Manganese abiotic oxidation

The oxidation rate of As(III) in the presence of manganese and water may be substantially enhanced by manganese-oxidizing bacteria, such as Leptothrix ochracea (Katsoyiannis, Zouboulis and Jekel, 2004). Katsoyiannis, Zouboulis and Jekel (2004) found that the bacteria are important in oxidizing Mn(II) to Mn(IV), Fe(II) to Fe(III), and As(III) to As(V). The oxidation of Mn(II) leads to the precipitation of Mn(IV) (oxy)(hydr)oxides, which then abiotically oxidize additional As(III) and significantly sorb the As(V) that results from both abiotic and biotic oxidation. 

About 35% of the iron and 75% of the manganese in soils and sediments is in the form of free oxides (Canfield, 1997 Cornell and Schwertmann, 1996 Thamdrup, 2000). The remainder occurs as a minor constituent of silicate minerals. The lattice stmcture of Fe(III) oxide minerals varies widely. Freshly oxidized Fe(III) precipitates rapidly as ferrihydrite (Fe(OH)3), a reddish-brown, amorphous, poorly crystalline mineral. Ferrihydrite is the dominant product of Fe(II) oxidation whether it occurs by abiotic oxidation, aerobic microbial oxidation, or anaerobic microbial oxidation (Straub et al., 1998). Over a period of weeks to months, amorphous ferrihydrite crystals undergo diagenesis to yield well-ordered, strongly crystalline, stable minerals such as hematite(a-Fe203) and goethite (a-FeOOH) (Cornell and Schwertmann, 1996). 

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. 

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. 

In natural systems microbiological oxidation may offer a faster pathway, particularly at pH < 8 and low concentrations (<5 jlM) of particulate oxides. Hastings and Emerson (17) showed that sporulated cultures of marine bacillus SG-1 at pH 7.5 accelerated the oxidation of Mn(II) by a factor of 104 with respect to the abiotic catalysis on a colloidal MnOz surface. A radiotracer study of microbial Mn oxidation in a marine fjord revealed half-lives as short as 2 days (18). Perhaps microorganisms can use the entire redox cycle of manganese. A study indicates that the vegetative cells of spores that mediate Mn(II) oxidation also reduce manganese oxides (19). 

---