Insight into the performance of novel kaolinite-cellulose/cobalt oxide nanocomposite as green adsorbent for liquid phase abatement of heavy metal ions: Modelling and mechanism

2.1 Material, methods, and instruments

Kaolinite (Shree Ram Minerals, Gujarat), cellulose acetate (Merck, India), lead nitrate, cadmium nitrate, cobalt nitrate (Merck, India), ethanol, dithizone, cetyltrimethylammonium bromide (CTAB), ammonium persulfate (APS) (all Merck, India), and HCl (Fischer Scientific, India) of analytical grade were used as acquired. The characterization of Kao-Cel/Co₃O₄ NC was accomplished using Fourier transform infrared spectroscopy, (FTIR) {Perkin-Elmer spectrometer, model BX spectrum, USA}, X-ray diffraction (XRD) {Philips Analytica PW 1830 apparatus, Philips, Netherlands}, Brunauer–Emmett–Teller (BET) {BET surface area analyzer (Quantachrome NovaWin}, Scanning electron microscope (SEM) {Carl Zeiss JOEL scanning electron microscope, Sigma 5.05, Germany}, energy dispersive X-Ray analysis (EDX) {Team EDS system, EDAX Inc.}, transmission electron microscopy (TEM) (Transmission electron microscope, HRTEM 200 kV model, FEI Tecnai), and the solution pHs was measured utilizing the Decibel pH meter (model DB-1011).

2.2 Synthesis of the cellulose functionalized kaolinite (kaolinite-cellulose)

The preparation of the modified kaolinite using cellulose as an intercalating biopolymer may be summarized as follows: Kaolinite (5 g) was dispersed in 100 mL of deionized water by stirring for 1 h, and then 0.1 g of CTAB solution was added and stirring further continued for 12 h. To this mixture, an ethanolic dispersion (50 mL) of cellulose (2.5 g) was mixed under continuous stirring for another 2 h. Thereafter, the kaolinite-cellulose mixture was exposed to ultrasonic irradiation for 5 h to accelerate the intercalation of cellulose between the kaolinite interlayers.

2.3 Green fabrication of kaolinite-cellulose/cobalt oxide nanocomposite

Plant leaf extract of Berberis lycium in ethanol was used as a capping and reducing agent for the precipitation of cobalt oxide over the kaolinite-cellulose clay (Kao-Cel). Firstly, 3.0 g of the pre-prepared Kao-Cel was suspended in 100 mL double-distilled water and sonicated for 1 h. Afterward, cobalt nitrate solution (2.5 g in 100 mL distilled water) was mixed with Kao-Cel suspension under constant stirring at 500 rpm. Then, 50 mL of Berberis lycium extract was added into the solution mixture under constant stirring and left for 48 h to affect the complete precipitation of nano cobalt oxide over Kao-Cel. The obtained residue of kaolinite-cellulose/cobalt oxide nanocomposite, abbreviated as Kao-Cel/Co₃O₄ NC, was separated by centrifugation, washed with deionized water, dried in an oven at 80 °C for 12 h, and finally preserved in a desiccator.

2.4 Equilibrium adsorption studies

In the present study, the adsorptive capacity or the removal effectiveness of the Kao-Cel/Co₃O₄ NC towards the target pollutants was evaluated by conducting a number of equilibrium adsorption experiments by agitating a constant adsorbent mass with a series of solutions of different [Pb²⁺] or [Cd²⁺] prepared from a stock solution of 500 mg/L each. The batch mode techniques were employed to study the impact of different process variables such as agitation time, adsorbent mass, initial metal concentration, pH, and temperature on adsorption. To an aliquot of 25 mL of metals solution (20–80 mg/L) in an Erlenmeyer flask (50 mL) was added a fixed adsorbent mass (0.4–2.4 g/L), and the solution mixture was shaken on a water bath shaker at a different time of agitation (10–60 min), pH (2.0–10) and temperature (303 K, 313 K and 323 K). After equilibrium was attained, the sorbent was separated by centrifugation and the centrifugate was investigated using T80 + UV/Vis spectrophotometer, PG Instrument Ltd, UK. The UV/Vis analyses of the test solutions for determining Pb²⁺/Cd²⁺ concentration was performed according to standard procedure using dithizone as a complexing agent (Khan et al., 2016; Khan et al., 2007). Briefly, 2.0 mL of dithizone solution and 1.0 mL of HCl (0.1 M) was mixed with Pb²⁺/Cd²⁺solution, followed by 2.0 mL of CTAB solution (0.3 M). The absorbance of the colored solutions thus developed was measured along with the corresponding blank at λ_max of 505 nm (Pb²⁺) and 540 nm (Cd²⁺). The remanent concentrations of Pb²⁺/Cd²⁺ post-adsorption was determined from the linear calibration curve obtained by plotting the absorbance versus Pb²⁺/Cd²⁺ concentration (5–30 mg/L) (Fig. S1) with R² = 0.994 showing good correlation. The limit of detection for Pb²⁺ and Cd²⁺ was appraised from the straight-line calibration plot, which was found to be 0.464 and 0.591 μg/L, respectively. For isotherm experiments at 303, 313, and 323 K, a 2.0 g/L Kao-Cel/Co₃O₄ NC was shaken with different initial Pb²⁺ or Cd²⁺ concentrations (20–80 mg/L) for 50 min extraction time at pH 6. However, the kinetic measurements were accomplished at constant temperature (303 K) by agitating 25 mL of Pb²⁺ or Cd²⁺ (40, 50, and 60 mg/L) by applying a fixed sorbent mass (2.0 g/L) for 10–60 min (10 min interval) and pH 6.

