Calcium-saturated montmorillonite (Ca-Mt) was collected from a mining area in Inner Mongolia and served as the raw material for preparing Li-exchanged montmorillonite (Li-Mt). Its cation exchange capacity (CEC) and layer charge density were 89 mmol/100 g and 0.55 per half-unit cell, respectively. The XRD pattern and FTIR spectrum of Ca-Mt are presented in Fig. S1. All reagents (anhydrous LiCl, Na₂CO₃, NaOH, and HCl) were of analytical grade (Tianjin Kermel Chemical Reagent Co., Ltd.). Cetyltrimethylammonium chloride (CTAC, 97.0%) was purchased from Aladdin Biochemical Technology Co., Ltd. (Shanghai, China).

The preparation process of Li-Mt involves dissolving lithium chloride in water to create a solution with a concentration equivalent to 15% of the mass of Ca-Mt. This solution is then mixed with 100 g of Ca-Mt to form a 10% suspension, which is stirred in a 70°C water bath for 4 hours. Afterward, the mixture is centrifuged to remove the supernatant, and the solid is washed three times. The final product is dried at 105°C and ground to pass through a 200-mesh sieve for later use.

Subsequently, the Li-Mt samples were subjected to microwave irradiation. The samples were placed in a microwave system and irradiated at 2.45 GHz for 22 minutes, with power levels set at 100 W, 800 W, and 1000 W, respectively. After cooling, the irradiated montmorillonite samples, designated as Li-Mt(p100), Li-Mt(p800), and Li-Mt(p1000), were obtained.

In this work, a sodium exchange and sequential organic modification method was employed to organically modify Li-Mt, Li-Mt(p100), Li-Mt(p800), and Li-Mt(p1000) using CTAC as the alkylammonium modifier. First, a sodium carbonate solution, corresponding to 5% of the mass of the montmorillonite to be modified, was dissolved in deionized water. The Li-Mt was dispersed into the solution to form a 4.3% suspension, which was maintained at 70 °C with stirring for 1.5 h and subsequently aged for 2 h to produce sodium-modified montmorillonite. Then, CTAC at a dosage equal to four times the montmorillonite’s CEC was added to the suspension. The system was further agitated at 70 °C for 4 h. After that, the suspension was centrifuged, and the recovered solid was dried and pulverized through a 200-mesh sieve. The resulting CTAC modified montmorillonite samples were designated as CTAC/Li-Mt, CTAC/Li-Mt(p100), CTAC/Li-Mt(p800), and CTAC/Li-Mt(p1000).

Adsorption efficacy of CTAC modified montmorillonite composites for anionic species was determined by measuring post-adsorption anion concentrations in the filtrates. In this study, nitrate ions (NO₃⁻) were targeted as the principal species. The adsorption process was probed systematically across a range of contact times, temperatures, initial pH values, and starting concentrations, in four separate experimental phases. In the first three phases, 0.10 g of the montmorillonite sample was introduced into 50 mL of anionic solution and subjected to continuous shaking. The experimental details were as follows: Adsorption time process: The specific analysis times were 5, 10, 30, 120, and 240 minutes, conducted at 25 °C with pH = 5.3. Temperature process: Conducted at 20, 30, 40, 50, and 60 °C for 30 minutes, with pH = 5.3. Initial pH process: Conducted at pH levels of 3, 5, 8, and 11 at 25 °C for 120 minutes. Unlike the first three processes, the initial concentration process involved shaking 50 mL of NO₃⁻ solution at concentrations of 0.1, 0.5, 1.2, 2, and 3 mmol/L at 25 °C with pH = 5.3 for 120 minutes. In addition, the experimental method of coexistence ions was as follows: The common anions NO₃⁻, SO₄²⁻, PO₄³⁻, and ClO₄⁻, which were frequently found in wastewater, were selected as target pollutants. A 50 mL mixed solution containing 1 mmol/L of each anion (NO₃⁻, SO₄²⁻, PO₄³⁻, and ClO₄⁻) was prepared. Then, 0.10 g of Ca-Mt, Li-Mt, Li-Mt(px), and CTAC/Li-Mt(px) were added to the solution. The mixture was stirred at 25 °C with a pH of 11 for 120 minutes and then filtered. The equilibrium concentrations of NO₃⁻, SO₄²⁻, PO₄³⁻, and ClO₄⁻ in the filtrate were measured.

