic contaminants by manganese oxides in the natural environment.

Cd(II) retention and remobilization on δ-MnO2 and Mn(III)-rich δ-MnO2 affected by Mn(II)

Birnessite owing to its negative surface charge and defective structure exhibits high sorption affinities for Cd(II). However, Mn(II) can not only compete for the sorption sites with Cd(II), but also react with structural Mn(IV) in birnessite to form Mn(III), and thus, affect Cd(II) immobilization by birnessite. Herein, we investigate effects of Mn(II) on Cd(II) retention and remobilization on two birnessite δ-MnO₂ and Mn(III)-rich δ-MnO₂ (denoted as HE-MnO₂). At pH 5.5, Cd(II) sorption to birnessite was inhibited by Mn(II) addition. Mn(II) addition to δ-MnO₂ led to Cd(II) migration from vacant sites to edge sites, forming double-corner sharing (DCS) complexes. Mn(II) introduction to δ-MnO₂ led to less stable Cd(II) species formed on birnessite, indicating that Cd(II) was more firmly bound to vacant sites than edge sites of birnessite. Cd(II) formed double-edge sharing (DES) and DCS complexes on HE-MnO₂. Mn(II) addition to HE-MnO₂ increased the CdMn distance in DES complexes. The stability of adsorbed Cd(II) on HE-MnO₂ was slightly elevated due to Mn(II) addition. At pH 7.5, Mn(II) had no effect on Cd(II) sorption and desorption amounts on birnessite. However, low concentration of Mn(II) added to δ-MnO₂ induced partial migration of Cd(II) from vacant sites to edge sites while high concentration of Mn(II) added to birnessite led to the formation of amorphous Cd(II)-Mn(III) coprecipitate. These findings imply that aqueous Mn(II) is an important factor in influencing Cd(II) immobilization by birnessite in the environment.

Metal Adsorption Controls Stability of Layered Manganese Oxides

Hexagonal birnessite, a typical layered Mn oxide (LMO), can adsorb and oxidize Mn(II) and thereby transform to Mn(III)-rich hexagonal birnessite, triclinic birnessite, or tunneled Mn oxides (TMOs), remarkably changing the environmental behavior of Mn oxides. We have determined the effects of coexisting cations on the transformation by incubating Mn(II)-bearing δ-MnO₂ at pH 8 under anoxic conditions for 25 d (dissolved Mn < 11 μM). In the Li⁺, Na⁺, and K⁺ chloride solutions, the Mn(II)-bearing δ-MnO₂ first transforms to Mn(III)-rich δ-MnO₂ or triclinic birnessite (T-bir) due to the Mn(II)-Mn(IV) comproportionation, most of which eventually transform to a 4 × 4 TMO. In contrast, Mn(III)-rich δ-MnO₂ and T-bir form and persist in the Mg²⁺ and Ca²⁺ chloride solutions. However, in the presence of surface adsorbed Cu(II), Mn(II)-bearing δ-MnO₂ turns into Mn(III)-rich δ-MnO₂ without forming T-bir or TMOs. The stabilizing power of the cations on the δ-MnO₂ structure positively correlates with their binding strength to δ-MnO₂ (Li⁺, Na⁺, and K⁺ < Mg²⁺ and Ca²⁺ < Cu(II)). Since metal adsorption decreases the surface energy of minerals, our finding suggests that the surface energy largely controls the thermodynamic stability of LMOs. Our study indicates that the adsorption of divalent metal cations, particularly transition metals, can be an important cause of the high abundance of LMOs, rather than the more stable TMO phases, in the environment.

