Study on the preparation of calcium modified coal gangue and its adsorption performance of phosphate

doi:10.21203/rs.3.rs-4961055/v1

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Study on the preparation of calcium modified coal gangue and its adsorption performance of phosphate

https://doi.org/10.21203/rs.3.rs-4961055/v1

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Excessive phosphate in water leads to eutrophication, and to address this, a new calcium-modified coal gangue (CaMCG) was developed using coal gangue and calcium chloride for phosphate removal. The optimum preparation conditions of CaMCG were obtained by response surface test: m_{calcium chloride}:m_{coal gangue}=1, calcination temperature 735℃, calcination time 135 min. Batch adsorption experiments showed that when the phosphate concentration was 100 mg/L, the optimal CaMCG dosage was 0.5 g and the optimal reaction time was 48 h. At pH 3-7, the adsorption capacity of CaMCG for phosphate was always good. The order of the strength of coexisting anions affecting the adsorption of phosphate by CaMCG was: CO₃^2- > SO₄^2- > HCO₃^- > NO₃^- ≈ Cl^-. Kinetic isotherm analysis showed that the adsorption of phosphate by CaMCG had both physical adsorption and chemical adsorption. The theoretical maximum adsorption capacity of CaMCG for phosphate was 17.85 mg/g. The adsorption process of CaMCG on phosphate conformed to the Langmuir model. The main mechanisms of CaMCG adsorbing phosphate are surface precipitation, adsorption exchange and complexation. This study shows that CaMCG has great potential in adsorbing phosphate, which can provide technical reference for the efficient utilization of coal gangue and the treatment of phosphate wastewater.

Biological sciences/Biochemistry

Biological sciences/Biotechnology

Calcium modification

Coal gangue

Adsorption

Phosphate

Preparation

A novel CaMCG was optimized for phosphate adsorption.
The alkaline environment is not conducive to the adsorption of phosphate by CaMCG.
CaMCG adsorbs phosphate through precipitation, exchange and complexation.

Phosphorus is present in human cells and is essential for the maintenance of bones and teeth. It is involved in many physiological reactions. Phosphate is very important for human activities. However, in recent years, industrial and agricultural activities have caused varying degrees of phosphate pollution, resulting in eutrophication of water bodies [1, 2]. Therefore, a large amount of phosphate can easily lead to serious pollution of water quality and eutrophication of water bodies, which is one of the major environmental problems recognized worldwide [3]. Interestingly, phosphate rock is also a non-renewable resource, and the current mining speed will cause a shortage of phosphorus resources [4, 5]. Therefore, in order to alleviate the problem of water pollution caused by excessive waste of phosphate, it is necessary to remove excess phosphate in water. At present, the main methods commonly used to alleviate phosphate pollution are microbial treatment, chemical precipitation, and adsorption [6]. Compared with the microbial treatment method and chemical precipitation method, the adsorption method has the advantages of low treatment cost, low environmental requirements, high phosphate treatment efficiency, and simple operation, and has received a lot of attention [7]. Commonly used adsorbents include activated carbon, zeolite, coal gangue, adsorption resin, etc., but these materials have the disadvantage of weak ability to adsorb phosphate [8]. In the face of this problem, the modification of traditional adsorbents is an effective way to improve the adsorption performance of phosphate. In particular, the use of industrial waste residue as an adsorbent for phosphate removal has become the focus of attention [9].

Coal gangue is an industrial solid waste produced by mining activities. The accumulation of coal gangue in China is 6 billion tons [10]. Therefore, how to efficiently develop and utilize coal gangue is an urgent problem to be solved. It has been reported that coal gangue is rich in metal, and pyrolysis modified coal gangue can be used to remove phosphate in solution [10]. However, the adsorption capacity of pyrolysis modified coal gangue to phosphate is limited, and the adsorption capacity of coal gangue to phosphate can be improved by pyrolysis combined with chemical modification [11]. Among them, by pyrolysis or calcination, coal gangue loses bound water and organic matter, forms a porous microstructure, and obtains high chemical activity [12, 13]. Through the modification of different chemical activators, coal gangue can obtain different functional groups and surface charges, thereby improving the adsorption capacity of coal gangue [11, 14]. At present, based on the pyrolysis combined with chemical modification method, it is a wise technology to improve the adsorption[15]. It has been proved that Ca²⁺, Mg²⁺, Fe³⁺, La²⁺ and other metal cations can improve the adsorption capacity of the adsorbent to phosphate [16, 17]. Compared with other metals, Ca is non-toxic and can be widely used. Calcium modification is a promising pyrolysis combined chemical modification method that enhances phosphate adsorption capacity [18]. Mitrogiannis et al. [19] showed that the removal rate of 10 mg/L phosphate by calcium-modified zeolite formed by Ca(OH)₂ modified natural zeolite increased from 1.7–97.6%. Zhang et al [20] showed that Ca²⁺ could enhance the phosphate removal ability of polystyrene-based nano-iron oxide composites. The nano-calcium oxide/biochar composite prepared by Li et al. [21] had a good adsorption effect on phosphate in a wide pH and temperature range. Based on the calcium modification technology, a new adsorption material HO-CaBen was prepared by Ma et al. [22] using bentonite as raw material, and its adsorption capacity for phosphate reached 29.1 mg/g. In summary, the calcium modification technology based on pyrolysis combined with chemical modification is a good modification method to improve the adsorption performance of the adsorbent for phosphate. However, the existing research focuses on calcium-modified zeolites, biochars, and bentonites. Compared with the above materials, the cost of coal gangue is lower and the stockpile is larger. However, there are few studies on the phosphate adsorption performance of calcium modified coal gangue (CaMCG).

