The modification and characterization of thermal-treated sericite by fluorosilicate

In this article, the thermal-treated sericite was modified by both fluorosilicate and the combination of fluorosilicate and nitric acid in order to reduce its layer charge and gain cation exchange capabilities for the preparation of sericite/polymer nanocomposites. After several orthogonal experiments and single factor experiments, the optimal experimental conditions were set up and we found that the combination of nitric acid and fluorosilicate is much more effective than fluorosilicate alone. Chemical composition analysis showed Al3+ was dissolved out from sericite and the dissolving amount is 65 mg/g under optimal experimental conditions. Combining the NMR test, it is considered that the Si/Al ratio in the tetrasheet of the modified product increased from 3.48 to 10. The layer charge reduced and the CEC value increased after fluorosilicate modification, which means the modified sericite is a promising matrix for clay-polymer nanocomposites.


Results and Discussion
The modification of thermal-treated sercite by fluorosilicate only. As to the modification of thermal-treated sericite by fluorosilicate, the main four factors, the concentration of fluorosilicate (factor A), reaction temperature (factor B), reaction time (factor C) and solid-to-liquid ratio (factor D) were researched and each control parameter has three experimental levels ( Table 2) 38,39 . The modification effect was evaluated by the dissolving-out amount of Al 3+ . The dissolution of Al 3+ and the reduction of negative layer charge allow sericite to show ion exchange capacity. Usually, the larger the dissolving amount of Al 3+ , the better the modification effect.
According to Table 2, the factors' levels of significance are as follows: solid-to-liquid ratio > reaction temperature > reaction time > fluorosilicate concentration. When the concentration of sodium fluorosilicate increased from 0.025 mol/L to 0.05 mol/L until 0.075 mol/L, the relevant influential effect decreased first, then increased, while the total influential effect was quite trivial. Considering the slight solubility of sodium fluorosilicate, 0.025 mol/L was chosen as the optimal fluorosilicate concentration.
Similarly, when the reaction time increased from 1 h to 3 h until 5 h, the relevant influential effect decreased first, then increased, while the total influential effect was quite trivial. Therefore, 1 h was chosen as the optimal reaction time. The optimal solid-to-liquid ratio is 4:200 (g:ml) and the optimal reaction temperature is 40 °C. Considering all the factors, trail number A101 is the combination of optimal reaction conditions and the dissolving-out amount of Al 3+ reached 2.5545 mg/g. Figure 1 shows the XRD patterns of the raw sericite (S 0 ), thermal-treated sericite (S 1 ) and fluorosilicated sericite (the optimal combination of reaction conditions) and Table 3 shows the relevant data.
Compared with raw sericite (S 0 ), the thermal-treated sericite (S 1 ) still kept complete structure of sericite, while the intensities of major reflections decreased and FWHM increased accordingly, which means that the raw sericite was activated after thermal modification. The thermal-treated sericite is more suitable to be used for the subsequent modification. After fluorosilicate modification, the intensity of corresponding peaks reduced further, while complete crystal form still kept. From Table 3, it can be seen that the FWHM of fluorosilicated sericite decreased, which indicated that the dissolving-out of Al 3+ decreased the crystallization degree of sericite while increasing the symmetry degree.
A second orthogonal experiment was done based on the results of the first orthogonal experiment in order to have more detailed information about the relevant reaction factors. Four factors, the concentration of fluorosilicate (factor A), reaction temperature (factor B), reaction time (factor C) and solid-to-liquid ratio (factor D) were researched and each control parameter has three experimental levels (Table 4).  Table 2. Design and results of the orthogonal experiment of fluorosilicate modification of sericite a . a K ij is defined as the sum of the evaluation indexes of all levels (i, i = 1, 2, 3) in each factor (j, j = A, B, C, D) and ω ij (mean value of K ij ) is used to determine the optimal level and the optimal combination of factors. The optimal level for each factor could be obtained when ω ij is the largest; R j is defined as the range between the maximum and minimum value of ω ij and is used for evaluating the importance of the factors.   According to Table 4, the factors' levels of significance are as follows: solid-to-liquid ratio > fluorosilicate concentration > reaction temperature > reaction time. Solid-to-liquid ratio is still the most significant factor, which corresponds to the results in the first orthogonal experiment, while other factors' influence order has changed. According to the results, the optimal combination of reaction conditions is A2B3C2D1, that is fluorosilicate concentration 0.025 mol/L, reaction temperature 50 °C, reaction time 1 h and solid-to-liquid ratio 3 g: 200 ml. Since no experiment was done using an optimal reaction condition, an extra experiment was conducted and it was found that the dissolving-out amount of Al 3+ reached 2.181 mg/g under such optimal reaction condition. Figure 2 shows the XRD patterns of thermal-treated sericite (S 1 ) and fluorosilicated sericites (A101 is the optimal product in the first orthogonal experiment and A210 is the optimal product in the second orthogonal experiment) and Table 5 shows the relevant data.
