2.1 Materials

Montmorillonite K 10 clay (cation exchange capacity (CEC) = 48 m_eq/100 g) was purchased from Nanosany Co., the chemical compositions of montmorillonite (wt%) are SiO₂ 50.95 %, Al₂O₃ 19.60 %, Fe₂O₃ 5.62 %, MgO 3.29 %, CaO 1.97 %, Na₂O 0.98 %, K₂O 0.86 %, TiO₂ 0.62 %, LOI 15.45 %. The chemicals used in this study copper nitrate trihydrate Cu(NO₃)₂·3H₂O (>95 %), magnesium nitrate hexahydrate Mg(NO₃)₂·6H₂O (>98 %), aluminum nitrate nonahydrate Al(NO₃)₃·9H₂O (>98 %), and cadmium nitrate tetrahydrate Cd(NO₃)₂·4H₂O (>98 %), also NaOH and HCL were used without further purification. The cadmium solutions were prepared using deionized water.

2.2 Synthesis of CuMgAl-LDH/MMt

The CuMgAl-LDH/MMt nanocomposite is considered a superior adsorbent for cationic ion removal. It was synthesized using the low-supersaturation co-precipitation method at constant pH giving a metal composition in the range (0.25 < x < 0.33). The cation salts of Mg(NO₃)₂·6H₂O, Cu(NO₃)₂·3H₂O, and Al(NO₃)₃·9H₂O had molar ratios (Cu²⁺, Mg²⁺: Al³⁺) and (Cu²⁺: Mg²⁺) of 2:1 and 1:1 respectively, and were added to 40 ml deionized water, and the mixture stirred for 2 h at room temperature. 5 g of MMt was dispersed in 500 ml of water and heated at 50 °C for 30 min with vigorous agitation. The cation salts solution and MMt solution were simultaneously added dropwise to the 200 ml of deionized water and stirred for 3 h. The reaction pH was maintained at a constant value of 9 ± 0.1 by adding 1 M NaOH (sketch 1). The mixture was aged for 12 h at 100 °C then filtered, washed three-times with deionized water, and oven-dried at 65 °C for 12 h. The claybank has appeared, and the possible reaction can be described in the following Eq. (1).

MMt is used as a base for the direct growth of CuMgAl-LDH. The interaction between the Cu²⁺, Mg²⁺, and Al³⁺ and the ${\mathit{OH}}^{-}$ leads to the formation of CuMgAl-LDH on the MMt, in which ${\mathit{NO}}_3^{2-}$ ions are intercalated into the interlayer structure to balance the charge.

2.3 Characterization

Transmission electron microscopy was used to investigate the nanostructures of the synthesized CuMgAl-LDH/MMt (TEM, Philips, CM120). The crystallinity of the samples were described by Powder X-ray diffraction patterns (XRD), which were recorded by a Siemens-D500 diffractometer with CuK_α radiation, operating at 40 kV, 40 mA, and λ_CuKα = 1.5418 Å at a scan rate of 0.05° s⁻¹ across a 2θ range of 0 – 80°. A Brunauer–Emmett–Teller analysis (BET) was used to determine specific surface areas and pore size distributions. Field-Emission Scanning Electron Microscopy (SEM, ZEISS, UK) outfitted with energy dispersive X-ray spectroscopy (EDX) was used to characterize the surface morphology and elemental distribution of the samples. Fourier transform infrared spectra in the range of 380–4000 cm⁻¹ were used to describe the surface functional groups of the materials (FIIR, Jasco 4200, Japan). The point zero charge (pH pzc) of the CuMgAl-LDH/MMt adsorbents were identified by preparing a 40 ml solution of (0.1 M) NaNO₃ and adjusting the pH of the solution with 0.1 M NaOH and/or HCl to obtain pH values in the range 2–12. An amount of adsorbent was added to the solution which was then agitated at a speed of 150 rpm at a temperature of 30 °C for 24 h. The mixture was then filtered to separate the solid particles, and the final solution pH was tested. The pH_PZC of the CuMgAl-LDH/MMt adsorbent is the intersection point in the plot of final by (initial-final) pH (Begum et al., 2021; Mohammed and Kareem, 2019).

2.4 Batch experiments for Cd²⁺ removal

Batch adsorption experiments were carried out to determine the Cd²⁺ removal performance of the as-prepared adsorbent. The effects of pH (2–8), CuMgAl-LDH/MMt (0.05–0.3 gm/100 ml Cd²⁺ solution dosage, agitation speed (100–250 rpm), particle size (87, 122, 194 μm), and initial concentration of Cd²⁺ (20–70 ppm) on the removal of cadmium ions was studied. The Cd²⁺ uptake on the CuMgAl-LDH/MMt nanocomposite was evaluated as a function of time (5–180 min) to determine the time required to reach equilibrium. A kinetic adsorption process was employed to evaluate the equilibrium time of Cd²⁺ adsorption on LDH to examine its mechanism and rate-controlling step. This was done by combining 0.25 mg of LDH with 100 ml of Cd²⁺ solution in concentrations ranging from 20 mg/l to 70 mg/l at 298 K and an initial pH of 5, adjusted with 0.1 M HCL and 0.1 M NaOH. This process is primarily time-dependent.

