Preparation of Coal Gangue-Based Porous Ceramics and Its Application on Pb²⁺ Cycling Adsorption

Abstract

The presence of lead in wastewater poses a significant threat to human health. To address this issue, coal gangue-based porous ceramics (CGPC) were developed to remove Pb²⁺ in wastewater. Coal gangue (CG) waste from Lvliang City, Shanxi province in China was used as raw material, and porosity was introduced through the addition of a pore-forming agent and an extrusion molding process. Properties of CGPC were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET) surface area analysis, Fourier transform infrared spectroscopy (FTIR), and thermogravimetric analysis (TGA) to explore its adsorption mechanism. The researchers examined the impact of pH, dosage of adsorbent, initial concentration, duration of adsorption, and temperature on the adsorption efficiency of CGPC. The CGPC of best performance had a porosity of 32.91% and compressive strength of 20.5 MPa prepared at 800 °C under nitrogen atmosphere with 10 wt% Na₂CO₃ pore-forming agent and 8 wt% CaO-MnO₂ combined fluxing agent. The removal rate of Pb²⁺ in simulated lead-containing wastewater with a concentration of 200 mg/L reached 99.63%, and the maximum adsorption capacity was 32.15 mg/g. The adsorption process of Pb²⁺ by CGPC involves chemical adsorption and ion exchange. After being regenerated for seven cycles with 1 mol/L NaOH as the desorption agent, the removal rate of Pb²⁺ by CGPC still had 72%.

1. Introduction

Lead is one of the most toxic heavy metal ions and can enter the human body through various means, such as coatings, dust, soil, water pollution, etc. [1]. The Pb²⁺ ion is introduced into the ecological system via industrial activities and can induce significant pathologies by disrupting vital physiological processes in living organisms [2,3,4]. The discharge of wastewater containing lead can cause damage to the immune and nervous systems, and in severe cases can lead to organ damage and even endanger life. The limits for Pb²⁺ in drinking water are set by various authorities, The acceptable limits are set by various international organizations and standards, including the World Health Organization (WHO), the European Union (EU), the US Environmental Protection Agency (USEPA), and the Guidelines for Canadian Drinking Water Quality are as follows: 0.01 mg/L, 0.01 mg/L, 0.015 mg/L, and 0.01 mg/L, respectively [5,6]. Zhu et al. [7] employed a facile intercalation method to prepare a layer-structured cationic framework material loaded with phosphonate salts. The functionalized layered double hydroxide prepared in this way exhibits excellent chelating and adsorption properties, allowing for highly efficient adsorption of Zn²⁺ and Fe³⁺ ions from aqueous solutions. The composite material prepared by Shi et al. [8] using the hydrothermal method possesses features of extremely low toxicity, exceptional stability, and high biocompatibility. It exhibits outstanding adsorption capabilities toward heavy metal ions. The objective of removing Cu²⁺ and Pb²⁺ is achieved through the formation of ternary surface complexes, involving hydrogen bonding and electrostatic interactions. Porous ceramics have gradually shown their advantages in wastewater treatment due to their high porosity, extensive surface area, excellent heat resistance, and durability against wear, good mechanical properties, and strong regenerability [9]. The activated carbon porous ceramics prepared by Aprianti et al. [10] has good adsorption properties for lead ions in papermaking wastewater, with a removal rate of 92.45% within 3 h. However, the manufacturing cost is relatively high, and the application conditions are more stringent. Fu et al. [11] prepared porous ceramics using coal as a raw material and modified it by adding magnetite. According to the research findings, the incorporation of magnetite demonstrates a substantial enhancement in the removal efficiency of Pb²⁺ in lead-containing wastewater, from 74.94% to 95.97%. However, this method still faces the challenges of complex preparation processes and harsh application conditions. Furthermore, extensive research has been conducted on the adsorption of lead ions using porous materials prepared from solid waste, as demonstrated in

Table 1. Comparison of adsorption performance of different activated carbon on Pb²⁺.

