Vernadite is a nanocrystalline manganese oxide, which controls the fate of many trace elements in soils and sediments through sorption and oxidative-degradation mechanisms. This exceptional reactivity directly results from its crystal structure, which may however evolve upon contact with redox-sensitive species. Understanding these changes is a prerequisite to predict and model the geochemical cycle of trace elements in the environment. Here, the structural and morphological modifications affecting synthetic nanocrystalline vernadite (δ-MnO2) upon contact with increasing concentrations of Mn2+ were investigated using wet chemistry, synchrotron X-ray diffraction and transmission electron microscopy. Fresh δ-MnO2 crystals had an Mn oxidation state of 3.94 ± 0.05 and a ∼10 Å layer-to-layer distance. Crystal size was ∼10 nm in the layer plane, and ∼1 nm perpendicular to that. Upon contact with aqueous Mn2+ under anoxic conditions, δ-MnO2 crystals underwent several morphological and mineral evolutions, starting with the stacking, perpendicular to the layer plane, of δ-MnO2 crystals to form crystals ∼10 nm × 2 nm which were then subjected to oriented aggregation both along and perpendicular to the layer plane to form lath-like crystals with dimensions of ∼100 nm × 20 nm. Finally, these laths stacked perpendicular to the layer plane to form synthetic feitknechtite (β-MnOOH) crystals with sizes up to ∼100 nm × 500 nm when the Mn2+ loading reached 31.9 mmol g−1. Structural transformation from δ-MnO2 to synthetic feitknechtite was detected at Mn2+ loading equal to or higher than 3.27 mmol g−1.
These mechanisms are likely to influence the geochemical fate of trace elements in natural settings where Mn2+ is abundant. Firstly, the systematic increase in crystal size with increasing Mn2+ loading may impact the sorption capacity of vernadite and feitknechtite by reducing the density of reactive edge sites. Secondly, the fate of trace elements initially sorbed at the vernadite surface is unclear, as they could either be released in solution or incorporated into the feitknechtite lattice.
Feitknechtite, whose structure is built of layers of (Mn3+O6)9− octahedra, may upon ageing convert to manganite (γ-MnOOH) at pH 7.0–7.5 or to hausmannite (Mn3O4) at pH > 8 (Elzinga, 2011; Lefkowitz et al., 2013; Lefkowitz & Elzinga, 2015).
The interplay between these structural transformations certainly influences the geochemical cycle of trace elements by modifying the reactivity of Mn oxides through a change in the nature and density of local crystal charges, but also by modifying the fate of trace elements initially adsorbed at the vernadite surface which, by analogy with the phyllomanganate-to-tectomanganate transformation, could either be incorporated in the structure during transformation or released to solution (Atkins et al., 2014, 2016; Grangeon et al., 2015).
The present study aimed at contributing to a better understanding of the vernadite-to-feitknechtite transformation, and in particular at elucidating the reaction mechanisms at the crystal scale, using wet chemistry, X-ray diffraction (XRD) and transmission electron microscopy (TEM).
2. Materials and methods
2.1. Sample synthesis, sorption experiments and chemical analyses
The dialysis membrane was opened and the solid separated by filtration (0.01 μm cut-off) before being washed with bi-distilled water. An aliquot was freeze-dried for TEM analysis, whereas the remaining was sealed in a polyimide capillary for XRD analysis in the wet state within 48 h. Samples were labelled MndBi-XX, where XX is the Mn2+ loading (in mmol Mn2+ per g of dry δ-MnO2).
2.2. Synchrotron X-ray diffraction
The XRD experiments were carried out at station CRISTAL (SOLEIL synchrotron in Orsay, France), using an energy of 28.44 keV (λ = 0.436 Å) and an XPad hybrid pixel detector. Data were recorded over the 1.2–124.5° 2θ range with a total collection time of 30 min and processed with a specific software (Ounsy et al., 2013). To ease comparison with previous studies (e.g., Grangeon et al., 2015, 2017), diffraction data will be expressed relative to the scattering vector q, where q = [4 × π × sin(θ)]/λ.
2.3. Transmission electron microscopy
The TEM experiments were carried out using a Philips CM20 microscope operated at 200 kV. Prior to observation, samples were embedded in epoxy resin, left to polymerize for 48 h in the dark and cut with an ultramicrotome (Reichert-Jung Ultra-cut E) equipped with a diamond knife. The ∼80 nm thick sections were picked up on lacey carbon films loaded on Cu grids. In addition, to assess possible preparation-induced artefacts (use of an ultramicrotome), a sample identical to MndBi-0.5 (Table 1) was prepared according to the above described protocol, filtered and re-suspended in alcohol. A drop of the obtained suspension was deposited on a Cu grid. Fast-Fourier transform (FFT) analysis of the micrographs was done with ImageJ (Schneider et al., 2012).
