Conditions for the formation of pure birnessite during the oxidation of Mn(II) cations in aqueous alkaline medium

Birnessite was synthetized through redox reaction by mixing MnO4-, Mn2+ and OH- solutions. The Mn(VII): Mn(II) ratio of 0.33 was chosen and three methods were used consisting in a quick mixing under vigorous stirring of two of the three reagents and then on the dropwise addition of the third one. The obtained solids were characterized by XRD, FTIR and XPS spectroscopies. Their average oxidation states were determined from ICP and CEC measurements while their surface properties were investigated by XPS. This study provides an increased understanding of the importance of dissolved oxygen in the formation of birnessite and hausmannite and shows the ways to obtain pure birnessite. The role of counter-ion ie. Na+ or K+ was also examined.


Introduction
Birnessite is a mixed Mn(III)-Mn(IV) oxide constituted by layers of edge-sharing MnO6 octahedra separated by planes of hydrated cations (e.g. Na + , K + , Ca 2+ ) and water molecules. It was found to be one of the most common and active occurring Mn oxide in soils and sediments [1,2]. Jones and Milnes [3] proposed the formula Na0.7Ca0.3Mn7O14, 2.8H2O. This natural form presents a disordered structure [2,4,5]. Thereby, many studies were focused on ways to obtain birnessite. Besides the existing biogenic ways, the chemical methods could be divided in four groups based on the mechanism involved.
The first method is based on the Mn(II) oxidation in alkali media producing Mn(OH)2 and its oxidation by O2 bubbling to form buserite, the hydrated form of birnessite [6,7]. To avoid hausmannite formation as by-product many authors studied the mechanism involved. Intriguingly, Yang and Wang [8] prepared pure birnessite by O2 oxidation of Mn(OH)2 precipitate that was prepared from deoxygenated NaOH and MnCl2 solutions. They suggested that the use of deoxygenated water prevented the presence of Mn(III) responsible of the hausmannite formation. Feng et al. [9] showed that the formation of hausmannite was avoided when NaOH and MnCl2 were mixed below 10°C; this study also showed that an oxygen flow rate of 5 L min -1 during 5 hours is required to produce pure birnessite. Cai et al. [10] proposed a variant of Giovanoli's method by replacing O2 by air and they pointed that, in that case, a flow rate of 24 L min -1 is essential to avoid hausmannite formation. Pure birnessite was also obtained by using H2O2 instead of O2 as oxidant [11,12] The second way to obtain birnessite relies on the reduction of MnO4in concentrated HCl medium [13,14]. Numerous organic reducing agents were used such as fumaric acid [15], sugars [16,17], alcohols [12,18], ethylene glycol [19] and more recently epoxypropane [20] and lactate [21]. This method could be constraining as it involved a relatively long time aging step [12,18], a calcination step at temperature above 400-450°C [16,17] or a hydrothermal treatment [20].
The third method is based on a direct conversion of hausmannite to birnessite involving a dissolution/recrystallization mechanism [22] in alkaline medium. Birnessite was obtained for concentrations of OHabove 2 mol L -1 . The reaction required several weeks to be complete and the dissolution of hausmannite was found to be the rate-limiting step in this reaction.
The fourth process is based on a redox reaction between MnO4and Mn 2+ in alkaline conditions. A manganese salt (i.e. MnCl2, MnSO4) and sodium or potassium permanganate were generally used as Mn(II) and Mn(VII) suppliers respectively while NaOH or KOH were used to provide alkaline medium. This method was first reported by [23]; it consisted on adding slowly the MnCl2 solution to NaOH/NaMnO4 solution. The product presented a poor crystallinity similar to natural birnessite. Luo et al., [24] and Luo and Suib, [25] studied the influence of many parameters (temperature, basicity, MnO4 -/Mn 2+ ratio, presence of magnesium and anion effect) and showed that pure birnessite could be obtained in strong alkaline conditions with a MnO4 -/Mn 2+ ratio of 0.28-0.36 and at 40-65°C. The authors also pointed that feitknechtite (β-MnOOH) was an intermediate in birnessite formation. Villalobos et al. [26] followed this work and showed the presence of small amount of manganite (γ-MnOOH) in addition to birnessite.
The redox method gained more interest on the past decades as it overcomes the inconvenient of the other methods such as high consumption of O2, use of boiling solution with concentrated acid and strong oxidant and long reaction times. In spite of an extensive literature [27][28][29][30] the role of dissolved oxygen on the redox method remains unclear. Actually a reactive medium with an average oxidation state (AOS) of manganese close to 3.25 (Mn(VII):Mn(II) molar ratio of 0.33) led to birnessite with an AOS of Mn equal to 3.53 [29]. This suggests that the oxidation of Mn(II) leading to birnessite is not related to a unique redox reaction between Mn(II) and Mn(VII) and that dissolved oxygen may intervene in it as already observed in other methods such as Giovanoli's studied by Yang and Wang [8]. Besides, the mixing order of the three solutions generally involved (i.e. manganese salt, permanganate and base) varied from one study to another [23,29,30] and to our knowledge, no study has focused on the influence that could have this order of mixing on the nature of the obtained products. Moreover, the occurrence and possible role of hausmannite (Mn3O4), which can be observed as a by-product, is not clear.
The goals of this study are: i) to optimize the experimental conditions to obtain pure Nabirnessite or K-Birnessite through the redox method and ii) to get an insight into birnessite formation mechanism. A special attention is paid to the role of dissolved oxygen and to the mixing order of the reagents to prevent the side reaction leading to hausmannite. The X-ray photoelectron spectrometry is used in order to gain a more in-depth understanding of the target reaction mechanism.

