Ultrasonic-enhanced phosphorus sludge excited O₃ coupled red mud simultaneous desulfurization and denitrification

2.1 Preparation of materials and analysis system

The RM (Sample picture as shown in Fig. S3 (b)) utilized in this study was sourced from an alumina plant in Wenshan, Yunnan Province. The RM sample underwent drying at 80 °C for 48 h and was ground to 200 mesh (particle size < 74 μm). The PS (Sample picture as shown in Fig. S3 (a)), a hazardous solid waste, was produced during the production of yellow phosphorus at a phosphorus chemical enterprise in Qujing City, Yunnan Province. Typically, PS was placed in Ultrapure water to prevent spontaneous combustion. During the experimental process, the PS was initially used to absorb excess water with filter paper. Throughout the experiment, PS was first absorbed with filter paper to remove excess water and weighed to dry weight for the experiment.

The experimental system comprises four parts: simulated flue gas preparation system, O₃ generation system, desulfurization and denitrification system, and analysis system. The schematic diagram of the experimental setup is depicted in Fig. 1. The experimental device of ultrasonic-enhanced phosphorous sludge excitation is shown in Fig. S4 in Supporting Information (SI). In the simulated flue gas preparation system, N₂ (99.999 %) and O₂ (99.999 %) were procured from Kunming Stoneman Gas Products Co., Ltd; SO₂ (1.75 %) and NO_X (4997.4 mg/Nm³) were obtained from Kunming Guangreda Gas Products Co., dispersed in a gas mixing tank to meet the experimental requirements (C_SO2 = 2000 ppm, C_NOX = 250 ppm, flue gas flow rate 400 mL/min). In the O₃ generation system, a certain amount of PS was weighed and placed in a four-necked flask containing 200 mL of Ultrapure water and then introduced into an ultrasonic reactor (JLT 400, Shenzhen Jielite Co., Ltd.) for preheating. Simultaneously, anhydrous air was inputted into the four-necked flasks (400 mL/min) to ensure complete contact between the air and the PS slurry, after which the O₃-containing gases were condensed and inputted to the subsequent steps of the reaction. In the desulfurization and denitrification system, a certain amount of RM was weighed, placed in a four-necked flask containing 200 mL of Ultrapure water, and introduced into an ultrasonic reactor. The simulated flue gas was inputted into the four-necked flask with the O₃-containing gas, which fully reacted with the RM absorbing solution and then transported to the flue gas analysis system. In the analysis system, the inlet and outlet flue gas concentrations were measured using a flue gas analyzer (Ecom-J2KN, RBR Co., Ltd., Germany), the O₃ concentration was detected using an ozone detector (GT-1000 Shenzhen Corno Electronic Technology Co., Ltd.), and the pH of the slurry before and after the reaction was tested using a portable pH meter (Ramag PHS-3C, INESA Scientific Instruments Shanghai Co. Ltd.) for testing. A detailed description of the characterization methodology employed in this study and the associated description of the response surface model used can be found in the SI. A comprehensive description of the characterization methods used in this study and a related description of the RSM model used can be found in SI.

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Fig. 1

Experimental setup diagram: (1) anhydrous air bottle; (2) ultrasonic reactor; (3) O₃ detector; (4) PS slurry bottle; (5) mechanical stirrer; (6) pH meter; (7) RM slurry bottle; (8) flue gas analyser; (9) tail gas absorption bottle; (10) gas mixing tank; (11) gas mass flow meter; (12) N₂ gas cylinder; (13) O₂ gas cylinder; (14) SO₂ gas cylinder; (15) NO_X gas cylinder.

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2.2 Desulfurization and denitrification calculation

The efficiency of desulfurization and denitrification is calculated based on the inlet and outlet flue gas concentrations and is used as a standard for evaluating the effectiveness of desulfurization and denitrification. The specific calculation formula is shown in Eqs. (1) and (2) (Kuropka, 2008):

2.3 Materials characterization

The detailed characterization procedures of all the sample are described in the SI file.

2.4 Response surface methodology (RSM)

The experimental response surface methodology is described in the SI file.

3.1 Study of ultrasonic-enhanced phosphorus sludge excited O₃

The concentration of O₃ in the reaction system is crucial for NO oxidation in NO_X. The flue gas with a NO_X concentration of 250 ppm, in which the NO concentration was approximately 200–220 ppm, was selected as the subject of the study. For gases containing both SO₂ and NO, the O₃ passed through preferentially oxidized NO (Li et al., 2020; Wang et al., 2020), but not SO₂. Therefore, to prevent environmental pollution caused by O₃ overflow during subsequent experiments, the molar ratio of O₃ to NO during the reaction process was controlled between 0.8:1–1:1 using the single factor experimental method combined with the RSM experimental design method in this study.

Fig. 2 (a–b) illustrates the effect of ultrasonic frequency on the concentration of O₃ generation. In this section, the experimental conditions are set to: PS concentration: 40 g/L; ultrasonic frequency: 0, 28, 40 kHz; US power density: 0.4 W/cm²; Stirring intensity: 1000 rpm; Reaction temperature: 50 °C; Air flow: 400 mL/min. Ultrasonic significantly enhances the increase of O₃ concentration generated by PS, primarily due to the ultrasonic cavitation and dispersion effects. Ultrasonic waves can increase the specific surface area of PS reactants and the gas–liquid contact area. Simultaneously, the ultrasonic can break down the microscopic agglomerate structure of PS so that the P₄ lurking in the PS aggregate can fully contact and react with O₂ in the air, improving the utilization rate of the P element in PS (Li et al., 2021). The cavitation effect caused by the ultrasonic can enhance the mass transfer process of chemical reactions on one hand. On the other hand, it can enable the tiny bubbles in the slurry and the medium (such as H₂O) at the liquid interface to be heated and decomposed into potent oxidizing free radicals (such as OH·), thus promoting the reaction process between P₄ in PS and O₂ in the air. The main reactions occurring in the ultrasonic-enhanced O₃ generation process in this study are shown in Eqs. (3)–(6) (Li et al., 2021; Liu et al., 2014; Liu et al., 2018; Rooze, 2013):

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Fig. 2

The O₃ over time concentration and the total concentration under different conditions of ultrasonic frequency (a–b), Reaction temperature (c–d), PS concentration (e–f) and ultrasonic power density (g–h).

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As depicted in Fig. 2(a–b), the utilization of low-frequency ultrasonic (28 kHz) has a more pronounced effect on enhancing the concentration of O₃ during the reaction process. However, the O₃ concentration does not increase with the rise in the ultrasonic frequency. This is attributed to the fact that at the same ultrasonic power, the ultrasonic cavitation threshold is proportional to the ultrasonic frequency, and an increase in frequency necessitates more power to initiate the cavitation effect, resulting in a decrease in the cavitation intensity. Furthermore, the increase in ultrasonic frequency will also diminish the time required for the expansion and rupture of tiny bubbles in the PS slurry, thus reducing the emulsification and dispersion of ultrasonic to the slurry (Du and Chen, 2021; Wang et al., 2023).

