Manganese oxides dissolution

Various chemical extraction techniques have been introduced in order to selectively remove metals from the different adsorption or complexation sites of natural sediments (e.g., Tessier et al, 1979 Erel et al, 1990 Leleyter et al., 1999). It is, for example, shown by Leleyter et al. (1999) that between 20% and 60% of REE in various suspended river sediments are removed by successive extractions by water, by Mg(N03)2 (exchangeable fraction), sodium actetate (acid-soluble fraction), NH2OH - - HCl (manganese oxide dissolution) ammonium oxalate (iron oxide dissolution) and a mixture of H2O2 + HNO3 (oxidizable fraction). The complexity of... 

The dissolution of carbon in molten iron in the lower part of the furnace, leads to the reduction of manganese oxide (eq. 15) and some sihea (eq. 14), both in the slag, whereby the subsequent dissolution of these metals occurs in the molten iron. 

In addition to effects on the concentration of anions, the redox potential can affect the oxidation state and solubility of the metal ion directly. The most important examples of this are the dissolution of iron and manganese under reducing conditions. The oxidized forms of these elements (Fe(III) and Mn(IV)) form very insoluble oxides and hydroxides, while the reduced forms (Fe(II) and Mn(II)) are orders of magnitude more soluble (in the absence of S( — II)). The oxidation or reduction of the metals, which can occur fairly rapidly at oxic-anoxic interfaces, has an important "domino" effect on the distribution of many other metals in the system due to the importance of iron and manganese oxides in adsorption reactions. In an interesting example of this, it has been suggested that arsenate accumulates in the upper, oxidized layers of some sediments by diffusion of As(III), Fe(II), and Mn(II) from the deeper, reduced zones. In the aerobic zone, the cations are oxidized by oxygen, and precipitate. The solids can then oxidize, as As(III) to As(V), which is subsequently immobilized by sorption onto other Fe or Mn oxyhydroxide particles (Takamatsu et al, 1985). 

Chao T. T. Selective dissolution of manganese oxides from soils and sediments with acidified hydroxylamine hydrochloride. Soil Sci Soc Am Proc 1972 36 704-768. 

In Limnodrilus sp., an oligochaete worm, copper bioavailability from surhcial freshwater sediments is associated with the amount of copper present in the manganese oxide fraction of the sediment. The redox potential and pH in the gut of Limnodrilus allows the dissolution of the manganese oxide coating, making copper and other metals available for uptake (Diks and Allen 1983). 

Stone, A. T. (1987), "Microbial Metabolites and the Reductive Dissolution of Manganese Oxides Oxalate and Pyruvate", Geochim. Cosmochim. Acta 51, 919-925. 

Of more apparent significance in the aquatic environment are redox processes induced or enhanced on absorbance of light by chromophores at metal oxide surfaces in which the metal of the oxide lattice constitutes the cationic partner. Light induced electron transfer within such a chromophore often results in disruption of the oxide lattice. The photoredox-induced dissolution of iron and manganese oxides by such a mechanism has been proposed as a possible means of supply of essential trace-metal nutrients to plants and aquatic organisms (29-31). ... 

Based upon thermodynamic data given in Table I, oxidant strength decreases in the order NijO > Mn02 > MnOOH > CoOOH > FeOOH. Rates of reductive dissolution in natural waters and sediments appear to follow a similar trend. When the reductant flux is increased and conditions turn anoxic, manganese oxides are reduced and dissolved earlier and more quickly than iron oxides (12, 13). No comparable information is available on release of dissolved cobalt and nickel. 

Experiments examining the influence of calcium and phosphate on the reductive dissolution of manganese oxides by hydroquinone have, in fact, shown inhibition by adsorbed ions (33). As the total phosphate in solution is increased, the rate of Mn + release diminished in proportion to the phosphate surface coverage. 

The most direct evidence for surface precursor complex formation prior to electron transfer comes from a study of photoreduc-tive dissolution of iron oxide particles by citrate (37). Citrate adsorbs to iron oxide surface sites under dark conditions, but reduces surface sites at an appreciable rate only under illumination. Thus, citrate surface coverage can be measured in the dark, then correlated with rates of reductive dissolution under illumination. Results show that initial dissolution rates are directly related to the amount of surface bound citrate (37). Adsorption of calcium and phosphate has been found to inhibit reductive dissolution of manganese oxide by hydroquinone (33). The most likely explanation is that adsorbed calcium or phosphate molecules block inner-sphere complex formation between metal oxide surface sites and hydroquinone. 

