Effect and mechanism of coal desulfurization using a surfactant-assisted NaClO-NaOH system | Scientific Reports

Feb 21, 2025

Scientific Reports volume 15, Article number: 5116 (2025) Cite this article

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This study employed both experimental and theoretical approaches to explore the desulfurization effects and mechanisms of surfactants in the NaClO-NaOH system. The effects of five different surfactants on the coal desulfurization rate, wettability, electrostatic potential, and chemical bonding were analyzed. The results revealed that all five surfactants enhanced the coal desulfurization rate and significantly improved its wettability. However, wettability alone was not the sole determinant of desulfurization effectiveness. For individual surfactants, improved wettability correlated with a greater desulfurization effect. Electrostatic potential analysis showed that oxygen-containing functional groups in the surfactants had high electron density, which attracted weakly electrophilic chlorine and carbon atoms. This interaction increased the reactivity of ClO⁻ with compounds such as benzenethiol or dibenzothiophene, making these regions preferential sites for reaction. A higher number of oxygen-containing functional groups in a surfactant was associated with better desulfurization performance. Among the surfactants, AEC-9Na was the most effective due to its high content of oxygen functional groups. Furthermore, surfactants were found to influence the bond lengths of Cl-O, S-H, and S-C to varying degrees. These findings provide insights into the microscopic mechanisms of surfactant-assisted coal oxidation desulfurization and offer guidance for improving reaction efficiency.

Coal is a fundamental energy source for economic stability. Given the limitations of petroleum resources and the increasing depletion of low-sulfur coal, the development and utilization of medium- to high-sulfur coal have become crucial strategies for addressing energy challenges1. It is widely recognized that sulfur in coal constitutes a detrimental impurity, particularly in industrial processes such as coking and combustion. Sulfur in coking coal can cause hot brittleness in steel, compromising the structural integrity and mechanical properties of the resulting steel products2. During coal combustion, sulfur dioxide (SO₂), a corrosive gas, is generated. This gas can cause significant damage damage to industrial equipment, reducing its lifespan and efficiency. Furthermore, the release of SO2 into the atmosphere poses serious health risks, contributing to respiratory problems and other adverse health effects3. Recognizing these impacts, various international policies and applications have been implemented to mitigate sulfur emissions. In the European Union, stringent regulations under the Industrial Emissions Directive (IED) mandate specific sulfur emission limits, encouraging the adoption of advanced desulfurization technologies4. In China, the world’s largest consumer of coal, recent Five-Year Plans have prioritized reducing sulfur content in coal and promoting cleaner, more efficient coal combustion technologies5. Given the numerous hazards associated with sulfur in coal, research on coal desulfurization has garnered significant attention from many scholars6.

Current pre-combustion desulfurization methods can be categorized into physical, biological, and chemical techniques7,8. Although physical desulfurization technology is widely applied in industry, its effectiveness in removing organic sulfur from coal is limited9. This limitation arises from the presence of complex sulfur structures in medium- to high-sulfur coals, such as thiophenes, mercaptans, and disulfide bonds10. While microbial desulfurization technology is capable of removing organic sulfur from coal, the prolonged reaction time and slow microbial growth rate pose significant challenges for industrial applications11. Chemical methods can efficiently and rapidly remove both organic and inorganic sulfur from coal12. Among chemical desulphurization methods, oxidative desulfurization, due to its mild reaction conditions and high efficiency, has received widespread attention13. Common oxidizing agents used in desulfurization reactions include KMnO4, Fe2(SO4)3, H2O2, and NaClO. Among these, NaClO can be produced from industrial wastewater and is easily applicable in the chemical industry, making it a promising desulfurization agent with significant development potential14. Li et al.15 found that pH is a crucial factor affecting the desulfurization activity of NaClO. Irum et al.16 effectively improved desulfurization efficiency by controlling coal particle size, NaClO concentration, and pH levels. Gao et al.17 studied the effects of NaClO + NaOH concentration, reaction temperature, and coal particle size on NaClO desulfurization using a combination of response surface methodology and experimental validation.