The increase in the reaction temperature is favorable to enhance the chemical reaction and mass transfer process, which escalates the chemical reaction rate and directs the reaction towards O₃ production, thus increasing the concentration of O₃ (Sahu et al., 2019). Fig. 2(c–d) illustrates that when the reaction temperature is 30–50 °C, the O₃ concentration increases with the temperature rise. In this section, the experimental conditions are set to: PS concentration: 40 g/L; ultrasonic frequency: 28 kHz; ultrasonic power density: 0.4 W/cm²; Stirring intensity: 1000 rpm; Reaction temperature: 30, 40, 50, 60, 70 °C; Air flow: 400 mL/min. However, when the reaction temperature escalates to 60–70 °C, the concentration of the O₃ gradually decreases. This phenomenon can be explained by the following two aspects: (1) The reaction temperature significantly influences the decomposition rate of O₃ in the PS slurry. The lower the temperature, the slower the decomposition rate of O₃, and the most stable O₃ is when the reaction temperature is close to 0 °C (Li et al., 2020). When the temperature rises to 60 °C or more, accompanied by the ultrasonic cavitation effect on the interior of the slurry resulting in the enhancement of the local thermal effect, the decomposition of O₃ accelerates, and continuing to raise the temperature will further increase the decomposition rate of O₃. (2) In the process of ultrasonic emulsification, an appropriate increase in the temperature of the reaction system will reduce the surface tension and viscosity of the PS slurry so that the substances in the PS slurry are more evenly dispersed. However, the increasing reaction temperature will also impede the emulsification effect. The increase in reaction temperature will elevate the air pressure inside the tiny bubbles in the PS slurry, resulting in the attenuation of ultrasonic diffusion and the generation of many bubbles. The pressure caused by bubble rupture will decrease, weakening the ultrasonic dispersion effect, which is not conducive to forming an O₃ reaction (Udepurkar et al., 2023). Therefore, after exceeding the appropriate temperature for O₃ generation, the concentration of O₃ generation tended to decrease.

PS concentration directly affects the generation of O₃ and reaction rate, as seen in Fig. 2(e–f). In this section, the experimental conditions are set to: PS concentration: 20, 30, 40, 50, 60 g/L; ultrasonic frequency: 28 kHz; ultrasonic power density: 0.4 W/cm²; Stirring intensity: 1000 rpm; Reaction temperature: 50 °C; Air flow: 400 mL/min. At PS concentrations between 20 and 50 g/L, the concentration of O₃ increased with time and PS concentration. However, as the PS concentration increased above 60 g/L, the concentration of O₃ generation slightly decreased compared to that at 50 g/L. The reason is that the addition of PS can increase not only the content of P₄ required in the O₃ generation reaction but also other impurities not involved in the O₃ generation reaction, such as SiO₂. The content of impurities in the PS slurry increases with the increase in the addition of PS, which makes it challenging to release the P₄ contained in the PS, hindering the mass transfer process of the O₃ generation reaction, thus leading to the decrease in the concentration of O₃ generation. Therefore, a reasonable concentration of PS addition can promote the O₃ generation reaction, thus generating more O₃.

As depicted in Fig. 2(g–h), the trend of O₃ generation concentration exhibits a positive correlation with the ultrasonic power density. In this section, the experimental conditions are set to: PS concentration: 50 g/L; ultrasonic frequency: 28 kHz; ultrasonic power density: 0.08, 0.16, 0.24, 0.32, 0.4 W/cm²; Stirring intensity: 1000 rpm; Reaction temperature: 50 °C; Air flow: 400 mL/min. An increase in ultrasonic power density will yield a high energy density and accelerate the collision rate of droplets within the PS slurry, thereby enhancing the turbulence and cavitation of the PS slurry. This, in turn, accelerates the gas–liquid phase mass transfer process of the O₃ generation reaction (Zhou et al., 2022). This study demonstrates that power density is one of the principal factors controlling liquid emulsification efficiency. With the escalation of ultrasonic power density, more intense dissolution and dispersion motion occur in the material inside the PS slurry, forming a flow field of high temperature, high pressure, and high turbulence. This stabilizes the emulsification effect inside the PS slurry (Taha et al., 2020). This study investigated the effect of rotational speed on excited O₃, and the results are detailed in the SI (Fig. S1).

3.2 Response surface analysis of ultrasonic-enhanced PS excited O₃

3.3 Quadratic regression fitting and variance analysis

Based on the design and results of the response surface experiment in SI, a multiple regression fit analysis of the quadratic model can be derived in Eq. (7):

The design model underwent evaluation and statistical analysis to ascertain the influence of the three selected factors on O₃ concentration (ppm) (R₁). The results of the Analysis of Variance (ANOVA) for the design model are presented in Table 2. The ANOVA was conducted using the F-test, a statistical method to determine the significance of each variable’s effect in the regression equation on the response value (O₃ concentration) (Jia et al., 2020; Tian et al., 2023). The principle of the F-test is as follows: when P_Prob > F, the corresponding variable’s level of significance increases (P < 0.001, highly extremely significant; P < 0.01, highly significant; P < 0.05, significant; and P > 0.1, not significant) (Wang et al., 2017). Therefore, the ANOVA was conducted using the F-test in this study, and the results are presented in Table 2. As shown in Table 2, the experiments’ results were analyzed using the response surface quadratic variance model, and the model’ s significance level was P < 0.0001, indicating that the model is highly significant and statistically significant (Chen et al., 2012). The significance analysis revealed that the effects of B (power density), C (PS concentration), A², and B² on O₃ concentration were highly significant, P < 0.0001; the effects of A (temperature) and AC were significant, P < 0.05; the effect of C² was significant, P < 0.01; and the effects of AB and BC were insignificant, P > 0.1. Furthermore, the larger the F-value, the more significant the effect on the response value; by analyzing each factor’s F-value, it can be seen that F_A > F_C > F_B, indicating that the order of influence of each factor on O₃ concentration is reaction temperature > PS concentration > ultrasonic power density (Kumar et al., 2020).

The proportion of the total variance of the responses predicted by the model can also be seen through R² (in Eq. (8)), which is calculated and expressed as follows, and the results of this calculation are placed in Table 2(Mofidi and Shahhosseini, 2022):

As indicated in Table S1, the model determination coefficient R² = 0.9912 suggests that the model accounts for 99.12 % of the variability in the experimental data. The model calibration coefficient of determination R²_adj = 0.9798 implies that the model explains 97.98 % of the variation in response values. As shown in Table 2, both R² and R²_adj (in Eq. (9)) are proximate to 1, signifying a high degree of consistency between the experimental and predicted results. This consistency validates that the established response surface model can comprehensively explain the designed relevant experiments. The model’s coefficient of variation, C.V% = 1.04 % <10 %, indicates a relatively small experimental error, thus affirming a good fit of the model. This good fit further substantiates that the analysis of the established correlation model is more reliable (Chen et al., 2019).

