Kinetic and thermodynamic factors controlling manganese concentrations in oceanic waters

Data are presented describing the changes in the distribution of dissolved and particulate Mn observed over a 16-month period in the periodically anoxic waters of Saanich Inlet, a fjord located on the coast of Vancouver Island, British Columbia. During the spring and summer when the bottom waters were anoxic, a dense cloud of particulate Mn was found at mid-depths where Mn²⁺ enriched anoxic bottom waters were mixing with oxygenated waters; then, during the autumn aand winter following an intrusion which reoxygenated the bottom water, an intense precipitation of Mn was observed throughout the entire water column. During this latter period, dissolved Mn concentrations in the bottom water, which exceeded 1000 nmol/l under anoxic conditions, decreased towards a lower limit of 1.6 nmole/l, a value comparable to that observed in Pacific Ocean waters of similar pH and dissolved oxygen content. Mn in the particulate matter collected just above the oxic-anoxic interface was found to have an average oxidation number of +3.05; and, on this basis, it is proposed that dissolved Mn concentrations in oceanic waters are controlled by the precipitation of the metastable oxide mineral manganite (γ-MnOOH), a hypothesis consistent with the fact that dissolved Mn values in subsurface Pacific Ocean waters closely approach the equilibrium solubility of this phase. Temporal and spatial gradients in the particulate Mn distribution were used to calculate the in situ rate of Mn precipitation, and the results of these calculations then were fitted to theoretical rate equations which suggest that the precipitation of Mn is controlled by 2 parallel processes: bacterial oxidation and an inorganic autocatalytic oxidation reaction.     

Dissolution and transport of manganese by organic acids and their role in sedimentary Mn ore formation

Dissolution experiments of Mn and Fe under natural conditions from fresh basalt, weathered basalt and Mn laterite by different organic acids show that Mn is highly enriched over Fe in solutions from the weathered rocks but that more Fe than Mn is dissolved from the fresh basalt. The enrichment of Mn is caused by sparingly soluble Fe-oxides and hydroxides and more soluble Mn-oxides. In addition from the weathered rocks the Mn concentration dissolved by the organic acids is up to 1000 times higher than in inorganic solutions. Mn enrichment is caused by acid attack, organic reduction of Mn⁴+ to Mn²+ and complexing by the organic acids. The complexed Mn is not attacked as easily by oxidation as free Mn ions. Higher concentrations of manganese in the organic dissolved stage can therefore be transported by rivers over greater distances. Organic complexed Mn, derived from lateritic weathered rocks may therefore contribute to the formation of low iron marine sedimentary Mn deposits.     

Oxidative Ce3+ sequestration by fungal manganese oxides with an associated Mn(II) oxidase activity

