3.1. Characterization of NanoMuscovite
3.1.1. XRD analysis
XRD provides information about the degree of the exfoliation of the organoclay. Activated muscovite shows two intense diffraction peaks at 2θ values of 8.9˚ and 26.8˚, less intense peaks at 2θ of 17.8˚, 26.6˚, 35.9˚and at 2θ 45.3˚ . The main peak at 8.87° corresponding to the interplanar spacing of 9.98 Å, which are all associated with muscovite as determined by JCPDS data (JCPDS: 00-058-2016). Crystallite size was determined by the Scherer's equation D = (Kλ)/ (βcos θ), where D is particle diameter size, λ is the wavelength of the X-ray radiation (λ = 0.154056 nm), K the Scherer constant (K = 0.891), θ the characteristic angle of diffraction (2θ =8.9º) and β is the full-width-at-half- maximum of the (001) plane after correcting with instrumental broadening using the Warren function. The XRD pattern for nanomuscovite samples prepared in the presence of of DTPA, PA, PN, TTAB and DTAB are obtained in Figure 1. From this Figure, it was noticed that the nanomuscovite samples display the same characteristic peaks as activated muscovite, suggesting the maintenance of the original crystallinity for the inorganic matrix when the reactions take place, where it shows two intense peaks at 8.9˚ and 26.8˚, less intense peaks at 2θ of 17.8˚, 26.6˚, 35.9˚and at 2θ 45.3˚ as determined by JCPDS data (JCPDS: 00-058-2016). But in the case of TTAB treated muscovite a new form of Rectorite (JCPDS: 00-014-0183) was appeared. It was observed that the crystalline size became smaller, which indicates that all intercalating agents reduced the crystallite size as summarized in Table (1). The high intensity and smallest crystallite size were obtained for the sample Muscovite/DTPA.
3.1.2. TEM examination
Figure 2 shows the TEM image of Mus/DTPA nanomuscovite sample. The face size of the majority of the particles is less than 100 nm, demonstrating the plate-like form of the muscovite particles and the compactness of the aggregated plates from the margins. Strong intermolecular forces cause the muscovite layers to be tightly packed together, and the smaller size of nanomuscovite particles than that of activated muscovite suggests that organic molecules may be intercalated between the muscovite layers.
From Table (2) and Figure 3, EDX spectra reflect the muscovite purity and end to that the particle is composed of O, Mg , Si, Al, and K.
3.1.3. FTIR analysis
Figure (4) shows the FTIR spectra of all nanomuscovite samples which prepared by different intercalating agents between 4000 cm-1 and 400 cm-1. The spectrum exhibits the typical appearance, with a peak at 1016 cm-1 corresponding to the stretching of the Si-O bond, and the bands at 2922 and 2855 cm-1 corresponding to the stretching vibration modes of the alkyl chain from the quaternary ammonium modifier. The peak at 440 cm-1 corresponding to the bending of the Mg-O bond. The bands of Al-O bond bending are observed at 918 cm−1. The peaks at 1635 and 3298 cm−1 are assigned to hydration HOH and –OH vibration groups between the tetrahedral and octahedral sheets (Nooshabadi et al., 2014).The band at ∼793 cm−1 is for the OH bending vibrations with respect to the silicate plate in the muscovite (Chen et al., 2020). The bands at 1403 cm-1, 1542 cm-1 are corresponding to the symmetric and asymmetric stretching vibration of the CH3COO- group, respectively. The 1395-1440 cm-1 band is attributed to the formation of coupling between C-O vibration and deformation vibration mode of the O-H groups (Ding et al., 2011). In the case of PN/Muscovite, not all peaks appear and have a new band around 1385 cm-1 which is attributed to N-O stretching
3.1.4. Surface area, BET and BJH analysis
The surface area of the prepared samples was also estimated by the BET method, and it
was found that surface area values of prepared samples have the following order: Muc/DTPA (195.15 m2g-1) > Muc/DTAB (190.97 m2g-1) >Muc/TTAB (182.46 m2g-1) > Muc/PA(178.45 m2g-1) > Muc/PN (172.62 m2g-1) >AM(137.659 m2g-1). Table (3) shows that the pore size of Muc/DTPA was smaller one due to decreasing the particles of Muscovite.