Therefore, based on the calcium modification technology of pyrolysis combined with chemical modification, a new adsorbent CaMCG was prepared by using coal gangue as raw material. The preparation conditions of CaMCG were optimized by single factor test and response surface test. The effects of CaMCG dosage, pH value, reaction time, phosphate concentration and coexisting ions on the adsorption of phosphate by CaMCG were investigated by batch adsorption experiments. At the same time, combined with the principle of adsorption kinetics and adsorption isotherm, the mechanism of phosphate adsorption by CaMCG was revealed based on X-ray diffraction (XRD) and fourier transform infrared spectroscopy (FTIR). The research results provide a technical reference for the efficient resource utilization of coal gangue and the treatment of phosphate wastewater.

2.1. Materials and instruments

Materials: The test water was deionized water. Coal gangue was taken from Haizhou Open-pit Mine in Fuxin City, Liaoning Province, and dried at 105℃ for 48 h. NaOH, HCl, anhydrous calcium chloride, KH₂PO₄ and other drugs were purchased from Sinopharm Chemical Reagent Co., Ltd..

Instruments: GZX-9246MBE Blower Dryer (Shanghai Boxun Medical Biological Instrument Co., Ltd., China). SX2-5-12 box type muffle furnace (Shanghai Leiyun Test Instrument Manufacturing Co., Ltd., China). V-1600PC visible spectrophotometer (Shanghai Jingke Industrial Co., Ltd., China). PHS-3C pH meter (Shanghai Yidian Scientific Instrument Co., Ltd., China). HZ-9811K thermostatic oscillator (Shanghai Yiheng Scientific Instrument Co., Ltd., China). X-ray diffractometer (Bruker D8 advance, Germany). Fourier transform infrared spectroscopy (Bruker ALPHAII, Germany).

2.2. Preparation of CaMCG

Table 1

Preparation conditions of CaMCG
Single factor test	Test condition
Single factor test	A-m_{calcium chloride}:m_{coal gangue}	B-calcination temperature (℃)	C-calcination time (h)
Mass ratio test of calcium chloride and coal gangue	0:1、1:2、1:1、2:1、3:1	600	2
Calcination temperature test	1:1	500、600、700、800、900	2
Calcination time test	1:1	700	1、2、3、4、5

The lump coal gangue was crushed and sieved through 100 mesh. The coal gangue powder was washed several times with deionized water to remove the floating dust on the surface of the coal gangue, and dried at 105℃ for 24 h. Calcium chloride and coal gangue were mixed according to a certain mass ratio, and added to 1 mol/L NaOH solution, stirred at 150 r/min for 4 h. The mixture was filtered and dried at 105℃. The dried material was placed in a crucible and tightly wrapped with tin paper (forming an oxygen-limited environment). The crucible was placed in a muffle furnace and heated to a certain temperature at 5℃/min. Pyrolysis at this temperature for a certain time, take out the material to cool. The specific mass ratio of calcium chloride to coal gangue, calcination temperature and calcination time parameters in the preparation test of CaMCG were shown in Table 1. The material was washed with deionized water to neutral, and dried at 105℃ to constant weight to obtain CaMCG. Based on the effect of CaMCG on the removal of phosphate and the influence of various factors on the preparation of CaMCG. Based on the single factor test, three factors were selected: A-m_{calcium chloride}:m_{coal gangue} (1, 2, 3), B-calcination temperature (600℃, 700℃, 800℃), C-calcination time (1 h, 2 h, 3 h). Using Design-Expert V8.0.6.1, the BBD module was selected to design and carry out the response surface test to optimize the preparation conditions of CaMCG.