Sequestration of Ce³⁺ by biogenic manganese oxides (BMOs) formed by a Mn(II)-oxidizing fungus, Acremonium strictum strain KR21-2, was examined at pH 6.0. In anaerobic Ce³⁺ solution, newly formed BMOs exhibited stoichiometric Ce³⁺ oxidation, where the molar ratio of Ce³⁺ sequestered (Ce_seq) relative to Mn²⁺ released (Mn_rel) was maintained at approximately two throughout the reaction. A similar Ce³⁺ sequestration trend was observed in anaerobic treatment of BMOs in which the associated Mn(II) oxidase was completely inactivated by heating at 85 °C for 1 h or by adding 50 mM NaN₃. Aerobic Ce³⁺ treatment of newly formed BMO (enzymatically active) resulted in excessive Ce³⁺ sequestration over Mn²⁺ release, yielding Ce_seq/Mn_rel > 200, whereas heated or poisoned BMOs released a significant amount of Mn²⁺ with lower Ce³⁺ sequestration efficiency. Consequently, self-regeneration by the Mn(II) oxidase in newly formed BMO effectively suppressed Mn²⁺ release and enhanced oxidative Ce³⁺ sequestration under aerobic conditions. Repeated treatments of heated or poisoned BMOs under aerobic conditions confirmed that oxidative Ce³⁺ sequestration continued even after most Mn oxide was released from the solid phase, indicating auto-catalytic Ce³⁺ oxidation at the solid phase produced through primary Ce³⁺ oxidation by BMO. From X-ray diffraction analysis, the resultant solid phases formed through Ce³⁺ oxidation by BMO under both aerobic and anaerobic conditions consisted of cerianite with crystal sizes of 5.00–7.23 Å. Such nano-sized CeO₂ (CeO_2,BMO) showed faster auto-catalytic Ce³⁺ oxidation than that on well-crystalized cerianite under aerobic conditions, where the normalized pseudo-first order rate constants for auto-catalytic Ce³⁺ oxidation on CeO_2,BMO was two orders of magnitude higher. Consequently, we concluded that Ce³⁺ contact with BMOs sequesters Ce³⁺ through two oxidation paths: primary Ce³⁺ oxidation by BMOs produces nano-sized crystalline cerianite, and subsequent auto-catalytic Ce³⁺ oxidation efficiently occurs using dissolved oxygen as the oxidizing agent. Pretreatment of newly formed BMOs with La³⁺ solution resulted in decreased rate constants for primary Ce³⁺ oxidation by BMO due to site blocking by La³⁺ sorption. The results presented herein increase our understanding of the role of BMO in oxidative Ce³⁺ sequestration process(es) through enzymatic and abiotic paths in natural environments and provide supporting evidence for the potential application of BMOs towards the recovery of Ce³⁺ from contaminated waters.     

The kinetics of the oxidation of ferrous iron in synthetic and natural waters

The rate of oxidation of ferrous iron in a seasonally anoxic lake was measured on 39 occasions with respect to both depth and time. Sample disturbance was minimal as only oxygen had to be introduced to initiate the reaction. The data were consistent with the simple rate law for homogeneous chemical kinetics previously established for synthetic solutions. The rate constant for the oxidation reaction in lake water was indistinguishable from that measured in synthetic samples. It did not appear to be influenced by changes in the microbial populations or by changes in any particulate or soluble components in the water, including iron and manganese. Analysis of the errors inherent in the kinetic measurements showed that the estimation of pH was the major source of inaccuracy and that values of the rate constant determined by different workers could easily differ by a factor of six.The present data, together with a comprehensive survey of the literature, are used to suggest a ‘universal’ rate constant of ca. 2 × 10¹³ M^?2 atm^?1 min^?1 (range 1.5–3 × 10¹³) in the rate law $\text{?d[Fe II]}\text{dt}\text{= k[Fe II]pO}{}_2\text{(OH?)}{}^2$ for natural freshwaters in the pH range 6.5–7.4. Discrepancies in the effects of ionic strength and interfering substances reported in the literature are highlighted. Generally substances have only been found to interfere at concentrations which far exceed those in most natural waters.     

Preparation,characterization and aging of δ-MnO2, for use in trace metal speciation studies

Well characterized synthetic δ-MnO₂ can be used successfully for trace metal speciation studies in natural waters. Characteristics and adsorption behavior of δ-MnO₂ depend on the preparation method and subsequent treatment and aging.Three different recipes were used to prepare δ-MnO₂. Methods involving a redox reaction between Mn²⁺ and Mn⁷⁺ at neutral pH (δ-MnO₂-redox), and reduction of Mn⁷⁺ at low pH (δ-MnO₂-red), yielded compounds with characteristics in agreement with literature data; they also have a high adsorption capacity for Cu. A method involving oxidation of Mn²⁺ at high pH yielded a δ-MnO₂(ox), with some variant characteristics and a lower adsorption capacity for Cu.An artificial aging process, using heat treatment at neutral pH, produced cryptomelane from δ-MnO₂(ox), but not from δ-MnO₂(redox) or δ-MnO₂(red). Conversion of δ-MnO₂(redox) to cryptomelane and loss of approximately one-half of the adsorption capacity, as a result of a natural aging process, can be avoided for at least four years if the K content is 10% in the solid, and the solid is stored at low temperature. Potassium content, and not a low pH, appears to control the conversion to cryptomelane.Adsorption capacity of δ-MnO₂ for Cu appears to be related to the K and H₂O content of the solid, which is structure dependent.Because of their higher adsorption capacity for Cu, δ-MnO₂(redox) or δ-MnO₂(red) are more suitable for trace metal studies in natural waters, than δ-MnO₂(ox).     