Sorbent Type	Ion Species	Concentration	Condition		Adsorption Capacity	Reference
			pH	T
Geopolymer foam	Pb	50 mg/L	5	24 h	6.3 mg/g	[19]
Pyrophyllite mine waste-based geopolymer	Pb	50 mg/L	7.8	1.5 h	7.54 mg/g	[20]
Porous phosphoric acid activated eopolymer	Pb	50 mg/L	7	24 h	11.99 mg/g	[21]
Hollow gangue microsphere	Pb	50 mg/L	6	4 h	138.89 mg/g	[22]
Porous chromium carbon biomorphic ceramics	Pb	200 mg/L	6	2.5 h	8.97 mg/g	[23]
Metallic iron/carbon ceramsites	Pb	50 mg/L	6	3 h	112.36 mg/g	[11]
Coal gangue microsphere	Pb	50 mg/L	6	2 h	18.90 mg/g	[24]
Coal gangue-based porous ceramics	Pb	200 mg/L	6	18 h	32.15 mg/g	This work

T: adsorption time.

.

Coal serves as a primary energy resource in numerous countries. The environmental impacts of coal mining and energy production pose considerable threats to the sustainable utilization of coal as a primary energy source [12]. Coal gangue is the primary industrial solid waste produced during coal production and typically accounts for around 15%–20% in terms of the entire coal’s output produced weight [13]. Improper disposal of coal gangue, such as landfill and long-term storage, has caused serious environmental pollution. Therefore, enhancing the utilization of resources and maximizing the value of coal gangue has become an urgent problem to be solved. Current research shows that coal gangue contains a large amount of SiO₂ and A₂O₃, as well as trace amounts of alkali metal oxides [14,15]. This is similar to the raw material composition required for the preparation of porous ceramics such as mullite and spinel. Porous ceramics often use a small amount of metal oxides as a co-solvent. Therefore, using low-cost and widely available coal gangue as a raw material to prepare porous ceramics can effectively replace expensive reagents and greatly reduce the preparation cost of porous ceramics. At the same time, it can effectively solve the problem of soil pollution caused by coal gangue stacking. Currently, there are some studies on the preparation of porous ceramics using coal gangue as a raw material. The coal gangue porous ceramics prepared by Zhou et al. [16] have been applied to the adsorption of industrial dyes, and the removal rate of cationic blue X-GRRL reaches 99.7% at pH = 8. Yuan et al. [17] successfully fabricated a novel adsorbent based on ceramics by combining bentonite, iron powder, and activated carbon. This innovative adsorbent exhibited an impressive adsorption capacity of 17.5 mg/g for lead ions and 7.4 mg/g for zinc ions. a novel ceramic-based adsorbent using bentonite, iron powder, and activated carbon, which demonstrated a maximum adsorption capacity of 17.5 mg/g and 7.4 mg/g for lead and zinc ions, respectively. Lu et al. [18] prepared ceramics from solid waste and rice husk, resulting in a material with a richer pore structure, higher polymerization degree, and greater zeolite content. The fundamental properties of the material met industrial standards, but its saturation adsorption capacity for Pb²⁺ was only 18.53 mg/g. The aforementioned study establishes a solid groundwork for the development of porous ceramics derived from coal gangue. Therefore, the application of porous ceramics made from coal gangue in wastewater treatment can achieve the goal of resource utilization of coal gangue, as well as turning waste into treasure and treating waste with waste.

In this research, coal gangue was used as a precursor, and materials such as calcium oxide, manganese dioxide, sodium carbonate, and clay were added. The coal gangue porous ceramic adsorbent was synthesized using a one-step molding and sintering process. The researchers examined the impacts of various factors on the production of porous ceramic adsorbents using coal gangue and explored the adsorption capacity and mechanism of Pb²⁺ in wastewater. This study further expands the high value-added utilization of coal gangue and effectively solves the problem of lead-containing wastewater pollution.