The hydrophobic nature of coal during the chemical desulfurization process restricts the contact between the oxidizing agent and the coal, thereby impacting the efficiency of the desulfurization process. Surfactants enhance the wettability of coal by reducing surface tension, which improves the interactions between the oxidizing agent, water, and coal particles18. This facilitates better contact between the coal and the oxidizing agent, thereby improving the efficiency of the oxidation process. Ge et al.19 explored the combined effects of ionic liquids (ILs) and hydrogen peroxide (H2O2) on the desulfurization of coal. They found that ILs significantly improved the wettability of coal, thereby facilitating the penetration and interaction of H2O2 with sulfur-containing compounds. Pan et al.20 reported that adjusting the concentration of surfactants can modify the surface tension of leaching solutions, thereby improving the bioleaching of sulfide minerals and enhancing microbial desulfurization of sulfide-bearing ores. Zhang et al.21 demonstrated research showing that the non-ionic surfactant Tween 20 significantly enhances the biodesulfurization of coal. Specifically, when Tween 20 was added at a concentration of 1100 mg/L, the total desulfurization rate reached 29.7% within 16 days. Surfactants have been shown to significantly improve the wetting properties of coal and enhance the leaching of sulfur from it. However, current analyses of surfactants’ ability to enhance coal desulfurization rates are primarily limited to their effect on improving coal wettability. Beyond wetting, whether surfactants have other effects on coal desulfurization reactions requires further investigation. Moreover, there are no reported studies on the application of surfactants in NaClO desulfurization reactions.

In this study, five surfactants were introduced into NaClO-NaOH composite desulfurizers to investigate their respective impacts on desulfurization efficiency. The objective was to optimize the surfactant-to-desulfurizer ratio and evaluate how different surfactant components influenced the effectiveness of NaClO-NaOH desulfurization. Materials Studio 2020 (MS) molecular simulation software was employed to analyze the electrostatic potential (ESP) energies of substances involved in the reaction and the changes in sulfur-containing group bond lengths in coal after the addition of surfactants. Revealing the mechanism by which surfactants enhance the reaction between oxidants and sulfur elements in coal at a microscopic level is crucial for improving NaClO desulfurization efficiency and reducing reagent costs.

Coal sample was obtained from Shanxi Ciyaogou (CYG) Coal Mine. The CYG coal was crushed and sieved to 200 mesh, then dried at 40 °C for 12 h under vacuum. Industrial analysis and sulfur forms analysis of the CYG coal were conducted in accordance with the Chinese National Standards GB/T 212–2008 and GB/T 215–2003. The results are shown in Table 1. The total sulfur content of CYG coal was 2.29%, categorizing it as medium- to high-sulfur coal.

The oxidizing agent used was a 10 wt% NaClO solution. The alkaline environment was provided by NaOH. The surfactants used included nonionic surfactants, such as fatty alcohol polyoxyethylene ether 9 (AEO9) and coconut oil diethanolamide (CDEA 1:1), as well as anionic surfactants such as secondary alkane sulfonate sodium (SAS-60), sodium alpha-olefin Sulfonate (AOS), and sodium fatty alcohol polyoxyethylene ether carboxylate (AEC-9Na). All experimental reagents were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd., and molecular structures were shown in Table 2.

In the desulfurization experiments assisted by surfactants, six representative samples of CYG coal, each weighing 3 g, were placed in beakers. Five samples were prepared by adding 0.101 mL of AEO9, 0.102 mL of CDEA 1:1, 0.167 mL of SAS-60, 0.286 mL of AOS, and 0.357 mL of AEC-9Na, respectively, followed by the addition of 100 mL of composite desulfurizer to each. The composition of the composite desulfurizer was 5 wt% NaOH and 5 wt% NaClO. At this point, the concentration of surfactants in each composite desulfurizer was 0.1 wt%. One remaining coal sample was prepared without adding any surfactant and was directly mixed with 100 mL of composite desulfurizer. The six solutions were stirred using a magnetic stirrer at a speed of 200 rpm for 1 h at 20 °C, followed by filtration. The coal samples were then washed with distilled water until the pH of the filtrate reached 6.5–7, and no foam was observed. Finally, the coal samples were vacuum-dried for 12 h, and the sulfur content of each coal sample was measured.

The surfactant category exhibiting the most effective desulfurization performance was selected for experiments aimed at optimizing surfactant concentration. By varying the amount of surfactant added to the desulfurizing agent, the mass fractions of the surfactant were controlled at 0 wt%, 0.1 wt%, 0.2 wt%, 0.3 wt%, 0.4 wt%, and 0.5 wt%. The previously outlined experimental procedures were meticulously repeated to measure the sulfur content of coal after desulfurization at different surfactant concentrations, thereby assessing the efficacy of the surfactant in aiding the desulfurization process.