2.3. Batch adsorption test

In order to determine the adsorption performance of CaMCG for phosphate, based on the preparation conditions of CaMCG optimized by response surface test, the effects of CaMCG dosage, pH value, reaction time, phosphate concentration and coexisting ions on the adsorption of phosphate by CaMCG were investigated by batch adsorption test. In the batch adsorption test, a certain amount of CaMCG was added to 100 mL, 100 mg/L, pH = 7 phosphate solution. After 48 h of adsorption at 25℃ and 150 r/min, the residual phosphate concentration in the solution was determined by filtration with a 0.45 µm filter membrane. The removal rate and adsorption capacity of phosphate were calculated. The detailed experimental conditions of batch adsorption test were shown in Table 2. Each experiment was repeated three times, and the mean value was used to draw the image.

Table 2

Batch test conditions for phosphate adsorption by modified coal gangue
Numbering	Experimental factors	Test condition
Numbering	Experimental factors	CaMCG dosage (g)	pH value	Reaction time (h)	Phosphate concentration (mg/L)	Coexisting ions
1	CaMCG dosage	0.1、0.3、0.5、0.7、0.9	7	48	100	—
2	pH value	0.5	3、5、7、9、11	48	100	—
3	Reaction time	0.5	7	6、12、18、24、30、36、42、48、54、60	100	—
4	Phosphate concentration	0.5	7	48	25、50、75、100、150、200、250	—
5	Coexisting ions	0.5	7	48	100	The concentrations of Cl⁻, NO₃⁻, HCO₃⁻, SO₄²⁻, CO₃²⁻were all 10 mg/L.

2.4. Analysis method

Phosphate concentration in water samples was determined by ammonium molybdate spectrophotometric method (GB 11893-89). In order to quantitatively reveal the correlation between CaMCG and phosphate concentration, Lagergren first-order kinetics (Formula 1) and Lagergren second-order kinetics (Formula 2) models were used to study the phosphate adsorption behavior of CaMCG. At the same time, the adsorption process of CaMCG on phosphate was evaluated by fitting the Langmuir (Formula 3) and Freundlich (Formula 4) isotherm models.

$${q_t}={q_e} \times (1 - \exp ( - {k_1} \times t))$$

$$\frac{{\text{t}}}{{{q_t}}}=\frac{1}{{{k_2}{q_e}^{2}}}+\frac{1}{{{q_e}}}t$$

$$\frac{{{C_e}}}{{{q_e}}}=\frac{1}{{{q_m}{K_L}}}+\frac{{{C_e}}}{{{q_m}}}$$

$$ln{q_e}=ln{K_F}+\frac{1}{n}ln{C_e}$$

In the equation: C_e (mg/L) is the equilibrium concentration of phosphate. q_e (mg/g) is the adsorption capacity of CaMCG for phosphate. q_t (mg/g) is the adsorption capacity of CaMCG to phosphate at t (h). k₁ (h^− 1) and k₂ (mg/(g·h)) are the adsorption rate constants of the first-order and second-order kinetic models, respectively. q_m (mg/g) represents the adsorption capacity of CaMCG for phosphate at adsorption saturation. K_L (L/mg) and K_F (mg^(1−1/n)·L^1/n·g^− 1) were the adsorption constants of Langmuir and Freundlich models, respectively. n is the adsorption strength constant of Freundlich model.

CaMCG and CaMCG after phosphate adsorption were detected by XRD and FTIR, respectively. Among them, the samples to be tested were ground and sieved to obtain 300 mesh powder. The 2θ detection range of XRD is 5–90°, which is used to judge the composition and crystal form of minerals. The detection range of FTIR is 500-4000cm^− 1, which is used to judge the change of functional groups. Combined with the detection results of CaMCG and CaMCG adsorbed phosphate samples, the mechanism of CaMCG adsorbing phosphate was revealed.

3.1. Preparation of CaMCG

3.1.1. Calcium chloride and coal gangue mass ratio test

Figure 1(a) shows the phosphate removal efficiency and adsorption capacity of CaMCG prepared under different mass ratios of calcium chloride to coal gangue. It can be seen that the removal efficiency of phosphate by coal gangue is only 12.79%. Compared with coal gangue, the removal efficiency of CaMCG on phosphate was significantly improved. When the mass ratio of calcium chloride to coal gangue was 1:2, 1:1, 2:1 and 3:1, the removal efficiency of phosphate by CaMCG was 50.59%, 60.87%, 57.58% and 55.17%, respectively. It can be seen from the removal effect that when the mass ratio of calcium chloride to coal gangue is small, the mass ratio of calcium chloride to coal gangue affects the phosphate removal effect obviously. When the mass ratio of calcium chloride to coal gangue is large, excessive calcium chloride cannot be fully loaded on the surface of coal gangue, and even masks some effective adsorption sites of coal gangue, resulting in stable adsorption effect. When the mass ratio of calcium chloride to coal gangue is 1:1, the prepared CaMCG has the largest adsorption capacity for phosphate, which is 12.17 mg/g.

