Adsorption and Desorption of Coal Gangue toward Available Phosphorus through Calcium-Modification with Different pH

Abstract

The chemical reaction between calcium ions (Ca²⁺) and phosphate in the soil is the main way to maintain the availability of soil phosphorus. Thus, we believe stimulating coal gangue with Ca²⁺ solution would be an effective way to improve its adsorption and desorption capacity toward phosphate. In order to explore the effects of different pH of Ca²⁺ solution on the modified effect of coal gangue, we conducted mechanical grinding (<1 mm), high temperature calcination (800 °C), and the stimulation of Ca²⁺ solution with different pH (2, 7, 13), to prepare acidic calcium-modified coal gangue (Ac-CG) (Ac-CG, acidic calcium-modified coal gangue; Ne-CG, neutral calcium-modified coal gangue; Al-CG, alkali calcium-modified coal gangue; RCG, raw coal gangue), neutral calcium-modified coal gangue (Ne-CG), and alkali calcium-modified coal gangue (Al-CG); raw coal gangue (RCG) was regarded as the control. The results indicated that Al-CG had better phosphate adsorption (3.599 mg g⁻¹); this favorable adsorption performance of Al-CG was related to the formation of hydrated calcium silicate gel and ettringite, which provided more Ca²⁺, Al³⁺, and hydroxyl groups, and a larger specific surface area (9.497 m² g⁻¹). Moreover, Al-CG not only held more phosphate but also maintained its availability longer for plants. It is suggested that stimulating coal gangue with Ca²⁺ solution under alkaline conditions is a perfect way to enhance its adsorption and desorption capacity toward phosphate; the Al-CG we prepared could be used as filling material and soil conditioner in the reclamation area.

1. Introduction

Coal gangue is the main solid waste generated during coal mining and washing, accounting for about 10–25% of total coal production [1,2,3,4]. With the progress of coal mining activities, coal gangue accumulates in large quantities; it has become one of the largest industrial residues [5,6]. The accumulation of coal gangue not only occupies large land resources but also causes serious environmental problems, such as soil pollution, soil degradation, geological disasters, and air pollution [7,8,9,10,11,12,13,14,15,16]. Therefore, it is an urgent international task to improve the comprehensive utilization efficiency of coal gangue and eliminate its negative impact on the ecological environment [17].

Coal mining causes massive land destruction, which is exhibited by the subsidence of surface morphology and the rising of the groundwater [18,19,20], and reclamation is regarded as the critical method to restore the subsidence land. Moreover, coal gangue has large storage capacity and is used as the common alternative soil substrate to filling the coal mining subsidence area [21,22,23], which not only reduces the land occupation but also realizes waste reuse [21]. However, some scholars reported that the filling of coal gangue is not conducive to nutrient sequestration: the soil nutrients are lost rapidly after passing through the coal gangue filling layer, resulting in in low nutrient content of reclaimed soil [9,24]. In addition, the primary utilization of coal gangue inevitably causes secondary environmental pollution such as water eutrophication [21]. Therefore, improving the retention capacity of coal gangue toward soil nutrient elements is significant in the reclamation of coal mining area. Thus, it can be seen that increasing the retention capacity of coal gangue to nutrients is of great significance to improve the application of coal gangue as an alternative soil substrate the in coal mining subsidence area.