The juxtaposition of the predicted O₃ concentration outcomes and the actual experimental results is depicted in Fig. 3. The experimental results exhibit a linear correlation with the predicted values, and the actual values align with the predicted values within the permissible experimental error. This congruence substantiates the reasonableness of the design model (Urbina-Suarez et al., 2023).

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Fig. 3

Predicted and experimental values of O₃ concentration.

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3.3.1

3.3.1 Contour map and three-dimensional (3D) map of response surface analysis

The roles of different influencing factors were further scrutinized, and the interactions between pairs of factors were visually analyzed by plotting observational contour plots and three-dimensional response surfaces. The influence of these factors on O₃ concentration can be visualized by observing the color shifts and inclination changes in the contour plots and the three-dimensional response surface. The response surface's apex corresponds to the center point of the most minor ellipse on the contour line. In instances where the contour line assumes an elliptical shape, the stronger the interaction between the two factors, the more pronounced the effect. A contoured shape approximating a circle indicates insignificant interaction between the two factors. The steeper the slope of the three-dimensional response surface, the more significant the interaction between the two factors, and the darker the color, the higher the O₃ concentration production rate.

As depicted in Fig. 4, the contours of AB, BC, and AC closely resemble circles, and the inclination angle of the three-dimensional response surface is more pronounced, indicating a significant interaction between A (temperature), B (power density), and C (PS concentration). The results derived from the contour lines and the three-dimensional response surface align with the results of the ANOVA of the quadratic regression equation, thereby validating the reliability of the analysis results (Wei et al., 2019).

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Fig. 4

The contour and 3D response surface plots of interaction for O₃ concentration of temperature and power density (a–b), power density and PS concentration (c–d), temperature and PS concentration (e–f).

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3.4 Performance of O₃ coupled red mud for desulfurization and denitrification

First of all, this research conducted a blank experiment research. This research conducted relevant experiments on desulfurization and denitrification of only ultrasonic + RM and desulfurization and denitrification of only ultrasonic + PS, and the results are shown in Fig. S7. As can be seen from ultrasonic + RM desulfurization and denitrification (Fig. S7(a)), adding RM under ultrasonic conditions for desulfurization and denitrification has basically no effect on denitrification. For desulfurization, the desulphurization effect can be maintained above 90 % in the first 200 min of the reaction, but the desulphurization efficiency decreases significantly after 200 min. The desulphurization efficiency is only 47.8 %, and the desulphurization effect is not good. As can be seen from ultrasonic + PS desulfurization and denitrification (Fig. S7(b)), when PS is added under ultrasonic conditions for desulfurization and denitrification, the denitrification efficiency can be maintained above 90 % only in the first 80 min of the reaction, and after 240 min to 480 min, the denitrification efficiency is basically maintained at about 60 % and no longer changes significantly. For the desulphurization reaction, the desulphurization efficiency is maintained above 90 % only in the first 180 min, and the desulphurization efficiency decreases continuously after 180 min, and the desulphurization efficiency is basically maintained at about 52 % when 360 min–480 min. To sum up, the desulfurization and denitrification effect of RM or PS alone is not good, so follow-up experiments are conducted.

Based on the optimal O₃ excitation concentration explored in Chapter 3.4, we investigated the impact of ultrasonic frequency on NO_X and SO₂ removal. The experiment was conducted at three commonly used ultrasonic frequencies of 28, 40, and 60 kHz and under conditions without ultrasonic. The results are presented in Fig. 5(a–b). In this section, the experimental conditions are set to: Stirring intensity: 1000 rpm; ultrasonic power density: 0.4 W/cm²; RM concentration: 15 g/L; Reaction temperature: 50 °C; Flue gas flow rate:400 mL/min;SO₂ inlet concentration:2000 ppm; NO_X inlet concentration:250 ppm; ultrasonic frequency: 0, 28, 40, 60 kHz. The desulfurization efficiency demonstrates that the removal efficiency of SO₂ is significantly enhanced by the two lower ultrasonic frequencies, 28 kHz and 40 kHz, compared to the 0 kHz and 60 kHz ultrasonic frequencies. The removal rate of SO₂ can be sustained at over 95 % for 250 min. This is attributed to the high solubility of SO₂ in water, which facilitates its reaction with the alkaline components in the RM slurry (Eqs. (10)–(11) and Eqs. (13)–(15)) (Nie et al., 2023). Therefore, the ultrasonic enhancement on the RM desulfurization process is primarily physical. The free radicals generated by the ultrasonic play a specific auxiliary role in the RM desulfurization process (Eq. (12). Low-frequency ultrasonic waves can generate large bubbles in the RM slurry, and these bubbles have a more extended expansion and contraction period. The robust movement of bubbles in the liquid can better dissolve the flue gas components in the RM slurry and accelerate the reaction mass transfer process, thereby improving the efficiency of the desulfurization reaction. As seen in Fig. 5(b), the ultrasonic frequency has a minimal influence on denitrification efficiency because the primary influencing factor of the NO_X removal effect is the oxidant O₃. In this study, O₃ is derived from the PS-generated O₃ stage, and the molar ratio of O₃/NO is controlled at approximately 1:1 by the RSM method in this procedure. Therefore, during the denitrification reaction, O₃ can oxidize NO to high-valent NO_X, such as NO₂ and NO₃, and simultaneously generate a large amount of acid. The central role of the RM slurry in NO_X removal is to provide an alkaline environment and act as a pH buffer to maintain stable and efficient NO_X removal efficiency (Eqs. (16)–(17)) (Liu et al., 2022).

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Fig. 5

The performance of desulfurization and denitrification under different conditions for ultrasonic frequency (a–b), Reaction temperature (c–d), RM concentration (e–f) and ultrasonic power density(g–h).

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Additionally, the ultrasonic has a specific promotional effect on the denitrification process of RM slurry (Patil and Raut-Jadhav, 2022; Zhang et al., 2022). The free radical (OH·) generated by the ultrasonic cavitation effect also plays an auxiliary role in NO oxidation. This is also why the denitrification effect of RM at 28 kHz is slightly higher than other conditions; the removal efficiency of NO_X can be maintained at more than 90 % for 250 min (Eq. (18)).