2. Materials and Methods

2.1. Materials

Coal gangue (CG) is from Xingxian, Lvliang City, Shanxi Province in China; white clay, sodium carbonate, manganese dioxide, and calcium oxide are analytical grade provided by China National Pharmaceutical Group Chemical Reagent Co., Ltd. (Shanghai China) Deionized water is prepared by ultrapure water system.

2.2. Main Instruments

The thermal stability of coal gangue was evaluated by subjecting it to a heating rate of 10 °C/min, ranging from 50 to 1200 °C, using a NETZSCH STA449 F5 simultaneous thermal analyzer STA449 F5 is from NETZSCH AG, Selb, Germany. The microstructure of coal gangue porous ceramics was observed using a TESCAN MIRA LMS which is from Czechoslovakia field emission scanning electron microscope, with a voltage range of 200 eV to 30 keV. The compressive strength was measured using a YHS-229WJ-50kN electronic universal testing machine which is from Shanghai Yihuan Instrument Technology Co., Ltd., Shanghai, China. The sample composition was analyzed using a Rigaku Smartlab SE X-ray diffractometer which is from Rigaku, Tokyo, Japan, with a scanning range of 2θ = 10–80°. The adsorption properties were tested using an Avio500 (PerkinElmer, Waltham, MA, USA) inductively coupled plasma optical emission spectrometer (ICP-OES). The pore size distribution was measured using an ASAP 2020 Plus microporous physical adsorption analyzer which is from Micromeritics, Norcross, GA, USA. The adsorption mechanism was further analyzed through surface functional group analysis using a Thermo Scientific Nicolet iS20 Fourier (Thermofly Corporation, Waltham, MA, USA) transform infrared spectrometer.

2.3. Synthesis of Coal Gangue-Based Porous Ceramic Adsorbent

This study first investigated the effects of factors such as the quality fraction of the pore-forming agent (2~20%), the type and amount of fluxing agent (2~10%), and firing temperature (600~1400 °C) on the adsorption efficiency, porosity, and compressive strength of coal gangue-based porous ceramics (CGPC), and further studied its microstructure, composition, and adsorption mechanism. The influence of various factors, including solution pH, dosage of adsorbent, initial concentration of the solution, reaction time, and temperature, were investigated to assess their impact on the efficiency of adsorption and the ability for cyclic regeneration. Raw coal gangue from Lvliang City was crushed using a crusher and passed through a 200-mesh sieve. The sieved coal gangue was activated by planetary ball milling for 1 h. The activated coal gangue was mixed with clay, fluxing agent, and pore-forming agent in a mass ratio of 10:2:0.5:1. MnO₂ was chosen as the fluxing agent, and Na₂CO₃ was chosen as the pore-forming agent for the adsorption experiment. The mixture was then pressed and formed into porous ceramic sheets using a pressing method. The compacted specimens were subjected to a drying process at a temperature of 80 °C for a duration of 1 h in an oven. Subsequently, the desiccated specimens were transferred into a pristine alumina crucible and subjected to firing in a tube furnace while maintaining a nitrogen atmosphere. The firing process involved a temperature range of 600 to 1400 °C in order to produce porous ceramics derived from coal gangue.

Weighed 1.0~5.0 g of CGPC samples were placed in a centrifuge tube containing 100 mL of simulated wastewater with different Pb²⁺ concentrations (50~800 mg/L) and pH values. The centrifuge tubes were shaken on a shaker for 5~1400 min and stirred for 30 min~12 h to obtain the supernatant. The concentration of Pb²⁺ ions was assessed using an inductively coupled plasma optical emission spectrometer (ICP-OES) to determine their concentration both prior to and following the adsorption process. The adsorption capacity was calculated using the formula below:

$$\mathsf{\delta }=\frac{{(\mathrm{C}}_0-{\mathrm{C}}_1)\mathrm{V}}{\mathrm{m}}\text{×}100\%$$

(1)

In which, C₀ is the initial concentration of wastewater containing Pb²⁺, C₁ is concentration at Pb²⁺ adsorption equilibrium, V is volume of solution used for adsorption, m is quality of adsorbent and $\mathsf{\delta }$ is the adsorption capacity. Desorption was performed using NaOH solutions of different concentrations (0.1~3 mol/L) for 5~1400 min, and after drying at room temperature, 7 cycles of experiments were conducted to investigate the cyclic regeneration performance of CGPC.