3.1.2. Calcination temperature test

Figure 1(b) shows the phosphate removal efficiency and adsorption capacity of CaMCG prepared at different calcination temperatures. With the increase of calcination temperature, the removal effect of CaMCG on phosphate increased first and then stabilized. When the calcination temperature is 700℃, the prepared CaMCG has the highest phosphate removal efficiency and adsorption capacity, which are 74.48% and 14.90 mg/g, respectively.

3.1.3. Calcination time test

Figure 1(c) shows the phosphate removal efficiency and adsorption capacity of CaMCG prepared under different calcination time conditions. When the calcination time was 1 h, 2 h, 3 h, 4 h and 5 h, the removal efficiency of phosphate by CaMCG was 54.15%, 74.48%, 67.65%, 67.46% and 67.19%, respectively. When the calcination time is 2 h, the prepared CaMCG has the largest adsorption capacity for phosphate.

3.1.4. Response surface experiment of CaMCG preparation

The response surface test results of the preparation of CaMCG are shown in Table 3 and Fig. 2.

Table 3

Response surface experimental design and results of CaMCG preparation
Numbering	Variable			The removal rate of PO₄³⁻ (%)
Numbering	A-m_{calcium chloride}:m_{coal gangue}	B-calcination temperature (℃)	C-calcination time (h)	The removal rate of PO₄³⁻ (%)
1	1	600	2	60.87
2	3	600	2	55.17
3	1	800	2	71.95
4	3	800	2	57.56
5	1	700	1	51.39
6	3	700	1	41.11
7	1	700	3	69.57
8	3	700	3	55.66
9	2	600	1	32.37
10	2	800	1	42.09
11	2	600	3	43.83
12	2	800	3	56.98
13	2	700	2	70.85
14	2	700	2	70.15
15	2	700	2	70.35
16	2	700	2	70.54
17	2	700	2	70.65

Based on Design-Expert V8.0.6.1, the second-order polynomial model was used to fit the results of Table 3. The multiple quadratic regression equation between the factors of CaMCG A-m_{calcium chloride}:m_{coal gangue}, B-calcination temperature, C-calcination time and phosphate removal rate was obtained (The removal rate of PO₄³⁻(%) = 70.51–5.54*A + 4.54*B + 7.39*C-2.17*A*B-0.91*A*C + 0.86*B*C + 0.75*A^2-9.87*B^2-16.82*C^2, R² = 0.9928). The above multivariate quadratic regression equation was analyzed by variance analysis and significance test. It was found that the F value of the model was 107.21, and the P value was < 0.0001, indicating that the regression equation was significant. At the same time, the R² of the model is 0.9928, indicating that the model has high accuracy. The effect of each factor on the adsorption of phosphate by CaMCG was obtained by F value:C > A > B. Combined with Fig. 2 and BBD model analysis, the optimal preparation conditions of CaMCG are as follows: m_{calcium chloride}:m_{coal gangue}=1, calcination temperature 735℃, calcination time 135 min. Under this condition, the removal rates of phosphate by CaMCG prepared by software prediction and experiment were 79.02% and 80.50%, respectively. The two are similar, indicating that the above model of CaMCG removal of phosphate is reliable.

3.2. Influencing factors of phosphate adsorption by CaMCG

3.2.1. Dosage of CaMCG

It can be seen from Fig. 3(a) that the effect of CaMCG dosage on phosphate adsorption. Under the condition of 5 kinds of dosage, CaMCG can effectively adsorb phosphate. When the dosage of CaMCG is low, the phosphate removal rate and equilibrium adsorption capacity are small. When the dosage increases, the removal effect is significantly improved. At this stage, the dosage of CaMCG is the main factor affecting the adsorption effect of phosphate. This is because CaMCG has a strong attraction to phosphate, and the effective adsorption sites of the adsorbent CaMCG are easily saturated [10]. When the dosage increases to 0.5 g, the phosphate removal rate and equilibrium adsorption capacity reach the best, which are 80.50% and 16.10 mg/g, respectively. When the dosage continues to increase to 0.9 g, the phosphate removal rate and equilibrium adsorption capacity tend to be stable. This is because when the adsorption reaction continues, the phosphate concentration in the solution continues to decrease, resulting in a decrease in the chance of attraction and collision between CaMCG and phosphate, an increase in mass transfer resistance, and a stabilization of the adsorption capacity. Therefore, when the phosphate concentration is 100 mg/L, the optimal CaMCG dosage is 0.5 g.