The main components of coal gangue are SiO₂ and Al₂O₃, accounting for 60–95%, and the its chemical structure is stable, with limited adsorption capacity for chemical elements [8,25,26]. However, it is rich in porous structures and clay minerals and is used as adsorbent or adsorbent feedstock by related scholars, and mechanical grinding, thermal activation, and chemical activation are common methods to enhance the reactivity of coal gangue [10,27,28,29,30,31,32]. Mechanical activation is a physical change of coal gangue by grinding processes. Grinding transformed the crystal structure, changed the crystalline degree, and increased the active reaction site of coal gangue and therefore enhanced the reaction activity of coal gangue [33,34,35]. The thermal activation is an effective method to stimulate the activity of coal gangue. The generated amorphous reactive SiO₂ and Al₂O₃ after thermal activation improved the reactivity of coal gangue [36,37,38]. Chemical activation is a method of causing chemical agents to react with amorphous aluminosilicates, which produces inorganic binders with chemical and physical properties. Han et al. [39] prepared coal gangue–slag composite materials with sodium silicate as an activator; some scholars have also improved the reactivity of coal gangue by acid leaching [40,41]. Although the above literature has proved that activation can enhance the adsorption performance of gangue, few studies have reported the effect of activated gangue on the adsorption and desorption capacity of nutrient ions; especially, whether the adsorbed nutrients can be effectively released is still unknown, which affects the application of gangue in agricultural production. Phosphorus is one of the main soil-nutrient elements; even though related scholars have prepared lanthanum-modified and sodium-modified coal gangue to remove phosphate from water [42], the adsorption of phosphate with those modified methods is irreversible, which affects their application in agricultural production. It is well known that combining soil phosphate with calcium ion (Ca²⁺) to form calcium phosphate salts is the main method of soil phosphorus retention, and calcium addition benefits the sequestering of soil active phosphorus and the maintaining of its effectiveness for plants [43]; thus, we believe activating coal gangue with Ca²⁺ would increase its adsorption and desorption capacity toward phosphorus. Currently, few works have been completed so far to study modified coal gangue with calcium to adsorb phosphorus; especially, the effect of the pH of calcium solutions on the adsorption and desorption characteristics has remained unclear now, so this study will fill the gap of related studies.

Here, we prepared three different calcium-modified coal gangues through grinding, calcination, and stimulating, with CaCl₂ solutions with different pH, to explore the effect of CaCl₂ solutions pH on the adsorption and desorption capacity of modified coal gangues toward phosphate and its mechanism. Therefore, (i) the structure, morphology, composition, and porosity of the raw and calcium-modified coal gangues were systematically characterized; and (ii) their adsorption and desorption characteristics toward phosphate were systematically studied. This study further improved the activation method of coal gangue and broadened the application scope of activated coal gangue, and these results are expected to improve the utilization of coal gangue in agriculture and reclamation in coal mining area.

2. Materials and Methods

2.1. Material Collection and Preparation

The coal gangue used in this study was collected from Yanzhou mine area in Jining city, Shandong province. After removing impurities, the samples were crushed and sieved by 1 mm sieve to get raw coal gangue (RCG). The RCG with muffle furnace (Nabertherm LT15/12, Lilienthal, Germany) were calcined in 800 °C for 2 h and cooled to room temperature, and then the samples were reacted with CaCl₂ solutions with a solid–liquid ratio of 0.12 and a reacting temperature of 98 °C for 4 h at different pH conditions (2, 7, and 13), which were adjusted with HCl and NaOH solutions. After that, the samples were rinsed to neutral with distilled water and air-dried to obtain acidic calcium-modified coal gangue (Ac-CG), neutral calcium-modified coal gangue (Ne-CG), and alkaline calcium-modified coal gangue (Al-CG).

2.2. Characterization

Evaluated in conjunction with the detailed clay mineral analysis of the <2 μm fraction of raw and modified samples by oriented-aggregate X-ray diffraction methods (XRD, Bruker D8 ADVANCE, Karlsruhe, Germany), the contents of elemental components were measured by X-ray fluorescence spectrometer (XRF, PANalytical Axios, Amsterdam, Netherlands). The distribution of functional groups was measured by Fourier transform infrared spectroscope (FTIR, Nicolet iS50, New York, USA). The structural morphology of the sample surface was observed by scanning electron microscope (SEM, Zeiss Sigma 300, Oberkochen, Germany). The specific surface area and pore structure were measured based on the multipoint Brunauer–Emmett–Teller (BET, Micromeritics Gemini 2.0, Norcross, GA, USA) method.

2.3. Adsorption Experiments

2.3.1. Adsorption Kinetic Experiments

In this study, KH₂PO₄ solution (AR, KCRC, Tianjin, China) was used to simulate soil-available phosphorus; 300 mg/L KH₂PO₄ solution was prepared, and 1 g each of RCG, Al-CG, Ne-CG, and Ac-CG was weighed and put into 50 mL centrifuge. We added 25mL KH₂PO₄ solution (pH = 6.0) in the centrifuge tubes, which were shaken by oscillator (220 rpm/min, Ronghua KS, Shanghai, China) for different times (0.5, 2.5, 5, 7.5, 10, 20, 30, and 40 h); after that, we filtered the mixture with 0.45 μm aqueous filters and measured the phosphate concentration of the filtrates using a UV-Vis spectrophotometer (UV-5500, Shanghai, China). The phosphate adsorption capacities were described by the adsorption kinetic model and calculated using Equations (1) and (2):

$$\mathrm{Pseudo} \mathrm{first}-\mathrm{order} \mathrm{model} q_t {=q}_e\left({1-e}^{{-k}_{1}t}\right)$$