3. Results and Discussion

3.1. Composition and Morphology of Raw Materials

Figure 1a shows the SEM image of coal gangue, which shows smooth, dense, and irregular in shape, with irregular particles scattered on the surface. The composition of coal gangue was characterized by XRF testing, and

Table 2. Chemical composition percentage of coal gangue determined by XRF.

Composition	Al2O3	SiO2	Fe3O4	SO3	CaO	TiO2	K2O	Na2O
Wt./%	34.54	48.36	5.65	4.057	2.73	2.29	1.36	0.1092

shows the results. SiO₂ and Al₂O₃ are the main components of coal gangue, accounting for as much as 82.9%, indicating their potential as raw materials for synthesizing porous ceramics. In addition, the trace amounts of oxides present in coal gangue can serve as flux agents in the production of porous ceramics. Analysis shows that the Si/Al ratio of coal gangue is 1.23, and clay can be added appropriately during synthesis to adjust the Si/Al ratio and optimize the performance of porous ceramics [25]. Figure 1b shows the TG–DSC curve of coal gangue. From the TG curve, it can be seen that the weight loss rate is 14.01% at 900 °C. From the DSC curve, an endothermic peak appears at 590 °C, indicating the transformation of dehydroxylated kaolin in coal gangue to metakaolin. Another endothermic peak appears at 805 °C, indicating the transformation of metakaolin to more ordered mullite [26]. Based on the TG–DSC curve, it can be inferred that the phase transition temperature range for preparing porous ceramics from coal gangue is between 800 and 900 °C.

3.2. Factors Affecting the Preparation of CGPC

3.2.1. The Effect of the Proportion of Pore Forming Agent on the Performance of CGPC

The incorporation of a pore-generating agent greatly influences the mechanical characteristics and adsorption effectiveness of porous ceramics [27]. By selecting appropriate pore-forming agents, substantial advantages can be brought to porous ceramics [28]. As shown in Figure 2, the red curve indicates that the internal porosity of coal gangue-based porous ceramics (CGPC) increases as the amount of Na₂CO₃ pore-forming agent increases, resulting in an increase in porosity. This is mainly because Na₂CO₃ undergoes thermal decomposition at high temperatures, leading to the formation of pores during CO₂ escape. The larger amount of pore-forming agent is added, the more pores are formed [29]. As shown by the blue curve in Figure 2, the compressive strength of CGPC decreases significantly with increasing Na₂CO₃ content. The corresponding black curve indicates that the removal rate of Pb²⁺ ions first increases and then decreases, indicating that the adsorption capacity of the prepared coal gangue-based porous ceramics follows the same trend. This is mainly due to the increase in the number of pores, which provides more adsorption sites, leading to an increase in adsorption efficiency. However, when the amount of Na₂CO₃ is too high, low-melting compounds are formed with substances such as calcium feldspar and quartz in the body, and the liquid phase products occupy the adsorption sites, resulting in a reduction in adsorption capacity. In addition, excessive pores also greatly reduce the compressive strength. Therefore, the amount of Na₂CO₃ pore-forming agent added should be comprehensively considered in terms of its impact on both porosity and compressive strength.