Contributions to the mineralogy of authigenic manganese phases from marine manganese deposits

The formation of authigenic manganese minerals and ores in the pelagic regions of the ocean is related to oxidation of Mn²⁺ extracted from basalts and other rocks with heated seawater. For littoral parts of the ocean and lakes mobilization of Mn²⁺ and Fe²⁺ is admitted finding its way to the bottom sediments (along with the organic substances) from land in the form of Mn⁴⁺. The main manganese mineral of oceanic and continental basins is vernadite. Its deposition is considered a result of the activity of microorganisms.     

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.     

Electron spin resonance and related studies of lignite and ball clay from South Devon,England

Electron spin resonance and infrared spectroscopic studies of lignite and ball clay from South Devon, and of extracts obtained from them by solvent fractionation, revealed similarities between corresponding organic components associated with both materials. All fractions exhibited a free radical resonance at g = 2.0037, which occurred with greatest intensity in the humic acids. Additional ESR features due to Fe³⁺, Mn²⁺ and VO²⁺ complexes were observed. Ferric ions give rise to resonances at g = 4.2 which have not been previously reported in the case of natural carbonaceous materials. It is shown that the paramagnetic species associated with the ball clay and lignite extracts do not significantly contribute to the observed ESR spectra of kaolinites, the latter being attributable to substituted Fe³⁺ ions and defect centres within the kaolinite lattice.     

Screening Effect of the Diffusive Boundary Layer in Sediments of Lake Aha in the Suburbs of Guiyang City,Guizhou Province

The redox cycle of iron and manganese is a major geochemica process at the boundary layers of lake sediments.Lake Aha,which lies in the suburbs of Guiyang City,Guizhou Province,China,is a medium-sized artificial reservoir with seasonally anoxic hypolimnion,Long-term sedimentary accumulation of iron and manganese resulted in their enrichment in the upper sediments,In the anoxic season,Fe^2 and Mn^2 ,formed by diological oxidation,would diffuse up to overlying waters from sediments.However,the concentration of oxidation,would diffuse up to overlying waters from sediments,However,the concentration of Fe^2 increased later and decreased earlier than that of Mn^2 .Generally,sulfate reduction occurred at 6 cm below the sediment-water interface.Whereas,in the anoxic season.the reduction reached upper sediments,inhibiting the release of Fe^2 ,The Fe concentration of anoxic water is quickly decreased from high to low as a result of reduction of the suplhur system.     

The citric acid-MnO2(birnessite) reaction. Electron transfer, complex formation, and autocatalytic feedback

Citrate released by plants, bacteria, and fungi into soils is subject to abiotic oxidation by MnO₂(birnessite), yielding 3-ketoglutarate, acetoacetate, and Mn^II. Citrate loss and generation of products as a function of time all yield S-shaped curves, indicating autocatalysis. Increasing the citrate concentration decreases the induction period. The maximum rate (r_max) along the reaction coordinate follows a Langmuir-Hinshelwood dependence on citrate concentration. Increases in pH decrease r_max and increase the induction time. Adding Mn^II, Zn^II, orthophosphate, or pyrophosphate at the onset of reaction decreases r_max. Mn^II addition eliminates the induction period, while orthophosphate and pyrophosphate addition increase the induction period. These findings indicate that two parallel processes are responsible. The first, relatively slow process involves the oxidation of free citrate by surface-bound Mn^III,IV, yielding Mn^II and citrate oxidation products. The second process, which is subject to strong positive feedback, involves electron transfer from Mn^II-citrate complexes to surface-bound Mn^III,IV, generating Mn^III-citrate and Mn^II. Subsequent intramolecular electron transfer converts Mn^III-citrate into Mn^II and citrate oxidation products.     

