A Novel Synthesis Method of Porous Calcium Silicate Hydrate Based on the Calcium Oxide / Polyethylene Glycol Composites

This paper proposed a novel method to prepare porous calcium silicate hydrate (CSH) based on the calcium oxide/polyethylene glycol (CaO/PEG 2000 ) composites as the calcium materials. The porosity formation mechanism was revealed via X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), Brunauer-Emmett-Teller (BET), and Fourier transformed infrared spectroscopy (FT-IR).The reactivity of silicamaterials (SiO 2 ) enhanced by increasing pH value. Ca could not sustain release from CaO/PEG 2000 and reacted with SiO 3 2− caused by silica to form CSH until the hydrothermal temperature reached to 170C, avoiding the hardly dissolved intermediates formation efficiently. The as-prepared CSH, due to the large specific surface areas, exhibited excellent release capability of Ca and OH. This porous CSH has potential application in reducing the negative environmental effects of continual natural phosphate resource depletion.


Introduction
Phosphate, as an irreplaceable and nonrenewable resource, has an important contribution to industry and agriculture [1][2][3][4].But this precious resource will be exhausted as a result of increased consumption in the near future [5][6][7][8].The sustainable utilization of phosphate has become a severe challenge for human beings.Calcium silicate hydrate, due to the unique release capability of Ca 2+ and OH − , has caused international extensive concern in the field of "recovery of phosphate from wastewater" [9][10][11][12][13][14].This is because the released Ca 2+ and OH − can react with the phosphate ions to form hydroxyapatite (HAP) on the surface of CSH when the concentration of these ions reached to the supersaturated conditions [15][16][17].Therefore, the release capability of Ca 2+ and OH − of CSH plays a key role in the field of phosphate recovery.
It is worthy to notice that the release capability is related to the specific surface area ( BET ) and pore structure.Large  BET and porous structures are beneficial to enhance the solubility of CSH [18,19].The current CSH samples were prepared by dynamic hydrothermal synthesis using CaO materials and SiO 2 materials [20][21][22][23].However, there two critical problems that affect the solubility of CSH.On the one hand, the reactivity of SiO 2 was too poor to participate in the formation of CSH.The residual SiO 2 precipitated on the surface of CSH is easy to block the pore structure and decrease the solubility of CSH.According to the previous study, the proper temperature to synthesize CSH was 170 ∘ C [18,19].However, there were abundant hard dissolve intermediates such as calcium silicate, formed during the heating process.These intermediates coated on the surface of CSH and affected the solubility of CSH [24,25].Therefore, enhancing the reactivity of SiO 2 materials and avoiding the formation of intermediates are the critical factors for the formation of CSH with porous structure.
A synthesis strategy based on calcium oxide/polyethylene glycol (CaO/PEG 2000 ) composites was developed for the formation of porous CSH.Under the dynamic hydrothermal condition, massive SiO  material to form a 300 mL slurry (liquid/solid mass ratio is 30/1; Ca/Si molar ratio is 1.75/1), respectively.1 mol/L of NaOH was used to maintain the pH values of the slurry at 13.0.Mixtures were agitated at 90 rpm, and the resulting slurry was put into a high-pressure kettle for hydrothermal synthesis at 170 ∘ C for 6 h.The as-prepared CSH samples obtained from CaO/PEG 2000 composites and CaO materials were labeled as CSH (CaO/PEG 2000 ) and CSH (CaO), respectively.

Dissolution Experiment.
The release of Ca 2+ and OH − from CSH was investigated via a series of batch experiments.For each experiment, 1 g of CSH was poured into 1 L of deionized water in glass bottles, thus leading to a sample to solution ratio of 1 g/L.The bottles were then placed on an agitation table and mixed at 40 rpm at 20 ∘ C for 5, 10, 15, 20, 40, 60, 80, and 100 min.The resulting Ca 2+ concentration was determined using the EDTA coordination titration method (the relative derivation of data is 0.05%).Solution pH value was measured (±0.1) using precise pH paper (pH 7.0-10.0,Sanai-si reagent Co., Ltd., Shanghai, China).The accuracy of pH measurement is 0.1.