The adsorptive capability of Pb²⁺ and Cd²⁺ (q_t, mg/g) onto the sorbent surface at a given time (min) and removal efficiency (R) was deduced from the equilibrium adsorption data based on the mass balance relationship (Eq. (1)) and Eq. (2), respectively:

The fitting of the equilibrium data to various isotherm model equations affords many physically meaningful constants, which may provide insight into the surface properties, sequestration capacity, heterogeneity of the adsorbent, mechanism of adsorption, affinity towards adsorbate, and energy of adsorption. The data was evaluated using Langmuir (Langmuir, 1918); Freundlich (Freundlich, 1906); Temkin (Temkin, 1940), and Dubinin–Radushkevich (D–R) (Dubinin and Radushkevich, 1947) isotherm model equations at 303 K, 313 K, and 323 K by applying nonlinear regression analysis. The corresponding nonlinear isotherm equation is listed in Table 1. The study of adsorption kinetics is essential to acquire information regarding the pollutants uptake rate, which controls the contaminant's residence period at the sorbent-sorbate interface, and interpret the plausible adsorption mechanism. The kinetic data is analyzed by applying Lagergren pseudo-first-order (Lagergren, 1898), Ho’s pseudo-second-order (Ho and McKay, 2003), Weber-Morris intraparticle diffusion (Weber and Morris, 1963), and Boyd liquid film diffusion (Boyd et al., 1947) model equations (Table 3) to determine the rate of Pb²⁺ or Cd²⁺ adsorption and the underlying mechanism thereof. Thermodynamic parameters such as a change in entropy (ΔS^o), enthalpy (ΔH^o), and Gibbs free energy (ΔG^o) were determined by van't Hoff equation ( $\mathrm{l}\mathrm{n}{k}_c=-\frac{\mathrm{\Delta }{H}^0}{\mathit{RT}}+\frac{\mathrm{\Delta }{S}^0}{R}$ ) and Gibbs equation (ΔG^◦ = ΔH^◦ $-$ TΔS^◦) to evaluate the change in randomness at the sorbate-sorbent interface, endothermic/exothermic nature, feasibility and spontaneity of the sorption procedure, respectively.

In desorption studies, 2.0 g/L of Kao-Cel/Co₃O₄ NC was saturated with Pb²⁺ or Cd²⁺ solution (50 mg/L) for 1 h. The used adsorbent was regenerated by agitating it with HCl (0.1 M), NaOH (0.1 M), or ethanol for 10 min, washed, dried in an oven, and reused. The Kao-Cel/Co₃O₄ NC shows good regeneration ability towards NaOH. To check the recyclability of the present adsorbent, the adsorption-desorption experiments were performed up to five cycles, and the confiscation capacity of the adsorbent was assessed. Similarly, in order to appraise the competence of Kao-Cel/Co₃O₄ NC to adsorb Pb²⁺ and Cd²⁺ from real wastewater, water sample from the light vehicles washing area of a local garage (pH = 7.6; COD = 207 mg/L; Total solids = 1275 mg/L; Total suspended solids = 700 mg/L; Oil and grease = 50 mg/L) was collected, allowed to stand for 24 h. The supernatant (5.0 mL) was diluted to 100 mL, and spiked with Pb²⁺ or Cd²⁺ solutions (30–80 mg/L).