To determine the effect of temperature, the experiments were performed under the optimal conditions determined by previous experiments, and the temperature was adjusted (25, 30, 35, 40 °C) using an incubator shaker.

In each experiment, a quantity of adsorbate (mg) was reacted with 100 ml of Cd²⁺ solution and shaken for 180 min. After adsorption, the mixture of CuMgAl-LDH/MMt and cadmium ions was separated using 0.45 μm filter paper. The residual metal ions in the solution were analyzed using an atomic absorption spectrophotometer (AAS) (AA-7000, Shimadzu Corporation, Japan). The removal efficiency, and adsorption amount of adsorbate for Cd²⁺, were calculated by using equations (2) and (3), respectively.

2.5 Statistical analysis

To compute the isotherm and kinetic model parameters, trial and error nonlinear regression analysis was performed using the Solver add-in in Microsoft Excel. Eq. (4) was used to calculate the determination coefficients (R²) of the nonlinear optimization models. To identify the best fitting adsorption model, chi-square values (X²) were determined alongside non-linear R² (Tran et al., 2018).

Where: q_e,exp is experimental solute uptake at equilibrium (mg/g), q_e,cal is the solute uptake calculated from the corresponding isotherm and kinetic model(mg/g), and q_e,mean is the mean of q_e,exp values (mg/g).

3.1 Characterization of MMT, CuMgAl-LDH, CuMgAl-LDH/MMt

The morphology of the prepared nanoadsorbent’s internal structure was investigated using transmission electron microscopy (TEM). The TEM micrographs in Fig. 1 demonstrate that the CuMgAl-LDH/MMt nanocomposites have irregular shapes, but the vast majority of them are roughly integrated spherical hierarchical particles with variable crystallite sizes. The LDH was well dispersed across the MMt substrate surface with minimal agglomeration, and it was neither too dense to block neighboring particles nor too sparse to provide enough active sites. LDH acts like monodispersed particles, rather than multidispersed particles, thus maximizing the surface area available for contaminant molecule adsorption (Mohammed and Kareem, 2019). The inclusion of a MMt substrate may significantly inhibit the LDHs aggregation due to the oxygen-containing functional groups on the MMt nanoparticles that provide nucleation sites for the growth of LDH (Yang et al., 2013). It can be concluded that by growing LDH on MMt, the ion exchange and surface adsorption abilities of LDH may be effective in removing heavy metal ions from liquid solutions.

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Fig. 1

TEM image of synthesized CuMgAl-LDH/MMt adsorbent.

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The crystallinity of the LDHs was described by their Powder X-ray diffraction patterns (XRD). The XRD patterns of the substrate MMt, the precursor CuMgAl-LDH, and the prepared CuMgAl-LDH/MMt are shown in Fig. 2. The XRD pattern in Fig. 2a, of the (00 l) series diffraction peaks at (0 0 3), (0 0 6), and (0 0 9), are observed as narrow symmetric lines at low 2θ angles at 20.5, 31.6, and 41.5°, respectively, indicating a lamellar structure (Rathee et al., 2020).

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Fig. 2

XRD characterization of (a) CuMgAl-LDH, (b) MMt, (c) CuMgAl-LDH/MMt.

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The diffraction peaks of MMt in Fig. 2b, observed at 19.9, 26.6, 35.0, 39.6 and 45.9°, confirm the crystalline structure of MMt (Ahmadi et al., 2020) and are consistent with the standard pattern (JCPDS no. 03–0015), suggesting the presence of MMt in their structure. In addition, a peak related to the crystalline structure of (0 0 1) at 6.3° was seen in the MMt structure, indicating a layer structure in the MMt matrix(Foroutan et al., 2020a; Peighambardoust and Pourabbas, 2007).

The characteristic sharp peaks of CuMgAl-LDH/MMt at 2θ values of 12.4, 19.9, 20.9, 26.8, 27.8, 35.2, 36.7, 39.6, 50.3, 60.1, 62.1, and 68.3° (corresponding to the hydrotalcite lattice planes (hkl) planes of (1 0 0), (0 0 2), (2 0 0), (1 1 2), (2 1 0), (2 1 2), (2 2 0), (3 1 1), (4 1 1), (3 3 2), (3 2 4), and (5 0 4), respectively (JCPDS no. 012–0232)), were identified, and the interlayer spacing for CuMgAl-LDH/MMt calculated for these diffraction angles. The findings are shown in Fig. 2c. The well-defined diffraction peaks of CuMgAl-LDH/MMt can be indexed to a series of crystal planes, which is characteristic of hydrotalcite-like materials (Jiang et al., 2019b). and the narrow peaks are attributed to the high crystallinity of the prepared adsorbent (Rathee et al., 2020).

