Order and Disorder in Layered Double Hydroxides: Lessons Learned from the Green Rust Sulfate─Nikischerite Series

Abstract

Layered double hydroxides (LDHs) occur naturally and are synthesized for catalysis, drug delivery, and contaminant remediation. They consist of Me­(II)–Me­(III) hydroxide sheets separated by hydrated interlayers and weakly held anions. Often, LDHs are nanocrystalline, and sheet stacking and Me­(II)–Me­(III) arrangement can be disordered, which influences the reactivity and complicates structural characterization. We have used pair distribution function (PDF) analysis to provide detailed information about local and medium range order (≤9 nm) and to determine the structure of synthetic Fe­(II)–Fe­(III)/Al­(III) LDH. The data are consistent with ordered Me­(II) and Me­(III) in hydroxide sheets, where structural coherence along the c axis decreases with increasing Al content. The PDF for Fe­(II)–Al­(III) LDH (nikischerite) is best matched by a pattern for a single metal hydroxide sheet. Parallel to decreased structural coherence between layers, coherence within layers decreased to ∼6 nm for synthetic nikischerite. Thus, the length scale of atomic ordering decreased within and between the sheets, resulting in mosaic crystals with coherent scattering domains decreasing in all directions. The high density of grain boundary terminations would affect reactivity. Based on classical nucleation theory and the Kossel crystal growth model, we propose that loss of structural coherence stems from increased supersaturation and the presence of Al-hydroxides during the formation of the Al-rich LDH.

Effects of Mn or Al incorporation on the structure, composition, and As(III) adsorption of oxidized green rust

“…1a), the presence of Al 3+ remarkably inhibited the crystallization of GR (Fig. 1c), consistent with the previous studies (Dideriksen et al, 2022;Refait et al, 2017;Ruby et al, 2008). After the addition of H2O2, the GR was quickly oxidized to Ox-GR, resulting in the rapid decrease of suspension pH and increase of suspension Eh (Fig.…”

“…The XRD patterns showed the characteristic peaks of GR(SO4 2-) at d-spacing values of ~10.5 Å, ~5.4 Å, and ~3.6 Å (i.e., black lines in Fig. 1a-1c) (Dideriksen et al, 2022;Perez et al, 2019), indicating the formation of GR at all conditions. Compared with the peak intensities of control system (Fig.…”

“…This should be ascribed to that the presence of Al substantially inhibits the crystal-growth of GR precursor (Fig. 1c) (Dideriksen et al, 2022;Refait et al, 2017), thus resulting in the poor crystalized of Ox-GR. In addition, with the increase of Mn or Al content, the d-spacing of (001) plane along the c-axis direction gradually shifted to a higher value (i.e., lower 2θ angle) (Fig.…”

“…A lot of studies have indicated that the Al(III) ions can replace Fe(III) in the layer of GR during its formation, thereby remarkably decreasing the crystallinity and coherent scattering domain size, varying the cell parameters, and favoring its transformation to goethite (Dideriksen et al, 2022;Genin and Ruby, 2004;Refait et al, 2017;Ruby et al, 2008;Trolard and Bourrié, 2006). In contrast, the effect of Mn on the crystallization of GR depends on its concentration, i.e., low Mn concentration slightly favors its crystallization but high Mn concentration slightly inhibits (Wang et al, 2019); the presence of Mn promotes the formation of lepidocrocite and magnetite during GR transformation (Inoue et al, 2007;Wang et al, 2019).…”

Dynamics and Mechanisms of Mn(II), Co(II), Ni(II), Zn(II), and Cd(II) Sorption onto Green Rust Sulfate

“…The coupled mechanisms of adsorption, exchange, and precipitation described above make the overall process of Me­(II) sorption sensitive to changes in GR sorbent properties, in particular, particle size (which controls the abundance of exchangeable Fe­(II) at edge sites , ) and crystallinity (which determines the solubility and stability of the GR sorbent). The variations observed between repeat experiments likely are due to such differences in sorbent GR properties, as noted above.…”