Low-Cost Pathways to Synthesize Silica-Smectite Clay-Based Composites

The present study aimed to use rice husk as a natural silica precursor in the fabrication of silica-smectite composites. A local smectite clay was respectively mixed with 1) silica sludge from rice husk ash after an acid treatment, 2) an aqueous sodium silicate solution from the alkaline dissolution of silica sludge, and 3) a nanosilica powder obtained after the hydroxylation/polymerization of a sodium silicate solution. Products from the three different synthetic pathways were investigated by X-ray diffraction; Fourier infrared spectroscopy, scanning electron microscopy coupled with energy dispersive X-ray spectroscopy, and BET specific surface area (SSA) measurements. All techniques showed a heterogeneous morphology, where the distribution of silica particles in the clay matrix changed with each synthetic pathway. For the silica sludge synthetic pathway, a predominantly three-dimensional-like structure with a phyllosilicate matrix skeleton was obtained. For the pathway using a silicate solution, an amorphous compound with limited intergranular cohesion containing silicate agglomerates intercalated between clay sheets was found. The nanosilica reinforced pathway led to a packed morphology with a regular distribution of silica phases in the clay matrix. In all the synthesized composites, the amorphous silica phase was identified, with a potential higher reactivity and SSA of 228, 257, and 300 m2/g for pathways 1, 2, and 3, respectively. Correspondingly, the microstructure evidenced both an increased porosity and an increase in chemically active sites. Consequently, the obtained products are potential multifunctional materials.


Introduction
Composites materials are a combination of at least two materials with different properties and have a heterogeneous microstructure.In general, they present enhanced properties compared to the individual components [1,2].They are of great interest as their fabrication can be easily designed according to specific applications.Their electrical, optical, thermal, conductive, medical, dielectric, mechanical, or surface properties can then be targeted [3][4][5][6][7][8][9][10]. Nowadays, composites based on smectite clays are subjected to more attention due to their very large specific surface that is highly reactive.Different applications can be found in multiple domains, such as depollution, catalysis, synthesis of new products, and the treatment of oils.Smectite clays display accessible inter-sheet spaces that allow the intercalation of organic polymers or monomers, as well as inorganic compounds such as TiO 2 , SiO 2 and TiO 2 -SiO 2 [11][12][13][14][15][16][17][18][19][20].Amorphous silica can be used as an inorganic additive, with a considerable advantage due to its low toxicity, great reactivity, and possible functionalization of the surface.
Natural clays with a mixture of smectite and silica gel have extensive industrial applications, such as water treatment, removal of undesirable substances, and catalysis [21,22].However, this type of clay is sporadic or exists in very limited quantity on site, making it difficult to address the high demand.In the synthesis of silica-smectite composites, it has been reported that silica can either be regularly intercalated in the inter-sheet space of the smectite or highly dispersed in the smectite phase, resulting in a core-shell structure [23][24][25][26].In most cases, silica is originated from manufactured chemical precursors, mainly silicate alkoxides such as tetramethyl orthosilicate (TMOS) and tetraethyl orthosilicate (TEOS).In the current context of sustainable development, greener synthesis methods, avoiding expensive processes and the use of hazardous compounds, are preferred.The use of natural resources and the synthesis of amorphous silica from natural precursors has been considered in several studies [4,[27][28][29][30][31][32][33][34].
Rice husk is an agricultural waste from rice processing.It is composed of 15-20 wt.% SiO 2 , 74 wt.% organics (cellulose, lignin, and hemicellulose, and a mixture of D-xylose, L-arabinose, methyl glucuronic acid, and D-galactose), and 4 wt.%A1 2 O 3 + Fe 2 O 3 + CaO + MgO.Calcination under controlled conditions leads to the formation of rice husk ash (RHA), which mostly contains between 61 and 71 wt.% silica, and 36 wt.% unburnt carbon compounds.Rice husk is thus a natural alternative to synthetized materials and has great potential as a bio-sourced silica precursor [29,[35][36][37][38][39].In addition, the wide availability of rice husk as an agricultural waste and its similarity in composition to smectite clays that can be mined in many localities in Cameroon [40][41][42][43][44] make this material a valuable low-cost source of silicon.Such materials are extremely important for the sustainable valorization of local resources.Numerous applications of nanosilica from rice husk have been largely reported, including their utilization as carriers, desiccants, preservation tools to control humidity, catalysts, medical additives, fillers in composite materials, and drug delivery synthetic adsorption materials [4,[36][37][38]45].
In this study, low-cost silica-smectite clay-based composites were synthesized, using eco-friendly pathways and materials, and characterized by different techniques in order to propose potential applications.Such composites are of interest due to their low-cost and availability, their relatively simple synthesis method, and their potential use as adsorbents for wastewater treatment.

