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Article

Synergistic Chemical Modification and Physical Adsorption for the Efficient Curing of Soluble Phosphorus/Fluorine in Phosphogypsum

by
Junsheng Zhou
1,2,
Yue Yang
1,3,
Huiquan Li
1,2,
Ganyu Zhu
1,* and
Haoqi Yang
1,4,*
1
CAS Key Laboratory of Green Process and Engineering, National Engineering Research Center of Green Recycling for Strategic Metal Resources, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China
2
School of Chemical Engineering, University of Chinese Academy of Sciences, Beijing 100049, China
3
College of Resources and Environmental Engineering, Wuhan University of Science and Technology, Wuhan 430081, China
4
Institute of Technology for Carbon Neutralization, College of Electrical, Energy and Power Engineering, Yangzhou University, Yangzhou 225127, China
*
Authors to whom correspondence should be addressed.
Appl. Sci. 2025, 15(2), 780; https://doi.org/10.3390/app15020780
Submission received: 10 December 2024 / Revised: 7 January 2025 / Accepted: 8 January 2025 / Published: 14 January 2025
(This article belongs to the Special Issue Resource Utilization of Solid Waste and Circular Economy)
Browse Figures
Figure 1
<p>Picture of Hubei Xingfa Chemical Group Co., Ltd.’s PG.</p> "> Figure 2
<p>Comparison of soluble phosphorus and fluorine impurity contents in PG before and after treatment with zeolite powder and CaO.</p> "> Figure 3
<p>Morphological changes in PG under different treatment conditions: (<b>a</b>) untreated PG; (<b>b</b>) PG treated with CaO; (<b>c</b>) PG treated with zeolite powder; (<b>d</b>) PG treated with a combination of zeolite powder and CaO.</p> "> Figure 4
<p>XRD spectra of PG under different treatment conditions.</p> "> Figure 5
<p>FT-IR spectra of PG under different treatment conditions.</p> "> Figure 6
<p>(<b>a</b>) Specific surface area and (<b>b</b>) pore size analyses of PG samples under different treatment conditions.</p> "> Scheme 1
<p>Schematic illustration of physical adsorption and chemical modification method to immobilize soluble phosphorus and fluorine in PG.</p> ">
Versions Notes

Abstract

:
Phosphogypsum (PG), a by-product of phosphoric acid production, contains high levels of fluorine and phosphorus impurities, which negatively impact the strength and setting time of PG-based cement materials and pose environmental risks. This study explores a dual approach combining physical adsorption using zeolite powder and chemical modification with quicklime (CaO) to immobilize these impurities. The composition of 90 wt.% PG, 5 wt.% zeolite powder, and 5 wt.% quicklime reduces the soluble phosphorus to below the detection limits and significantly lowers the free water content in the PG. Through SEM, XRD, and FT-IR analyses, it was found that zeolite powder adsorbs fluorine and phosphorus through encapsulation, while quicklime chemically reacts to form insoluble calcium phosphate and calcium fluoride. This transformation decreases the solubility, mitigating potential environmental contamination. The combination of physical adsorption and chemical conversion provides a sustainable strategy to reduce environmental hazards and enhance PG’s suitability for cement-based materials. The findings from this research offer a promising pathway for the sustainable utilization of PG, providing a mechanism for its safe incorporation into building materials, while addressing both environmental and material performance concerns.