The diffraction peaks of (3 2 4) at high angles (near 2θ = 62.1˚) reflect that the presence of three metal cations (Cu²⁺, Mg²⁺ and Al³⁺) in the host layer. The crystallite size for MMt, CuMgAl-LDH and CuMgAl-LDH/MMt was calculated using the Scherrer equation (Kareem and Mohammed, 2020) and found to be 2, 27.74, and 30.89 nm, respectively. This confirms the loading of MMt and the successful fabrication of particles in nano dimensions with a crystallinity order of 44.64 %.

As illustrated in Fig. 3, N2 sorption–desorption isotherms at 77 K were used to investigate the main textural (microstructural) properties of the substrate MMt, the precursor CuMgAl-LDH, and the as-synthesized CuMgAl-LDH/MMt adsorbent. The specific surface area was estimated by applying the BET equation with the total pore volume taken at P/Po = 1, and the pore size distribution was estimated by the BJH method. Based on the IUPAC classification, all tested samples were classified as IV-type isotherms and have a H3 type hysteresis loop (which extends down to P/Po = 0.42). It indicates that all adsorbents have the characteristic structure of mesopores (Santamaría et al., 2020) with average pore diameters of 6.83, 19.813, and 26.238, for the MMt-, LDH, and as-synthesized LDH-MMt hybrid, respectively. The extensive hysteresis loop demonstrates the successful fabrication of a mesoporous structure that permits the fast diffusion of inorganic pollutants during the adsorption process. This accelerates the reaction rate and provides a more active site for adsorption (Ahmed et al., 2017).

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Fig. 3

N₂ adsorption/desorption isotherms and pore-size distribution of (a) MMt, (b) CuMgAl-LDH (c) CuMgAl-LDH/MMt.

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The BET surface areas of MMt and LDH as listed in Table 1 are 17.23 and 12.545 m²/g, respectively, while that of the CuMgAl-LDH/MMt hybrid is higher at 77.62 m²/g with a BET constant of 94.022. This indicates that precipitation of LDH into the MMt increased its specific surface area and improved its porosity, thus resulting in a comparatively greater uptake of Cd²⁺ ions (Foroutan et al., 2021a). This could be due to decreased agglomeration from the mixing of the two counterparts and preparation of the hybrid nano-filler (Mallakpour and Naghdi, 2020). As the BET results showed, a CuMgAl-LDH/MMt would has higher catalytic and adsorption activity also play an important role in the Cd²⁺ adsorption mechanism(Chen et al., 2012) than for MMt and LDH alone.

Fig. 4a depicts the surface morphology of native MMt prior to the nonabsorbent preparation. This figure shows an irregular sphere-shaped possessing lamellar aggregates. Several minute cavities and pores can be seen suggesting that the surface of MMt facilitates the precipitation of metal salts during the synthesis of the nanocomposite. Fig. 4b shows the porous structure of CuMgAl-LDH/MMt. It can be clearly seen that layered materials grow directly on the surface of the spherical MMt, and chemical bonding ensures a strong adhesion of CuMgAl-LDH to MMt. The results above indicate that the CuMgAl-LDHs film successfully modified the MMt. The layered structure embedded cavities of sizes ranging from 1.2 to 61.3 nm (as defined in N₂ sorption–desorption isotherms) that were scattered across the complete sample matrix. These cavities are large enough to enable heavy metal ions to penetrate the LDH structure where they are then able to interact with the surface groups. The morphology of CuMgAl-LDH/MMt was also observed using SEM after the sorption of Cd²⁺ ions (Fig. 4c). This figure suggests that the sorption of cadmium ions did not substantially alter the surface morphological characteristics of CuMgAl-LDH/MMt adsorbents. These results confirm that the CuMgAl-LDH/MMt has considerable stability in acidic environments(Ahmed et al., 2017). Furthermore, in all experiments the primary mechanism for cadmium removal by the synthesized LDHs was by ion exchange (Ma et al., 2014).

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Fig. 4

SEM image of (a) MMt, (b) CuMgAl-LDH/MMt before adsorption, (c) CuMgAl-LDH/MMt after Cd²⁺ sorption.

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Energy dispersive spectrum (EDS) maps were taken on the SEM. The EDS mapping image in Fig. 5a shows the surface elements of MMt, whereas in Fig. 5b shows that the CuMgAl-LDH/MMt surface has elements of O, Si, Al, Mg, and Cu. Considerable amounts of C were also present, implying the need for more thorough washing of the synthesis solution to eliminate carbonates, or that the water used for washing was poorly distilled (Vulić and Bošković, 2010). There was also a decrease in the initial molar ratio of (Cu + Mg)/Al, which is probably the result of Mg and Al being elements in the chemical composition of both MMt and LDH, and thereby producing an overlap in the molar ratio. The change in the elements distribution of CuMgAl-LDH/MMt and decrease in the initial molar ratio are very strong proof of the success of structuring LDH with MMt by a co-precipitation method. The EDS spectrum after sorption of Cd²⁺ is illustrated in Fig. 5b. The figure shows the EDS spectrum of LDH/MMt loaded with Cd²⁺ and also that the CuMgAl-LDH/MMt content and elements distribution had changed. This proved the sorption of Cd²⁺ onto CuMgAl-LDH/MMt.