3.2. A comparative study of the prepared NanoMuscovite samples for Cd2+ and Pb2+ removal efficiency.
For comparison between the prepared NanoMuscovite samples by adsorption of cadmium and lead (Table 4). It is clear that Muc/DTPA is the best one for the removal of cadmium and lead and these results have been confirmed previously.So, the Muc/DTPA will be used in all the other experiments.
3.3. Optimal parameters for the adsorption of Cd2+ and Pb2+ on nanomuscovite (Muc/DTPA)
3.3.1. Effect of pH on metals adsorption
One of the key factors affecting how metal ions adsorb is the pH of the system, which can alter the metal forms in solution and have an impact on the surface characteristics of the adsorbent in terms of the dissociation of functional groups and surface charges. The outcomes demonstrated that as the pH rose, the amount of heavy metal ions that could be removed by adsorption on nanomuscovite (Muc/DTPA) increased (Figure 5 ). The adsorption removal percentage increased from 49 to 84% when the pH increased from 2 to 7 for Cd2+ and increased from 52 to 89.7% when the pH increased from 2 to 6 for Pb2+. At a low pH, the competition at the binding surfaces between heavy metal ions and H+ ion occur which results in a lower repulsion of the adsorbed metal (Sdiri et al., 2012). With the increase of solution pH, the deprotonation of the nanomuscovite surface occurs, that results in increase the number of negative charge sites , and so resulting in the high adsorption of metal ions on nanomuscovite (Zhu et al., 2013).
Liu et al., (2018) found that by using nano-montmorillonite (NMMT) at pH 7 , maximum Cd ions adsorption from aqueous solutions occurs. Eloussaief and Benzina (2010) investigated the efficiency of removing lead (II) from aqueous solutions using natural and acid-activated clays and discovered that the Pb(II) ions adsorption increased with pH up to a neutral pH. (7.0). Yin et al. (2018) revealed that Pb(II) was effectively adsorbed in solution at pH 4 using a nano illite/smectite clay as adsorbent. Al-a'qarbeh et al., (2020) investigated the Pb(II) ions adsorption from aqueous solution using the nano nanoplatelets client and found that the maximum adsorption capacity of Pb(II) ions at pH 7. Using Saudi Arabian clay, Ahmad et al. (2014) investigated the removal of Pb2+ and discovered that pH 6 had the highest adsorption capability.
3.3.2. Effect of adsorbent dosage on heavy metal adsorption
To investigate the effect of varying nanomuscovite adsorbent doses on the uptake of Cd2+ and Pb2+ from aqueous solution , different nanomuscovite doses (0.05, 0.1, 0.2, 0.3, 0.5 and 1.0 g) were studied with constant parameters (intial metal concentration, pH, contact time , temperature). The results (Figure 6) show that increase in adsorbent dose from 0.05 to 0.2 g increased the adsorption percentage of the metal ions 82% to 97.1% for Pb2+ and 80.2% to 96.1% for Cd2+; as a result of increased negative charge on the surface and the electrostatic potential decrease near the solid surface which led to sorbent-solute interaction.
Cadmium and lead removal percentages therefore dropped when the effects of accumulation on the nanomuscovite adsorbent occurred with increases in its dose of more than 0.2g, and increases in active sites with higher surface area also had little effect after equilibrium was reached (Gerçel and Gerçel, 2007). Similar results to our study have been reported by Unuabonah et al. (2007) for the adsorption of Cd and Pb ions from solutions by adsorbent (tripolyphosphate-impregnated Kaolinite clay).
