The interaction of cobalt with hydrous manganese dioxide

Adsorption of cobalt on synthetic hydrous manganese dioxide was studied as a function of pH and surface area in NaCl solutions and solutions containing sea water concentrations of Na, Ca and Mg. The amount of cobalt adsorbed increased sharply at pH 6, a significantly lower pH than that required for significant hydrolysis of Co(II) or precipitation of Co(OH)₂(S) in bulk solution. Sea water concentrations of Na, Ca and Mg have little effect on adsorption until the cobalt concentration is less than 10^?7 M.Micro-electrophoresis experiments from 1 × 10^?3 M to 1 × 10^?5 M to Co(II) show three charge reversals. The first is the pH of zero point charge of hydrous manganese dioxide. The second correlates well with the abrupt increase in adsorption at pH 6 and may reflect both specific adsorption of Co(II) and precipitation of Co(OH)₂ on the surface. The third agrees well with literature values for the pH of zero point of charge of Co(OH)₂.An adsorption isotherm was constructed for cobalt and these data were used to test the hypothesis that the enrichment of cobalt in the suspended matter of the Black Sea is due to adsorption of cobalt from sea water by manganese dioxide. The calculations indicate that adsorption is a feasible explanation for this example.     

Pyrolusite surface transformations measured in real-time during the reactive transport of Co(II)EDTA2−

Oxidation of Co(II)EDTA²⁻ to Co(III)EDTA⁻ by manganese and iron hydrous oxide minerals enhances the transport of ⁶⁰Co in subsurface environments. Until now, reduction of the oxidant MnO₂ has not been identified in hydrodynamic systems, leaving the fate and transport mechanisms involving ⁶⁰Co in natural environments unresolved. We investigated the transport of Co(II)EDTA²⁻ through packed beds of β-MnO₂ and identified the reaction mechanism using a novel hydrodynamic flow cell coupled with X-ray absorption near edge structure (XANES) spectroscopy. Using this technique we are able to determine both solution and solid-phase species of cobalt and manganese in real-time. Co(II)EDTA²⁻ is produced while Mn(IV) is reduced to Mn(III) which forms an α-Mn₂O₃layer on pyrolusite. This layer passivates the surface after an initial reaction period and ultimately limits the production of Co(III)EDTA⁻. As a consequence, the enhanced transport of ⁶⁰Co by oxidative processes may be diminished by continual exposure to pyrolusite—an advantage from an environmental quality perspective. It has also been clarified that Mn(III) is formed rather than Mn(II) resulting in formation of a stable trivalent manganese solid (α-Mn₂O₃).     

The oxidation of cobalt(II) adsorbed on manganese dioxide

X-ray photoelectron spectroscopy (XPS) measurements of cobalt adsorbed on MnO₂ reveal strong evidence that Co(II) has been oxidized to Co(III). The manganese spectra are characteristic of Mn(IV). Model calculations suggest that Co(II) cannot be oxidized by O₂ to Co(III) in bulk solution at seawater concentrations but that the oxidation can proceed in the presence of the strong electric field at the MnO₂-solution interface. Ni(II), however, cannot be oxidized at the interface except at very high concentrations. These calculations suggest that the oxidation of Co(II) can explain the geochemical separation of cobalt from nickel.     

Cobalt(II) sequestration on fungal biogenic manganese oxide enhanced by manganese(II) oxidase activity

We examined the ability of biogenic manganese oxide (BMO) formed in the cultures of a Mn(II) oxidizing fungus, Acremonium strictum strain KR21-2, to sequester Co(II) and found that the newly formed BMO effectively sequestered Co(II) under aerobic conditions with virtually no release of Mn(II). Under anaerobic conditions, smaller amounts of Co(II) were sequestered and a significant amount of Mn(II) was released. Similar trends were observed when the BMOs were poisoned with 50 mM NaN₃ or heated at 85 °C for 1 h. X-ray absorption near-edge structure spectroscopy and two-step extraction confirmed that oxidation of Co(II) to Co(III) occurs with BMOs with higher oxidation efficiency under aerobic conditions. These results demonstrate that BMOs can reoxidize Mn(II) through the Mn(II) oxidase associated with the BMO phase and can subsequently provide a new reaction site for Co sequestration. The ability of BMO to sequester Co(II) was also found to be long lasting in 20 mM 4-(2-hydroxyethyl)-1-piperazineethanesulphonic acid (HEPES) buffer (pH 7.0) containing no nutrients to maintain fungal growth, because sequential treatment of BMOs with the Co(II) solution every 24 h for at least 8 days led to Co(II) sequestration. In addition, Co accumulation in the solid phase was eventually 2.48-fold relative to the accumulation of Mn oxide (molar ratio). X-ray diffraction results suggest that the continuing Co(II) oxidation by newly formed BMOs results in the formation of heterogenite (β-CoOOH) aggregates. Assays using the concentrated Mn(II) oxidase crude solution showed that the preformed Mn oxide phase was important for further Mn(II) oxidation in coexisting Co(II). The fact that the coexisting Co(II) was less inhibitory to Mn(II) oxidation if the preformed Mn oxide phase was present suggests a possible electron path from Co(II) to the final electron acceptor O₂ through BMO and Mn(II) oxidase in BMO/enzyme aggregation. These results suggest that fungal BMOs supporting Mn(II) oxidase activity can serve as an effective Co(II) sequestering material, without the need for additional nutrients.     