As depicted in Fig. 5(c–d), an increase in reaction temperature facilitates the removal of SO₂. In this section, the experimental conditions are set to: Stirring intensity: 1000 rpm; ultrasonic power density: 0.4 W/cm²; RM concentration: 15 g/L; Reaction temperature: 30, 40, 50, 60 °C; Flue gas flow rate: 400 mL/min; SO₂ inlet concentration: 2000 ppm; NOx inlet concentration: 250 ppm; ultrasonic frequency: 28 kHz. This is attributed to the fact that a rise in reaction temperature enhances the gas–liquid-solid mass transfer process of the reaction system, thereby accelerating the rate of chemical reaction and promoting the reaction towards SO₂ absorption. When the reaction temperature exceeds 50 °C, the desulfurization efficiency can be sustained above 95 % for 250 min. Fig. 5(d) illustrates that within the temperature range of 40–50 °C, the oxidation removal efficiency of NO_X remains above 85 % for 250 min, and the denitrification efficiency at 30 °C is marginally lower than that at 40–50 °C. This validates that an increase in temperature can enhance the chemical reaction rate of NO_X absorption by the RM slurry to a certain extent, thereby improving the removal efficiency of NO_X. However, when the temperature rose to 60 °C, the NO_X removal efficiency rapidly declined after 100 min. This is due to the gradual decomposition of O₃ with rising temperature, leading to a decrease in NO oxidation and NO_X removal efficiency (Zhang et al., 2023).

As seen in Fig. 5(e–f), an increase in RM concentration benefits the SO₂ absorption reaction. In this section, the experimental conditions are set to: Stirring intensity: 1000 rpm; ultrasonic power density: 0.4 W/cm²; RM concentration: 5, 10, 15, 20 g/L; Reaction temperature: 50 °C; Flue gas flow rate: 400 mL/min; SO₂ inlet concentration: 2000 ppm; NO_x inlet concentration: 250 ppm; ultrasonic frequency: 28 kHz. The desulfurization efficiency significantly increases when the concentration of RM slurry is between 5 and 15 g/L. This is because an increase in the addition of RM implies that more alkaline components come into contact with the acidic flue gas and react, thereby enhancing the overall desulfurization efficiency of the reaction system. However, when the concentration of RM increased from 15 g/L to 20 g/L, the impurities in the RM slurry that were not involved in the reaction gradually increased due to the increase in the addition of RM, which hindered the mass transfer process of the reaction and led to a slight decrease in the desulfurization efficiency. In the denitrification process, a low concentration of RM cannot provide sufficient alkaline substances and pH buffer function for the nitrogen oxide absorption process, resulting in a low NO_X removal efficiency. The experimental results show that when the concentration of RM is increased to 15 g/L, the denitrification efficiency can be maintained above 85 % for 250 min. Therefore, the optimal concentration of 15 g/L RM is selected in this study (Bang and Kang, 2022).

Based on previous studies, it is known that ultrasonic power density is one of the critical parameters for the generation and intensity of ultrasonic cavitation. In this study, the effect of ultrasonic power density on the removal efficiency of SO₂ and NO at four intensity gradients was investigated, and the results are shown in Fig. 5(g–h). In this section, the experimental conditions are set to: Stirring intensity: 1000 rpm; ultrasonic power density: 0.16, 0.24, 0.32, 0.4 W/cm²; RM concentration: 15 g/L; Reaction temperature: 50 °C; Flue gas flow rate: 400 mL/min; SO₂ inlet concentration: 2000 ppm; NO_x inlet concentration: 250 ppm; ultrasonic frequency: 28 kHz. It can be observed that the removal efficiency of SO₂ increases with the increase in ultrasonic power density. The removal efficiency of SO₂ can be stabilized at more than 98 % when the power density is increased to 0.4 W/cm². This is because the ultrasonic cavitation and emulsification effects increase with the power increase under the same ultrasonic frequency. The higher ultrasonic power density can better break the local inhomogeneity in the liquid phase of the RM slurry, promote the mixing and dispersion of reactants, and then promote the reaction mass transfer process to enhance the desulfurization effect of the reaction system. Since O₃ dominates the NO oxidation process, the denitrification effect under different power densities can be maintained at more than 80 % within 250 min. The high ultrasonic power density will bring more intense ultrasonic cavitation and produce more vital oxidant free radicals, which have a specific enhancement effect on NO oxidation, thus improving the removal effect of NO_X (Li et al., 2022).

4.1 Structure and composition of materials

Fig. 6(a) depicts the XRD patterns of RM samples pre- and post-reaction. The initial RM predominantly consists of calcite (CaCO₃), hematite (Fe₂O₃), and Na₆Al₆Si₆O₂₄, manifesting as stable minerals. A dissolution equilibrium is observed during the desulfurization and denitrification process, providing an optimal alkaline environment for the reaction system to attain stable and efficient desulfurization and denitrification performance. Post-reaction, the phase products primarily comprise desulfurization gypsum (CaSO₄·H₂O). It is speculated that the reaction related to Eq. (19) occurs. At 38°, the corresponding peak is Ca₃(PO₄)₂, and this change is due to the occurrence of the Eq. (23). During the O₃ generation stage, P₂O₅ dissolves in water to form PO₄³⁻, which reacts with Ca²⁺ in water to yield Ca₃(PO₄)₂.

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Fig. 6

(a)XRD comparison of PS before and after reaction; (b)XRD comparison of RM before and after reaction; (c)N₂ adsorption/desorption isotherms of ultrasonic and unultrasonic RM; (d)Barrett-Joyner-Halendar (BJH) pore size distribution ofultrasonicated and unultrasonicated RM.

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A comparison of the XRD patterns of the PS samples pre- and post-reaction, as presented in Fig. 6, reveals that the primary components of the PS pre-reaction are P₂O₅, CaF₂, SiO₂, and P₄O₇. In contrast, the main constituents of the PS post-reaction are Ca₃(PO₄)₂, CaF₂, CaAl₂O₄·19H₂O, SiO₂, and AlPO₄. In Fig. 6(a), the peaks corresponding to 37° and 50° mainly represent CaF₂, and the intensity of the peaks increases after the reaction. A notable observation from comparing the PS pre- and post-reaction XRD patterns is the disappearance of the P₂O₅ peaks post-reaction. This is attributed to the dissolution of P₂O₅ in water during the reaction process, leading to the formation of phosphate post-reaction, as delineated in Eq. (20) (Li et al., 2024).

