7.2.2 Association of Main Soil Mn Oxides with Trace Metals
The ability of Mn oxides to sorb transition metals and actinides, thus controlling or
influencing significantly their speciation and mobility, has been recognized for long,
in virtually all environments, including soils as well as freshwater and marine
sediments (e.g., Aplin and Cronan 1985; Bodeï et al. 2007; Chukhrov et al. 1985;
Duff et al. 1999; Exon et al. 2002; Friedl et al. 1997; Hochella Jr et al. 2005; Isaure
et al. 2005; Koschinsky and Halbach 1995; Koschinsky and Hein 2003; Lei and
Boström 1995; Manceau et al. 2004, 2003; Marcus et al. 2004; McKenzie 1980;
Ostwald and Frazer 1973). This exceptional reactivity, which explains why Mn
oxides are often described as “scavengers” (Goldberg 1954), is generally only
competed by iron oxides, depending on the trace metal considered. For example,
Dong et al. (2002) studied the relative abilities of the three main components of
surface coatings (Mn oxides, Fe oxides, and organic material) to uptake Pb, and
found that Mn oxides were the most efficient Pb sorbing phase. Such exceptional
reactivity is exemplified in Fig. 7.5, which shows the Pb content of a Mn oxide grain
from the top horizon of a carbonated soil. In this study, the soil material has been
artificially contaminated using PbS particles (final Pb content % 300 mg kg
À1 ). Soil
and pollutant have first been mechanically dry mixed. Then, the mixture has been
reaggregated by hand with progressive addition of determined volumes of deionized
aerated water and kept wet at 85% of its water holding capacity for 3 months at 25
C
(Bataillard et al. 2003). Scanning electron microscopy, equipped with an energy
dispersive spectrometer (EDS), shows that Pb, from PbS particles, has been partly
redistributed at the periphery of Mn oxides rather than Fe oxides (see Bataillard
(2002) for details).
Moreover, vernadite possess a redox potential that allows for the degradation of
many natural organic acids (e.g., fulvic and humic acids) and organic pollutants.
Although beyond the scope of the present section and consequently not detailed
here, it must be noted that vernadite is capable of degrading humic substances and
pollutants such as herbicides and phenolic compounds, but also pathogenic prions
(Barrett and McBride 2005; Chang Chien et al. 2009; Russo et al. 2009; Sunda and
Kieber 1994). This might stem from possible coexistence, in its structure, of Mn(II),
Mn(III), and Mn(IV) oxidation degrees.
7.2.3 Relation Between Layered and Tunnel Structures:
Mechanisms, Relevance for Soils and Implication
for Soil Mn Reactivity
In the previous sections, it has been emphasized that phyllomanganates and
tectomanganates are the common Mn oxides. Because these two groups are often
found associated in a given soil sample, and because some structural similarities
exist (Bodeï et al. 2007), it has been suspected for long that a link exists between
406
S. Grangeon et al.
The ability of Mn oxides to sorb transition metals and actinides, thus controlling or
influencing significantly their speciation and mobility, has been recognized for long,
in virtually all environments, including soils as well as freshwater and marine
sediments (e.g., Aplin and Cronan 1985; Bodeï et al. 2007; Chukhrov et al. 1985;
Duff et al. 1999; Exon et al. 2002; Friedl et al. 1997; Hochella Jr et al. 2005; Isaure
et al. 2005; Koschinsky and Halbach 1995; Koschinsky and Hein 2003; Lei and
Boström 1995; Manceau et al. 2004, 2003; Marcus et al. 2004; McKenzie 1980;
Ostwald and Frazer 1973). This exceptional reactivity, which explains why Mn
oxides are often described as “scavengers” (Goldberg 1954), is generally only
competed by iron oxides, depending on the trace metal considered. For example,
Dong et al. (2002) studied the relative abilities of the three main components of
surface coatings (Mn oxides, Fe oxides, and organic material) to uptake Pb, and
found that Mn oxides were the most efficient Pb sorbing phase. Such exceptional
reactivity is exemplified in Fig. 7.5, which shows the Pb content of a Mn oxide grain
from the top horizon of a carbonated soil. In this study, the soil material has been
artificially contaminated using PbS particles (final Pb content % 300 mg kg
À1 ). Soil
and pollutant have first been mechanically dry mixed. Then, the mixture has been
reaggregated by hand with progressive addition of determined volumes of deionized
aerated water and kept wet at 85% of its water holding capacity for 3 months at 25
C
(Bataillard et al. 2003). Scanning electron microscopy, equipped with an energy
dispersive spectrometer (EDS), shows that Pb, from PbS particles, has been partly
redistributed at the periphery of Mn oxides rather than Fe oxides (see Bataillard
(2002) for details).
Moreover, vernadite possess a redox potential that allows for the degradation of
many natural organic acids (e.g., fulvic and humic acids) and organic pollutants.
Although beyond the scope of the present section and consequently not detailed
here, it must be noted that vernadite is capable of degrading humic substances and
pollutants such as herbicides and phenolic compounds, but also pathogenic prions
(Barrett and McBride 2005; Chang Chien et al. 2009; Russo et al. 2009; Sunda and
Kieber 1994). This might stem from possible coexistence, in its structure, of Mn(II),
Mn(III), and Mn(IV) oxidation degrees.
7.2.3 Relation Between Layered and Tunnel Structures:
Mechanisms, Relevance for Soils and Implication
for Soil Mn Reactivity
In the previous sections, it has been emphasized that phyllomanganates and
tectomanganates are the common Mn oxides. Because these two groups are often
found associated in a given soil sample, and because some structural similarities
exist (Bodeï et al. 2007), it has been suspected for long that a link exists between
406
S. Grangeon et al.