Enhanced removal of antimony by acid birnessite with doped iron ions: Companied by the structural transformation

In the environment, antimony as a priority control pollutant is mainly associated with Fe- or Mn- related minerals. In this work, acid birnessite (AB) doped with iron was synthesized as the artificial mineral to study the adsorption and oxidation of antimony. As compared to the pristine birnessite, Fe-doping birnessites show a markedly enhanced removal efficiency for both Sb(III) and Sb(V), where 10% Fe exhibited an excellent adsorption capacity of 759 mg/g Sb(III). The removal of Sb(III) clearly underwent a novel kinetic process of adsorption-desorption- (re-adsorption). By monitoring the kinetics with XRD, XPS, and IR, it is demonstrated that the three-stage kinetics were attributed to the strong interaction between Sb(III) and birnessite, including Sb(III) oxidation, followed by destruction of birnessite and then phase transformation into vernadite. Furthermore, the increase of iron content doped into birnessite enhanced the rate of its phase transition, which led to an increased adsorption of the oxidized antimony on the surface of vernadite by substituting iron and manganese associated with hydroxyl group. This work suggested that the strong interactions between heavy metal ions and mineral particles, more than adsorption, are critical to the transformation, mobility and biotoxicity of antimony in nature.

Mandelic acid and phenyllactic acid "Reaction Sets" for exploring the kinetics and mechanism of oxidations by hydrous manganese oxide (HMO)

At pH 4.0, hydrous manganese oxide (HMO) oxidizes mandelic acid by two mole-equivalents of electrons, yielding phenylglyoxylic acid and benzaldehyde. These intermediates, in turn, are oxidized by two mole-equivalents of electrons to the same ultimate oxidation product, benzoic acid. The four compounds of the "reaction set" just described are conveniently monitored using capillary electrophoresis (CE) and HPLC. Extents of adsorption are negligible and their sum exhibits mass balance. Concentrations of phenylglyoxylic acid, benzaldehyde, and benzoic acid can therefore be used to calculate mole-equivalents delivered to HMO for comparison with experimentally-determined dissolved MnII concentrations. Semi-log plots (ln[substrate] versus time) and numerical analysis can also be used to explore rates of oxidation of the functional groups represented, i.e. an α-hydroxycarboxylic acid, an α-ketocarboxylic acid, and an aldehyde. Inserting a -CH2- group between the benzene ring and the functional groups just described yields a new reaction set comprised of phenyllactic acid, phenylpyruvic acid, and phenylacetaldehyde, plus the C-1 ultimate oxidation product, phenylacetic acid. At pH 4, mass balance for phenyllactic acid oxidation fell short by ∼9%. Phenyllactic acid was oxidized 2.7-times more slowly than mandelic acid, while phenylpyruvic acid was oxidized 12.7-times faster than phenylglyoxylic acid. Unlike benzaldehyde, oxidation rates for phenylacetaldehyde were too fast to measure. Under pH 4.0 conditions, this reaction set approach was used to explore the acceleratory effects of increases in HMO loading and inhibitory effects of 500 μM phosphate and pyrophosphate additions. Mandelic acid and phenyllactic acid were oxidized by HMO far more slowly at pH 7.0 than at pH 4.0. At pH 7.0, 2 mM MOPS and phosphate buffers did not yield appreciable dissolved MnII, despite oxidation of organic substrate. 2 mM pyrophosphate, in contrast, solubilized HMO-bound MnII and MnIII.

Geochemical Stability of Dissolved Mn(III) in the Presence of Pyrophosphate as a Model Ligand: Complexation and Disproportionation