3.2.2. pH value

From Fig. 3(b), it can be seen that the removal rate and adsorption capacity of CaMCG on phosphate under different pH (3, 5, 7, 9, 11) conditions. Interestingly, the adsorption capacity of CaMCG for phosphate is always good at pH 3–7. At pH 9–11, the adsorption capacity of CaMCG for phosphate is significantly reduced. In particular, compared with pH = 7, the removal rate of phosphate by CaMCG decreases by 29.53% at pH = 11. This indicates that the adsorption capacity of CaMCG for phosphate in acidic and neutral pH environments is significantly due to the alkaline pH environment. It is reported that phosphorus mainly exists in the form of H₂PO₄⁻ at pH 2.5–7.2, and phosphorus mainly exists in the form of HPO₄²⁻ at pH 7.2–11.5 [7]. Compared with HPO₄²⁻, H₂PO₄⁻ is more easily adsorbed by the adsorbent [23]. In addition, in the alkaline environment, the concentration of OH⁻ is higher, OH⁻ competes with phosphate, and OH⁻ will occupy the effective adsorption site of the adsorbent.

3.2.3. Reaction time

Under different reaction time conditions, the removal rate and adsorption amount of phosphate by CaMCG are shown in Fig. 3(c). With the increase of reaction time, the effect of CaMCG on phosphate increases first and then stabilizes. Finally, the phosphate removal rate is stable at about 80%, and the adsorption capacity is stable at 16 mg/L. Compared with the reaction of 6 h, the removal rate of phosphate by CaMCG increased by 48.82% and the adsorption capacity increase by 9.76 mg/g after 48 h. It can be seen from the Figure that as the reaction time increases, the adsorption effect of phosphate per unit time gradually decreases. This is mainly because as the reaction time increases, the active sites on the surface of the adsorbent CaMCG gradually decrease, which reduces the attraction of the surfactant to phosphate. At the same time, the concentration of phosphate in the solution continues to decrease, resulting in increased mass transfer resistance and reduced phosphate removal. When the reaction time is 60 h, the removal rate and adsorption amount of phosphate by CaMCG are 81.13% and 16.23 mg/g, respectively. Therefore, considering the adsorption effect and time saving, the optimal reaction time for CaMCG to adsorb phosphate is 48 h.

3.2.4. Phosphate concentration

From Fig. 3(d), it can be seen that the removal rate and adsorption capacity of CaMCG on phosphate under different phosphate concentration conditions. Interestingly, when the phosphate concentration is low (< 75 mg/L), the removal rate of phosphate by CaMCG is relatively large, stable at more than 95%. However, the adsorption capacity is only 4.87–14.23 mg/g. The reason for this phenomenon is that the number of adsorbents added in the phosphate solution is the same, that is, the effective adsorption sites of the adsorbents are similar. However, the initial concentration of phosphate in the solution is different, and the excess adsorbent can almost all attract phosphate to achieve a large removal rate of phosphate. When the concentration of phosphate is low (> 100 mg/L), the adsorption capacity of CaMCG to phosphate reaches saturation, which is stable at 16.10-17.74 mg/g. However, the removal rate of phosphate decreases significantly. When the initial phosphate concentration is 250 mg/L, the removal rate of phosphate by CaMCG is only 35.47%. Compared with the initial phosphate concentration of 100 mg/L, the removal rate of phosphate decreased by 45%. This is mainly because the adsorption effect of the adsorbent CaMCG gradually tends to be saturated.

3.2.5. Coexisting ions

Phosphate-containing water environment is often a variety of ions exist at the same time. In particular, the coexistence environment of anions may compete with phosphate, resulting in a decrease in the removal efficiency of phosphate by adsorbent CaMCG. Therefore, the effects of Cl⁻, NO₃⁻, HCO₃⁻, SO₄²⁻ and CO₃²⁻ on the phosphate removal performance of CaMCG were investigated. The results are shown in Fig. 3(e). Cl⁻ and NO₃⁻ have no effect on the phosphate adsorption performance of CaMCG. Therefore, compared with CaMCG adsorption of phosphate, the adsorption competition of Cl⁻ and NO₃⁻ is weaker. However, HCO₃⁻, SO₄²⁻ and CO₃²⁻ have a significant effect on the phosphate adsorption performance of CaMCG. This may be because HCO₃⁻, SO₄²⁻, CO₃²⁻ are easy to combine with Ca²⁺ on the surface of CaMCG and produce slightly soluble or insoluble calcium salts [24]. Therefore, the adsorption of phosphate by CaMCG will be reduced. It can be seen from the removal effect of phosphate that the order of coexisting anions affecting the adsorption of phosphate by CaMCG is: CO₃²⁻ > SO₄²⁻ > HCO₃⁻ > NO₃⁻ ≈ Cl⁻.

