This paper proposed a novel method to prepare porous calcium silicate hydrate (CSH) based on the calcium oxide/polyethylene glycol (CaO/PEG2000) 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 silica materials (SiO2) enhanced by increasing pH value. Ca2+ could not sustain release from CaO/PEG2000 and reacted with caused by silica to form CSH until the hydrothermal temperature reached to 170°C, avoiding the hardly dissolved intermediates formation efficiently. The as-prepared CSH, due to the large specific surface areas, exhibited excellent release capability of Ca2+ and OH. This porous CSH has potential application in reducing the negative environmental effects of continual natural phosphate resource depletion.

1. Introduction

Phosphate, as an irreplaceable and nonrenewable resource, has an important contribution to industry and agriculture [14]. But this precious resource will be exhausted as a result of increased consumption in the near future [58]. The sustainable utilization of phosphate has become a severe challenge for human beings. Calcium silicate hydrate, due to the unique release capability of Ca2+ and OH, has caused international extensive concern in the field of “recovery of phosphate from wastewater” [914]. This is because the released Ca2+ 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 [1517]. Therefore, the release capability of Ca2+ 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 () and pore structure. Large 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 SiO2 materials [2023]. However, there two critical problems that affect the solubility of CSH. On the one hand, the reactivity of SiO2 was too poor to participate in the formation of CSH. The residual SiO2 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 SiO2 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/PEG2000) composites was developed for the formation of porous CSH. Under the dynamic hydrothermal condition, massive (released from the SiO2 materials due to the increased pH values) reacted with Ca2+ (sustained released from CaO/PEG2000 composites) at a proper temperature to form CSH efficiently. Compared with previously reported synthesis methods, this new synthesis method herein avoided the formation of the hardly dissolved intermediates. Thus, the as-prepared CSH with porous structure exhibited excellent release capabilities of Ca2+ and OH. In addition, a novel porosity formation mechanism was revealed in the present paper.

2. Materials and Methods

2.1. Raw Materials

The CaO material (carbide residue, content of CaO > 75%) and SiO2 material (Silica, content of SiO2 > 98%) were obtained from Chongqing Changshou Chemical Co. Ltd. PEG2000 (the chemical formula is HO(CH2CH2O)nH) and NaOH were obtained from Chengdu Kelong chemical Co. Ltd. The Chemical composition of the carbide residue and silica is presented in Table 1. The above materials and chemicals were placed into sealed bottles for storage.

2.2. Dynamic Hydrothermal Synthesis of Porous CSH

Prior to CSH synthesis, PEG2000 was put into 300 mL deionized water with strong stirring to obtain the PEG solution, and the mass fraction of PEG2000 was 2% w/v. Subsequently, 6 mg of carbide residue was added into the PEG solution with strong stirring and reacted 60 min at 80°C. Then, the solid segments were centrifugal separated from the PEG solution and were dried at 105°C for 2 h to obtain the CaO/PEG2000 composites.

Subsequently, CaO/PEG2000 composites and CaO materials were mixed with SiO2 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/PEG2000 composites and CaO materials were labeled as CSH (CaO/PEG2000) and CSH (CaO), respectively.

2.3. Dissolution Experiment

The release of Ca2+ 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 Ca2+ 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, San-ai-si reagent Co., Ltd., Shanghai, China). The accuracy of pH measurement is 0.1.

2.4. 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).

2.5. 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 field-emission scanning electron microscopy (FESEM, IUE, Hitachi, Japan) and transmission electron microscope (TEM, JEOL JEM-2010, Japan). The 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).

3. Results and Discussion

3.1. The Effect of pH on the Reactivity of SiO2

In the water solution, the main existence forms of silicon are different with the changes of pH values. Silicon exists in the form of when the pH value is over 13.0 [2628]. This result can be verified according to the fraction formula of H2SiO3, and under the given pH value as follows: where [H+] is the concentration of hydrogen ions and and are the first and second dissociation constants, respectively. When pH = 12.0, the distribution coefficients of H2SiO3, , and 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 that is beneficial to the formation of CSH.