After identifying the surfactant with the best desulfurization performance, the following mass fractions of this surfactant were set: 0 wt%, 0.1 wt%, 0.2 wt%, 0.3 wt%, 0.4 wt%, and 0.5 wt%. The above experimental steps were repeated, and the sulfur content of the coal after desulfurization with different concentrations of the surfactant was measured.

The determination of the total sulfur content in coal samples was conducted using the coulometric titration method with an LCS-430 automatic sulfur analyzer (Hunan Lichen Instrument Technology Co., Ltd.). A coal sample mass of 50 mg was used, with tungsten trioxide serving as the catalyst. The electrolyte solution consisted of potassium iodide, potassium bromide, and glacial acetic acid. The sulfur analyzer was operated at a rated temperature of 1050 °C, and the desulfurization rate was calculated using Eq. (1).

Where \(\:\eta\) is the coal desulfurization efficiency, %; \(\:{\upomega\:}({\text{S}}_{1}\text{)}\) is the total sulfur content of the coal before desulfurization, %; \(\:{\upomega\:}({\text{S}}_{2}\text{)}\) is the total sulfur content of the coal after desulfurization, %.

To determine the effect of surfactants on the wettability of coal, a JC2000D2M contact angle meter (Shanghai Zhongchen Digital Technology Equipment Co., Ltd.) was used to measure the contact angles of the composite desulfurizers on the coal surface. To minimize experimental error, the contact angle of each sample was measured three times, and the average value was calculated.

Benzenethiol and dibenzothiophene were chosen to simulate the mercaptan and thiophene structures in coal. The ESP of NaClO, benzenethiol, dibenzothiophene, SAS-60, CDEA 1:1, and AEC-9Na were calculated. The initial models were geometrically optimized using the DMol3 module in MS software, based on the COMPASS II force field. The GGA-BLYP gradient correction function was selected, and electronic density calculations were performed using DFT Semi-core Pseudopotentials. The ESP maps were then plotted.

Models of composite desulfurizer solutions with 0.1 wt% SAS-60, 0.1 wt% CDEA 1:1, 0.1 wt% AEC-9Na, 0.2 wt% AEC-9Na, and 0.3 wt% AEC-9Na were constructed respectively. The models included H2O, benzenethiol, dibenzothiophene, surfactant molecules, Na+, ClO-, and OH-. The solution systems were optimized to simulate their stable states. The specific optimization procedures were as follows: Using the Amorphous Cell module, periodic models of each solution structure were constructed. Annealing dynamics simulations were then performed on the models to obtain the globally minimized energy configurations. Finally, the bond lengths of Cl-O, C-S, and S-H in each composite desulfurizer solution model were recorded.

Figure 1 shows the comparison of the desulfurization effects of NaOH-NaClO with five surfactants. As shown in the figure, the desulfurization rate of coal treated with NaOH-NaClO without any surfactant is 30.0%. The addition of surfactants can improve the desulfurization rate of coal to varying degrees. Among them, the anionic surfactant AEC-9Na exhibits the most pronounced auxiliary desulfurization efficacy, increasing the desulfurization rate to 36.8%. This is followed by the nonionic surfactant CDEA 1:1, which increases the desulfurization rate to 33.4%. AOS, SAS-60, and LAS-30 have relatively weaker abilities to improve the desulfurization rate, with desulfurization effects of 32.8%, 32.1%, and 31.6%, respectively.

Additionally, from the contact angle data of the various desulfurization agents on coal in Fig. 1, it can be observed that the contact angle of coal without surfactant is 63.3º, indicating poor wettability. Five surfactants significantly improve the wettability of coal. The anionic surfactant SAS-60 achieves the best wettability, with a contact angle of 29.1º. AEO9, CDEA 1:1, and AEC-9Na show moderate improvements in wettability, with contact angles of 35.1º, 42.5º, and 43.7º, respectively. Although AOS has a relatively poorer wettability effect compared to other surfactants, its contact angle of 47.1º still significantly enhances the penetration effect of NaOH-NaClO on coal.

