**3. Results**

#### *3.1. XRD Analysis of Aged Precipitates*

Figure 1 displays the XRD pattern of precipitate M0 after 1 week of ageing. In this first case, Mg(II) cations were not present and the obtained compound is then a Fe(II)- Fe(III) LDH.

**Figure 1.** XRD pattern of reference precipitate M0 ([MgII] = 0) after 1 week of ageing at room temperature (RT). GR = GR(SO4<sup>2</sup>−), GRCl = GR(Cl−), with the corresponding Miller index.

In agreemen<sup>t</sup> with the previous work [11], the XRD pattern reveals that the solid phase is mainly composed of GR(SO4<sup>2</sup>−), i.e., the Fe(II)-Fe(III) SO4-LDH. The two main peaks of the chloride green rust are seen together with those of GR(SO4<sup>2</sup>−), but their intensity is very low. Using the fitting procedure described in Section 2.2, the intensity ratio between the main peak of GR(SO4<sup>2</sup>−) (GR001, at 2*θ* = 9.2◦) and the main peak of GR(Cl−) (GRCl003, at 2*θ* = 12.9◦) is determined at 93:1.

Figure 2 displays the XRD pattern of precipitate M4 after 1 week of ageing. This second case corresponds to the situation where Fe(II) cations are not present. The obtained compound is consequently a Mg(II)-Fe(III) LDH. Strikingly, its XRD pattern drastically differs from that of GR(SO4<sup>2</sup>−). The main diffraction peak, which corresponds to the distance between two consecutive Fe planes in the LDH structure, is located at about 2*θ* = 13◦. This leads to an interplanar distance of 8 Å, rather typical of GR(Cl−). By comparison, the main diffraction peak of GR(SO4<sup>2</sup>−) is found at 9.2◦ (Figure 1), which corresponds to an interplanar distance of 11.15 Å. The diffraction peaks of the obtained Mg(II)-Fe(III) LDH actually correspond to the mineral iowaite, that is the Mg(II)-Fe(III) Cl-LDH similar to GR(Cl−) [14,15] with the chemical formula Mg6Fe2(OH)16Cl2·4H2O [15]. In the experimental conditions considered here, when Mg(II) is substituted for Fe(II), a Cl-LDH is formed rather than a SO4-LDH. Note that the solid phase was analyzed as a dry powder so that the diffraction lines of NaCl are also seen.

**Figure 2.** XRD pattern of precipitate M4 ([FeII] = 0) after 1 week of ageing at RT. The precipitate was analyzed as a dry powder. Io: Iowaite, H: Halite NaCl, with the corresponding Miller index.

It can finally be observed that the diffraction peaks of the obtained iowaite are much broader than those of the sulfated GR obtained in the absence of Mg(II) (Figure 1). This shows that the average crystal size, or more exactly the mean coherent domain size, of the Mg(II)-Fe(III) Cl-LDH is much smaller than that of GR(SO4<sup>2</sup>−), i.e., the Fe(II)-Fe(III) SO4-LDH.

The XRD pattern of the precipitate obtained with equal amounts of Fe(II) and Mg(II), i.e., precipitate M2, is displayed in Figure 3. Both GR(SO4<sup>2</sup>−) and GR(Cl−) are identified, and found in similar proportions according to the respective intensity of their main peaks. Note that both compounds are likely to comprise not only Fe(II) cations, but Mg(II) cations too. Consequently, they may not be green rust compounds sensu stricto. However, for clarity, this terminology will be used in the following to designate the FeII-(MgII)-FeIII SO4-LDH and Cl-LDH.

**Figure 3.** XRD pattern of precipitate M2 ([MgII]/[FeII] = 1) after 1 week of ageing at RT. GR = GR(SO4<sup>2</sup>−), GRCl = GR(Cl−), M = Fe3O4, with the corresponding Miller index.

Magnetite, the Fe(II-III) mixed valence oxide with chemical formula Fe3O4, is also identified. This shows that the presence of Mg(II) cations has induced in this case the formation of both GR(Cl−) and Fe3O4.