From the XRD patterns it can be seen that both A210 and A101 still kept the crystalline form of sericite, which means that sodium fluorosilicate didn't have substantial destruction on the structure of sericite while dissolving Al 3+ out. This provides guarantee for further modification and application of sericite with its own mineral properties remained. Compared with S 0 , both the intensities of (002) crystal plane of A210 and A101 decreased by a large margin. However, when compared with S 1 , it can be seen that the intensities of A101 decreased slightly while the intensities of A210 increased instead, which means the usage of sodium fluorosilicate restored the integrity  The modification of thermal-treated sericite by the combination of fluorosilicate and nitric acid. From the above experiment results, it can be seen that the modification of fluorosilicate can dissolve Al 3+ out from sericite, however, the dissolving amount of Al 3+ is quite limited (only 2.181 mg/g under an optimal reaction condition), which of course, can hardly reduce the layer charge of sericite to the ideal state. Therefore, the combination of fluorosilicate and nitric acid was used to modify thermal-treated sericite. First, an orthogonal experiment was done with four main factors at three levels ( Table 6). According to the above experiment, it can be seen that the smaller the solid-to-liquid ratio, the better the result. Therefore, the solid-to-liquid ratio was set as 3:200 (g:ml) in this experiment. Still, the modification effect was evaluated by the dissolving-out amount of Al 3+ . According to Table 6, the factors' levels of significance are as follows: reaction temperature > reaction time > nitric acid volume > fluorosilicate concentration. The most significant factor is reaction temperature. The optimal combination of reaction conditions is A1B3C3D3, that is fluorosilicate concentration 0.025 mol/L, reaction temperature 90 °C, reaction time 5 h and nitric acid volume 50 ml. The joint action of both fluorosilicate and nitric acid can dissolve large amount of Al 3+ from sericite (the dissolving-out amount of Al 3+ is 120.403 mg/g), while these two factors restrained each other, which can be seen from the effect of the volume of nitric acid. Nevertheless, the combination modification effect of fluorosilicate and nitric acid is much more remarkable than fluorosilicate itself (the dissolving-out amount of Al 3+ by fluorosilicate alone was 2.5545 mg/g).
XRD patterns of the above 9 products shows that they all kept crystal state, while the intensities and the shapes of the peaks changed at different levels. Among them, H103 (low concentration of fluorosilicate and high concentration of nitric acid) and H109 (high concentration of fluorosilicate and low concentration of nitric acid) all have high dissolving amount of Al 3+ , while their XRD patterns have wide background, which means that amorphous substances have emerged after modification (Fig. 3). Table 7 shows the crystallinity degree and the relative crystallinity degree of the products. Both the crystallinity degree of H103 and H109 declined obviously compared with A210, with the relative crystallinity degree at about 60%, which means the modification by the combination of fluorosilicate and nitric acid in high concentration can dissolve large amount of Al 3+ out as well as destroy the integrity of crystal form seriously. Although the CEC value of H109 after sodium chloride modification is 9.40 mmol/100 g, higher than raw sericite (4.94 mmol/100 g), still it is not that much, which confirms the modification effect is not that ideal. Therefore, neither the concentration of fluorosilicate nor the concentration of nitric acid should be too high.  Considering the importance of fluorosilicate and nitric acid in this experiment, single factor experiments were conducted with these two factors. Table 8 showed the effect of nitric acid volume on the product. Other experiment conditions were set as: the concentration of fluorosilicate 0.025 mol/L, reaction temperature 90 °C, reaction time 5 h and solid-to-liquid ratio 3 g/200 ml. When the volume of nitric acid was 30 ml, the dissolving amount of Al 3+ reached 133.002 mg/g, with a large increasement compared with the other two. XRD patterns show that all the three samples kept the crystal form of sericite, while the intensities and the shapes of peaks changed (Fig. 4). This is especially true for H303, with its peak intensities declined obviously and background grew wider than the other two, which means that more amorphous substances have emerged in H303 after modification.
