7.4.1 Use of Mn Oxides for Oxidation of both Organic
and Inorganic Compounds
Mn oxides can be used as oxidants for soil remediation of a wide range of organic
and inorganic pollutants: azo compounds (Clarke et al. 2010), bisphenol A (Lin et al.
2009), or bisphenol F (Lu et al. 2011), pharmaceutical chemicals such as hormones
(Han et al. 2015; Xu et al. 2008) or antibiotics (Gao et al. 2012; Rubert and Pedersen
2006; Wang et al. 2015a; Zhang and Huang 2005), trivalent arsenic (Lafferty et al.
2010a; Wang et al. 2015a).
The reaction pathways are similar for the oxidation of all the phenolic compounds. Electron transfer from the phenolic groups to the Mn oxide generates
phenoxy radicals which undergo resonance rearrangement (Clarke et al. 2010; Lin
et al. 2009; Lu et al. 2011). The radicals are then coupled and forms intermediate
products, which could be oxidized again in the presence of MnO 2 . A wide variety of
final products can be formed (up to 11 compounds identified, see Lin et al. 2009),
both aromatic and nonaromatic. Environmental implications of such degradation are
not well-known, even if the degradation rates are generally high. The toxicity of the
newly formed phenolic compounds is not understood and some final products can
still have estrogenic properties (Lin et al. 2009). Moreover, some soil parameters can
inhibit the reactions, such as dissolved organic matter for phenolic compounds
(Lu et al. 2011) or high pH (>6) for some antibiotics and phenolic compounds
(Gao et al. 2012; Lu et al. 2011). Transformation of these organic compounds is also
accompanied by the dissolution of the Mn oxides and the release of Mn(II) in the
waters.
Oxidation could also have reverse effects on remediation in specific conditions.
For instance, a common remediation method for hexavalent chromium is reduction,
under biotic or abiotic conditions (e.g., Dresel et al. 2011). It has been demonstrated
that in subsurface environments, abiotically formed Cr(III) solids are more susceptible to reoxidation of Cr(III) to mobile Cr(VI) in the presence of the manganese
oxide birnessite (Butler et al. 2015). More generally, various studies have pointed
out that a wide variety of Mn oxides are able to oxidize Cr(III) (Eary and Rai 1987).
The use of Mn oxides for trivalent arsenic [As(III)] oxidation is of interest since
As(III) has been reported to be the most labile and toxic form of As (Smedley and
Kinniburgh 2002). As(III) oxidation kinetics in natural conditions is very low (e.g.,
⁄
ä
Fig. 7.6 (continued) experimental data (solid black line) with the best simulation obtained (solid
red line). From this simulation, the structure is turbostratic, has a crystallite size in the ab plane of
7.4 nm, has a ¼ 4.9208 Å (a ¼ √3 Â b), the abundance of interlayer Mn capping a layer vacancy is
equal to 0.24 per layer octahedron, and the interlayer also contains 0.15 Na
+ and 0.45 H 2 O per layer
octahedron. (c), (d), (e), and (f) exemplify the sensitivity of calculated XRD patterns to main layer
characteristics: in (c), interlayer Mn has been omitted. In (d), a has been reduced by 2.5%. In (e), the
size of the crystallites has been reduced by 0.7 nm, and in (f), the abundance of random stacking
faults has been reduced to 80%. In each of these sensitivity tests, arrows point out to main
discrepancies between experiment and calculations
7 The Nature of Manganese Oxides in Soils and Their Role as Scavengers of Trace. . .
415
and Inorganic Compounds
Mn oxides can be used as oxidants for soil remediation of a wide range of organic
and inorganic pollutants: azo compounds (Clarke et al. 2010), bisphenol A (Lin et al.
2009), or bisphenol F (Lu et al. 2011), pharmaceutical chemicals such as hormones
(Han et al. 2015; Xu et al. 2008) or antibiotics (Gao et al. 2012; Rubert and Pedersen
2006; Wang et al. 2015a; Zhang and Huang 2005), trivalent arsenic (Lafferty et al.
2010a; Wang et al. 2015a).
The reaction pathways are similar for the oxidation of all the phenolic compounds. Electron transfer from the phenolic groups to the Mn oxide generates
phenoxy radicals which undergo resonance rearrangement (Clarke et al. 2010; Lin
et al. 2009; Lu et al. 2011). The radicals are then coupled and forms intermediate
products, which could be oxidized again in the presence of MnO 2 . A wide variety of
final products can be formed (up to 11 compounds identified, see Lin et al. 2009),
both aromatic and nonaromatic. Environmental implications of such degradation are
not well-known, even if the degradation rates are generally high. The toxicity of the
newly formed phenolic compounds is not understood and some final products can
still have estrogenic properties (Lin et al. 2009). Moreover, some soil parameters can
inhibit the reactions, such as dissolved organic matter for phenolic compounds
(Lu et al. 2011) or high pH (>6) for some antibiotics and phenolic compounds
(Gao et al. 2012; Lu et al. 2011). Transformation of these organic compounds is also
accompanied by the dissolution of the Mn oxides and the release of Mn(II) in the
waters.
Oxidation could also have reverse effects on remediation in specific conditions.
For instance, a common remediation method for hexavalent chromium is reduction,
under biotic or abiotic conditions (e.g., Dresel et al. 2011). It has been demonstrated
that in subsurface environments, abiotically formed Cr(III) solids are more susceptible to reoxidation of Cr(III) to mobile Cr(VI) in the presence of the manganese
oxide birnessite (Butler et al. 2015). More generally, various studies have pointed
out that a wide variety of Mn oxides are able to oxidize Cr(III) (Eary and Rai 1987).
The use of Mn oxides for trivalent arsenic [As(III)] oxidation is of interest since
As(III) has been reported to be the most labile and toxic form of As (Smedley and
Kinniburgh 2002). As(III) oxidation kinetics in natural conditions is very low (e.g.,
⁄
ä
Fig. 7.6 (continued) experimental data (solid black line) with the best simulation obtained (solid
red line). From this simulation, the structure is turbostratic, has a crystallite size in the ab plane of
7.4 nm, has a ¼ 4.9208 Å (a ¼ √3 Â b), the abundance of interlayer Mn capping a layer vacancy is
equal to 0.24 per layer octahedron, and the interlayer also contains 0.15 Na
+ and 0.45 H 2 O per layer
octahedron. (c), (d), (e), and (f) exemplify the sensitivity of calculated XRD patterns to main layer
characteristics: in (c), interlayer Mn has been omitted. In (d), a has been reduced by 2.5%. In (e), the
size of the crystallites has been reduced by 0.7 nm, and in (f), the abundance of random stacking
faults has been reduced to 80%. In each of these sensitivity tests, arrows point out to main
discrepancies between experiment and calculations
7 The Nature of Manganese Oxides in Soils and Their Role as Scavengers of Trace. . .
415