Figure 4 displays the XRD patterns of precipitates M1 and M3 after 1 week of ageing. These data confirm that Mg(II) favors the formation of GR(Cl−) and magnetite. Actually, for the high substitution ratio [MgII]/[FeII] = 3 (precipitate M3), the main obtained LDH is GR(Cl−). The intensity of the diffraction peaks of GR(SO4<sup>2</sup>−) is very weak, even with respect to that of the main peak of magnetite (M311, at 2*θ* = 41.3◦). The intensity ratio between the main peak of GR(SO4<sup>2</sup>−) and the main peak of GR(Cl−) is now determined at 1:32. Conversely, for the low substitution ratio [MgII]/[FeII] = 1/3 (precipitate M1), the diffraction peaks of both GR(Cl−) and magnetite remain very small. However, the intensity ratio between the main peak of GR(SO4<sup>2</sup>−) and the main peak of GR(Cl−) is equal to 22:1 in this case, while it was 93:1 in the absence of Mg(II) cations. The influence of Mg(II) is small but nonetheless detectable.

**Figure 4.** XRD pattern of precipitates M1 ([MgII]/[FeII] = 1/3) and M3 ([MgII]/[FeII] = 3) after 1 week of ageing at RT. GR = GR(SO4<sup>2</sup>−), GRCl = GR(Cl−), M = Fe3O4, with the corresponding Miller index.

A detailed analysis of the XRD data was achieved to obtain further information, in particular about a possible variation of the GR lattice parameters with the Mg(II):Fe(II) concentration ratio. For that purpose, the angular regions where the two main peaks of GR(SO4<sup>2</sup>−) and GR(Cl−) are present were computer fitted (see Section 2.2). The result obtained for precipitate M2 in the 24–30◦ 2*θ* region of the GRCl006 peak is displayed in Figure 5 as an example.

Since the GRCl006 peak overlaps slightly with the GR003 peak, both peaks were taken into account. However, the experimental curve could not be adequately fitted and an additional broad peak had to be added. The position of this peak was determined through the fitting procedure at 2*θ* = 27.52◦, a diffraction angle associated with an interplanar distance of 3.76 Å. It corresponds exactly to the 006 diffraction peak of the carbonated green rust GR(CO3<sup>2</sup>−) [13,16]. This finding actually shows that a very small amount of GR(CO3<sup>2</sup>−) has formed together with GR(SO4<sup>2</sup>−), GR(Cl−), and magnetite, although carbonate species were not added specifically to the system. These carbonate species could originate in (i) the dissolution of CO2 in the solution and (ii) some impurities present in the chemicals used. It happened that the NaOH pellets used for this study contained a small proportion of Na2CO3.

**Figure 5.** Fitting of the XRD pattern of precipitate M2 ([MgII]/[FeII] = 1) after 1 week of ageing at RT: Detail of the 24–30◦ angular region. GR = GR(SO4<sup>2</sup>−), GRCl = GR(Cl−), GRC = GR(CO3<sup>2</sup>−), with the corresponding Miller index.

However, the presence of the weak GRC006 peak cannot explain the important asymmetry of the 006 diffraction peak of GR(Cl−). As it can be seen in Figure 5, the computer fitting procedure had to be achieved with two pseudo-Voigt functions in the case of the GRCl006 diffraction peak. Such an asymmetry was not observed for the diffraction peaks of GR(SO4<sup>2</sup>−), as illustrated by the GR003 peak in Figure 5.

All the results obtained with the fitting of the XRD patterns are listed in Table 2. The data corresponding to the traces of carbonate GR, identified in each case, are omitted as they are only the consequence of the presence of carbonate traces (CO2 and impurities) in the system.

**Table 2.** Characteristics of the two main diffraction peaks of GR(SO4<sup>2</sup>−) and GR(Cl−)/iowiate for the aged M0-M4 precipitates; *d*: Interplanar distance (Å), *I*: Peak intensity, with *I* = 100 for the most intense peak of the considered compound, and FWHM: Full width at half maximum, in degrees. GR = GR(SO4<sup>2</sup>−) and GRCl = GR(Cl−)/iowaite.


First, these results show that the 001 and 002 interplanar distances of GR(SO4<sup>2</sup>−), linked to the *c* parameter of the hexagonal cell, are not influenced by the [MgII]/[FeII] substitution ratio. They vary slightly around an average of 11.16 ± 0.02 Å for *d*001 and 5.52 ± 0.01 Å for *d*002 with no apparent link with [MgII]/[FeII]. However, a clear trend is observed for the width of those peaks. FWHM increases significantly with the proportion of Mg(II), which shows that the growth of the GR(SO4<sup>2</sup>−) crystals, and/or the increase of crystallinity of GR(SO4<sup>2</sup>−), is hindered by the presence of the Mg(II) cations.

