Simple process for synthesis of layered sodium silicates using rice husk ash as silica source

Accepted Manuscript Simple process for synthesis of layered sodium silicates using rice husk ash as silica source Miao Deng, Guoyong Zhang, Yu Zeng, Xiangjun Pei, Runqiu Huang, Jinhui Lin PII:

S0925-8388(16)31448-7

DOI:

10.1016/j.jallcom.2016.05.115

Reference:

JALCOM 37635

To appear in:

Journal of Alloys and Compounds

Received Date: 1 April 2016 Revised Date:

10 May 2016

Accepted Date: 12 May 2016

Please cite this article as: M. Deng, G. Zhang, Y. Zeng, X. Pei, R. Huang, J. Lin, Simple process for synthesis of layered sodium silicates using rice husk ash as silica source, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.05.115. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Simple process for synthesis of layered sodium silicates using rice husk ash as silica source Miao Denga, Guoyong Zhanga, Yu Zenga, Xiangjun Peib, Runqiu Huangb, Jinhui

a

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Lina,*

College of Materials and Chemistry & Chemical Engineering, Chengdu

b

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University of Technology, Chengdu, Sichuan, 610059, China

State Key Laboratory of Geohazard Prevention and Geoenvironment Protection,

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Chengdu University of Technology, Chengdu, Sichuan, 610059, China E-mail address: [email protected] *Corresponding author

College of Materials and Chemistry & Chemical Engineering, Chengdu University

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of Technology, Chengdu, Sichuan, 610059, China

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E-mail address: [email protected]

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Tel: +86 2884078239

ACCEPTED MANUSCRIPT Abstract β, δ, and α phases of layered sodium silicates were synthesized using waste rice husk ash (RHA) as the raw material by a simple sintering process. X-ray diffraction

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(XRD) measurements revealed a phase transformation from β through δ to α as the reaction temperature and time of synthesis was increased. Raman studies confirmed the

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synthesized products have a layered SiO4 tetrahedral structure. Further, a Si–O–Si (Na) band bond force constant sequence for the three phases was speculated (δ > β > α) that

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agrees with the ion binding sequence of the phases. Scanning electron microscopy (SEM) elucidated the typical micromorphology of the synthesized products, which was structurally similar to that of RHA. Optimal synthesis conditions produced layered sodium silicates that exhibited Ca2+ and Mg2+ binding capacities of 406 and 453 mg/g,

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respectively. This method can therefore be used to simply and cheaply convert

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agricultural waste into valuable products.

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Keywords: layered sodium silicates, rice husk ash, Raman, ion binding capacity

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ACCEPTED MANUSCRIPT 1. Introduction Layered sodium silicates have a polymeric layered crystalline structure with complicated polymorphism [1]. At least five phases have been acknowledged, though

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the common framework phases are δ, α, and β [2]. Their foundations are built from the condensation of silicate tetrahedra into a single sheet, with interlamination achieved

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through sodium cations. This makes them structurally similar to the natural mineral bentonite, and they therefore have the potential for application as a grinding aid,～

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adsorbent, dispersant, or catalyst carrier [3]. Furthermore, their high sodium content gives them a pH buffering ability, strong absorption, and excellent ion binding capacity [4], providing a suitable alternative for phosphorus-based detergents that would be less

industry.

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polluting. Layered sodium silicates are therefore of great interest to both academia and

Commercial layered sodium silicates are typically mixtures of δ ( 90%), α ( 10%) phases [2] prepared using sodium silicate solutions as raw

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10%), and β (

materials. These solutions are thermally spray dried to produce granules that are then

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sintered at high temperature under different conditions [4-6]. However, researchers have found that this conventional approach is not suitable for producing a high content of single-phase product [2, 5]. To address this, Falamaki incorporated an appropriate quantity of pure-phase seeds with a specific particle size to induce the direct formation of a particular phase and suppress the formation of other phases [7]. Kahlenberg et al. have also used the pretreatment of raw materials (slowly drying liquid sodium silicate at

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ACCEPTED MANUSCRIPT 75 °C for eight weeks), followed by sintering for 20 h to obtain pure-phase products [8]. However, the high cost of the raw materials, prolonged production cycle, and wide range of control variables in these processes greatly decreases the industrial value of the

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products produced, which has limited their application. Researchers have therefore concentrated on finding an economically viable approach for the synthesis of layered

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sodium silicates based on a low-cost source and simple process. Sun et al. used zirconium oxychloride waste effluent for the preparation of layered sodium silicates to

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reduce the production cost associated with the raw materials [9]. Yang et al. synthesized layered sodium silicates from sodium silicate solutions using the mirabilite method, which provides a new resource for their synthesis [10]. The Korea Research Institute

production [11].