Combining the analysis of desulfurization effects with surfactant assistance and wettability, there was no clear correlation between the ability of the five surfactants to promote coal desulfurization and their improvement in wettability. Despite SAS-60 achieving the best wettability with the lowest contact angle, it did not exhibit the highest desulfurization efficiency. This anomaly suggests that simply enhancing the coal surface’s wettability does not directly translate to higher sulfur removal rates. AEC-9Na, while only moderately improving wettability, demonstrated the most significant enhancement in desulfurization efficiency. This implies that AEC-9Na may have facilitated a more effective chemical interaction at the coal surface, potentially by enabling better penetration of the NaClO-NaOH solution into sulfur-rich sites or by altering the electronic environment at the surface to promote sulfur oxidation. Similarly, CDEA 1:1 only moderately improved wettability but still achieved a better enhancement in coal desulfurization rates. On the other hand, AOS and LAS-30, despite improving wettability to some extent, did not correspondingly achieve desulfurization rates that would align with their wettability enhancement. These findings indicate a complex relationship in which not all mechanisms driven by improved wettability are equally effective in sulfur removal.

To explore the optimal concentration of AEC-9Na in the composite desulfurizer for aiding desulfurization, the concentration of the surfactant in the desulfurizing agent was changed, and the results are shown in Fig. 2. The data reveal that as the mass concentration of AEC-9Na increases, the desulfurization rate initially rises before subsequently declining. Specifically, as the AEC-9Na concentration increases from 0.0 wt% to 0.2 wt%, the desulfurization rate progressively climbs from 30.0 to 42.5%. However, upon further increasing the concentration to 0.3 wt%, the desulfurization rate begins to diminish and eventually plateaus. At an AEC-9Na concentration of 0.5 wt%, the desulfurization rate stabilizes at approximately 38.5%. Correspondingly, the contact angle measurements in Fig. 2 demonstrate a similar trend; as the AEC-9Na concentration increases, the contact angle initially decreases and then rises. At an AEC-9Na concentration of 0.2 wt%, the contact angle reaches 34.6º, indicative of optimal wettability. Further increases in AEC-9Na concentration led to larger contact angles, signifying diminished wettability. This suggests that there exists an optimal concentration range for AEC-9Na to effectively aid the desulfurization process with NaClO-NaOH. Furthermore, for dexsulfurizing agents, the greater the wettability of coal imparted by the same surfactant, the more effective the desulfurization process.

Based on these observations, the causes of this phenomenon were analyzed. As the concentration of AEC-9Na in the desulfurizing solution increased, it eventually reached the critical micelle concentration (CMC), where surfactant molecules began forming micelles. Below the CMC, surfactant molecules primarily adsorbed at the coal-water interface, reducing surface tension and improving wettability, thereby enhancing the contact between the desulfurizer and the sulfur in coal. Once the AEC-9Na concentration exceeded the CMC (between 0.2 wt% and 0.3 wt%), surfactant molecules formed micelles, which could encapsulate portions of the desulfurizing agent. This encapsulation reduced the agent’s availability to react with sulfur in coal, thereby decreasing the desulfurization efficiency beyond the CMC. At higher AEC-9Na concentrations, the formation of micelles trapped the desulfurizing agent within the micellar structures, reducing the surface area available for reaction with sulfur and ultimately limiting the desulfurization rate.

The disparity in the impact of different surfactant categories on coal wettability and desulfurization efficacy suggests that wettability is not the sole determinant influencing the desulfurization effect of surfactants on coal. However, within the same category of surfactant, the changes in coal wettability and desulfurization effectiveness exhibit a consistent correlation. This demonstrates that wettability is one of the factors by which surfactants influence the desulfurization rate of coal.

Comparison of desulfurization effect and wettability of different surfactants.

Comparison of the auxiliary desulfurization effect and wettability of AEC-9Na with different concentrations.

Sulfur forms analysis was performed on the CYG coal according to the Chinese National Standard GB/T 215–2003. Based on the morphological sulfur content test, the coal sample with the best desulfurization results was selected to measure the content of morphological sulfur after desulfurization. After desulfurization using 0.2 wt% AEC-9Na-assisted NaClO-NaOH, there was a reduction in the mass ratios of total sulfur, as well as sulfate sulfur, pyritic sulfur, and organic sulfur in the coal. The removal of sulfate sulfur was particularly significant, with its mass ratio decreasing from 0.1 to 0.05%, achieving a desulfurization rate of 50%. The pyritic sulfur content decreased from 0.75 to 0.40% after desulfurization, resulting in a removal rate of 46.7%. There was also a notable decrease in the mass ratio of organic sulfur, which dropped from 1.44 to 0.87%, corresponding to a desulfurization rate of 39.6%.

