Mn oxide minerals

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. 

FIGURE 7.5 Pb adsorption of biogenic Mn oxide compared to that of colloidal Fe oxyhy-droxide and abiotic Mn oxide minerals (pyrolusite). (From Nelson, Y.M. et al., Appl. Environ. Microbiol., 65, 175, 1999b. With permission.)... 

The black layer is a 3.5 cm thick, black to very dark gray clayey silt with weak lamination. The most notable feature of this layer is the abundance of euhedral, starburst shaped Mn oxide minerals that average 0.5 to 0.1 mm in diameter (Figs. 4E and 4F). The detrital component of this unit is dominated by quartz silt. The upper contact of this unit is slightly eroded and marked by clay clasts and a transition to red laminated clay. 

Feng, X.H. et al., Adsorption and redox reactions of heavy metals on synthesized Mn oxide minerals, Environ. Poll., 147, 366, 2007. 

Three principal Mn oxide minerals are found in Mn nodules and crusts. Their principal X-ray diffraction peaks are given below together with their alternative mineral names (in parenthesis) ... 

To date, there have been few studies on the mineral structure and the environment fate of the Mn oxide precipitates that form widiin in situ chemical oxidation (ISCO) schemes with Mn04. Identification of the structure of Mn oxide mineral, together with the determination of die chemical conqmsition, is necessary to lay the foundation for further studies on how Mn oxide precipitation can be controlled. There are more than 30 different types of Mn oxide/hydroxide minerals widi different structures and conq>ositions. 

Generally, there are two major structural forms for these minerals chain or tunnel structures, and layer structures. All of these forms are comprised of MnOs octahedras. Water molecules and/or other cations (8) are ofien present at various sites in the structures. Mn oxides having a chain or tunnel structure include pyrolusite, ramsdellite, hollandite, romanechite, and todorokite. Typical structures for the chain or tunnel type Mn oxide mineral are presented in Figure 1. Lithiophorite, chalcophanite, and bimessite are examples of Mn oxide minerals havii a layer structure. Typical structural maps are shown in Figure 2. 

Identifying the particular Mn oxide mineral is not straightforward because the sanq)les are usually not crystallized well enough for sii e-crystal diffiaction studies, especially witii synthetic oxides of the type formed here. In this study, we will apply powder x-ray dif action and transmission electron microscopy (TEM), along with chemical analysis, to identify the type of structure and to determine the properties of the Mn oxide. 

Figure 1. Polyhedral representations of crystal structures of Mn oxide minerals with chain or tunnel structures. (A) Pyrolusite. (B) Ramsdellite.

The X-ray diffiaction (XRD) spectrum (Figure 4) for die synthetic Mn oxide has no prominent peak. However, die pattern matched bimessite in the database with a characterisdc small peak at 7.3 A. Anmng all the Mn oxide minerals, the bimessite family has layer spacings of approximately 7 A (5, 14) with Na, Ca, or K as die interlayer elements. Because the mineral is formed in a potassium rich environment, the mineral formed in our study is semi-amorphous potassium-rich bimessite. The basic structural unit for bimessite is a sheet of MnO octahedra (Crystal structure C in Figure 2). The interlayer cations and water molecules generally are known to occupy different positions inside the mineral. 

Thermal analysis showed that structural water in the mineral was 12.49% by weight of die total solid. The combination of lOP-MS analysis of die dissolved Mn oxide and a charge balance of die different elements provides a formula for the Mn oxide mineral as Ko.i54Mni.7ge04 l.S5H20. This interpretation assumes that the oxidation state for all Mn inside the mineral is IV. As compared with other results (74), our interpretation seems reasonable. Four independent measurements using the BET method yielded a specific surfitce area for Mn oxide of 23.6 0.82 m /g. Potentiometric titrations of Mn oxide solid found the pzc for Mn oxide to be 3.7 0.4, which is con arable to the range reported (2.0 to 4.5) for different forms of MnOa (75). 

To accurately model contaminant transport when Al, Fe, and Mn oxide minerals are present, intraparticle diffusivities are needed. Additionally, as we tried to point out in this ehapter, there are a number of implieations in using the diffusion model with amorphous oxides. Some of these implications of intraparticle diffusion have been observed by researchers in macroscopic studies of both model and real systems. However, as only a small number of studies have been conducted on metal eontaminant diffusion in aqueous oxide systems, many implications need yet to be addressed sueh as the long-term effect of contaminants sorbed in micopores of metastable minerals and desorption of contaminants from both coprecipitated oxides and oxides exposed to contaminants over long periods of time. Therefore, future studies are needed to study and improve our understanding of this slow sorption proeess, intraparticle diffusion. 

Figure 12.7 Eh-pH and pe-pH relations in the system Mn-HjO-Oj at 25 C, 1 bar. Mn + activities and stability fields of Mn-oxide minerals are included. The pe and Eh axes are related by the formulas pe = 50400)/ ...

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