Reduction of Colloidal Manganese Dioxide by Manganese(II)

Direct observation of Mn distribution/speciation within and surrounding a basidiomycete fungus in the production of Mn-oxides important in toxic element containment

Biogenic manganese (Mn) oxides occur ubiquitously in the environment including the uranium (U) mill tailings at the Ningyo-toge U mine in Okayama, Japan, being important in the sequestration of radioactive radium. To understand the nanoscale processes in Mn oxides formation at the U mill tailings site, Mn²⁺ absorption by a basidiomycete fungus, Coprinopsis urticicola, isolated from Ningyo-toge mine water samples, was investigated in the laboratory under controlled conditions utilizing electron microscopy, synchrotron-based X-ray analysis, and fluorescence microscopy with a molecular pH probe. The fungus' growth was first investigated in an agar-solidified medium supplemented with 1.0 mmol/L Mn²⁺, and Cu²⁺ (0-200 μM), Zn²⁺ (0-200 μM), or diphenyleneiodonium (DPI) chloride (0-100 μM) at 25 °C. The results revealed that Zn²⁺ has no significant effects on Mn oxide formation, whereas Cu²⁺ and DPI significantly inhibit both fungal growth and Mn oxidation, indicating superoxide-mediated Mn oxidation. Indeed, nitroblue tetrazolium and diaminobenzidine assays on the growing fungus revealed the production of superoxide and peroxide. During the interaction of Mn²⁺ with the fungus in solution medium at the initial pH of 5.67, a small fraction of Mn²⁺ infiltrated the fungal hyphae within 8 h, forming a few tens of nm-sized concentrates of soluble Mn²⁺ in the intracellular pH of ∼6.5. After 1 day of incubation, Mn oxides began to precipitate on the hyphae, which were characterized as fibrous nanocrystals with a hexagonal birnessite-structure, these forming spherical aggregates with a diameter of ∼1.5 μm. These nanoscale processes associated with the fungal species derived from the Ningyo-toge mine area provide additional insights into the existing mechanisms of Mn oxidation by filamentous fungi at other U mill tailings sites under circumneutral pH conditions. Such processes add to the class of reactions important to the sequestration of toxic elements.

Oxidative transformation of emerging organic contaminants by aqueous permanganate: Kinetics, products, toxicity changes, and effects of manganese products

Permanganate (Mn(VII)) has been widely studied for removal of emerging organic contaminants (EOCs) in water treatment and in situ chemical oxidation process. Studies on the reactive intermediate manganese products (e.g., Mn(III) and manganese dioxide (MnO₂)) generated from Mn(VII) reduction by EOCs in recent decades shed new light on Mn(VII) oxidation process. The present work summarizes the latest research findings on Mn(VII) reactions with a wide range of EOCs (including phenols, olefins, and amines) in detailed aspects of reaction kinetics, oxidation products, and toxicity changes, along with special emphasis on the impacts of intermediate manganese products (mainly Mn(III) and MnO₂) in-situ formed. Mn(VII) shows appreciable reactivities towards EOCs with apparent second-order rate constants (k_app) generally decrease in the order of olefins (k_app = 0.3 - 2.1 × 10⁴ M^-1s^-1) > phenols (k_app = 0.03 - 460 M^-1s^-1) > amines (k_app = 3.5 × 10^-3 - 305.3 M^-1s^-1) at neutral pH. Phenolic benzene ring (for phenols), (conjugated) double bond (for olefins), primary amine group and the N-containing heterocyclic ring (for amines) are the most reactive sites towards Mn(VII) oxidation, leading to the formation of products with different structures (e.g., hydroxylated, aldehyde, carbonyl, quinone-like, polymeric, ring-opening, nitroso/nitro and C-N cleavage products). Destruction of functional groups of EOCs (e.g., benzene ring, (conjugated) double bond, and N-containing heterocyclic) by Mn(VII) tends to decrease solution toxicity, while oxidation products with higher toxicity than parent EOCs (e.g., quinone-like products in the case of phenolic EOCs) are sometimes formed. Mn(III) stabilized by model or unknown ligands remarkably accelerates phenolic EOCs oxidation by Mn(VII) under acidic to neutral conditions, while MnO₂ enhances the oxidation efficiency of phenolic and amine EOCs by Mn(VII) at acidic pH. The intermediate manganese products participate in Mn(VII) oxidation process most likely as both oxidants and catalysts with their generation/stability/reactivity affecting by the presence of NOM, ligand, cations, and anions in water matrices. This work presents the state-of-the-art findings on Mn(VII) oxidation of EOCs, especially highlights the significant roles of manganese products, which advances our understanding on Mn(VII) oxidation and its application in future water treatment processes.