1. Introduction

The wet phosphoric acid (WPA) process is the predominant industrial method for producing phosphoric acid (H3PO4) from phosphate ore using sulfuric acid [1,2,3]. The main chemical reaction can be expressed as:
Ca5F(PO4)3 + 5H2SO4 + nH2O → 3H3PO4 + 5CaSO4·nH2O + HF
Phosphogypsum (PG) is the primary by-product of phosphoric acid production [2,4], with approximately 4.55 tons of PG generated for every ton of P2O5 produced (calculated as P2O5) [5,6]. Globally, the annual production of PG reaches around 300 million metric tons [7]. However, the comprehensive utilization rate of PG remains alarmingly low at only 15%, with the remaining 85% being landfilled, stockpiled, or discarded [8]. This practice not only occupies vast land areas but also poses significant environmental risks [9]. The environmental threat is primarily driven by the presence of soluble phosphates and fluorides, which can leach into soil and water, causing severe ecological damage [10,11]. Consequently, the low utilization rate and environmental hazards highlight the urgent need for effective strategies to stabilize these impurities and enable the sustainable use of PG [12,13].
Various efforts have explored PG’s application in soil conditioning [14], new materials [15,16], and construction [17,18]. While these approaches offer some mitigation, the actual demand for PG in these areas is insufficient to achieve large-scale utilization. Moreover, impurities in PG, such as heavy gold ions [19], fluoride [11,20], phosphoric acid [21], and radioactive elements [7,16,22], are the main reasons limiting its application. Particularly, phosphorus and fluorine impurities directly affect the compressive strength and setting time of PG-based cement materials [23,24,25]. Therefore, the effective stabilization of these harmful substances is key to solving the environmental pollution problems associated with PG and improving its utilization rate. The current PG pretreatment methods, such as water washing, are widely used [25,26,27]. However, these techniques are resource-intensive, requiring large amounts of water and energy, further exacerbating water scarcity issues. Additionally, water washing generates significant wastewater, posing risks of secondary pollution if not adequately treated. Moreover, these methods involve complex processes, high equipment costs, and significant labor requirements, reducing their economic viability [28]. In contrast, dry treatment techniques present several advantages, including resource conservation, cost reductions, and the avoidance of wastewater generation [29]. These methods also simplify the operational process, lowering the equipment and labor costs. Given their efficiency, environmental compatibility, and economic feasibility, dry reaction methods show significant potential for large-scale PG treatment, offering a sustainable and cost-effective solution for the utilization of this industrial by-product.
Phosphorus and fluorine impurities in PG can be effectively stabilized through advanced curing technologies [29,30,31]. These techniques immobilize impurities within specific crystalline structures or chemical compounds [32,33], significantly reducing their mobility and solubility. As a result, curing treatments enhance the environmental safety and utilization potential of PG. Curing technology has been recognized as an efficient, rapid, and cost-effective method for managing phosphorus and fluorine impurities [34]. By encapsulating these impurities in dense matrices or converting them into stable, insoluble compounds, the risk of leaching is substantially mitigated. Recent advancements have shown that incorporating specific materials can significantly improve the curing performance [35,36,37]. For instance, Al2O3 demonstrates exceptional adsorption capacity for phosphorus and fluorine due to its large specific surface area and efficient mass transfer properties [38]. Similarly, integrating polymeric aluminum chloride (PAC) into PG binder formulations has shown promising results in enhancing the fluoride retention through high adsorption capacity and encapsulation efficiency [39]. While these studies demonstrate that the leaching rate of fluorine and phosphorus can be effectively reduced by adding binders to PG, it is important to note that the current methods require significant quantities of other materials, such as fly ash [40], red clay [41], and lime [42], to prepare PG-based cement materials. This substantially reduces the PG consumption, increases process costs, and limits the environmental compatibility. Therefore, identifying suitable curing materials for fluorine and phosphorus is a critical scientific challenge in unlocking the full resource potential of PG and advancing its large-scale utilization.
Zeolite has emerged as a widely utilized material for pollutant treatment, owing to its unique nanoscale pore structure and excellent adsorption properties [43,44,45]. Its large specific surface area and well-developed pore network provide a promising platform for the solidification of phosphorus and fluorine impurities, effectively reducing their mobility and environmental risks. In addition to zeolite, inexpensive and readily available curing agents such as quicklime (CaO) and slaked lime (Ca(OH)2) have been identified as key materials for stabilizing phosphorus and fluorine impurities. These agents react with phosphorus and fluorine impurities to form insoluble composite precipitates, thereby achieving effective immobilization [46,47,48]. The synergistic use of zeolite and lime-based additives offers an efficient approach to addressing the challenges associated with impurity leaching in PG. By leveraging their complementary properties, these materials hold significant potential for scalable, cost-effective, and environmentally friendly applications in PG treatment and resource recovery.
In this study, quicklime and zeolite powder were selected as curing agents to address the challenges of stabilizing soluble phosphorus and fluorine impurities in PG. This selection was guided by their complementary properties; zeolite powder offers exceptional physical adsorption capabilities due to its nanoscale pore structure and large specific surface area, while quicklime is highly reactive and facilitates chemical modification through the formation of insoluble compounds. By examining the individual and combined roles of quicklime and zeolite powder, this study provides critical insights into the optimization of curing conditions. To deepen our understanding, the curing mechanisms of quicklime and zeolite powder were thoroughly analyzed using advanced characterization techniques. These analyses revealed the specific pathways through which physical adsorption and chemical reactions contribute to impurity stabilization. The findings of this study provide a theoretical foundation and technical guidance for optimizing PG treatment processes. They highlight the potential of integrating physical and chemical methods to develop economical, efficient, and environmentally friendly strategies for large-scale impurity stabilization and PG resource utilization.

