Impact of hydrothermal nitric acid treatment on the composition of CGFSImpact on elemental compositionTable S2 presents the concentrations of more than 60 inorganic ash-forming elements detected in CGFS and CGFS-X. The analysis results indicate that, aside from Si, which shows an enrichment trend in CGFS-X, only a few elements, such as Ag in CGFS-2 M and CGFS-4 M , Bi in CGFS-2 M, and In and Se in the CGFS-1 M, exhibit enrichment. Conversely, all other elements display a significant leaching trend, suggesting that nitric acid hydrothermal leaching is effective the extraction of minerals.To ensure data consistency in the classification of the 60 elements with concentrations above the detection limit, we standardized the raw data. Based on this standardized data, hierarchical clustering was performed. The results, depicted in the dendrogram in Fig. 1, categorize the data into two clusters, labeled A and B. Cluster A includes up to 45 elements, whose leaching rates do not improve significantly with increasing nitric acid concentration. In contrast, Cluster B comprises 15 elements, whose leaching rates show a marked increase with higher nitric acid concentrations.Fig. 1Clustering dendrogram of elements.Comparative analysis of the two clusters reveals differences in their response to changes in nitric acid concentration. This variability can be attributed to the distinct modes of occurrence of these elements in the original gasification slag10. Elements in Cluster A are more likely to be associated with aluminosilicate, carbonate, and sulfate minerals, whereas elements in Cluster B are probably present as iron oxides or phosphates11. This differentiation underscores the influence of the initial mineralogical composition on the leaching efficiency of nitric acid hydrothermal treatment.The impact on the phase compositionThe XRD patterns of CGFS and CGFS-X (X = 1 M, 2 M, 4 M, 10 M) are illustrated in Fig. 2. The primary inorganic minerals detected in CGFS include gehlenite (Ca₂Al₂SiO₇), quartz (SiO₂), hematite (Fe₂O₃), and magnetite (Fe₃O₄). With increasing nitric acid concentration, the characteristic diffraction peaks of Ca₂Al₂SiO₇ in CGFS-2 M, CGFS-4 M, and CGFS-10 M nearly disappear, while the characteristic diffraction peaks of SiO₂ increase. This indicates that higher concentrations of nitric acid lead to the precipitation of more free SiO₂ from Ca₂Al₂SiO₇12, which is consistent with the increase in Si content with rising nitric acid concentration as shown in Table S2. Furthermore, the characteristic diffraction peaks of Fe₃O₄ also diminish with increasing nitric acid concentration. This is due to Fe₃O₄, being a basic oxide, undergoing redox reactions with the strong oxidizing acid nitric acid. After the dissolution of Fe₃O₄ by nitric acid, the remaining nitric acid oxidizes Fe2⁺ in the acid leachate to Fe3⁺13. The characteristic diffraction peaks of Fe₂O₃ and SiO₂ do not show significant enhancement with increasing nitric acid concentration, indicating that concentrations above 2.0 mol/L do not markedly improve the leaching efficiency of Fe₂O₃ and SiO₂.Fig. 2XRD patterns of CGFS and CGFS-X. (Mineral abbreviations: Q = quartz, G = gehlenite, H = hematite, M = magnetite.)As the concentration of nitric acid increases, CGFS-X exhibits distinct “bread-like” peak structures in the diffraction angle range of 2θ = 20−30°, suggesting the presence of substantial amorphous substances in these acid leaching residues14. The evolution of amorphous substances in CGFS-X is associated with the partial degradation of inorganic minerals (such as Ca₂Al₂SiO₇ and Fe₃O₄) and the formation of poorly crystalline or non-crystalline materials15. In addition to containing amorphous aluminosilicates and SiO₂, CGFS also includes a significant amount of amorphous carbon.Impact of hydrothermal nitric acid treatment on the microstructure of CGFSImpact on the microscopic surface morphologyAs shown in Fig. 3, SEM images of the CGFS and CGFS-X samples illustrate their microstructural characteristics. In Fig. 3a, the untreated CGFS is observed to comprise several components: partially clogged porous char particles (A), smooth-surfaced fully molten inorganic components (B), and rough-surfaced partially molten agglomerated inorganic components (C). After nitric acid demineralization