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Fig. 5

EDS analysis of (a)CuMgAl-LDH/MMt before adsorption, (b)CuMgAl-LDH/MMt after Cd²⁺ sorption.

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FTIR spectra of MMt, CuMgAl-LDH/MMt before and after Cd²⁺ ion adsorption, is shown in Fig. 6. It shows the existence of functional groups that might be responsible for the adsorption process. Some peaks were seen at 3420, 1635 and 1033 cm⁻¹ in MMt, indicating the stretch vibrations of Al–OH, –OH and Si–O–Si respectively (Ahmadi et al., 2020). Also, other peaks were observed at 796, 532, 470 cm⁻¹, which are attributed to Mg–Al–OH, Si–O–Al, and Si–O–Si (Ahmadi et al., 2020; Madejova, 2003). A shown in Fig. 6b, a characteristic band at 3625 cm⁻¹ can be ascribed to Al-OH groups located between the octahedral and tetrahedral layers (Madejova, 2003; Shattar et al., 2017). The broad vibrating band at 3444 cm⁻¹, and the weak band at 1634 cm⁻¹, are ascribed to the H—O—H bending vibration of free water and the stretching vibration of –OH groups of interlaminar water molecules in the LDH structure, respectively (Jiang et al., 2019a; Kostic et al., 2018; Madejova, 2003; Mishra et al., 2012). The peaks at 2956 and 2853 cm⁻¹ are assigned to the asymmetric C—H and symmetric C—H stretching vibrations (Mishra et al., 2012). The nitrate in the layers of LDH can be observed at a wavenumber of 2359 cm⁻¹(Seddighi et al., 2017). The broad absorption band at 1034 cm⁻¹ is ascribed to Si–O stretching vibrations of the tetrahedral-layer (Breen et al., 1995; Madejova, 2003), whereas the vibrating bands at 530 cm⁻¹ and 472 cm⁻¹ are attributed to Si–O–Al and Si–O–Si groups, respectively. The absorption bands in the range of 650–850 cm⁻¹ are attributed to the Al–M − OH (M = Mg and Cu) that are present on the edges of LDH sheets (Hossein Panahi et al., 2017; Mishra et al., 2012). The bands of metal-hydroxide octahedral complexes (M − O, M = Mg and Cu) can be detected at lower wavenumbers in the 520–380 cm⁻¹ region (Seddighi et al., 2017). It indicates that the CuMgAl-LDH grew on the MMt, which is consistent with the characterization provided by the XRD (Jiang et al., 2019a). The distinctive bands of the CuMgAl-LDH/MMt structure were clearly detected in the FTIR spectra after Cd²⁺ ion sorption (Fig. 6c), however their intensities were drastically reduced compared to those shown in Fig. 6b. This indicates that the solid particles of the adsorbent maintain their chemical structure throughout the interaction between the solid surfaces and the solution (Khitous et al., 2015). Some peaks behaved differently, with observable shifts in their wavenumbers, indicating that the studied metals had a strong affinity for the functional groups on their surface (Mohammed et al., 2020; Shahid et al., 2021). As shown in Fig. 6c, the intensities of the bands of metal-hydroxide octahedral complexes were significantly changed. This suggests that the mechanism of ion exchange with octahedral ions is responsible for the removal of cadmium ions from the solution (Khitous et al., 2015).

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Fig. 6

FTIR of (a) MMt, CuMgAl-LDH/MMt (b) before adsorption, and (c) after Cd²⁺ sorption.

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3.2 Parameters affecting Cd²⁺ adsorption

The sorption of Cd²⁺ is affected by several factors including pH, adsorbent dose, agitation speed, particle size, initial Cd²⁺ concentration, and adsorbate–adsorbent contact time.

The pH substantially affects both the adsorbent chemistry and surface charge of the LDH-based composites. This greatly influences its tendency toward adsorbent particles, the speciation of ions in the solution, and the conditioning of the adsorption mechanism (Chen et al., 2019; Zubair et al., 2017). In this study, the effect of initial pH (2–8) on the adsorption of 50 ppm of Cd²⁺ on 0.25 g of adsorbent was investigated. The adsorption capacity increased up to a maximum value of 14.3 mg/g at a pH of 5 (see Fig. 7a). As pH increases, the electrostatic attraction, or chemisorption, will occur more readily since the active groups in LDH, such as –OH groups, are gradually deprotonated, accompanied by the negatively charged surface(Seddighi et al., 2017). On the other hand, the adsorption capacity decreased with further increases in pH beyond 5 to 8.0 (12.2 mg/g). This can be attributed to the electrostatic repulsion between the Cd²⁺ species and the CuMgAl-LDH/MMt surface charges, which is influenced by the pH-solution, as well as to the metal complexes lower solubility in higher pH solutions decreasing sufficiently to allow precipitation. The results showed the removal rate of Cd²⁺ was more pH-dependent and that while CuMgAl-LDH/MMt can adsorb Cd²⁺ in acidic, neutral, and basic media, acidic media have the highest removal rate. The pH effect can be explained in terms of the point zero charge (pH_PZC = 4.1), which indicates the pH at which the surface of CuMgAl-LDH/MMt is electrically neutral (Nava-Andrade et al., 2021). The pH effect could be explained in terms of the point zero charge (pH_PZC = 4.1), which indicates the pH at which the surface of CuMgAl-LDH/MMt is electrically neutral (Nava-Andrade et al., 2021).