The chemistry of the anoxic waters in the Framvaren Fjord,Norway

In the summer of 1993, a number of chemical parameters (H₂S, O₂, pH, TA, TCO₂, NH ₄ ⁺ , PO ₄ ^3– , SiO₂, Mn²⁺ and Fe²⁺) were measured in the Framvaren Fjord, a permanently super-anoxic fjord in southern Norway. The extremely steep gradient of sulfide near the interface suggests that other than downward flux of oxygen, three other possible oxidants, particulate manganese and iron oxides, phototrophic sulfur oxidation bacteria and horizontally transported oxygen account for the oxidation of the upward flux of H₂S. Water intrusion through the sill accounts for the temperature inflection above the interface, which, together with internal waves (Stigerbrandt and Molvaer, 1988), may cause fluctuations of the depth of interface. Significant gradients of hydrographic properties and chemical species between 80–100 m suggest that there is a second interface at about 90 m that separates the deep and older bottom waters. A stoichiometric model is applied to examine the biogeochemical cycles of S, C, N and P in the Framvaren. High C:S, C:N and C:P ratios are found while the nutrients (N, P) have Redfield ratio. Based on the C:N:P ratio of 155:16:1 in organic matter, about 30% of sulfide produced by sulfate reduction is estimated to be removed by processes such as oxidation, formation of FeS₂, degassing and incorporation into organic matter. The rates of oxidation of H₂S by Mn and Fe oxides in the water near the interface were slightly faster than the observed values in the laboratory, probably due to the presence of bacteria.     

An evaluation of copper remobilization from mine tailings in sulfidic environments

A laboratory-based assessment of copper remobilization from Cu-rich mine tailings exposed to anoxic, sulfide rich waters was performed. The results from incubation experiments, conducted over a 20 day period, were compared to thermodynamic modelling calculations of copper speciation in sulfidic waters. The tailings materials were observed to react rapidly with added sulfide, consuming 159 μmol HS⁻ g⁻¹ (dry wt) within a 24 h period. The consumption of sulfide was attributed to a two stage process involving the reduction of Fe-hydroxy phases by sulfide followed by reaction with available Fe²⁺ and Cu²⁺ resulting in the formation Fe- and Cu-sulfide phases. During incubation experiments, the dissolved copper concentrations in the absence of sulfide were approximately 0.31 μmol l⁻¹, whereas in the presence of sulfide (0.5–5 mM) concentrations were typically 0.24 μmol l⁻¹. The experiments did not indicate enhanced solubility owing to the formation of soluble copper sulfide species. The predictions (based on the most recent thermodynamic data for aqueous Cu-sulfide and Cu-polysulfide species) did not accurately explain the laboratory observations. Model predictions were greatly influenced by the assumptions made about the oxidation state of copper under anoxic conditions and the solid sulfide phase controlling copper solubility. The study emphasizes the limitations of modelling copper speciation in sulfidic waters and the need for laboratory or field verification of predictions.     

Silicate and carbonate mineral weathering in soil profiles developed on Pleistocene glacial drift (Michigan, USA): Mass balances based on soil water geochemistry

