Manganese abiotic reduction

The data in Figure 7.13 show reductive-dissolution kinetics of various Mn-oxide minerals as discussed above. These data obey pseudo first-order reaction kinetics and the various manganese-oxides exhibit different stability. Mechanistic interpretation of the pseudo first-order plots is difficult because reductive dissolution is a complex process. It involves many elementary reactions, including formation of a Mn-oxide-H202 complex, a surface electron-transfer process, and a dissolution process. Therefore, the fact that such reactions appear to obey pseudo first-order reaction kinetics reveals little about the mechanisms of the process. In nature, reductive dissolution of manganese is most likely catalyzed by microbes and may need a few minutes to hours to reach completion. The abiotic reductive-dissolution data presented in Figure 7.13 may have relative meaning with respect to nature, but this would need experimental verification. 

In soils and sediments rich in sulfides, abiotic reduction of Fe(III) and Mn(IV) is possible. For example, sulfides produced during sulfafe reduction can reduce Fe(III) to Fe(II) and Mn(IV) to Mn(ll). Manganese oxides are known to be more reactive with reduced sulfur compounds than with Fe(lll) oxides. 

Based on critical reviews, Lovley (1991, 2004) concluded that there are potential mechanisms for the abiotic reduction of Fe(III) and Mn(IV), but the significance of this process is minimal as compared to biotic reduction catalyzed by microbial activities. Typically, the end products of Fe(II) and Mn(II) are measured as indicators of the biotic and abiotic reduction of Fe(III) and Mn(IV) in anaerobic environments. The reduction of Fe(III) and Mn(IV) as a function of Eh is shown in Figures 10.10 and 10.11. Sodium acetate extractable iron and manganese in anaerobic soils represents Fe(II) and Mn(II), end products of reduction. As expected, extractable Mn(II) and Fe(II) concentrations are low nnder oxidized conditions and increase with a decrease in the Eh of soil. The accumulation of Mn(II) occurs at higher Eh values than the accumulation of Ee(II), suggesting Mn(IV) reduction precedes Fe(III) reduction. Because the reduction of Ee(III) and Mn(IV) occurs... 

Shindo, H., and P.M. Huang. 1985b. The catalytic power of inorganic components in the abiotic synthesis of hydroquinone-derived humic polymers. Appl. Clay Sci. 1 71-81. Stone, A.T. 1987. Reductive dissolution of manganese (III)/(IV) oxides by substituted phenols. 

Similar to the possibility of concurrent reduction of sulfate and ferric iron by a culture of a single bacteria (Coleman et al. 1993 see section 7.4.3.4) other iron reducing bacteria were found to additionally maintain dissimilation with more than one electron acceptors under suboxic conditions (Lovley and Phillips 1988) or even under oxic conditions (Myers and Nealson 1988a). In the presence of Fe(III) and Mn(IV) strain MR-1 was found to reduce both but additional manganese reduction occurred due to the immediate abiotic reaction with released Fe (Myers and Nealson 1988b). The interactions of biotic and abiotic reactions are shown in Fig. 7.17. 

Wetlands exhibit distinct redox gradients between the soil and overlying water column and in the root zone (Chapter 4), resulting in aerobic interfaces. For example, the aerobic layer at the soil-floodwater interface is created by a slow diffusion of oxygen and the rapid consumption at the interface. The thin aerobic layer at the soil-floodwater interface and around roots functions as an effective zone for aerobic oxidation of Fe(ll) and Mn(II). Below this aerobic layer there exists the zone of anaerobic oxidation of Fe(ll) and Mn(ll) and reduction of Fe(III) and Mn(IV). The juxtaposition of aerobic and anaerobic zones creates conditions of intense cycling of iron and manganese mediated by both biotic and abiotic reactions. 

Explain the difference between biotic and abiotic iron and manganese reduction. Which is the dominant process ... 

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