Influence of pH

The results reported here show that the synthesis of pure saponite does not depend heavily on the pH of the solution in which the gels of saponite composition are immersed (saponite was obtained in solutions at pH ranging from 5.5 to 14). This can be due to the stability of the starting gel of trioctahedral smectites, as previously observed by Reference Huertas, Cuadros, Huertas and LinaresHuertas et al. (2000). Of note, the saponites produced in the present study contain tetrahedral Al and no octahedral Al. In fact, the ATR-FTIR spectra of the saponites produced were very similar to the ATR-FTIR spectrum of saponite presented by Reference Kloprogge and PonceKloprogge and Ponce (2021), with a feature at 800–815 cm^–1 attributed to δAl–O of tetrahedral Al (absent from the ATR-FTIR spectrum of talc which contains no tetrahedral Al; Reference Kloprogge and PonceKloprogge & Ponce, 2021). Plus, the absence of the FTIR band attributed to vMg₂-Al–OH at 3625 cm^–1 is consistent with the absence of octahedral Al (Reference Kloprogge and PonceKloprogge & Ponce, 2021).

The XRD pattern of the saponite synthesized in solutions at pH 14 exhibited narrower 001 reflections than those of the saponites synthesized in solutions at lower pH. Such a decrease in width indicates a decrease in the d ₀₀₁ distance and can be interpreted as an increase in crystallinity (Reference Zhang, Fu, Wang and LingZhang et al., 2022). Consistently, high-pH conditions during syntheses have been reported to improve both the crystallinity and crystallization rate of saponites (Reference Blukis, Schindler, Couasnon and BenningBlukis et al., 2022), as is the case for high-temperature conditions (Reference Kloprogge and FrostKloprogge & Frost, 2000; Reference Kloprogge and PonceKloprogge & Ponce, 2021). Still, such a decrease in width may also indicate an increase in the permanent charge, i.e. a higher degree of ^[4]Al³⁺ for ^[4]Si⁴⁺ tetrahedral substitutions. Consistently, with increasing pH (from 5.5 to 14), the ѵSi–O–Si and δAl–O absorption bands shifted from 960 to 944 cm^–1 and from 800 to 815 cm^–1, respectively, while the ѵMg₃OH and δMg₃OH absorption bands of the synthesized saponites shifted from 3674 to 3685 cm^–1 and from 655 to 640 cm^–1, respectively (Fig. 2), such shifts having been interpreted previously as an increase in the permanent charge (Reference Meyer, Bennici, Vaulot, Rigolet and DzeneMeyer et al., 2020; Reference Pelletier, Michot, Humbert, Barrès, de la Caillerie and RobertPelletier et al., 2003). Note that the production of saponites with a higher charge should leave available Si and Mg in the system, possibly leading to the production of other phases such as brucite (Mg(OH)₂), serpentine (Mg₃Si₂O₅(OH)₄), or talc (Si₄Mg₃O₁₀(OH)₂), i.e. an assemblage similar to what is obtained with a gel produced using Na₂SiO₃ and AlCl₃ solutions at pH 14 (Fig. S3). The absence of these minerals in the final residue (only saponite is present according to XRD and FTIR data; Figs. 1 and 2) suggests either that both Si and Mg in excess have been leached during filtration or that some Mg has replaced Na as the interlayer cation. Additional data (such as elemental or nuclear magnetic resonance – NMR data) would be required to determine if the saponite produced by immersing the gel in a solution at pH 14 has exactly the same composition as those produced at lower pH.

In contrast to that of saponite, the synthesis of beidellite is highly sensitive to the pH of the AlCl₃ solution and the pH of the solution in which gels are immersed. A gel of beidellite composition produced using an AlCl₃ solution at pH 3.4 seems to be very unstable and does not lead to the formation of pure beidellite, irrespective of the pH of the solution used for the synthesis (cf Fig. S4). In contrast, a gel of beidellite composition produced using an AlCl₃ solution at pH 10.6 will lead systematically to the formation of boehmite (cf Fig. S4). Syntheses using gels of beidellite composition immersed in pure water will lead to the formation of kaolins, while only zeolites form in solutions at high pH, as also reported by Reference De KimpeDe Kimpe (1976) and Reference Huertas, Cuadros, Huertas and LinaresHuertas et al. (2000). The cause resides in the (pH-dependent) form of Si and Al species. In pure water, H₄SiO₄, Al(OH)₂ ⁺, and Al(OH)²⁺ prevail, and Si and Al ions form tetrahedral and octahedral complexes, respectively, while at high pH (> 13), H₂SiO₄ ^– and Al(OH)₄ ^– prevail, leading to the formation of only tetrahedral complexes (Table 1). Thus, the successful synthesis of pure beidellite occurs only at a very specific pH (i.e. 12), where fourfold and sixfold coordinated Al species exist, as previously suggested (Reference De KimpeDe Kimpe, 1976).