After shaking and adsorption, the solid-liquid separation was performed. The supernatant was collected and analyzed for residual anion concentrations using an ion chromatograph. The amount of anion adsorbed q_t was calculated using the (Eq. (S1)).

To probe the mechanism underlying NO₃⁻ uptake by CTAC/Li-Mt(px), kinetic modeling was performed using the pseudo-first-order (Eq. (S2)), pseudo-second-order (Eq. (S3)), Elovich (Eq. (S4)), and intraparticle diffusion (Eq. (S5)) equations to discern both the controlling reaction steps and diffusion regimes.

Through fitting adsorption isotherm data, the maximum adsorption capacity of the material was determined—this reflects the equilibrium state when the distribution of adsorbate reaches dynamic balance between solid and solution phases. The Langmuir (Eq. (1)) [21], Freundlich (Eq. (2)) [22], and Sips (Eq. (3)) [23] models were employed to describe the adsorption isotherms.

where Q_e (mmol/g) is the adsorption capacity at equilibrium; Q (mmol/g) is the maximum adsorption capacity; C_e (mmol/L) is the equilibrium concentration of the adsorbate in solution; b is the Langmuir adsorption equilibrium constant, which is related to temperature or the enthalpy change of the adsorption process and reflects the adsorption affinity [21]. K_F (L/g) are empirical constants related to adsorption capacity, 1/n is the Freundlich constant [22], q_s (mmol/g) is the saturation adsorption capacity, k_s represents the Sips adsorption equilibrium constant, and m represents dissociation parameter.

Adsorption thermodynamics involved investigating the adsorption behavior of adsorbents toward adsorbates under varying temperature conditions. This approach enabled the determination of key thermodynamic parameters, such as enthalpy change (ΔH), entropy change (ΔS), and Gibbs free energy change (ΔG). These parameters provided insights into the adsorption tendency, the extent of adsorption, and the driving forces governing the adsorption process.

Research indicated that, during the liquid-phase adsorption process, the solid-liquid distribution coefficient (K_D) of the adsorbate on the adsorbent is calculated using the following equation (Eq. (4)) [24]:

where K_D (mL/g) is the solid-liquid distribution coefficient; C₀ (mmol/L) is the initial concentration of the solution; C_t (mmol/L) is the remaining concentration of the solution at time t; V (mL) is the volume of the solution; m (g) is the mass of the adsorbent [24].

The relationship between the thermodynamic parameters (ΔG, ΔH, ΔS) and the solid-liquid distribution coefficient is given by Eqs. (5), (6) [25]:

where ΔG (kJ/mol) is the change in Gibbs free energy; ΔH (kJ/mol) is the change in reaction enthalpy; ΔS (kJ/mol/K) is the change in reaction entropy; K_D (mL/g) is the solid-liquid distribution coefficient; T (K) is the absolute temperature; R (kJ/mol/K) is the ideal gas constant [25].

X-ray diffraction (XRD) analysis of montmorillonite powder samples was conducted on a Rigaku D/MAX 2500V diffractometer operated at 40 kV and 150 mA. Measurements were performed over a 2 range of 3–80° at a scan rate of 8°/min. Fourier transform infrared (FTIR) spectroscopy was carried out on dried solid samples pressed into KBr pellets using a Nicolet iS 50 FTIR spectrometer. Spectra were collected at a resolution of 4 cm⁻¹ with 32 scans across the 400 to 4000 cm⁻¹ wavenumber range. Specific surface area and pore size distribution were determined via the Brunauer–Emmett–Teller (BET) method using nitrogen adsorption on a BSD-PS(M) analyzer (Best Instrument Technology, Beijing). Prior to analysis, samples were dried and degassed under appropriate conditions. Zeta potential measurements were performed on aqueous suspensions of the powder using a NanoBrook Omni analyzer (Nashua, USA), equipped for high-sensitivity electrophoretic mobility determination. X-ray photoelectron spectroscopy (XPS) measurements were conducted utilizing a Thermo Fisher Scientific Escalab 250Xi spectrometer (Waltham, USA). The survey scan range was 0 to 1400 eV, and high-resolution scans were conducted for elements such as C, O, N, and Cl. Additionally, the CTAC loading on CTAC/Li-Mt(px) was calculated based on the carbon content obtained from elemental analysis, the molar mass of carbon, and the number of carbon atoms in the organic modifier by solving the relevant equations.