In contrast, more important changes are observed for the diffraction peaks of GR(Cl−). The data obtained for precipitate M4, that is for the Mg(II)-Fe(III) Cl-LDH, are indeed characteristic of iowaite [14]. It can then be noted that the lattice parameters of iowaite differ from those of GR(Cl−). The *d*003 and *d*006 interplanar distances are linked to the *c* parameter of the conventional hexagonal cell. They lead to an average *c*/3 value of 8.11 ± 0.03 Å (average of *d*003 and 2 × *d*006) comparable to the values reported in previous works for iowaite, which are between 8.04 [14] and 8.11 Å [15]. The *c*/3 parameter of GR(Cl−) is smaller, about 7.95 Å [17].

The main peak GRCl003 of the chloride GR, though slightly asymmetric, could be fitted in any case with only one pseudo-Voigt function. However, the corresponding interplanar distance was observed between 7.96 Å for M3 and 8.04 Å for M2, and up to 8.14 Å for M4. The two extreme values are typical of GR(Cl−) and iowaite [14,15,17]. The important asymmetry of the GRCl006 diffraction peak implied the use of two pseudo-Voigt functions. Actually, variations of *d*hkl are associated with larger variations of 2*θ*hkl in the angular region corresponding to the GRCl006 peak, which may explain that the asymmetry of the GRCl003 peak was smaller. The phenomenon was more pronounced in the case of precipitate M2 (Figure 5) and led to two peaks with a similar intensity (Table 2). The corresponding *d*006 distances were determined at 4.01–4.02 and 3.95–3.96 Å. They lead to values of 8.03 ± 0.01 and 7.91 ± 0.01 Å, respectively. Though the asymmetry of the GRCl006 peak may have various origins, a heterogeneous Mg(II) content could lead to a variation of the *c* lattice parameter of the conventional hexagonal cell, this parameter increasing with the Mg(II) content, as illustrated by the difference between the *c* lattice parameter of GR(Cl−) and that of iowaite.

The width of the GRCl peaks also varies with the [MgII]/[FeII] substitution ratio. As already noted, FWHM is very high in the absence of Fe(II), that is for iowaite. The influence of Mg(II) is also illustrated by the increase of FWHM from M2 to M3. However, the width of the GR(Cl−) peaks is larger for M1 even though the [MgII]/[FeII] ratio is smaller.

#### *3.2. XRD Analysis of Unaged Precipitates*

Some solid phases may result from the precipitation reactions, while other phases may form during ageing via the transformation of initially precipitated compounds. The evolution with time of precipitate M0, previously studied [11], showed, for instance, that the amount of GR(Cl−) decreased upon ageing, which implied that part of the initially formed GR(Cl−) transformed to GR(SO4<sup>2</sup>−). Consequently, only traces of GR(Cl−) remained after 1 week (as seen in Figure 1). Similarly, it was observed that, in the absence of excess dissolved Fe(II) species, part of the initially precipitated GR(SO4<sup>2</sup>−) could transform into magnetite [11].

Figure 6 displays the XRD patterns of unaged precipitates M1 and M2. In both cases, the diffraction peaks are clearly broader than those of the aged compounds (Figures 3 and 4). This illustrates a well-known effect of ageing, i.e., the increase of crystallinity and crystal size with time. In the case of M1, only two phases are detected, namely GR(SO4<sup>2</sup>−) and GR(Cl−). After 1 week of ageing, magnetite was present. This result shows that magnetite results in this case from the ageing procedure. The intensity ratio between the main peak of GR(SO4<sup>2</sup>−) and the main peak of GR(Cl−) is determined before ageing at 5.5:1. It was determined (see previous Section 3.1) at 22:1 after 1 week of ageing. This shows that the proportion of GR(Cl−) decreased significantly during ageing, as observed for M0 [11], i.e.,

intheabsenceofMg(II)cations.Forthelowest[MgII]/[FeII]ratioof1/3,GR(Cl−)may

**Figure 6.** XRD pattern of unaged precipitates M1 ([MgII]/[FeII] = 1/3) and M2 (MgII]/[FeII] = 1). GR = GR(SO4<sup>2</sup>−), GRCl = GR(Cl−), M = Fe3O4, with the corresponding Miller index.