3.1 Characterization of Kao-Cel/Co₃O₄ nanocomposite

3.1.1

3.1.1 Fourier transform infrared (FTIR) spectra

The FTIR spectra of kaolinite (Kao), cellulose (Cel), kaolinite-cellulose clay (Kao-Cel), and Kao-Cel/Co₃O₄ NC are shown in Fig. 1. The absorption bands appearing at 3670 cm⁻¹ and 1640 cm⁻¹ denote Al—OH stretching vibration of kaolinite, and H—O—H bond of water retained by the silica matrix (Fig. 1a) (Sari and Tuzen, 2014). The spectral band at 1122 cm⁻¹ is assigned to Si—O stretching vibration (out of plane), while those at 1057 cm⁻¹ and 948 cm⁻¹ are attributed respectively to the stretching vibration of Si—O and —OH deformations of the inner surface hydroxyl groups. The bands corresponding to Si—O (deformation) and Al—O—Si (deformation) of the kaolinite is noticed at 748 cm⁻¹ and 540 cm⁻¹, respectively (Khan et al., 2021). The Si—O—Si stretching peak is observed at 481 cm⁻¹ (Khan et al., 2017). The characteristic absorption bands of cellulose (Fig. 1b) appear at 3465 cm⁻¹ corresponding to O—H stretching vibration (Liu et al., 2017), and at 1368 cm⁻¹, 1225 cm^−1, and 1036 cm⁻¹ assigned respectively to C—CH₃ symmetric bending, asymmetric stretching of O—C⚌O group, and to asymmetric C—O—C stretching arising from the pyranose ring. Moreover, the band at 1738 cm⁻¹ emerges due to the C⚌O of the acetate group in the cellulose (Ali et al., 2014; Yan et al., 2014; Khatri et al., 2012). The FTIR spectrum of Kao-Cel (Fig. 1c) validates the formation of hybrid material containing complex bands related to the functional groups of both kaolinite and cellulose, with observable deviations of characteristic groups from main positions. The fabrication of green Kao-Cel/Co₃O₄ nanocomposite is validated by the IR peaks in the spectrum of the composite material. Two new absorption peaks are observed for the Co₃O₄ structure as depicted in Fig. 1d. The peak at 541 cm⁻¹ is attributed to Co—O stretching vibration and the band at 658 cm⁻¹ is ascribed to O-Co-O bridging vibration (Manigandana et al., 2013; Farhadi et al., 2016). To scrutinize the functional groups that interact with Cd²⁺ and Pb²⁺ ions, the FTIR spectra of loaded-Kao-Cel/Co₃O₄ NC are revealed in Fig. 2a and b. Change in the position of some bands at 1734 and 1370 cm⁻¹ and diminished intensities of bands at 1124, 1040, and 952 cm⁻¹ support the successful uptake of Cd²⁺ and Pb²⁺ by the nanocomposite. The shifting or weakening of absorption bands corresponding to the O—H band and C⚌O/C—O stretching vibration indicates that free electron pairs on oxygen atoms in the hydroxyl and/or carboxyl groups interact with the empty orbitals of Pb²⁺ and Cd²⁺ to form complexes mediated by coordinative bonds, that alters the density of electron cloud causing the vibrational bands to shift or reduce in intensity (Wang et al., 2017; Haung et al., 2018; Jiang et al., 2019).

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Fig. 1

FTIR spectra of Kao, Cel, Kao-Cel and Kao-Cel/Co₃O₄ nanocomposite.

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Fig. 2

FTIR spectra of Pb²⁺ and Cd²⁺ loaded Kao-Cel/Co₃O₄ nanocomposite.

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3.1.2

3.1.2 Brunauer–Emmett–Teller (BET) analysis

The specific surface area and porosity of the solid Kao-Cel/Co₃O₄ nanocomposite were determined from the nitrogen adsorption/desorption isotherm curves using the BET equation. The N₂-adsorption/desorption isotherm and pore size distribution of Kao-Cel/Co₃O₄ NC are displayed in Fig. 3a and 3b, respectively, which reveals a small hysteresis loop that is categorized as type IV (Nguyen et al., 2017). The BET surface area of Kao-Cel/Co₃O₄ NC is 85.92 m²/g, pore size determined by BJH is about 0.024 cm³/g and a pore diameter is 7.221 nm, depicting the mesoporous nature of Kao-Cel/Co₃O₄ NC. The adsorbent surface area is an influential feature governing the adsorption of contaminants and thereby its effectiveness. Generally, a larger surface area means access to more active sites, and hence admirable adsorption aptitude.

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Fig. 3

N₂ adsorptions-desorption (a) and pore size (b), XRD spectra of Cel, Kao, Kao-Cel and Kao-Cel/Co₃O₄ nanocomposite (c).

