High Co-doping promotes the transition of birnessite layer symmetry from orthogonal to hexagonal

Transformation of Co-containing birnessite to todorokite: Effect of Co on the transformation and implications for Co mobility

“…For the CoB samples, the distribution of Co between incorporated (~80%) and surface adsorbed (~20%) species is consistent with previous reports (Manceau et al, 1997;Kwon et al, 2013;Yin et al, 2013Yin et al, , 2015. For the CoT samples, which contain a 592 mixture of layered (~80-70%) and tunnel (~20-30%) structure Mn oxides, the 593 distribution of Co between incorporated (~80%) and surface (~20%) species reflects 594 the average coordination environment.…”

“…As a result, the a/b 294 ratio decreases from 1.818 to 1.736 (Table 2). Taken together, these changes indicate that the layer symmetry of Bir, CoB5 and CoB10 is orthogonal (a/b > √3), while the layer symmetry of CoB15 and CoB20 is hexagonal (a/b ≈ √3), as reported in Yin et al (2015). The orthogonal layer symmetry in Bir is associated with the substitution of Mn(III) (0.58 Å, low spin) for Mn(IV) (0.53 Å) (Manceau et al, 1997), and the 299 systematic elongation of Jahn-Teller distorted Mn(III) octahedra parallel to the a* axis (Gaillot et al, 2007).…”

“…The orthogonal layer symmetry in Bir is associated with the substitution of Mn(III) (0.58 Å, low spin) for Mn(IV) (0.53 Å) (Manceau et al, 1997), and the 299 systematic elongation of Jahn-Teller distorted Mn(III) octahedra parallel to the a* axis (Gaillot et al, 2007). Here, hexagonal symmetry is likely caused by the 301 replacement of Mn(III) by Co(III) in the layer, where the ionic radius of Co(III) (0.54 Å, low spin) is sufficiently similar to that of Mn(III) to allow isomorphic substitution (Yin et al, 2015). With an increase in Co content however, the peaks of the CoB 304 samples shift to higher angles, compared to Bir, and this is enhanced in the high-angle 305 region ( Fig.…”

“…Table Table Fractional and average valence states of Mn obtained from a Combo fit of Mn K-edge XANES 1st derivative spectra of layered precursors and reflux products. The estimated error for Combo method is ±4% (Manceau et al, 2012;Yin et al, 2015) Coordination numbers (CN) and Debye-Waller factors ( 2 ) of Co-O shells were refined together, as were Debye-Waller factors of the two Co-Me shells.…”

“…A combination of polarized extended X-ray absorption fine structure (EXAFS) 141 spectroscopy and X-ray diffraction (XRD) shows that both layer and interlayer Mn(III) 142 can oxidize Co(II) and lead to Co(III) migration into the octahedral layers (Manceau et al, 1997;Yu et al, 2012). Both Co(II) oxidation and the subsequent migration of 144 Co(III) into the mineral structure appear to be key to the high capacity of layered Mn 145 oxides for Co scavenging (Yin et al, 2014) and their resulting Co-enrichment 146 (Loganathan and Burau, 1973;Davies and Morgan, 1989).As Co(III)O 6 octahedra are 147 not Jahn-Teller distorted, Co-enrichment in birnessite layers may modify layer 148 symmetry from orthogonal to hexagonal (Yin et al, 2015). This enrichment may also 149 impact the ability of Co-containing phyllomanganates to transform into 150 tectomanganates as the abundance of distorted Mn(III) octahedra within the 151 octahedral layers appears to be a key factor for the formation of tunnel structures 152 (Burns et al, 1985;Bodeï et al, 2007;Cui et al, 2008;Atkins et al, 2014Atkins et al, , 2016Grangeon et al, 2014Grangeon et al, , 2015Zhao et al, 2015).…”