Synthesis and Kinetic Adsorption Characteristics of Zeolite/CeO2 Nanocomposite

Adsorption is an effective method for treating polluted water bodies. In this study, Zeolite/Cerium oxide nanocomposite (Z/CeO2-NC) was hydrothermally synthesize and its adsorption capacity on methylene blue organic dye (MB) studied. The as-synthesized nanocomposite (Z/CeO2–NC) were characterized using X-Ray Diffraction (XRD), Fourier Transform Infra-Red (FT-IR), Energy Dispersive X-Ray Spectroscopy (EDX) and Scanning Electron Microscopy (SEM). Kinetic models and sorption isotherms were used to predict the adsorption rate constants and process mechanisms. The synthesized Z/CeO2-NC showed an excellent adsorption kinetics of methylene blue and the characteristics of adsorption best fitted a pseudo-second-order model. The rate parameters of other models were evaluated and compared to establish the adsorption mechanisms. The Langmuir isotherm model best fitted the adsorption process indicating a homogeneous monolayer of the dye on the surface of the adsorbent. The thermodynamic Gibbs free energy (ΔG) parameter was determined to be negative, indicating a spontaneous adsorption process. The synthesized Z/CeO2-NCs showed strong adsorption for methylene blue dye with increasing amount of CeO2. The maximum adsorption efficiency was calculated as 93.9% and the maximum regeneration efficiency of the adsorbent was found to be 78.8% higher after it had


INTRODUCTION
Dyes are important coloring agents with application in several industries including printing, textiles, food, plastic etc. The release of toxic-colored wastewater from these industries into natural water bodies has led to harmful effects on humans, oceanic life and photosynthesis through the reduction of solar irradiation [1,2].
Numerous technologies, including biological degradation which uses fungal decolourization as well as chemical and physical methods such as coagulation, electrochemical techniques and ozonation approaches, have been broadly studied to treat wastewater polluted with dyes [1,2,3].
None of these strategies has however been effective in dye molecules removal from polluted water [1]. This has heightened interest in the search for novel materials and simpler methods for purification of water bodies affected by wastewater from industries that uses dye [3].
One of such techniques capable of removing dyes from polluted waterbodies is the use of nanocomposite materials to adsorb the dye molecules [1]. An ideal adsorbent should have a porous structure, high surface area, good physical, mechanical and chemical stability and high affinity for pollutant molecules [3]. The adsorption of an organic molecule by an adsorbent depends on various parameters such as pH of the solution medium, the structure and concentration of the adsorbate, degree off ionization of the adsorbate, temperature, the ionic strength of dispersion and the structure and surface charge of the adsorbent. An adsorbent surface with highly ionic character is effective for removing organic molecule of opposite ionic character [4]. Research works have shown that Zeolite; a microporous aluminosilicate material with regular structures consisting of well-defined molecular-sized pores and channels can be engineered to be efficient in removing dye molecules from wastewater. Zeolites frameworks are built by [SiO4] 4− and [AlO4] 5− tetrahedral, linked together to form cages connected by pore openings of defined size. The presence of [AlO 4 ] − in the zeolite framework introduces negative charges which is balanced by cations such as Ca 2+ , K + and Na + which are also exchangeable in solution through ionic exchange [5,6]. Several research works have revealed that the nature of their porous framework has a strong effect on its efficiency as an absorbent and as a host material in biomedical and catalytic applications [7].
Dyes are classified as anionic, cationic and nonionic with different molecular sizes [8].
Adsorption of anionic dyes by zeolite has been reported to be low because both zeolite and anionic dyes have similar surface charge characteristic [8,9]. It is also reported that the adsorption capacity of zeolite can be enhanced by modifying the surface with hexamethylenediamine (HMDA), hexadecyltrimethylammonium bromide (HTAB) and cetyltrimethylammonium bromide (CTAB) [9,10,11]. In addition, the charge characteristics of zeolite can be modified to enhance its adsorption of cationic dyes by introducing functional groups or by encapsulation of complexes in the zeolites framework to increase the number of active sites [12,13,14].
Currently, much attention is paid to nanoparticle adsorbents, such as Manganese oxide (MnO 2 ), Iron (IV) Oxide (Fe 3 O 4 ), Titanium dioxide (TiO 2 ), MnFe 2 O 4 and Cerium oxide (CeO 2 ), due to their ability to exist in multivalent state and the formation of complexes within host materials.
Interestingly, one of the abundant and less costly rare earth metal oxides is Cerium oxide which has been studied as a catalyst providing excellent physical and chemical properties [15]. Among the rare earth metal oxides, Ceria has the lowest solubility in acidic media and does not elute during its use in the removal of harmful ions from aqueous solutions. In addition, cerium is a multivalent element, capable of forming several metal complexes [16]. One interesting property of the CeO 2 nanocrystals is that they exhibit a point zero charge at pH 8.0 implying that they are positively charged under pH less than 8.0 and negatively charged at pH values greater than 8.0 [17]. In many environmental remediation applications, cerium oxide had shown a high adsorption capacity for various anions and cations including fluoride and arsenic which makes it attractive as an adsorbent [18,19]. The authors are hypothesizing that, with tunable surface charge characteristics of zeolite and CeO 2 , when developed into a nanocomposite, CeO 2 and zeolite may act synergistically to enhance their capacity to adsorb cationic methylene blue dye from water bodies.
In this work, natural aluminosilicate-based deposit Kaolin have been sampled from Saltpond, in the Central region of Ghana. Zeolite/Cerium oxide nanocomposite (Z/CeO 2 -NC) was hydrothermally synthesize. The as-synthesized Zeolite and its nanocomposite (Z/CeO 2 -NC) were characterized by X-Ray Diffraction (XRD), Fourier Transform Infra-Red (FT-IR), Energy Dispersive X-Ray Spectroscopy (EDX) and Scanning Electron Microscopy (SEM) to confirm the formation of Zeolite and Cerium Oxide nanoparticles. The adsorption kinetics of methylene dye molecules onto the Z/CeO 2 -NC and the thermodynamic Gibb's free energy parameter was studied to understand the adsorption characteristics of the zeolite nanocomposite.