Dissolved Mn(III) species have recently been recognized as a significant form of Mn in redox transition zones, but their speciation, stability, and reactivity are poorly understood. Besides acting as the intermediate for Mn redox chemistry, Mn(III) can undergo disproportionation producing insoluble Mn oxides and aqueous Mn(II). Using pyrophosphate(PP) as a model ligand, we evaluated the thermodynamic and kinetic stability of Mn(III) complexes. They were stable at circumneutral pH and were prone to (partial) disproportionation at acidic or basic pH. With an initial lag phase, the kinetics of Mn(III)-PP disproportionation was autocatalytic with the produced Mn oxides promoting the disproportionation. X-ray diffraction and the average Mn oxidation state indicated that the solid products were not pure Mn(IV) oxides but a mixture of triclinic birnessite and δ-MnO₂. Addition of synthetic analogs of the precipitates eliminated the lag phase, confirming their catalytic roles. Thermodynamic calculations adequately predicted the stability regime of Mn(III)-PP. The present results refined the constant for Mn(PP)₂^5- formation, which allows a consistent and quantitative prediction of equilibrium speciation of Mn(III)-Mn(II)-birnessite with PP. A simple and robust model, which incorporated the thermodynamic constraints, autocatalytic rate law, and verified reaction stoichiometry, successfully simulated all kinetic data.

Biogenic and Synthetic MnO2 Nanoparticles: Size and Growth Probed with Absorption and Raman Spectroscopies and Dynamic Light Scattering

MnO₂ nanoparticles, similar to those found in soils and sediments, have been characterized via their UV-visible and Raman spectra, combined with dynamic light scattering and reactivity measurements. Synthetic colloids were prepared by thiosulfate reduction of permanganate, their sizes controlled with adsorbates acting as capping agents: bicarbonate, phosphate, and pyrophosphate. Biogenic colloids, products of the manganese oxidase, Mnx, were similarly characterized. The band-gap energies of the colloids were found to increase with decreasing hydrodynamic diameter, D_h, and were proportional to 1/ D_h², as predicted from quantum confinement theory. The intensity ratio of the two prominent Mn-O stretching Raman bands also varied with particle size, consistent with the ratio of edge to bulk Mn atoms. Reactivity of the synthetic colloids toward reduction by Mn²⁺, in the presence of pyrophosphate to trap the Mn³⁺ product, was proportional to the surface to volume ratio, but showed surprising complexity. There was also a remnant unreactive fraction, likely attributable to Mn(III)-induced surface passivation. The band gap was similar for biogenic and synthetic colloids of similar size, but decreased when the enzyme solution contained pyrophosphate, which traps the intermediate Mn(III) and slows MnO₂ growth. The band gap/size correlation was used to analyze the growth of the enzymatically produced MnO₂ oxides.

Natural Attenuation of Mn(II) in Metal Refinery Wastewater: Microbial Community Structure Analysis and Isolation of a New Mn(II)-Oxidizing Bacterium Pseudomonas sp. SK3

Natural attenuation of Mn(II) was observed inside the metal refinery wastewater pipeline, accompanying dark brown-colored mineralization (mostly MnIVO2 with some MnIII2O3 and Fe2O3) on the inner pipe surface. The Mn-deposit hosted the bacterial community comprised of Hyphomicrobium sp. (22.1%), Magnetospirillum sp. (3.2%), Geobacter sp. (0.3%), Bacillus sp. (0.18%), Pseudomonas sp. (0.03%), and non-metal-metabolizing bacteria (74.2%). Culture enrichment of the Mn-deposit led to the isolation of a new heterotrophic Mn(II)-oxidizer Pseudomonas sp. SK3, with its closest relative Ps. resinovorans (with 98.4% 16S rRNA gene sequence identity), which was previously unknown as an Mn(II)-oxidizer. Oxidation of up to 100 mg/L Mn(II) was readily initiated and completed by isolate SK3, even in the presence of high contents of MgSO4 (a typical solute in metal refinery wastewaters). Additional Cu(II) facilitated Mn(II) oxidation by isolate SK3 (implying the involvement of multicopper oxidase enzyme), allowing a 2-fold greater Mn removal rate, compared to the well-studied Mn(II)-oxidizer Ps. putida MnB1. Poorly crystalline biogenic birnessite was formed by isolate SK3 via one-electron transfer oxidation, gradually raising the Mn AOS (average oxidation state) to 3.80 in 72 h. Together with its efficient in vitro Mn(II) oxidation behavior, a high Mn AOS level of 3.75 was observed with the pipeline Mn-deposit sample collected in situ. The overall results, including the microbial community structure analysis of the pipeline sample, suggest that the natural Mn(II) attenuation phenomenon was characterized by robust in situ activity of Mn(II) oxidizers (including strain SK3) for continuous generation of Mn(IV). This likely synergistically facilitated chemical Mn(II)/Mn(IV) synproportionation for effective Mn removal in the complex ecosystem established in this artificial pipeline structure. The potential utility of isolate SK3 is illustrated for further industrial application in metal refinery wastewater treatment processes.