To elucidate the mechanism by which surfactants facilitate alkaline desulfurization, this section focuses on three representative surfactants: SAS-60, which exhibits excellent wettability but negligible desulfurization efficacy; CDEA 1:1, characterized by neither significant wettability nor desulfurization performance; and AEC-9Na, which demonstrates poor wettability yet the highest desulfurization efficacy. Composite desulfurizer solution models were constructed, and the reactive sites and chemical bond lengths of the involved substances were meticulously analyzed.

Figure 3 illustrates the ESP distribution maps of NaClO, benzenethiol, dibenzothiophene, SAS-60, CDEA 1:1, and AEC-9Na molecules. In these maps, red regions denote high electron density and concentrated negative charges, indicative of nucleophilic character. Conversely, blue regions signify low electron density and concentrated positive charges, indicative of electrophilic character27. Light-colored regions represent areas of lower polarity, capable of functioning as both electron donors and acceptors, thereby exhibiting weak electrophilic and nucleophilic characteristics.

It can be observed that the negative charges of the three surfactants are primarily concentrated in regions containing oxygen functional groups. For instance, in SAS-60, the ESP values of the three oxygen atoms in the -SO3H group are − 0.522, − 0.506, and − 0.506, respectively; in CDEA 1:1, the ESP values of the oxygen atoms in the -OH and -CON- groups are − 0.522 and − 0.479, respectively; in AEC-9Na, the ESP values of the oxygen atoms in the -O- and -COOH groups are − 0.320 and − 0.450, respectively. Among these, the oxygen atoms in SAS-60 and CDEA 1:1 exhibit a higher concentration of negative charges, indicating stronger nucleophilicity, whereas the oxygen atoms in AEC-9Na show a lower concentration of negative charges, indicating weaker nucleophilicity. In benzenethiol and dibenzothiophene, the positive charges are mainly concentrated on the carbon atoms, with positive ESP values, indicating electrophilicity; the negative charges are concentrated on the sulfur atoms, with ESP values of − 0.259 and − 0.205, respectively, indicating weak nucleophilicity. In NaClO, the positive charges are essentially concentrated on the sodium atom, while the negative charges are concentrated on the oxygen atom. The ESP value of the chlorine atom in the light-colored region is − 0.076, indicating both weak electrophilicity and nucleophilicity.

One can deduce that the chlorine atom in NaClO and the carbon atoms in benzenethiol and dibenzothiophene have the potential to bond with the oxygen-containing functional groups of the surfactants via the electrostatic attraction between their positive charges and the negative charges on the functional groups. Under the influence of ESP, surfactants link originally heterogeneous reactants, enhancing the likelihood of contact between ClO⁻ and sulfur-containing functional groups. The oxygen-containing groups in the surfactants provide active sites for desulfurization reactions. Therefore, among the three surfactants, the oxygen atoms in SAS-60 and CDEA 1:1 possess higher ESP values and stronger electrostatic attractions, rendering them more prone to attract ClO-, as well as benzenethiol or dibenzothiophene. However, the AEC-9Na molecule offers a greater number of active sites due to its abundance of oxygen-containing functional groups. This effectively increases the contact opportunities between ClO- and sulfur-containing functional groups, resulting in better desulfurization performance compared to the other surfactants.

Potential energy distribution of reactive substances.

The mixed solution models composed of the three surfactants and the desulfurizing agent are shown in Fig. 4. To analyze the effect of electrostatic attraction on the interaction between ClO- and the molecules of benzenethiol and dibenzothiophene mediated by the surfactants, the bond lengths of Cl-O, S-H, and C-S in the mixed solution were measured. The normal distribution of bond lengths for ClO- and sulfur-containing functional groups in each composite desulfurizing agent solution model is shown in Fig. 5.