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3.1.4

3.1.4 Scanning electron microscopy-energy dispersive X-ray (SEM- EDX) analyses

The morphological features, shape, size, and elemental contents of the synthesized Kao-Cel/Co₃O₄ NC were scrutinized by SEM and EDX analysis. The SEM micrographs of Kao-Cel/ Co₃O₄ NC, before and after adsorption of Pb²⁺ and Cd²⁺, are revealed in Fig. 4. The surface of the nanocomposite is figured out as irregular, rough, and highly porous due to the existence of sufficient pores and grooves of distinctive sizes and shapes, which are generally accountable for the larger surface area and higher adsorption efficacy of Kao-Cel/Co₃O₄ NC. However, the SEM images of Kao-Cel/Co₃O₄ NC post-sorption of Pb²⁺ and Cd²⁺ (Fig. 4b, 4c) display an almost smooth texture that validates the sorption of Pb²⁺ and Cd²⁺ onto Kao-Cel/Co₃O₄ NC surface. However, the EDX analysis recognizes the presence of additional elements, Pb or Cd in the Kao-Cel/Co₃O₄ NC adsorbent. after adsorption signifying the sorbed Pb²⁺ and Cd²⁺ onto the surface of the NC, and also validates the existence of Al, Si, O, C, and Co as the foremost components of the nanocomposite.

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Fig. 4

SEM and EDX micrographs of (a) Kao-Cel/Co₃O₄ nanocomposite (b) Pb²⁺, (c) Cd²⁺loaded nanocomposite.

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3.1.5

3.1.5 Transmission electron microscopy (TEM) analysis

The incorporation of green-synthesized cobalt oxide (Co₃O₄) nanoparticles, their size and shape, and size distribution were affirmed by TEM images, as depicted in Fig. 5a. The TEM images reveal the spherical Co₃O₄ nanoparticles randomly distributed within the kaolinite/cellulose (Kao-Cel) matrix, which may have resulted in significant augmentation in the textural properties and provide the nanocomposite with a high surface area. The particle distribution curve signifies that the typical particle size of Kao-Cel/Co₃O₄ NC is 50 nm as shown in Fig. 5b. However, the average crystallite size of the NC calculated from the XRD data is 43 nm, which is in close agreement with the TEM acquired particle size (50 nm).

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Fig. 5

TEM micrographs of (a) Kao-Cel/Co₃O₄ nanocomposite and (b) histogram (based on 95 particles).

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3.2 Effect of adsorption variables on the uptake of Pb²⁺ and Cd²⁺ by Kao-Cel/Co₃O₄ nanocomposite

3.2.1

3.2.1 Variation in Kao-Cel/Co₃O₄ nanocomposite dose

In the adsorptive removal process, the quantity of adsorbed pollutants and removal efficiency are influenced by the adsorbent amount, therefore, optimization of adsorbent dosage is an important aspect that ensures usage of minimum adsorbent, which may regulate the overall operations cost in an efficient adsorption system. The effect of adsorbent mass for the removal competency of Pb²⁺ and Cd²⁺ was considered by varying the amount of Kao-Cel/Co₃O₄ NC from 0.4 to 2.4 g/L, and the results of the percentage removal of both the metal ions with change in adsorbent dosage are shown in Fig. 6a and 6b. The uptake effectiveness increases gradually up to 96.84% Pb²⁺ and 96.32% Cd²⁺ with an increment in the nanocomposite dose until 2.0 g/L where optimal uptake is observed. The rise in decontamination of Pb²⁺ and Cd²⁺ from the aqueous solution with an increase in adsorbent loading up to 2.0 g/L may be attributed to the increased number of available active adsorption sites. However, the removal efficiency presents a decreasing trait after that, which may either be due to a lesser number of accessible surface sites as a result of improved collision rate between nanocomposite particles, augmented active sites available for a fewer number of metal cations for adsorption or a drop in the overall surface area/or overlapping of surface-active sites, and an enhancement in the diffusional path length for the metal ions to grasp onto the adsorbent’s surface (Khan et al., 2022). Hence, a 2.0 g/L adsorbent dosage was used for conducting the rest of the experiments. The Kao-Cel/Co₃O₄ NC illustrates an admirable adsorption aptitude in small dosage with good removal effectiveness. Similar results were accounted for by Huang et al. (Haung et al., 2020), and Tabesh et al. (Tabesh et al., 2018 for the adsorption of Pb²⁺ and Cd²⁺ onto CS/Cu₃(BTC)₂-SH nanocomposite and γ-Al₂O₃ nanoparticles.