Antimony oxidation and sorption behavior on birnessites with different properties (δ-MnO2 and triclinic birnessite)

Birnessites are abundant naturally occurring minerals with high sorption and oxidation capacity that could therefore play an important role in antimony (Sb) migration and transformation. There are various types of birnessites in the environment. However, little is known about the similarities and differences in Sb oxidation and sorption on birnessites with different properties. In this study, the behavior of Sb oxidation and sorption on two contrasting birnessites (δ-MnO₂ and triclinic birnessite (TrBir)) were investigated via batch and kinetic experiments and various spectroscopic techniques. Our results showed that the reaction mechanisms between Sb and the two birnessites were similar. The edge sites of birnessites were responsible for Sb(III) oxidation. Mn(IV) was reduced to Mn(III) and Mn(II), bound with birnessites and released to the solution, respectively. Because of the rapid rate of electron transfer of adsorbed Sb(III) to birnessites, the only Sb species on δ-MnO₂ after the oxidation reaction was Sb(V). Sb(V) was adsorbed at the edge sites of birnessites by replacing the OH group of birnessites, forming corner-sharing complexes with birnessites. However, the Sb sorption and oxidation capacities of the two birnessites were significantly different. Poorly-crystallized δ-MnO₂ exhibited a much higher oxidation and sorption capacity than well-crystallized TrBir because the former had many more edge sites than the latter. This study reveals the general mechanism of the reaction between Sb and birnessite and indicates that birnessite with a high number of edge sites would exhibit a huge capacity in Sb oxidation and sorption.

Effect of Manganese Oxide Aging and Structure Transformation on the Kinetics of Thiol Oxidation

The kinetics and mechanism of thiol oxidation by Mn oxides undergoing dynamic structural transformation under environmentally relevant conditions remain poorly understood. In this study, thiol/disulfide pair concentrations were simultaneously determined in situ using voltammetric microelectrodes during the interaction of four common thiols (cysteine, homocysteine, cysteamine, and glutathione) with fresh and aged δ-MnO₂ at pH 7.0. The reaction kinetics was first order with respect to thiol and zero order with respect to Mn oxides. A transient intermediate sulfur surface species observed during the reaction provides evidence for a mechanism involving two successive one-electron transfer steps. The reaction kinetics of fresh and aged δ-MnO₂ was investigated with cysteine and compared to that of manganite, a Mn(III) oxyhydroxide phase. The reactivity of aged δ-MnO₂ decreased as a result of structural transformation to cryptomelane but remained higher than that of manganite, suggesting the potential roles of transient Mn(III) surface intermediate in promoting the reduction of Mn(IV) in δ-MnO₂ and cryptomelane (compared to Mn(III) in manganite). This study demonstrates the importance of correlating Mn oxide mineral structure and redox reactivity and extends the potential for thiols commonly found in sedimentary environments to be utilized as electron shuttles during dissimilatory Mn reduction.

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.