From the data in Table 9, it can be seen that the relative crystallinity of H303 is much lower than H301 and H302 (only 0.492). Among the three products, H301 has the largest intensity of (002) peak together with smaller FWHM, which means the crystal structure of H301 was kept much better than the other two after the modification. Therefore, the optimal usage of nitric acid is 10 ml. Table 10 shows the effect of fluorosilicate concentration to the product. Other experiment conditions were set as: the volume of nitric acid 10 ml, reaction temperature 90 °C, reaction time 5 h and solid-to-liquid ratio 3 g/200 ml. It can be seen that the dissolving-out amount of Al 3+ increased with the concentration of fluorosilicate.
XRD tests show that all the three products kept crystal form with relatively high crystal state (Table 11), much better than H301. Among them, H402 has the largest (002) peak intensities together with the lowest FWHM, which indicates its best crystal integrity among these products. Therefore, the optimal experimental conditions are: reaction temperature 90 °C, reaction time 5 h, solid-to-liquid ratio 3 g/200 ml, the volume of nitric acid 10 ml and the concentration of fluorosilicate 0.015 mol/L. The optimal product has 65 mg/g Al 3+ dissolved out after the modification by the combination of nitric acid and fluorosilicate.
The properties of the modified sericite. The chemical composition analysis was done for both raw sericite (S 0 ) and fluorosilicated sericite by the combination of nitric acid and sodium fluorosilicate (S 2 ) (Table 12). Compared with S 0 , the SiO 2 contents and Na 2 O contents increased while the content of Al 2 O 3 decreased in S 2 . The modification by the combination of nitric acid and fluorosilicate can dissolve Al 3+ out from sericite, which of course, will improve Si/Al ratio to a large extent. The increase of Na 2 O content was induced by the Na + in the sodium fluorosilicate by ion exchanges. Other oxide contents, such as Fe 2 O 3 , FeO and K 2 O, decreased.
The filtrate during the modification was also detected in order to know the dissolution of different components from sericite (Table 13). Large amount of Al 3+ and K + have been dissolved together with Fe 3+ and Mg 2+ during the modification. Since sodium fluorosilicate also contains Na + , therefore, the detection of Na + couldn't reflect the dissolution of Na + from sericite. This is in accordance with the chemical analysis of sericite after modification.       Table 9. XRD data of the related products (single factor experiment of nitric acid volume).   Table 11. XRD data of the related products (single factor experiment of fluorosilicate concentration).

Trail
No. Zeta potential tests (Fig. 5) were conducted for sericite before and after modification. Both zata potential of raw sericite and modified sericite decreased with the increasing pH value of the solution. The absolute zeta potential value of the modified sericite was always lower than raw sericite, which means that the modification reduces the layer charge of sericite. This is also in accordance with data in Table 12 and Table 13, which shows that the modification dissolves Al 3+ out from sericite. The substitution of Si 4+ by Al 3+ in the tetrahedral sheet leads to the negative interlayer charge and makes sericite have no ion exchange capacity. After fluorosilicate modification, the layer charge of sericite decreased, which indicated that the modified sericite may gain cation exchange capacity, which of course, will help it to be used in the sericite/polymer nanocomposites.

Main Composition (ω B /%)
CEC value was tested in order to evaluate the cation exchange capacities of sericite before and after modification (Table 14). The CEC value of raw sericite is 4.94 mmol/100 g, which is caused by the unsaturated bonds on the surfaces and ends during mineral processing and chemical analysis (Table 12) shows that a certain amount of Na + exists in the interlayer places. After fluorosilicate modification, the CEC value of S 2 increased to 14.84 mmol/100 g, which means the fluorosilicate modification improved the cation exchange capacities and the structure of sericite. This result is also in accordance with the zeta potential tests before.