Raw Materials
All the chemicals used in this study were of analytical grade and were used without purification.

Synthesis of Zeolite A
The method used for the synthesis of Zeolite A is similar to the one reported by Nyankson et al. [20]. At the initial stages of the synthesis, the kaolin particles were sized using Retsch-VS1000 mechanical shaker to dimensions ≤ 75 µm. The fine raw kaolin sample was then calcined at a temperature of 600 ℃ for 2 hours to convert it to metakaolin, which is more reactive. This stage was immediately followed by alkaline treatment with a 2 M NaOH aqueous solution using a Solid-to-Liquid (S/L) ratio of 10 g/50 mL. The mixture was stirred continuously using a magnetic stirrer for 30 min to give a homogeneous mixture and then allowed to age for 24 hours to allow for nucleation of the zeolite phases. After this, the mixture was then placed in an oven at 100 ℃ for 7 hours to allow the growth of the zeolite crystals. The next stage involved filtration of the mixture and the subsequent washing of the residue until the pH of the filtrate was near neutrality. The residue separated was then further dried in an oven at 60 ℃ overnight. The dried sample was then crushed and grinded in a mortar with a pestle and sent for characterization.

Synthesis of Zeolite/Cerium Oxide Nanocomposites
In the synthesis of the Zeolite/Cerium Oxide Nanocomposites (Z/CeO 2 -NCs), two optimal masses; 300 mg and 400 mg masses of the Ce(NO 3 ) 3 •6H 2 O salt were used based on ion exchange calculation in order to avoid overdose and blocking of the zeolite pores. The Z/CeO 2 -NCs prepared with 300 and 400 mg of Ce(NO 3 ) 3 •6H 2 O were designated as Z-300 mg Ce and Z-400 mg Ce, respectively. The measured masses of salt were each dissolved in water and stirred continuously for 30 minutes to form a homogenous solution. This was immediately followed by the addition of 6 g of the as-synthesized zeolite into each of the aqueous solution and stirred continuously for 20 hours and at 150 rpm under room condition. Each mixture was treated slowly with 50 mL 0.1 M aqueous solution of NaOH under vigorous stirring for 1 hour to allow for precipitation. The precipitate was filtered, washed by centrifuging with deionized water and dried in an oven at ℃ overnight. The final stage involved calcination of the dried nanocomposites at ℃ for 2 hours to allow for the CeO 2 nanoparticles formation.