Characterization of manganese oxide precipitates from Appalachian coal mine drainage treatment systems

The removal of Mn(II) from coal mine drainage (CMD) by chemical addition/active treatment can significantly increase treatment costs. Passive treatment for Mn removal involves promotion of biological oxidative precipitation of manganese oxides (MnO_x). Manganese(II) removal was studied in three passive treatment systems in western Pennsylvania that differed based on their influent Mn(II) concentrations (20–150 mg/L), system construction (±inoculation with patented Mn(II)-oxidizing bacteria), and bed materials (limestone vs. sandstone). Manganese(II) removal occurred at pH values as low as 5.0 and temperatures as low as 2 °C, but was enhanced at circumneutral pH and warmer temperatures. Trace metals such as Zn, Ni and Co were removed effectively, in most cases preferentially, into the MnO_x precipitates. Based on synchrotron radiation X-ray diffraction and Mn K-edge extended X-ray absorption fine structure spectroscopy, the predominant Mn oxides at all sites were poorly crystalline hexagonal birnessite, triclinic birnessite and todorokite. The surface morphology of the MnO_x precipitates from all sites was coarse and “sponge-like” composed of nm-sized lathes and thin sheets. Based on scanning electron microscopy (SEM), MnO_x precipitates were found in close proximity to both prokaryotic and eukaryotic organisms. The greatest removal efficiency of Mn(II) occurred at the one site with a higher pH in the bed and a higher influent total organic C (TOC) concentration (provided by an upstream wetland). Biological oxidation of Mn(II) driven by heterotrophic activity was most likely the predominant Mn removal mechanism in these systems. Influent water chemistry and Mn(II) oxidation kinetics affected the relative distribution of MnO_x mineral assemblages in CMD treatment systems.     

Oxidation of aqueous Cr(III) at birnessite surfaces: constraints on reaction mechanism

X-ray Photoelectron Spectroscopy (XPS) was used to investigate oxidation of aqueous Cr(III) at the surface of 7 Å-birnessite [MnO_1.75(OH)_0.25]. Special emphasis was placed on detection of intermediate oxidation states of chromium due to their critical environmental significance. No previous studies have been able to identify these intermediate oxidation states of chromium (namely, Cr[IV] and Cr[V]) on mineral surfaces or in natural solutions. Mn(2p_3/2), Cr(2p_3/2) and O(1s) spectra of the reacted surfaces reveal that Mn(IV) of synthetic birnessite undergoes reductive dissolution in two steps. The first step involves Mn(IV) reduction to Mn(III),that forms at the oxide surface probably as an oxyhydroxide (MnOOH), and in the second step Mn(III) is reduced to Mn(II) that is subsequently taken into solution. Each reductive reaction step involves transfer of only one electron to the Mn ion. After Cr(III)_aq is adsorbed onto the MnO₂ surface, it undergoes oxidation in three separate steps, each involving the loss of one electron to Mn ions, so that Cr(IV), Cr(V) and Cr(VI) are produced. The intermediate reaction products, namely Mn(III), and Cr(V) were positively identified by XPS spectral analyses. Similarity in XPS binding energy values of Cr(III) and Cr(IV) as well as that of Cr(V) and Cr(VI), however, preclude separate identification of Cr(III) from Cr(IV) and Cr(VI) from Cr(V) multiplets on the near-surface of the solid. A parallel reaction scheme (exclusive of sorption reactions) best describes the birnessite-Cr(III)_aq redox reactions. The two parallel reactions proceed by separate mechanisms with a monodentate complex formed in one mechanism and a bidentate complex in another. The bulk of Cr(IV) probably is formed via the monodentate complex and Cr(V) via the bidentate complex. The rate expressions associated with these reactions display near-perfect correlation with changing surface abundances of Cr(IV) and Cr(V) as a function of reaction time. Copyright © 1999 Elsevier Science Ltd.     