Materials and Sampling
The smectite clay was quarried in the locality of Bana (West, Cameroon).The clay fraction was increased by wet sieving at 75 μm to remove undesirable larger particles (quartz, plant residues, etc.).SiO 2 (50.94 wt.%) and Al 2 O 3 (22.39wt.%) were identified as major chemicals, and smectite (Ca-Montmorillonite) was identified as the major mineral phase [44].The grain size of the 75 μm fine fraction was subsequently reduced to under 20 μm by sedimentation.The process was performed in an aqueous suspension, with the addition of sodium hexametaphosphate.The particles' settling velocity is correlated to the diameter d (m) and the relative density ρ p (kg/m 3 ) by Stoke's model, assuming that the fluid flow in the vicinity of the particles remains laminar and following Eq.( 1).
where ρ p (kg/m 3 ) is the relative density of the particle, ρ f (kg/m 3 ) is the relative density of the water, μ f (Pa•s) is the dynamic viscosity of water, g (N/kg) is the gravity parameter, x (m) is the flow distance, and t (s) is the flow time.
In this way, the particle size distribution of the final fraction of the enriched clay fraction (ECF) was controlled.
The rice husk was provided by SEMRIZ Company (Far-North, Cameroon).After washing in deionized water to remove the remaining soil and sand, the suspension was sieved and dried at 110 °C for 24 h.The obtained rice husk was calcined in a muffle furnace at 700 °C for 2 h (5 °C/min heating rate).This thermal treatment program was found to be optimum according to previous work investigating the calcination of rice husk and leads to the formation of chemically reactive amorphous silica.At higher temperatures, crystalline phases such as cristobalite were formed, while temperatures below 500 °C led to incomplete carbonization [28,29,33,39].In this study, the objective of the heat treatment was to remove all organic matter and to obtain a silicarich ash, denoted RHA.
The method described by Le et al. [4] was modified to obtain enriched silica sludge from the RHA.RHA was firstly treated by a hydrothermal acid reflux process at 90 °C for 2 h, with a ratio of 100 g of ash per 200 mL of hydrochloric acid (10%).A second treatment with 300 mL of sulfuric acid (30%) dissolved the undesirable minerals.After filtration, the sludge was washed by centrifugation at 2000 rpm with distilled water over several cycles until the complete removal of chloride compounds (negative (1) test with AgNO 3 0.01 M) and sulfate compounds (negative test with BaCl 2 0.01 N) was realized.The enriched silica sludge was then dried at 110 °C to constant mass.The obtained product was kept in a closed Plexiglas container for future experiments.

Synthesis of Composites
Three different pathways were investigated for composite synthesis and are described as follows:

Pathway (1): Mixture of silica sludge and smectite clay in water
First of all, 100 mL of distilled water and 10 g of clay were introduced into a beaker and stirred at 60-65 °C for 30 minutes.Then, 5 g of silica sludge was added to the suspension and stirred at the same temperature for 90 minutes.The mixture was finally separated by centrifugation at 2000 rpm.The solid fraction was dried at 110 °C for 24 h and designated as CSS1.

Pathway (2): In situ precipitation of silica in a smectite clay suspension
A sodium silicate solution was previously prepared by introducing 5 g of silica sludge into a beaker containing 100 mL of 3.5 N NaOH.The mixture was stirred at 90 °C for 4 h to dissolve the sludge.After filtration, a clear sodium silicate solution was obtained.The pH of this solution was maintained at a value greater than 9 to avoid the formation of silicic acid (Si(OH) 4 ), which polymerizes at a pH between 3 and 4, with the formation of SiO 2 .In another beaker, a suspension of 2.5 g of smectite clay, 50 mL of water/ethanol solution (v/v), and 5 mL of H 2 SO 4 30% was prepared by stirring at room temperature.The sodium silicate solution was added to the prepared suspension.The pH of the mixture was quickly reduced and maintained between 3 and 4 by adding H 2 SO 4 30% dropwise.The obtained gel phase was kept in solution for 24 h, and was then recovered and dispersed in dry ethanol.After several washing/centrifugation cycles with distilled water to ensure the total removal of sulfate ions (using the BaCl 2 test), the gel was dried at 110 °C for 24 h, and the powder obtained was designated as CSS2.

