complicates the determination of their exact mineralogical nature. Consequently, Mn
oxides are often termed “amorphous Mn oxides,” “HMO” (hydrous manganese
oxides), or “wad” (Post 1999), this latter being an ill-defined termed referring to a
manganese-rich black and soft substance, often found deposited by springs. However, these ambiguous terms hide the real nature of the precipitates, and consequently prevent a sound understanding of the environmental Mn mineralogy.
This chapter aims first at providing an introduction regarding the complex
mineralogy of Mn oxides and at highlighting the role of Mn oxides in the geochemical cycle of trace elements in pristine and polluted environments. Studying the
association between Mn oxides and trace elements that exists in natural settings and
which are representative of long-term interaction between Mn oxides and their
surrounding environments may provide insights regarding the design of efficient
engineered reactive barriers able to immobilize trace elements in polluted or
man-made environments. The use of Mn oxides for soil remediation is reviewed in
the last section of this chapter.
7.2 The Structural Variety of Manganese Oxides Found
in Soils, Their Relationships, and Their Affinity
for Trace Elements
7.2.1 Main Mn Oxides Identified in Soils
The term “Mn oxide” hides an impressive variety of minerals. At least 30 different
crystal structures occurring in a large variety of geological settings have been
reported (Post 1999). This exceptional diversity stems from a combination of several
factors, the two main being (1) the large diversity of temperature/pressure systems
that can be encountered on Earth and (2) the fact that Mn occurs under many
different oxidation degrees, depending on the Eh/pH conditions prevailing in the
medium; in the specific case of surficial environment, Mn is commonly reported to
exist under the Mn(II), Mn(III), and Mn(IV) oxidation degrees, either forming a
mineral of homogeneous (e.g., pyrolusite, feitknechtite, chalcophanite) or heterogeneous (e.g., vernadite) Mn oxidation degree.
The two main types of crystal structures that are observed in surficial environment
are tunnel and layered structures (e.g., Bodeï et al. 2007; Post 1999). In both of them,
the “elementary unit” is the MnO 2 octahedron, with Mn having mainly an oxidation
degree of IV, but the presence of the Mn(III) and, to a minor extent, Mn(II) is
common. In layered structures, the octahedra are connected by their edges to form
sheets that are separated from each other by a hydrated interlayer space containing
exchangeable cations, whereas the tunnel structures are built of the regular arrangement of MnO 6 octahedra that form [n  m] tunnel structures, where n is the number
of octahedra connected to form the walls and m is the number of octahedra forming
the ceiling and the floor. The space within the tunnel is filled by water and cations
7 The Nature of Manganese Oxides in Soils and Their Role as Scavengers of Trace. . .
401
oxides are often termed “amorphous Mn oxides,” “HMO” (hydrous manganese
oxides), or “wad” (Post 1999), this latter being an ill-defined termed referring to a
manganese-rich black and soft substance, often found deposited by springs. However, these ambiguous terms hide the real nature of the precipitates, and consequently prevent a sound understanding of the environmental Mn mineralogy.
This chapter aims first at providing an introduction regarding the complex
mineralogy of Mn oxides and at highlighting the role of Mn oxides in the geochemical cycle of trace elements in pristine and polluted environments. Studying the
association between Mn oxides and trace elements that exists in natural settings and
which are representative of long-term interaction between Mn oxides and their
surrounding environments may provide insights regarding the design of efficient
engineered reactive barriers able to immobilize trace elements in polluted or
man-made environments. The use of Mn oxides for soil remediation is reviewed in
the last section of this chapter.
7.2 The Structural Variety of Manganese Oxides Found
in Soils, Their Relationships, and Their Affinity
for Trace Elements
7.2.1 Main Mn Oxides Identified in Soils
The term “Mn oxide” hides an impressive variety of minerals. At least 30 different
crystal structures occurring in a large variety of geological settings have been
reported (Post 1999). This exceptional diversity stems from a combination of several
factors, the two main being (1) the large diversity of temperature/pressure systems
that can be encountered on Earth and (2) the fact that Mn occurs under many
different oxidation degrees, depending on the Eh/pH conditions prevailing in the
medium; in the specific case of surficial environment, Mn is commonly reported to
exist under the Mn(II), Mn(III), and Mn(IV) oxidation degrees, either forming a
mineral of homogeneous (e.g., pyrolusite, feitknechtite, chalcophanite) or heterogeneous (e.g., vernadite) Mn oxidation degree.
The two main types of crystal structures that are observed in surficial environment
are tunnel and layered structures (e.g., Bodeï et al. 2007; Post 1999). In both of them,
the “elementary unit” is the MnO 2 octahedron, with Mn having mainly an oxidation
degree of IV, but the presence of the Mn(III) and, to a minor extent, Mn(II) is
common. In layered structures, the octahedra are connected by their edges to form
sheets that are separated from each other by a hydrated interlayer space containing
exchangeable cations, whereas the tunnel structures are built of the regular arrangement of MnO 6 octahedra that form [n  m] tunnel structures, where n is the number
of octahedra connected to form the walls and m is the number of octahedra forming
the ceiling and the floor. The space within the tunnel is filled by water and cations
7 The Nature of Manganese Oxides in Soils and Their Role as Scavengers of Trace. . .
401