structure and induces a change in layer symmetry, from hexagonal to orthogonal
(Zhao et al. 2016). When the abundance of Mn
2+
(s) is further increased and/or when
pH is lower, vernadite irreversibly transforms to feitknechtite (β-Mn
3+ OOH),
another layered Mn oxide. Finally, upon aging, feitknechtite converts to hausmanite
(Mn 3 O 4 ) at pH > 8 or to manganite (γ-MnOOH) at lower pH 7–75 (Elzinga 2011,
2016; Lefkowitz et al. 2013). As these transformations significantly change Mn
oxides reactivity (Lefkowitz and Elzinga 2015; Matocha et al. 2001; Weaver and
Hochella 2003), they should be investigated with special care.
7.3 Layered Manganese Oxides: Origin of the Reactivity
and Difficulties of Characterization
7.3.1 Structural Characteristics Responsible for the Reactivity
of Layered Mn Oxides
As discussed in Sect. 7.2, phyllomanganates, and in particular vernadite, are one of
the most common soil Mn oxides. They may also be the precursor of tunnel
structures and play a major role in the geochemical cycle of both Mn and trace
elements.
Numerous studies were conducted that aimed at contributing to a better understanding of the structural features responsible for the exceptional reactivity of
vernadite (e.g., Appelo and Postma 1999; Drits et al. 2002; Grangeon et al. 2008;
Lanson et al. 2002b, 2008; Manceau et al. 1997; Peacock and Sherman 2007b; Peña
et al. 2010; Rihs et al. 2014; Zhu et al. 2010). A full description of these structural
characteristics is beyond the scope of the present chapter, and only the two main
parameters governing vernadite reactivity will be briefly mentioned. The first one is
the presence of sorption sites at the edges of vernadite layers. Indeed, with a typical
size in the layer plane of 10 nm (e.g., Bargar et al. 2009), ~10% of layer Mn atoms
are located at the edges of the crystals, with part of the O atoms, coordinating these
Mn atoms, being located on the border of the particle and subjected to protonation/
deprotonation mechanisms. These edge sites are accessible to trace metals, for
example for Ni
2+ and Pb
2+ (Simanova et al. 2015; Takahashi et al. 2007). The
second factor responsible for vernadite reactivity toward trace metals is the presence
of layer vacancies above which cations can sorb by forming inner-sphere complexes.
When the medium is free of any trace metal, this sorption site can be occupied by
Mn
3+ , as discussed previously. When metals are introduced in the solution at
equilibrium with vernadite, they may adsorb above the vacancies, and can replace
Mn
3+ initially present above the vacancies. This has for example been observed for
Zn
2+ and Pb
2+ (Grangeon et al. 2012; Zhao et al. 2009). When the layers do not only
contain Mn
4+ but also Mn
3+ , the sorption of the trace metal may, in certain cases, be
accompanied by the expulsion of a layer Mn
3+ , thus generating a new layer vacancy.
This has been observed for example for Zn
2+ (Grangeon et al. 2012) and Ni
2+
7 The Nature of Manganese Oxides in Soils and Their Role as Scavengers of Trace. . .
411
(Zhao et al. 2016). When the abundance of Mn
2+
(s) is further increased and/or when
pH is lower, vernadite irreversibly transforms to feitknechtite (β-Mn
3+ OOH),
another layered Mn oxide. Finally, upon aging, feitknechtite converts to hausmanite
(Mn 3 O 4 ) at pH > 8 or to manganite (γ-MnOOH) at lower pH 7–75 (Elzinga 2011,
2016; Lefkowitz et al. 2013). As these transformations significantly change Mn
oxides reactivity (Lefkowitz and Elzinga 2015; Matocha et al. 2001; Weaver and
Hochella 2003), they should be investigated with special care.
7.3 Layered Manganese Oxides: Origin of the Reactivity
and Difficulties of Characterization
7.3.1 Structural Characteristics Responsible for the Reactivity
of Layered Mn Oxides
As discussed in Sect. 7.2, phyllomanganates, and in particular vernadite, are one of
the most common soil Mn oxides. They may also be the precursor of tunnel
structures and play a major role in the geochemical cycle of both Mn and trace
elements.
Numerous studies were conducted that aimed at contributing to a better understanding of the structural features responsible for the exceptional reactivity of
vernadite (e.g., Appelo and Postma 1999; Drits et al. 2002; Grangeon et al. 2008;
Lanson et al. 2002b, 2008; Manceau et al. 1997; Peacock and Sherman 2007b; Peña
et al. 2010; Rihs et al. 2014; Zhu et al. 2010). A full description of these structural
characteristics is beyond the scope of the present chapter, and only the two main
parameters governing vernadite reactivity will be briefly mentioned. The first one is
the presence of sorption sites at the edges of vernadite layers. Indeed, with a typical
size in the layer plane of 10 nm (e.g., Bargar et al. 2009), ~10% of layer Mn atoms
are located at the edges of the crystals, with part of the O atoms, coordinating these
Mn atoms, being located on the border of the particle and subjected to protonation/
deprotonation mechanisms. These edge sites are accessible to trace metals, for
example for Ni
2+ and Pb
2+ (Simanova et al. 2015; Takahashi et al. 2007). The
second factor responsible for vernadite reactivity toward trace metals is the presence
of layer vacancies above which cations can sorb by forming inner-sphere complexes.
When the medium is free of any trace metal, this sorption site can be occupied by
Mn
3+ , as discussed previously. When metals are introduced in the solution at
equilibrium with vernadite, they may adsorb above the vacancies, and can replace
Mn
3+ initially present above the vacancies. This has for example been observed for
Zn
2+ and Pb
2+ (Grangeon et al. 2012; Zhao et al. 2009). When the layers do not only
contain Mn
4+ but also Mn
3+ , the sorption of the trace metal may, in certain cases, be
accompanied by the expulsion of a layer Mn
3+ , thus generating a new layer vacancy.
This has been observed for example for Zn
2+ (Grangeon et al. 2012) and Ni
2+
7 The Nature of Manganese Oxides in Soils and Their Role as Scavengers of Trace. . .
411