UV-Vis spectroscopy
The process of adsorption of the dye molecules onto Zeolite/Cerium oxide nanocomposite surface was monitored using UV-Vis spectroscopy. A scan between 200 -900 nm wavelengths using a GENESYS 10S UV-Vis (version v4.005 2L5S048209) was carried out. The maximum adsorption peak for MB was observed at 665 nm wavelength and the corresponding absorbance measured were used to extract concentration values from a calibration curve.

X-Ray Diffraction (XRD)
Powder XRD patterns were recorded using the Empyrean PANalytical series 2 X-ray Diffractometer (XRD) with CuKα (1.54Ǻ) radiation source and a tube operating at 40 mA and 40 kV. The phases in the samples were identified using X'Pert Highscore plus database software.

Fourier Transform Infra-Red (FTIR) Spectroscopy
In this work Attenuated Total Reflectance (ATR) was employed with single bounce diamond anvil ATR accessory fitted to a Thermo-Fisher Nicolet IS50 FT-IR spectrometer.

Scanning Electron Microscopy (SEM) and Energy Dispersive X-Ray Analysis (EDX)
The structural morphology and empirical elemental compositions were estimated using energy dispersive X-ray spectroscopy (EDX)-Zeiss EVO LS10 scanning electron microscopy (SEM) equipped with Oxford INCA X-act detector.

Adsorption Experiments and Kinetics
The aim of the present study is to investigate the ability of the Z/CeO 2 -NCs adsorbent to remove MB dye molecules from solution as well as describe the adsorption mechanisms using kinetic and isotherm models.
A series of experiments were conducted before the optimal adsorbent loading of 2.8 mg Z/CeO 2 -NC in 3.5 mL MB solution was arrived at [20]. All adsorption experiment was performed in a batch sequence using adsorbent loading of 2.8 mg, adsorbate pH of 9 and at a temperature of 25 °C. In the batch adsorption experiments, 2.8 mg of the Z/CeO 2 -NCs was placed into a cuvette followed by the addition of 3.5 mL of MB solution (2 mg/L). The cuvette with the mixture was then placed in the Genesys 10S UV-Vis spectrophotometer and a set of absorbance data taken over a period of 3 hours at the MB characteristic monochromatic wavelength (λ max ) of 665 nm.
pH is a parameter capable of having effect on the adsorption and desorption process in a batch adsorption study through altering the charge surface characteristics an adsorbent [21]. Desorption of MB molecules from an adsorbent in an alkaline medium is as a result of the excess OHreacting with the cationic sites of the MB molecules in solution [22,23].
From the adsorption experiments, the adsorption capacity, q t (mg/g) which represents the amount of dye adsorbed per unit weight of zeolite, mg/g and the dye removal efficiency was then calculated using the equation as shown below: Removal Efficiency (%), R = Where,

Adsorption Kinetics
The purpose of carrying out an adsorption kinetic study is to describe the solute uptake rate from the solute-solution interface. The Pseudo-First-Order (PFO) Lagergren equation, Pseudo-Second-Order (PSO) rate equation and the intra-particle diffusion model are among the common models for investigating the mechanism for dye molecule uptake by adsorbents [24]. In this work, the PFO kinetic, PSO kinetic and an Intra-particle diffusion models were all employed.
The PFO kinetic model by Lagergren states that the solute uptake rate is directly proportional to the difference in concentration of the solute and the equilibrium saturation concentration on the adsorbent [24]. Thus, Where; -Amount of dye adsorbed at time "t" in minutes (mg/g) A linear graph of against t is plotted and the value of (The rate constant of pseudosecond order) were deduced.
The third model, intra-particle diffusion model (Eq. 7), was proposed by Weber and Morris in 1962 has been widely applied for the analysis of adsorption kinetics to determine the ratecontrolling steps. The intra-particle diffusion is controlled by film diffusion, pore diffusion or surface diffusion or a combination during the adsorption phenomenon on the pore surface [26].
The model is described as; Where; -The intercept which corresponds to the boundary layer thickness (mg/g) -Intra-particle diffusion rate constant (mg/g• ) and can be directly determined from the liner plot of against as the slope and intercept respectively. The larger is the value of , the greater is the boundary layer effect.