treatment, the smooth spherical inorganic components in the CGFS-X (X = 1 M, 2 M, 4 M) samples are almost entirely absent (Fig. 3b−d). This disappearance can be attributed to two major factors: first, some of these spheres consist of metal oxides (such as Ca, Mg, Na) that are readily dissolved by nitric acid16; second, the leaching of soluble metal elements from these spheres renders their surfaces rough and porous (as seen in D), rather than smooth. The partially molten agglomerates, enriched with acid-insoluble elements (such as Si and Al), remain even after treatment with nitric acid.Fig. 3SEM images of (a) CGFS, (b) CGFS-1 M, (c) CGFS-2 M, and (d) CGFS-4 M.Compared to CGFS, the CGFS-X samples obtained after nitric acid treatment exhibit a more open pore structure. As the acid concentration increases, the extent of surface agglomeration decreases, leading to a more developed porous structure with a noticeable reduction in pore size. Figure 3c shows that the surface of the residue treated with 2.0 mol/L nitric acid is the smoothest, with the least residual agglomerated inorganic components. However, when the concentration of nitric acid is increased to 4.0 mol/L, the severe corrosiveness of the acid significantly erodes the char framework, resulting in denser pore channels, thinner carbon layers, and a rougher surface.Influence on the microstructure of carbon crystalsRaman spectroscopy is extensively employed for the structural characterization and defect analysis of carbonaceous materials17. Figure S3 displays the Raman spectra of CGFS and CGFS-X (X = 1 M, 2 M, 4 M) samples. Prominent D and G peaks are observed in the first-order region (1100−1800 cm⁻1), along with comparatively weaker vibrational peaks of C–H and other groups in the second-order region (2200−3400 cm⁻1)18. A higher D band, located between 1327 and 1350 cm⁻1, indicates a greater content of amorphous carbon in the residual material, whereas the G band, observed at 1579−1598 cm⁻1, is attributed to the stretching vibrations of aromatic layers in the graphite structure19. This implies that a stronger G band signifies a higher degree of graphitization.It is highlighted that the overlap between the D and G bands can lead to the loss or neglect of characteristic information pertaining to highly disordered carbonaceous materials when relying solely on the D and G band Raman spectra20. Therefore, further deconvolution (peak fitting) of the Raman spectra of coal is necessary to extract hidden information about the skeletal carbon structures in the overlapping region. To achieve precise spectral parameters, particularly the integral intensity ratio, the Raman spectra were deconvoluted into five peaks (D1, D2, D3, D4, G) according to the method proposed by Sadezky et al.21, as depicted in Fig. 4.Fig. 4Raman spectra of the CGFS and CGFS-X.The D1 band typically represents defect structures and is associated with the planar vibrations of sp2 carbon atoms with structural defects, edge carbon atoms, and impurity atoms22. The D2 band appears as a shoulder of the G band and corresponds to the E 2 g vibrational mode at the surface of graphite layers23. The G peak arises from the vibrations of the ideal graphite lattice. The D3 peak corresponds to sp2-bonded amorphous carbon, including organic molecules and functional groups23. The D4 peak’s formation is attributed to sp2-sp3 hybrid bonds or C–C and C=C stretching vibrations at the periphery of microcrystallites, forming structures similar to polyolefins 24.Using the peak intensity ratio ID/IG as an indicator for evaluating the structural order of carbon in the samples, an increase in the ID/IG ratio suggests a higher degree of disorder in the carbon microcrystalline structure and a lower degree of graphitization19. As shown in Table 1, the ID/IG ratios for CGFS-X are all lower than that for CGFS, indicating that the degree of graphitization of CGFS has improved following hydrothermal nitric acidacid leaching. The ID/IG ratio of CGFS-2 M is lower than those for CGFS-1 M and CGFS-4 M, suggesting that CGFS-2 M has the highest degree of graphitization. When the concentration of nitric acid increases from 2.0 to 4.0 mol/L, the ID/IG value increases. This is because, during the acid leaching process, the increasing concentration of nitric acid can