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has also applied for a patent on improved technology for layered sodium silicate

Rice husk (RH) is a common agricultural residue, with a global annual output of

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~1.2 billion tons [12, 13], but contains 10–21% silicon in a hydrated amorphous form. This amorphous silica is a desirable raw material for the preparation of silica and

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silicates, making RH a potential biomass resource [14-17]. However, most RH is currently discarded as waste because it lacks any direct value for commercial application. This not only wastes a valuable silica resource, but can also create environmental issues due to its extremely slow decomposition rate [18]. There is therefore an urgent need to find a way of using RH more effectively. Incinerating RH under controlled conditions can enrich its amorphous silica

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ACCEPTED MANUSCRIPT content, with the rice husk ash (RHA) obtained being one of the most silica-rich raw materials [19-22]. In addition to being cheap and abundant, RHA has many other advantages such as high reactivity, large surface area, and a superfine size. The use of

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RHA as a silica source for the preparation of economical products [23-28] has been explored, but there are few reports pertaining to the preparation of layered sodium

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silicates from RHA.

In this paper, we report the synthesis of β-, δ-, and α-phase layered sodium silicates

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from RHA using a simple sintering process. The effect of changing the reaction temperature and time is investigated, and the structure and morphology of the silicates produced are characterized and their ion binding capacities are determined. This study is

and simple process.

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2. Experimental

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advantageous as it converts agricultural waste into valuable products using a low cost

2.1. Materials and reagents

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RH was collected locally, from which RHA was prepared in-house by pyrolysis.

Analytically pure NaOH (Kelong Chemical, Sichuan, Chengdu) was used in the process.

2.2. Preparation of layered sodium silicates The RH was washed with distilled water to remove any dust or impurities. This

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ACCEPTED MANUSCRIPT was incinerated at 600 °C for 120 min to obtain RHA, which was subsequently mixed with a 1 M NaOH solution (molar ratio of Si/Na = 1) using a mechanical stirrer. The mixture was then poured into a ceramic crucible and sintered at 680−800 °C for

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5−210min. The samples were cooled to room temperature as the oven cooled naturally. The experimental process is shown schematically in Fig.1. The molar ratio of Si/Na in each sample was established using the following reaction:

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2 SiO + 2 NaOH = Na O⋅2 SiO + H O

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(1)

2.3. Characterization

The chemical composition of the RHA was analyzed by X-ray fluorescence (XRF-1800). The crystal structures of the synthesized samples were determined by

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X-ray powder diffraction (XRD) (Rigaku DmaxIIIc diffractometer) with Ni-filtered Cu Kα radiation. Raman spectra were obtained using a Renishaw inVia Raman instrument.

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Morphological characterization was performed by scanning election microscopy (SEM, model: Quanta 250 FEG, USA). N2 adsorption/desorption measurements were

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performed using a QUADRASORB SI surface area and pore size analyzer. Transmission electron microscopy (TEM) images were obtained with a JYT011–1996 instrument.

The

Ca2+

and

Mg2+

binding

ethylenediaminetetraacetic acid (EDTA) titration.

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capacities

were

determined

by

ACCEPTED MANUSCRIPT 3. Results and discussion The chemical composition of the RHA shown in Table 1 indicates that the major ingredient is SiO2, along with small amounts of Al2O3, Fe2O3, CaO, MgO, TiO2, K2O,

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and Na2O. The broad intense peak at 2θ=22° in Fig. 2 indicates that the RHA has a high content of amorphous silica, with the absence of any sharp peaks confirming that the

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solid has no ordered crystalline structure. As the RHA clearly contains pure silica in an amorphous state with few impurities, it is a viable raw material for the preparation of

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layered sodium silicates.

The effects of reaction temperature and time on the synthesis of layered sodium silicate were investigated by XRD (Fig. 3). The phase contents of the synthesized samples were determined based on Lucas's study on quantitative phase analysis [2, 5].

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Based on Fig. 3a and Table 2, the reaction temperature for phase evolution can be divided into two sections. In the interval from 680 °C to 720 °C, a high content of β

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phase is obtained at 680 °C, but this decreases with an increase in δ phase up to 720 °C. In the interval from 720 °C to 800 °C the δ phase has an analogous evolution to the β

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phase, with the α phase tending to form at higher temperatures until its phase content reaches a maximum at the extinction of the δ phase. The effect of reaction time on the synthesis is shown in Fig. 3b and Table 3. Here, a similar phase transformation is observed, with an evolution from β to δ up to 120 min and stable α phase starting to be detected at 150 min. According to the XRD patterns, it is reasonable to speculate that β-Na2Si2O5

