Removal of Cd2+ from aqueous solution by zeolite synthesized from Egyptian kaolin

Zeolite Na-Y was prepared from kaolin located in Wadi-Hagul, Suez, Egypt. The synthetic zeolite prepared by the hydrothermal reaction of kaolin was characterized by X-ray fluorescence (XRF), X-ray diffraction (XRD), scanning electron microscopy (SEM) and cation exchange capacity (CEC). Cadmium ion removal was investigated using the synthetic solution of Cd2+ ions with different concentrations at room temperature (25 °C ± 0.2), initial pH of the solution and contact times. The optimum contact time for removal of Cd2+ ion was 10 min, with 0.1g of synthetic zeolite and pH 7.57. The experimental data were correlated using Langmuir, Freundlich and HarkinsJura adsorption isotherms. The maximum adsorption capacity Qm obtained from the Langmuir isotherm was 14.006 mg/g for Cd2+ ion. Kinetic studies reveal that synthetic zeolite is more effective as adsorbent for removing cadmium ions.


Introduction
Heavy metal pollution seriously harms the environment and human health resulting in reduced growth and development, as well as diseases such as cancer, organ and nervous system damage, along with the ecosystem (Yuna, 2016;Hong et al., 2019). Environmental pollution became one of the common important subjects that attract the attention of many scientists at present (Fathy et al., 2016). The removal of heavy metals (Pb 2+ , Zn 2+ , Cd 2+ , Cu 2+ and Cr 2+ ) from wastewater has been received great attention, because of the adverse effect on the health and the natural environment (Ismael et al., 2012;Merrikhpour & Jalali, 2013;Guan et al., 2019).Various treatment processes are available, among which ion-exchange is considered to be cost-effective if low-cost ion-exchangers such as zeolites are used (El-Naggar et al., 2019).
The use of alternative low-cost materials with high selectivity as potential sorbents has been emphasized recently (Ji et al., 2020).
Adsorption is a special characteristic of zeolites which is usually ion exchange occurs into the pore openings of the ion exchanger and active side on the surface. The amount of metal ion to be adsorbed is strongly affected by the nature and concentration of the metal ion, pH, and metal solubility, presence of competing and complexion ions (El-Naggar et al., 2019).
Natural kaolin is the most common clay mineral of a group of hydrous aluminum silicate, has a typical 1 : 1 layered structure, with one tetrahedral and one octahedral sheet, which results from the breaking of aluminum-rich silicate rock, such as feldspar (Aveen & Kafia, 2014;Aragaw & Ayalew, 2018). Thus, Kaolinite mineral can be a potential adsorbent owing to its low cost, rich natural abundance, and high mechanical and chemical stability.
Zeolite NaY is a material with regular architecture and micropores consist of cavities and channels. Zeolite NaY is a material with regular architecture and micropores consist of cavities and channels. Zeolite NaY has a wider application as catalyst and adsorption (Ayoola et al., 2017;Warzybok & Warchol, 2018).
Zeolites have satisfactory capacity adsorption for heavy metals removal due to their high cation exchange capacity and surface sorption properties (Aragaw & Ayalew, 2019;Guan et al., 2019). The exchangeable cation (sodium, potassium, or calcium) balances the net negative charge. These cations are exchangeable with certain cations in solutions such as lead, cadmium, zinc, and manganese.
However, synthetic Zeolites are preferred for industrial applications, such as adsorption, catalysis, and ion exchange, then their natural counterparts due to more uniform pore sizes, relative ease of manufacture, low costs and can be tailored to specific shape and size needs. Many synthetic molecular sieves have a pore size range of 3.5-4.5Å for zeolite LTA (Z-A), 4.5-6.0 Å for ZSM-5 and 6.0-8.0 Å for zeolite X and Y (Khalifah et al., 2018).
The objective of this study is to prepared synthetic zeolite-Y from Egyptian kaolin, Wadi Hagul area and to investigate the capacity and performance of synthetic zeolite-Y for removal of Cd 2+ from prepared solution.

Prepared samples
Raw representative sample of natural, poorly crystalline clay mineral (kaolin) was collected from Wadi-Hagul, Suez, Egypt. The location of the study area (Latitude 29° 44' 13.5" N and longitude 32° 14' 08.2" E) is shown in (Fig. l). Clay samples were air dried at 80 °C and grinded to fine powder by using standard sieves to obtain the fine powder less than (40 µm) (Black et al., 1985).