Reversible surface-sorption-induced electron-transfer oxidation of Fe(II) at reactive sites on a synthetic clay mineral

The sorption of ⁵⁷Fe(II) onto an Fe-free, mineralogically pure and Ca-saturated synthetic montmorillonite sample (structural formula: Ca_0.15(Al_1.4Mg_0.6)(Si₄)O₁₀(OH,F)₂), was studied as a function of pH under strictly anoxic conditions (N₂ glove box atmosphere, O₂ content <1 ppm), using wet chemistry and cryogenic (T = 77 K) ⁵⁷Fe Mössbauer spectrometry. No Fe(III) was detected in solution at any pH. However, in pH conditions where Fe(II) is removed from solution, a significant amount of surface-bound Fe(III) was produced, which increased with pH from 0% to 3% of total Fe in a pre-sorption edge region (i.e. at pH < 7.5 where about 15% of total Fe is sorbed) to 7% of total Fe when all Fe is sorbed. At low pH, where the pre-sorption edge plateau occurs (2 < pH < 7.5), the total sorbed-Fe amount remained constant but, within this sorbed-Fe pool, the Fe(III)/Fe(II) ratio increased with pH, from 0.14 at pH 2 up to 0.74 at pH 7. The pre-sorption edge plateau is interpreted as cation exchange on interlayer surfaces together with a sorption phenomenon occurring on highly reactive (i.e. high affinity) surface sites. As pH increases and protons are removed from the clay edge surface, we propose that more and more of these highly reactive sites acquire a steric configuration that stabilizes Fe(III) relative to Fe(II), thereby inducing a Fe to clay particle electron transfer. A sorption model based on cation exchange combined with surface complexation and electron transfers reproduces both wet chemical as well as the Mössbauer spectrometric results. The mechanism is fully reversible: sorbed-Fe is reduced as pH decreases (Mössbauer solid-state analyses) and all Fe returned to solution is returned as Fe(II) (solution analyses). This would not be the case if the observed oxidations were due to contaminant oxidizing agents in solution. The present work shows that alternating pH may induce surface redox phenomena in the absence of an electron acceptor in solution other than H₂O.     

Permanganate oxidation of arsenic(III): Reaction stoichiometry and the characterization of solid product

Permanganate (MnO₄⁻) has widely been used as an effective oxidant for drinking water treatment systems, as well as for in situ treatment of groundwater impacted by various organic contaminants. The reaction stoichiometry of As(III) oxidation by permanganate has been assumed to be 1.5, based on the formation of solid product, which is putatively considered to be MnO₂(s). This study determined the stoichiometric ratio (SR) of the oxidation reaction with varying doses of As(III) (3-300 μM) and MnO₄⁻ (0.5 or 300 μM) under circumneutral pH conditions (pH 4.5-7.5). We also characterized the solid product that was recovered ∼1 min after the oxidation of 2.16 mM As(III) by 0.97 mM MnO₄⁻ at pH 6.9 and examined the feasibility of secondary heterogeneous As(III) oxidation by the solid product. When permanganate was in excess of As(III), the SR of As(III) to Mn(VII) was 2.07 ± 0.07, regardless of the solution pH; however, it increased to 2.49 ± 0.09 when As(III) was in excess. The solid product was analogous to vernadite, a poorly crystalline manganese oxide based on XRD analysis. The average valence of structural Mn in the solid product corresponded to +III according to the splitting interval of the Mn3s peaks (5.5 eV), determined using X-ray photoelectron spectroscopy (XPS). The relative proportions of the structural Mn(IV):Mn(III):Mn(II) were quantified as 19:62:19 by fitting the Mn2p_3/2 spectrum of the solid with the five multiplet binding energy spectra for each Mn valence. Additionally, the O1s spectrum of the solid was comparable to that of Mn-oxide but not of Mn-hydroxide. These results suggest that the solid product resembled a poorly crystalline hydrous Mn-oxide such as (Mn^II_0.19Mn^III_0.62Mn^IV_0.19)₂O₃·nH₂O, in which Mn(II) and Mn(IV) were presumably produced from the disproportionation of aqueous phase Mn(III). Thermodynamic calculations also show that the formation of Mn(III) oxide is more favorable than that of Mn(IV) oxide from As(III) oxidation by permanganate under circumneutral pH conditions. Arsenic(III), when it remained in the solution after all of the permanganate was consumed, was effectively oxidized by the solid product. This secondary heterogeneous As(III) oxidation consisted of three steps: sorption to and oxidation on the solid surface and desorption of As(V) into solution, with the first step being the rate-limiting process as observed in As(III) oxidation by various Mn (oxyhydr)oxides reported elsewhere. We also discussed a potential reaction pathway of the permanganate oxidation of As(III).     