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ACCEPTED MANUSCRIPT (JCPDS: 22-1123) can be obtained as an initial product at low temperature and with a short reaction time, while higher temperatures and longer times favor the formation of stable α-Na2Si2O5 (JCPDS: 22-1397). The transitional δ-Na2Si2O5 (JCPDS: 22-1396)

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can only be prepared within a narrow range of conditions, suggesting the following phase transformation sequence occurs during the process: β→δ→α (Tables S1, S2, and

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S3). This is consistent with the phase evolution process reported for the synthesis of layered sodium silicates from sodium silicate solutions [6]. More importantly, this

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means that pure phases of layered sodium silicate can be obtained from RHA using this preparation technology, which should be of benefit to its industrialization. Raman spectroscopy was used to identify the functional groups of typical samples (Fig. 4). All spectra exhibit two major peaks around 540 cm-1, corresponding to the

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bending vibration of Si–O–Si bonds, and around 1050 cm-1, which are attributed to the symmetric stretching vibration of Si–O–Si (Na) [1]. These peaks suggest the existence

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of layered SiO4 tetrahedra [29] in the samples. Furthermore, the intensive peaks around 1050 cm-1 corresponding to Si–O–Si (Na) of the layered sodium silicates display

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different Raman shifts (β-Na2Si2O5, 1072 cm-1; δ-Na2Si2O5, 1063 cm-1; and α-Na2Si2O5, 1080 cm-1), implying that the bond force constants in the different phases are different. The values for β-Na2Si2O5, δ-Na2Si2O5, and α-Na2Si2O5 are calculated using Equations (2) and (3) and found to be 8.49 N/cm, 8.35 N/cm, and 8.62 N/cm respectively. It can thus be concluded that the Si–O–Si (Na) bond force constants can be arranged in the following order: δ > β > α.

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∆v =  

=

   



(2)

(3)



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where ∆v (cm-1) and k (N/cm) are the Raman shift and diatomic bond force constant,

respectively; u (g) is the atomic mass; m1 and m2 are the relative atomic mass of silica

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and sodium, respectively; and N0 is the Avogadro constant.

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The morphologies of the synthesized β-Na2Si2O5, δ-Na2Si2O5, and α-Na2Si2O5 were determined by SEM (Fig. 5, Fig. S1). All three phases are aggregates of irregular crystals and display analogous micromorphologies by aggregation of small layers in particles [4, 5]. This is similar to the structural characteristics of RHA, which clearly

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supplies the basis for the crystal growth of layered sodium silicates. The morphology and structure characteristics of δ-Na2Si2O5 were further analyzed

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by TEM (Fig. 6). The lamellar structures of the as-synthesized samples are shown in Fig. 6(a), while a high-resolution TEM (HRTEM) image is shown in Fig. 6(b). Two types of

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regular fringes with different orientations (A and B) can be observed. The interplanar distance between the adjacent lattice fringes in A and B are 0.4204 nm and 0.4934 nm, respectively, which correspond to d011 and d210 spacings. The ion binding capacities are primarily influenced by two processes: (a) the exchange of cations of the intermediate layers and shows the ion exchange properties (b) and the dissolution of a part of the product in water and the subsequent freeing of the

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ACCEPTED MANUSCRIPT silicate and hydroxyl precipitates with Ca2+ and Mg2+ cations. Layered sodium silicates generally behave as finely dispersed solids of low solubility in the wash liquor. The dissolution process is a slow one, especially at room temperature. Instead, the sodium

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exchange is relatively fast. Therefore, the Ca2+ and Mg2+ binding process exhibits characteristics corresponding to ion exchange and precipitation [7]. The Ca2+ and Mg2+

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binding capacities of the samples synthesized under different reaction conditions are exhibited in Figs. 7 and 8. Note that the ion binding capacities of all samples are

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significantly impacted by the reaction temperature and time. The sample synthesized at 720 °C demonstrates an excellent Mg2+ binding value, which is more than twice that of the sample synthesized at 800 °C. Combined with the XRD results, the different phases of layered sodium silicate clearly demonstrate distinct ion binding capacities. The

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sample prepared at 720 °C for 120 min, corresponding to δ-Na2Si2O5, has the highest binding capacities (Ca2+: 406 mg/g, Mg2+: 453 mg/g), whereas those of the sample

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prepared at 800 °C for 120 min, corresponding to α-Na2Si2O5, are quite low (Ca2+: 258 mg/g, Mg2+: 200 mg/g). Accordingly, the ion binding capacity sequence of the different