As depicted in Fig. 5, SAS-60 emerges as the surfactant exerting the most pronounced influence on the Cl-O bond length, with an average value of 2.933. This observation underscores SAS-60’s exceptional efficacy in augmenting the activity of ClO⁻ ions. Conversely, CDEA 1:1 is noteworthy for its impact on the C-S bond length, averaging 1.844, which suggests that the composite desulfurizing solution containing CDEA 1:1 fosters a longer and more susceptible C-S bond, facilitating its cleavage. AEC-9Na stands out as the surfactant most significantly affecting the S-H bond length, with an average measurement of 1.345, followed by CDEA 1:1 and SAS-60. This indicates that, within a composite desulfurizing solution containing AEC-9Na, the S-H bond is more readily cleaved. When AEC-9Na concentrations are at 0.1 wt%, 0.2 wt%, and 0.3 wt%, the average S-H bond lengths are 1.345, 1.382, and 1.344, respectively. Notably, the 0.2 wt% AEC-9Na composite desulfurizing solution model exhibits the longest average S-H bond length. In conclusion, different surfactants exert varying degrees of influence on the bond lengths of the chemical bonds involved in the reaction. SAS-60 is most effective in extending the Cl-O bond length, CDEA 1:1 significantly affects the C-S bond length, and AEC-9Na notably influences the S-H bond length. This differential impact of surfactants highlights their tailored roles in optimizing desulfurization processes by modulating bond stability and reactivity within the reaction milieu. It can thus be inferred that the influence of different surfactants on the bond lengths of oxidants and sulfur-containing functional groups is one of the factors affecting the desulfurization rate.

Solution model of compound desulfurizer.

The normal distribution of bond lengths of ClO- and sulfur-containing functional groups.

When there was only NaClO-NaOH in the desulfurizer, the mercaptans and thioethers in coal could directly react with NaOH to form ROH and Na2S, the relatively stable mercaptans and thioethers could also react with ClO− to form sulfoxides and RSClO−, and the Na2S and sulfoxides were further oxidized to form SO42−28. After the surfactant was added, utilizing AEC-9Na as a representative example, the mechanism by which surfactants enhance the oxidation desulfurization of coal was elucidated in Fig. 6. This process is primarily driven by the ESP difference that facilitates interactions between different molecular components. The nucleophilic oxygen-containing functional groups within the surfactant exhibit a strong affinity for the electrophilic carbon atoms present in benzenethiol and dibenzothiophene, as well as the weakly electrophilic chlorine atoms in ClO⁻. This affinity increases the likelihood of interactions between ClO⁻, benzenethiol, and dibenzothiophene within the solution, thereby elevating the probability of ClO⁻ oxidizing the sulfur atoms. AEC-9Na contains numerous oxygen-containing functional groups (-O-, -COOH) along its ether and carboxylate chains. These oxygen-rich groups increase the electron density, enhancing the surfactant’s ability to attract and react with electrophilic species. This feature makes AEC-9Na the most effective in facilitating desulfurization. SAS-60 features a sulfonate group (-SO3H) linked to an alkyl chain. The sulfonate group confers strong hydrophilic properties. Although it significantly improves coal’s wettability, its structure offers fewer reactive sites for desulfurization compared to AEC-9Na. CDEA 1:1 features hydroxyl and amide groups on a fatty acid derivative, enhancing its interaction with sulfur compounds, albeit less effectively than AEC-9Na. AEO9, composed of a fatty alcohol with ethylene oxide units, improves hydrophilicity and wettability, modestly boosting desulfurization. AOS features a long-chain alpha-olefin with a sulfonate group at one end. Similar to SAS-60, the sulfonate group imparts strong hydrophilic properties but differs in the structure of the hydrocarbon chain, which affects its ability to interact with the coal matrix.