Geochemistry of soil, soil water, and soil gas was characterized in representative soil profiles of three Michigan watersheds. Because of differences in source regions, parent materials in the Upper Peninsula of Michigan (the Tahquamenon watershed) contain only silicates, while those in the Lower Peninsula (the Cheboygan and the Huron watersheds) have significant mixtures of silicate and carbonate minerals. These differences in soil mineralogy and climate conditions permit us to examine controls on carbonate and silicate mineral weathering rates and to better define the importance of silicate versus carbonate dissolution in the early stage of soil-water cation acquisition.Soil waters of the Tahquamenon watershed are the most dilute; solutes reflect amphibole and plagioclase dissolution along with significant contributions from atmospheric precipitation sources. Soil waters in the Cheboygan and the Huron watersheds begin their evolution as relatively dilute solutions dominated by silicate weathering in shallow carbonate-free soil horizons. Here, silicate dissolution is rapid and reaction rates dominantly are controlled by mineral abundances. In the deeper soil horizons, silicate dissolution slows down and soil-water chemistry is dominated by calcite and dolomite weathering, where solutions reach equilibrium with carbonate minerals within the soil profile. Thus, carbonate weathering intensities are dominantly controlled by annual precipitation, temperature and soil pCO₂. Results of a conceptual model support these field observations, implying that dolomite and calcite are dissolving at a similar rate, and further dissolution of more soluble dolomite after calcite equilibrium produces higher dissolved inorganic carbon concentrations and a Mg²⁺/Ca²⁺ ratio of 0.4.Mass balance calculations show that overall, silicate minerals and atmospheric inputs generally contribute <10% of Ca²⁺ and Mg²⁺ in natural waters. Dolomite dissolution appears to be a major process, rivaling calcite dissolution as a control on divalent cation and inorganic carbon contents of soil waters. Furthermore, the fraction of Mg²⁺ derived from silicate mineral weathering is much smaller than most of the values previously estimated from riverine chemistry.     

Outer-sphere electron transfer kinetics of metal ion oxidation by molecular oxygen

Density functional theory molecular orbital calculations and Marcus theory have been combined to assess the rates and physicochemical factors controlling the outer-sphere oxidation of divalent V, Cr, Mn, Fe, and Co aquo and hydroxo ions by O₂ in homogeneous aqueous solution. Key quantities in the elementary oxidation step include the inner-sphere component of the reorganization energy, the thermodynamic driving force, and electrostatic work terms describing the interactions occurring, in this case, between the net charges on the product species. Collectively, these factors and their interplay have a large influence on the rate of the oxidation cross-reaction.An inner-sphere pathway for the self-exchange reactions and oxidation by O₂ of Mn²⁺ and Cr²⁺ ions has been supported indirectly in this study by comparing predicted outer-sphere rates with the results of previous experiments. Likewise, an outer-sphere pathway is suggested for the similar sets of reactions involving the V, Fe, and Co ions. An assessment of the self-exchange reaction for the oxygen/superoxide couple has led to predicted rates in excellent agreement with direct measurements. Predicted rates of oxidation for the hexaquo Fe ion are also in agreement with experiment, while the predicted rates for the outer-sphere oxidation of its hydrolysis products are ∼2 to 3 (monohydroxo) and ∼4 (dihydroxo) orders of magnitude slower than the observed rates. This suggests an inner-sphere pathway is appropriate to explain the relatively fast rates observed for the hydrolyzed Fe species.     

The Removal of Dissolved Metals by Hydroxysulphate Precipitates during Oxidation and Neutralization of Acid Mine Waters, Iberian Pyrite Belt

This study examines the removal of dissolved metals during the oxidation and neutralization of five acid mine drainage (AMD) waters from La Zarza, Lomero, Esperanza, Corta Atalaya and Poderosa mines (Iberian Pyrite Belt, Huelva, Spain). These waters were selected to cover the spectrum of pH (2.2–3.5) and chemical composition (e.g., 319–2,103 mg/L Fe; 2.85–33.3 g/L SO₄⁼) of the IPB mine waters. The experiments were conducted in the laboratory to simulate the geochemical evolution previously recognized in the field. This evolution includes two stages: (1) oxidation of dissolved Fe(II) followed by hydrolysis and precipitation of Fe(III), and (2) progressive pH increase during mixing with fresh waters. Fe(III) precipitates at pH < 3.5 (stages 1 and 2) in the form of schwertmannite, whereas Al precipitates during stage 2 at pH 5.0 in the form of several hydroxysulphates of variable composition (hydrobasaluminite, basaluminite, aluminite). During these stages, trace elements are totally or partially sorbed and/or coprecipitated at different rates depending basically on pH, as well as on the activity of the SO₄⁼ anion (which determines the speciation of metals). The general trend for the metals which are chiefly present as aqueous free cations (Pb²⁺, Zn²⁺, Cu²⁺, Cd²⁺, Mn²⁺, Co²⁺, Ni²⁺) is a progressive sorption at increasing pH. On the other hand, As and V (mainly present as anionic species) are completely scavenged during the oxidation stage at pH < 3.5. In waters with high activities (> 10⁻¹) of the SO ₄⁼ ion, some elements like Al, Zn, Cd, Pb and U can also form anionic bisulphate complexes and be significantly sorbed at pH < 5. The removal rates at pH 7.0 range from around 100% for As, V, Cu and U, and 60–80% for Pb, to less than 20% for Zn, Co, Ni and Mn. These processes of metal removal represent a significant mechanism of natural attenuation in the IPB.     