2. Materials and Methods

2.1. Materials

The PG used in this study was sourced from Hubei Xingfa Chemical Group Co., Ltd., Yichang City, Hubei Province, China. The material appeared off-white in color (Figure 1), indicative of its typical composition and purity. The primary chemical composition of PG, as analyzed and detailed in Table 1, includes calcium sulfate dihydrate (CaSO4·2H2O) as the dominant phase, along with impurities such as soluble phosphorus and fluorine compounds. These impurities were key targets for stabilization in this study due to their environmental mobility and associated risks.
The zeolite powder, utilized as a key curing agent, was procured from Chaoyang Xinhe Zeolite Science and Technology Co., Ltd., Chaoyang, China. For experimental consistency and enhanced adsorption performance, 100–120 mesh zeolite powder was selected. This particle size was chosen to maximize the material’s surface area and facilitate efficient interaction with impurities. The chemical composition of the zeolite powder, listed in Table 2, highlights its significant content of aluminosilicate minerals, which are critical for adsorption and encapsulation mechanisms. The well-developed porous structure and high cation exchange capacity of the zeolite contribute to its effectiveness in stabilizing impurities. All other chemicals used in the experiments, including the quicklime and analytical-grade reagents, were sourced from certified suppliers to ensure the reliability and reproducibility of results.

2.2. Experimental Methods

In this study, several experimental and control groups were established to investigate the effects of different material combinations on the stabilization of phosphorus and fluorine impurities in PG (Scheme 1). Samples of PG, zeolite powder, and quicklime were mixed in a mass ratio of 90:5:5 and subjected to thorough grinding using a mortar and pestle to ensure uniformity in the particle size distribution. After grinding, the samples were placed in an oven at 45 °C for 24 h to facilitate the curing process, forming what was designated as experimental group-1. This experimental setup aimed to evaluate the combined effect of zeolite powder and quicklime on the stabilization of impurities in PG. As part of the study, additional sample combinations were prepared for comparison purposes. In control group-1, the PG and zeolite powder were mixed in a 95:5 mass ratio, ground thoroughly, and then placed in the oven under the same conditions as experimental group-1. This group was designed to isolate the effects of zeolite on impurity stabilization, without the influence of quicklime. In control group-2, a mixture of PG and quicklime in a 95:5 mass ratio was prepared and ground, and then subjected to the same curing conditions. This group aimed to assess the impact of quicklime alone on the stabilization of the phosphorus and fluorine impurities. Finally, control group-3 consisted solely of PG, which was ground and placed in the oven without the addition of any curing agents. This group served as the baseline for evaluating the natural behavior of PG under the same temperature conditions.

2.3. Methods of Analysis

2.3.1. Chemical Composition and Mineral Phase Analysis of Materials

The elemental composition of the PG was analyzed using an X-ray fluorescence spectrometer (XRF) with an AXIOS-MAX instrument (PANalytical B.V., Tokyo, Japan). This non-destructive technique allows for the quantitative analysis of the elemental composition of PG, providing detailed information on the concentrations of key elements such as calcium (Ca), sulfur (S), phosphorus (P), fluorine (F), and other trace elements. The mineralogical composition of the PG was determined using an X-ray diffractometer (XRD), employing an Empyrean diffractometer from PAN-alytical, Tokyo, Japan. The scanning range of 2θ was set between 5° and 90°, with a scanning speed of 5°/min. This mineralogical analysis provides essential insights into the dominant crystalline phases present in PG, such as calcium sulfate dihydrate (CaSO4·2H2O), and helps identify other phases that may influence the material’s stability, reactivity, and potential for reuse in various applications.

2.3.2. Micro-Morphological Analysis

The microstructural characteristics of the PG were investigated using a scanning electron microscope (SEM) with a thermal field emission electron microscope (JSM-7610F, JEOL, Tokyo, Japan). A secondary electron image (SEI) detector was used to capture high-quality images of the PG’s surfaces. The SEM analysis was conducted at an accelerating voltage of 15 kV and a working distance of 10.0 mm to optimize the image resolution and depth of focus. The images obtained through the SEM provide valuable information about the texture, porosity, and surface features of the PG particles, which are crucial for understanding the material’s reactivity and its interactions with curing agents such as zeolite powder and quicklime.