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Fig. 6

Effect of adsorbent dose on (a) Pb²⁺, (b) Cd²⁺ and contact time on (c) Pb²⁺, (d) Cd²⁺ uptake.

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3.2.3

3.2.3 Alteration in the initial concentration of Pb²⁺ and Cd²⁺

The initial contaminant molecules/ions concentration (C_i) is a significant operational variable as it governs the adsorption competency of an adsorbent. The impact of C_i on the uptake ability of Kao-Cel/Co₃O₄ NC for the elimination of Pb²⁺ and Cd²⁺ was explored in C_i range of 20–80 mg/L with constant sorbent mass (2.0 g/L) for 50 min, and the results are displayed in Fig. 7a and 7b, respectively. As the C_i of M²⁺ (Pb²⁺ and Cd²⁺) is increased from 20 to 60 mg/L, the aptitude of adsorption decreases from 95.60 to 92.58% for Pb²⁺ and 97.34 to 93.34% for Cd²⁺. A given amount of Kao-Cel/Co₃O₄ NC possesses a fixed number of surface binding sites. Initially, at lower M²⁺ concentration, the ratio of the adsorption sites to the number of M²⁺ ions are relatively high leading to boosted retention capacity. But as the Ci gradually increases, the available binding sites are progressively occupied by M²⁺ thus diminishing the adhering of M²⁺ onto fewer remaining active sites at higher C_i. However, only a slight change in adsorption behavior on further increasing the C_i beyond 60 mg/L is an indication of the saturation of sorbent binding sites. This suggests that a higher removal efficiency can be realized from the high concentration of liquid wastes by diluting the solution. Moreover, a rise in the adsorption capacity from 17.92–52.04 mg/g for Pb²⁺ and 12.16–46.67 mg/g for Cd²⁺ with an increment in C_i is due to the substantial driving force created by higher M²⁺ concentration exceeding the mass transfer resistance (Egbosiuba et al., 2022). These results are commensurate with Pb²⁺ and Cd²⁺ adsorption by chemically modified algal biomass and polyacrylamide/bentonite hydrogel nanocomposite (Khan et al., 2016; Khan et al., 2020).

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Fig. 7

Effect of initial concentration on (a) Pb²⁺, (b) Cd²⁺ and effect of pH on Pb^2+, (d) Cd²⁺ removal.

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3.3 Analysis of the adsorption isotherm data

Langmuir's model supposes that on the sorbent surface there is no saturated atomic force field, and once the sorbent surface is covered by sorbate molecules, the force field gets saturated and no further adsorption can occur, which corresponds to monolayer adsorption (Langmuir, 1918). From the slope and intercept of the plot of the quantity of Pb²⁺/Cd²⁺ adsorbed at equilibrium (q_e) as a function of solute’s equilibrium concentration in bulk phase (C_e), shown in Fig. 8a and b, the values of Langmuir saturation capacity, Q_m and Langmuir adsorption energy, b_L was determined. The deduced values of Q_m (mg/g), b_L (L/mol), R² and separation factor, R_L ( $=\frac{1}{1+b{C}_0}$ ) are summarized in Table 1. The Q_m values (in mg/g) for the confiscation of Pb²⁺ and Cd²⁺ at 303 K, 313 K, and 323 K are in the 155.78–293.68 and 132.35–267.85 range, respectively. This increasing trend in the Q_m values may probably be a consequence of the rise in temperature providing the energy needed for the higher mobility of adsorbate ions across the external layer for enhanced interaction with the surface sites, and/or an increased rate of diffusion into the interior pores, which is the characteristic of the endothermic removal process. The maximum removal capacities of Kao-Cel/Co₃O₄ NC are 293.68 mg Pb²⁺/g and 267.85 mg Cd²⁺/g, respectively at 323 K. The magnitude of Q_m is of interest to many researchers in establishing the adsorption competence of different adsorbents. But the direct comparison of Q_m may not always lead to unequivocal conclusions because of differing optimal experimental conditions. A comparative account of Q_m of the present adsorbent with those of other adsorbents along with experimental conditions is summarized in Table 2. The Q_m of Kao-Cel/Co₃O₄ NC for Pb²⁺/Cd²⁺ are considerably higher than other stated adsorbents (Table 2), which propounds its superior adsorption performance.

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Fig. 8

Langmuir isotherm plots for (a) Pb²⁺, (b) Cd²⁺, and pseudo-second order kinetic plots for (c) Pb²⁺, (d) Cd²⁺ adsorption.