3. Results and discussion
3.1. Determination of Mn2+ retention coefficient
3.2. Mineralogical evolution
Such an evolution was consistent with previous studies (Elzinga, 2011, 2016; Lefkowitz et al., 2013; Lefkowitz & Elzinga, 2015). In the following section, the mechanisms of transformation were studied at the crystal scale, hypothesizing that an increased concentration of Mn2+ led to an increased conversion of δ-MnO2 to synthetic feitknechtite without significant dissolution of δ-MnO2. In other words, it was hypothesized that the structural and morphological evolution of samples following their contact with a given concentration of Mn2+(aq) underwent intermediate recrystallization steps similar to those observed during post-mortem analysis of samples in contact with lower concentrations of Mn2+(aq). This hypothesis is supported by previous TEM observations, from which it was concluded that there was a lack of significant dissolution/reprecipitation during the recrystallization of δ-MnO2 when in contact with aqueous MnSO4 (Tu et al., 1994). Absence of δ-MnO2 dissolution is further substantiated by the study of Elzinga & Kustka (2015) showing that the first steps of transformation involves solely comproportionation and disproportionation reactions induced by interfacial electron transfer between adsorbed Mn2+ and Mn4+(s). Experimental validation of this hypothesis would require performing a kinetic experiment at high Mn2+(aq) concentration to check whether recrystallization steps observed as a function of Mn2+ concentration are similar to those occurring as a function of time. Such a procedure was applied to the study of the δ-MnO2 to synthetic todorokite transformation (Atkins et al., 2014, 2016), but was not possible in the present case because of reaction kinetics. The RD values obtained in the present study after 1 d of contact time were identical to those reported previously for a contact time of 8 d (Fig. 1). Steady-state was thus reached in less than 1 d of contact time, consistent with previous observations showing that feitknechtite is formed in less than 20 min (Johnson et al., 2016) and that sorption of Mn2+ at the δ-MnO2 surface is completed within less than 1 s (Fendorf et al., 1993).
3.3. Morphological evolution during δ-MnO2 to synthetic feitknechtite transformation
Observation of MndBi-0.5 evinced that a Mn2+ loading as low at 0.49 mmol g−1 led to the formation of crystals with contrasting morphologies and sizes, ranging ∼10–200 nm in the layer plane (Fig. 4a). In addition to type-1 crystals, three types of such crystals were identified:
- Type-2 crystals had a size of ∼10 nm in the layer plane, ∼2–3 nm perpendicular, and were bent (Fig. 4b), probably as a result of the incorporation of Mn3+(s) in layers containing mainly Mn4+. The layer-to-layer distance was ∼5.5 Å, typical for dehydrated phyllomanganates (Cygan et al., 2012; Wegorzewski et al., 2015). Dehydration was possibly induced by sample freeze-drying or by the secondary vacuum in the TEM. For type-1 and type-2 crystals having similar lateral dimensions, it is proposed that the latter formed from the coherent stacking of the former;
Type-3 crystals were ∼50 nm in the layer plane, and were built of ∼10–20 layers stacked parallel to each other, with a layer-to-layer distance of ∼5.5 Å (Fig. 4c, d). As type-2 crystals, they were bent. Images of the lateral terminations of type-3 crystals showed that they consisted of stacked type-2 crystals (Fig. 4c). Layer dislocations were observed also on some crystals (Fig. 4d), suggesting that type-2 crystals connected also within the layer plane to form type-3 crystals. Most likely, the type-3 crystals thus originated from the oriented aggregation of type-2 crystals, both within the layer plane and perpendicular to it;
Type 4 crystals had lath-like shape, dimensions of ∼50–100 nm in the layer plane and ∼10–20 nm perpendicular (Fig. 4e), layer-to-layer distance of 5.5 Å (inset in Fig. 4e), and were rarely found as stacks (Fig. 4f). It is speculated that they resulted from the same aggregation mechanisms proposed for the type-3 crystals, but with fewer defects between connected type-2 crystals and/or a specific distribution of Mn3+(s) so as to minimize layer strains and bending. Note that layer bending and dislocations observed in type-3 crystals cannot be related to sample preparation, as crystals deposited from a drop of a δ-MnO2 suspension exhibited similar microstructural features (Fig. 4g, h).