Experiments on Phosphate
Recovery from Synthetic Solutions.Phosphate recovery property of the as-synthesized samples were investigated in a series of batch experiments.The pH values of phosphate-content solution were in the range of 7.0-7.5 before the CSH samples was added into this solution.For each one, one glass bottle containing 1 L of a synthetic solution with initial phosphate concentration (100 mg/L) was prepared.Then 1 g of synthesized sample was put into this bottle, thus leading to a sample-to-solution ratio of 1 g/L.The bottle was placed on an agitation table and shaken at 40 r/min under given temperature conditions (20 ∘ C) for 60 min.The solid samples after reaction were then separated from the removed synthetic solution, and were added again to synthetic solution with initial phosphate concentration of 100 mg/L.This experiment was repeated for six times until the phosphate concentration was kept unchanged with the addition of samples.The content of phosphate in the recovered products was identified with atomic absorption spectrophotometry (Atomic Absorption Spectrometer, AA800, USA).

Characterization Instruments.
The phase component and crystal structure of CSH are determined using X-ray diffraction with Cu   radiation (XRD, model XD-2 instrument, China).The morphology was observed by fieldemission scanning electron microscopy (FESEM, IUE, Hitachi, Japan) and transmission electron microscope (TEM, JEOL JEM-2010, Japan).The  BET and pore structure was investigated using adsorption-desorption measurements.Nitrogen adsorption-desorption isotherms were obtained on a nitrogen adsorption apparatus (ASAP-2010, USA).The microstructures are evaluated by Fourier transformed infrared spectroscopy (FT-IR, IR Prestige-21FT-infrared spectrometer, Shimadzu, Japan).when the pH value is over 13.0 [26][27][28].This result can be verified according to the fraction formula of H 2 SiO 3 , HSiO 3 − and SiO 3 2− under the given pH value as follows:

The Effect of pH on the Reactivity of SiO
where [H + ] is the concentration of hydrogen ions and  1 and  2 are the first and second dissociation constants, respectively.When pH = 12.0, the distribution coefficients of H 2 SiO 3 , HSiO 3 − , and SiO 3 2− are 0%, 39%, and 61%, respectively.When pH = 13.0,these coefficients are 0%, 6%, and 94%, respectively, [29].This trend indicated that silicon exists only in the form of SiO 3 2− that is beneficial to the formation of CSH.CaO and PEG 2000 .The reaction mechanism between CaO and PEG 2000 was revealed via FT-IR analysis.Figures 1(a) and 1(b) show the FT-IR spectra of neat CaO and CaO/PEG 2000 composites, respectively.As shown in Figure 1(a), a broad and sharp peak at 1402∼ 1546 cm −1 and 870 cm −1 can be attributed to the characteristic peak of CaO.The stretching vibration band of C-O-C at 1090 cm −1 and the characteristic absorption band occurred at about 2270 cm −1 due to a bent oscillation peak of C-H bond in Figure 1(b) can be assigned to bands of PEG 2000 , and the absorption peak of C-O-C moves to a low band.This phenomenon indicated that the asymmetric stretching vibration frequency of C-O-C group of PEG 2000 decreased due to the effect of Ca 2+ .Furthermore, this result demonstrated that Ca 2+ reacted with oxygen atom in PEG molecule to form complexation structure; that is, CaO and PEG 2000 existed together in the form of complex.