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Freundlich's model describes that the surface of the sorbent is heterogeneous with a continuous dispersal of active sites, and the adsorption is multi-layered (Freundlich, 1906). The values of the Freundlich adsorption efficiency, K_F, and Freundlich isotherm constant, 1/n_F, R^2, and SEE, computed from the q_e versus C_e plot (Fig. S2), are given in Table 1. The gradual increase in K_F {(mg/g)(mg/ L)^1/n} from 17.48–20.52 for Pb²⁺ and 17.01–18.67 for Cd²⁺ with change in temperature from 303 to 323 K substantiates the endothermic adsorption phenomenon and better adsorption performance at elevated temperature. The Freundlich parameter, n_F is used to authenticate whether the adsorption is linear (n = 1), physical and favorable (n_F > 1) or chemical (n_F < 1), and further explicates the deviation from the linear adsorption (Della Puppa et al., 2013). Its values greater than unity (Pb²⁺ = 1.23–1.13; Cd²⁺ = 1.91–1.96) highlight the favorable and physical uptake process. The parameter, 1/n_F is a measure of the surface heterogeneity, its value approaching nearer to zero dictates a more heterogeneous surface. The values (Pb²⁺ = 0.813–0.885; Cd²⁺ = 0.523–0.511) proposes low to moderate surface heterogeneity.

Temkin adsorption model describes the interaction between sorbent and sorbate, which explore the heat of adsorption in term of temperature in layers assuming that the heat of adsorption decreases with the degree of the adsorption process (Temkin, 1940). The model equation (Table 1) is generally used to estimate the equilibrium binding constant, K_T (L/g), and the heat of adsorption, b_T (=RT/β_T) (kJ/mol) from q_e versus C_e curve (Fig. S3) for the adsorption process. The calculated values of these parameters along with R² and SEE are depicted in Table 1. The positive values of b_T (0.074–0.071 kJ/mol for Pb²⁺ and 0.199–0.183 kJ/mol for Cd²⁺) at all studied temperatures provide for favorable physical and endothermic adsorption. Only a slight variation in K_T (Table 2) reflects that the binding energy is not much influenced by the changing temperature.

D-R isotherm model usually envisages the nature of sorbate adsorption over the surface of sorbent (physisorption or chemisorption), and also examines both homogeneous and heterogeneous adsorption (Dubinin and Radushkevich, 1947). The D–R model constants, q_D (mg/g) related to the degree of adsorbate adsorption and K_D (mol²/kJ²) associated with the average adsorption energy per mole, evaluated from the q_e versus C_e isotherm plot (Fig. S4) together with R² and SEE, is provided in Table 1. The probable energy of adsorption, E ( $=\frac{1}{{\left(2B_D\right)}^{1/2}}$ ), which is used to extricate physical or chemical adsorption mechanism was estimated from $K_D$ $(=B_D{R}^2{T}^2$ ). The increase in the estimated q_D values (mg/g) for Pb²⁺ (117.09–121.83) and Cd²⁺ (117.24–120.81) with elevation in temperature elucidates a high extent of adsorption capacity at higher temperature. Less than 8.0 kJ/mol values of E for Pb²⁺ (0.34–0.31 kJ/mol) and Cd²⁺ (0.35–0.31 kJ/mol) entitles physical sorption.

To comprehend the most fitted isotherm model, the higher R² and lower SEE parameters are generally considered. From the results in Table 1, it is concluded that the Langmuir model presents a higher R² (0.998–0.989 for Pb²⁺ and 0.996–0.985 for Cd²⁺) and lower SEE (0.340–0.260 for Pb²⁺ and 0.682–0.296 for Cd²⁺) values relative to rest of the models signifying that Langmuir isotherm best explain the equilibrium data. The adsorption process of Pb²⁺/Cd²⁺ ions, therefore, is believed to be a homogenous monolayer onto the surface of Kao-Cel/Co₃O₄ NC in accordance with the Langmuir model.