(1)

$$\mathrm{Pseudo} \mathrm{second}-\mathrm{order} \mathrm{model} q_t=k_2{q}_e^{2}t/\left(1+k_2{q}_{e}t\right)$$

(2)

where q_e and q_t correspond to phosphate adsorption capacity at equilibrium time and at time t, respectively. k₁ and k₂ correspond to model-specific rate constants.

2.3.2. Adsorption Isotherm Experiments

KH₂PO₄ solutions of different concentrations (10, 20, 50, 100, 150, 200, 250, and 300 mg L⁻¹) were configured separately; 1 g samples of RCG, Ac-CG, Ne-CG, and Al-CG were weighed and put into 50 mL centrifuge tubes with 25 mL KH₂PO₄ solutions (pH = 6.0) with different concentrations and shaken by oscillator at 220 r/min for 24 h. The mixture was filtered through 0.45 μm aqueous filter, and the phosphate content in the filtrate was determined using the UV-Vis spectrophotometer. The phosphate adsorption capacities were described by the isotherm adsorption model and calculated using Equations (3) and (4):

$$\mathrm{Langmuir} \mathrm{isotherm} \mathrm{model} q_e=K_l{q}_m{C}_e/\left(1+K_l{C}_e\right)$$

(3)

$$\mathrm{Freundlich} \mathrm{isotherm} \mathrm{model} q_e=K_f{C}_e^{1/n}$$

(4)

where k_l and k_f are the Langmuir and Freundlich constants, respectively; q_m denotes the Langmuir maximum adsorption capacity; C_e represents the equilibrium solution concentration; and n is the Freundlich linearity constant.

2.4. Desorption Experiments

The samples after the adsorption isotherm experiment were collected, air-dried, and desorbed with distilled water (pH = 7) and 0.5 mol/L NaHCO₃ solutions (pH = 8.5, AR, KCRC, Tianjin, China) for 10 h, respectively. NaHCO₃ solution can effectively extract phosphate and is a common solution for extracting soil available phosphorus; thus, we selected it as desorbent to measure the availability of the adsorbed phosphorus of the samples. Then, we collected the filters through 0.45 μm aqueous filter and measured their phosphate concentrations. Moreover, the samples after 40 h of adsorption in the adsorption kinetics experiment were also collected, air-dried, and desorbed with two desorbents for different times (0.5, 2, 4, 6, 8, and 10 h), and the phosphate concentration of the filter was measured.

2.5. Statistical Analysis

The measured data are expressed as the mean value of triplicate experiments. All data were analyzed by one-way variance (ANOVA) using SPSS 19.0 software (SPSS Inc., Chicago, IL, USA), and images were drawn using Origin 2021 software (Origin Lab., Northampton, MA, USA).

3. Results

3.1. Characterization of Raw and Calcium-Modified Coal Gangues

3.1.1. XRD Analysis

XRD patterns of RCG and modified coal gangue samples are presented in Figure 1a. As shown in the XRD patterns, the dominant mineral phases of RCG were quartz (SiO₂), kaolinite (Al₂(Si₂O₅)(OH)₄), and illite (KAl₂[(OH)₂AlSi₃O₁₀]), while the characteristic diffraction peaks of kaolinite and illite completely disappeared in the patterns of calcium-modified coal gangues. This may have been due to the hydroxyl (-OH) removal process during grinding, and high temperature calcination [44,45,46], where kaolinite is transformed to metakaolinite, and metakaolinite and illite are further decomposed into amorphous SiO₂ and Al₂O₃ [47]. Moreover, new phases were observed in the XRD pattern of Al-CG, which were the calcium silicate hydrate (C-S-H gel) phase and the ettringite phase (AFt); the appearance of C-S-H was attributed to a hydration reaction among SiO₂, Al₂O_3, and Ca²⁺, and the AFt was associated with the reaction of [Al(OH)₆]³⁻, Ca^2+, and sulphate (SO₄²⁻).