It has been reported that the value of the cubic lattice parameter, which is directly related to the average oxidation state of the manganese, is critical to obtain effective cycling. The lattice parameter should preferably be 8.23A or less, and such values are associated with lithium-rich materials, Lii+Mriz-jOi, where the average manganese oxidation state is 3.58 or higher this value minimizes dissolution of manganese and also the impact of the... 

Ratieuville Y, Wu BE, Lincot D, Vedel J, Yu LT (1999) Voltammetric and electrogravimetric study of manganese dioxide thin film electrodes. J Electrochem Soc 146 S-1-17/23. Bakardjieva S, Bezdicka P, Grygar T, Vorm P (2000) Reductive dissolution of microparticulate manganese oxides. J Solid State Electrochem 4 306-313. 

All measured profiles of sulfate reduction in sediments indicate that much sulfide production and, by inference, oxidation occurs in permanently anaerobic sediments (78, 73, 90,101). The two most likely electron acceptors for anaerobic sulfide oxidation are manganese and iron oxides. Burdige and Nealson (151) demonstrated rapid chemical as well as microbially catalyzed oxidation of sulfide by crystalline manganese oxide (8-Mn02), although elemental S was the inferred end product. Aller and Rude (146) documented microbial oxidation of sulfide to sulfate accompanied by reductive dissolution... 

Iron and manganese oxides are characterized by high specific surface areas and high affinity of their surface hydroxyl groups for adsorption of a variety of trace elements. In addition to adsorption processes, oxidation reactions are catalyzed by these surfaces (18-20). The in situ precipitation and dissolution of these oxides are thus significant for the fate of various trace... 

There is no indication of a release of Zn from the sediments during the development of anoxia, unlike the release of phosphate and dissolved silicate. Zn bound to algal material may be dissolved upon mineralization of this material and Zn bound to manganese oxides upon dissolution of manganese oxides. It appears, however, that Zn is efficiently retained in the sediments, probably through bonding to other less soluble particles, such as iron oxides and silica parts of diatoms. In the presence of sulfide, Zn is probably retained in association with sulfide-containing particles. 

Oxidation of arsenic-bearing pyrite with adsorption onto iron oxides and/or other metal (oxy)(hydr)oxides Nitrate reduction by pyrite oxidation (note that Appelo and Postma, 1999 referred to pure rather than arsenian pyrite) Manganese oxide reduction and release of sorbed arsenic Fe(lll) reduction on oxide surfaces changes net charge leading to arsenic desorption Iron oxide reductive dissolution and release of sorbed arsenic catalyzed by NOM degradation... 

Perhaps one of the most unknown areas in kinetics of soil chemical processes is redox dynamics (Chapter 8). Some work on reductive dissolution of manganese oxides [Mn(III/IV)] and oxidation of As(V), Cr(III), and Pu(III/IV) by oxides has appeared. However, a comprehensive understanding of redox kinetics in heterogeneous systems is lacking. 

Figure 8.2. Rates of manganese oxide reductive dissolution by 1.00 x 10 4 M oxalate as a function of pH. Reactions were performed in 5.0 x 10 2 M NaCl using either acetate (O) or constant -Pco2 (P) buffers. ([MnOx]0 is 4.81 x 10 5 M.) Numerical values are apparent reaction orders with respect to [H+], [From Stone (1987a), with permission.]...

Among the interesting results of such endeavors for 304 stainless steel is a stress upon inclusions involving manganese sulfide and manganese oxide. Surprisingly, traces ofCu in the steel are detected and form flowerlike deposits prior to the initiation of a pit. Metallic inclusions tend to dissolve anodically in 0.1 MNaCl at 400 mV SCE. There is some evidence for their readsorption. However, to be the cause ofpit initiation, the inclusions have to be at least 0.7 jum in size. It may be that the local current density for dissolution at smaller inclusions is too high to sustain CF adsorption there. 

Reduction-dissolution kinetics of manganese-oxides in excess H202 plus H2S04 (assuming complete Mn-oxide surface coverage by the reductant, irreversible electron transfer, and instantaneous product release) can be expressed by... 

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