2.3.3. Infrared Spectroscopy Analysis

Changes in the reactive functional groups in PG, particularly those involved in the stabilization of phosphorus and fluorine impurities, were analyzed using Fourier transform infrared (FTIR) spectroscopy, performed with a Nicolet iS50 spectrometer (Thermo Fisher Scientific, Waltham, MA, USA). The FTIR spectra were collected in the range of 4000 to 500 cm−1, and changes in peak intensities or shifts in absorption bands were carefully analyzed to assess the impact of curing treatments on the chemical structure of the PG.

2.3.4. Particle Size and Surface Area Analysis

The specific surface area of the PG was determined using the Brunner–Emmett–Taylor (BET) method, employing a Micromeritics ASAP 2020 PLUS HD88 analyzer (ASAP2460, Micromeritics, Norcross, GA, USA). The BET method is based on nitrogen adsorption, and the surface area is calculated from the amount of nitrogen gas adsorbed at different relative pressures (P/P0 = 0.01 to 0.95). The BET analysis provides critical data on the surface area available for adsorption, which is a key factor in evaluating the efficiency of curing agents such as zeolite powder in immobilizing phosphorus and fluorine impurities. Additionally, the pore size distribution of the PG was obtained using the Barret–Joyner–Halenda (BJH) method, which is based on nitrogen desorption data from the isotherm. These data are critical for understanding how the porosity of PG influences the adsorption of curing agents and the immobilization of impurities. By analyzing both the surface area and pore size distribution, a more comprehensive understanding of the physical properties can be gained, informing the design of more efficient treatment strategies.

2.3.5. Specimen Preparation

We weighed 5 g of test material, accurate to 0.0001 g, placed this into the mortar, added 50 mL of water, and ground this in the mortar for 15 min. After, grinding all specimens were transferred to a 250 mL volumetric flask to be fixed, and after 30 min of resting were filtered with a medium-speed qualitative filter paper for the determination of the water-soluble phosphorus pentoxide (P2O5) and water-soluble fluorine (F−)

2.3.6. Analysis of Water-Soluble Phosphorus Content

We accurately withdrew 50 mL of the solution prepared in step 2.3.5, placed it into a 300 mL beaker, added 10 mL of nitric acid solution (V(H2O): V(HNO3) = 1:1), and diluted it with water to 100 mL. At the same time, we performed a blank test (the blank test was the same as the above except that no reagent solution was added), covered the surface dish, and heated it up to boiling on the electric stove, then took it off and used a small amount of water to rinse the surface dish and the wall of the glass. Under constant stirring, we added 30 mL of quinhydrone solution, then continued to heat it gently to a slight boil for 1 min. We removed the beaker, stirring 3~4 times during the cooling process, and left it to precipitate.
A filter was drawn using a glass crucible pre-dried to a constant weight in a constant temperature oven at 180 °C. The upper layer of clear liquid was first filtered, and then the precipitate was washed using the pouring method 1~2 times (each with about 25 mL of water). The precipitate was then transferred to the crucible and washed with water 5~6 times.
The water at the bottom of the crucible was sucked up with filter paper and then placed in a constant-temperature drying oven at 180 °C, dried to a constant weight, placed in a desiccator and cooled for 30 min, and weighed accurately to 0.0001 g.
Calculation of water-soluble phosphorus content:
ω 1 = m 14 − m 13 − m 16 − m 15 × 0.03207 m 12 1 − ω 50 250
Here, ω indicates that the mass fraction of attached water was measured for the same sample; m12 = the mass of the specimen in grams (g); m13 = the mass of the crucible in grams (g); m14 the mass of the quinone phosphomolybdate precipitate and crucible in grams (g); m15 the mass of the crucible used for the blank test in grams (g); m16 the mass of the blank test precipitate and crucible in grams (g).
The test results were taken as the arithmetic mean of two parallel tests, accurate to 0.01%.