Secondly, it is observed that a significant quantity of CaAl₂O₄·19H₂O transforms into Ca₃(PO₄)₂ post-reaction, while a substantial amount of AlPO₄ is generated, likely due to the reaction as outlined in Eqs. (21)–(26) (Duan et al., 2023). It is evident that CaAl₂O₄·19H₂O initially produces a minor quantity of AlO₂⁻ via hydrolysis, which is further hydrolyzed to yield OH⁻. This OH⁻ is neutralized with the H⁺ generated from the dissolution of P₂O₅ in water, resulting in the formation of H₂O. Throughout the reaction process, the large quantity of P₂O₅ dissolved in water leads to the generation of H⁺ far exceeding the quantity of OH⁻, rendering the overall reaction environment acidic (Liu et al., 2022). The enhancement of the CaF₂ peak is attributed to the presence of fluorine in the PS form, which is bifurcated into F⁻ and CaF₂. The presence of Ca²⁺ in the PS slurry and the dissolution of soluble fluorine in the reaction generate a new CaF₂, making the CaF₂ peak more prominent post-reaction.

The BET results are presented in Table 4 and Fig. 6(c–d). The pore size, pore volume, and specific surface area of the original and post-ultrasonic treatment RM samples were analyzed and evaluated. It is discernible that ultrasonic treatment enhances the specific surface area, pore diameter, and pore volume, thereby improving the surface properties of RM (Li et al., 2022). As depicted in Fig. 6(c), the N₂ adsorption and desorption isotherms of the RM, both pre- and post-reaction, exhibit Type IV adsorption curves, all characterized by double hydroxide structures and H3-type hysteresis loops (López et al., 2011). This outcome indicates that the RM, both pre-and post-ultrasonic treatment, possesses mesoporous material characteristics, albeit with irregular results. The hysteresis loops of the RM, both pre-and post-ultrasonic treatment, appear at a relative pressure of P/P₀, approximately 0.50, signifying the existence of micro and mesopores in the RM structure (Deihimi et al., 2018).

Regarding the pore size distribution of the RM, both pre-and post-ultrasonic treatment, as shown in Fig. 6(c–d), it is observed that the particle size of the RM pre-sonication is primarily concentrated in the 10–20 nm range, while post-ultrasonic treatment, it is predominantly concentrated in the 0–10 nm range. This suggests that ultrasonic treatment can effectively reduce the particle size of RM, thereby exposing a larger specific surface area and more active sites. Concurrently, the BET results also confirm that the RM is primarily mesoporous, with the presence of mesoporous and microporous structures, indicating that the material possesses a multilevel pore structure post-ultrasonication. This structure could provide more binding sites conducive to the adsorption of NO_X and SO₂, thereby demonstrating that ultrasonic treatment can endow the RM with a more robust structural foundation for desulfurization and denitrification (Li et al., 2024).

Table 5 presents the XRF compositions of PS in its original state and after undergoing reactions under various conditions. As can be observed from Table 5, elements such as O, F, Na, Mg, Al, Si, P, K, Ca, and Fe primarily exist in the PS both pre- and post-reaction. Compared with the original PS samples, the content of P elements is reduced post-reaction, attributable to the generation of O₃ by P during the reaction process. Furthermore, Ca and Al decompose along with CaAl₂O₄·19H₂O to form AlO₄⁻, Ca₃(PO₄)₂, and CaF₂ (Li et al., 2024). The XRF results of the original RM sample pre- and post-reaction are displayed in Table 6. Table 6 reveals that O, Na, Al, Si, S, P, Ca, and Fe dominate the original RM samples. A comparison of the RM samples post-reaction with the original RM samples indicates a significant decrease in the content of Na, Al, and Si elements. This could be due to the large-scale consumption of the chemical binding base of the free base, composed of the elements above, during the reaction (Liu et al., 2022). The notable increase in the content of O and S elements may be ascribed to the absorption of NO_X and SO₂ flue gas by the slurry during the reaction. The increase in the content of P element is due to the formation of Ca₃(PO₄)₂ by partial P₂O₅ in the flue gas containing O₃, which results in the detection of P in the solid phase post-reaction (Li et al., 2024; Liu et al., 2023).

In order to further study the removal mechanism of SO₂ and NO_X, XPS analysis of RM samples before and after desulfurization and denitrification was carried out. The XPS full spectrum of the sample before and after the reaction is shown in Fig. 7(a–b), the percentage content of each element in the full spectrum is shown in Table S6, and the XPS spectrum of Ca 2p is shown in Fig. 7(a). As can be seen from the full spectrum of XPS (Fig. 7(a)), the characteristic peaks of Fe, Al, Si, O, Ca, and Na were detected in the untreated RM samples. After the reaction, the peaks of Fe 2p and Si 2p were weakened, while Na 1 s was not detected, which indicates that the free base and chemically bound base in the RM were consumed and decomposed during the reaction, which is consistent with the results of XRD. We carried out peak fitting of Ca 2p (Fig. 7(b)), and it can be seen that the binding energies corresponding to Ca 2p before and after the reaction are 347.2 eV and 347.5 eV respectively, which belong to the Ca-containing alkaline components (CaCO₃, etc.) and desulfurization gypsum (CaSO₄·2H₂O) in RM, respectively. These results indicate that the alkaline components are converted to CaSO₄·2H₂O after the reaction, consistent with the XRD results.

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Fig. 7

(a) Full Xps spectrum of absorbent sample before and after reaction; (b) XPS spectra of Ca 2p in RM before and after desulfurization and denitrification; (c) Anion concentration in absorption solution under different ultrasonic power density conditions.

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IC is used for testing to explore the concentration of anions in the absorption solution after the reaction. Under different ultrasonic power density conditions, the sample was pretreated: first, the sample was filtered by centrifugation, and then the supernatant was taken and reserved. In the detection process, the sample is diluted 50 times according to the concentration of the standard line, and then the type and concentration of anions in the absorption solution are determined. Three parallel tests were performed on each sample. Fig. 7(c) shows the measured results.

It can be observed from Fig. 7(c) that the concentrations of NO₃⁻ and SO₄²⁻ both increase with the increase of ultrasonic power density, but the concentration of NO₂⁻ decreases with the increase of ultrasonic power density, and its concentration decreases from 0.227 g/L to 0.173 g/L-1. NO₃⁻ concentration increased from 0.108 g/L to 0.139 g/L because with the increase of ultrasonic power density, more hydroxyl radicals were generated in the system, which promoted the reaction and made more NO absorbed by oxidation, which was conducive to the transformation of NO₂⁻ to NO₃⁻, resulting in the reduction of NO₂⁻ concentration. The concentration of SO₄²⁻ increased from 1.732 g/L to 1.921 g/L, indicating that when the ultrasonic power density increased, the physicochemical effect caused by it made the absorption reaction of SO₂ proceed forward, and more SO₂ was absorbed (Liu et al., 2022).

The change in fluorine content of PS samples before and after the reaction of ultrasonic-induced PS to produce O₃ was compared in the leaching experiment. The main reason is that ultrasonic-induced PS to produce O₃ may lead to fluoride conversion. By comparison, the effects of ultrasound on fluoride can be better understood, and the chemical reactions and substance transfer processes that may occur during the reaction can be inferred. The fluorine content in PS is related to the risk of environmental pollution. By comparing the fluorine content in PS before and after the reaction, we can further judge whether there will be a potential environmental pollution risk.