The synthesis of ^[4]Al-nontronite is also very sensitive to the pH of solutions, as is the case for ^[4]Fe-nontronite as acknowledged earlier (Reference Andrieux and PetitAndrieux & Petit, 2010; Reference Boumaiza, Dutournié, Le Meins, Limousy, Brendlé, Martin, Michau and DzeneBoumaiza et al., 2020; Reference Dzene, Dutournie, Brendle, Limousy, Le Meins, Michelin, Vidal, Gree, Abdelmoula, Martin and MichauDzene et al., 2022). Only the gel produced using an AlCl₃ solution at pH 10.6 and immersed in a solution at pH 12.5 leads to pure ^[4]Al-nontronite. The syntheses conducted at pH <12.5 from the gels of nontronite composition produced using an AlCl₃ solution at pH 10.6 led to the production of nontronite together with bayerite. Gels produced using an AlCl₃ solution at pH 10.6 and immersed in solutions at pH 13 crystallized into pure nontronite, but these are ^[4]Fe-nontronites as indicated by the shift of the 06,33 reflection to lower angles and the shift of the ѵSi–O to lower wavelengths (Reference Baron, Petit, Tertre and DecarreauBaron et al., 2016a). A high pH, thus, seems to favor ^[4]Fe³⁺ for ^[4]Si⁴⁺ tetrahedral substitutions, as historically suggested by Reference GrubbGrubb (1969). The use of gels produced using an AlCl₃ solution at pH 3.4 did not lead to the production of pure, well crystallized ^[4]Al-nontronite (only poorly crystalline nontronite is obtained at pH 12.5). The immersion of such gels in solutions at lower pH leads to the production of hisingerite (i.e. a precursor of nontronite containing no Al; Reference KohyamaKohyama, 1975; Reference Farmer, McHardy, Elsass and RobertFarmer et al., 1994; Reference EggletonEggleton, 1998; Reference Milliken and BishMilliken & Bish, 2014), while their immersion in solutions at higher pH leads to the production of a zeolite (analcime) together with ^[4]Fe-nontronite.

Mechanistic Considerations

This section concerns the reactions that have probably occurred during the syntheses of saponite, beidellite, and nontronite. Please note that the equations presented below refer to the theoretical products of the syntheses carried out in the present study.

Pure saponite can be synthesized following various protocols, at various temperatures (Reference Kloprogge and FrostKloprogge & Frost, 2000; Reference Meyer, Bennici, Vaulot, Rigolet and DzeneMeyer et al., 2020) and for different durations (Reference Zhang, Petit, He, Villiéras, Razafitianamaharavo, Baron, Tao and ZhuZhang et al., 2020), whatever the pH (Reference Blukis, Schindler, Couasnon and BenningBlukis et al., 2022). Here, saponite was obtained by immersing gels in solutions at pH ranging from 5.5 to 14, probably because the rather large Mg²⁺ ions did not compete with Al species for tetrahedral substitutions. The major species in the Na₂SiO₃ solution at 0.2 M at pH 13.1 are H₂SiO₄ ^2– and H₃SiO₄ ^– (Table 1). Although Al was present mainly as Al³⁺ in the AlCl₃ solution, it converted to Al(OH)₄ ^– when this solution was poured into the Na₂SiO₃ solution, leading to the polymerization of an aluminosilicate network (i.e. tetrahedral complexes) at high pH (as suggested by Reference Besselink, Stawski, Freeman, Hövelmann and ToBesselink et al., 2020). The subsequent addition of the MgCl₂ solution was responsible for a slight decrease in the pH (down to 10 – Table 2), probably converting the remaining H₂SiO₄ ^2– ions into H₃SiO₄ ^–, thereby leading to the polymerization of Mg-rich octahedral complexes. The gel produced crystallized into saponite whatever the pH of the solution into which it will be immersed, following the overall simplified reaction, written with respect to the main species present in each starting solution at 20°C (cf Table 1).

(1) $\begin{array}{c}3.6N{a}_2{\mathrm{SiO}}_{3\left(s\right)}+0.4\mathrm{AlC}{l}_{3\left(s\right)}+3\mathrm{MgC}{l}_{2\left(s\right)}+5.04H_{2}O\\ \to 2.16H_2\mathrm{Si}{O_4^{2-}}_{\left(\mathrm{aq}\right)}+1.44H_3\mathrm{Si}{O_4^{-}}_{\left(\mathrm{aq}\right)}+0.4A{l^{3+}}_{\left(\mathrm{aq}\right)}+3{{\mathrm{Mg}}^{2+}}_{\left(\mathrm{aq}\right)}+7.2N{a^{+}}_{\left(\mathrm{aq}\right)}+7.2C{l^{-}}_{\left(\mathrm{aq}\right)}+1.44O{H^{-}}_{\left(\mathrm{aq}\right)}\\ \to {\mathrm{Na}}_{0.4}\left({\mathrm{Si}}_{3.6}{\mathrm{Al}}_{0.4}\right){\mathrm{Mg}}_3{O}_{10}{\left(\mathrm{OH}\right)}_{2\left(s\right)}+6.8\mathrm{NaC}{l}_{\left(s\right)}+0.4{H^{+}}_{\left(\mathrm{aq}\right)}+0.4{{\mathrm{Cl}}^{-}}_{\left(\mathrm{aq}\right)}+3.84H_{2}O\end{array}$