2.3.7. Analysis of Water-Soluble Fluoride Content

We accurately withdrew 10 mL of the solution prepared in Section 2.3.5, added 5 drops of citric acid–sodium citrate buffer (pH: 5.5~6) and two drops of bromocresol green indicator, neutralized the solution with sodium hydroxide solution (200 g/L) until the solution turned blue, and then adjusted the solution with nitric acid solution (V(H2O):V(HNO3) = 1:5) until it was precisely yellow. We then added 20 mL of citric acid–sodium citrate buffer solution, diluted it with water to scale, shook it well, and poured it into a dry 50 mL beaker. Next, we inserted the fluoride-ion-selective electrode and the saturated calomel electrode, turned on the magnetic stirrer and stirred it at a constant speed for 2 min, stopped for 30 s, measured the potential value at equilibrium, and identified the mass of the fluorine (F−) on the working curve c4.
Calculation of water-soluble fluoride content:
ω 2 = 2.5 × c 4 m 12 1 − ω
Here, ω indicates that the mass fraction of the attached water was measured for the same sample; m12 = the mass of the specimen in grams (g).
The test results were taken as the arithmetic mean of two parallel tests, accurate to 0.01%.

3. Results

3.1. Curing Performance of Zeolite Powder Synergized with CaO on Pollutants in PG

As shown in Figure 2, the raw PG material used in this study contained approximately 0.3894 wt.% soluble phosphorus impurities. This high concentration of soluble phosphorus is problematic, as it can negatively impact the properties of PG-based products, such as the strength and durability, and poses a potential environmental risk due to leaching. The use of zeolite powder as a physical adsorbent for soluble phosphorus impurities in PG has proven to be highly effective. Zeolite, with its well-developed nanoscale pore structure and large specific surface area, adsorbs soluble phosphorus, significantly reducing its concentration. Following the treatment with zeolite powder, the soluble phosphorus content in the PG decreased to 0.3432 wt.%. A further improvement in the removal of soluble phosphorus impurities was achieved through the chemical modification of PG with CaO. When the PG was treated with both zeolite powder and CaO, the soluble phosphorus content was reduced to below the detection limit, demonstrating the combined effectiveness of physical adsorption and chemical modification.
In addition to phosphorus, PG also contains a considerable amount of soluble fluorine impurities, with an initial concentration of approximately 0.3659 wt.%. Fluorine compounds, particularly soluble fluoride, are known to be environmentally hazardous and can lead to soil and water contamination if not properly treated. Zeolite powder was again employed to adsorb the soluble fluorine impurities present in PG, achieving a reduction in fluorine content to 0.2857 wt.%. Although this represents a reduction in fluorine concentration, the removal efficiency was not as high as that achieved for phosphorus. To further reduce the fluorine content, the PG was subjected to chemical modification with CaO. The application of CaO alone reduced the soluble fluorine content to 0.0365 wt.%. When zeolite powder and CaO were simultaneously used in a synergistic treatment process, both physical adsorption and chemical modification contributed to a more substantial reduction in soluble fluorine content. This combined treatment reduced the fluorine concentration to 0.0361 wt.%, a significant improvement over either treatment applied individually. These results demonstrate that the physical adsorption of pollutants, when coupled with chemical modification, offers a highly effective method for reducing both soluble phosphorus and fluorine impurities in PG. The combination of zeolite powder and CaO treatment provides a robust, synergistic approach to improving the environmental safety and resource potential of PG.