3.1.2. XRF Analysis

The chemical components in RCG and calcium-modified coal gangues were shown in

Table 1. Physiochemical properties of coal gangue samples.

Sample ID	Elemental Composition (Weight%)								Surface Area (m2·g−1)		Pore Volume (cm3·g−1)		Pore Diameter (nm)
	SiO2	Al2O3	Fe2O3	K2O	CaO	P2O5	SO3	MgO	SBET	At-Plot	Vmeso	Vt-Plot	Davr
RCG	64.414 ± 0.24	26.468 ± 0.22	3.576 ± 0.11	2.269 ± 0.08	1.051 ± 0.05	0.150 ± 0.007	1.570 ± 0.08	0.677 ± 0.04	4.017 ± 1.72	0.295 ± 0.51	0.0253 ± 0.007	0.0012 ± 0.0004	16.193 ± 1.97
Ac-CG	63.185 ± 0.23	26.281 ± 0.23	3.571 ± 0.10	2.424 ± 0.08	1.179 ± 0.03	0.147 ± 0.004	0.775 ± 0.006	0.716 ± 0.04	4.545 ± 1.69	0.082 ± 0.37	0.0268 ± 0.004	0.0010 ± 0.0005	15.609 ± 1.83
Ne-CG	62.820 ± 0.24	26.336 ± 0.22	3.492 ± 0.09	2.402 ± 0.08	1.307 ± 0.04	0.177 ± 0.003	0.605 ± 0.004	0.744 ± 0.05	5.590 ± 1.68	0.094 ± 0.56	0.0283 ± 0.003	0.0015 ± 0.0002	13.665 ± 1.77
Al-CG	62.826 ± 0.24	26.534 ± 0.22	3.598 ± 0.10	2.212 ± 0.07	1.628 ± 0.03	0.149 ± 0.004	0.569 ± 0.005	0.749 ± 0.04	9.497 ± 1.63	1.515 ± 0.47	0.0160 ± 0.002	0.0074 ± 0.0002	5.628 ± 1.46

Note: S_BET, BET surface area; A_t-Plot, t-Plot micropore area; V_meso, BJH desorption cumulative volume of pores; V_t-Plot, t-Plot micropore volume; and D_avr, BJH desorption average pore diameter.

; these elements were represented by oxides forms. It can be seen that the major constituents in the RCG sample were SiO₂ (64.414%), Al₂O₃ (26.468%), Fe₂O₃ (3.576%), and K₂O (2.269%), and minor constituents contained CaO, P₂O₅, SO_3, and MgO. After modification, the content of major chemical components rarely changed, while CaO and SO₃ in minor components changed noticeably. CaO increased from 1.051% in RCG to 1.179% in Ac-CG, and from 1.307% in Ne-CG to 1.628% in Al-CG, respectively, while SO₃ decreased from 1.570% in RCG to 0.775% in Ac-CG, 0.605% in Ne-CG, and 0.569% in Al-CG, indicating that SO₄²⁻ in RCG was released into solution after modification and reacted with Ca²⁺ to form new phases.

3.1.3. FTIR Analysis

The FTIR spectra of RCG and the modified coal gangues are shown in Figure 1b. According to the different wavenumber ranges of different functional groups, the functional groups of coal gangue samples were detected, and the vibration bands were very different from each other. For the RCG, the peaks at 3698, 3654, and 3621 cm⁻¹ were the stretching vibration peaks that were attributed to the -OH bonds [48], which were generated from the inner and outer-hydroxyl stretching vibrations of kaolinite. The broad peak at 3443 cm⁻¹ was associated with -OH bond stretching vibration absorption, and the band at 1625 cm⁻¹ may be due to H-O-H bending vibrations from physisorbed water [49,50]. The peaks at 1033 cm⁻¹ and 1006 cm⁻¹ referred to Si-O-Si stretching vibration, the peaks at around 798 cm⁻¹ and 695 cm⁻¹ were attributed to Si-O bond symmetric stretching vibration, and the bands at 470 cm⁻¹ were attributed to the stretching vibration of Si-O-Si [51]. In addition, the peaks at 912 cm⁻¹ and 537 cm⁻¹ referred to Al-OH bending vibration and stretching vibration in the octahedral aluminum structure, respectively.