Manganese(III) binding to a pyoverdine siderophore produced by a manganese(II)-oxidizing bacterium

The possible roles of siderophores (high affinity chelators of iron(III)) in the biogeochemistry of manganese remain unknown. Here we investigate the interaction of Mn(III) with a pyoverdine-type siderophore (PVD_MnB1) produced by the model Mn(II)-oxidizing bacterium Pseudomonas putida strain MnB1. PVD_MnB1 confirmed typical pyoverdine behavior with respect to: (a) its absorption spectrum at 350-600 nm, both in the absence and presence of Fe(III), (b) the quenching of its fluorescence by Fe(III), (c) the formation of a 1:1 complex with Fe(III), and (d) the thermodynamic stability constant of its Fe(III) complex. The Mn(III) complex of PVD_MnB1 had a 1:1 Mn:pvd molar ratio, showed fluorescence quenching, and exhibited a light absorption spectrum (A_max = 408-410 nm) different from that of either PVD_MnB1-Fe(III) or uncomplexed PVD_MnB1. Mn(III) competed strongly with Fe(III) for binding by PVD_MnB1 in culture filtrates (pH 8, 4°C). Equilibration with citrate, a metal-binding ligand, did not detectably release Mn from its PVD_MnB1 complex at a citrate/PVD_MnB1 molar ratio of 830 (pH 8, 4°C), whereas pyrophosphate under the same conditions removed 55% of the Mn from its PVD_MnB1 complex. Most of the PVD_MnB1-complexed Mn was released by reaction with ascorbate, a reducing agent, or with EDTA, a ligand that is also oxidized by Mn(III). Data on the competition for binding to PVD_MnB1 by Fe(III) vs. Mn(III) were used to determine a thermodynamic stability constant (nominally at 4°C) for the neutral species MnHPVD_MnB1 (log K = 47.5 ± 0.5, infinite dilution reference state). This value was larger than that determined for FeHPVD_MnB1 (log K = 44.6 ± 0.5). This result has important implications for the metabolism, solubility, speciation, and redox cycling of manganese, as well as for the biologic uptake of iron.     

Sorption of metals on humic acid

The sorption on humic acid (HA) of metals from an aqueous solution containing Hg(II). Fe(III), Pb, Cu, Al, Ni, Cr(III), Cd, Zn, Co and Mn, was investigated with special emphasis on effects of pH, metal concentration and HA concentration. The sorption efficiency tended to increase with rise in pH, decrease in metal concentration and increase in HA concentration of the equilibrating solution. At pH 2.4. the order of sorption was: Hg? Fe? Pb? CuAl ? Ni ? CrZnCdCoMn. At pH 3.7. the order was: Hg and Fe were always most readily removed, while Co and Mn were sorbed least readily. There were indications of competition for active sites (CO₂H and phenolic OH groups) on the HA between the different metals. We were unable to find correlations between the affinities of the eleven metals to sorb on HA and their atomic weights, atomic numbers, valencies, and crystal and hydrated ionic radii. The sorption of the eleven metals on the HA could be described by the equation $\text{Y =}\text{100}\text{[1 + exp ? (A + BX)]}$, where Y = % metal removed by HA; X = mgHA; and A and B are empirical constants.     

Stable Fe isotope fractionations produced by aqueous Fe(II)-hematite surface interactions

Stable Fe isotope fractionations were investigated during exposure of hematite to aqueous Fe(II) under conditions of variable Fe(II)/hematite ratios, the presence/absence of dissolved Si, and neutral versus alkaline pH. When Fe(II) undergoes electron transfer to hematite, Fe(II) is initially oxidized to Fe(III), and structural Fe(III) on the hematite surface is reduced to Fe(II). During this redox reaction, the newly formed reactive Fe(III) layer becomes enriched in heavy Fe isotopes and light Fe isotopes partition into aqueous and sorbed Fe(II). Our results indicate that in most cases the reactive Fe(III) that undergoes isotopic exchange accounts for less than one octahedral layer on the hematite surface. With higher Fe(II)/hematite molar ratios, and the presence of dissolved Si at alkaline pH, stable Fe isotope fractionations move away from those expected for equilibrium between aqueous Fe(II) and hematite, towards those expected for aqueous Fe(II) and goethite. These results point to formation of new phases on the hematite surface as a result of distortion of Fe-O bonds and Si polymerization at high pH. Our findings demonstrate how stable Fe isotope fractionations can be used to investigate changes in surface Fe phases during exposure of Fe(III) oxides to aqueous Fe(II) under different environmental conditions. These results confirm the coupled electron and atom exchange mechanism proposed to explain Fe isotope fractionation during dissimilatory iron reduction (DIR). Although abiologic Fe(II)_aq - oxide interaction will produce low δ⁵⁶Fe values for Fe(II)_aq, similar to that produced by Fe(II) oxidation, only small quantities of low-δ⁵⁶Fe Fe(II)_aq are formed by these processes. In contrast, DIR, which continually exposes new surface Fe(III) atoms during reduction, as well as production of Fe(II), remains the most efficient mechanism for generating large quantities of low-δ⁵⁶Fe aqueous Fe(II) in many natural systems.     