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.

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.

A chemiluminescence reaction consisting of manganese(IV), sodium sulfite, and sulfur- and nitrogen-doped carbon quantum dots, and its application for the determination of oxytetracycline

Sulfur- and nitrogen-doped carbon quantum dots (S,N-CQDs) were prepared by a solid-phase hydrothermal method from cysteine and citric acid and characterized by transmission electron microscopy, energy-dispersive X-ray spectroscopy, and FTIR spectroscopy. These QDs were exploited as enhancers to amplify the chemiluminescence (CL) of manganese(IV)-sodium sulfite reaction. S,N-CQDs exceptionally enhanced the CL intensity of this system, around 900-fold. This effect was attributed to the energy transfer from SO₂^*, produced by reaction of Mn(IV) with SO₃^2-, to S,N-CQDs. The maximum wavelength of CL emission was 480 nm, which confirmed that the final emitting species was S,N-CQDs. After optimization of reaction conditions, the analytical applicability of S,N-CQD-Mn(IV)-SO₃^2- CL system was studied. In the presence of oxytetracycline, the CL intensity was significantly diminished. A linear relationship was observed between CL signal and the logarithm of oxytetracycline concentration in the range of 0.075-3.0 μM with a detection limit of 25 nM. This CL assay for oxytetracycline was used for analysis of spiked milk and water samples. Graphical abstractSchematic representation of the amplified chemiluminescence (CL) reaction consisting of sulfur- and nitrogen-doped carbon quantum dots (S,N-CQDs) Mn(IV) and Na2SO3. Sub-micromolar levels of oxytetracycline can be determined by using this system.

Influence of Pyrophosphate on the Generation of Soluble Mn(III) from Reactions Involving Mn Oxides and Mn(VII)

The detection of soluble Mn(III) is typically accomplished using strong complexing agents to trap Mn(III), but the generation of soluble Mn(III) induced by strong complexing agents has seldom been considered. In this study, pyrophosphate (PP), a nonredox active ligand, was chosen as a typical Mn(III) chelating reagent to study the influence of ligands on soluble Mn(III) formation in reactions involving Mn oxides and Mn(VII). The presence of excess PP induced the generation of soluble Mn(III)-PP from α- and δ-MnO₂ and led to the conproportionation reaction of α-, β-, δ-, or colloidal MnO₂ with Mn(II) at pH 7.0. Compared to MnO₂ minerals, colloidal MnO₂ showed much higher reactivity toward Mn(II) in the presence of PP and the conproportionation rate of colloidal MnO₂ with Mn(II) elevated with increasing PP dosage and decreasing pH. The generation of Mn(III) was not observed in MnO₄^-/S₂O₃^2- or MnO₄^-/NH₃OH⁺ system without PP while the introduction of excess PP induced the generation of Mn(III)-PP. Thermodynamic calculation results were consistent with the experimental observations. These findings not only provide evidence for the unsuitability of using strong ligands in quantification of soluble Mn(III) in manganese-involved redox reactions, but also advance the understanding of soluble Mn(III) generation in aquatic environment.

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.

Degradative treatment of bispyribac sodium herbicide from synthetically contaminated water by colloidal MnO2 dioxide in the absence and presence of surfactants

Bispyribac sodium (BS) is one of the most commonly used herbicides used to kill selectively unwanted herbs particularly in rice plantation. However, the increasing use of herbicides in agricultural field is associated with a potential risk to water resources and aquatic system. Thus, the treatment of such pesticides after fulfillment of their herbicidal activity is of quite interest to minimize the contamination of water. The degradation kinetics of BS from synthetic contaminated water by water-soluble colloidal MnO₂ in acidic medium (HClO₄) has been studied spectrophotometrically in the absence and presence of different surfactants. The degradation has been observed to be fractionally ordered in both BS and HClO₄ under pseudo-first-order reaction condition with respect to MnO₂. The anionic surfactant (sodium dodecyl sulfate) has been observed to be ineffective whereas the cationic surfactant (cetyltrimethyl ammonium bromide) causes flocculation with oppositely charged colloidal MnO₂ and therefore could not be studied further. However, the non-ionic surfactant (Triton X-100) has been observed to accelerate the reaction rate. The catalytic effect of this surfactant has been analyzed and discussed in the light of the available mathematical model. The kinetic data have been used to generate the various activation parameters accompanying the degradation process of BS in the absence and presence of the non-ionic surfactant, Triton X-100.