Pathway (3): Mixture of silica nanoparticles and smectite clay in an aqueous solution
Silica nanoparticles were prepared according to the modified sol-gel method described by Tchanang et al. [33] from the clear solution of sodium silicate.A suspension of 10 g of smectite clay in 10 mL of distilled water was prepared and held under stirring for 30 minutes.Then, 4 g of nanosilica powder was added to the suspension and kept at a constant stirring rate at 60-65 °C for 1 h.The mixture was then centrifuged, and the final residue was dried at 110 °C and denoted CSS3.
Table 1 presents the main differences between the synthesis, particularly concerning the silica source and the process type.

Characterization of Composites
The surface morphology and chemical surface analysis of the synthesized composites were investigated by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) using a Thermo Scientific Prisma E. SEM with an Oxford EDS system.The samples were coated with a gold layer of 10 nm using a sputter coater (Leica ACE600, Wetzler, Germany) [46].The SEM-EDS analysis was conducted at a speed scan of 7 s with a LaB 6 filament at 40 μA.The images were collected with a magnification of 2500x.
The X-ray diffraction (XRD) analysis was performed using Empyrean series 2 equipment and the diffractograms for the synthesized composites were recorded using the PAN analytical (BBHD10-90.xrdmp) measurement software.The analysis was done using CuKα radiation at a wavelength of 1.54060 Å (40 kV and 45 mA), with a scan step of 25 s at 25 °C, in the 2θ range of 20°-90°.
The Fourier-transform infrared (FTIR) spectra were recorded at room temperature using an Alpha-P FT-IR BRUKER spectrophotometer.Samples were prepared by encapsulating 1.2 mg of fine grounded dried powder particles in 300 mg KBr [32].The spectral ranges were 500-4000 cm −1 for the composites and 370-1500 cm −1 for RHA and nanosilica.The specific surface area (SSA) was obtained by the BET method, from N 2 adsorption and desorption isotherms at 77 K, using a Carlo Erba Sorptomatic 1990 volumetric device, after outgassing the samples overnight at room temperature at a pressure lower than 10 −4 Pa.

Particle Size Distribution of Enriched Clay
The particle size of the ECF used for the work is shown in Fig. 1.It can be seen that about 10% of particles have a diameter Ø ≤ 0.002 mm, while about 85% have a diameter Ø ≤ 0.02 mm.This overall low particle size is necessary to attain a large specific surface for the synthesized composite material.

X-ray Diffraction (XRD) Analysis of Rice Husk Ash (RHA) and Nanosilica
Figure 2 shows the X-ray patterns of the RHA and the synthesized nanosilica.The two diffractograms were characterized by a large broad band centered at 2θ = 22.5°, demonstrating the amorphous nature of the analyzed samples.However, two peaks were observed at 2θ = 24.33°and 26.84° on the RHA diffractogram, as well as a single peak at 2θ = 26.71° on the nanosilica diffractogram.In both cases, these peaks can be attributed to quartz, determined with the Minerals JCPDS(ASTM) file [47].This may possibly be formed during the treatments applied.

Fourier-Transform Infrared (FTIR) Analysis of RHA and Nanosilica
The FTIR spectra of RHA and nanosilica are given in Fig. 3.The two spectra exhibited similar bands, with higher intensities for the nanosilica.The bands at 1060 cm −1 and 788 cm −1 correspond to the symmetric and antisymmetric Si-O bond vibrations, respectively.The band at 456 cm −1 is assigned to the absorption band of the vibration of the Si-O-Si bond [32,48,49].This analysis confirmed that the RHA and nanosilica are mainly composed of silica.

XRD Structural Phases
The XRD patterns of the synthesized composites are presented in Fig. 4, together with that of the ECF.The small peak at 2θ = 6° corresponds to a basal distance of 14.6 A°, which can be assigned to montmorillonite (smectite) [50,51].Kaolinite was also identified, with a basal distance of 7.2 A° at 2θ = 13.6°[47].Some other minerals are also identified, such as micas, anatase, quartz, zeolite, and K-feldspars.It can also be seen that no different mineral phases are identified in the three composites CSS1, CSS2, and CSS3, compared to those of ECF.The CSS1 composite shows sharp peaks attributed to montmorillonite (smectite) and quartz, which suggests the presence of a higher crystalline structure.For the CSS2 composite, the pattern is characterized by a wide hump, indicating the occurrence of an amorphous phase.The CSS3 sample shows slightly more crystallinity, but without clear sharp peaks, indicating crystalline silicon oxide.