Pure beidellite cannot be synthesized over a large range of pH. The pH of the solution in which the gel is immersed for crystallization has to be controlled precisely. Although the formation of tetrahedral complexes occurred as for saponite when pouring the AlCl₃ solution at pH 3.9 into the Na₂SiO₃ solution at 0.2 M at pH 13.1, the subsequent addition of the second AlCl₃ solution was responsible for a large decrease in the pH (down to 4.5 – Table 2), destabilizing the tetrahedral complexes already formed and converting the H₂SiO₄ ^2– ions into H₄SiO₄ and the Al(OH)₄ ^– ions into Al(OH)₂ ⁺, Al(OH)²⁺, and Al³⁺. The gel obtained with the starting AlCl₃ solution at pH 3.9 will crystallize into pure beidellite only if immersed in solutions at pH 12 (allowing the conversion of Al species into Al(OH)₄ ^–, and thus the incorporation of tetrahedral Al) followed the overall simplified reaction, written with respect to the main species present in each starting solution at 20°C (cf Table 1):

(2) $\begin{array}{c}3.6N{a}_2{\mathrm{SiO}}_{3\left(s\right)}+2.4\mathrm{AlC}{l}_{3\left(s\right)}+0.4\mathrm{NaO}{H}_{\left(\mathrm{aq}\right)}+5.04H_{2}O\\ \to 2.16H_2\mathrm{Si}{O_4^{2-}}_{\left(\mathrm{aq}\right)}+1.44H_3\mathrm{Si}{O_4^{-}}_{\left(\mathrm{aq}\right)}+2.4A{l^{3+}}_{\left(\mathrm{aq}\right)}+7.6N{a^{+}}_{\left(\mathrm{aq}\right)}+7.2C{l^{-}}_{\left(\mathrm{aq}\right)}+1.84O{H^{-}}_{\left(\mathrm{aq}\right)}\\ \to {\mathrm{Na}}_{0.4}\left({\mathrm{Si}}_{3.6}{\mathrm{Al}}_{0.4}\right){\mathrm{Al}}_2{O}_{10}{\left(\mathrm{OH}\right)}_{2\left(s\right)}+7.2\mathrm{NaC}{l}_{\left(s\right)}+4.24H_{2}O\end{array}$

If the gel is immersed in a solution at pH <12, only kaolins will be produced following Eq. 3, while if the gel is immersed in a solution at pH >12, only zeolites will be produced following Eq. 4 (together with SiO₂ or Al(OH)₃, respectively, which are probably leached during filtration). Note that a starting AlCl₃ solution at pH >8, which would contain Al(OH)₄ ^– ions, cannot be used to produce a gel because of the inevitable precipitation of AlOOH or Al(OH)₃ (Fig. S4).

(3) $\begin{array}{c}3.6N{a}_2{\mathrm{SiO}}_{3\left(s\right)}+2.4\mathrm{AlC}{l}_{3\left(s\right)}+5.04H_{2}O\\ \to 2.16H_2\mathrm{Si}{O_4^{2-}}_{\left(\mathrm{aq}\right)}+1.44H_3\mathrm{Si}{O_4^{-}}_{\left(\mathrm{aq}\right)}+2.4A{l^{3+}}_{\left(\mathrm{aq}\right)}+7.2N{a^{+}}_{\left(\mathrm{aq}\right)}+7.2C{l^{-}}_{\left(\mathrm{aq}\right)}+1.44O{H^{-}}_{\left(\mathrm{aq}\right)}\\ \to 1.2A{l}_2{\mathrm{Si}}_2{O}_5{\left(\mathrm{OH}\right)}_{4\left(s\right)}+7.2\mathrm{NaC}{l}_{\left(s\right)}+1.2\mathrm{Si}{O}_{2\left(s\right)}+2.64H_{2}O\end{array}$