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.

Mn(II) Oxidation by the Multicopper Oxidase Complex Mnx: A Binuclear Activation Mechanism

The bacterial protein complex Mnx contains a multicopper oxidase (MCO) MnxG that, unusually, catalyzes the two-electron oxidation of Mn(II) to MnO₂ biomineral, via a Mn(III) intermediate. Although Mn(III)/Mn(II) and Mn(IV)/Mn(III) reduction potentials are expected to be high, we find a low reduction potential, 0.38 V (vs Normal Hydrogen Electrode, pH 7.8), for the MnxG type 1 Cu²⁺, the electron acceptor. Indeed the type 1 Cu²⁺ is not reduced by Mn(II) in the absence of molecular oxygen, indicating that substrate oxidation requires an activation step. We have investigated the enzyme mechanism via electronic absorption spectroscopy, using chemometric analysis to separate enzyme-catalyzed MnO₂ formation from MnO₂ nanoparticle aging. The nanoparticle aging time course is characteristic of nucleation and particle growth; rates for these processes followed expected dependencies on Mn(II) concentration and temperature, but exhibited different pH optima. The enzymatic time course is sigmoidal, signaling an activation step, prior to turnover. The Mn(II) concentration and pH dependence of a preceding lag phase indicates weak Mn(II) binding. The activation step is enabled by a pK_a > 8.6 deprotonation, which is assigned to Mn(II)-bound H₂O; it induces a conformation change (consistent with a high activation energy, 106 kJ/mol) that increases Mn(II) affinity. Mnx activation is proposed to decrease the Mn(III/II) reduction potential below that of type 1 Cu(II/I) by formation of a hydroxide-bridged binuclear complex, Mn(II)(μ-OH)Mn(II), at the substrate site. Turnover is found to depend cooperatively on two Mn(II) and is enabled by a pK_a 7.6 double deprotonation. It is proposed that turnover produces a Mn(III)(μ-OH)₂Mn(III) intermediate that proceeds to the enzyme product, likely Mn(IV)(μ-O)₂Mn(IV) or an oligomer, which subsequently nucleates MnO₂ nanoparticles. We conclude that Mnx exploits manganese polynuclear chemistry in order to facilitate an otherwise difficult oxidation reaction, as well as biomineralization. The mechanism of the Mn(III/IV) conversion step is elucidated in an accompanying paper .

Oxidation of arsenite to arsenate on birnessite in the presence of light

The effect of simulated solar radiation on the oxidation of arsenite [As(III)] to arsenate [As(V)] on the layered manganese oxide, birnessite, was investigated. Experiments were conducted where birnessite suspensions, under both anoxic and oxic conditions, were irradiated with simulated solar radiation in the presence of As(III) at pH 5, 7, and 9. X-ray absorption spectroscopy (XAS) was used to determine the nature of the adsorbed product on the surface of the birnessite. The oxidation of As(III) in the presence of birnessite under simulated solar light irradiation occurred at a rate that was faster than in the absence of light at pH 5. At pH 7 and 9, As(V) production was significantly less than at pH 5 and the amount of As(V) production for a given reaction time was the same under dark and light conditions. The first order rate constant (k_obs) for As(III) oxidation in the presence of light and in the dark at pH 5 were determined to be 0.07 and 0.04 h^-1, respectively. The As(V) product was released into solution along with Mn(II), with the latter product resulting from the reduction of Mn(IV) and/or Mn(III) during the As(III) oxidation process. Post-reaction XAS analysis of As(III) exposed birnessite showed that arsenic was present on the surface as As(V). Experimental results also showed no evidence that reactive oxygen species played a role in the As(III) oxidation process.