etch the carbon structure and expand the spacing within the carbon network, leading to a decrease in the degree of graphitization for CGFS-4 M.Table 1 Carbon microstructure parameters of samples.AD3+D4/AAll can be used to indicate the active site of carbon, the higher the ratio, the more active site19. AD1/AG is related to the degree of disorder in the carbon microcrystalline structure25, AD4/AAll can be described as the relative number of cross-linking bonds, and AD3/AG+D2+D3 is used to characterize the degree of amorphousness in the carbon microcrystalline structure19. CGFS-4 M showed an increase in AD1/AG and AD1+D2+D3+D4/AG compared to CGFS, implying that acid leaching with 4.0 mol/L of nitric acid could change the microstructure of CGFS. In contrast, CGFS-2 M exhibited lower AD1/AG values, indicating that the carbon microcrystalline structure of CGFS acid-impregnated with nitric acid concentration of 2.0 mol/L had a low degree of disorder. CGFS-X showed a decrease in all AD3/AG+D2+D3 ratios as compared to CGFS, with the smallest ratio of 0.2370 for CGFS-2 M This indicates that nitric acid has a significant effect on the amorphous carbon structure of CGFS, with 2.0 mol/L of nitric acid having a more pronounced effect on the degree of crystallinity of the carbon microcrystalline structure. Additionally, since nitric acid treatment of CGFS resulted in a significant increase in AD3+D4/AAll, it illustrated that nitric acid also had a significant impact on the active sites of carbon.Effect of hydrothermal nitric acid treatment on pore structureN2 adsorption/desorption isotherm analysisAs shown in Fig. 5a, N2 adsorption/desorption isotherms of CGFS and acid-impregnated slag CGFS-X are similar to the type IV isotherms. In the lower relative pressure (P/P0) interval, the adsorption/desorption isotherm curves of CGFS-X basically overlap and show a significant upward bulge, a phenomenon that implies that the surface of CGFS-X is dominated by monolayer adsorption. The turning point at this bulge marks the saturation point of monolayer adsorption, which in turn indicates the presence of microporous structure, in agreement with the discussion in the literature26. With the increase of P/P0 to approximately 0.4, the phenomenon of hysteresis loops appears, which marks the transition from monolayer adsorption to multilayer adsorption, as well as the accompanying generation of a large number of mesopores.Fig. 5(a) N2 adsorption/desorption isotherms and (b) pore size distribution of CGFS and CGFS-X.In the high P/P0 region, CGFS-X undergoes capillary coalescence, which is manifested by a sharp rise in isotherms. The presence of a small amount of macroporous formation can be inferred at conditions close to the unit relative pressure27. It is noteworthy that the isotherms of the desorption process do not replicate the trajectory of the adsorption process, which is mainly due to the adsorption hysteresis phenomenon caused by capillary coalescence28. This deviation reflects the irreversible character of the adsorption and desorption processes and implies a significant influence exerted by the microstructure of the material on its adsorption properties.In N2 adsorption/desorption isotherm analyses of the four samples, it was observed that the nitrogen adsorption at consistent relative pressure points showed a clear sequential ordering, from high to low, of CGFS-4 M, CGFS-2 M, CGFS-1 M, and CGFS.This ordering revealed a trend: as the concentration of the nitric acid treatments increased, the pore structure of the samples became progressively more developed, and accordingly, the specific surface area also increased significantly. This phenomenon characterizes the significant influence of nitric acid concentration on the pore characteristics and specific surface area of the materials, suggesting that nitric acid treatment plays a key role in optimizing the pore structure and enhancing the adsorption performance of the materials.Pore size distribution analysisFigure 5b demonstrates the pore size distribution diagrams of the four materials, revealing that all of these materials exhibit a hierarchical porous structure of macropores, mesopores and micropores, with a high distribution density especially at the mesopore level. According to the statistical results of the