The uptake of cobalt into ferromanganese nodules,soils, and synthetic manganese (IV) oxides

Strong enrichments of cobalt occur in marine manganese nodules, soils, wads, and natural and synthetic minerals such as hollandite, cryptomelane, psilomelane, lithiophorite, birnessite, and δ-MnO₂. Previously, it was suggested that Co³⁺ ions in these minerals replace either Mn³⁺ or substitute for Fe³⁺ in incipient goethite epitaxially intergrown with δ-MnO₂. Neither of these interpretations is now considered to be satisfactory on account of the large discrepancy of ionic radius between octahedrally coordinated low-spin Co³⁺ and high-spin Mn³⁺ or Fe³⁺ in oxide structures. The close agreement between the ionic radii of Co³⁺ and Mn⁴⁺ suggests that some cobalt substitutes for Mn⁴⁺ ions in edge-shared [MnO₆] octahedra in many manganese(IV) oxide mineral structures. It is proposed that hydrated cations, including Co²⁺ ions, are initially adsorbed on to the surfaces of certain Mn(IV) oxides in the vicinity of essential vacancies found in the chains or sheets of edge-shared [MnO₆] octahedra. Subsequently, fixation of cobalt takes place as a result of oxidation of adsorbed Co²⁺ ions by Mn⁴⁺ and replacement of the displaced manganese by low-spin Co³⁺ ions in the [MnO₆] octahedra or vacancies.     

High-resolution parallel electron energy-loss spectroscopy of Mn L2,3-edges in inorganic manganese compounds

Parallel electron energy-loss spectroscopy (PEELS) in a scanning transmission electron microscope (STEM) was used to record the Mn L_2,3-edges from a range of natural and synthetic manganese containing materials, covering valences 0, II, III, IV and VII, with an energy resolution of ca. 0.5 eV. The Mn L_2,3 electron-loss near-edge structure (ELNES) of these edges provided a sensitive fingerprint of its valence. The Mn²⁺ L_2,3-edges show little sensitivity to the local site symmetry of the ligands surrounding the manganese. This is illustrated by comparing the Mn L_2,3-edges from 4-, 6- and 8-fold coordinated Mn²⁺. In contrast, the Mn L₃-edges from Mn³⁺ and Mn⁴⁺ containing minerals exhibited ELNES that are interpreted in terms of a crystal-field splitting of the 3d electrons, governed by the symmetry of the surrounding ligands. The Mn L₃-edges for octahedrally coordinated Mn²⁺, Mn³⁺ and Mn⁴⁺ showed variations in their ELNES that were sensitive to the crystal-field strength. The crystal-field strength (10Dq) was measured from these edges and compared very well with published optically determined values. The magnitude of 10Dq measured from the Mn L₃-edges and their O K-edge prepeaks of the manganese oxides were almost identical. This further confirms that the value of 10Dq measured at the Mn L₃-edge is correct. Selected spectra are compared with theoretical 2p atomic multiplet spectra and the differences and similarities are explained in terms of the covalency and site symmetry of the manganese. The Mn L₃-edges allow the valence of the manganese to be ascertained, even in multivalent state materials, and can also be used to determine 10Dq.     