(54)Mn Radiotracers Demonstrate Continuous Dissolution and Reprecipitation of Vernadite (δ-MnO2) during Interaction with Aqueous Mn(II)

(54)Mn radiotracers were used to assess Mn atom exchange between aqueous Mn(II) and vernadite (δ-MnO2) at pH 5.0. Continuous solid-liquid redistribution of (54)Mn atoms occurred, and systems are near isotopic equilibrium after reaction for 3 months. Despite this extensive exchange, X-ray diffraction and X-ray absorption spectroscopy data showed no major changes in vernadite bulk mineralogy. These results demonstrate that the vernadite-Mn(II) interface is dynamic, with the substrate undergoing continuous dissolution and reprecipitation mediated by aqueous Mn(II) without observable impacts on its mineralogy. Interfacial redox reactions between adsorbed Mn(II) and solid-phase Mn(IV) and Mn(III) are proposed as the main drivers of this process. Interaction between aqueous Mn(II) and structural Mn(III) likely involves interfacial electron transfer coupled with Mn atom exchange. The exchange of aqueous Mn(II) and solid-phase Mn(IV) is more complex and is proposed to result from coupled interfacial comproportionation-disproportionation reactions, where electron transfer from adsorbed Mn(II) to lattice Mn(IV) produces transient Mn(III) species that disproportionate to regenerate aqueous Mn(II) and structural Mn(IV). These findings provide further evidence of the importance of Mn(II)(aq)-MnO2(s) interactions and the attendant production of transient Mn(III) intermediates to the geochemical functioning of phyllomanganates in environments undergoing Mn redox cycling.

Oxygen Isotope Evidence for Mn(II)-Catalyzed Recrystallization of Manganite (γ-MnOOH)

Manganese is biogeochemically cycled between aqueous Mn(II) and Mn(IV) oxides. Aqueous Mn(II) often coexists with Mn(IV) oxides, and redox reactions between the two (e.g., comproportionation) are well known to result in the formation of Mn(III) minerals. It is unknown, however, whether aqueous Mn(II) exchanges with structural Mn(III) in manganese oxides in the absence of any mineral transformation (similar to what has been reported for aqueous Fe(II) and some Fe(III) minerals). To probe whether atoms exchange between a Mn(III) oxide and water, we use a (17)O tracer to measure oxygen isotope exchange between structural oxygen in manganite (γ-MnOOH) and water. In the absence of aqueous Mn(II), about 18% of the oxygen atoms in manganite exchange with the aqueous phase, which is close to the estimated surface oxygen atoms (∼11%). In the presence of aqueous Mn(II), an additional 10% (for a total of 28%) of the oxygen atoms exchange with water, suggesting that some of the bulk manganite mineral (i.e., beyond surface) is exchanging with the fluid. Exchange of manganite oxygen with water occurs without any observable change in mineral phase and appears to be independent of the rapid Mn(II) sorption kinetics. These experiments suggest that Mn(II) catalyzes manganese oxide recrystallization and illustrate a new pathway by which these ubiquitous minerals interact with their surrounding fluid.

A five-year performance review of field-scale, slow-release permanganate candles with recommendations for second-generation improvements

In 2009, we identified a TCE plume at an abandoned landfill that was located in a low permeable silty-clay aquifer. To treat the TCE, we manufactured slow-release potassium permanganate cylinders (oxidant candles) that had diameters of either 5.1 or 7.6 cm and were 91.4 cm long. In 2010, we compared two methods of candle installation by inserting equal masses of the oxidant candles (7.6-cm vs 5.1-cm dia). The 5.1-cm dia candles were inserted with direct-push rods while the 7.6-cm candles were housed in screens and lowered into 10 permanent wells. Since installation, the 7.6-cm oxidant candles have been refurbished approximately once per year by gently scraping off surface oxides. In 2012, we reported initial results; in this paper, we provide a 5-yr performance review since installation. Temporal sampling shows oxidant candles placed in wells have steadily reduced migrating TCE concentrations. Moreover, these candles still maintain an inner core of oxidant that has yet to contribute to the dissolution front and should provide several more years of service. Oxidant candles inserted by direct-push have stopped reducing TCE concentrations because a MnO2 scale developed on the outside of the candles. To counteract oxide scaling, we fabricated a second generation of oxidant candles that contain sodium hexametaphosphate. Laboratory experiments (batch and flow-through) show that these second-generation permanganate candles have better release characteristics and are less prone to oxide scaling. This improvement should reduce the need to perform maintenance on candles placed in wells and provide greater longevity for candles inserted by direct-push.