In comparison with RCG, the intensity of the absorption peak of the modified gangue was weakened, indicating that structural water reduced. For Ac-CG and Ne-CG, the absorption peaks at 3698, 3654, and 3621 cm⁻¹ disappeared and the Si-O-Al absorption peaks at 695 and 537 cm⁻¹ were weakened, which indicated that kaolinite was conversed to metakaolinite [52], while for Al-CG, the intensity of the absorption peaks diminished, and two bands at 1033 and 1006 cm⁻¹ disappeared, which was related to the depolymerization and collapse of the silica tetrahedral structure. The band of Al-CG at 464 cm⁻¹ was the bending vibration mode of Si-O-Al [53], and a new absorption peak at 1089 cm⁻¹ indicated the formation of a new phase [54], which might be caused by the asymmetric stretching vibration of Si-O-Si in the C-S-H gels.

3.1.4. SEM Analysis

The SEM images of RCG and modified coal gangue samples are presented in Figure 2. It can be seen that RCG had an irregular lamellar structure and a rough surface, which might be associated with the flaky structure of kaolinite (Figure 2a). As observed, the kaolinite microstructure obviously changes with the calcium-modified coal gangue (in Figure 2b–d), indicating that its original structure was destructed and active reaction sites were increased [35]. In Figure 2b,c, the surfaces of Ac-CG and Ne-CG became dense and smooth, which was associated with the transformation of kaolinite to metakaolinite at high temperatures; they did not further react with Ca²⁺ under acidic and neutral solutions. When the coal gangue was modified with alkali situation (in Figure 2d), the surface structure became complicated and clusters (C-S-H gels) occurred on the surface of Al-CG; thus, the potential gelation activity was improved [55]. Additionally, some small needle-shaped crystals (AFt) could be detected in the pores, and the results were consistent with the results of Mola Abasi et al. [56] and Han et al. [57], which make the surface structure more complicated; thus, the adsorption process was facilitated. The SEM images were consistent with the XRD results.

3.1.5. BET Analysis

The textural characteristic of RCG and modified coal gangue samples are shown in Table 1. The results of BET analysis indicated that Al-CG had larger specific surface area (9.497 m² g⁻¹), smaller pore size (5.628 nm), larger micropore area (1.515 m² g⁻¹), and bigger volume (0.0074 cm² g⁻¹) than other samples. This was attributed to the more complex surface structure of Al-CG, which was associated with the generation of needle-like crystals (AFt) and C-S-H gels under alkaline modification conditions. The enlarged specific surface area would provide coal gangue with more adsorption sites, thereby increasing its physical adsorption capacity.

3.2. Phosphate Adsorption of Raw and Calcium-Modified Coal Gangues

3.2.1. Adsorption Kinetics of Phosphate

The adsorption kinetics based on the pseudo first-order and pseudo second-order model are shown in Figure 3, and the fitting parameters are listed in

Table 2. Parameters of phosphate adsorption kinetic and isotherm of coal gangue samples.

Sample ID	Pseudo First-Order Model			Pseudo Second-Order Model			Langnuir Model			Freundlish Model
	k1 (h−1)	qe (mg·g−1)	R2	k2(g·mg−1·h−1)	qe(mg·g−1)	R2	Kl (L·mg−1)	qm (mg·g−1)	R2	n	Kf [mg(1−n)Lng−1]−1	R2
RCG	0.267	0.126	0.878	2.910	0.139	0.845	0.029	0.221	0.980	1.019	0.033	0.981
Ac-CG	1.524	0.336	0.732	5.457	0.357	0.939	0.003	2.510	0.992	1.008	0.023	0.992
Ne-CG	0.845	0.625	0.750	2.094	0.665	0.935	0.004	2.734	0.994	1.006	0.035	0.994
Al-CG	0.400	2.129	0.856	0.311	2.283	0.946	0.004	3.599	0.995	1.005	0.045	0.994

. According to Table 2, it can be seen that the experimental data of the pseudo-second-order model fit better than the pseudo-first-order model, indicating that the pseudo-second-order model provides good fitness between the experimental values and the model. In the pseudo-second-order model, the R² of modified gangue is higher than that of RCG, which indicates that the fitting effect of RCG is poor and the fitting effect of activated gangue is better. It can be seen from Table 2 that the fitting R² of the pseudo second-order model for Al-CG is the highest (R² = 0.946), showing the largest equilibrium adsorption capacity, which indicates that the pseudo second-order model can better describe the phosphate adsorption process. The value calculated with this model was 2.283 mg g⁻¹, which was 16.42, 6.39 and 3.43 times higher than that of RCG, Ac-CG, and Ne-CG, respectively.