porous structure parameters of the materials in Table 2, the specific surface area and total pore volume of the acid-impregnated residue showed an increasing trend with the increase of nitric acid concentration, while the mean pore diameter showed a decreasing trend. Specifically, the specific surface area and total pore volume of the 4 mol/L acid-impregnated residue were 2.39 and 1.74 times that of the original gasified fine residue, respectively; while its mean pore diameter was 0.73 times that of the original gasified fine residue.Table 2 Specific surface area and pore distribution of CGFS and CGFS-X.It is crucial to note that as the concentration of nitric acid increases, the specific surface area and total pore volume of CGFS-X exhibit an increasing trend. For example, when the nitric acid concentration was raised from 1 to 2 mol/L, these parameters increased by 175.18 m2/g and 0.122 cm3/g, respectively. However, with a further increase in concentration from 2 to 4 mol/L, the increments in specific surface area and total pore volume were only 44.01 m2/g and 0.053 cm3/g, respectively. Additionally, when the nitric acid concentration increased from 1 to 2 mol/L, the pore diameter decreased by 0.36 nm. In contrast, when the concentration increased from 2 to 4 mol/L, the pore diameter did not decrease but instead increased by 0.1 nm. This observation suggests that excessive nitric acid concentrations may compromise the carbon structural skeleton during the process of intense pore etching, corroborating the results of SEM analysis.Pore fractionation characterizationUsing low-temperature nitrogen adsorption data, the fractal Frenkel-Halsey-Hill (FHH) model was used to calculate the fractal dimension values (D) that could reflect the degree of pore development of the four materials29. The fractal dimension values were calculated as Eqs. (1) and (2)30:$${\text{ln}}(V) = b + A\ln \left(\ln \left(\frac{{p_{0} }}{p}\right)\right)$$
(1)
where V is the volume of N2 adsorbed at the equilibrium pressure P, and P0 represents the saturation pressure of N2. A is the power-law exponent dependent on the fractal dimension (D), and b is a constant. D takes values between 2 and 3.N2 adsorption/desorption curves at low (P/P0 < 0.4) and high (P/P0 > 0.4) pressures showed significant differences, and their fractal dimensions had different physical interpretations in different pressure intervals. Therefore, a critical point was set at P/P0 = 0.4, and the curves were linearly fitted by the least squares method, respectively, and the fitting results are shown in Fig. 6. In the low-pressure interval, the calculated fractal dimension reflects the roughness of the microporous surface or the complexity of the microporous structure; while in the high-pressure interval, the fractal dimension reflects the roughness and complexity of the mesoporous and macroporous pore surfaces30. The low and high pressure fractal dimension values (denoted as D1 and D2, respectively) were calculated according to the fitted equations, and the results are shown in Table 3.Fig. 6Representative plots of CGFS and CGFS-X of lnV versus ln (ln (P0/P)) on the basis of the N2 adsorption isotherms.Table 3 Fractal dimensions of FHH model for CGFS and CGFS-X.From the analysis presented in Table 3, it is evident that the fitted correlation values for various materials across different pressure stages are all greater than 0.9000, indicating a strong fit. Nitric acid treatment significantly enhances the D1 value and exhibits an increasing trend with higher acid leaching concentrations, suggesting that nitric acid treatment facilitates the development of micropores in CGFS. This treatment also increases the roughness and structural complexity of the microporous surface, with the improvement effect intensifying with higher nitric acid concentrations. Regarding the D2 value, nitric acid treatment similarly enhances the development of mesopores and macropores, although this improvement diminishes with excessive increases in nitric acid concentration. For instance, when the nitric acid concentration was elevated from 2 to 4 mol/L, the D2 value markedly decreased. This indicates that a moderate nitric acid concentration is beneficial for enhancing the development of mesopores and macropores, thereby improving