These observations demonstrate that the surface of δ-MnO2 is modified at loadings as low as 0.5 Mn2+ per nm2. As a consequence, surface complexation models, which rely on a reversible sorption hypothesis (Tournassat et al., 2013), may not be suited to describe Mn2+ interaction with δ-MnO2.
3.4. The role of oriented aggregation in nanoparticle growth
The present description of the vernadite-to-feitknechtite transformation, from macroscopic to crystal scales, is consistent with our current understanding of this transformation (Elzinga, 2011, 2016; Lefkowitz et al., 2013; Lefkowitz & Elzinga, 2015). One of the key findings is the major role of oriented aggregation in the nucleation and growth of feitknechtite from δ-MnO2 precursors. This process is increasingly recognized as one of the main processes responsible for nanoparticle growth (Penn, 2004; Niederberger & Colfen, 2006; Yuwono et al., 2010): carbonates (Shen et al., 2006), iron (Jia & Gao, 2008a and b) and manganese (Portehault et al., 2007) oxides. It was also shown to be responsible for the phase transformations of nanoparticle precursors (e.g., Hockridge et al., 2009). Recently, oriented aggregation was shown to be involved in the formation of synthetic todorokite and cryptomelane from a δ-MnO2 precursor (Chen et al., 1986; Atkins et al., 2014, 2016; Grangeon et al., 2015) and in the change of δ-MnO2 layer symmetry, from hexagonal to orthogonal (Zhao et al., 2016). It is thus possible to speculate that Mn oxides found in surficial environments were to a large extent formed from vernadite precursors (Bodeï et al., 2007; Xu et al., 2010), possibly through oriented aggregation mechanisms.
4. Concluding remarks: environmental implications
Vernadite with hexagonal layer symmetry is a sink for many trace elements in the environment. In natural settings were Mn2+(aq) is abundant, such as in lake and marine water columns and in sediments (Burdige, 1993), interactions between vernadite and Mn2+(aq) may induce morphological and mineralogical changes that could impact its reactivity and the fate of trace elements sorbed at its surface. Two main structural transformations were identified during reaction of δ-MnO2 (synthetic vernadite) with Mn2+(aq). At a low Mn2+ to δ-MnO2 ratio (<3.27 mmol g−1), δ-MnO2 crystal size increased with Mn2+(aq). At a Mn2+ to δ-MnO2 ratio of 3.27 mmol g−1 or higher, δ-MnO2 was converted to synthetic feitknechtite, and the size of synthetic feitknechtite crystals was observed to increase with Mn2+(aq). δ-MnO2 with orthogonal layer symmetry was not detected as a transformation intermediate, possibly because experiments were conducted at slightly acidic pH (Zhao et al., 2016).
When crystal size increases, the specific surface area, and especially the contribution of particle edges to this surface, decreases. Vernadite edge sites being reactive (Simanova et al., 2015), the increase in vernadite crystal size when in contact with Mn2+(aq), even at low concentrations, can be detrimental to its trace-metal scavenging ability. By analogy, the same is expected for feitknechtite at high Mn2+ to δ-MnO2 ratio. The impact of the vernadite-to-feitknechtite transformation on the fate of trace elements remains unclear, however. The lack of any significant dissolution of δ-MnO2 potentially minimizes the release of trace metal elements along the reaction pathway, but their possible incorporation in the newly formed feitknechtite structure remains undocumented. Unlike tunnel structures, feitknechtite has been observed seldom in natural settings, probably because of its limited chemical stability and high sensitivity to fluctuation in redox conditions, which lead to its back-conversion to vernadite (Bargar et al., 2005). Consequently, no data are currently available to evaluate the affinity of trace elements for feitknechtite in natural settings. Similar to tunnel structures (Liu et al., 2004; Kumagai et al., 2005; Cui et al., 2011; Atkins et al., 2014), additional insights into vernadite reactivity in suboxic systems and its impact on the fate of trace metals could probably be sought experimentally as feitknechtite is frequently observed in laboratory experiments mimicking natural conditions (Hem et al., 1982; Murray et al., 1985; Wang et al., 2015).
S.G. acknowledges funding by the French National Research Agency (ANR, grant ANR-14-CE01-0006) and thanks Jacques Deparis, Nicolas Marty and Julie Philibert for fruitful discussions. SOLEIL data were acquired in the frame of proposal 20141260. A. Fernandez-Martinez and T. Conte are respectively thanked for help during XRD data collection and Mn2+ measurement. This article benefited from comments and suggestions by an anonymous reviewer and Alain Baronnet.