3.2.2. Adsorption Isotherms of Phosphate

The adsorption isotherm based on the Langmuir and Freundlich models were shown in Figure 3, and the fitting parameters were listed in Table 2. Based on R² values, it can be seen that the adsorption for all samples fit better with the Langmuir model than the Freundlich model, and the calcium-modified coal gangues showed better phosphate adsorption capacity than RCG. According to the fitting results of the Langmuir model, the maximum theoretical adsorption capacity of the available phosphorus of Al-CG was the highest (3.599 mg g⁻¹), which was approximately 16.29, 1.43, and 1.32 times higher than that of the RCG, Ac-CG, and Ne-CG samples, respectively.

3.3. Phosphate Desorption of Raw and Calcium-Modified Coal Gangues

The desorption curves of the RCG and modified samples desorbed by distilled water and NaHCO₃ solutions are shown in Figure 4 and Figure 5, respectively. Among them, the gangues used in Figure 4 were the samples after phosphate adsorption in the adsorption isotherm experiment, while the gangues in Figure 5 were the samples after 40 h of phosphate adsorption in the adsorption kinetics experiment.

It can be seen from Figure 4 that the desorption curve of distilled water was obviously different than that of NaHCO₃ for modified coal gangues, while those two curves changed little for RCG. Moreover, compared with other samples, the phosphate concentration was the smallest when the Al-CG was desorbed by distilled water, and the phosphate concentration was the largest when the Al-CG was desorbed by NaHCO₃. For example, at the concentration of 300 mg/L, the filtrate concentration was 8.280 mg/L for RCG and 4.833 mg/L for Al-CG when distilled water was used as the desorbent, while the filtrate concentration was 11.027 mg/L for RCG and 32.240 mg/L for Al-CG when the NaHCO₃ solution was used as the desorbent.

Furthermore, it can be seen from Figure 5 that phosphate concentrations of filtrate gradually increased, and the concentration desorbed by NaHCO₃ solution was higher than that desorbed by H₂O, namely, when Al-CG was desorbed by distilled water, the phosphate concentration of filtrate was the smallest, which was lower than that of other coal gangues, while when Al-CG was desorbed by NaHCO₃, the phosphate concentration of the filtrate was the largest. For example, at a desorption time of 10 h, the filtrate concentration was 13.090 mg/L for RCG and 6.560 mg/L for Al-CG when distilled water was used as the desorbent, while the filtrate concentration was 14.500 mg/L for RCG and 34.933 mg/L for Al-CG when NaHCO₃ solution was used as the desorbent.

4. Discussion

4.1. Analysis of Phosphate Adsorption Mechanism by Calcium-Modified Coal Gangue

It is reported that new phases were generated; the kaolinite and illite disappeared in coal gangue after the grinding and high-temperature calcination; and the Al-O, Si-O-Si, and Si-O bonds in coal gangue broke, producing an expansion in crystal spacing and an increase in activity of SiO₂ and Al₂O₃ [42,58,59,60,61]. In this study, we found that when stimulating the calcined coal gangue with alkaline Ca²⁺ solution, Ca²⁺ would react with the amorphous SiO₂ of coal gangue to generate C-S-H gel (Equation (5)) and reacted with SO₄²⁻ and [Al(OH)₆]³⁻ to form AFt (Equation (6)).

$$4\mathrm{Ca}{\left(\mathrm{OH}\right)}_2+6{\mathrm{SiO}}_2\to {\mathrm{Ca}}_4{\mathrm{Si}}_6{\mathrm{O}}_{14}{\left(\mathrm{OH}\right)}_4·2{\mathrm{H}}_2\mathrm{O}$$

