Porous calcium silicate hydrate (PCSH) was synthesized by carbide residue and white carbon black. The influence of hydrothermal temperature on phosphorus recovery efficiency was investigated by Field Emission Scanning Electron Microscopy (FESEM), Brunauer-Emmett-Teller (BET), and X-Ray Diffraction (XRD). Hydrothermal temperature exerted significant influence on phosphorus recovery performance of PCSH. Hydrothermal temperature 170°C for PCSH was more proper to recover phosphorus. PCSH could recover phosphorus with content of 18.51%. The law of Ca2+ and OH− release was the key of phosphorus recovery efficiency, and this law depended upon the microstructure of PCSH. When the temperature of synthesis reached to 170°C, the reactions between CaO and amorphous SiO2 were more efficient. Solubility of SiO2 was a limiting factor.
Phosphorus recovery from wastewater in the form of hydroxyapatite is an effective method [
Phosphorus recovery on the condition of alkalescency not only decreased the significant competition between carbonate and calcium, but also decreased the cost of chemical treatment and increased the effective phosphorus composition of the final products [
From a theoretical and practical point of view, the synthesis, properties, and structure of calcium silicate hydrate have been analyzed in detail [
The main aim of the research is to find a proper hydrothermal temperature for calcium silicate hydrate to recover phosphorus. The originality and importance of this paper are highlighted by the following three points. PCSH was synthesized by carbide residue and white carbon black with a dynamic hydrothermal method. The influence of hydrothermal temperature on phosphorus recovery performance was investigated. The relationship between pore structure and the law of The mechanism of phosphorus recovery was studied by FESEM, BET, and XRD on the basis of an in-depth critical investigation.
PCSH was synthesized with carbide residue (providing Ca) and white carbon black (providing Si). Carbide residue (calcareous, hoar, and powdery) was obtained from Chongqing Changshou Chemical Co. Ltd. and calcined at 700°C for 2 h. White carbon black (particles present spherical with homogeneous diameter) was purchased from Chongqing Jianfeng chemical Co. Ltd. Chemical constituents of carbide residue and white carbon black are shown in Table
Chemical components of carbide residue and white carbon black.
Chemical components (contents)/% | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|
CaO | SiO2 | Al2O3 | SO2 | MgO | Fe2O3 | SrO | NaOH | CuO | H2O | |
Carbide residue | 79.34 | 3.57 | 2.14 | 1.22 | 0.62 | 0.21 | 0.26 | — | — | 12.64 |
White carbon black | 0.08 | 97.46 | 0.16 | 1.82 | — | 0.03 | — | 0.29 | 0.02 | 0.14 |
Carbide residue and white carbon black were mixed, and the Ca/Si molar ratios were controlled at 1.6 : 1. The mixture was then added to prepared slurries. The slurry was hydrothermally reacted at 110°C, 140°C, 170°C, and 200°C, respectively, and the reaction time was 6 h. The samples were taken out when the temperature was reduced to the natural condition. The hydrothermal reaction was carried out with a liquid/solid ratio of 30. The obtained products were dried at 105°C for 2 h, and then were ground through a sieve of 200 meshes. The prepared samples that were hydrothermally reacted at 110°C, 140°C, 170°C, and 200°C were denoted as PCSH: 110°C, PCSH: 140°C, PCSH: 170°C, and PCSH: 200°C, respectively.
Firstly, synthetic solution (1 L) was added into several bottles. 4 g of samples were added to these bottles, respectively, and shaken at 40 r/min under controlled temperature conditions (20°C). Phosphorus concentration of supernatant was measured according to the molybdenum blue ascorbic acid method (the relative error of data is 0.3%) with a Unico spectrophotometer (UV-2012PCS, Shanghai Unico Instruments Co., Ltd., China). The solid samples were then separated from the removed synthetic solution with the addition of samples after reaction. Finally, the produced sediments were separated from removed synthetic solution, dried, and weighted. Phosphorus was contented by
4 g of samples (PCSH: 110°C, PCSH: 140°C, PCSH: 170°C, and PCSH: 200°C) were immersed in 1 L of demonized water, respectively, contained in a glass bottle, generating samples with a solution concentration of 4 g/L. The bottle was placed on an agitation table and shaken at 40 r/min under controlled temperature conditions (20°C). Samples of solution were taken after 5, 10, 15, 20, 40, 60, and 80 mins of agitation. Ca2+ concentration of the samples was determined by EDTA titration (the relative error of data is 0.05%).
XRD patterns were collected in an XD-2 instrument (Persee, China) using Cu
The PCSH samples were separated from the removed synthetic solution after phosphorus removal, and these samples were added into synthetic solution with initial phosphorus concentration 100 mg/L again. This process was repeated for several times in order to explore the phosphorus recovery performance of PCSH. Changes of restrained phosphorus concentration are shown in Figure
Changes of restrained phosphorus concentration by circulation of phosphorus removal.
Changes of pH values by circulation of phosphorus removal.
Specific surface area and pore size distribution were calculated by BET equation and Barrett-Joyner-Halenda method, respectively (Figure
Nitrogen adsorption-adsorption isotherms on samples.
The morphology of PCSH: 110°C, PCSH: 140°C, PCSH: 170°C, and PCSH: 200°C was examined by FESEM observations (Figure
FESEM photographs of samples. (a) PCSH: 110°C; (b) PCSH: 140°C; (c) PCSH: 170°C; (d) PCSH: 200°C.
The experiments showed that Ca2+ concentration dissolved from PCSH: 110°C, PCSH: 140°C, PCSH: 170°C, and PCSH: 200°C was 2.70, 3.11, 4.91, and 3.76 mg/g, respectively (Figure
Correlation equations and rate constants for the Avrami kinetic model describing Ca2+ release.
Samples | Avrami kinetic equations | Kinetic constant ( |
Correlation coefficient ( |
---|---|---|---|
PCSH: 110°C |
|
0.039 | 0.973 |
PCSH: 140°C |
|
0.052 | 0.985 |
PCSH: 170°C |
|
0.085 | 0.998 |
PCSH: 200°C |
|
0.066 | 0.988 |
Concentrate of Ca2+ dissolved from samples.
According to (
According to (
The mechanism of hydrothermal temperature effect on phosphorus recovery performance could be further investigated by XRD. The XRD patterns of PCSH samples were compared (Figure
XRD patterns of samples.
As the siliceous material, white carbon black exhibited high activity [
Porous calcium silicate hydrate was synthesized by carbide residue and white carbon black with a dynamic hydrothermal method. This material could be considered a tenable material for phosphorus removal and recovery from wastewater. Hydrothermal temperature showed significant influence on phosphorus recovery performance of PCSH. Hydrothermal temperature 170°C for PCSH was more proper to recover phosphorus. PCSH could recover phosphorus with content of 18.51%.
The law of Ca2+ and OH− release was the key of phosphorus recovery efficiency. Changes of hydrothermal temperature led to the different pore structures. The increase of specific surface area and the increase in concentration of Ca2+ release were in good agreement with each other.
Further analysis by XRD indicated that hydrothermal reaction process depended on the dissolution of SiO2. And hydrothermal temperature affected the solubility of SiO2.