Degradation and Isotope Source Tracking of Glyphosate and Aminomethylphosphonic Acid

Glyphosate [N-(phosphonomethyl) glycine], an active ingredient of the herbicide Roundup, and its main metabolite, aminomethylphosphonic acid (AMPA), have been frequently reported to be present in soils and other environments and thus have heightened public concerns on their potential adverse effects. Understanding the fate of these compounds and differentiating them from other naturally occurring compounds require a toolbox of methods that can go beyond conventional methods. Here, we applied individual isotope labeling technique whereby each compound or mineral involved in the glyphosate and AMPA degradation reaction was either synthesized or chosen to have distinct (18)O/(16)O ratios so that the source of incorporated oxygen in the orthophosphate generated and corresponding isotope effect during C-P bond cleavage could be identified. Furthermore, we measured original isotope signatures of a few commercial glyphosate sources to identify their source-specific isotope signatures. Our degradation kinetics results showed that the rate of glyphosate degradation was higher than that of AMPA in all experimental conditions, and both the rate and extent of degradation were lowest under anoxic conditions. Oxygen isotope ratios (δ(18)OP) of orthophosphate generated from glyphosate and AMPA degradation suggested that one external oxygen atom from ambient water, not from dissolved oxygen or mineral, was incorporated into orthophosphate with the other three oxygen atoms inherited from the parent molecule. Interestingly, δ(18)OP values of all commercial glyphosate products studied were found to be the lightest among all orthophosphates known so far. Furthermore, isotope composition was found to be unaffected due to variable degradation kinetics, light/dark, and oxic/anoxic conditions. These results highlight the importance of phosphate oxygen isotope ratios as a nonconventional tool to potentially distinguish glyphosate sources and products from other organophosphorus compounds and orthophosphate in the environment.

Oxidative degradation of l-histidine by manganese dioxide (MnO2) nano-colloid in HClO4 medium with/without using TX-100 catalyst: a kinetic approach

The kinetics of the oxidative degradation of l-histidine (His) in perchloric acid medium by freshly prepared manganese dioxide (MnO₂) nano-colloid have been investigated with and without using TX-100 catalyst.

Biocompatible natural sugar-based surfactant assisted oxidation of citric acid by MnO4− in absence and presence of SDS

Crocin, a natural carotenoid with antioxidant properties, was used in the present investigation as a surfactant in the citric acid–MnO₄⁻ redox system for the first time.

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

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

A Mn-54 radiotracer study of Mn isotope solid-liquid exchange during reductive transformation of vernadite (δ-MnO2) by aqueous Mn(II)

We employed Mn-54 radiotracers to characterize the extent and dynamics of Mn atom exchange between aqueous Mn(II) and vernadite (δ-Mn(IV)O2) at pH 7.5 under anoxic conditions. Exchange of Mn atoms between the solid and liquid phase is rapid, reaching dynamic equilibrium in 2-4 days. We propose that during the initial stages of reaction, Mn atom exchange occurs through consecutive comproportionation-disproportionation reactions where interfacial electron transfer from adsorbed Mn(II) to lattice Mn(IV) generates labile Mn(III) cations that rapidly disproportionate to reform aqueous Mn(II) and solid-phase Mn(IV). Following nucleation of Mn(III)OOH phases, additional exchange likely occurs through electron transfer from aqueous Mn(II) to solid-phase Mn(III). Our results provide evidence for the fast and extensive production of transient Mn(III) species at the vernadite surface upon contact of this substrate with dissolved Mn(II). We further show that HEPES buffer is a reductant of lattice Mn(IV) in the vernadite structure in our experiments. The methods and results presented here introduce application of Mn-54 tracers as a facile tool to further investigate the formation kinetics of labile Mn(III) surface species and their impacts on Mn-oxide structure and reactivity over a range of environmentally relevant geochemical conditions.