Leaching experiment procedure: A certain amount of PS before and after the reaction was placed in a 100 mL polyethylene serum bottle, and a certain amount of ultra-pure water was added and placed in a constant temperature oscillation chamber. Three parallel samples were set after oscillating at 250 rpm at room temperature for some time. At the corresponding time, 3 mL of samples were taken and filtered with a 0.45 μm organic filter membrane. After that, the fluoride content in the filtrate was determined by HJ 999–2018, “Determination of Fluorine in Solid Waste by alkali fusion - ion selective Electrode Method,” and the content was determined by ion chromatography. Then, it was compared with the national standard GB 5085.6–2007 “Identification standard of hazardous waste Toxic substance content Identification” to determine whether the fluorine ion content in PS after reaction met the relevant standards.

The PS samples before and after the ultrasound-induced PS-produced O₃ reaction were levered, and the results are listed in Table S7. It can be found that the F⁻ leaching content of PS can reach 446.78 mg/L before O₃ excitation, and the F⁻ ion concentration drops to 133.65 mg/L after ultrasound excitation PS to produce O₃, which may be because P₄ is released in the ultrasonic process of PS, exposing more active sites. There is a large amount of Ca²⁺ in the active site, and F⁻ reacts, as shown in Eq. (25), to combine and generate CaF precipitation, thereby reducing the content of F⁻ in it. However, the national standard GB 5085–2007 stipulates that the limit of inorganic fluoride in the leaching solution is 100 mg/L (Xu et al., 2021). In this study, the F⁻ content in the PS leaching solution before the reaction exceeds the standard by 346.786 mg/L, while the F⁻ content in the PS after the reaction decreases and is lower than the limit and meets the standard requirements. The results indicated that the fluoride-leaching toxicity of PS did not exceed the standard.

Organic element analysis was performed on the RM before and after the desulfurization and denitrification reaction, and the results are listed in Table S8. In combination with RM’s XRF before and after the reaction (Table 6), it can be found that the RM after desulfurization and denitrification is rich in P, S, K, and N elements (Liu, 2021). P element can make plants grow better and promote germ differentiation; K element can make plants have the ability to resist lodging, but also can make plant roots thrive and have the ability to resist insects, diseases, and drought. The S element can promote plant growth and chlorophyll synthesis and make plants thrive. N element can promote plant height, leaf number, root length, biomass, and other agronomic traits and has an essential effect on plant growth. It can be found that the RM, after desulphurization and denitrification, has relevant nutrient elements necessary for plant growth. The coordinated desulfurization and denitrification of RM with O₃ stimulated by ultrasound can effectively transform SO₂ and NO_X into S and N elements required for plant growth. This provides a corresponding theoretical basis for the soil utilization of RM after desulphurization and denitrification.

4.2 Morphology analysis

SEM characterization was conducted on the original PS sample, the PS reaction sample without ultrasonic, and the PS reaction sample under ultrasonic conditions of 28 kHz, 0.4 W/cm². The surface morphology of the PS, both pre- and post-reaction, was observed and analyzed. Selected areas of the surface layer of the PS samples were subjected to spot-scanning via the EDS analytical method. Fig. 8(a–d) displays the SEM images of the original PS sample, revealing that the PS primarily comprises a variety of spherical and massive agglomerated structures, bound together by numerous spherical and porous granular structures of varying sizes, with an amorphous granular agglomerate of impurities adhering to the P element. Fig. 8(b–e) presents the samples post-O₃ production from PS under non-ultrasonic conditions, indicating no significant change in the micro-morphology of PS pre- and post-reaction. Fig. 8(c) and Fig. 8(f) depict the PS samples under ultrasonic conditions. As shown in Fig. 8(c), introducing ultrasonic waves disrupts the original agglomerate structure, enhances the dispersion of PS particles, and provides a larger reaction area for PS and air, thereby promoting the O₃ generation reaction. Upon increasing the magnification to 20 K, it was discovered that the membrane-like material on the surface of the porous spherical particles within the PS had disappeared, as shown in Table S2 (EDS). Post-EDS spot-scanning test on the site, it was found that the content of elemental P in the area had significantly decreased from 7.40 wt% to 1.80 wt%, further substantiating that the introduction of the ultrasonic external field could facilitate the release of more latent P₄ within the PS, and enhance the utilization of the P element in PS. As can be inferred from Fig. S2(a), the microstructure of the ultrasonic-treated RM desulfurization samples was more loosely packed, and the desulphurized gypsum (CaSO₄) was fragmented into fine particles by the ultrasonic under the magnification of 2 K, 20 K. According to prior studies, one of the primary reasons for the gradual decrease in desulfurization efficiency of the RM wet desulfurization process is the extensive deposition of desulfurization product CaSO₄ blocking the pores of RM and reducing the pore size between particles (Mohal et al., 2023). The surface of the RM cannot adsorb more foreign substances, leading to a rapid decrease in the rate of desulfurization and denitrification. The introduction of the ultrasonic external field can break and disperse the generated CaSO₄ crystals and increase the reaction contact area between RM and flue gas, which is one of the main reasons for maintaining high desulfurization efficiency under the action of the ultrasonic external field.

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Fig. 8

SEM image of PS under different conditions: (a) 20 K scanning electron microscope image of PS raw sample; (b) 20 K scanning electron micrograph of PS without ultrasonic; (c) 20 K scanning electron micrograph of PS under ultrasonic; (d) 2 K scanning electron micrograph of PS raw sample; (e) 2 K scanning electron micrograph of PS without ultrasonic;(f) 2 K scanning electron micrograph of PS under ultrasonic.

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4.3 Free radical of materials surface analysis

EPR characterization was performed on the original RM samples and the RM samples that reacted under varying ultrasonic power densities to elucidate the reaction mechanism and analyze the impact of ultrasonic on the reaction. The results are presented in Fig. 9. The critical radical OH· was captured utilizing EPR spectroscopy in conjunction with DMPO. As depicted in Fig. 9, the EPR spectroscopy successfully captured the radical signals under different ultrasonic power densities. The measured ultrafine splitting constants (a_N = 15.4 G and a_N = 14.8 G) align with the relevant literature, suggesting the production of OH· (Liu et al., 2018; Nie et al., 2021). The EPR test results suggest that OH· is produced during ultrasonic treatment, with the highest OH· intensity observed when the power density was 0.40 W/cm². This indicates that the ultrasonic can promote OH· production, and the OH· intensity becomes more vigorous as the power density increases (Li et al., 2023; Tian et al., 2022).

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Fig. 9

EPR results of original RM samples and RM samples treatment under different power densities.