3.3.3. Effect of heavy metals concentration on the adsorption
The initial metal concentration effect on the removal of Cd2+ and Pb2+ by the adsorbent was represented in Figure (7). The data evidence that the removal percentage increases from 74.3%,76 % to 96%, 97.1% then decreases to 91.5%, 92% for Cd2+ and Pb2+, respectively with increased metal concentration. This might be because Cd2+ and Pb2+ are more mobile on the surface of nanomuscovite due to the concentration differential between the adsorbate and its outer surface, increasing the driving force. (Zhu et al., 2013). The reason why the efficiency of removing metals decrease; is because when metal concentration rises, the active surface sites of the adsorbent become saturated, which lowers the percentage of metal removal. Additionally, additional adsorption sites are needed when Cd2+ and Pb2+ are present in the solution at greater starting concentrations (Masindi and Gitari, 2016). The final result refers to that the best concentration of Cd2+ and Pb2+ was 50 ppm for the adsorption on nanomuscovite adsorbent.
Liu et al. (2018) studied the cadmium ions adsorption from aqueous solutions using nano-montmorillonite (NMMT) at an initial Cd ion concentration (22.4–224 mg/L) and found that the maximum Cd adsorption (17.61 mg/g ) was at 22.4 mg/L Cd2+ concentration . David et al. (2016) prepared intercalated clay nanoparticles using Cetylpyridinium Chloride (CPC) and Tetradecyltrimethylammonium Bromide (TTAB) . The resulted nanoparticles C-TTAB and C-CPC were applied to remove Cd , Pb and pentachlorophenol (PCP) through batch process.They reported the intial concentration for removal of Pb by CNP was 80 ppm and 50 ppm for Cd .
3.3.4. Effect of contact time on heavy metals adsorption
The results of contact time effects on the efficiency of Cd and Pb ion removal are presented in Figure 8. The results revealed that the adsorption of Cd2+ and Pb2+ increased rapidly with time up to 60 min in which it reached the metal removal 95%, after then the metal removal rate decreased as the result of the Cd2+ and Pb2+ accumulation on the vacant sites of nanomuscovite. The rapid increase of metal adsorption with time resulted a large number of vacant sites.
3.3.5. Effect of temperature on heavy metals adsorption
The temperature effect on the adsorption of Cd2+ and Pb2+ on nanomuscovite had been studied with different temperatures (25oC, 35oC, 4 5oC and 60oC). The results (Figure 9) showed that the removal of Cd2+ and Pb2+ on nanomuscovite was highly at room temperature (25 oC), after then it was decreased gradually.The adsorption effectiveness decreased from 99% to 81% for Pb and from 97.2% to 79% for Cd when the temperature was raised to 55 oC. Since the Van derWaals bonds break down with increased temperature, the active sites are reduced (Negm et al., 2022). Because of this, Pb and Cd ions can be better adsorbed by nanomuscovite at room temperature (25 oC) than at any other temperature .
3.4. Adsorption Isotherm Study
Studies of adsorption isotherms are helpful for understanding how ions move from the adsorbents during adsorption processes.Characterization of Pb and Cd adsorption on the nanomuscovite approach was done using Freundlich , Langmuir , Temkin and Dubinin-Raduskevich (D-R) isotherm models.
The experimental data ( solution temperature 25oC , initial metals concentration 50 ppm /50 ml, 0.2 g adsorbent dosage and 60 min contact time) are presented in Table ( 5 ).
3.4.1. Langmuir isotherm
Langmuir isotherm model described the saturated monolayer adsorption on the homogenous surface at constant energy . Langmuir equation can be quantified by equation:
Ce/qe = 1/Qob + Ce/Qo ( 1)
Ce, the equilibrium concentration of heavy metals (mg/L), Qo adsorbent monolayer adsorption capacity of (mg/g) , qe metal equilibrium capacity at adsorbent (mg/g), and b is the Langmuir bonding energy coefficient (L/mg).