3.3. Adsorption kinetics and isotherms

3.3.1. Adsorption kinetics

Figure 4(a) is the kinetic fitting curve of CaMCG adsorbing phosphate, and the fitting parameters are shown in Table 4. The first-order kinetic fitting equation of CaMCG adsorption of phosphate is q_t=16.46054×(1-exp(-0.08289×t)), R² = 0.99746. The second-order kinetic equation of phosphate adsorption by CaMCG is t/q_t=0.05146t + 0.51267, R² = 0.99312. The correlation coefficients R² of the first-order adsorption kinetic model and the second-order adsorption kinetic model are 0.99746 and 0.99312, respectively. The difference between the two is not large, and both are close to 1, indicating that the adsorption of phosphate by CaMCG has both physical adsorption and chemical adsorption.

Table 4

Adsorption kinetics and adsorption isotherm fitting parameter table of CaMCG for phosphate
Adsorption model	Parameter	Numerical value
First-order adsorption kinetic model	q_e (mg/g)	16.46054
	k₁ (h^− 1)	0.08289
	R²	0.99746
Second-order adsorption kinetic model	q_e (mg/g)	19.43257
	k₂ (mg/(g·h))	0.00517
	R²	0.99312
Freundlich adsorption isotherm model	K_F (mg^(1−1/n)·L^1/n·g^− 1)	7.52959
	n	5.05638
	R²	0.76122
Langmuir adsorption isotherm model	q_m (mg/g)	17.85077
	K_L (L/mg)	0.51188
	R²	0.99986

3.3.2. Adsorption isotherms

Figure 4(b) and (c) are the adsorption isotherm fitting curves of CaMCG for phosphate, and the fitting parameters are shown in Table 4. The correlation coefficient of Langmuir model (R² = 0.99986) is significantly higher than that of Freundlich model (R² = 0.76122). Therefore, the Langmuir model can more effectively describe the adsorption process of CaMCG on phosphate. This also indicates that the adsorption of phosphate by CaMCG conforms to a uniform monolayer adsorption [18]. It can be seen from Table 4 that the maximum adsorption capacity of CaMCG for phosphate is 17.85 mg/g predicted by Langmuir model.

3.4. Mechanism of phosphate adsorption by CaMCG

The XRD diffraction peak spectrum of phosphate adsorbed by CaMCG and CaMCG is shown in Fig. 5(a). Quartz, kaolinite, albite, calcite, vaterite, calcium hydroxide and calcium chloride appeare in the diffraction peaks of CaMCG. Among them, quartz, kaolinite and albite are mainly from coal gangue. Calcite, vaterite, calcium hydroxide and calcium chloride are formed in the process of preparing CaMCG based on pyrolysis combined chemical modification technology, using coal gangue as raw material and calcium chloride as modifier. Calcite and vaterite are all calcium carbonate, but the crystal forms are different. The high temperature calcination environment makes the carbon-containing organic matter in coal gangue combine with calcium ions to form calcium carbonate. The presence of calcium hydroxide is mainly due to the decomposition of some calcium carbonate into calcium oxide during high temperature calcination, and calcium oxide absorbs water vapor in the air to form calcium hydroxide. After CaMCG adsorbed phosphate, the characteristic diffraction peaks of calcium phosphate and hydroxylapatite appeare. The diffraction peaks near 31.0°, 27.8° and 34.3° correspond to the (021) crystal plane, (214) crystal plane and (220) crystal plane of calcium phosphate, respectively. The diffraction peaks near 31.9° and 33.2° correspond to the (211) and (300) crystal planes of hydroxylapatite, respectively. At the same time, after CaMCG adsorbing phosphate, the diffraction peak intensity of kaolinite, calcite, vaterite and calcium hydroxide decreases significantly, indicating that these minerals play an important role in adsorbing phosphate. Therefore, kaolinite, calcite, vaterite and calcium hydroxide in CaMCG participated in the reaction of phosphate adsorption, and finally form calcium phosphate and hydroxylapatite.

The FTIR spectra of phosphate adsorbed on CaMCG and CaMCG are shown in Fig. 5(b). At 1430.9 cm^− 1 and 667.2 cm^− 1, Ca-O stretching vibration peaks appeared in CaMCG [25]. The out-of-plane deformation vibration peak of CO₃²⁻ appeared at 875.5 cm^− 1. The hydroxyl vibration peak appeares at 3590–3671 cm^− 1. The appearance of the above peaks indicates that CaMCG contains Ca-O, CO₃²⁻ and hydroxyl groups, and it is inferred that there are calcium carbonate and calcium hydroxide. This result is consistent with the results of XRD analysis. At the same time, after CaMCG adsorbed phosphate, the absorption peaks of PO₄³⁻ appeares at 601.7 cm^− 1, 962.3 cm^− 1 and 1039.4 cm^− 1. Among them, the absorption peak at 601.7 cm^− 1 is due to the bending of PO₄³⁻ v4. The absorption peak at 962.3 cm^− 1 is caused by the bending of PO₄³⁻ v1. The absorption peak at 1039.4 cm^− 1 corresponds to the bending of PO₄³⁻ v3 in hydroxylapatite crystals. These absorption peaks indicate that calcium phosphate and hydroxylapatite appeare after CaMCG adsorbed phosphate.