FTIR Spectra of the Composites
In Fig. 5, the FTIR spectra of composites CSS1, CSS2, and CSS3 are plotted against that of ECF.The O-H stretching domain depicts two bands of hydroxyl stretching vibrations at 3710 and 3643 cm −1 .The bands at 3643 and 915 cm −1 respectively [32,42,52].The absorption band at 465 cm −1 corresponds to the deformation vibration of the Si-O-Si bond, but it is broader for CSS2 due to the higher amount of silica.The spectrum of CSS2 has some differences due to the disappearance of the OH stretching bands at 3710-3643 cm −1 .This may be from the polymerization of the silanol groups during the synthesis process.Moreover, for the same product, the bands of the Si-O and Si-O-Si stretching vibrations are broader, which can be correlated to the amorphous phase identified in the XRD patterns (Fig. 4).The FTIR spectra provide further evidence of the increased amount of silica in the synthesized composites.

Specific Surface Area (SSA)
The SSA value of CSS1 (228 m 2 /g) is lower and comparable to that of the initial clay (233 m 2 /g), which is related to the clay sheet-like structure, as demonstrated by the XRD and FTIR analyses.In contrast, CSS2 (257 m 2 /g) and CSS3 (300 m 2 /g) exhibited higher SSA than that of the initial clay sample.The two products are assumed to be nanocomposites in nature, since the clay matrix reinforced by silica is a nanosized compound.For CSS2, the one-step synthesis of the clay-nanosilica composite favors the in situ formation of nanosilica, which is effectively distributed in the matrix phase.

Scanning Electron Microscopy (SEM)/Energy Dispersive Spectroscopy (EDS) of the Composites
Figure 6 shows SEM images of the three composites from the three synthetic pathways, and Table 2 the EDS elemental analyses of the three main nuances of each images (#1, #2 and #3).Both reaveal heterogeneous morphologies, with specificities in each case: For the CSS1 composite, a microstructural arrangement of the clay crystallites and silica agglomerates is shown.Individual silica clusters are distributed within the skeleton of the crystalline phases  identified by XRD.The whole arrangement evidences that the density is reduced by the porosity; For CSS2, a predominant number of silica agglomerates is randomly scattered in the clay matrix.The whole system is less heterogeneous and more compact than CSS1; CSS3 exhibits multiphase microstructure with a more homogeneous repartition of the phyllosilicate matrix and agglomerated silica particles.The quasi organized microstructure induces an increased compacity, but with some entrapped porosity.