3.4 Evaluation of the adsorption kinetics data

The rate constants for pseudo-first order (k₁; 1/min), pseudo-second order (k₂; g/mg/min) and q_e (mg/g) were calculated from the respective kinetic plots of q_t against t (Fig. S5) and (Fig. 8c and 8d), respectively. The determined values of k₁, k₂, and other related parameters at three different initial concentrations (C_i = 40, 50, and 60 mg/L) are given in Table 3. The best-fitted kinetic model was evaluated based on R² and SEE values. The pseudo-second order model with close to unity values of R² (Pb²⁺: 0.975–0.978; Cd²⁺: 0.971–0.919) and lower SEE (Pb²⁺: 0.033–0.018; Cd²⁺: 0.061–0.082) for all Ci stipulates that this model most appropriately describes the kinetic data in comparison with pseudo-first order kinetics. The best fitting of kinetic data by the pseudo-second order model illustrates that the sorption mechanism depends on the adsorbent and adsorbate, and chemisorption and/or physicochemical interactions between species on the surface may be rate-limiting (Abd El Aal et al., 2019). The k₂, which represents the adsorbate’s affinity for the adsorbent surface (Della Puppa et al., 2013) is higher for Pb²⁺ (0.250–0.241 g/mg/min) in comparison to that of Cd²⁺ (0.136–0.115 g/mg/min), indicating a greater affinity of Pb for Kao-Cel/Co₃O₄ NC. A similar result of greater affinity of Pb²⁺ for diatomite has been explained based on its capability to occupy both the interlayer and surface edge sites (Wan Ngah et al., 2011).

The adsorptive uptake rate of M²⁺ ions usually depend on external liquid film diffusion or intraparticle diffusion of solute or a combination of both these steps. So, to gain further acuity into the mechanism and to clarify the rate-dominating steps impacting the adsorption kinetics, the data was scrutinized by intra-particle diffusion and liquid-film diffusion models. The Weber-Morris intra-particle diffusion model assumes the probable transportation of the dissolved pollutants (Pb²⁺/Cd²⁺) from the aqueous system onto the solid adsorbent as a function of the intra-particle conveyance mechanism, while the Boyd liquid-film diffusion model considers the transfer of adsorbate molecules/ions through a liquid film encompassing the adsorbent. The extent to which the adsorption data fit the models is predicted by the linear plot of q_t versus t^0.5 (Fig. S6) and ln(1 − q_t/q_e) versus t (Fig. S7). The values of intra-particle diffusion constant, k_i (mg/g min^0.5), liquid film diffusion rate constant, k_A (1/min), intercept that is associated closely with the thickness of the boundary layer, C_i, R^2, and SEE for Pb²⁺ and Cd²⁺ are presented in Table 3. The intra-particle diffusion plot (Fig. S6) reveals that adsorption of Pb²⁺ and Cd²⁺ is not linear over the studied concentration range, and no straight-line curve passes through the origin. because the curves do not pass through the origin shows. Hence, the accomplishment of the Pb²⁺/Cd²⁺ adsorption onto Kao-Cel/Co₃O₄ NC is not solely controlled by the intra-particle diffusion mechanism. Similarly, the straight-line curves based on the liquid-film diffusion model with a non-zero intercept recommend that the Pb²⁺/Cd²⁺ diffusion through the liquid-film is not the solitary rate-controlling step. The values of C_ip for Pb²⁺ and Cd²⁺ at 40–50 mg/L are 31.43–39.83, and 31.60–39.83, respectively. The higher C_ip demonstrates a greater boundary layer effect, and consequently liquid-film diffusion as the dominant rate-controlling factor. The observed enhancement in C_ip with an increase in initial Pb²⁺ and Cd²⁺ concentration reflects that the boundary layer effect is more pronounced at higher initial sorbate concentration. It may, therefore, be proposed that the adsorption mechanism is not confined merely to intra-particle and liquid-film diffusion but is controlled by numerous other operating mechanisms.

3.5 Thermodynamic studies

The adsorption thermodynamic parameters were evaluated at four temperatures (303, 313, 323, and 333 K). Adsorption of a solute at a solid-solution interface, in general, is accompanied by negative free energy change (ΔG^°), which is suggestive of the spontaneity of the process. The positive entropy (ΔS^°) and enthalpy change (ΔH^°) attribute to an increase in randomness of displaced water molecules from a solid surface due to the gain of translational entropy, and endothermic adsorption. The negative values of entropy and enthalpy changes, however, impute a decrease in translational entropy as a result of regular attachment/association of water molecules and adsorbate fixation or immobilization onto the adsorbent surface, and the exothermic nature of the adsorbent. The negative ΔG^° (in kJ/mol) at four studied temperatures (between −7.83 to −9.00 for Pb²⁺; −7.86 to −9.33 for Cd²⁺), and positive ΔH^° (Pb²⁺: 3.98 kJ/mol; Cd²⁺: 6.99 kJ/mol) and ΔS^° (Pb²⁺: 0.038 kJ/mol/K; Cd²⁺: 0.049 kJ/mol/K), depicted in Table 4, advocate that the adsorption of Pb²⁺ and Cd²⁺ onto Kao-Cel/Co₃O₄ NC is spontaneous, feasible and endothermic (Fig. S8). An increment in the ΔG^° with increasing temperature from 303 K to 333 K illustrates more preferred adsorption at higher temperature. The values ΔG^° are well within the range (zero and −20 kJ/mol) for the physisorption mechanism, which is further supported by the ΔH^° values (3.68 and 6.99 kJ/mol) in the 2.1–20.9 kJ/mol range in accordance with physisorption involving van der Waals interactions and hydrogen bonding. The positive ΔS^° values (0.038–0.049 kJ/mol/K) foresee a slight enhancement in the randomness at the interface between two phases probably due to displaced water molecules gathering more translational entropy. However, the low ΔS° values point to no significant change in entropy.