Synthesis of birnessite
Three solutions were used for the synthesis of Na-birnessite: 125 mL of NaOH (8.8 mol L -1 ), 250 mL NaMnO4 (0.1 mol L -1 ) and 125 mL MnCl2 (0.6 mol L -1 ). Some syntheses were performed by using KMnO4 instead of NaMnO4 and KOH instead of NaOH. The Mn(VII):Mn(II) ratio of 0.33 was chosen accordingly to a previous study [29]. The typical procedure used in the present work consisted on a quick mixing under vigorous stirring of two of the three reagents and then on the addition of the third in one go (0 h) or during 2, 4 or 8 hours. The reaction mixture was then stirred for another 30 min and aged at 60ºC for 14 h. The product was finally centrifuged and washed until the pH of the solution was between 9 and 10 and dried at 60°C during 16 h. The designation of the three methods refers to the nature of the third reagent added. Thus, the first method, named the "reductant-last method", consists on a dropwise addition of the reductant (MnCl2) solution to the NaMnO4/NaOH mixture. The second one, the "alkali-last method" consists on a dropwise addition of the basic solution (NaOH) to the MnCl2/NaMnO4 mixture. The third one, called the "oxidant-last method", consists on a dropwise addition of the oxidant (NaMnO4) solution to the NaOH/MnCl2 mixture. Samples were denoted ∶ C = Na or K when NaMnO4 and NaOH or KMnO4 and KOH were used respectively, m = R, A or O when the reductant-last, the alkali-last or the oxidant-last method was used respectively and t corresponds to the addition time, in hour, of the third reactant. Three extra samples were synthesized using pre-deoxygenated solutions and N2 bubbling to study the influence of dissolved oxygen. These samples were synthesized following the oxidant-last method and the bubbling was maintained during only the mixture of NaOH and MnCl2 for the first sample ( 2 -a) and during all the reaction time of 2 hours for the second one ( 2 -b). A sample of hausmannite was prepared by the mixture of 250 mL NaOH (5.5 mol L -1 ) and 200 mL of MnCl2 (0.5 mol L -1 ) under air bubbling at a flow rate of 2 L min -1 during 5 hours.

Characterization techniques
The X-ray powder diffraction data were collected from an X'Pert MPD diffractometer (Panalytical AXS) with a goniometer radius 240 mm, fixed divergence slit module (1/2° divergence slit, 0.04 rd Sollers slits) and an X'Celerator as a detector. The powder samples were placed on zero background quartz sample holders and the XRD patterns were recorded at room temperature using Cu Kα radiation (λ = 0.15418 nm). FTIR analysis was performed using a Bruker Vertex 70v equipped with a DLaTGS detector. Spectra were recorded in transmission mode using KBr pellets containing 2-5 mg of sample. For each sample, 100 scans were collected in the 5000-220 cm −1 wavenumber with 4 cm −1 resolution. The background spectrum of KBr was also recorded at the same conditions. X-ray photoelectron spectra were recorded on a KRATOS Axis Ultra X-ray photoelectron spectrometer with Al Kα source monochromated at 1486.6 eV (spot size 0.7 mm × 0.3 mm). Photoelectrons were detected by a hemispherical analyzer at an electron emission angle of 90° and pass energy of 160 eV (survey spectra) and 20 eV (core level spectra). For the core-level spectra, the overall energy resolution, resulting from monochromator and electron analyzer bandwidths, was 800 meV. As an internal reference for the absolute binding energies, the C1s peak of hydrocarbon contamination set at 284.  Table 1 gives the nature of the products identified by XRD for all prepared samples.