The loading capacity of CTAC on CTAC-Mt was determined by measuring the carbon (C) content using a Vario EL Cube elemental analyzer (Elementar, GER). The calculation was performed according to Eq. (7):

where W_C represents the mass fraction of organic carbon in the alkylammonium-modified montmorillonite; M_C (g/mol) is the molar mass of carbon; N_C denotes the number of carbon atoms in the organic modifier molecule; and A (mmol/g) corresponds to the loading capacity of alkylammonium on montmorillonite.

To investigate the effects of different microwave power levels on the crystal structure and surface functional groups of montmorillonites, XRD and FTIR analyses were conducted on Li-Mt, Li-Mt(p100), Li-Mt(p800), and Li-Mt(p1000), as well as CTAC/Li-Mt, CTAC/Li-Mt(p100), CTAC/Li-Mt(p800), and CTAC/Li-Mt(p1000). The resulting XRD and FTIR spectra were shown in Fig. 1 and Fig. S2.

Fig. 1(a) illustrated that the interlayer spacing of CTAC/Li-Mt(px) diminished as the microwave power increased during the modification of montmorillonite. The sequence of interlayer spacings was as follows: CTAC/Li-Mt > CTAC/Li-Mt(p100) > CTAC/Li-Mt(p800) > CTAC/Li-Mt(p1000). Specifically, CTAC/Li-Mt exhibited a maximum interlayer spacing of 1.91 nm, whereas CTAC/Li-Mt(p1000) showed a reduced spacing of 1.78 nm. When compared to Fig. S1 and Fig. S2, the interlayer spacing of CTAC/Li-Mt(px) was notably larger than that of Ca-Mt (1.50 nm) and Li-Mt(px) (1.44–1.47 nm), as shown in Fig. S2(a), confirming the successful intercalation of CTAC into the montmorillonite interlayers. After further modification with CTAC, the diffraction peaks remained essentially unchanged in form, suggesting that the organic modification with CTAC had minimal impact on the basic layered structure of montmorillonite.

Fig. 1(b) revealed a series of infrared characteristic bands for CTAC/Li-Mt(px) at approximately 3625 cm⁻¹, 3425 cm⁻¹, and 1035 cm⁻¹, corresponding to the stretching vibration of the Al-OH bands in the octahedral hydroxyl groups of montmorillonite [26], the stretching vibration of -OH groups in interlayer water molecules, and the asymmetric stretching vibration of Si-O-Si bands in the tetrahedral structure of montmorillonite, respectively. These findings indicated that the fundamental structure of montmorillonite remained intact. Additionally, new characteristic bands of the -CH₂- functional groups of CTAC appeared at approximately 2928 cm⁻¹, 2850 cm⁻¹, and 1480 cm⁻¹, corresponding to the -CH₂- asymmetric stretching vibration, -CH₂- symmetric stretching vibration, and -CH₂- bending vibration [27], respectively, further confirming the successful incorporation of CTAC into the montmorillonite structure, consistent with the XRD results [28]. Furthermore, Fig. S2(b) showed that the band position of the hydroxyl group in the octahedral structure of montmorillonite shifted from 3620 cm⁻¹ in Li-Mt to 3625 cm⁻¹ in lithium-treated montmorillonite, indicating that the lithium treatment promoted the migration of lithium ions into the montmorillonite layers and their combination with the octahedral lattice. Nitrogen adsorption -desorption measurements were conducted to assess the specific surface area and pore size distribution of CTAC-modified montmorillonite composites, including CTAC/Li-Mt, CTAC/Li-Mt (p100), CTAC/Li-Mt(p800), and CTAC/Li-Mt(p1000). Fig. 2 illustrated the adsorption-desorption isotherms and pore size distribution profiles of CTAC/Li-Mt(px) synthesized under varying microwave power levels (0 W, 100 W, 800 W, and 1000 W). Table 1 and Table S1 summarized the specific surface area, pore structure characteristics, zeta potential, and loading capacity data for CTAC/ Li-Mt(px), Li-Mt, Li-Mt(p100), Li-Mt(p800), and Li-Mt(p1000), respectively.