3.6 Adsorption mechanism

Generally, the adsorption of metal ions onto clay-based nanocomposite occurs through electrostatic interactions and ion exchange mechanisms between the contaminants and biosorbent. The functional groups of the Kao-Cel/Co₃O₄ NC can act as active sites for metal ions sorption. The solution pH influences the mechanism of ion exchange which is prevalent between Pb²⁺/Cd²⁺ and Si²⁺ of the Kaolinite phase. Also, the increase in H⁺ ions in an acidic medium competes with positively charged contaminants (Pb²⁺ and Cd²⁺) leading them to be adsorbed onto the surface of Kao-Cel/Co₃O₄ NC (Iqbal et al., 2009) validates the involvement of electrostatic attraction (physical adsorption). The FTIR studies display the weakening and shifting of the O—H and C⚌O/C—O bands at 952 and 1040 cm⁻¹ confirming the formation of the coordinated —OH and C⚌O/CO— metal ion complexes. Moreover, the change of the absorption band at 952 cm⁻¹ assigned to stretching vibration of —OH deformations of the inner surface hydroxyl group of kaolinite is the indication that —OH groups are involved in binding to metals ions, which is in accordance with previously reported works (Irani et al., 2015) between metals ions and the functional groups on the adsorbent surface. Therefore, the adsorption of Pb²⁺ and Cd²⁺ onto Kao-Cel/Co₃O₄ NC might be due to the combined effect of electrostatic, ion-exchange, and chelation mechanisms respectively. The plausible mechanisms of Pb²⁺ and Cd²⁺ sequestration onto the Kao-Cel/Co₃O₄ NC are schematically demonstrated in Fig. 9.

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Fig. 9

Plausible interaction mechanism of Pb²⁺/Cd²⁺ with Kao-Cel/Co₃O₄ nanocomposite surface.

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3.7 Desorption of Pb²⁺ and Cd²⁺

To elucidate the practical utility and adsorption mechanism of Kao-Cel/Co₃O₄ NC, desorption studies were performed. If the sorbed M²⁺ ions show water-induced desorption, it can be inferred that the attachment of metal ions onto the adsorbent surface involves weak interactive forces or bonds. However, if desorption is achieved with a strong base or acid (NaOH/HCl) then the adsorption of M²⁺ ions onto the adsorbent surface may include electrostatic attraction or ion exchange (Baskar et al., 2022). After the fifth adsorption/desorption cycle, 84.31% and 83.02% desorption and 77.99% and 72.93% adsorption occur for Pb²⁺ and Cd²⁺, respectively (Table 5) indicating the excellent regeneration property of the Kao-Cel/Co₃O₄ NC adsorbent.

3.8 Removal efficiency for metal ions from real wastewater sample spiked with Pb²⁺/Cd²⁺

Under optimized conditions, the %removal is reduced from 90.86–83.17 for Pb²⁺ and 87.75–76.93 for Cd²⁺ with increment of initial concentrations (Fig. 10). The decrease in the removal efficacy may be due to the presence of different contaminants in the real source, which interferes with the uptake of both metal ions. However, the decrease is not so obvious that recommends that Kao-Cel/Co₃O₄ NC can be used effectively for Pb²⁺ and Cd²⁺ removal from the real wastewater.

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Fig. 10

Effect of real wastewater on the removal efficiency of Kao-Cel/Co₃O₄ nanocomposite.

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3.9 Disposal of the spent adsorbent

The sustainable management of the exhausted adsorbents is one of the major tasks in the adsorption-based treatment system. Their disposal through landfills, regeneration by desorption process for use (ceramics, catalyst/catalyst support, capacitor)/reuse and recycle, and ultimately incineration are viable strategies adopted for managing the spent adsorbents. However, the present adsorbent poses no disposal issues because of its green characteristics, and good regeneration potential and recyclability.