3.2.2. Effect of Flux on the Performance of CGPC

Figure 3a reflects the effect of the addition of different fluxing agents on the compressive strength and adsorption performance of prepared coal gangue-based porous ceramic materials. The results show that adding only MgO significantly reduces the compressive strength, indicating that MgO inhibits the formation of mullite [30], leading to a decrease in material strength. Adding only Fe₂O₃ results in high compressive strength and low removal rate, which is attributed to the fact that Fe₂O₃ mainly promotes the formation of mullite and corundum through a solid solution, making the pore structure denser and increasing its compressive strength [31]. However, at the same time, the dense pore structure also inhibits adsorption efficiency. The addition of CaO+MnO₂ combined fluxing agent results in excellent mechanical properties and the strongest adsorption capacity of the prepared coal gangue-based porous ceramics [32]. This is due to the synergistic effect between liquid-phase fluxing and lattice distortion strengthening mechanisms. The analysis of Figure 3b indicates that the adsorption capacity initially rises and subsequently declines as the CaO-MnO₂ content increases. When the ratio of CaO-MnO₂ is 1:1 and the total proportion is 8%, the adsorption amount of the adsorbent for Pb²⁺ reaches 29.71 mg/g, indicating the best adsorption effect. However, a further increase in the content of alkaline earth metals leads to a decrease in adsorption capacity, as the excessive addition of these metals results in an increase in the liquid phase during melting, making the pore structure even denser. Although the compressive strength of coal gangue-based porous ceramic materials is improved, their adsorption efficiency is significantly reduced.

3.2.3. Effect of Sintering Temperature on the Performance of CGPC

Figure 4 reflects the influence of calcination temperature on the adsorption performance and compressive strength of coal gangue-based porous ceramics. The adsorption capacity of the ceramics showed an upward trend between 600~800 °C, reaching a maximum adsorption amount of 32.15 mg/g at 800 °C followed by a decrease in adsorption capacity. This is mainly because at temperatures below 800 °C, free water quickly pass through the air pores, and a melting reaction occurs between the particles inside the CGPC, causing an increase in the degree of combination, making the material denser and improving the compressive strength [33]. The pore-forming agent Na₂CO₃ decomposes into mesoporous and macroporous structures with increasing temperature, which benefits the adsorption of Pb²⁺ by CGPC [34]. At temperatures above 800 °C, a high-temperature transformation of kaolinite to mullite occurs in coal gangue, making the CGPC denser and increasing its strength [35,36]. However, excessive densification negatively affects the adsorption of Pb²⁺, resulting in a decrease in adsorption capacity. As the temperature continues to rise, Na₂CO₃ in the CGPC reacts more violently, resulting in a significant decomposition of CO₂ and the formation of larger pores. These larger pores are not conducive to the adsorption of Pb²⁺ by coal gangue-based porous ceramics, resulting in a decrease in both adsorption capacity and compressive strength at high temperatures.

3.3. XRD Analysis of CGPC

Figure 5a shows that quartz (SiO₂) and kaolinite (Al₂Si₂O₅(OH)₄) are the main components of CG. Figure 5b is the XRD pattern of CGPC, from which SiO₂, Al₂Si₂O₅(OH)₄, NaAlSiO₄, Ca[AlSi₂O₆]·4H₂O, CaSiO₃ can be identified as the skeletal framework of CGPC that lead to improved mechanical properties of the prepared coal gangue-based ceramic materials. In comparison with Figure 5a, it can be found that the content of quartz and kaolinite decreases in Figure 5b. This is mainly because the addition of Na₂CO₃ and CaO into CG leads to a molten state where SiO₃²⁻ in coal gangue transforms into NaAlSiO₄ and Ca[AlSi₂O₆]·4H₂O, which facilitates the adsorption of Pb²⁺ by CGPC [37]. Moreover, the kaolinite in CG dissociates into large amounts of Ca₂SiO₄ and Ca₃SiO₅ upon high-temperature sintering when CaO was added as a flux agent [38], which enhances the cementitious activity of coal gangue and stabilizes the Si-O tetrahedral and Al-O octahedral structures, thereby improving the adsorption capacity [39].