3.2. Mechanism of Zeolite Powder Synergized with CaO for Immobilization of Pollutants in PG

The original PG sample exhibited a characteristic tetragonal rhombic structure (Figure 3a), which is typical of PG. This structure is formed during the production process when PG crystals stack in horizontal layers. While this arrangement imparts some structural integrity to the material, it significantly restricts the flow of interstitial liquids. This limitation hinders the efficient removal of soluble impurities, such as phosphorus and fluorine, through traditional methods such as water washing. Furthermore, PG contains a large number of free hydroxyl groups on its surface, which enhance its reactivity but also promote the agglomeration of particles, especially in the presence of high impurity concentrations. This agglomeration results in a more heterogeneous material, where particles tend to cluster together, leading to an uneven distribution of the impurities throughout the sample. Our microscopic observation of the PG structure revealed that it predominantly consisted of plate-like crystals, which form a spatial network through weak physical interactions, such as van der Waals forces, at their contact points. The weak intercrystal bonding and low compaction of PG contribute to its high porosity, which allows for fluid circulation and pollutant retention. However, this also makes PG susceptible to stress concentration when subjected to external forces, leading to material fracture and reduced mechanical strength. The low compaction and large pore size further contribute to poor water resistance and make it prone to rapid erosion and degradation, especially under wet conditions.
CaO has been demonstrated to effectively react with soluble phosphorus and fluorine impurities in PG, converting them into insoluble phosphorus and fluorine precipitates. This chemical reaction occurs during the solid-phase milling process, where it interacts with the impurities, facilitating the formation of stable compounds such as calcium phosphate (Ca3(PO4)2) and calcium fluoride (CaF2). However, while this chemical modification is effective in reducing the solubility of these pollutants, it does not significantly alter the overall microscopic morphology of PG (Figure 3b). After treatment, the PG particles largely retain their characteristic rhombohedral plate-like shape. Some fine particles, likely consisting of the newly formed insoluble precipitates, can be observed to be dispersed around the edges of the PG crystals but the core structure remains largely unchanged. In contrast, when PG is treated with zeolite powder, a noticeable change in the microstructure can be observed. Zeolite particles appear to surround the PG crystals, as the zeolite physically adsorbs soluble impurities, such as phosphorus and fluorine, from the surface of the PG particles (Figure 3c). When both zeolite powder and CaO are used in combination to treat PG, a more complex morphological transformation occurs (Figure 3d). The crystals of PG exhibit fine cracks, suggesting that both the physical adsorption by the zeolite and the chemical reaction with CaO contribute synergistically to the immobilization of soluble impurities. This dual approach of physical adsorption coupled with chemical modification not only reduces the mobility of pollutants but also alters the microstructure of the PG in a way that enhances its environmental stability and suitability for further applications.
The treatment of PG using zeolite powder and CaO does not significantly alter the overall crystal structure of the PG, as confirmed by our XRD analysis (Figure 4). The XRD patterns of the treated PG samples closely resemble those of the untreated PG, indicating that the primary crystalline phases remain intact after treatment. This observation is consistent with our SEM imaging, which showed no drastic changes in the morphology of the PG particles upon treatment. In the XRD spectra, several characteristic peaks were identified. The peaks at 11.52°, 20.64°, 23.43°, 31.15°, 47.86°, and 56.81° correspond to the calcium sulfate dihydrate (CaSO4·2H2O) phase, which is the main crystalline component of PG. The presence of additional peaks at 29.34°, 31.14°, 40.23°, and 50.55° corresponds to the anhydrite phase of calcium sulfate (CaSO4·0.5H2O), which is also a significant constituent of PG. Furthermore, the peaks at 26.65° and in the range of 32°~37° are attributed to the presence of silicon dioxide (SiO2) crystals, Additionally, the XRD pattern revealed some minor peaks between 60° and 90°, indicating the presence of trace impurities such as calcium fluorophosphate (Ca5F(PO4)3) and sodium hexafluorosilicate (Na2SiF6). These impurities, although present in small amounts, may contribute to the environmental concerns associated with PG. Despite the treatment with zeolite powder and CaO, these impurities remained detectable, suggesting that the treatment process primarily focuses on reducing the solubility and mobility of soluble pollutants, while the overall crystalline framework of the PG remains largely unaltered.
As illustrated in Figure 5, FT-IR spectroscopy was employed to analyze the functional groups and molecular interactions within the PG samples before and after treatment. In the FT-IR spectra of the untreated PG, the absorption peaks observed at 597 cm−1 and 665 cm−1 are attributed to the asymmetric bending vibrations of sulfate ions (SO42−), which are a prominent feature of PG due to its calcium sulfate content. Additionally, the absorption peak at 1096 cm−1 corresponds to the asymmetric stretching vibration of SO42−, further confirming the presence of calcium sulfate and its characteristic functional groups [49,50]. The two absorption bands at 1620 and 1682 cm−1 are indicative of the symmetric stretching vibrations of hydroxyl groups (OH−) in free water molecules, which are present in PG as part of its hydrated structure [51]. These peaks suggest the presence of adsorbed water in the crystalline structure of PG, which is often linked to the crystallization water in CaSO4·2H2O. However, when zeolite powder and CaO were used in combination to treat PG, noticeable changes in the FT-IR spectra were observed. Specifically, the intensity of the broad absorption peak at 3392 cm−1, which corresponds to the stretching vibration of hydroxyl groups in free water, was significantly weakened. This suggests that the treatment effectively reduced the amount of free water in the PG structure. The decrease in the intensity of this absorption band is attributed to the removal or immobilization of free water molecules during the physical adsorption by zeolite and the chemical reaction with CaO. The reduction in free water content highlights the successful interaction between the treatment agents and the PG, leading to a more stable material with less water retention, which could enhance the material’s mechanical properties and reduce its environmental impact.
Following the treatment of PG with zeolite powder and CaO, the nitrogen adsorption–desorption isotherms of all treated samples showed typical IV-type isotherms, characteristic of mesoporous materials (Figure 6a). The corresponding pore size distributions were primarily in the range of 8–12 nm (Figure 6b), indicating the presence of mesopores in the PG samples. This behavior suggests that the treatment methods, including physical adsorption and chemical modification, did not significantly alter the mesoporous nature of the PG but rather enhanced its adsorption properties without substantially changing its overall pore structure. The specific surface area of the untreated PG was measured at 21.63 m2 g−1, with an average pore size of 11.54 nm, reflecting the inherent characteristics of the PG. After treatment with zeolite powder, the specific surface area slightly increased to 22.02 m2 g−1, accompanied by a small reduction in the average pore size to 10.81 nm. In contrast, the treatment with CaO resulted in a minor decrease in the specific surface area to 20.94 m2 g−1, with an increase in the average pore size to 12.25 nm. This may suggest that the CaO treatment had a slight effect on the structure of the PG, possibly due to the formation of insoluble precipitates that did not significantly block or alter the mesopores. When both zeolite powder and CaO were applied synergistically, the specific surface area was 20.84 m2 g−1, with an average pore size of 8.71 nm. Although there was a slight decrease in specific surface area compared to the untreated PG, the combined effect of zeolite and CaO did not dramatically alter the pore size distribution or surface area, indicating that the physical adsorption and chemical modification strategies employed had a minimal impact on the fundamental textural properties of the PG. Overall, these results suggest that while the treatment methods effectively reduce the soluble impurities in PG, they do not significantly affect the material’s mesoporosity, preserving its potential for further applications.