On the model of Mn,Fe, Ni,Co ore formation in recent basins: Experiments with the synthesis of Me-hydroxide phases on Mn3O4

Experiments on the sorption of dissolved Ni, Co, Mn, Fe from seawater by Mn₃O₄ reveal a sequence of reactions taking place: Ion exchange, hydrolysis, then autocatalytic oxidation and layer formation on the interface. The composition of the new compounds depends on the kinetics of i) sorption, and ii) interface oxidation. The highest oxidized Me ions accumulate at low sorption rates, i. e. when sorption does not inhibit interface oxidation: 60% Mn⁴⁺, 30% Ni³⁺ & 30% Co³⁺ are a representative example for that layer type. Iron is present in this layer as amorphous FeOOH·xH₂O according to Mössbauer spectra. Specific for the Me sorption by Mn₃O₄ is the interaction of Ni & Co with Mn²⁺ and Mn³⁺ of the sorbent lattice. Mn is found in the solute phase equivalent to 16, 14% of the adsorbed Co or 17, 96% of the adsorbed Ni. These results confirm the earlier presented model on the transition metal accumulation in recent basins as taking place in distinct stages with interface autocatalysis for the Me oxidation playing the main role.     

Modeling of subsurface calcite dissolution, including the respiration and reoxidation processes of marine sediments in the region of equatorial upwelling off Gabon

Mineralization of organic matter and the subsequent dissolution of calcite were simulated for surface sediments of the upper continental slope off Gabon by using microsensors to measure O₂, pH, pCO₂ and Ca²⁺ (in situ), pore-water concentration profiles of NO₃⁻, NH₄⁺, Fe²⁺, and Mn²⁺ and SO₄²⁻ (ex situ), as well as sulfate reduction rates derived from incubation experiments. The transport and reaction model CoTReM was used to simulate the degradation of organic matter by O₂, NO₃⁻, Fe(OH)₃ and SO₄²⁻, reoxidation reactions involving Fe²⁺ and Mn²⁺, and precipitation of FeS. Model application revealed an overall rate of organic matter mineralization amounting to 50 μmol C cm⁻² yr⁻¹, of which 77% were due to O₂, 17% to NO₃⁻ and 3% to Fe(OH)₃ and 3% to SO₄²⁻. The best fit for the pH profile was achieved by adapting three different dissolution rate constants of calcite ranging between 0.01 and 0.5% d⁻¹ and accounting for different calcite phases in the sediment. A reaction order of 4.5 was assumed in the kinetic rate law. A CaCO₃ flux to the sediment was estimated to occur at a rate of 42 g m⁻² yr⁻¹ in the area of equatorial upwelling. The model predicts a redissolution flux of calcite amounting to 36 g m⁻² yr⁻¹, thus indicating that ∼90% of the calcite flux to the sediment is redissolved.     

Geochemical modeling of coal mine drainage, Summit County, Ohio

 Geochemical modeling was used to investigate downstream changes in coal mine drainage at Silver Creek Metro-park, Summit County, Ohio. A simple mixing model identified the components that are undergoing conservative transport (Cl^–, PO₄ ^3–, Ca²⁺, K⁺, Mg²⁺ and Na⁺) and those undergoing reactive transport (DO, HCO₃ ^–, SO₄ ^2–, Fe²⁺, Mn²⁺ and Si). Fe²⁺ is removed by precipitation of amorphous iron-hydroxide. Mn²⁺ are removed along with Fe²⁺ by adsorption onto surfaces of iron-hydroxides. DO increases downstream due to absorption from the atmosphere. The HCO₃ ^– concentration increases downstream as a result of oxidation of organic material. The rate of Fe²⁺ removal from the mine drainage was estimated from the linear relationship between Fe⁺² concentration and downstream distance to be 0.126 mg/s. Results of this study can be used to improve the design of aerobic wetlands used to treat acid mine drainage. Received: 4 June 1996 · Accepted: 17 September 1996