A specific Ce oxidation process during sorption of rare earth elements on biogenic Mn oxide produced by Acremonium sp. strain KR21-2

Sorption of rare earth elements (REEs) and Ce oxidation on natural and synthetic Mn oxides have been investigated by many researchers. Although Mn(II)-oxidizing microorganisms are thought to play an important role in the formation of Mn oxides in most natural environments, Ce oxidation by biogenic Mn oxide and the relevance of microorganisms to the Ce oxidation process have not been well understood. Therefore, in this study, we conducted sorption experiments of REEs on biogenic Mn oxide produced by Acremonium sp. strain KR21-2. The distribution coefficients, K_d(REE), between biogenic Mn oxide (plus hyphae) and 10 mmol/L NaCl solution showed a large positive Ce anomaly and convex tetrad effect variations at pH 3.8, which was consistent with previous works using synthetic Mn oxide. The positive Ce anomaly was caused by oxidation of Ce(III) to Ce(IV) by the biogenic Mn oxide, which was confirmed by analysis of the Ce L_III-edge XANES spectra. With increasing pH, the positive Ce anomaly and convex tetrad effects became less pronounced. Furthermore, negative Ce anomalies were observed at a pH of more than 6.5, suggesting that Ce(IV) was stabilized in the solution (<0.2 μm) phase, although Ce(III) oxidation to Ce(IV) on the biogenic Mn oxide was confirmed by XANES analysis. It was demonstrated that no Ce(III) oxidation occurred during sorption on the hyphae of strain KR21-2 by the K_d(REE) patterns and XANES analysis. The analysis of size exclusion HPLC-ICP-MS showed that some fractions of REEs in the filtrates (<0.2 μm) after sorption experiments were bound to organic molecules (40 and <670 kDa fractions), which were possibly released from hyphae. A line of our data indicates that the negative Ce anomalies under circumneutral pH conditions arose from Ce(III) oxidation on the biogenic Mn oxide and subsequent complexation of Ce(IV) with organic ligands. The suppression of tetrad effects is also explained by the complexation of REEs with organic ligands. The results of this study demonstrate that the coexistence of the biogenic Mn oxide and hyphae of strain KR21-2 produces a specific redox chemistry which cannot be explained by inorganic species.     

Rapid photo-oxidation of Mn(II) mediated by humic substances

The oxidation of Mn(II) by O₂ to Mn(III) or Mn(IV) is thermodynamically favored under the pH and pO₂ conditions present in most near surface waters, but the kinetics of this reaction are extremely slow. This work investigated whether reactive oxygen species, produced through illumination of humic substances, could oxidize Mn at an environmentally relavent rate. The simulated sunlight illumination of a solution containing 200 μM Mn(II) and 5 mg/L Aldrich humic acid buffered at pH 8.1 produced ∼19 μM of oxidized Mn (MnO_x where x is between one and two) after 45 minutes. The major oxidants reponsible for this reaction appear to be photoproduced superoxide radical anion, O₂⁻, and singlet molecular oxygen, ¹O₂. The dependencies of MnO_x formation on Mn(II), humic acid, and H⁺ concentration were characterized. A kinetic model based largely on published rate constants was established and fit to the experimental data. As expected, analysis of the model indicates that the key reaction rate controlling MnO_x production is the rate of decomposition of a MnO₂⁺ complex formed from the reaction of Mn(II) with O₂⁻. This rate is strongly dependent on the Mn(II) complexing ligands in solution. The MnO_x production in the seawater sample taken from Bodega Bay, USA and spiked with 200 μM Mn(II) was well reproduced by the model. Extrapolations from the model imply that Mn photo-oxidation should be a significant reaction in typical surface seawaters. Calculated rates, 5.8 to 55 pM h⁻¹, are comparable to reported rates of biological Mn oxidation, 0.07 to 89 pM h⁻¹. Four fresh water samples that were spiked with 200 μM Mn(II) also showed significant MnO_x production. Based on these results, it appears that Mn photo-oxidation could constitute a significant, and apparently unrecognized geochemical pathway in natural waters.     