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Fig. 7

Effect of expremental conditions on Cd²⁺ sorption onto CuMgAl-LDH/MMt. (a) pH-solution at Co = 50, dose = 0.25 g, rpm = 200, and particle size = 194 μm; (b) CuMgAl-LDH/MMt dose at pH = 5, Co = 50, rpm = 200, and particle size = 194 μm; (c) agitation speed at pH = 5, Co = 50, dose = 0.2g, and particle size = 194 μm; (d) CuMgAl-LDH/MMt particle size at pH = 5, Co = 50, dose = 0.2g, and rpm = 150; and (e) initial Cd²⁺ concentration at pH = 5, dose = 0.2g, and rpm = 150, and 87 μm,

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When pH < pH_PZC, the CuMgAl-LDH/MMt surface is protonated and the adsorbent surface becomes positively charged causing the electrostatic repulsion between Cd²⁺ and the adsorbent surface and it become strongly at the highly alkali media. In contrast, when the pH is greater than pH_PZC the surface becomes deprotonated, and its resulting negative charge increases the electrostatic attraction of the binding sites to Cd²⁺, thereby enabling the surface to adsorb more Cd²⁺ (Mittal, 2021).

The effects of different dosages of CuMgAl-LDH/ MMt (0.05, 0.1, 0.15, 0.2, 0.25, and 0.3 gm/100 ml) on Cd²⁺ adsorption are shown in Fig. 7b. It can be observed that the Cd²⁺ removal efficiency increases from 38.4 % to 77.9 % when the sorbent dose rises from 0.05 to 0.2 g. Furthermore, the high sorbent dosage resulted in a reduced adsorption capacity of CuMgAl-LDH/MMt at an initial Cd²⁺ concentration of 50 ppm. This behavior was most likely caused by the low sorbent dose, which caused the dispersion of CuMgAl LDH/MMt particles in the liquid phase. In this situation, adsorption at the surface was rapidly saturated, showing a high capacity for adsorption. At increased particle concentrations, however, the availability of sites with higher energy declines, and a greater proportion of the lower-energy sites get occupied, resulting in a diminished adsorption capability(Bo et al., 2015). As a result, future experimental of adsorption will use 0.20 g as the effective dose.

As indicated in Fig. 7c, four agitation speeds (100, 150, 200, and 250 rpm) were considered to examine the influence of external diffusion at the optimum pH and CuMgAl-LDH/MMt dose. As agitated speed rose, Cd²⁺ ion removal increased, plateauing at 150 rpm (20.11 mg/g). As the agitation speed increased, the liquid stationary layer around the adsorbent particle decreased. The external diffusion coefficient increases, reducing mass transfer resistance. This increases cadmium's diffusivity from the bulk liquid phase via the boundary layer to the sorbent surface (Evans et al., 2002; Shen and Duvnjak, 2005). Higher agitation speeds may reduce contaminant removal and adsorption time owing to the vortex phenomena (Arif et al., 2022).

The sorbent size also plays a significant role in the sorption process (Vijayaraghavan and Yun, 2008). At optimum pH, agitation speed, and sorbent dose, the influence of three CuMgAl-LDH/MMt particle sizes (87, 122, and 194 μm) on Cadmium ion adsorption was examined (Fig. 7d). As explained in the figure, Cd²⁺ uptake decreased with increasing sorbent particle size, with the smallest particle size (87 μm) yielding the highest cadmium uptake (22.36 mg/g). Smaller particles have greater specific surface areas, more binding sites, and exhibit faster adsorption (Mohammed and Kareem, 2021). Hence, a particle size of 87 μm was used, in conjunction with the other previously established optimal parameters, to examine the effect of initial concentration and contact time.

Fig. 7e and f shows the uptake of CuMgAl-LDH/MMt as a function of different initial Cd²⁺ concentrations and contact time. The uptake of Cd²⁺ (qe) increases from 8.15 to 32.59 mg/g when the initial Cd²⁺ concentration increases from 20 to 70 ppm (Fig. 7e.) This is because at higher concentrations, the driving force increases and overcomes the mass transfer resistance of the sorbate between the liquid phase and the solid phase. The higher uptake is thus attributed to adsorption occurring in a monolayer at a low initial concentrations but changing to a multilayer at a high initial concentrations (Mittal, 2021; Nava-Andrade et al., 2021). The curves also illustrated a rapid increase in the adsorption of Cd²⁺ at the first stage. After that, the adsorption process increased slowly until equilibrium was reached at minute 120. This behavior can be described as follows: in the first 30 min, the adsorbent has more active sites and most adsorption sites become saturated. and the rest of them are hard to occupy owing to the repulsive force between the bulk phases and solid sorbate molecules (Jiang et al., 2019a)(Mohammed and Kareem, 2021).