Calculating the Qo and b1 from equation (1) requires graphing Ce/qe against Ce and determining the intercept and slope.
From the data in Table (7), the correlation coefficients (R2) of Cd2+ and Pb2+ were found as 0.986 and 0.982 for nanomuscovite adsorbent, respectively indicating that the adsorption mechanism of Cd2+ and Pb2+ ions onto the nanomuscovite surface is carried out via monolayer. The maximum monolayer adsorption capacities (Qo) of Cd2+ and Pb2+ were found to be 26.6 and 25.2 mg g−1, respectively and close to the experimental capacities.
Also, from the results bL<1 for the studied metal adsorption reveal that the adsorption of Cd2+ and Pb2+ ions on nanomuscovite are favorable according to Langmuir isotherm. So, the Langmuir model is best fits the experimental data .
3.4.2. Freundlich isotherm
The Freundlich isotherm is applied to determine the applicability of heterogeneous surface energy in the adsorption process. The convenient formula for Freundlich is :
Log qe = log Kf + 1/n log Ce (2 )
The constants Kf, and n are the adsorption capacity and the adsorption intensity , respectively. So, when 1/n values is < 1 indicates a normal adsorption with a strong interaction between adsorbent and the metal, while if 1/n < 1 it indicates cooperation adsorption.
From equation (2), the intercept and slope of the linear plot of log qe against log Ce can be used to compute the Kf and n.
From equation (2), the Kf and n can be calculated from the intercept and slope of the linear plot of log qe against log Ce, and tabulated in the Table (7). The results obtained of Kf and 1/n were 0.36 and 2.09 for Cd2+ and 0.26 and 2.29 for Pb2+, respectively. The resulted 1/n values indicate that the adsorption of Cd and Pb ions onto nanomuscovite was favorable at studied conditions. The Freundlish isotherm model did not provide a better fitting than the Langmuir isotherm, as indicated by the lower correlation coefficient (R2) values (0.805 and 0.773 for Cd2+ and Pb2+, respectively) than that in the Langmuir model.
3.4.3. The Temkin isotherm
The Temkin isotherm model suggests that the population of surface binding adsorption will have a uniform distribution of binding energies
The Temkin equation's linear form is written as:
qe = B ln K + B ln Ce (3)
B = RT/b , where b is the heat of sorption-related Temkin constant. A plot of qe versus ln Ce allows one to estimate the constants B, which is related to the heat of sorption (J/mol), and K, which is the Temkin isotherm equilibrium binding constant.
The values KT = 4.3 and 8.9 L g−1, R2 = 0.98 and 0.96 and B = 5.44 and 4.62 J mol−1 for Cd2+and Pb2+, respectively were estimated from the Temkin plot (Table 7). The heat of sorption (bT) indicates a physical adsorption process as the values of bT are less than 8kJ/mol. This means that the adsorption of Cadmium and Lead on nanomuscovite follows the Temkin model.
3.4.4. Dubinin-Radushkevich isotherm model
On a heterogeneous surface, the Dubinin-Radushkevich isotherm is typically used to express the adsorption mechanism with Gaussian energy distribution. It is used to differentiate between metal adsorption that is physical and chemical.
The linearized D-R equation :
Ln qe = ln qm – BE2 (4)
By graphing ln qe versus E2 using equations (4), it is able to calculate the β values from the slope and qm from the intercept.
Information regarding chemical and physical adsorption is provided by the mean adsorption energy (E). The obtained values of the mean free energy (E) for cadmium and lead, respectively, are 0.32 kJ/mol and 0.5 kJ/mol, respectively (Table 7).
E value for Cd2+ and Pb2+ is less than the 8–16 kJ /mol range of the adsorption reaction. Therefore, physical adsorption was used to describe adsorption of Cd2+ and Pb2+ onnanomuscovite.
From Table (7) the adsorption of Cd2+ and Pb2+ on nanomuscovite fitted well Langmiur Isotherm due to the high correlation factor (R2) than the other Isotherm models.