Comparison of the three used methods
The three different orders of mixing were studied with an identical dropwise addition time (2 hours) of the third reactant. The XRD patterns of the samples 2 , 2 and 2 are shown in Fig. 1. The main product identified for the three methods is Na-birnessite (PDF 00-043-1456) with two main reflections at 7.14 and 3.57 Å corresponding to d001 and d002 respectively. In addition to birnessite, hausmannite, Mn3O4, (PDF 04-007-9635) was also identified as byproduct appearing clearly in the oxidant-last method (Fig. 1c) and slightly in the two other methods ( Fig. 1a and 1b). Feitknechtite, β-MnOOH, (PDF 00-018-0804) with a small diffraction peak around 4.6 Å appeared as a second by-product for the oxidant method. Thus, when following the reductant-last and the alkali-last methods, the main product observed was Na-birnessite with only trace of hausmannite. In Fig. 2 are shown the XRD patterns of samples 2 , 2 and 2 . In this series, KMnO4 and KOH were used instead of NaMnO4 and NaOH in order to investigate the role of the counter-ion. As in the previous series, two products were identified: K-birnessite (PDF 01-080-1098) with two main diffraction peaks corresponding to d001 = 7.02 and d002 = 3.51 Å and hausmannite. But, in contrast to the Na-birnessite, the formation of K-birnessite single phase, with only traces of a crystalline by-products, was only possible in the alkali-last method (sample 2 ).
The syntheses leading to the formation of Na-birnessite identified by XRD were further characterized by FTIR. The birnessites showed similar IR fingerprint ( Fig. 3a and 3b) [34][35][36][37][38][39]. In particular, the absence of absorption band close to 345 cm -1 rule out any contribution of hausmannite phase to 2 and 2 spectra. In contrast, the small band observed at 594 cm -1 for the samples prepared following oxidant-last or alkalilast method could be attributed to traces of MnOOH [32,38,40,41], the role of this manganese oxyhydroxide in birnessite formation will be discussed below.  (Fig. 1b).

Influence of dissolved oxygen on the product formation
This part is aimed to determine the influence of dissolved oxygen on the formation of Nabirnessite and hausmannite in the oxidant-last and alkali-last methods. As previously shown, the formation of pure birnessite was not possible for the oxidant-last method as the product consisted of a mixture of hausmannite and birnessite regardless of the addition rate used (Fig.   4). The reaction was carried out with N2 bubbling to remove oxygen from the solutions.
Thereby, the diffractogram of the 2 -a sample, prepared by N2 bubbling in the mixture of NaOH and MnCl2 solutions before adding the NaMnO4 solution, gives only birnessite (Fig. 5a).
The diffractogram of the 2 -b sample, prepared by N2 bubbling during all the reaction time gives both birnessite and manganite (γ-MnOOH) (Fig. 5b) [12], it is assumed that in oxygenated water, Mn(OH)2 oxidizes to hydrohausmannite which is a mixture of feitknechtite (β-MnOOH), and hausmannite (Mn3O4) and then the formation of birnessite occurs by the oxidation of hydrohausmannite. The pathways leading to birnessite and hausmannite are summarized in Fig. 6.