3.4. Pore Structure of CGPC

Figure 6a–c show the microstructures of samples 1 (coal gangue + pore-forming agent), 2 (coal gangue + fluxing agent), and 3 (CGPC) at 500 times magnification, respectively. Sample 1, after adding the pore-forming agent, has various sizes of interconnected pore structures, which are formed by the pore-forming agent escaping during high-temperature decomposition. This indicates that the prepared coal gangue-based porous ceramics have a large number of connected pores [40]. Sample 2, under the action of the fluxing agent, has various particles adhered to the surface of the sample, and the particles are bonded together, but the surface is slightly loose and not compact enough. Sample 3, under the joint action of the fluxing and pore-forming agents, has particles adhered to the surface that are better bonded together, which also improves its mechanical properties. Figure 6c is more compactly arranged than Figure 6b, indicating that CG has been fully activated and has high activity. The irregular surface morphology greatly increases the material’s specific surface area, which is beneficial to adsorption. Figure 6d shows the microstructure of the coal gangue-based porous ceramics at 1000 times magnification, which has good porosity, smooth pore channels, and the particles attached to the surface greatly enhance its adsorption capacity.

Figure 7 shows the N₂ adsorption–desorption isotherms and pore size distribution curve of the coal gangue-based porous ceramic. The adsorption isotherm of CGPC belongs to type IV, exhibiting an H₃ hysteresis loop. Type IV isotherms show low initial uptake at low pressures, which increases gradually with pressure until it reaches a maximum, after which it again rises due to capillary condensation in the mesoporous and macroporous regions of the adsorbent [41]. The H₃ hysteresis loop is mainly caused by the irregular structure of particle materials, indicating the irregular pore structure of the coal gangue-based porous ceramic [42], consistent with the microstructure characterization shown in Figure 6d, which indicates that the pore size distribution of the coal gangue-based porous ceramic is mainly dominated by mesopores and macropores.

3.5. Mechanism of Pb²⁺ Removal by CGPC

Through infrared spectroscopy characterization of the coal gangue-based porous ceramics, the adsorption mechanism of Pb²⁺ on the coal gangue-based porous ceramics is further analyzed and explained. By comparing and analyzing the FTIR spectra of CG and CGPC materials (Figure 8), it was found that in CG, the observed bands at 3690 cm⁻¹, 3620 cm⁻¹, and 3445 cm⁻¹, as well as in CGPC at 3445 cm⁻¹, are related to the bending vibration of -OH produced by the combination of water molecules with oxygen atoms on the surface [43], while the band at 1630 cm⁻¹ corresponds to the stretching and flexure vibrations of H-O bonds of adsorbed surface water [44]. The bands at 1099 cm⁻¹ and 470 cm⁻¹ in CG and at 795 cm⁻¹ and 475 cm⁻¹ in CGPC correspond to the characteristic absorption peaks of amorphous SiO₂, while the broadening and weakening of this feature peak in CGPC indicate that the amorphous SiO₂ in CG plays a skeleton role during the synthesis of CGPC [45].

The peak observed at 1435 cm⁻¹ corresponds to the asymmetric stretching of Si-O-Si bonds. The absorption band detected at 2922 cm⁻¹ corresponds to the vibration of CH stretching [46]. The peaks observed at 1123 cm⁻¹ and 1041 cm⁻¹ in CGPC correspond to the asymmetric stretching vibrations of T-O-T (T represents Si or Al) [47]. Additionally, the significant broadening and weakening of the above-mentioned feature peak at 1630 cm⁻¹ in the material after adsorption indicate that Pb²⁺ in the wastewater forms chemical bonds with the oxygen atoms in the coal gangue-based porous ceramics.

Under the action of the pore-forming agent Na₂CO₃, CGPC forms many interconnected pores, and the ion exchange between free Na⁺, Ca²⁺, and Pb²⁺ improves the adsorption performance and promotes the adsorption of lead ions [48]. Furthermore, there is a chemical bond formed between Pb²⁺ and the (-OH) functional groups in CGPC. The hydroxyl (-OH) groups on the surface of CGPC can decompose into -O-, among which -O- acts as an active binding site for Pb²⁺ [49]. The strong affinity between Pb²⁺ and the porous ceramic framework leads to the formation of Pb-O-Si and Pb-O-Al [50], promoting the adsorption of Pb²⁺ and playing a role in “synergistic adsorption”.