The R2 values of Cd2+ and Pb2+ using the evaluated isotherms confirm the following sequence Langmuir (best fit) > Temkin > Dubinin-Radushkevich > Freundlich.
3.4.5. Adsorption kinetic studies
Kinetic models have been proposed to identify the adsorption process' mechanism. These models offer important data that can improve adsorption effectiveness and process scale viability (Eftekhari et al., 2010). The Pseudo-first order, Pseudo-second order, Elovich models , and intra-particle diffusion were applied to examine the adsorption kinetics that characterize the effectiveness of Pb and Cd adsorption on nanomuscovite. All adsorption steps, including external film diffusion, internal particle diffusion, and adsorption, are included in these models. Tables (8 and 9) show the kinetic information for Pb and Cd adsorption on nanomuscovite.
3.4.5.1 Pseudo–first order model
The following is a representation of the pseudo-first order kinetic model:
Log (qe –qt) = log qe – (K1/2.303) t (5)
K1 is the rate constant of the pseudo-first order and qe and qt (mg/g) are the adsorption capacities at equilibrium and time (min), respectively.
The pseudo-first order kinetic constants for the adsorption of Cd2+ and Pb2+ on nanomuscovite were calculated from equation (5) using the slope and intercept of the linear relation between time (t) and Log (qe –qt). The correlation coefficient (R2) values were 0.843 and 0.854 for Cd2+ and Pb2+, respectively .The k1values were 0.02 and 5.8 × 10−3 min−1 for Cd2+ and Pb2+, respectively (Table 10). The pseudo first-order model's R2 value for Cd2+ and Pb2+ is lower than the R2 values in the other models, indicating that the model is irrelevent. This indicates that pseudo-first order equation might not be adequate to construe the mechanism of Cd2+ and Pb2+ onto nanomuscovite interactions.
3.4.5.2. Pseudo–second order model
It is confirmed that the pseudo second-order model is the rate-limiting stage in the formation of the vaderwalls bond between Cd2+ and Pb2+ via electron sharing or exchange. The following is a representation of the pseudo-second order kinetic model:
t/qt = 1/K2qe2 + 1/qet (6)
K2 , the rate constant of pseudo–second order adsorption (g/mg min).
From equation (6), the plots of t/qt vs time (t) for the adsorption of Cd2+ and Pb2+ are derived using the pseudo-second order model. Two parameters (qe and K2) were determined using the intercept and slope, respectively. High correlation coefficient (R2) values were obtained for Cd2+ and Pb2+ ions (0.996 and 0.997, respectively) (Table 10). For Cd2+ and Pb2+, the calculated equilibrium adsorption capacity (qe) values were 12 and 12.1 mgg-1, respectively. The adsorption of Cd2+ and Pb2+ followed pseudo second order model, as seen by the high conformity between the qe, cal and qe exp values.
3.4.5.3. Intra-particle diffusion model
The solute transfer for solid-liquid adsorption is often characterised by an intra-particle diffusion model, which was used to determine the adsorption mechanism :
qt = Kit ½ + C (7)
Where C (mgg-1) indicates the boundary layer effect and Ki (mg g-1 min-1/2) is the rate constant of the intra-particle diffusion model.