Specific Surface Area and Pore
Structure.The  BET and pore structure of the as-prepared samples were investigated by adsorption-desorption measurements.As shown in Table 2, the  BET of CSH (CaO/PEG 2000 ) increased to 133 m 2 /g compared to CSH (CaO) (62 m 2 /g).In comparison to CSH (CaO) (0.16 cm 3 /g), the pore volume of CSH (CaO/PEG 2000 ) increased to 0.36 cm 3 /g.Figure 3(a) shows the N 2 adsorption-desorption isotherms of the CSH samples.According to the Brunauer-Deming-Deming-Teller (BDDT) classification, the majority  of physisorption isotherms can be grouped into six types.The isotherms of all the samples belonged to type IV, including the pore-size distributions in the mesoporous regions [30].
The shapes of hysteresis loops were of the type H3, which was associated with mesopores formed due to aggregation of plates-like particles [31].Figure 3(b) shows the corresponding PSD of the samples.For the CSH (CaO), the PSD curve is bimodal with smaller (∼2.54 nm) and larger (∼45.42 nm) mesopores.For CSH (CaO/PEG 2000 ), the PSD curve exhibits small (∼6.52 nm) mesopores.The small mesopores and larger ones came from the aggregation of primary particles and secondary particles, respectively.This result was consistent with the result of N 2 adsorption-desorption isotherms.A large number of small mesopores contribute to the large  BET .The porosity formation mechanism can be revealed as follows: (1) during the heating process, SiO     2− quickly to form CSH (CaO/PEG 2000 ).To the synthesis of CSH (CaO), massive Ca 2+ was released from neat CaO materials during the heating process before the hydrothermal temperature reached to 170 ∘ C, leading to formation of abundant hardly dissolved intermediates.By comparison, the new synthesis method herein avoided the formation of the hard dissolve intermediates.

The Enhanced Phosphate Recovery Property of CSH.
The phosphate content of the product recovered by CSH (CaO/PEG 2000 ) increased to 117.6 mg/g, compared with CSH (CaO) (84.5 mg/g).This result indicated that the as-prepared porous CSH, without the hardly dissolved intermediates, exhibited highly enhanced phosphate recovery property.Meanwhile, the recovered phosphate products, due to their abundant phosphate content, can be reused as phosphate rock or phosphate fertilizer.

Conclusion
Porous CSH was prepared based on the CaO/PEG 2000 composites as the calcium materials; Ca 2+ could not sustain release from CaO/PEG 2000 and reacted with SiO 3 2− caused by silica to form CSH until the hydrothermal temperature reached to 170 ∘ C, avoiding the formation of hardly dissolved intermediates compared with previously reported synthesis methods.The as-prepared CSH, due to the large specific surface areas, exhibited excellent release capability of Ca 2+ and OH − .Thus, the phosphate recovery property of CSH enhanced.The recovered phosphate products, due to their abundant phosphate content, can be reused in industry and agriculture instead of phosphate rock.Therefore, the asprepared porous material has potential application value in recovering phosphate from wastewater to solve the environmental problems caused by the shortage of phosphate resource.

2 .
In the water solution, the main existence forms of silicon are different with the changes of pH values.Silicon exists in the form of SiO 3 2− Mechanism of CSH 3.3.1.Morphological Structure.The surface morphology of CSH (CaO) and CSH (CaO/PEG 2000 ) was examined by FESEM, as shown in Figures 2(a) and 2(b).It can be seen that CSH (CaO) possessed a dense surface and compact structure (Figure 2(a)).In contrast, pore size of CSH (CaO/PEG 2000 ) tended to be larger (Figure 2(b)).The morphological structure of CSH (CaO) and CSH (CaO/PEG 2000 ) was further examined by TEM, as shown in Figures 2(c) and 2(d).The TEM image shows that the surface of CSH (CaO) was compact (Figure 2(c)) consistent with the FESEM observation.In contrast, CSH (CaO/PEG 2000 ) possesses hollow microspheres due to the absence of the hardly dissolved intermediates (Figure 2(d)).

Figure 5 :
Figure 5: Concentration of Ca 2+ released from CSH samples (a) and pH in deionized water kept by CSH samples (b).

Figure 5
shows the variations of concentration of Ca 2+ and OH − released from the as-synthesized CSH samples and pH-values in deionized water.According to Figure 5(a), CSH (CaO/PEG 2000 ) releases more Ca 2+ than CSH (CaO).Compared with the CSH (CaO) (2.83 mg/L), the concentration of Ca 2+ was released from CSH (CaO/PEG 2000 ) and increased to 5.74 mg/L.Figure 5(b) shows that the pH value of the solution can be kept at 9.5 by CSH (CaO/PEG 2000 ); however, CSH (CaO) can only maintain the pH value at 8.2.The as-prepared CSH (CaO/PEG 2000 ) with porous structure exhibited enhanced release capability of Ca 2+ and OH − .

Table 1 :
Chemical components of carbide residue.

Table 2 :
Specific BET surface areas and pore parameters of CSH samples.