(5)

$$2{\left[\mathrm{Al}{\left(\mathrm{OH}\right)}_6\right]}^{3-}+6{\mathrm{Ca}}^{2+}+3{\mathrm{SO}}_4^{2-}+26{\mathrm{H}}_2\mathrm{O}\to {\mathrm{Ca}}_6{\mathrm{Al}}_2{\left({\mathrm{SO}}_4\right)}_3{\left(\mathrm{OH}\right)}_{12}·26{\mathrm{H}}_2\mathrm{O}$$

(6)

C-S-H and AFt benefited the improving of the adsorption capacity of coal gangue toward phosphate, which was because Ca²⁺ and Al³⁺ could be dissociated from C-S-H gels and AFt in KH₂PO₄ solution (pH = 6), with the corresponding chemical equations as follows:

$${\mathrm{Ca}}_4{\mathrm{Si}}_6{\mathrm{O}}_{14}{\left(\mathrm{OH}\right)}_4·2{\mathrm{H}}_2\mathrm{O}+8{\mathrm{H}}^{+}\to 4{\mathrm{Ca}}^{2+}+6{\mathrm{H}}_2{\mathrm{SiO}}_3+2{\mathrm{H}}_2\mathrm{O}$$

(7)

$${\mathrm{Ca}}_6{\mathrm{Al}}_2{\left({\mathrm{SO}}_4\right)}_3{\left(\mathrm{OH}\right)}_{12}·26{\mathrm{H}}_2\mathrm{O}\to 2{\left[\mathrm{Al}{\left(\mathrm{OH}\right)}_6\right]}^{3-}+6{\mathrm{Ca}}^{2+}+3{\mathrm{SO}}_4^{2-}+26{\mathrm{H}}_2\mathrm{O}$$

(8)

$${\left[\mathrm{Al}{\left(\mathrm{OH}\right)}_6\right]}^{3-}+6{\mathrm{H}}^{+}\to {\mathrm{Al}}^{3+}+6{\mathrm{H}}_2\mathrm{O}$$

(9)

The adsorption mechanism of Al-CG toward phosphate was related to four processes: physical adsorption, electrostatic adsorption, ion exchange, and chemical precipitation, which can be seen in Figure 6. (i) The internal structures of Al-CG were more complex after the modified process [62], and then its physical adsorption capacity toward ions improved. (ii) The negatively charged phosphates were adsorbed by free Ca²⁺ and Al³⁺ through electrostatic adsorption. (iii) AFt provides more -OH, and its presence could participate in phosphate adsorption reactions through ion exchange [63]. (iv) The chemical reaction between Ca²⁺, OH-, and phosphate forms Ca₅(PO₄)₃OH precipitates, and Ca²⁺ and Al³⁺ react with phosphate to form Ca-P and Al-P precipitates, which are attached to the surface of gangue. The reaction equations are as follows:

$$5{\mathrm{Ca}}^{2+}+7{\mathrm{OH}}^{-}+3{\mathrm{H}}_2{\mathrm{PO}}_4{}^{-}\to {\mathrm{Ca}}_5{\left({\mathrm{PO}}_4\right)}_3\mathrm{OH}\downarrow +6{\mathrm{H}}_2\mathrm{O}$$

(10)

$${\mathrm{Ca}}^{2+}+2{\mathrm{H}}_2{\mathrm{PO}}_4^{-}\to \mathrm{Ca}-\mathrm{P}\downarrow$$

(11)

$${\mathrm{Al}}^{3+}+3{\mathrm{H}}_2{\mathrm{PO}}_4^{-}\to \mathrm{Al}-\mathrm{P}\downarrow$$

(12)