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4.4 Adsorption kinetics model

This study used Pseudo-first-order and pseudo-second-order kinetic models and Weber-Morris internal diffusion models to study the adsorption of NO_X and SO₂ in flue gas by ultrasonic enhanced RM, and corresponding kinetic models were established. The adsorption mechanism of NO_X and SO₂ by ultrasonic enhanced RM was explored through model fitting analysis. 4 g of RM was added to the absorption solution, and the absorption was enhanced by ultrasound. The initial concentration of NO_X was 625.0 mg/m³, and the initial concentration of SO₂ was 5235.17 mg/m³. Pseudo-first-order and Pseudo-second-order kinetic equations and Weber-Morris internal diffusion models were used to fit and analyze the adsorption equilibrium data. The results are shown in Fig. 10(a–b), and the relevant parameters are shown in Table 7.

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Fig. 10

Adsorption kinetics fitting of (a)SO₂ and (b)NO_X by RM; Ultrasonic enhanced isotherm model fitting for (c)SO₂ and (d)NO_X adsorption by RM.

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As can be seen from the fitting curve of adsorption kinetics (Fig. 10(a–b)), when the adsorption time is prolonged, the adsorption capacity of NO_X and SO₂ by ultrasonic enhanced RM also increases gradually. When the reaction reaches 45 h, the adsorption equilibrium state is basically reached, and the saturated adsorption capacity of NO_X is 71.80 mg/g, and the saturated adsorption capacity of SO₂ is 585.82 mg/g. It can also be seen from Table 7 that the determination coefficients R² of adsorption of NO_X and SO₂ by the Pseudo-first-order kinetic model are 0.99770 and 0.99891. The determination coefficients R² of adsorption of R² and SO₂ by the Pseudo-second-order kinetic model are 0.99853 and 0.99877. In the Weber-Morris internal diffusion model, the determination coefficients R² for adsorption of NO_X and SO₂ are 0.95086 and 0.98573. The correlation coefficients R² of the above three models are all greater than 0.93, indicating that the kinetic models adopted can better describe the whole process of adsorption in this study, and there are physical, chemical adsorption and internal diffusion processes in the adsorption process (Sun et al., 2021). However, the theoretical adsorption quantity fitted in the Weber-Morris internal diffusion model differs greatly from the actual value, and the correlation coefficient R² is lower than that of the Pseudo-first-order kinetic model, indicating that internal diffusion is not the main step to control adsorption in this study (Ma et al., 2011). The theoretical adsorption capacity of the Pseudo-first-order and Pseudo-second-order kinetic models is closer to the actual value, and the correlation coefficient is R² > 0.99, indicating that both primary and secondary levels can well explain the adsorption behavior of NO_X and SO₂ by ultrasonic enhanced RM. External and external diffusion can play a certain role, the adsorption process is mainly chemical and physical adsorption, and the alkaline substances in the RM play a leading role. Through ultrasonic strengthening, more free alkali and bound alkali in the RM can react with NO_X and SO₂, and the specific surface area of the RM can be increased after ultrasound. It is conducive to the physical adsorption of NO_X and SO₂ (Wang and Zhong, 2016). At the same time, in this experiment, a higher concentration of SO₂ is also conducive to the adsorption reaction, because the sulfate substances generated by SO₂ and the alkaline substances in the RM can increase the adsorption site on the surface of the adsorbent, providing a relevant driving force for the external diffusion reaction.

4.5 Adsorption isotherm model analysis

Langmuir and Freundlich's adsorption isotherm models were used to explore further the adsorption data of ultrasound-enhanced RM on SO₂ and NO_X. The adsorption isotherm model was established to explore the adsorption principle of ultrasound-enhanced RM on SO₂ and NO_X. Langmuir's adsorption isotherm model assumes that the adsorption process is carried out at homogeneous adsorption sites, and there is no interaction between adsorption sites and adsorbed material molecules (Yi et al., 2018) The Langmuir model is suitable for monolayer adsorption and is more suitable for highly uniform adsorption sites with no interaction. The Freundlich adsorption model assumes that the adsorption is carried out at heterogeneous adsorption sites, that there are interactions between adsorption sites, and that there are interactions between adsorbent molecules in the adsorption layer. The Freundlich model is suitable for multilayer adsorption and is more suitable for the interaction between heterogeneous adsorption sites and adsorbent molecules (Gaudino et al., 2016). The Langmuir and Freundlich adsorption isothermal models fitted by ultrasonic-enhanced RM are shown in Fig. 10(c–d), and the fitting parameters are shown in Table 8.

As seen from Table 8, the correlation coefficients R² of adsorption of NO_X and SO₂ by ultrasonic enhanced RM fitted by Langmuir adsorption isotherm model are 0.9963 and 0.9981, respectively. The correlation coefficients R² of adsorption of NO_X and SO₂ by ultrasonic enhanced RM fitted by Freundlich adsorption isotherm model are 0.9968 and 0.9989, respectively, indicating that the adsorption of NO_X and SO₂ by ultrasonic enhanced RM is more in line with the Freundlich adsorption isotherm model, and the adsorption is multi-layer adsorption (Gaudino et al., 2016).

Moreover, the n value fitted by the Freundlich adsorption isotherm model was within the range of 0.1 < 1/n < 0.5, indicating that the adsorption reaction was relatively easy to carry out, and the reaction corresponded to the heterogeneous surface due to the complex composition of the RM. Combined with the adsorption kinetics model, it can be found that the co-action of chemisorption, physical adsorption and internal diffusion model mainly causes the influence of ultrasonic enhanced RM adsorption. In contrast, the Langmuir adsorption isothermal model can explore the maximum adsorption capacity under the optimal theoretical conditions. The theoretical maximum adsorption capacities of NO_X and SO₂ fitted by Langmuir adsorption isothermal model are 74.30 mg/g and 5565.07 mg/g, which are close to the actual values in the experiment, indicating that the actual values in the experiment are credible (Huang et al., 2020 & Nie et al., 2020).

The desulphurization performance of this study was compared with that of the same type of research, and the results are shown in Table S5. It can be seen that the desulphurization and denitrification effect in this study is better, and the adsorption capacity is higher than that found in other studies. Such differences may be due to the strong interaction between O₃ and related free radicals with SO₂ and NO_X generated by ultrasonic excitation of PS production, resulting in strong adsorption force to improve the adsorption capacity, and this study can be maintained for a longer time under high desulphurization and denitrification efficiency. And the produced byproducts are less, which also shows that the effect of this study is better, and the RM and PS can be used as resources without secondary pollution.