In the case of M2, magnetite is already present among the solid phases that compose the unaged precipitate. Consequently, the three phases observed after ageing, i.e., GR(SO4<sup>2</sup>−), GR(Cl−), and Fe3O4, result from the precipitation process. The intensity ratio between the main peak of GR(SO4<sup>2</sup>−) and the main peak of GR(Cl−) is determined at 1:1.5 for the unaged precipitate and 1:1.8 for the aged precipitate (Figure 3). The variation is slight and may not be significant. In any case, it shows that the proportion of GR(Cl−) remained constant or increased slightly upon ageing, in contrast with what was observed without Mg(II) (precipitate M0, [11]) or with the lowest [MgII]/[FeII] substitution ratio (precipitate M1). This shows that the presence of Mg(II) not only favors the precipitation of the Cl-LDH, but also increases its stability with respect to the SO4-LDH.

#### *3.3. Analysis of the Mg(II)-Fe(III) Solid Phases Obtained in the Absence of Chloride*

The first XRD pattern, shown in Figure 7, is that of precipitate M4s aged 1 week and analyzed immediately after filtration as a wet paste. The obtained Mg(II)-Fe(III) compound is poorly crystallized and its pattern is similar to that of GR(SO4<sup>2</sup>−), i.e., the main diffraction peak is located at 9.0◦. This pattern was indexed according to the ICCD-JCPDS file of wermlandite Mg7Al1,14Fe0,86(OH)18Ca0,6Mg0,4(SO4)2(H2O)12, a mineral structurally similar to GR(SO4<sup>2</sup>−) [18]. Wermlandite includes Al3+ and Ca2+ ions and not only Mg2+ and Fe3+ cations. In our experiment, Al3+ and Ca2+ ions were not present and the obtained compound is then a Mg(II)-Fe(III) SO4-LDH. Other SO4-LDH are also characterized by this type of structure, where two consecutive metal cations planes are separated by ~11 Å. An example is hydrohonnessite, where the cations present in the hydroxide layers are Ni2+ and Fe3+ [19].

**Figure 7.** XRD pattern of precipitate M4s ([FeII] = 0 and [Cl−] = 0) after 1 week of ageing at RT. W = Mg(II)-Fe(III) hydroxysulfate similar to wermlandite, with the corresponding Miller index.

This result clearly shows that a Mg(II)-Fe(III) SO4-LDH similar to GR(SO4<sup>2</sup>−) can be obtained if Cl− ions are not available for the formation of a Cl-LDH. The distance between two consecutive planes of metal cations is determined at 11.56 Å, which shows that, as for the Cl-LDH, the substitution of Fe(II) by Mg(II) cations leads to an increase of the *c* lattice parameter of the hexagonal cell. Actually, the ionic radius of Mg(II) is smaller than that of Fe(II) [20], which induces a decrease of the *a* lattice parameter of Mg(II)-Fe(III) LDHs with respect to Fe(II)-Fe(III) LDHs [21]. However, the *c* lattice parameter is nonetheless higher with Mg(II) [21]. This illustrates how the cationic composition of the hydroxide layer influences the electrostatic interactions that bind together the hydroxide sheets and the interlayers and ensures the stability of the crystal structure [21]. This crucial point is further discussed in Section 4.

The second XRD pattern, shown in Figure 8, was obtained with the same aged M4s precipitate. However, the wet paste obtained after filtration was dried in air and the solid phase was analyzed as a dry powder 10 days later. The result of the drying is a change in the structure of the Mg(II)-Fe(III) SO4-LDH. This new compound can be considered as a second form of SO4-LDH and will be called in the following the Mg(II)-Fe(III) hydroxysulfate-b. Its new structure seems similar to that of GR(Cl−) and iowaite and was then indexed similarly. The main peak of the Mg(II)-Fe(III) hydroxysulfate-b is then the 003 peak. Actually, this second type of SO4-LDH was already reported [19]. Honessite, a Ni(II)-Fe(III) SO4-LDH, for example, is characterized by a distance between two consecutive planes of metal cations of 8.7 Å [19].

The main diffraction peak of the Mg(II)-Fe(III) hydroxysulfate-b obtained here is located at a position 2*θ* = 11.74◦. This corresponds to a *c*/3 distance of 8.75 Å, very similar to that of honessite. The transformation from one type of structure to the other, associated with the drying of the solid phase, is due to the release of water molecules initially present in the interlayers [22].

**Figure 8.** XRD pattern of precipitate M4s ([FeII] = 0 and [Cl−] = 0) after 1 week of ageing at RT, filtration, and drying in air (10 days). HSb = Mg(II)-Fe(III) hydroxysulfate-b (see text) with the corresponding Miller index.