i. Influence of dissolved oxygen
Regardless of the method employed, oxidation conditions appear to be a key factor in directing the reaction towards the obtaining of a product or another. Thus, when producing birnessite from the oxidation of Mn(OH)2 with oxygen bubbling in alkali medium, Feng et al., [9] showed that, for a flow rate lower than 5 L min -1 , hausmannite was obtained as by-product. This is in good line with our experiment in which hausmannite was obtained as a single phase when bubbling Mn(OH)2 with synthetic air under a flow rate of 2 L min -1 .
In the oxidant-last method, MnO4ions are used as oxidizer instead of oxygen bubbling.
In addition to birnessite, hausmannite is observed as by-product and its amount raised with the MnO4addition time (samples 0 , 2 , 4 and 8 ). This could be explained considering two competitive reaction pathways: the slow transformation of Mn(OH)2 to hausmannite by dissolved oxygen before the addition of MnO4via pathway A (cf. Fig. 6) as previously observed [43] or the reaction leading to birnessite via pathway B (cf. Fig. 6). Lowering the MnO4addition rate indeed left time for the slow formation of hausmannite. Based on these observations, it can be assumed that Mn3O4 formation rate (pathway A, cf. Fig .6) is lower than the birnessite one (pathway B, cf. Fig. 6). It was thus possible to obtain pure birnessites following two methods: alkali-last and reductantlast. The obtaining of birnessite following the oxidant-last method was not successful as previously demonstrated [29]. Indeed, to obtain pure birnessite via the oxidant-last method, special precautions regarding the presence of dissolved oxygen when mixing NaOH and MnCl2 must be taken, otherwise, hausmannite appears as by-product.

ii. Thermodynamics of birnessite and mechanism of formation
Thermodynamic properties of birnessite were assumed to be equal to -MnO2 [44]. In fact, -MnO2, like its natural analog vernadite, is a turbostratic variety of birnessite. For sake of simplicity, the solid phases are assumed immiscible and activity of dissolved or ionic species were considered as equal to their respective concentrations. Table 3 6). This assumption is confirmed by the work of Cornell and Giovanoli [22], who pointed that the transformation of hausmannite into birnessite, in alkaline medium, proceeds very slowly, involves a dissolution/re-precipitation mechanism and the dissolution of hausmannite is the rate limiting step.

iii. NaOH and NaMnO4 vs KOH and KMnO4
Solids obtained with KMnO4/KOH solutions were different from the ones obtained with NaOH/NaMnO4. Indeed, for the reductant-last method, with KOH and KMnO4, in addition to birnessite, hausmannite was observed (sample 2 ). Jin et al., [45] have shown that the nature of the alkaline solution influences the O2 solubility and its diffusion which increase when NaOH is changed by KOH. In this case, the easy diffusion of dissolved oxygen can enhance the rate of pathway A in aqueous solution in the expense of pathway B (Fig. 6).
Na-birnessite and K-birnessite prepared by reductant-last, oxidant-last and alkali-last method are quite similar in their Mn 2p3/2 spectra (Fig. 7) with a maximum at 642 ± 0.2 eV and two broad shoulders, at 643 eV and 641 eV. For samples prepared with oxidant-last method, additional weak contribution is observed at lower binding energy. According to literature [46][47][48][49], Mn 2p3/2 spectrum of birnessite is composed of multiplet peaks associated to Mn(III) and Mn(IV). Experimental spectra, shown in Fig. 7, were fitted considering the multiplet peak characteristics (positions and relative intensities) determined previously [49]. For the samples prepared with oxidant-last method (Na 2 O and K 2 O ) the weak contribution at low binding energy can be attributed to a third multiplet associated to Mn(II). Similar component is observed for K-birnessite prepared by reductant-last method (K 2 R ). This is consistent with the presence of hausmannite whose reference Mn 2p3/2 spectrum is given in Fig. 7g giving the necessary amount of Na + to counter-balance the negative charge of the layers. The Mn(III)/Mntot bulk ratio was found closed to 0.3 (Table 2). This is in line with previous reports [46,49,50] pointing to an Mn(III) excess at birnessite surface with respect to bulk content. This suggests that birnessite formation occurs through a dissolution/precipitation process rather than a solid state reaction. Actually, in the latter case, oxidation would progress inward from the particle surfaces leading to higher oxidation state of the surface with respect to the bulk.

Conclusion
Na-birnessites with different chemical compositions were synthesized following three different  Samples denotation ∶ C = Na or K when NaMnO 4 and NaOH or KMnO 4 and KOH were used, m = R, A or O when the reductant-last, the alkali-last or the oxidant-last method is used and t corresponds to the time addition, in hour, of the third reactant. a and b under N 2 bubbling (during the initial mixture for "a" and 2 h for "b"). bir for birnessite, haus for hausmannite, mang for manganite and feit for feitknechtite. * : trace  Table 3. Standard Gibbs free energies of formation of different species involved in studied reactions (Dean, 1999