3.2. Complexation between CaO and PEG2000

The reaction mechanism between CaO and PEG2000 was revealed via FT-IR analysis. Figures 1(a) and 1(b) show the FT-IR spectra of neat CaO and CaO/PEG2000 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 PEG2000, 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 PEG2000 decreased due to the effect of Ca2+. Furthermore, this result demonstrated that Ca2+ reacted with oxygen atom in PEG molecule to form complexation structure; that is, CaO and PEG2000 existed together in the form of complex.

3.3. The Porosity Formation Mechanism of CSH
3.3.1. Morphological Structure

The surface morphology of CSH (CaO) and CSH (CaO/PEG2000) 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/PEG2000) tended to be larger (Figure 2(b)). The morphological structure of CSH (CaO) and CSH (CaO/PEG2000) 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/PEG2000) possesses hollow microspheres due to the absence of the hardly dissolved intermediates (Figure 2(d)).

3.3.2. Specific Surface Area and Pore Structure

The and pore structure of the as-prepared samples were investigated by adsorption-desorption measurements. As shown in Table 2, the of CSH (CaO/PEG2000) increased to 133 m2/g compared to CSH (CaO) (62 m2/g). In comparison to CSH (CaO) (0.16 cm3/g), the pore volume of CSH (CaO/PEG2000) increased to 0.36 cm3/g.

Figure 3(a) shows the N2 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/PEG2000), 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 N2 adsorption-desorption isotherms. A large number of small mesopores contribute to the large .

3.3.3. Phase Structure

Figure 4 shows the XRD patterns of CSH (CaO) and CSH (CaO/PEG2000). Multiple phases, such as Jennite (PDF card 18-1206, chemical formula Ca9Si6O18(OH)6 · 8H2O), xonotlite (PDF card 23-0125, chemical formula Ca6Si6O17(OH)2), and Ca3Si2O7 (PDF card 11-0317), appear in the XRD pattern of Figure 4(a). By comparison, the phase of CSH (CaO/PEG2000) was only Jennite (Figure 4(b)). Combined with the above analysis, the as-prepared CSH (CaO/PEG2000) (i.e., Jennite), without intermediates, exhibited large and porous structure.

The porosity formation mechanism can be revealed as follows: (1) during the heating process, was released from SiO2 materials due to the increased pH value (13.0), and Ca2+ cannot be released from CaO/PEG2000 composites due to the coating of PEG2000 below the hydrothermal temperature (170°C); (2) when the hydrothermal temperature reached to a proper condition (170°C), the molecular chain broke between CaO and PEG2000. At this time, Ca2+ released from CaO/PEG2000 composites and reacted with quickly to form CSH (CaO/PEG2000). To the synthesis of CSH (CaO), massive Ca2+ 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.

3.4. The Enhanced Solubility of CSH

Figure 5 shows the variations of concentration of Ca2+ and OH released from the as-synthesized CSH samples and pH-values in deionized water. According to Figure 5(a), CSH (CaO/PEG2000) releases more Ca2+ than CSH (CaO). Compared with the CSH (CaO) (2.83 mg/L), the concentration of Ca2+ was released from CSH (CaO/PEG2000) 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/PEG2000); however, CSH (CaO) can only maintain the pH value at 8.2. The as-prepared CSH (CaO/PEG2000) with porous structure exhibited enhanced release capability of Ca2+ and OH.

3.5. The Enhanced Phosphate Recovery Property of CSH

The phosphate content of the product recovered by CSH (CaO/PEG2000) 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.

4. Conclusion

Porous CSH was prepared based on the CaO/PEG2000 composites as the calcium materials; Ca2+ could not sustain release from CaO/PEG2000 and reacted with 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 Ca2+ 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 as-prepared porous material has potential application value in recovering phosphate from wastewater to solve the environmental problems caused by the shortage of phosphate resource.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.