4. Discussion

The results of this study are consistent with previous research on phosphogypsum PG treatments, demonstrating the effectiveness of zeolite powder and CaO in reducing soluble phosphorus and fluorine impurities. The observed reductions in impurity concentrations support the hypothesis that physical adsorption and chemical modification can work synergistically to mitigate the environmental risks associated with PG. However, the findings also highlight areas requiring further investigation. For instance, while the study successfully reduced the impurity concentrations, it did not assess the long-term stability of these reductions under variable environmental conditions. Additionally, the minimal impact on the PG’s structural properties suggests that although the treatment preserves its porosity, further studies are needed to evaluate how these changes might affect the mechanical strength of PG, particularly in construction or soil applications. PG-based cementitious materials are becoming increasingly important for large-scale PG utilization. This technology typically involves using PG as an aggregate, mixed with cementitious materials and water to prepare a paste, which is then used for backfilling in underground mines or open pits. However, phosphorus and fluoride can retard the hardening of PG and weaken its structure. The soluble phosphorus also significantly reduces the hydration rate. Fluoride, in particular, impacts the strength development by interfering with the pore structure and formation of hydration products, which results in the need for increased amounts of cementitious materials to compensate for the strength loss, thereby raising costs. Future research should focus on optimizing the treatment process for large-scale applications, evaluating the long-term leaching behavior of treated PG, and assessing the economic viability and environmental impact of using zeolite and CaO on an industrial scale. Furthermore, exploring alternative, more sustainable treatment agents could further enhance the effectiveness and cost-efficiency of PG resource recovery efforts.

5. Conclusions

In this paper, we have presented a novel approach combining physical adsorption with chemical modification, demonstrating its effectiveness in reducing soluble impurities in phosphogypsum (PG), particularly soluble phosphorus and fluorine. By adding only 5 wt.% zeolite powder and 5 wt.% CaO, the soluble phosphorus content in the PG was reduced to below the detection limit. This treatment not only immobilized the fluorine and phosphorus but also enhanced the compressive strength and setting time of the PG-based cementitious materials, making it one of the most viable options for large-scale PG remediation. Importantly, the treatment did not significantly alter the crystal structure or micromorphology of the PG, preserving its potential for direct use in the production of α-hemihydrate gypsum and maintaining its suitability for applications such as building materials. Additionally, the inclusion of CaO reduced the free water content in the PG, improving its handling and storage properties, which is beneficial for subsequent processing and utilization. Our solid-phase treatment method avoids complications such as wastewater disposal, further simplifying the process. Moreover, the proposed curing technology is cost-effective, easy to operate, and well-suited for large-scale industrial applications. These findings underscore the environmental benefits of this approach and open new pathways for the sustainable utilization of PG across various industries. Our future research will focus on optimizing the process for large-scale operations and assessing the long-term performance of treated PG under real-world conditions.