4.6 Analysis of adsorption thermodynamic parameters

The thermodynamic parameters of the adsorption process were evaluated, including the change of Gibbs free energy (ΔG), entropy (ΔS) and enthalpy (ΔH). The Van't Hoff diagram of ultrasound-enhanced adsorption of NO_X and SO₂ by RM was drawn using the temperature experimental data, and the fitting equations were shown in Eqs. (S1)–(S2), as shown in Fig. S5. The equation can calculate the thermodynamic parameters of the adsorption process. Tables S3 and S4 shows the results. It can be seen from the figure that ΔG increases negatively with the increase of temperature, which proves that the increase of temperature leads to the increase of mobility of NO_X and SO₂ molecules, indicating that adsorption is a spontaneous process (Han and Chen, 2016). The positive value of ΔS indicates that the adsorption has a high randomness, and the internal structure of the adsorption increases randomly, indicating the feasibility of the adsorption, and the driving force of the adsorption of NO_X and SO₂ under the action of ultrasonic enhancement of RM is due to the entropy effect (Sedaghat et al., 2019). ΔH₀ < 0, indicating that the absorption of NO_X by ultrasonic-enhanced RM is an exothermic reaction, and higher temperature is not conducive to improving the absorption effect of ultrasonic-enhanced RM on NO_X. ΔH₀ > 0 indicates that the absorption of SO₂ by ultrasonic-enhanced RM is an endothermic reaction (Sedaghat et al., 2019).

4.7 Penetration curve fitting

In order to study the effect of pH value on the removal efficiency of sulfur nitrate pollutants in ultrasonic-enhanced RM desulfurization and denitrification, a penetration curve experiment was carried out. The control test conditions are: stirring intensity 1000 rpm; Ultrasonic power density 0.4 W/cm²; RM concentration: 15 g/L; Reaction temperature: 50 °C; Flue gas flow rate: 400 mL/min; SO₂ import concentration: 2000 ppm; NO_X import concentration: 250 ppm; Ultrasonic frequency: 28 kHz. The influence of pH value changing over time on NO_X and SO₂ simultaneous removal efficiency was investigated, as shown in Fig. 11(c).

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Fig. 11

(a)The effect of pH value variations on NO_X and SO₂ removal efficiency with time extension; Ultrasonic enhancement of adsorption penetration fitting curve of (b)NO_X and (c)SO₂ in RM.

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According to the results in Fig. 11(a), the pH value of the RM slurry presents a certain change with the extension of reaction time. At first, the pH drops sharply, then stabilizes and slowly declines. At the same time, the simultaneous removal efficiency of NO_X and SO₂ showed an “anti-S” -type declining trend. In the range of 0–25 h, the removal efficiency slowly decreased, and in the range of 25–40 h, the removal rate dropped sharply and then stabilized. The reaction process can be divided into two stages according to the change in pH value. The first stage occurs within the first 10 h of the reaction, with a sharp drop in pH, from an initial value of 9.89 directly to 2.10. At this stage, the desulfurization and denitrification efficiency is stable and can be maintained at more than 95 %. This rapid drop in pH value is due to the hydrolysis of SO₂ to SO₃²⁻, HSO₃⁻ and H⁺, and the dissolution of NO_X to NO₂⁻, NO₃⁻ and H⁺. P₂O₅ produced in the O₃ phase stimulated by ultrasonic enhanced PS dissolved in water to produce PO₄³⁻ and H⁺. At this time, the free alkali substance present in the RM slurry can neutralize the generated H⁺, resulting in a sharp drop in pH value. The second stage occurs when the pH value drops from 2.10 to 1.26, a slow and steady decline. At this stage, the desulfurization rate dropped from 98.24 % to 0.56 % and the denitrification rate from 90.34 % to 0.91 %. This is due to the decomposition of chemically bound bases in the RM (including hydrocalcite, calcite and calcite nepheline), so the pH value of the RM slurry is reduced. In summary, in the process of ultrasonic enhanced desulfurization and denitrification of RM, the change in pH value impacts the removal efficiency of sulfur nitrate pollutants. An efficient desulfurization and denitrification process can be achieved by controlling the reaction time and pH value.

The logistic model was used to fit the adsorption penetration curve, as shown in Fig. 11(b–c), and the fitting parameters are shown in Table 9. In the early stage of adsorption, the outlet concentration is 0, but with the progress of the reaction, the mass transfer zone changes and gradually moves upward, while the outlet concentration also increases. However, when the outlet concentration increases to a certain level, the growth rate becomes slow, which indicates that this adsorption process belongs to fixed bed adsorption. When the penetration curve tends to be flat, the fixed bed adsorption reaches the saturation saturation adsorption state, and the mass transfer zone moves to the top of the fixed bed. Moreover, it can be seen from Table 9 that the correlation coefficients R² obtained from the fitting curves of the adsorption penetration of NO_X and SO₂ by ultrasonic enhanced RM are 0.99849 and 0.99809, indicating that the fitting degree is good and the fitting regression equation is reliable. This formula can also be applied in actual industry.

4.8 Reaction mechanism

Based on the experimental results and pertinent characterization analysis, the process of ultrasonic-enhanced PS-generating O₃ and ultrasonic-enhanced RM desulfurization and denitrification can be summarized as follows: (1) ultrasonic-enhanced PS excited O₃ process: The application of an ultrasonic external field can liberate more latent P₄ in the PS, augment the dispersion degree of PS reactants, thereby facilitating better contact and reaction between O₂ in the air and P₄ in PS. P₄ reacts with O₂ to form P₂O₅ and releases active oxygen, which can oxidize O₂ to O₃. The hydroxyl radical (OH·) generated by ultrasonic cavitation can oxidize water-soluble O₂ to hydroperoxyl radical (HO₂·) and active oxygen, which has a specific promotional effect on the O₃ formation process. After P₂O₅ is dissolved in water, PO₄³⁻ is formed. Acidic substances can neutralize the alkaline components in PS and release Ca²⁺ and Al³⁺, forming Ca₃(PO₄)₂ and AlPO₄. Simultaneously, Ca²⁺ can combine with soluble F to form stable CaF₂. (2) ultrasonic-enhanced RM desulfurization and denitrification process: The O₃ and the OH· introduced by ultrasonic cavitation can oxidize NO in the flue gas to water-soluble NO_X. Concurrently, P₂O₅ from the O₃ formation stage generates PO₄³⁻ after being dissolved in water, and SO₂ generates SO₃²⁻ after being dissolved in water, which induces more soluble free alkali (sodium base) and calcite to decompose in RM slurry, neutralizing the acidic substances in the reaction system. The final product of the reaction is primarily CaSO₄. At the same time, introducing the ultrasonic wave will crush and disperse the agglomerated CaSO₄, enabling the reaction system to maintain a high efficiency of desulfurization and denitrification. According to the principles above and the test results, the potential reaction mechanism is roughly inferred, as depicted in Fig. 12.

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Fig. 12

Reaction mechanism diagram.

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