Sorption Isotherm Models
Two different isotherm models; Langmuir and Freundlich Isotherms are used to describe the equilibrium adsorption data and to understand the extent of favorability of the adsorption. The applicability of the isotherm equation to the adsorption study done was compared by analyzing the correlation coefficients [27].
The Langmuir isotherm is a useful isotherm that assumes that adsorption takes place at specific homogeneous sites within the adsorbent, with a process of homogeneous monolayer adsorption of adsorbate on the surface of the adsorbent [28]. It also assumes that one active site is responsible for adsorbing a dye molecule, and once it is occupied, no additional adsorption can occur at that site. The saturated monolayer isotherm model can be expressed as; The linearized form of becomes: Where: It is well known that the separation factor (R L ) is crucial in determining the suitability of Langmuir isotherm to model the adsorption process. A value of 0 <R L < 1 indicates that it is a favorable adsorption process, while R L > 1 shows that it is an unfavorable adsorption process, mathematically from Eq. (10) Where: On the other hand, the Freundlich isotherm describes a heterogeneous multilayer adsorption system with interactions between adsorbed molecules. The Freundlich isotherm is an empirical equation that is derived by assuming a heterogeneous surface with a non-uniform distribution of heat over the surface of adsorbent [29] and it can be represented by (11) The linearized form of the Eq. (11) is: Where; -The adsorption capacity at equilibrium (mg/g) -The liquid phase concentration of adsorbate at equilibrium (mg/L)

X-Ray Diffraction (XRD)
XRD analyses were done on the as-synthesized pure zeolite (Z) and Z/CeO 2 -NCs in order to identify the phases present as shown in Figure 2.  Again, the intense XRD diffraction peaks for Figure 2  From the results, it was observed that the crystallite size of Pure Zeolite was 57.4 nm and that of Z/300 mg Ce and Z/400 mg Ce were 28.7 nm and 24.8 nm respectively. By comparing these three, it can be observed that the compositional variation led to a decreased crystallite size. Also, by comparing the two composites, it was further observed that the one with the higher composition of Ce(NO 3 ) 3 •6H 2 O salt had the smallest crystallite size. The smaller the crystallite size, the larger the surface area of the adsorbent exposed to the dye molecules and hence the better dye removal efficiency.

Energy Dispersive X-Ray Analysis (EDX)
The EDX shows the elemental composition present in a given sample. The EDX analysis of the composite detected aluminum, oxygen and silicon which indicates the presence of zeolite which is an aluminosilicate. The detection of cerium confirms the presence of the cerium oxide phase in the zeolite/ cerium oxide nanocomposite. The EDX results is summarized in Figure 3.

Fourier Transform Infra-Red (FTIR) Analysis
FTIR spectroscopy is used to identify the IR spectra corresponding to the functional groups and the vibrational bands present in both the pure zeolite and Z/CeO 2 -NCs in the spectral range of 4000-400 cm -1 , as shown in Figure 4.
The spectral of pure zeolite as shown in Figure 4  The shift in the characteristic peaks by 5-10 cm -1 is as a result of the CeO 2 incorporation [33].
That means that Ce(NO 3 ) 3 •6H 2 O salt addition has some influence on the zeolite framework.
From the analyses of the data, it is observed that the IR spectra confirmed the evidence of the formation of zeolite and Cerium Oxide phases.

Scanning Electron Microscopy (SEM) Analysis
The SEM gives information on the morphological structure of synthesized zeolite and the zeolite/cerium oxide nanocomposite. Mohammadi & Pak (2002) reported that zeolite A is cubic structured. From Figure 5 (a) the formation of cubic zeolite A structures can be observed [34].
By comparing the two diagrams ( Figure 5 (a and b)) it can be deduced that the presence of the spherical structure interspersed within the morphology of the composite can be attributed to the formation of cerium oxide phases [31,35].