Author Contributions

Conceptualization, G.Z. and H.Y.; methodology, J.Z. and Y.Y.; validation, J.Z., Y.Y. and H.Y.; investigation, J.Z.; data curation, J.Z. and Y.Y.; writing—original draft preparation, J.Z.; writing—review and editing, H.L. and H.Y.; supervision, G.Z.; project administration, H.Y.; funding acquisition, G.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This study was funded by the 14th Five-Year National Key Research and Development Plan Project (Grant No. 2022YFC3902704) and National Natural Science Foundation of China (22078344).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Acknowledgments

We would like to express our heartfelt gratitude to the National Engineering Research Center of Green Recycling for Strategic Metal Resources for providing the experimental instruments and workstations, as well as to Fade Wu and Ziheng Meng for their contributions to the experimental research and discussion.

Conflicts of Interest

The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of the data; in the writing of the manuscript; or in the decision to publish the result.

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Applsci 15 00780 g001
Figure 1. Picture of Hubei Xingfa Chemical Group Co., Ltd.’s PG.
Figure 1. Picture of Hubei Xingfa Chemical Group Co., Ltd.’s PG.
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Scheme 1. Schematic illustration of physical adsorption and chemical modification method to immobilize soluble phosphorus and fluorine in PG.
Scheme 1. Schematic illustration of physical adsorption and chemical modification method to immobilize soluble phosphorus and fluorine in PG.
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Figure 2. Comparison of soluble phosphorus and fluorine impurity contents in PG before and after treatment with zeolite powder and CaO.
Figure 2. Comparison of soluble phosphorus and fluorine impurity contents in PG before and after treatment with zeolite powder and CaO.
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Figure 3. Morphological changes in PG under different treatment conditions: (a) untreated PG; (b) PG treated with CaO; (c) PG treated with zeolite powder; (d) PG treated with a combination of zeolite powder and CaO.
Figure 3. Morphological changes in PG under different treatment conditions: (a) untreated PG; (b) PG treated with CaO; (c) PG treated with zeolite powder; (d) PG treated with a combination of zeolite powder and CaO.
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Figure 4. XRD spectra of PG under different treatment conditions.
Figure 4. XRD spectra of PG under different treatment conditions.
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Figure 5. FT-IR spectra of PG under different treatment conditions.
Figure 5. FT-IR spectra of PG under different treatment conditions.
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Figure 6. (a) Specific surface area and (b) pore size analyses of PG samples under different treatment conditions.
Figure 6. (a) Specific surface area and (b) pore size analyses of PG samples under different treatment conditions.
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Table 1. Analysis of major chemical constituents in PG raw materials (wt.%).
Table 1. Analysis of major chemical constituents in PG raw materials (wt.%).
SO3CaOSiO2P2O5FAl2O3K2OFe2O3
43.320736.25311.79750.59320.40280.22850.13150.1244
Table 2. Analysis of the main chemical composition of zeolite powder raw material (wt.%).
Table 2. Analysis of the main chemical composition of zeolite powder raw material (wt.%).
SiO2Al2O3CaOK2OFe2O3MgONa2OTiO2
57.245010.82092.79422.43371.86610.84540.41640.1378
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Zhou, J.; Yang, Y.; Li, H.; Zhu, G.; Yang, H. Synergistic Chemical Modification and Physical Adsorption for the Efficient Curing of Soluble Phosphorus/Fluorine in Phosphogypsum. Appl. Sci. 2025, 15, 780. https://doi.org/10.3390/app15020780

AMA Style

Zhou J, Yang Y, Li H, Zhu G, Yang H. Synergistic Chemical Modification and Physical Adsorption for the Efficient Curing of Soluble Phosphorus/Fluorine in Phosphogypsum. Applied Sciences. 2025; 15(2):780. https://doi.org/10.3390/app15020780

Chicago/Turabian Style

Zhou, Junsheng, Yue Yang, Huiquan Li, Ganyu Zhu, and Haoqi Yang. 2025. "Synergistic Chemical Modification and Physical Adsorption for the Efficient Curing of Soluble Phosphorus/Fluorine in Phosphogypsum" Applied Sciences 15, no. 2: 780. https://doi.org/10.3390/app15020780

APA Style

Zhou, J., Yang, Y., Li, H., Zhu, G., & Yang, H. (2025). Synergistic Chemical Modification and Physical Adsorption for the Efficient Curing of Soluble Phosphorus/Fluorine in Phosphogypsum. Applied Sciences, 15(2), 780. https://doi.org/10.3390/app15020780

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