The fluorosilicate modification mechanism of sericite. NMR analysis was done after fluorosilicate modification. The range of chemical shift (δ) of Al is 450 ppm. Generally, δ of octahedral Al (Al o ) species and tetrahedral Al (Al t ) species is −10 to 10 ppm and 50 to 70 ppm, respectively. Therefore 27 , A1 NMR is employed to distinguish the two kinds of Al in the clay. As shown in Fig. 6, δ of Al t and Al o of S 0 was 68.7 and 4.0, both of which were similar to theoretical values. The counterparts of S 2 turned to be 69.6 ppm and 2.0 ppm, respectively.
According to Fig. 6, the relative ratio of Al t and Al o was calculated for both S 0 and S 2 (Table 15). Combining with the data in Table 12, the contents of Al t and Al o in each sample can be known as well. For example, in S 0 , there are 30.02 g Al 2 O 3 existed in each 100 g of S 0 . Therefore, the amount of Al is 15.89 g and the contents of Al t and Al o are 6.11 g and 9.78 g, respectively. Similarly, there are 3.37 g Al t and 5.73 g Al o existed in 100 g S 2 . The Si/ Al t ratio for S 0 and S 2 are 3.48 and 10, respectively, which clearly indicates that the fluorosilicate modification by the combination of and nitric acid can improve the Si/Al t ratio in the tetrasheet of sericite. It is speculated that the fluorosilicate modification reduced the layer charge of sercite, leading to weak binding effect of sericite to its interlayer ions, which finally induced the relatively high ion exchange properties of sericite.
Based on the above characterization results, the fluorosilicate modification mechanism was proposed as follows: First, when sodium fluorosilicate was used alone to modify the thermal-treated sericite, it had a tiny effect. However, the combination of nitric acid and sodium fluorosilicate had an obvious effect on sericite, which is    caused by the strong interactions between the two reagents. The H + in the nitric acid has a strong effect on the structure of sericite and made ion exchanges 40 . It also promotes the effect of fluorosilicate on sericite, probably because fluorosilicate can dissociate much more easily in nitric acid solution and its dissociative products have better acts on sericite. Second, high reaction temperature, low solid-to-liquid ratio, long reaction time, high acid concentration and high fluorosilicate concentration all helped the dissolution of Al 3+ , which are in accordance with the general laws during chemical reaction progress and these conditions can improve the activation energy during the reactions. Therefore, the mechanism of sodium fluorosilicate to the Al t in sericite was conjectured as follows: ( (2) Fluorosilicate in acid solution: The hydrolyzed F − reacted with H + in the solution and generated HF, which reacted with sericite and dissolved large amount of Al from sericite. The relevant reaction is listed below: The Si 4+ and Al 3+ in sericite have different chemical stabilities and Si 4+ can hardly be dissolved out. Therefore, the Si/Al ratio increased as the reaction went on, which promoted the reduction of its layer charge.

Conclusion
In this article, the thermal-treated sericite was modified by both fluorosilicate and the combination of nitric acid and fluorosilicate in order to improve the Si/Al ratio and reduce the layer charge for the preparation of sericite/ polymer nanocomposite. Several orthogonal experiments and single factor experiments were done in order to obtain the optimal reaction conditions. After comparison, it is thought that the combination of nitric acid and fluorosilicate has more remarkable effect on sericite than fluorosilicate alone. The optimal experimental conditions are: reaction temperature 90 °C, reaction time 5 h, solid-to-liquid ratio 3 g/200 ml, the volume of nitric acid 10 ml and the concentration of fluorosilicate 0.015 mol/L. The dissolving-out amount of Al 3+ can reached 65 mg/g and 27 Al NMR test showed that both Al t and Al o were dissolved out from sericite, while Al t had a lower relative content after modification. After modification, the Si/Al ratio increased from 3.48 to 10 and the layer charge decreased, together with the CEC value increased, which of course, proved the fluorosilicate modification can effectively activate sericite for the preparation of sericite/polymer nanocomposites.