As depicted in Fig. 2, the nitrogen adsorption-desorption isotherms of CTAC-modified montmorillonite composites—CTAC/ Li-Mt, CTAC/Li-Mt(p100), CTAC/Li-Mt(p800), and CTAC/Li-Mt (p1000)—demonstrated an initial gradual increase in adsorption followed by a sharp rise, accompanied by distinct hysteresis loops. According to the IUPAC classification, these isotherms were categorized as type IV [29], with the hysteresis loops identified as type H3 [30]. This indicated that the adsorption of molecules by CTAC/Li-Mt(px) involved multilayer adsorption and that the pore structure of montmorillonite was composed of slit-shaped pores formed by plate-like structures. The pore size distribution profiles demonstrated that the desorption pore diameters of CTAC/Li-Mt(px) predominantly fell within the 2–50 nm range, suggesting that CTAC modification preserved the mesoporous characteristics of montmorillonite. Furthermore, the pore size distribution curves of CTAC/Li-Mt(px) exhibited distinct peaks, with the most probable pore diameter centered at approximately 4 nm, falling within the mesoporous range [31].

The specific surface area is a critical structural parameter for adsorbents, as a larger surface area provides more adsorption sites and enhances the diffusion and adsorption of ions [32]. The CTAC/Li-Mt(px) composites were confirmed to have a specific surface area twice as large as that of CTAC-modified montmorillonite reported in previous study [33]. This significant structural change demonstrated that microwave irradiation could enhance the adsorption potential of CTAC-modified montmorillonite. According to the data in Table 1 and Table S1, the BET specific surface area of CTAC-modified montmorillonites (CTAC/Li-Mt, CTAC/ Li-Mt(p100), CTAC/Li-Mt(p800), CTAC/Li-Mt(p1000)) was markedly lower than that of their lithium-modified counterparts (Li-Mt, Li-Mt(p100), Li-Mt(p800), Li-Mt(p1000)). As shown in Table 1, the BET specific surface areas increased progressively from 10.68 m²/g (CTAC/Li-Mt) to 11.33, 12.94, and 22.65 m²/g for CTAC/Li-Mt (p100), CTAC/Li-Mt(p800), and CTAC/Li-Mt(p1000), respectively, indicating that increased microwave power resulted in a higher specific surface area. This can be attributed to the fact that lithium modification through microwave irradiation occupied some of the permanent negative charge sites with Li⁺, resulting in a reduction in the adsorption sites for CTAC in the montmorillonite interlayer. As the microwave power increased, more adsorption sites were occupied, leading to a lower CTAC loading and consequently less CTAC being adsorbed onto the montmorillonite layers or surface. Furthermore, the adsorption of CTAC both within the interlayer and on the surface of montmorillonite led to significant pore blockage in the montmorillonite structure. As the microwave power increases, the degree of pore blockage decreased.

Analysis of the Zeta potential revealed that the Zeta potential of Li-Mt(px) reversed from negative to positive after modification with CTAC, indicating successful modification of CTAC/Li-Mt(px). This transformation was attributed to the presence of chloride ions (Cl⁻) and large organic cations (CTAC⁺) in the CTAC structure, which dissociated in water. Under electrostatic attraction, CTAC⁺ was introduced into the interlayers of lithium modified montmorillonite, neutralizing the inherent electronegativity of the montmorillonite layers and forming an organic phase within the interlayer. The formation of this organic phase further promoted the adsorption of additional CTAC onto the interlayer spaces or the surface of montmorillonite [34], resulting in the reversal of the surface potential. Moreover, the Zeta potential of the modified montmorillonite decreased as the microwave power increased. This trend arose because the CTAC loading on montmorillonite decreased with increasing microwave power. Consequently, CTAC not only neutralized the negative charges on the montmorillonite surface but also left an excess of positive charges. The quantity of residual positive charges exhibited an inverse correlation with the microwave power applied during modification.