The slop and intercept of the linear plot of qt vs. t1/2 allow for the determination of the Ki and C from equation (7) (Figures 10,11 ). Table 10 contains the estimated and collected intraparticle diffusion constants for adsorption of Cd2+ and Pb2+ onto nanomuscovite
The multilinearity forms shown in Figures 10 and 11 indicate that there may be two or more additional steps. The first shows that the line's sharper portion can be attributable to the boundary stratum diffusion of solutions to the diffusion of divalent ions (Cd2+ and Pb2+ ) from the solution to the surface of nanomuscovite. The progressive adsorption stage is described in the second section, with intraparticle diffusion serving as the rate limiting step. (Bany-Aiesh et al., 2015; Srivastava and Hasan, 2011; Dawodu and Akpomie, 2014). Kid values for Cd2+ and Pb2+ were 0.19 and 0.17 mg g−1 min−1/2 , while R2 equal 0.83 and 0.88, respectively. C (10.2 and 10.4 mgg−1 for for Cd2+ and Pb2+ , respectively) values provide information regarding the boundary layer's thickness. The impact of the boundary strata increases with increasing intercept (Bany-Aiesh et al., 2015). The variation in the rate of mass transfer at the first and final stages of adsorption may cause the straight lines' aberration from the origin. Furthermore, pore dispersion is not the only rate-controlling step indicated by such a divergence of the straight lines from the origin. (Srivastava and Hasan, 2011). Furthermore, there may be two distinct stages involved in the adsorption of two divalent ions onto nanomuscovite. The surface adsorption or rapid external diffusion stage was considered to be the first linear fraction. The piecemeal adsorption process, which is managed by intra-particle-diffusion, forms the basis of the second linear portion. (Dawodu and Akpomie, 2014). Due to low R2 values, the sorption of Cd2+ and Pb2+ on nanomuscovite did not follow the intraparticle diffusion equation..
3.4.5.4. Elovich model
Elovich equation is one of the best models for characterizing these activated chemisorptions, and describe as:
qt= 1/β ln[αβ] + 1/β lnt (8)
α, the initial adsorption, and β, the adsorption coefficient
In the equation (8), when qe is plotted against ln (t) , graphs are created from which the constants α, β and correlation coefficient (R2) are computed (Table 10). The rate-controlling step (RCS) of the Elovich model is the chemical reaction of Cd2+ and Pb2+ with the adsorbent surface, and the removal process involves multilayer adsorption. (Hosseini-Bandegharaei et al., 2016).
The results reveal that the initial Cd2+ and Pb2+ adsorption rates onto nanomuscovite were studied, and the Elovich values (α) were determined to be 2.1 and 2.4 mg min-1 Cd2+ and Pb2+, respectively. The values of β, which represent the number of adsorption sites, were 1.1 and 0.1.3 for Cd2+ and Pb2+ , respectively. The adsorption of Cd2+ and Pb2+ onto nanomuscovite did not follow the Elovich equation because of low R2 values
Last but not least, Table (10) shows that all four kinetic models used for Cd2+ and Pb2+ adsorption on nanomuscovite follow a pseudo-second order kinetic model.
3.4.6. Thermodynamics of adsorption
The actual indications for particle applications are thermodynamic variables such (ΔGo), entropy (ΔSo) , and enthalpy (ΔHo) . Thermodynamics of adsorption was assessed in relation to various temperatures (298, 313, and 333 K).
The following equation was used to compute the thermodynamic parameters:
Ln k = ΔSo /R-ΔHo /RT (9)
R is temperature (K) and T is the gas constant (8.314 J/mol K) .Table (11) lists the estimated thermodynamic parameters.
The findings demonstrate that the characteristic sign of the ΔGo values was negative at all temperatures, demonstrating the spontaneous nature of these adsorptions. Additionally, it was found that the ΔGo values drop as T rises, suggesting that there is less driving force for adsorption at higher T. The negative sign of ΔHo indicates that the adsorption processes are exothermic. The results show the ΔHo values for adsorption of Cd2+ and Pb2+ ions are smaller than 20.9 kJ/mol which implies that Cd2+ and Pb2+ ions are physically adsorbing to nanomuscovite . The ΔHo value in the range of 2.1-20.9 kJ/mol indicates physical adsorption, and 80-200 kJ/mol chemical adsorption processes. The organisation of the adsorbed ions becomes less random, and the negative ΔSo values reflect a drop in ion concentration at the solid-solution interface, indicating an increase in divalent ions adsorption on the nanocross linked solid phase.