Reduction of Manganese Oxides: Thermodynamic,Kinetic and Mechanistic Considerations for One- Versus Two-Electron Transfer Steps

Manganese oxides, typically similar to δ-MnO₂, form in the aquatic environment at near neutral pH via bacterially promoted oxidation of Mn(II) species by O₂, as the reaction of [Mn(H₂O)₆]²⁺ with O₂ alone is not thermodynamically favorable below pH of ~?9. As manganese oxide species are reduced by the triphenylmethane compound leucoberbelein blue (LBB) to form the colored oxidized form of LBB (λ_max?=?623 nm), their concentration in the aquatic environment can be determined in aqueous environmental samples (e.g., across the oxic–anoxic interface of the Chesapeake Bay, the hemipelagic St. Lawrence Estuary and the Broadkill River estuary surrounded by salt marsh wetlands), and their reaction progress can be followed in kinetic studies. The LBB reaction with oxidized Mn solids can occur via a hydrogen atom transfer (HAT) reaction, which is a one-electron transfer process, but is unfavorable with oxidized Fe solids. HAT thermodynamics are also favorable for nitrite with LBB and MnO₂ with ammonia (NH₃). Reactions are unfavorable for NH₄⁺ and sulfide with oxidized Fe and Mn solids, and NH₃ with oxidized Fe solids. In laboratory studies and aquatic environments, the reduction of manganese oxides leads to the formation of Mn(III)-ligand complexes [Mn(III)L] at significant concentrations even when two-electron reductants react with MnO₂. Key reductants are hydrogen sulfide, Fe(II) and organic ligands, including the siderophore desferioxamine-B. We present laboratory data on the reaction of colloidal MnO₂ solutions (λ_max?~?370 nm) with these reductants. In marine waters, colloidal forms of Mn oxides (<?0.2 µm) have not been detected as Mn oxides are quantitatively trapped on 0.2-µm filters. Thus, the reactivity of Mn oxides with reductants depends on surface reactions and possible surface defects. In the case of MnO₂, Mn(IV) is an inert cation in octahedral coordination; thus, an inner-sphere process is likely for electrons to go into the empty e _g ^* conduction band of its orbitals. Using frontier molecular orbital theory and band theory, we discuss aspects of these surface reactions and possible surface defects that may promote MnO₂ reduction using laboratory and field data for the reaction of MnO₂ with hydrogen sulfide and other reductants.     

Kinetics of Mn(II) oxidation by Leptothrix discophora SS1

The kinetics of Mn(II) oxidation by the bacterium Leptothrix discophora SS1 was investigated in this research. Cells were grown in a minimal mineral salts medium in which chemical speciation was well defined. Mn(II) oxidation was observed in a bioreactor under controlled conditions with pH, O₂, and temperature regulation. Mn(II) oxidation experiments were performed at cell concentrations between 24 mg/L and 35 mg/L, over a pH range from 6 to 8.5, between temperatures of 10°C and 40°C, over a dissolved oxygen range of 0 to 8.05 mg/L, and with L. discophora SS1 cells that were grown in the presence of Cu concentrations ranging from zero to 0.1 μM. Mn(II) oxidation rates were determined when the cultures grew to stationary phase and were found to be directly proportional to O₂ and cell concentrations over the ranges investigated. The optimum pH for Mn(II) oxidation was approximately 7.5, and the optimum temperature was 30°C. A Cu level as low as 0.02 μM was found to inhibit the growth rate and yield of L. discophora SS1 observed in shake flasks, while Cu levels between 0.02 and 0.1 μM stimulated the Mn(II) oxidation rate observed in bioreactors. An overall rate law for Mn(II) oxidation by L. discophora as a function of pH, temperature, dissolved oxygen concentration (D.O.), and Cu concentration is proposed. At circumneutral pH, the rate of biologically mediated Mn(II) oxidation is likely to exceed homogeneous abiotic Mn(II) oxidation at relatively low (≈μg/L) concentrations of Mn oxidizing bacteria.     

X-ray absorption fine structure study of As(V) and Se(IV) sorption complexes on hydrous Mn oxides