Influence of pH on the reductive transformation of birnessite by aqueous Mn(II)

We investigated the effect of pH (5.5-8.5) on the mineralogical transformation of hexagonal birnessite induced by reaction with aqueous Mn(II) (50-2200 μM), using batch sorption experiments, X-ray diffraction analyses, X-ray absorption and infrared spectroscopic measurements. Samples reacted at pH < 7.0 exhibited disrupted stacking of birnessite sheets, but no mineralogical transformation products were observed. At pH 7.0 and 7.5, reaction with Mn(II) under anoxic conditions caused reductive transformation of birnessite into manganite (γ-MnOOH), whereas at pH 8.0 and 8.5, conversion into hausmannite (Mn3O4) occurred. Feitknechtite (β-MnOOH) is a major transformation product at low Mn(II) inputs at pH 7.0-8.5, and represents a metastable reaction intermediate that is converted into manganite and possibly hausmannite during further reaction with Mn(II). Thermodynamic calculations suggest that conversion into hausmannite at alkaline pH reflects a kinetic effect where rapid hausmannite precipitation prevents formation of thermodynamically more favorable manganite. In oxic systems, feitknechtite formation due to surface catalyzed oxidation of Mn(II) by O2 increases Mn(II) removal relative to anoxic systems at pH ≥ 7. The results of this study suggest that aqueous Mn(II) is an important control on the mineralogy and reactivity of natural Mn-oxides, particularly in aqueous geochemical environments with neutral to alkaline pH values.

Reductive transformation of birnessite by aqueous Mn(II)

Reaction of aqueous Mn(II) with hexagonal birnessite at pH 7.5 causes reductive transformation of birnessite into feitknechtite (β-Mn(III)OOH) and manganite (γ-Mn(III)OOH) through interfacial electron transfer from adsorbed Mn(II) to structural Mn(IV) atoms and arrangement of product Mn(III) into MnOOH, summarized by Mn(II) + Mn(IV)O(2) + 2 H(2)O → 2 Mn(III)OOH + 2 H(+). Feitknechtite is the initial transformation product, and subsequently converted into the more stable manganite polymorph during ongoing reaction with Mn(II). Feitknechtite production is observed at Mn(II) concentrations 2 orders of magnitude below thermodynamic thresholds, reflecting uncertainty in thermodynamic data of Mn-oxide minerals and/or specific interactions between Mn(II) and birnessite surface sites facilitating electron exchange. Under oxic conditions, feitknechtite formation through surface-catalyzed oxidation of Mn(II) by O(2) leads to additional Mn(II) removal from solution relative to anoxic systems. These results indicate that Mn(II) may be an important moderator of the reductive arm of Mn-oxide redox cycling, and suggest a controlling role of Mn(II) in regulating the solubility and speciation of phyllomanganate-reactive metal pollutants including Co, Ni, As, and Cr in geochemical environments.

Arsenite oxidation by a poorly crystalline manganese-oxide 1. Stirred-flow experiments

Manganese-oxides (Mn-oxides) are quite reactive, with respect to arsenite (As(III)) oxidation. However, studies regarding the pathways of As(III) oxidation, over a range of time scales, by poorly crystalline Mn-oxides, are lacking. In stirred-flow experiments, As(III) oxidation by δ-MnO₂ (a poorly crystalline form of hexagonal birnessite) is initially rapid but slows appreciably after several hours of reaction. Mn(II) is the only reduced product of δ-MnO₂ formed by As(III) oxidation during the initial, most rapid phase of the reaction. There seems to be evidence that the formation of Mn(III) observed in previous studies is a result of conproportionation of Mn(II) sorbed onto Mn(IV) reaction sites rather than from direct reduction of Mn(IV) by As(III).The only evidence of arsenic (As) sorption during As(III) oxidation by δ-MnO₂ is during the first 10 h of reaction, and As sorption is greater when As(V) and Mn(II) occur simultaneously in solution. Our findings indicate that As(III) oxidation by poorly crystalline δ-MnO₂ involves several simultaneous reactions and reinforces the importance of studying reaction mechanisms over time.

