Manganese oxides, reductive dissolution

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.]...

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... 

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). 

Dissolved arsenic is correlated with ammonia (Fig. 4), consistent with a release mechanism associated with the oxidation of organic carbon. Other chemical data not shown here provide clear evidence of iron, manganese and sulfate reduction and abundant methane in some samples indicates that methanogenesis is also occurring. It is not clear however if arsenic is released primarily by a desorption process associated with reduction of sorbed arsenic or by release after the reductive dissolution of the iron oxide sorbent. Phreeqc analysis shows PC02 between 10"12 and 10"° bars and that high arsenic waters are supersaturated with both siderite and vivianite. 

Rates of reductive dissolution of amorphous manganese (111,1V) oxide particles decrease as the electrode half-wave potentials of the substituted phenols (as reported by Suatoni et al., 1961) increase (4.8 x 10 5 M total manganese, pH 4.4). 

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

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. 

Stone, AT. 1987. Reductive dissolution of manganese (lll/IV) oxides by substituted phenols. 

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... 

Reductive dissolution of iron and manganese (oxy)(hydr)oxides... 

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. 

Reductive Dissolution. Many substances in nature contain the same metal or metalloid, but under different oxidation states. For example, the metalloid arsenic may exist as arsenite (AsIII, As03) or arsenate (AsIV, As04) in the forms of ferrous-arsenite or ferric-arsenate, respectively. Ferrous-arsenite is more soluble than ferric-arsenate for this reason, one may be interested in studying the kinetics of arsenate reduction to arsenite. Similar chemistry applies to all elements present in soil-water systems with more than one oxidation state (e.g., iron, manganese, selenium, and chromium). 

Reductive dissolution kinetics of Mn02 (MnlV) and MnOOH (Mnlll) are presented below for demonstration purposes. The chemistry of manganese in nature is rather complex because three oxidation states are involved [Mn(II), Mn(III), and Mn(FV)] and form a large number of oxides and oxyhydroxides with various degrees of chemical stability (Bricker, 1965 Parc etal., 1989 Potter and Rossman, 1979a,b). One... 

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