From the above results, it can be concluded that under optimal conditions for the sorption of cadmium ions on a CuMgAl-LDH/MMt adsorbent, the maximum uptake for Cd²⁺ ion is 32.6 mg/g. Using these optimal conditions, Cd²⁺ ion adsorption of MMt and CuMgAl-LDH were investigated and the uptake was 10.7 mg/g and 12.5 mg/g, respectively. Therefore, CuMgAl-LDH/MMt can be considered a superior adsorbent for the removal of Cd²⁺ in aqueous solutions.

Cd²⁺ in aqueous solutions.

3.3 The effect of ionic strength and Co-existing

The existence of soluble ions may have a competing effect on the adsorption site (Johnston et al., 2021). The effect of ionic strength was investigated by adding different concentrations of NaCl (0.01, 0.1, 0.5, and 1 mol/l) to 50 ppm Cd²⁺ solutions at the previously determined optimum conditions. Fig. 8a shows that there was no significant effect of NaCl salt, at a concentration of 0.01 mol/L, on the uptake of Cd²⁺ ions, which indicates the formation of inner-sphere complexes (Sahu et al., 2019). The trapping of Cd²⁺ decreased gradually with the augmenting concentration of the NaCl salt, implying that the sorption was governed not by inner-sphere but by outer-sphere surface complexation (Wang et al., 2017).

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Fig. 8

Effect of (a) ionic-strength, (b) Co-existing ions, on Cd²⁺ sorption at pH = 5, dose = 0.2 g, and rpm = 150, 87 μm, and Co = 50 ppm, and t = 120 min.

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Further experiments were carried out, also at previously determined optimum conditions, to assess the elimination of 50 ppm of Cd²⁺ in the presence of 50 ppm of common interfering ions in water such as nitrate, chloride, sulfate, carbonate, and sulfite. The interference of competing anions on Cd²⁺ adsorption follows the decreasing order ${{\mathrm{N}\mathrm{O}}_3}^{2-}<{{\mathrm{S}\mathrm{O}}_4}^{2-}<{{\mathrm{C}\mathrm{O}}_3}^{2-}<{\mathrm{C}\mathrm{l}}^{-}$ indicating that nitrate has a relatively low impact on its removal (see Fig. 8b), which could be attributed to both charge density and the hydrated ionic radius. The distribution coefficient (K_d) of Cd²⁺ (ml/g) was calculated using Eq. (6), as it is important to know the affinity of the adsorbent toward the adsorbate molecule. The K_d values for the Cd²⁺ were in the range of 0.67–4.23 × 10³ ml/g. The high K_d values for LDH/MMt indicates the great affinity of the material toward Cd²⁺ (Foroutan et al., 2022; Sahu et al., 2019).

Where C₀ and Ce are the initial and equilibrium concentration of Cd²⁺ (mg/l), respectively; m is the mass of CuMgAl-LDH/MMt (g), and V is the volume of solute (l).

From this section, it can be concluded that the existence of high concentration soluble ions effect on adsorption (Low selectivity) and can be limited of using it.

3.4 Effect of temperature and adsorption thermodynamics

The results for the removal efficiency of Cd²⁺ as a function of solution temperature are shown in Fig. S1a. The removal efficiency dropped from 89.42 % to 73.16 % as the temperature was raised from 25 to 40 °C, owing to solute mobility increasing from the solid to the bulk phase. This causes a deactivation of the solid surfaces or the destruction of some binding sites, as well as desorption, which takes place in preference to sorption (Mohammed and Kareem, 2019).

The concept of thermodynamics presumes that entropy change considered a driving force for the adsorption in an isolated system (Shattar et al., 2017). The relationship of adsorption temperature and kinetic parameters can be analyzed by determining thermodynamic parameters (standard entropy change (ΔS^o), standard enthalpy change (ΔH^o), and Gibbs free energy change (ΔG^o)), using the following equations (Amin et al., 2020; Santos et al., 2017):

K_T is the equilibrium constants (the ratio of Cd²⁺concentration in the solid and liquid phase), R is the universal gas constant (8.314 J/mol K), and T is the temperature (K). Thermodynamic studies are typically conducted at a range of temperatures (298,303, 308, and 313 K) in order to analyze experimental data. The linear graphs of ln K₂ vs 1/T are shown in Fig. S1b, and the thermodynamic parameters for Cd²⁺ sorption (ΔS^o, ΔH^o, and ΔG^o) are provided in Table 2. The negative value of enthalpy change (ΔH^o) in Table 2 indicates the exothermic nature of the adsorption process of Cd²⁺ on CuMgAl-LDH/MMt. This implies that increases in temperature reduced the adsorption capacity of the adsorbent (hence reduced K_T values) (Zubair et al., 2018)(Mohammed et al., 2020), while the negative value of the energy change, ΔG^o, denotes the spontaneity of adsorption (George and Saravanakumar, 2018). Moreover, the negative values of ΔS^o designated higher order of reactions during the sorption process, which could be attributed to the adhesion of sorbate to the adsorbent, thereby reducing the degrees of freedom of the system (Shan et al., 2014).