Discussion
To obtain more sustainable composite materials, in addition to the extensive applications gained from their heterostructures, different versatile and easy-to-use production methods were investigated using a natural smectite clay and a rice husk agricultural waste.Figure 7 summarizes the three methods, from the initial resources to the final products.The synthesis methods differ mainly in respect to the chemical processes that change the size and the morphology of the silica clusters and their distribution and behavior in the clay matrix phase.Although starting from the same original rice husk sample, pathway 1 was found to be more favorable for micro sized particles, whereas the other two pathways produced nanosized silica, which was formed in situ for pathway 2 and preliminarily synthetized for pathway 3.
The product CSS1 appears to be a mixture of well crystalline phases and some amorphous phases, evidenced by the XRD pattern (Fig. 4).This feature suggests the occurrence of a tridimensional network in the microstructure.It can be assumed that the relatively large size of the microparticles of amorphous silica is not favorable for insertion into clay interlayers, since the basal spacing has been reported to be around 10-18.5 Å depending on the hydration state [53][54][55].Such products might have synergic properties, due to the presence of amorphous and crystalline phases, and may have a similar trend in performance as other structurally similar materials [21,42,56].Advanced mechanical properties are also expected from this type of composite, as stated by previous investigations on composites with tridimensional network structures.Fibrous reinforcement in the mineral matrix is a typically well-known for its good mechanical strength and limited drawbacks [57,58], which could be compared with the current composite made of clay mineral reinforced with silica sludge, described as fibrous by SEM images.
The composite silica-smectite clay CSS2, synthesized by pathway 2, was obtained via the in situ formation of nanosilica in the mixture of smectite clay with sodium silicate.The process includes an acid condensation and the precipitation of amorphous silica combined with the clay.A slight protonation process of the internal and external oxygen atoms of the clay sheets occurs, to form Si-OH groups in a similar process that leads to Si(OH) 4 synthesis from silicate solution at pH 3-4.Finally, a condensation reaction occurs between the Si-OH groups to form clusters of nanosilica.The reactions involved in pathway 2 can be expressed by the reactions below (Eqs.[2][3][4][5]. Protonation of the clay surface in acidic medium This pathway leads to the formation of nanosilica that is able to reinforce the clay matrix.CSS2 is an in situ nanocomposite material obtained with a limited number of chemical steps and, consequently, with some energy saving.The XRD pattern and SEM-EDS images (Figs. and 6) show that the product has a predominantly amorphous phase in the microstructure, derived from the crystalline mineral phases in the initial clay.Besides, the FTIR spectrum (Fig. 5) also evidences the absence of -OH vibration bands, which are involved in the polymerization process with the Si-OH groups of silicic acid.Such a compound is assumed to have various superior properties that are advantageous in many applications, such as hydrophobic surface coatings, catalysis, and adsorption [14,25].In fact, the amorphous character restricts the fixed boundaries usually found in rigid organized structures, increasing the versatile behavior of the product.Composite CSS3, synthesized using pathway 3, is a typical nanocomposite, characterized by the strong combination of nanosilica and clay evidenced in the SEM images (Fig. 6), with a compact but heterogeneous morphology.The insertion of nanosilica particles into the clay layers is evident, since their size is comparable to that of the interlayer gap of 10 Å. Silica can also be found in the vicinity of the clay external surfaces, where they are bound by hydrogen and Van der Waals bonds due to the occurrence of silanol groups (Si-OH) at the silica-water interface [59].This phenomenon is accentuated in aqueous acidic media (Eq.6).The hydrogen bonds in the interlayers might occur following the mechanism described in Fig. 8. Upon the possible diverse arrangements of the silica groups, the reinforced clay gains a high surface to volume ratio and a related high aspect ratio.The interesting properties of this nanocomposite might make its application outstandingly extensive.This material is able to exhibit innovative surface and inner physico-chemical properties that are necessary to understand their application.( 6)  3) shows the higher values of these products.This is an interesting result regarding the potential use of the current synthesized against those of previous works, which were applied in adsorption and in electrical, optical, and thermal treatments [23][24][25][26]60].

Conclusions
This work aimed to synthesize and characterize composites based on two local Cameroonian raw materials, namely smectite clay and rice husk.The raw materials were collected, prepared, and characterized, and the results obtained show that they were indeed interesting for the synthesis of porous composites potentially usable for many purposes.Three composites have been synthesized and characterized (X-Ray, FTIR, SEM-EDS, and BET SSA) and the conclusions are as follows: 1) Composite materials can be successfully obtained by mixing a purified RHA and a smectite clay.The micronized particles of silica from the RHA are relatively large, limiting possible intercalation into the gaps of the clay layers.They preferably reacted to form crystalline assemblies with clay sheets, which were distributed in a tridimensional network.However, amorphous silica clusters were embedded with a limited capacity and the occurrence of porosity; 2) Amorphous silica can be synthetized in situ from a silicate solution and a smectite clay mixture, resulting in a clay matrix reinforcement.Protonation of the crystalline clay led to the synthesis of a material having a prevalent amorphous structure and a microstructure favorable to the versatile behaviors related to amorphous materials.This in situ nanocomposite was obtained with limited chemical steps, favoring cost and energy savings; 3) The nanocomposite based on smectite clay and nanosilica from the RHA exhibited a compact heterogeneous morphology with a packed design that allowed the control of porosity.Nano-composites were characterized by the occurrence of internal and external chemically reactive surfaces due to the silica protonation at the interlayer surfaces and the formation of hydrogen bonds in the surroundings.
The presence of more reactive amorphous phases and nanoparticles of silica in the CSS2 and CSS3 composites may be valuable in the use of these products to remove pollutants, from wastewater for instance.

Fig. 3 Fig. 4
Fig. 3 Main peaks identified in the FTIR spectra of (a) RHA and (b) nanosilica

Formation of silicic acid 3 ) 2 Fig. 7
Fig. 7 General scheme of the 3 synthesis pathways used for obtaining composite materials

Fig. 8
Fig. 8 Illustration of Hydrogen interactions at the silica-clay interface