CaMCG has a good adsorption effect on phosphate, mainly because the adsorption of coal gangue and the calcium-containing substances in CaMCG are involved in the reaction. On the one hand, the coal gangue in CaMCG has rich pore structure and large specific surface area, which can be used as adsorbent to treat aqueous solution [26]. At the same time, kaolinite and other aluminosilicate minerals in coal gangue have adsorption exchange and degradation [27]. Therefore, the coal gangue in CaMCG can treat the phosphate in the solution by surface precipitation and adsorption exchange. On the other hand, kaolinite, calcite, vaterite, calcium hydroxide and other substances in CaMCG participate in the reaction of phosphate adsorption, and finally form calcium phosphate and hydroxylapatite. Among them, calcium phosphate is formed by chemical reaction between phosphate and calcium-containing substances in CaMCG. It has been reported that the calcium-based active sites on the surface of the adsorbent can bind to phosphate and form a surface complex Ca-O-P through a complexation reaction [28]. Liu et al. [29] reported that CaCO₃ on the surface of the adsorbent material can react slowly with phosphate through surface complexation to form calcium dihydrogen phosphate, calcium hydrogen phosphate, calcium phosphate, etc. Hydroxylapatite is mainly produced by the chemical reaction of calcium hydroxide and phosphate in CaMCG. Zeng and Kan [30] showed that hydroxylapatite had strong adsorption. This adsorption further promoted the adsorption of phosphate by CaMCG. In conclusion, the main mechanism of CaMCG adsorbing phosphate is surface precipitation, adsorption exchange and complexation.

3.5. Discussion

The maximum adsorption capacity of CaMCG for phosphate is 17.85 mg/g. Cheng et al. [31] prepared biochar from the plant Eupatorium adenophorum, and the theoretical maximum adsorption capacity of the biochar for ammonium and phosphate was 1.909 mg/g and 2.32 mg/g, respectively. Ajmal et al. [32] found that the phosphate adsorption capacity of biochar prepared from wood and rice husk was 12–15 mg/g. Jena et al. [33] found that the adsorption capacity of magnesium modified biochar for 3–25 mg/L phosphate was 8.44 mg/g. Compared with the biochar and magnesium-modified biochar studied above, the CaMCG prepared in this study has a significantly higher adsorption capacity for phosphate. However, the phosphate adsorption capacity of CaMCG prepared in this study was significantly lower than that of La(OH)₃ modified biochar. For example, Liu et al. [34] prepared La(OH)₃ modified canna biochar (CBC-La) by co-precipitation method, and its maximum adsorption capacity for phosphate was 37.37 mg/g. The theoretical maximum adsorption capacity of phosphate by La(OH)₃ modified biochar prepared by Luo et al. [35] was 75.08 mg/g. The reason for the large difference in phosphate adsorption is that La modification promotes the stronger ability of activated carbon to adsorb phosphate. Interestingly, compared with La(OH)₃ modified biochar, the raw material of CaMCG is coal gangue. Coal gangue is the solid waste of mining area. The preparation of CaMCG from coal gangue and its application in the field of phosphorus-containing wastewater treatment can further develop the resource utilization of coal gangue. This technology reduces the accumulation of coal gangue in the mining area, realizes the treatment of waste by waste, and provides a way for the construction of green mines. Ding et al. [36] reported that the maximum adsorption capacity of phosphate by newly discharged coal gangue and spontaneous combustion coal gangue were 2.504 mg/g and 7.079 mg/g, respectively. Wang et al. [10] prepared coal gangue modified rape straw biochar, and its maximum adsorption capacity for phosphate was 7.9 mg/g, which was 4.6 times that of the original biochar. Ye et al. [11] improved the adsorption capacity of coal gangue to phosphate based on the alkali activation of Ca(OH)₂, and the maximum adsorption capacity of phosphate was 11.796 mg/g. Xiong et al. [9] prepared a new type of calcined coal gangue loaded zirconia (CCG-Zr), and the maximum adsorption capacity of CCG-Zr for phosphate was 8.55 mg/g. Compared with the above-mentioned coal gangue and coal gangue activation modified materials, the adsorption capacity of phosphate by CaMCG prepared in this study was significantly improved. This shows that the new CaMCG adsorbent prepared based on pyrolysis combined with chemical modification can effectively improve the adsorption effect of coal gangue on phosphate, expand the comprehensive utilization range of coal gangue, and reduce the accumulation of coal gangue.