3.5 Kinetics of Cd²⁺ adsorption

Models of adsorption kinetics are important for fitting equilibrium data for adsorption because they help determine rate-limiting steps and the interaction mechanism between the adsorbate and the adsorbent (Mittal, 2021). Four kinetics models were used to predict the order and rate of the adsorption process. The pseudo-first-order model (PFO), pseudo-second-order model (PSO), Elovich model (EIH), and intraparticle diffusion model (IPD) assume that the adsorption rate is generally directly related to the equilibrium uptake difference and time, and expressed mathematically in Eqs. 10–13 (Chao et al., 2018; Li et al., 2021; Shen et al., 2016).

Where: q_e and q_t are the uptake rates of Cd²⁺ (mg/g) at equilibrium and time t, respectively; k₁ is the constant of the PFO (1/min) and k₂ is the constant of the PSO (g/mg.min); α is the initial rate of adsorption(mg/g.min); β is the constant of desorption (mg/g); K_id is the constant of intra-particle diffusion rate (mg/g.min ^(1/2)) and C is the constant of intra particle diffusion (mg/g).

From the pseudo-second-order model at time at t → 0, the initial rate of adsorption $\delta$ is obtained as follows: Eq. (14) and Fig. s2b (Mohammed and Kareem, 2019).

Where: $\delta$ is the initial adsorptive rate(mg/g. min), k₂ is the constant of PSO (g/mg.min).

The regression coefficients and slopes of the straight lines in the plots of ln (qe – qt) versus time (PFO), t/qt versus time (PSO), ${\mathrm{q}}_{\mathrm{t}}$ versus $\mathit{Ln}(t)$ (EIH), and ${\mathrm{q}}_{\mathrm{t}}$ versus $t^{0.5}$ (IPD), can be used to infer the order and rate constant of the adsorption process., respectively, The fitted lines for each model are depicted in Fig. S2a-c. Table 3 shows the kinetic parameters (k₁, k₂, α) and the correlation coefficients (R²) that correspond to these values. The value of R² for the PSO model was greater than those of the other models, and closer to 1. In addition to this, the calculated adsorption capacity $q_{\mathrm{e},\mathrm{c}\mathrm{a}\mathrm{l}}$ of the pseudo-second-order model (PSO) agreed with the experimental adsorption capacity $q_{\mathrm{e},\mathrm{E}\mathrm{x}\mathrm{p}.}$ . Thus, it was concluded that the PSO adsorption mechanism was dominant and that chemisorption process seemed to control the overall rate of the Cd²⁺ adsorption process (Hu et al., 2016).

The intra-particle diffusion Kinetic model (IPD) is also used to explain diffusion mechanisms. The IPD model parameters are listed in Table 4, and Fig. s2d shows the linear fit of the IPD model. As the figure explains, the entire kinetic process of Cd²⁺ diffusion is controlled by three adsorption mechanisms. The first phase (bulk-diffusion) is immediate adsorption, which happens on the exterior surface of the solid particles and takes about three minutes. This phase is induced by the large number of active adsorption sites as well as the high initial concentration of Cd²⁺, thus giving it enough driving force to diffuse rapidly to the adsorbent surface (Lei et al., 2017). The second phase (gradual-adsorption) primarily addresses the intraparticle diffusion within the CuMgAl-LDH/MMt adsorbent (Shin and Kim, 2016). K_id2 is related to pore volume and pore size distribution (Hu et al., 2020). The third phase is the equilibrium stage, in which the molecules are adsorbed at the interior surface of CuMgAl-LDH/MMt particles, and because of low Cd²⁺ concentrations, intraparticle diffusion slows until equilibrium is reached (Hu et al., 2020; Shin and Kim, 2016). Fitting the experimental data to the IPD model suggests that intraparticle diffusion may be the rate-limiting step in Cd²⁺ adsorption.

3.6 Adsorption isotherm models

Isotherm models are essential to predict the complete adsorption behavior in practical applications and to calculate the maximum contaminant removal from polluted water at a constant temperature. To investigate the precise adsorption mechanism and compute the adsorption capacity, a non-linear optimization Langmuir (Eq. (15)), Freundlich (Eq. (16)), Temkin (Eq. (17)), and Dubinin-Radushkevich (D-R) (Eq. (18)) isotherms models were used to analyze isothermal data.