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phases is δ > β > α. This variation may be caused by the characteristic crystalline structure [7] and different bonding modes [30] in the frameworks of the phases. It should also be noted that the bond force constant sequence of Si–O–Si (Na), as calculated from the Raman results, is consistent with the ion binding sequence of the phases. This suggests that the smaller Si–O–Si (Na) bond force constant of Na2Si2O5 corresponds to a lower bonding energy. Therefore, these bonds can be broken readily,

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ACCEPTED MANUSCRIPT resulting in cations; this is more conducive to cation exchange and should result in higher binding capacities. Consequently, the low Si–O–Si (Na) bond force constants could be one reason for the superior capacities of the synthesized δ-Na2Si2O5 when

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compared with the other phases. Furthermore, the ion binding values exhibited by the optimal synthesis conditions are superior to those of layered sodium silicates prepared

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from sodium silicate solutions by traditional processes [4, 31, 32]. This advantage may be attributed to the larger specific surface area of layered sodium silicates that is

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inherited from RHA and high δ phase content of the products [Fig S2, Table S4]. 4. Conclusion

Layered sodium silicates were prepared from RHA using a simple sintering procedure, and the effects of reaction temperature and time were investigated. A phase

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transformation sequence was identified from β through δ to α as the temperature and time of synthesis was increased. The synthesized products with a higher content of the δ

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phase exhibited superior Ca2+ and Mg2+ binding capacities. These results demonstrate that employing RHA as a raw material for the synthesis of layered sodium silicates is an

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effective method that is both theoretically and practically relevant to the commercial production of layered sodium silicates. It also has the added advantage of utilizing what would otherwise be waste as a raw material, thus reducing the environmental impact of the waste.

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ACCEPTED MANUSCRIPT Acknowledgements This work was financially supported by the National Natural Science Foundation

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of China (40402025).

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Figure captions:

Fig. 2. XRD pattern of rice husk ash (RHA).

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Fig. 1. Schematic of the experimental procedure used.

120 min and (b) for different reaction times at 720 °C.

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Fig. 4. Raman spectra of layered sodium silicates.

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Fig. 3. XRD patterns of the samples prepared at (a) different reaction temperatures for

Fig. 5. SEM micrographs of (a) β-Na2Si2O5, (b) δ-Na2Si2O5, (c) α-Na2Si2O5, and (d) RHA.

Fig. 6. (a) low and (b) high magnification TEM images of as-synthesized δ-Na2Si2O5.

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Fig. 7. Ca2+ and Mg2+ binding capacities of the samples prepared at different reaction temperatures for 120 min.

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reaction times.

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Fig. 8. Ca2+ and Mg2+ binding capacities of the samples prepared at 720 °C for different

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ACCEPTED MANUSCRIPT Table 1 Chemical composition of RHA (%)

Al2O3

Fe2O3

MgO

CaO

TiO2

K2O

Na2O

98.64

0.38

0.238

0.218

0.249

0.03

0.125

0.12

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SiO2

ACCEPTED MANUSCRIPT Table 2 Phase contents of the samples prepared at different reaction temperatures for 120 min

Reaction temperature （°C） 680

Phase content（w%） β

α

3

97

/

700

75

25

/

720

100

/

/

740

84

/

760

43

/

780

15

/

800

/

/

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δ

16 57 85

100

ACCEPTED MANUSCRIPT Table 3 Phase contents of the samples prepared at 720 °C for different reaction times

Phase content（w%） β

α

/ 57 66 74 100 89

100 43 34 26 / /

/ / / / / 11

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δ

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Reaction time （min） 0 30 60 90 120 150

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Fig. 1. Schematic of the experimental procedure used.

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Fig. 2. XRD pattern of rice husk ash (RHA).

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Fig. 3. XRD patterns of the samples prepared at (a) different reaction temperatures for 120 min

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and (b) for different reaction times at 720 °C.

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Fig. 4. Raman spectra of layered sodium silicates.

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Fig. 5. SEM micrographs of (a) β-Na2Si2O5, (b) δ-Na2Si2O5, (c) α-Na2Si2O5, and (d) RHA.

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Fig. 6. (a) low and (b) high magnification TEM images of as-synthesized δ-Na2Si2O5.

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Fig. 7. Ca2+ and Mg2+ binding capacities of the samples prepared at different reaction temperatures

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for 120 min.

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Fig. 8. Ca2+ and Mg2+ binding capacities of the samples prepared at 720 °C for different reaction

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times.

ACCEPTED MANUSCRIPT Highlights •

β, δ, and α phases of layered sodium silicates are prepared from rice husk ash using a simple process. The phases transform from β to δ to α as time and temperature increase.

•

Optimal synthesis conditions produced layered sodium silicates that exhibited excellent

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•

Ca2+ and Mg2+ binding capacities.

Agricultural waste is converted to valuable products for potential use as detergent

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builders.

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•