Equilibrium Curve
The capacity of Z/CeO 2 -NCs to adsorb MB dye molecules from solution was examined and compared to that of the pure zeolite as shown in Figure 6. Overall it was observed that the concentration of the MB dye decreased with time and the amount of dye uptake, q t (mg/g), increased with contact time. The graph depicts that the dye uptake occured in two phases with a first rapid initial phase of adsorption within the first 10 minutes followed by a gradual second phase attaining saturation after 3 hours, which all contributed to the total adsorption process. The rapid initial stage of adsorption was an instantaneous surface adsorption resulting from the presence of vacant adsorption site as well as the presence of high concentration gradient. The adsorption by pure zeolite can be attributed to the negative surface charge of the zeolite leading to a high electrostatic attraction between the negatively charged zeolite and the positively charged cationic MB [36]. It has been reported that pHzpc of zeolite is 7.8. Below pH 7.8 the surface of zeolite is positively charged and above 7.8 the zeolite surface is negatively charged.
The pH of the methylene blue dye used for the study was 9. Hence the zeolite possessed a negatively charged surface at pH of 9 resulting in a high electrostatic adsorption of the cationic MB molecules from solution [37].  Table 1. As can be seen in the Table 1, the adsorption efficiency of Z/400mb Ce was higher than that of Z/300 mg Ce. The relatively higher efficiency of the Z/400 mg Ce was due to the increase in cerium oxide content than that of Z/300 mg Ce with a lower amount of cerium oxide content . More adsorption sites were therefore available in the sample with higher amount of CeO 2 . Pure zeolite attained the least efficiency in the removal of the MB dye.

Kinetic Models
The pseudo first order (PFO), pseudo second order (PSO) and intra-particle models were used to describe the adsorption kinetics of the MB dye onto the adsorbent surfaces. The fitting of the adsorption data unto the models are shown in Figure 8 (a, b and c). The best-fit model was  Table 2 below.

Sorption Equilibrium Isotherm Models
Langmuir and Freundlich isotherms (Figure 9) were used to describe the adsorption mechanisms for the interaction of dye molecules on the adsorbent surface and to determine the adsorption capacity. The related correlation coefficients (R 2 ) and other constant values are given as shown in

Gibbs Free Energy Thermodynamic Adsorption Parameter
To estimate the effect of the 25 ᴼC (298 K) temperature on the adsorption of MB onto the Z/CeO 2 -NCs, the change in the Gibb's free energy (ΔG) was calculated using the Eq (14).
Where ΔGᴼ is the standard Gibb's free energy change (J/mol), R is the universal gas constant (8.314 J/mol K), T is the absolute temperature (K), and K L is the Langmuir adsorption equilibrium constant (L/mol) [39].
The standard Gibb's free energy was calculated to be -1.208 and -4.118 KJ/mol for Z/300 mg Ce and for Z/400 mg Ce, respectively indicating that the adsorption process was spontaneous [39].

Regeneration Capacity
Reusability of adsorbents is a subject of great relevance in the economic development for adsorption processes. The zeolite nanocomposite adsorbents after full adsorption were washed with different pH of 3, 5, 7, 9, 11 and 13 in order to regenerate the adsorbent for reuse. pH is a parameter capable of having effect on the adsorption and desorption process in a batch adsorption study through altering the charge surface characteristics of the adsorbent [21]. The regenerated adsorbents were utilized again for dye adsorption as shown in Figure 10 (a and b). It was observed that adsorbent washed with alkaline medium achieved a higher adsorption rather than those washed with acidic medium. Desorption of MB molecules from an adsorbent in an alkaline medium resulted from the excess OHreacting with the cationic sites of the MB molecules in solution [22,23]. Excess H + ions were not able to desorb the cationic MB to free the adsorptive sites leading to decrease in dye uptake due to the electrostatic repulsion of both cations. Also, it was observed that the used Z/400 mg Ce showed relatively high regeneration efficiency of about 78.8% as against a regeneration efficiency of 67.0% for Z/300 mg Ce. In both cases, it was realized that the pH at which maximum and minimum efficiency occurred was pH of 13 and 3, respectively. The results from the regeneration studies are summarized in Table 4.  67.0 ± 0.3 78.8 ± 0.8

CONCLUSION
The potential of Zeolite/ CeO 2 nanocomposite as a dye removal adsorbent has been successfully explored. From the experiment, it was observed that the adsorption of the MB dye molecules unto the Zeolite/ CeO 2 nanocomposite adsorbent (Z/CeO 2 -NCs) was controlled by the Pseudo Second Order implying that the overall rate of adsorption was by chemisorption. By further fitting the equilibrium adsorption data, it was found that the Langmuir isotherm model better describes the mechanism of adsorption process suggesting a single molecular layer adsorption of the adsorbate molecules unto the Zeolite/CeO 2 nanocomposite. In addition, the negative value obtained from the standard Gibb's free energy indicated a spontaneous adsorption process. In the regeneration studies, it was also observed that treating the composite adsorbents with solutions of higher pH values generally gave higher removal efficiencies of the dye molecules during the reuse of the adsorbents.