their surface roughness and structural complexity. The findings on the impact of nitric acid treatment on the pore structure of gasification slag align with the previous analysis in Section “Impact on the microscopic surface morphology”. Specifically, while higher concentrations of nitric acid treatment promote micropore etching, excessive treatment may compromise the stability of the carbon skeleton structure, adversely affecting the mesoporous and macroporous structures.Effect of hydrothermal nitric acid treatment on the surface composition of CGFSEffect on the composition of surface functional groupsThe FTIR profiles of CGFS and CGFS-X are shown in Fig. 7a. The peak positions were attributed as follows: 3444 cm−1 corresponds to the stretching vibration of –OH31; 1612 cm−1 corresponds to the stretching vibration of C=C32; the peak at 1380 cm−1 is the stretching vibration of –CH3 33; the range from 1073 cm−1 to 1050 cm−1 is the stretching vibration of Si–O–Al or Si–O–Si 34; 789 cm−1 corresponds to the symmetric stretching vibration of Si–O35; and 464 cm−1 corresponds to the symmetric stretching vibration of Si–O–Si36. By comparing CGFS and the sample CGFS-X after nitric acid treatment, it can be found that nitric acid treatment (especially under 2 M and 4 M treatment conditions) significantly enhanced the vibrational strengths of functional groups such as –OH, Si–O–Al, and Si–O–Si in the sample. This indicates that the nitric acid treatment can effectively promote the formation or exposure of these functional groups.Fig. 7(a) FTIR spectra and (b) XPS survey scan spectra of the CGFS and CGFS-X.Influence on surface elemental binding statesFigure 7b presents the full-spectrum scans of CGFS and CGFS-X, while Table 4 lists the atomic percentages of the major elements on the surface of each sample. With increasing concentrations of hydrothermal nitric acid treatment, the O/C ratio significantly increases, indicating that the strong oxidative properties of nitric acid facilitate the incorporation of oxygen-containing functional groups into CGFS. The atomic percentage of Si also shows a substantial increase with higher nitric acid treatment concentrations, which is consistent with the analysis in Section “Impact on elemental composition”. This suggests that hydrothermal nitric acid treatment is ineffective at leaching silicate-based inorganic materials. In contrast, elements such as Fe, Al, and Ca are more readily leached by nitric acid, with even a 1 mol/L nitric acid treatment showing a notable leaching effect.Table 4 The atomic percentage of the main elements on the surface of CGFS and CGFS-X.Figure 8a shows the deconvolution fitting results of the C 1 s high-resolution spectra for CGFS-X and CGFS. The peak positions correspond to non-oxidized carbon (C–C/C=C/C–H: 284.8 eV)37, oxygenated carbon (C–O: 285.4 eV)38, and carboxyl carbon (O–C=O: 289.2 eV)39. Among these, C–C/C=C/C–H is typically considered to be hydrophobic hydrocarbon chain functional groups40. Compared to CGFS, the content of C–C/C=C/C–H in CGFS-X is reduced, with CGFS-2 M exhibiting the lowest content at just 24.00%. This indicates that the hydrophobicity of CGFS decreases following nitric acid treatment, with the most significant reduction observed in CGFS-2 M.Fig. 8XPS spectra of (a) C1s, (b) O1s, (c) Fe2p, and (d) Si2p in CGFS and CGFS-X.The fitting results of the O 1 s spectra of the sample in Fig. 8b revealed the presence of the following chemical bonds: inorganic oxygen (such as Fe–O, i.e., at 530.3 eV)41, carbonyl (C=O, at 531.4 eV)42, hydroxyl (C–OH, at 532.5 eV)43, and ether (C–O–C, at 533.5 eV)44. Compared to the original CGFS, the inorganic oxygen content of CGFS-1 M increased by 0.73%, indicating that the nitric acid treatment can dissolve the iron oxides encapsulated in the carbon. As shown in Table 5, compared to CGFS, the CGFS-X series samples exhibited increases in the contents of C=O, C–OH, and C–O–C. Among them, the increase in C–OH was the most significant, with increases of 7.01%, 15.35%, and 20.92%, respectively, suggesting that the nitric acid treatment can significantly increase the content of acidic functional groups on the CGFS surface, thereby enhancing the hydrophilicity of the material and its affinity for polar organic compounds.Table 5 Functional group