3.6. Factors Affecting the Removal of Pb²⁺ by CGPC

Figure 9a shows the change in CGPC adsorption performance under different pH conditions. It was found that the adsorption performance of CGPC is better in weak acid and weak alkali environments. When the pH is too low, H⁺ in the solution competes with Pb²⁺ for adsorption sites [51], and when the pH is too high, a large amount of -OH combines with Pb²⁺ to form a large amount of Pb(OH)₂ precipitate, which blocks the pores and inhibits the adsorption capacity of CGPC itself. Industrial wastewater containing Pb²⁺ is typically around pH 6, at which point the adsorption capacity of CGPC reaches 32.15 mg/g. Figure 9b reflects the changes in CGPC adsorption efficiency and capacity with increasing adsorbent dosage. With the increasing dosage of CGPC, the number of adsorption sites available also increases, resulting in a higher adsorption rate but a diminishing adsorption capacity per unit area of Pb²⁺. Figure 9c shows that CGPC adsorption capacity gradually increases with time, reaching equilibrium after 1080 min, at which point the adsorption efficiency reaches 99.63%. Figure 9d simulates the effect of different concentrations of wastewater and different temperatures on adsorption. As the concentration increases, the adsorption capacity continues to increase, and when the concentration reaches a certain level, the available adsorption sites provided by CGPC are fully occupied, reaching adsorption equilibrium. For the same concentration at different temperatures, it was found that higher temperatures result in better adsorption performance, so increasing the temperature appropriately is beneficial for the reaction to proceed.

3.7. Study on Desorption Performance of CGPC

The cyclic adsorption performance is an important factor in evaluating the material’s environmentally benign and cost-effective [52]. In order to effectively recycle and reuse the coal gangue porous ceramic (CGPC) material after the adsorption of lead ions, thereby maximizing the resource utilization of coal gangue-based porous ceramics, its desorption performance was investigated. A comparative study of hydrochloric acid, water, and sodium hydroxide solutions revealed that the NaOH solution is a better desorbent. The influence of various concentrations of NaOH solution and duration of desorption on the desorption efficiency can be observed in Figure 10a. As shown in the figure, the desorption efficiency is best for a 1 mol/L NaOH solution, as -OH can combine with Pb²⁺ adsorbed by CGPC to achieve degradation, but a concentration that is too high can inhibit detachment. The desorption equilibrium was reached after 240 min, and the adsorption rate stabilized at around 90%. Figure 10b shows the effect of desorption cycles on the desorption efficiency, which decreases as the number of cycles increases. Following seven desorption cycles, the adsorption capacity reached 72%, potentially attributed to the reduction in surface area, blockage of pores, and depletion of available active sites [53], which indicates that CGPC has good regenerative abilities.

4. Conclusions

This article uses coal gangue solid waste as raw materials and applies an activator and flux to prepare coal gangue-based porous ceramics for the adsorption of Pb²⁺ in wastewater. It not only solves the environmental problems caused by the disposal of coal gangue but also reduces the heavy metal lead ions pollution problem in water. The results show that when calcined at 800 °C with 10% Na₂CO₃ pore-forming agent and 8% CaO-MnO₂ combined flux, the prepared coal gangue porous ceramics have the optimal adsorption performance, accompanied by a porosity of 32.91% and a compressive strength of 20.5 MPa. At pH = 6, removal time of 18 h, initial concentration of 200 mg/L, and adsorbent dosage of 4.5 g at 298K, the maximum adsorption amount of Pb²⁺ on the coal gangue porous ceramics was 32.148 mg/g, with a removal rate of 99.63%. The mechanism involves the synergy between chemical adsorption and ion exchange. Using 1 mol/L NaOH as the desorbent, equilibrium was reached after four hours, and even after being repeatedly desorbed seven times, the removal rate of Pb²⁺ could still reach 72%, indicating excellent regenerative properties of the coal gangue-based porous ceramics prepared. Therefore, coal gangue-based porous ceramics are promising as a sorbent to remove heavy metal ions in wastewater.