X-ray absorption fine structure (XAFS) spectroscopic analysis at the As, Se, and Mn K-edges was used to study arsenate [As(V)O₄³⁻] and selenite [Se(IV)O₃²⁻] sorption complexes on the synthetic hydrous manganese oxides (HMOs) vernadite (δ-MnO₂) and K-birnessite (nominal composition: K₄Mn₁₄O₂₇ · 9H₂O). No significant changes were observed in sorption complex structure as a function of sorbent, pH (5 to 8), surface coverage (0.04 to 0.73 μmol/m²), or reaction time (5 to 22 h) in the arsenate or selenite systems. In the arsenate/HMO system, extended XAFS parameters indicate an average second-neighbor As(V) coordination of 2.0 ± 0.4 Mn at an average distance of 3.16 ± 0.01 Å, which is consistent with formation of As(V)O₄ sorption complexes sharing corners with two adjacent Mn(IV)O₆ surface species (i.e., bidentate, binuclear). In the selenite/HMO system, selenite surface complexes are surrounded by two shells of Mn atoms, which could represent two different adsorption complexes or a precipitate. The first shell consists of 1.6 ± 0.4 Mn at 3.07 ± 0.01 Å, which is consistent with the selenite anion forming bidentate (mononuclear) edge-sharing complexes with Mn(II)O₆ or Mn(III)O₆ octahedra. The second shell consists of 1.4 ± 0.4 Mn at 3.49 ± 0.03 Å, consistent with selenite forming monodentate, corner-sharing complexes with Mn(II)O₆ or Mn(III)O₆ octahedra. Pauling bond valence analysis that uses the extended XAFS-derived bond lengths for As(V)-O, Se(IV)-O, and Mn-O bonds indicates that the proposed surface complexes of selenite and arsenate on HMOs should be stable. Although a nearly identical Se(IV) coordination environment is found in a crystalline Mn(II)-Se(IV) precipitate (which has a structure similar to that of MnSeO₃ · H₂O), there are significant differences in the X-ray absorption near-edge structure and extended XAFS spectra of this precipitate and the selenite/HMO sorption samples. These differences coupled with transmission electron microscopy results suggest that if a precipitate is present it lacks long-range order characteristic of crystalline MnSeO₃ · H₂O.     

Inter-relationships of MnO2 precipitation, siderophore-Mn complex formation, siderophore degradation, and iron limitation in Mn-oxidizing bacterial cultures

To examine the pathways that form Mn^(III) and Mn^(IV) in the Mn^(II)-oxidizing bacterial strains Pseudomonas putida GB-1 and MnB1, and to test whether the siderophore pyoverdine (PVD) inhibits Mn^(IV)O₂ formation, cultures were subjected to various protocols at known concentrations of iron and PVD. Depending on growth conditions, P. putida produced one of two oxidized Mn species - either soluble PVD-Mn^(III) complex or insoluble Mn^(IV)O₂ minerals - but not both simultaneously. PVD-Mn^(III) was present, and MnO₂ precipitation was inhibited, both in iron-limited cultures that had synthesized 26-50 μM PVD and in iron-replete (non-PVD-producing) cultures that were supplemented with 10-550 μM purified PVD. PVD-Mn^(III) arose by predominantly ligand-mediated air oxidation of Mn^(II) in the presence of PVD, based on the following evidence: (a) yields and rates of this reaction were similar in sterile media and in cultures, and (b) GB-1 mutants deficient in enzymatic Mn oxidation produced PVD-Mn^(III) as efficiently as wild type. Only wild type, however, could degrade PVD-Mn^(III), a process linked to the production of both MnO₂ and an altered PVD with absorbance and fluorescence spectra markedly different from those of either PVD or PVD-Mn^(III). Two conditions, the presence of bioavailable iron and the absence of PVD at concentrations exceeding those of Mn, both had to be satisfied for MnO₂ to appear. These results suggest that P. putida cultures produce soluble Mn^(III) or MnO₂ by different and mutually inhibitory pathways: enzymatic catalysis yielding MnO₂ under iron sufficiency or PVD-promoted oxidation yielding PVD-Mn^(III) under iron limitation. Since PVD-producing Pseudomonas species are environmentally prevalent Mn oxidizers, these data predict influences of iron (via PVD-Mn^(III) versus MnO₂) on the global oxidation/reduction cycling of various pollutants, recalcitrant organic matter, and elements such as C, S, N, Cr, U, and Mn.     

Sorption and precipitation of Co(II) in Hanford sediments and alkaline aluminate solutions

Sorption and precipitation of Co(II) in simplified model systems related to the Hanford site high-level nuclear waste tank leakage were investigated through solution studies, geochemical modeling, and X-ray absorption fine structure (XAFS) spectroscopy. Studies of Co(II) sorption to pristine Hanford sediments (ERDF and Sub), which consist predominantly of quartz, plagioclase, and alkali feldspar, show an adsorption edge centered at pH ≈ 8.0 for both sediments studied, with sorption >99% above pH ≈ 9.0. Aqueous SiO₂ resulting from dissolution of the sediments increased in concentration with increasing pH, though the systems remained undersaturated with respect to quartz. XAFS studies of Co(II) sorption to both sediment samples reveal the oxidation of Co(II) to Co(III), likely by dissolved O₂, although this oxidation was incomplete in the Sub sediment samples. The authors propose that Fe(II) species, either in aqueous solution or at mineral surfaces, partially inhibited Co(II) oxidation in the Sub sediment samples, as these sediments contain significantly higher quantities of Fe(II)-bearing minerals which likely partially dissolved under the high-pH solution conditions. In alkaline solutions, Al precipitated as bayerite, gibbsite, or a mixture of the two at pH > 7; an amorphous gel formed at pH values less than 7. Aqueous Co concentrations were well below the solubility of known Co-bearing phases at low pH, suggesting that Co was removed from solution through an adsorption mechanism. At higher pH values, Co concentrations closely matched the solubility of a Co-bearing hydrotalcite-like solid. XAFS spectra of Co(II) sorbed to Al-hydroxide precipitates are similar to previously reported spectra for such hydrotalcite-like phases. The precipitation processes observed in this study can significantly reduce the environmental hazard posed by ⁶⁰Co in the environment.     