content of CGFS and CGFS-X.The fitting results of the Fe 2p spectra shown in Fig. 8c indicate that the iron element exists in both divalent and trivalent chemical states45. Compared to the original GGFS sample, the relative content of trivalent iron in the GGFS-1 sample increased, which may be attributed to the strong oxidizing effect of nitric acid. In the CGFS sample treated with 2 mol/L nitric acid, the Fe 2p peak was barely observed, reflecting the excellent dissolution ability of nitric acid towards iron oxides.The fitting results of the Si 2p XPS spectra in Fig. 8d show peaks at 102.7 and 103.5 eV, corresponding to aluminosilicates and silicon dioxide (SiO2)46, respectively. With the increase in nitric acid concentration, the SiO2 content on the CGFS surface gradually increased, and the CGFS-4 M sample exhibited the highest SiO2 content of 13.38%. Furthermore, the aluminosilicate content on the CGFS-X sample surface first increased and then decreased, with the CGFS-2 M sample having the maximum aluminosilicate content of 9.19%. This suggests that when the nitric acid concentration is increased to 2.0 mol/L, it effectively extracts aluminosilicates from the pores and carbon-encapsulated regions. At a concentration of 4.0 mol/L, the nitric acid treatment further leaches these species, enhancing the purification of the material.Influence of nitric acid concentration on the adsorption performance of MBAs illustrated in Figure S4, CGFS-X significantly enhances the removal efficiency for MB, achieving a removal rate that is 2.7 to 2.9 times greater than that of untreated CGFS. To further investigate the effect of hydrothermal nitric acid treatment concentration on the adsorption performance of CGFS, the adsorption capacity (qt) and removal rate (R) of MB by CGFS-X were compared under the same experimental conditions, as illustrated in Fig. 9. It is evident that, after 30 min of adsorption, CGFS-2 M exhibits the highest removal performance for MB, whereas the differences between CGFS-1 M and CGFS-4 M after 60 min of adsorption are not significant. In the first 30 min of adsorption, the trend line for the removal rate of MB by CGFS-4 M shows the steepest slope, indicating the fastest adsorption rate, which is attributed to its largest specific surface area. The slope of the removal rate trend line for CGFS-2 M is the second highest, while CGFS-1 M demonstrates the steepest slope within the first 20 min, but this slope decreases significantly between 20 and 30 min. After 30 min of adsorption, the adsorption rate of the CGFS-2 M sample significantly slowed down. At an adsorption duration of 150 min, CGFS-2 M demonstrated exceptional adsorption capacity and removal efficiency, achieving maximum values of 210.20 mg/g and 82.78%, respectively, which are significantly superior to those of the CGFS-1 M and CGFS-4 M samples. Additionally, the adsorption capacity of CGFS-2 M also outperformed various adsorbents derived from industrial waste reported in the literature (Table 6).Fig. 9Adsorption of Methylene Blue by CGFS-X (X = 1 M, 2 M, 4 M).Table 6 Reported MB adsorption capacities of adsorbents prepared from wastes in the literature.Exploration of the mechanism of hydrothermal nitric acid treatment of CGFSSurface and composition modification by nitric acid treatmentUnder hydrothermal conditions, nitric acid undergoes a series of chemical reactions with the carbon materials and minerals in CGFS. Nitric acid acts as a strong oxidizing agent and undergoes oxidation reactions with the carbon in the carbon materials to produce oxygen-containing functional groups such as carboxyl, hydroxyl and nitro groups (as described in 3.4). These functional groups increase the polarity and chemical activity of the carbon surface, enabling it to form hydrogen bonds or electrostatic interactions with the pollutant molecules53, thus improving the adsorption efficiency. The specific chemical reaction equations are as follows.$$C + 2HNO_{3} \to CO_{2} + 2NO_{2} + H_{2} O$$
(1)
$$R + HNO_{3} \to R – OH + NO_{2} + H_{2} O$$
(2)
$$R + 2HNO_{3} \to R – COOH + 2NO_{2} + H_{2} O$$
(3)
$$R + HNO_{3} \to RC = O + NO_{2} + H_{2} O$$
(4)
Nitric acid undergoes acid–base neutralization reaction with alkaline components (e.g., oxides, carbonates) in CGFS to produce corresponding nitrates and water.$$MO_{x/2} + xHNO_{3} \to M(NO_{3} )x + \frac{x}{2}H_{2} O$$
