(Grangeon et al. 2017). If this is the case, sorption may be considered as autocatalytic, because it is accompanied by the creation of new sorption sites.
The sorption above vacancies is, for some trace metals, only the first step in their
interaction with vernadite. When the ionic radius of the trace metal sorbed is similar
that of Mn(III), the metal can migrate from its triple-corner sharing position to a layer
site, and fill the layer vacancy. This is for example the case for Ni, which only exists
in as Ni(II) in most environmental conditions, and which has an effective ionic
radius of 0.69 Å when sixfold coordinated to O, as compared to 0.645 Å for Mn(III)
in similar conditions (Shannon 1976). When observed, migration from the interlayer
to the layer is generally found to be favored by long interaction times and by neutral
to slightly basic pH condition (Peacock 2009; Peacock and Sherman 2007b). The
inhibition of migration in acidic conditions is ascribed to the protonation of O atoms
present on the other side of the vacancy as compared to the trace metal sorption site,
which creates electrostatic repulsion (Manceau et al. 2007b). When the trace metal of
interest possesses several possible oxidation states, the mechanisms are even more
complex and generally imply a change in vernadite structure. For example, when
Co(II) interacts with a phyllomanganate layer containing Mn(IV) and a proportion of
Mn(III), it first sorbs above a layer vacancy. Then, if a layer Mn(III) is adjacent,
Co(II) is oxidized to Co(III) and migrates from the interlayer to the layer, while
Mn(III) is reduced to Mn(II) and migrates either to the interlayer or in solution. This
creates a new layer vacancy, that will autocatalyze the mechanism of Co(II) sorption,
oxidation, and migration with the layer (Manceau et al. 1997).
These examples of trace metal interaction with vernadite surface highlight that
understanding why vernadite has such an exceptional affinity for trace metals
requires identifying and quantifying metal sorption sites, including the size of the
crystals, and the nature, abundance, and distribution of layer defects (in particular
layer isomorphic substitutions and vacancies).
7.3.2 Kinetics of Ion Uptake and Oxidation
The exceptional reactivity of phyllomanganates toward cations is further reinforced
by the uptake and oxidation kinetics. It has been shown that the mechanisms of
exchange of alkalis and alkaline earth metals, which sorb as outer-sphere complexes
in the interlayer space, proceed within the first hour of contact time (Lopano et al.
2011, 2009, 2007). The mechanisms of ion oxidation are even faster. For example,
less than 5 min are sufficient to oxidize more than 30% of As(III) at a concentration
of 5Á10
À3 M to As(V) by 10 g L
À1 vernadite (Ginder-Vogel et al. 2009). Consistently, Fendorf et al. (1993) could evince that the sorption of 40 10
À6 M Mn
2+ by
1 g L
À1 vernadite is completed within less than 1 s, with more than 80% of Mn
2+
being sorbed within the first 0.2 s. This sorption then induces redox reactions
between the sorbed Mn
2+ and vernadite (see Sect. 7.2.4). Finally, in the case of
transitions metals, with which there is no redox reaction occurring with vernadite,
412
S. Grangeon et al.
The sorption above vacancies is, for some trace metals, only the first step in their
interaction with vernadite. When the ionic radius of the trace metal sorbed is similar
that of Mn(III), the metal can migrate from its triple-corner sharing position to a layer
site, and fill the layer vacancy. This is for example the case for Ni, which only exists
in as Ni(II) in most environmental conditions, and which has an effective ionic
radius of 0.69 Å when sixfold coordinated to O, as compared to 0.645 Å for Mn(III)
in similar conditions (Shannon 1976). When observed, migration from the interlayer
to the layer is generally found to be favored by long interaction times and by neutral
to slightly basic pH condition (Peacock 2009; Peacock and Sherman 2007b). The
inhibition of migration in acidic conditions is ascribed to the protonation of O atoms
present on the other side of the vacancy as compared to the trace metal sorption site,
which creates electrostatic repulsion (Manceau et al. 2007b). When the trace metal of
interest possesses several possible oxidation states, the mechanisms are even more
complex and generally imply a change in vernadite structure. For example, when
Co(II) interacts with a phyllomanganate layer containing Mn(IV) and a proportion of
Mn(III), it first sorbs above a layer vacancy. Then, if a layer Mn(III) is adjacent,
Co(II) is oxidized to Co(III) and migrates from the interlayer to the layer, while
Mn(III) is reduced to Mn(II) and migrates either to the interlayer or in solution. This
creates a new layer vacancy, that will autocatalyze the mechanism of Co(II) sorption,
oxidation, and migration with the layer (Manceau et al. 1997).
These examples of trace metal interaction with vernadite surface highlight that
understanding why vernadite has such an exceptional affinity for trace metals
requires identifying and quantifying metal sorption sites, including the size of the
crystals, and the nature, abundance, and distribution of layer defects (in particular
layer isomorphic substitutions and vacancies).
7.3.2 Kinetics of Ion Uptake and Oxidation
The exceptional reactivity of phyllomanganates toward cations is further reinforced
by the uptake and oxidation kinetics. It has been shown that the mechanisms of
exchange of alkalis and alkaline earth metals, which sorb as outer-sphere complexes
in the interlayer space, proceed within the first hour of contact time (Lopano et al.
2011, 2009, 2007). The mechanisms of ion oxidation are even faster. For example,
less than 5 min are sufficient to oxidize more than 30% of As(III) at a concentration
of 5Á10
À3 M to As(V) by 10 g L
À1 vernadite (Ginder-Vogel et al. 2009). Consistently, Fendorf et al. (1993) could evince that the sorption of 40 10
À6 M Mn
2+ by
1 g L
À1 vernadite is completed within less than 1 s, with more than 80% of Mn
2+
being sorbed within the first 0.2 s. This sorption then induces redox reactions
between the sorbed Mn
2+ and vernadite (see Sect. 7.2.4). Finally, in the case of
transitions metals, with which there is no redox reaction occurring with vernadite,
412
S. Grangeon et al.