By observing the changes in loading capacity, it could be determined that montmorillonites subjected to different microwave powers successfully adsorbed a certain amount of CTAC, confirming the successful modification of montmorillonite with CTAC. Among these, CTAC/Li-Mt exhibited the highest CTAC loading, reaching 303.65 mmol/g. A clear trend was observed where lower microwave power corresponded to a higher loading of the modifying agent on CTAC/Li-Mt(px). This suggested that CTAC primarily entered the montmorillonite interlayers through ion exchange.

The adsorption capacity of Ca-Mt, Li-Mt, Li-Mt(px), and CTAC/ Li-Mt(px) for NO₃⁻ removal under varying adsorption conditions was presented in Fig. 3.

In Fig. 3(a), the adsorption capacities of both Li-Mt(px) and CTAC/Li-Mt(px) toward NO₃⁻ gradually rose within the first 30 minutes and thereafter leveled off. Among the tested montmorillonite samples, the nitrate adsorption capacity of Ca-Mt was only 0.034 mmol/g, while that of CTAC/Li-Mt reached 0.133 mmol/g, which was about 3.9 times that of the Ca-Mt (Fig. 3(a)). Even for CTAC/Li-Mt (p1000), its adsorption capacity (0.07 mmol/g) was still 2.1 times that of Ca-Mt, indicating that the synergistic effect of CTAC organic modification and lithium exchange significantly improved the anion adsorption capacity of montmorillonite. For CTAC/Li-Mt(px), its adsorption capacity for NO₃⁻ followed the order: CTAC/Li-Mt > CTAC/Li-Mt(p100) > CTAC/Li-Mt(p800) > CTAC/ Li-Mt(p1000). This result indicated that the increase in microwave power led to a decrease in the nitrate adsorption capacity of CTAC/Li-Mt, and its intrinsic mechanism could be clearly explained through the quantitative causal relationships between key factors. As microwave power increased, Li+ migration into the montmorillonite lattice was enhanced [35, 36], and the Li-exchanged precursors exhibited decreased layer charge density and CEC (Table S2), with layer charge density decreasing from 0.48 (p100) to 0.45 (p800) and 0.35 (p1000), and CEC decreasing from 59 to 56 and 55 mmol/100 g, respectively. Consistently, the CTAC loading decreased from 292.58 (p100) to 265.15 (p800) and 244.58 mmol/g (p1000), accompanied by a decrease in zeta potential from 30.74 to 26.58 and 25.13 mV for the corresponding samples (Table 1). These correlations indicated that as the microwave power increased, the effective CTAC loading and surface positive charges decreased, thereby resulting in a reduced adsorption capacity of CTAC/Li-Mt(px) for NO₃⁻ under higher microwave power conditions.

As shown in Fig. 3(b), the adsorption capacity of Li-Mt(px) increased with rising temperature. In contrast, CTAC/Li-Mt(px) exhibited a slight decline in adsorption as temperature increased, though the overall variation remained minimal. Fig. 3(c) indicated that for both Li-Mt(px) and CTAC/Li-Mt(px), the adsorption capacity for NO₃⁻ first increased with rising pH and then declined. Under acidic conditions (pH < 5), the relatively higher concentration of SO₄²⁻ ions used to adjust the pH competed with NO₃⁻ for adsorption sites. When pH exceeded 7, elevated OH⁻ concentrations also competed with nitrate for binding sites [37]. Moreover, it could be calculated from Fig. 3(d) that with an increase in the initial concentration of NO₃⁻ in the solution, the NO₃⁻ removal ratio of Li-Mt(px) and CTAC/Li-Mt(px) decreased, indicating that the number of available adsorption sites was limited. As the concentration of NO₃⁻ increased, the adsorption sites were gradually occupied, reaching adsorption saturation, which resulted in the remaining NO₃⁻ being unable to be effectively removed.