(5)
where M represents a metal ion.$$C{\text{a}}CO3 + 2HNO_{3} \to C{\text{a}}(NO_{3} )_{2} + CO_{2} + H_{2} O$$
(6)
Silicates and nitric acid can also undergo a variety of chemical reactions in hydrothermal environments, such as the acidolytic reaction of nitric acid on silicates54, the generation of different forms of silicon oxides, and the gelation reaction of silicates. The possible reaction equations are as follows:$$N{\text{a}}_{2} SiO_{3} + 2HNO_{3} \to H_{2} SiO_{3} + 2NaNO_{3}$$
(7)
$$N{\text{a}}_{2} SiO_{3} + 2HNO_{3} \to SiO_{2} + 2NaNO_{3} + H_{2} O$$
(8)
$$H_{2} SiO_{3} \to SiO_{2} \bullet nH_{2} O$$
(9)
These reactions contribute to the transformation of silicates, which can have an effect on the structure and properties of the material through the SiO2 and other products generated.Mechanisms of porous structure formationBased on the analysis of elemental composition and micro-morphology, it can be concluded that chemical etching is the key factor in the formation of a porous structure. The strong oxidizing properties of nitric acid lead to the oxidation and etching of the surface of carbon materials, thereby increasing the pore structure. Additionally, nitric acid can generate micropores and mesopores within the residual carbon and silica and silicate by leaching out some of the inorganic components in the gasification slag. The oxidative decomposition of organic matter and the dissolution of inorganic matter not only increase the specific surface area of the material but also create more active adsorption sites within the material.Under hydrothermal conditions, the gases generated by the nitric acid reaction (e.g., CO2 and NO2) generate pressure inside the carbon material, leading to swelling and cracking. While this process generates new pores, it may also cause the collapse of the carbon structure due to excessive force, as described in Section “Impact on the microscopic surface morphology”. Therefore, proper control of the nitric acid treatment concentration, duration, and temperature is crucial. The optimal hydrothermal nitric acid treatment conditions are provided in Figure S2.Nitric acid treatment disrupts the lattice structure of carbon materials, leading to an increase in amorphous carbon and the formation of defect sites, as described in Section “Influence on the microstructure of carbon crystals”. These amorphous carbon and defect sites contribute to the formation of a porous structure and provide more active sites for adsorption. Nitric acid treatment also significantly affects the lattice structure of the inorganic components, influencing the adsorption properties of the material during the mineral morphology transformation process. Therefore, the conditions of nitric acid treatment need to be carefully controlled to achieve the optimal preparation of adsorbent materials.Structure–property correlation mechanismNitric acid treatment significantly enhances the adsorption performance of CGFS-X. This treatment markedly increases the specific surface area of the material by generating a porous structure, thereby providing more active sites for the adsorption process. Additionally, nitric acid treatment introduces a substantial number of oxygen-containing functional groups (such as carboxyl, hydroxyl, and nitro groups), which increase the polarity and chemical reactivity of the carbon material, thereby facilitating more efficient adsorption of polar pollutants. Moreover, nitric acid treatment leads to an increase in the amorphous carbon content and the formation of defect sites within the carbon matrix. These defect sites act as active centers, significantly enhancing the adsorption capacity and selectivity of the material. By precisely controlling the concentration of nitric acid and treatment conditions, the pore size distribution of the material can be tailored to better accommodate the adsorption of molecules or ions of specific dimensions. Through the synergistic effects of these mechanisms, hydrothermal nitric acid treatment significantly improves the adsorption performance of CGFS-X for MB removal.
Mechanism of one-step hydrothermal nitric acid treatment for producing high adsorption capacity porous materials from coal gasification fine slag