A comparison study of the start-up of a MnOx filter for catalytic oxidative removal of ammonium from groundwater and surface water

As an efficient method for ammonium (NH₄⁺) removal, contact catalytic oxidation technology has drawn much attention recently, due to its good low temperature resistance and short start-up period. Two identical filters were employed to compare the process for ammonium removal during the start-up period for ammonium removal in groundwater (Filter-N) and surface water (Filter-S) treatment. Two types of source water (groundwater and surface water) were used as the feed waters for the filtration trials. Although the same initiating method was used, Filter-N exhibited much better ammonium removal performance than Filter-S. The differences in catalytic activity among these two filters were probed using X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and compositional analysis. XRD results indicated that different manganese oxide species were formed in Filter-N and Filter-S. Furthermore, the Mn3p XPS spectra taken on the surface of the filter films revealed that the average manganese valence of the inactive manganese oxide film collected from Filter-S (FS-MnO_x) was higher than in the film collected from Filter-N (FN-MnO_x). Mn(IV) was identified as the predominant oxidation state in FS-MnO_x and Mn(III) was identified as the predominant oxidation state in FN-MnO_x. The results of compositional analyses suggested that polyaluminum ferric chloride (PAFC) used during the surface water treatment was an important factor in the mineralogy and reactivity of MnO_x. This study provides the theoretical basis for promoting the wide application of the technology and has great practical significance.

Structural Transformation of Birnessite by Fulvic Acid under Anoxic Conditions

The structure and Mn(III) concentration of birnessite dictate its reactivity and can be changed by birnessite partial reduction, but effects of pH and reductant/birnessite ratios on the changes by reduction remain unclear. We found that the two factors strongly affect the structure of birnessite (δ-MnO₂) and its Mn(III) content during its reduction by fulvic acid (FA) at pH 4-8 and FA/solid mass ratios of 0.01-10 under anoxic conditions over 600 h. During the reduction, the structure of δ-MnO₂ is increasingly accumulated with both Mn(III) and Mn(II) but much more with Mn(III) at pH 8, whereas the accumulated Mn is mainly Mn(II) with little Mn(III) at pH 4 and 6. Mn(III) accumulation, either in layers or over vacancies, is stronger at higher FA/solid ratios. At FA/solid ratios ≥1 and pH 6 and 8, additional hausmannite and MnOOH phases form. The altered birnessite favorably adsorbs FA because of the structural accumulation of Mn(II, III). Like during microbially mediated oxidative precipitation of birnessite, the dynamic changes during its reduction are ascribed to the birnessite-Mn(II) redox reactions. Our work suggests low reactivity of birnessite coexisting with organic matter and severe decline of its reactivity by partial reduction in alkaline environment.

Recrystallization of Manganite (γ-MnOOH) and Implications for Trace Element Cycling

The recrystallization of Mn(III,IV) oxides is catalyzed by aqueous Mn(II) (Mn(II)_aq) during (bio)geochemical Mn redox cycling. It is poorly understood how trace metals associated with Mn oxides (e.g., Ni) are cycled during such recrystallization. Here, we use X-ray absorption spectroscopy (XAS) to examine the speciation of Ni associated with Manganite (γ-Mn(III)OOH) suspensions in the presence or absence of Mn(II)_aq under variable pH conditions (pH 5.5 and 7.5). In a second set of experiments, we used a ⁶²Ni isotope tracer to quantify the amount of dissolved Ni that exchanges with Ni incorporated in the Manganite crystal structure during reactions in 1 mM Mn(II)_aq and in Mn(II)-free solutions. XAS spectra show that Ni is initially sorbed on the Manganite mineral surface and is progressively incorporated into the mineral structure over time (13% after 51 days) even in the absence of dissolved Mn(II). The amount of Ni incorporation significantly increases to about 40% over a period of 51 days when Mn(II)_aq is present in solution. Similarly, Mn(II)_aq promotes Ni exchange between Ni-substituted Manganite and dissolved Ni(II), with around 30% of Ni exchanged at pH 7.5 over the duration of the experiment. No new mineral phases are detected following recrystallization as determined by X-ray diffraction and XAS. Our results reveal that Mn(II)-catalyzed mineral recrystallization partitions Ni between Mn oxides and aqueous fluids and can therefore affect Ni speciation and mobility in the environment.