Where: qe is an adsorbed amount under equilibrium condition (mg/g); ${\mathrm{q}}_{\mathrm{m}\mathrm{a}\mathrm{x}}$ a maximum uptake of sorbate (mg/g); ${\mathrm{K}}_{\mathrm{f}}$ a constant of the Freundlich isotherm (m/g)(l/mg)^1/n; K_L a constant of the Langmuir isotherm (l/mg); b_T is the Temkin isotherm constant related to the heat of adsorption (J/mol); and ${\mathrm{A}}_{\mathrm{T}}$ an is equilibrium binding constant (l/g); ${\mathrm{K}}_{\mathrm{D}\mathrm{R}}$ is a constant of D-R (mol/J)²; ε is a potential of Polanyi; Ce the concentration of the sorbate at equilibrium (mg/l), and n is the intensity of the Freundlich adsorption parameter (which indicates the existence of adsorption driving forces and the degree of surface heterogeneity). A non-linear regression analysis was performed using the Solver add-in in Microsoft Excel to calculate the parameters of each model. This enabled the best fitting model to be selected (Ahmed et al., 2017; Blaisi et al., 2018; Mittal, 2021; Mohammed and Kareem, 2019; Nava-Andrade et al., 2021; Zubair et al., 2017). The determined constants and variables for the sorption isotherms of Cd²⁺ on the CuMgAl-LDH/MMt surface study are presented in Table 5. The obtained R² value of CuMgAl-LDH/MMt composites for the Langmuir isotherm was 0.9933, which was lower than that of the Freundlich isotherm. This revealed that the Langmuir isotherm did not adequately describe Cd²⁺ sorption on CuMgAl-LDH/MMt. The Freundlich isotherm model assumes multilayer adsorption at heterogeneous surfaces and the nonuniform energy distribution to the active adsorbent site occurs(Dinari and Neamati, 2020).

If the value of 1/n is less than unity as in Table 5, it indicates the heterogeneity of site energies as well as higher adsorption intensity (Mu’azu et al., 2018). The maximum adsorption capacity of the CuMgAl-LDH/MMt was 174.87 mg/g, which was higher than other maximum adsorption capacities reported in previous studies and listed in Table 6. Despite the fact that there are many excellent materials used as adsorbents based LDH in Cd2 + sorption, CuMgAl-LDH/MMt has superior sorption properties for Cd2 + ions when compared to the other adsorbents.

In addition, the Langmuir, or equilibrium, parameter (R_L) is the most important parameter of the Langmuir isotherm and must be considered to estimate the favourability and unfavourability of the adsorption process (Alnasrawi et al., 2022; Shattar et al., 2017).

The process is unfavorable if R_L > 1, favorable if 0 < R_L < 1, linear if R_L = 1, and irreversible if R_L = 0. These results imply that the isotherm adsorption is controlled by the Langmuir model and that Cd²⁺ species adsorption is a favorable process (0 < R_L < 1 calculated from Eq. (19)).

The Temkin isotherm is used to explore the impact of indirect adsorbate–adsorbate interactions on the sorption system, whereas the D-R model determines if the sorption process is chemical or physical. Calculation of the Temkin model parameters showed that the interaction of Cd²⁺ with CuMgAl-LDH/MMt composite surfaces is weak, and most probably a physical sorption process. The mean free energy of surface adsorption for Cd²⁺ attraction of 2.8 kJ/mol confirm a physical sorption process (E < 8 kJ/mol) (Foroutan et al., 2021b, 2020b).

It can be concluded that isotherm adsorption is controlled by the Freundlich adsorption isotherm model, as well as that the adsorption of Cd species by the adsorbent is a favorable process (because the calculated R_L was in the range of 0 and 1).

3.7 Desorption of Cd²⁺ and reusability of CuMgAl-LDH/MMt nonabsorbent

To investigate the reusability of LDH/MMt, five successive cycles of regeneration (sorption–desorption) were carried out. The resorption process is tested by adding 0.5 g of spent CuMgAl-LDH/MMt to 100 ml of 0.2 M NaOH and another to 100 ml 0.2 M of HCL. The mixture was stirred for 180 min, filtered, washed three times with distilled water, and dried in the oven. This process was repeated five times. The adsorption capacity of the regenerated adsorbent in each cycle is calculated using Eq. (2). As displayed in Fig. 9, the trapping of Cd²⁺ dropped to 14.96 mg/g after five cycles of regeneration of adsorbent with the NaOH solution. The adsorption capacity also decreased to 7.06 mg/g after five cycles of regenerating the adsorbate with the HCL solution. The reduced uptake of reused CuMgAl-LDH/MMt was most likely attributable to the nanocomposite's decreasing crystallinity (Yuan et al., 2013). As stated in Section 3.2, increasing the solution pH resulted in drastically reduced cadmium uptake by the CuMgAl-LDH/MMt. These results demonstrate that relatively alkaline conditions (pH > pH_PZC) can facilitate cadmium desorption from exhausted adsorbent. Thus, the cadmium desorption process from cadmium-loaded CuMgAl-LDH/MMt was more efficient. These outcomes indicate that the recently developed nanocomposites have a high potential for recycling, resulting in cost savings and diminished environmental impact (Du et al., 2019).

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Fig. 9

Regeneration of cumgal-ldh/mmt samples (at ph = 5, dose = 0.2 g, and rpm = 150, and 87 μm).

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