(4) $\begin{array}{c}3.6N{a}_2{\mathrm{SiO}}_{3\left(s\right)}+2.4\mathrm{AlC}{l}_{3\left(s\right)}+1.8\mathrm{NaO}{H}_{\left(\mathrm{aq}\right)}+5.04H_{2}O\\ \to 2.16H_2\mathrm{Si}{O_4^{2-}}_{\left(\mathrm{aq}\right)}+1.44H_3\mathrm{Si}{O_4^{-}}_{\left(\mathrm{aq}\right)}+2.4A{l^{3+}}_{\left(\mathrm{aq}\right)}+9{{\mathrm{Na}}^{+}}_{\left(\mathrm{aq}\right)}+7.2C{l^{-}}_{\left(\mathrm{aq}\right)}+3.24O{H^{-}}_{\left(\mathrm{aq}\right)}\\ \to 1.8\mathrm{NaAlS}{i}_2{O}_6.H_2{O}_{\left(s\right)}+7.2\mathrm{NaC}{l}_{\left(s\right)}+0.6\mathrm{Al}{\left(\mathrm{OH}\right)}_{3\left(s\right)}+3.24H_{2}O\end{array}$

Pure nontronite cannot be synthesized over a large pH range. As for beidellite, the pH of the solution in which the gel is immersed for crystallization has to be controlled precisely, even though the pH of the starting AlCl₃ solution does not seem to be critical, given that pure ^[4]Fe-nontronite or ^[4]Al-nontronite were produced here from AlCl₃ solutions at either pH 3.4 or 10.6. However, here, only the use of an AlCl₃ solution at pH 10.6 led to the production of ^[4]Al-Nontronite. As for saponite and beidellite, the formation of tetrahedral complexes occurred when pouring the AlCl₃ solution into the 0.2 M Na₂SiO₃ solution at pH 13.1. The subsequent addition of the FeCl₃ solution was responsible for only a slight decrease in the pH (down to 9; Table 2), leading to the production of Fe-rich octahedral complexes forming a gel which would crystallize into pure ^[4]Al-nontronite only if immersed in solutions at pH 12.5 following the overall simplified reaction, written with respect to the main species present in each starting solution at 20°C (cf Table 1):

(5) $\begin{array}{c}3.6N{a}_2{\mathrm{SiO}}_{3\left(s\right)}+2.4\mathrm{AlC}{l}_{3\left(s\right)}+2\mathrm{FeC}{l}_{3\left(s\right)}+0.4\mathrm{NaO}{H}_{\left(\mathrm{aq}\right)}+5.49H_{2}O\\ \to 2.16H_2\mathrm{Si}{O_4^{2-}}_{\left(\mathrm{aq}\right)}+1.44H_3\mathrm{Si}{O_4^{-}}_{\left(\mathrm{aq}\right)}+0.4\mathrm{Al}{{\left(\mathrm{OH}\right)}_4^{-}}_{\left(\mathrm{aq}\right)}+0.99F{e^{3+}}_{\left(\mathrm{aq}\right)}+0.26F{e}_2{\left(\mathrm{OH}\right)}_2^{4+}+0.32\mathrm{FeC}{l^{2+}}_{\left(\mathrm{aq}\right)}+0.17\mathrm{FeO}{H^{2+}}_{\left(\mathrm{aq}\right)}+7.6N{a^{+}}_{\left(\mathrm{aq}\right)}+7.2C{l^{-}}_{\left(\mathrm{aq}\right)}+0.45O{H^{-}}_{\left(\mathrm{aq}\right)}\\ \to {\mathrm{Na}}_{0.4}\left({\mathrm{Si}}_{3.6}{\mathrm{Al}}_{0.4}\right){\mathrm{Fe}}_2{O}_{10}{\left(\mathrm{OH}\right)}_{2\left(s\right)}+7.2\mathrm{NaC}{l}_{\left(s\right)}+4.69H_{2}O\end{array}$

At high pH, the major species in solution were H₃SiO₄ ^–, H₂SiO₄ ^2–, Al(OH)₄ ^–, and Fe(OH)₄ ^– (Reference Guan, Dong, Ma and JiangGuan et al., 2009; Reference Perry and ShafranPerry & Shafran, 2001), with Al(OH)₄ ^– competing with Fe(OH)₄ ^– for tetrahedral substitutions (Reference Decarreau and PetitDecarreau & Petit, 2014). According to the present results, if the gel is immersed in a solution at pH 12.5, Al(OH)₄ ^– is preferentially incorporated into tetrahedral sites over Fe(OH)₄ ^–, leading eventually to the production of ^[4]Al-nontronite. In contrast, if the gel is immersed in a solution at pH >12.5, Fe(OH)₄ ^– is incorporated preferentially into tetrahedral sites over Al(OH)₄ ^–, eventually leading to the production of ^[4]Fe-nontronite rather than ^[4]Al-nontronite (cf Fig. 6). At pH <12.5, Al seems to be incorporated neither in tetrahedral nor octahedral sites and bayerite is produced together with ^[4]Fe-nontronite (Fig. 6).