Surface chemistry and relative Ni sorptive capacities of synthetic hydrous Mn oxyhydroxides under variable wetting and drying regimes

The surface binding site characteristics and Ni sorptive capacities of synthesized hydrous Mn oxyhydroxides experimentally conditioned to represent three hydrological conditions—MnO_XW, freshly precipitated; MnO_XD, dried at 37°C for 8 d; and MnO_XC, cyclically hydrated and dehydrated (at 37°C) over a 24-h cycle for 7 d—were examined through particle size analysis, surface acid-base titrations and subsequent modelling of the pK_a spectrum, and batch Ni sorption experiments at two pH values (2 and 5). Mineralogical bulk analyses by XRD indicate that all three treatments resulted in amorphous Mn oxyhydroxides; i.e., no substantial bulk crystalline phases were produced through drying. However, drying and repeated wetting and drying resulted in a non-reversible decrease in particle size. In contrast, total proton binding capacities determined by acid-base titrations were reversibly altered with drying and cyclically re-wetting and drying from 82 ± 5 μmol/m² for the MnO_XW to 21 ± 1 μmol/m² for the MnO_XD and 37 ± 5 μmol/m² for the MnO_XC. Total proton binding sites measured decreased by ≈75% with drying from the MnO_XW and then increased to ≈50% of the MnO_XW value in the MnO_XC. Thus, despite a trend of higher surface area for the MnO_XD, a lower total number of sites was observed, suggesting a coordinational change in the hydroxyl sites. Surface site characterization identified that changes also occurred in the types and densities of surface sites for each hydrologically conditioned Mn oxyhydroxide treatment (pH titration range of 2-10). Drying decreased the total number of sites as well as shifted the remaining sites to more acidic pK_a values. Experimentally determined apparent pH_zpc values decreased with drying, from 6.82 ± 0.06 for the MnO_XW to 3.2 ± 0.3 for the MnO_XD and increased again with rewetting to 5.05 ± 0.05 for the MnO_XC. Higher Ni sorption was observed at pH 5 for all three Mn oxyhydroxide treatments compared to pH 2. However, changes in relative sorptive capacities among the three treatments were observed for pH 2 that are not explainable simply as a function of total binding site density or apparent pH_zpc values. These results are the first to our knowledge, to quantitatively link the changes induced by hydrologic variability for surface acid base characteristics and metal sorption patterns. Further, these results likely extend to other amorphous minerals, such as Fe oxyhydroxides, which are commonly important geochemical solids for metal scavenging in natural environments.     

Lepidocrocite-catalyzed Mn(II) oxygenation by air and its effect on the oxidation and mobilization of As(III)

Manganese (oxy)hydroxides (MnO_X) play important roles in the oxidation and mobilization of toxic As(III) in natural environments. Abiotic oxidation of Mn(II) to MnO_X in the presence of Fe minerals has been proved to be an important pathway in the formation of Mn(III, IV) (oxy)hydroxides. However, interactions between Mn(II) and As(III) in the presence of Fe minerals are still poorly understood. In this study, abiotic oxidation of Mn(II) on lepidocrocite, and its effect on the oxidation and mobilization of As(III) were investigated. The results show that MnO_X species are detected on lepidocrocite and their contents increase with increasing pH values ranging from 7.5 to 8.4. After 10 days, an MnOx component, groutite (α-MnOOH) was found on lepidocrocite. During the simultaneous oxidation of Mn(II) and As(III), and the As(III) pre-adsorbed processes, the presence and oxidation of Mn(II) significantly promotes the removal of soluble As(III). In addition, MnOx formed on lepidocrocite also contributes to the oxidation of soluble and adsorbed As(III) to As(V), the latter being subsequently released into solution. In the process where Mn(II) is pre-adsorbed on lepidocrocite, less As(III) is removed, given that the active sites occupied by MnOx inhibit the adsorption of As(III). In all experiments, the removal percentages of As(III) and the release of As(V) are correlated positively with pH values and initial concentrations of Mn(II), although they are not apparent in the Mn(II) pre-adsorbed system.