Cation effects on the layer structure of biogenic Mn-oxides

Biologically catalyzed Mn(II) oxidation produces biogenic Mn-oxides (BioMnO(x)) and may serve as one of the major formation pathways for layered Mn-oxides in soils and sediments. The structure of Mn octahedral layers in layered Mn-oxides controls its metal sequestration properties, photochemistry, oxidizing ability, and topotactic transformation to tunneled structures. This study investigates the impacts of cations (H(+), Ni(II), Na(+), and Ca(2+)) during biotic Mn(II) oxidation on the structure of Mn octahedral layers of BioMnO(x) using solution chemistry and synchrotron X-ray techniques. Results demonstrate that Mn octahedral layer symmetry and composition are sensitive to previous cations during BioMnO(x) formation. Specifically, H(+) and Ni(II) enhance vacant site formation, whereas Na(+) and Ca(2+) favor formation of Mn(III) and its ordered distribution in Mn octahedral layers. This study emphasizes the importance of the abiotic reaction between Mn(II) and BioMnO(x) and dependence of the crystal structure of BioMnO(x) on solution chemistry.

Enhanced removal of organics by permanganate preoxidation using tannic acid as a model compound--role of in situ formed manganese dioxide

The effect of permanganate preoxidation on organic matter removal during the coagulation with aluminum chloride was investigated using tannic acid as a model compound. Results showed that a small amount of KMnO4 (0.75 mg/L) increased the removal efficiency of tannic acid up to 20%, as compared to the process of coagulation by aluminum chloride alone. The key factor enhancing the removal efficiency of tannic acid in preoxidation process was the in situ formation of a reductant manganese dioxide. The complexation model was used to describe the reaction between MnO2 and tannic acid. Under weak pH condition, tannic acid was difficult to be adsorbed by MnO2 due to the static electrical repulsive forces. The presence of Ca2+ served as a bridge to hold the negative charged MnO2 and tannic acid together, which could be a crucial factor influencing tannic acid adsorption by in-situ manganese dioxide.

Oxidation of gum arabic by soluble colloidal MnO2

In the present work, the oxidative degradation of gum arabic by colloidal manganese dioxide (MnO2) was carried out. Monitoring the disappearance of the MnO2 spectrophotometrically at 375 nm was used to follow the kinetics. The oxidation obeyed fractional-order kinetics with respect to the [gum arabic]. Effect of various experimental parameters such as the initial colloidal [MnO2], [HClO4], temperature, and complexing agents (P2O7(4-), F-, and Mn2+) for the oxidation of gum arabic was studied. The reaction was acid catalyzed. Addition of P2O(7)4-, F-, and Mn2+ ions enhances the rate of oxidation significantly. Gum arabic adsorbs onto the surface of the colloidal MnO2 through the equatorial -OH groups of the rhamnose moiety, and the complex breaks down into products. The Arrhenius equation was valid for the oxidation kinetics between 40 and 60 degrees C. To explain the observed kinetic results, a suitable mechanism and rate law for the reaction taking place at the surface of the colloidal particle has been proposed. The reducing nature of gum arabic is found be due to the presence of -OH group in the skeleton.

Spatially resolved characterization of biogenic manganese oxide production within a bacterial biofilm

Pseudomonas putida strain MnB1, a biofilm-forming bacterial culture, was used as a model for the study of bacterial Mn oxidation in freshwater and soil environments. The oxidation of aqueous Mn+2 [Mn+2(aq)] by P. putida was characterized by spatially and temporally resolving the oxidation state of Mn in the presence of a bacterial biofilm, using scanning transmission X-ray microscopy (STXM) combined with near-edge X-ray absorption fine structure (NEXAFS) spectroscopy at the Mn L2,3 absorption edges. Subsamples were collected from growth flasks containing 0.1 and 1 mM total Mn at 16, 24, 36, and 48 h after inoculation. Immediately after collection, the unprocessed hydrated subsamples were imaged at a 40-nm resolution. Manganese NEXAFS spectra were extracted from X-ray energy sequences of STXM images (stacks) and fit with linear combinations of well-characterized reference spectra to obtain quantitative relative abundances of Mn(II), Mn(III), and Mn(IV). Careful consideration was given to uncertainty in the normalization of the reference spectra, choice of reference compounds, and chemical changes due to radiation damage. The STXM results confirm that Mn+2(aq) was removed from solution by P. putida and was concentrated as Mn(III) and Mn(IV) immediately adjacent to the bacterial cells. The Mn precipitates were completely enveloped by bacterial biofilm material. The distribution of Mn oxidation states was spatially heterogeneous within and between the clusters of bacterial cells. Scanning transmission X-ray microscopy is a promising tool for advancing the study of hydrated interfaces between minerals and bacteria, particularly in cases where the structure of bacterial biofilms needs to be maintained.