Usually, it is a common way to use coal gangue for land reclamation in coal mining areas. This can not only restore the use value of the land in the mining area, but also effectively deal with the accumulated coal gangue, realize the recycling of coal gangue, and reduce the waste of resources. However, when coal gangue is directly used for land reclamation in mining areas, the fertilizer retention capacity is insufficient, resulting in low phosphorus and other nutrient contents in reclaimed topsoil. The lack of phosphorus and other nutrients in the environment is not conducive to the growth of plants and microorganisms, reducing the efficiency of land reclamation in mining areas. Interestingly, the CaMCG prepared in this study has high phosphate adsorption efficiency. In future applications, CaMCG after phosphate adsorption can replace coal gangue for land reclamation in mining areas, which can effectively utilize the phosphate adsorbed by CaMCG and improve the survival rate and diversity of plants and microorganisms. This material can not only solve the problem of eutrophication of phosphorus-containing wastewater, but also solve the problem of coal gangue accumulation. At the same time, the use of adsorbed phosphate for land reclamation in mining areas is in line with the concept of sustainable development and accelerates the construction of green mines. This technology will be a practical and safe new way of coal gangue resource utilization in the future.

(1) The optimal preparation conditions of CaMCG are obtained by response surface test analysis: m_{calcium chloride}:m_{coal gangue}=1, calcination temperature 735℃, calcination time 135 min. Under this condition, the removal rate of phosphate by the prepared CaMCG is 80.50%. The multivariate quadratic regression equation between the influencing factors of CaMCG preparation A-m_{calcium chloride}:m_{coal gangue}, B-calcination temperature, C-calcination time and phosphate removal rate is: the removal rate of PO₄³⁻ (%) = 70.51–5.54*A + 4.54*B + 7.39*C-2.17*A*B-0.91*A*C + 0.86*B*C + 0.75*A^2-9.87*B^2-16.82*C^2, R² = 0.9928.

(2) Batch experiments that affect the adsorption of phosphate by CaMCG showed that when the phosphate concentration is 100 mg/L, the optimal CaMCG dosage was 0.5 g. At pH 3–7, the adsorption capacity of CaMCG for phosphate was always good. At pH 9–11, the adsorption capacity of CaMCG for phosphate was significantly reduced. The optimum reaction time for phosphate adsorption by CaMCG was 48 h. CaMCG is suitable for adsorption of SO₄²⁻ > HCO₃⁻ > NO₃⁻ ≈ Cl⁻.

(3) The adsorption kinetics test and adsorption isotherm test showed that the adsorption of phosphate by CaMCG had both physical adsorption and chemical adsorption. The maximum adsorption capacity of CaMCG for phosphate was 17.85 mg/g. The adsorption process of CaMCG on phosphate conforms to the Langmuir model, which is consistent with uniform monolayer adsorption.

(4) The main mechanisms of CaMCG adsorbing phosphate are surface precipitation, adsorption exchange and complexation. The CaMCG after phosphate adsorption is used to replace coal gangue for land reclamation in mining areas, which is in line with the concept of sustainable development and the concept of building green mines. This technology will be a practical and safe new way of coal gangue resource utilization in the future.

Funding:

This work was supported by the National Natural Science Foundation of China, Youth Science Foundation Project, grant number 52304188; Liaoning Provincial Natural Science Foundation Project (Doctoral Research Start-up Program), grant number 2023-BS-201; Liaoning Provincial Department of Education Basic Research Project (Youth Project), grant number LJKQZ20222319.

Author Contribution

Yanrong Dong: Writing – original draft, Funding acquisitionZiqing gao: Writing – review & editingHongyu Zhai: Investigation and resourcesGuohao Gong: InvestigationFengjuan Li: InvestigationThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data Availability

The research data can be provided on request. If needed, please contact the corresponding author ([email protected])

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No competing interests reported.

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Study on the preparation of calcium modified coal gangue and its adsorption performance of phosphate

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Abstract

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Highlights

1. Introduction

2. Materials and Methods

2.1. Materials and instruments

2.2. Preparation of CaMCG

2.3. Batch adsorption test

2.4. Analysis method

3. Results and discussion

3.1. Preparation of CaMCG

3.1.1. Calcium chloride and coal gangue mass ratio test

3.1.2. Calcination temperature test

3.1.3. Calcination time test

3.1.4. Response surface experiment of CaMCG preparation

3.2. Influencing factors of phosphate adsorption by CaMCG

3.2.1. Dosage of CaMCG

3.2.2. pH value

3.2.3. Reaction time

3.2.4. Phosphate concentration

3.2.5. Coexisting ions

3.3. Adsorption kinetics and isotherms

3.3.1. Adsorption kinetics

3.3.2. Adsorption isotherms

3.4. Mechanism of phosphate adsorption by CaMCG

3.5. Discussion

4. Conclusion

Declarations

Funding:

Author Contribution

Data Availability

References

Additional Declarations

Supplementary Files

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