As shown in Fig. 3(e), Mt samples exhibited a marked selectivity for perchlorate anions (ClO₄⁻), attributed to the lower hydration energy of ClO₄⁻ (−214 kJ/mmol) [38]. Lower hydration energy correlated with lower hydration, which in turn increased adsorption affinity [39]. Overall, the relative order of anion adsorption was ClO₄⁻ > NO₃⁻ > SO₄²⁻ > PO₄³⁻.

The results above clearly indicated that both lithium exchange via microwave treatment and organic modification enhanced the anion adsorption capacity of montmorillonite. Notably, the unmodified CTAC/Li-Mt showed higher adsorption than its microwave-treated versions, implying that microwave exposure may have suppressed available adsorption sites for CTAC. This effect was likely due to microwave-induced migration of Li+ into montmorillonite’s tetrahedral or octahedral sheets, thereby lowering its cation exchange capacity (CEC) [20, 40]. Overall, microwave modification enhanced the adsorption capacity of Mt but was detrimental to the adsorption of CTAC modified montmorillonite.

To investigate the adsorption behavior of NO₃⁻ on CTAC/Li-Mt(px), the time-dependent uptake data (Fig. 3(a)) were fitted using the pseudo-first-order, pseudo-second-order, Elovich, and intraparticle diffusion kinetic models. The corresponding fitting curves and parameters were provided in Fig. S3 and Tables S3–S4. Considering the relatively low R² values obtained for these kinetic models, the fittings were used only as a comparative reference. The adsorption profiles (Fig. 3(a)) showed a rapid initial uptake within the first 30 min, followed by a slower stage with minor fluctuations at longer contact times. The weak kinetic fittings likely reflected a multi-step uptake process involving simultaneous mass-transfer effects and heterogeneous adsorption sites introduced by modification.

In addition, the equilibrium adsorption data at different initial nitrate concentrations (Fig. 3(d)) were fitted with the Langmuir, Freundlich, and Sips isotherm models. The isotherm fitting curves were presented in Fig. S4, and the fitted parameters were summarized in Table 2 and Table S5. For CTAC/Li-Mt and CTAC/Li-Mt(p100), the Langmuir model provided the best fit among the three models, suggesting that monolayer-type uptake was more dominant for these samples. For both CTAC/Li-Mt(p800) and CTAC/Li-Mt(p1000), the Freundlich model yielded higher R² values and lower χ² values, suggesting increased adsorption-site heterogeneity and a greater contribution from non-ideal adsorption behavior. These results indicated a shift from predominantly Langmuir-type behavior at low microwave power to more heterogeneous, non-ideal adsorption at higher microwave power [41, 42]. Moreover, the Langmuir monolayer capacity Q provided an estimate of the potential maximum nitrate uptake under the tested conditions. Due to the large uncertainties of the Sips parameters for some samples, only the Langmuir-derived Q was discussed quantitatively below. The Q value increased from 0.35 ± 0.02 (CTAC/Li-Mt) to 0.41 ± 0.08 (CTAC/Li-Mt(p800)), suggesting that appropriately controlled microwave-assisted Li-exchange preconditioning (e.g., p800) increased the capacity potential of the modified montmorillonite, in line with the experimentally observed improvement in nitrate uptake.

The measured adsorption capacity of NO₃⁻ (Fig. 3(b)) was analyzed using thermodynamic equations (Eqs. (4)-(6)) to calculate the corresponding thermodynamic parameters. The results were summarized in Table 3 and Table S6. In the adsorption process of NO₃⁻ by CTAC/Li-Mt(px), the thermodynamic parameters exhibited significant characteristics: As shown in Table 3, all enthalpy change (ΔH) values were negative, indicating that the adsorption process was exothermic. This implied that lower temperatures could facilitate the adsorption reaction [43], which was consistent with the experimentally observed temperature dependence of nitrate adsorption capacity (Fig. 3(b)). The Gibbs free energy change (ΔG) values were all negative and shifted to higher negative values throughout the experimental temperature range of 20 °C to 60 °C, thus confirming that the adsorption process was thermodynamically spontaneous [44, 45]. The positive entropy change (ΔS) reflected an overall increase in disorder during the adsorption process [43].