On the other hand, the oxygen structures in surfactants attract both chlorine and sulfur atoms, increasing the bond lengths of Cl-O, S-H, and C-S and making these chemical bonds more prone to breaking under the influence of the surfactant. Different surfactants affect various chemical bonds to differing extents: SAS-60 primarily affects the Cl-O bond length, CDEA 1:1 has a greater impact on the C-S bond length, and AEC-9Na significantly affects the S-H bond length. Previous research28 has shown that the predominant form of sulfur in CYG coal is thiols, accounting for more than 45%, thus the removal of thiols plays a dominant role in reducing the total sulfur content. In the oxidation desulfurization reaction, since the sulfur bonds in benzenethiol are more easily broken than those in dibenzothiophene, ClO- will preferentially promote the breaking of S-H bonds to form sulfoxides or sulfones, followed by the breaking of C-S bonds to form SO42-. Therefore, AEC-9Na, which has the greatest impact on the S-H bond length, exhibits the best desulfurization performance, with a desulfurization rate of up to 36.8%. Next is CDEA 1:1, which significantly affects the C-S bond length, with a desulfurization rate of 33.4%. Among the three surfactants, SAS-60 has the least significant impact on the desulfurization rate, as it mainly affects the oxidant Cl-O.

It is evident that surfactants play a multifaceted role in enhancing the desulfurization rate of coal, extending beyond their primary function of improving wettability. One significant contribution of surfactants is through the modulation of ESP effects, which facilitates more effective interactions between reactants. By altering the electrostatic environment, surfactants can increase the attraction between oxidizing agents and sulfur-containing compounds, thereby enhancing the reactivity of the system. Moreover, surfactants influence the desulfurization process by inducing changes in bond lengths. By elongating bonds such as Cl-O, S-H, and C-S, surfactants make these bonds more susceptible to breaking under reaction conditions. This bond length alteration is crucial because it lowers the energy barrier for bond cleavage, allowing the desulfurization reactions to proceed more efficiently.

Mechanism Diagram of AEC-9Na Promoting NaClO Oxidation Desulfurization.

All five surfactants can improve the desulfurization rate of coal and significantly enhance its wettability. SAS-60 has the best wettability but an average desulfurization effect. AEC-9Na shows average wettability but the best desulfurization assistance, indicating that wettability is not the only factor influencing the desulfurization effect of coal by surfactants. For a single surfactant, better wettability generally correlates with a higher desulfurization effect.

ESP analysis shows that the regions with oxygen functional groups in the surfactants possess high electron density, which readily attracts the weakly electrophilic Cl and C atoms. This increases the contact opportunities between ClO- and benzenethiol or dibenzothiophene, making these regions preferential sites for the reaction. The more oxygen-containing functional groups present in a surfactant, the more active sites it provides, resulting in better desulfurization effects.

In different solution models, SAS-60 significantly affects the Cl-O bond length, CDEA 1:1 notably impacts the C-S bond length, while AEC-9Na has a major influence on the S-H bond length. Additionally, as the concentration of AEC-9Na increases, 0.2 wt% AEC-9Na solution model exhibits the longest S-H bond length. Since mercaptan is the predominant form of sulfur in CYG coal, the surfactant that most affects the S-H bond length and has the highest number of oxygen-containing functional groups, AEC-9Na, provides the best desulfurization performance.

At present, there are few studies on the combination of surfactants and desulfurizers. In the future, it is imperative to explore the combination of various oxidants and surfactants to enhance our understanding of surfactant-assisted oxidative desulfurization. Additionally, the diffusion rate of oxidizers in coal requires further investigation. Furthermore, surfactant-assisted desulfurization is still in its exploratory stage. In the future, we aim to continue studying methods to achieve the recycling of desulfurizers and to reduce the overall cost of desulfurization.

The remaining data used to support the findings of this study are available from the corresponding author upon request.

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This work was supported by the National Natural Science Foundation of China (52074147), Excellent Youth Fund Project of Liaoning Natural Science Foundation (2023JH3/10200011).

School of Safety Science and Engineering, Liaoning Technical University, Huludao, 125100, China

Fei Gao & Yunming Zhang

Key Laboratory of Mine Thermodynamic disasters and Control of Ministry of Education, Liaoning Technical University, Huludao, 125100, China

Fei Gao & Yunming Zhang

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G.F.: provide Investigation, Project administration, Supervision, Editing. Z.Y.: carry out Data curation, Writing - Original Draft, Methodology.

Correspondence to Fei Gao.

The authors declare no competing interests.

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Gao, F., Zhang, Y. Effect and mechanism of coal desulfurization using a surfactant-assisted NaClO-NaOH system. Sci Rep 15, 5116 (2025). https://doi.org/10.1038/s41598-025-88994-2

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Received: 19 October 2024

Accepted: 03 February 2025

Published: 11 February 2025

DOI: https://doi.org/10.1038/s41598-025-88994-2

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