4.2. Enlightenment of Modified Coal Gangue on Phosphate Retention and Release Capacity

The adsorption experiments indicated that RCG had limited adsorption capacity toward phosphate, and the maximum adsorption capacity simulated by Langmuir isotherm model was just 0.221 mg g⁻¹. However, the adsorption capacity of modified coal gangues improved largely through calcium-modification; the maximum adsorption capacity toward phosphate by the Langmuir model of Ac-CG and Ne-CG was 2.510 mg g⁻¹ and 2.734 mg g⁻¹, respectively; and the maximum adsorption capacity of Al-CG reached 3.599 mg g⁻¹. The maximum adsorption capacity toward the phosphate of RCG was lower than that of some agricultural soils in the relevant literature, for example, Huang et al. [64] reported that the maximum phosphate adsorption capacity of Cixi paddy soils by Langmuir model is 0.235–0.186 mg g⁻¹, and that of Jinxian paddy soils is 0.44–1.55 mg g⁻¹; Eslamian et al. [65] reported that the maximum phosphate adsorption capacity of sandy is 0.250 mg g⁻¹ and the maximum phosphate adsorption capacity of clay loam is 0.222 mg g⁻¹; and Bubbaa et al. [66] reported that the maximum phosphate adsorption capacity of Nymolle sands by Langmuir model is 0.266mg g⁻¹. Thus, we believe that using RCG as filling material to replace soil substrates might accelerate nutrient loss and lead to the eutrophication of water bodies in the coal mining area. The maximum phosphate adsorption capacity of Ac-CG and Ne-CG was higher than that of the agricultural soils mentioned above but lower than that of some biochars. For example, the maximum phosphorus adsorption capacity of rice husk biochar is 2.91 mg g⁻¹, and the maximum phosphorus adsorption capacity of lanthanum modified biochar is 2.836 mg g⁻¹ [67], while Al-CG had comparable phosphate adsorption capacity with the biochars, indicating that it can be used as a favorable soil conditioner. Moreover, Al-CG also had perfect desorption characteristics toward phosphate, when we used distilled water to desorb the Al-CG sample that had absorbed phosphate, the phosphate concentrations of filtrate were lower than that of other samples, while when it was desorbed with NaHCO₃ solution, the phosphate concentrations were higher than other samples. It is indicated that the phosphate adsorbed by Al-CG could be held longer and released slowly, which is benefit of the long-term effectiveness of phosphorus for plant.

4.3. Application Advantages and Directions of Modified Coal Gangue

In this study, we explored the adsorption and desorption characteristics of calcium-modified coal gangues toward available phosphorus, and we believe that the Al-CG has some application advantages, including inexpensive, eco-friendly, and durable application. (i) Inexpensive: coal gangue is a common solid waste with a large number of emissions, the modified process of coal gangue is simple, and the energy consumption is low; thus, the production cost of Al-CG is relatively inexpensive. (ii) Eco-friendly: the use of coal gangue as a filling material would lead to the rapid loss of soil nutrients and cause the eutrophication of water bodies in coal mining area. However, after modification, the structure of coal gangue was changed and new substances were produced, which were conducive to the reclaimed soil nutrient sequestration and the control of water bodies eutrophication. (iii) Durable application: after the modification of alkali situation, C-S-H gel and AFt were generated, which made the structure of modified coal gangue more stable and chemically resistant, and the applicated life was increased.

All in all, Al-CG had preferable phosphate adsorption and desorption capacity, and its ecological and economic benefits were better than RCG. We proposed that Al-CG had perfect adsorption capacity toward phosphate, which could be used as an adsorbent to remove phosphate in water bodies in the environmental field. It could also maintain the effectiveness of phosphate, which could also be used as a soil conditioner to enhance soil available phosphorus content in agriculture field. Moreover, it could be used as a better filling material to improve reclaimed soil quality in the coal mining area.

5. Conclusions

In this study, we prepared three calcium-modified coal gangues (Ac-CG, Ne-CG, and Al-CG). We mainly investigated the phosphate adsorption and desorption characteristics of raw and modified coal gangues and explored the phosphate adsorption and desorption mechanisms of modified coal gangues. The main findings of this study are summarized as follows.

• Al-CG had better adsorption and desorption effects, and the adsorption process was more in line with the pseudo second-order model and the Langmuir isotherm adsorption model, where the Langmuir maximum adsorption capacity of Al-CG was the highest (3.599 mg g⁻¹).
• The better adsorption effect of Al-CG was contributed to the generation of C-S-H and AFt, which provided a larger specific surface area (9.497 m² g⁻¹) and more Ca²⁺, Al³⁺, and hydroxyl groups.
• When the Al-CG sample that absorbed phosphate was desorbed by distilled water, the desorption amount of phosphate was the lowest, and when using NaHCO₃ solution (pH = 8.5) to desorb it, the desorption amount was highest.
• The study demonstrated that calcium modification with alkali condition was an effective way to realize the resource utilization of coal gangue, which can be used as a filling material in the reclamation area, as a soil conditioner in agricultural production, and as a phosphorus removal agent in water bodies.