To investigate the changes in surface functional groups of the organically modified montmorillonite after nitrate ion (NO₃⁻) adsorption, FTIR analysis was performed on representative samples of CTAC/Li-Mt(p100) following NO₃⁻ adsorption. Fig. 4 presents the FTIR spectrum of CTAC/Li-Mt(p100) after adsorption. As shown in Fig. 4, the FTIR spectrum of CTAC/Li-Mt(p100) after adsorption retained the organic functional group bands at 2928 cm⁻¹, 2850 cm⁻¹, and 1480 cm⁻¹ associated with the -CH₂ group, while several changes in infrared characteristic bands were observed: the octahedral hydroxyl stretching vibration at around 3625 cm⁻¹ and the interlayer water molecule hydroxyl stretching vibration at around 3425 cm⁻¹ [46]. This indicated that the main structure of CTAC/Li-Mt(p100) was preserved after adsorption. A comparison of the transmittance curves before and after NO₃⁻ adsorption showed that after adsorption, characteristic bands for NO₃⁻ appeared around 1384 cm⁻¹ and 840 cm⁻¹, confirming the successful adsorption of NO₃⁻ onto CTAC/Li-Mt(p100) [47]. Furthermore, the characteristic bands at 2928 cm⁻¹, 2850 cm⁻¹, 1480 cm⁻¹, and 1035 cm⁻¹ showed varying degrees of attenuation after adsorption, primarily due to the saturation of NO₃⁻ adsorption, which led to a coverage effect and consequently a decrease in transmittance [28].

The XPS spectrum of the CTAC/Li-Mt(p100) sample after NO₃⁻ adsorption is shown in Fig. 5. It is evident from Fig. 5(a) that the two peaks around 402 eV and 399 eV in the XPS spectrum of CTAC/Li-Mt(p100) were attributed to the quaternary ammonium functional group -R₄ N⁺, which resulted from the interaction between CTAC and montmorillonite (Mt) [48]. This confirmed that CTAC was successfully loaded onto the lithium exchanged montmorillonite, consistent with the FTIR analysis results. The interaction between -R₄N⁺ and montmorillonite was notably weaker compared to that between -R₄N⁺ and CTAC, indicating that the majority of -R₄N⁺ groups were predominantly associated with CTAC, with a smaller fraction interacting with montmorillonite. After adsorption, a new peak corresponding to NO₃⁻ appeared at 405.97 eV, confirming the successful adsorption of NO₃⁻ by CTAC/Li-Mt(p100). Comparison of the Cl 2p spectra before and after adsorption (Fig. 5(b)) revealed that the Cl 2p_3/2 and Cl 2p_1/2 peaks corresponding to Cl⁻ shifted by 0.14 eV and 0.29 eV, indicating that Cl⁻ participated in the adsorption process. Furthermore, the significant decrease in the Cl⁻ peak area after adsorption further supported that the adsorption mechanism of the organic synergistically modified montmorillonite for NO₃⁻ was based on an ion exchange process between the Cl⁻ in the quaternary ammonium group and NO₃⁻ [38].

Overall, the present results suggested that integrating microwaveassisted Li-exchange with CTAC modification provided a practical route to enhance nitrate adsorption by montmorillonite through microwave-power preconditioning. In practical effluents, nitrate concentrations can vary widely, ranging from the mg/L level in some treated wastewaters to tens to hundreds of mg/L as NO₃⁻ in certain industrial streams. The best-performing sample, CTAC/ Li-Mt(p800), exhibited a Langmuir monolayer capacity of Q = 0.41 ± 0.08 mmol/g, which was higher than that of CTAC/Li-Mt prepared without microwave irradiation (Q = 0.35 ± 0.02 mmol/g) under identical conditions (Table 2). This indicated its potential value as a polishing adsorbent for nitrate-containing effluents, although the practical performance could be reduced in complex matrices due to competing anions and elevated ionic strength, and further evaluation of multi-cycle reusability was still required, including structural stability and potential surfactant loss.