Hydrothermal synthesis
Calcined of natural Egyptian Kaolin (NEK) for 6 h at 800 °C. Through this process kaolin converted into metakaolinite phase (Rahman et al., 2018;Warzybok & Warchoł, 2018). Metakaolinite was mixed with (1.0 M Na OH) solution and used as starting material without any additional silica to the reaction mixture. Na OH was utilized as homogeneous catalyst which is a nonrenewable and becomes consumable chemical in the process (Fatimah & Rubiyanto, 2018). The solid/liquid ratio of metakaolinite to alkaline solution was (10 g/100 mL) in covered Teflon cups. The suspension was stirred at 800 rpm at room temperature for 24 h to obtain a homogenous fusion of solid as presented in (Adeoye et al., 2017). The slurry was transformed to the dry oven for treatment at 90 °C/2d under ambient pressure to synthesize zeolite-Y according to (Tavasoli et al., 2014;Ayoola et al., 2018;Olaremu et al., 2018). The residue product was filtered and washed till (pH less than 8). The residue was dried overnight at 100 °C. The steps of synthesis zeolite are shown in (Fig. 2). The high crystallinity product zeolite-Y and the raw material are characterized by XRF, XRD, and SEM (Somderama et al., 2019).

Characterization of synthetic zeolite and kaolin
Chemical and mineralogical composition of raw material (kaolin), metakaolin, final zeolite-Y product were determined by X-ray fluorescence (XRF) instrument model AX-IOS, WD-XRF Sequential Spectrometer (for the elemental composition) (Iftitahiyah et al., 2018). X-ray diffraction analysis (XRD) was applied using an Empyrean PANalytical diffractometer with Cu, Kα radiation at 40 kV with the current 30 mA. The diffractograms were recorded in 0.0263 o 2θ. The diffraction data were analyzed of both the kaolinite and the resulting zeolite product and the morphology was examined using scanning electron microscope (SEM). The scanning electron microscope using SEM Model Quanta 250 FEG (Field Emission Gun) attached with EDX Unit (Energy Dispersive X-ray Analyses), with accelerating voltage 30 K.V., magnification14x up to 1000000 and resolution for Gun.1n.
pH and EC were determined in both aqueous solution and clay minerals by glass electrode and electrical conductivity respectively. Cation exchange capacity (CEC) is measured by the sodium saturation method are shown in Table 2.  The total cation exchange capacity (CEC) of zeolite-Y and kaolin is the amount of all exchangeable cations and it is defined by the number of equivalents of fixed negative charges per amount of zeolite (Yusof et al., 2010).

Batch Adsorption Experiments
Heavy metal ions removal was determined in terms of initial concentration, pH, and contact time. A stock solution of concentration 1000 mg/L from Cd 2+ solution was prepared by dissolving an appropriate amount of Cd(NO 3 )2.4H 2 O, diluted in deionized distilled water (DDW). Solution ionic strength was controlled by adding a 50 mL of 0.05 M KNO 3 , background electrolyte solution containing different concentrations of Cd 2+ ions (10, 15, 20, 25 and 30 mg/L). In the study of the impact of contact time, 0.1 g of synthetic zeolite (SEZ) mixed with 50 mL of the 10 mg/L of Cd 2+ solution at 5, 10, 20, 40, 60, 80,100, 120 and 180 min (Jamil et al., 2010) to establish the ability of synthetic zeolite to retained Cd 2+ ions (to determine the suitable time to adsorb the metal from the waste solution by synthetic zeolite). Supernatant aliquots were filtered through Whatman filter paper (No. 42) (particle retention 2.5 mL) before chemical analysis. The pH of the solution was measured before and after treatments, pH of initial solutions before the addition of adsorbent was determined 5.95. In these experiments, these solutions are simulated in terms of the pH of real wastewater. The concentrations of metal ions in the solutions before and after adsorption were determined using Inductively Coupled Plasma (ICP) 6500 Duo, Thermo Scientific, England.1000mg/L multi-elements certified standard solution; Merck, Germany).
For studying the influence of the different concentrations, 0.1g from SEZ and 50, mL of heavy metal solution in the concentration range (10-30 mg/L) (EL Zayat, 2014). The suspensions were shaken at 350 rpm on a reciprocating shaker at room temperature (25 ± 0.2 °C). After equilibration at the contact time, the supernatants were separated by centrifugation at 3000 rpm/10 min (RoTofix 32A ttettich ZentRifugen RPMx RcFx100) and filtered through a Whatman filter paper (No. 42) (particle retention 2.5 lm). The amount of metal removed from the solution was taken as the difference in concentration between the amount added initially and that remaining in solution after equilibration (Chen et al., 2011).
The removal efficiency (%) adsorption capacity (q e ) was calculated using the equation (1) and equation (2), respectively (Franus et al., 2019;Rahimi & Mahmoudi, 2020): Removal efficiency 1 Adsorption capacity 2 where: q e is the equilibrium adsorption capacity of the adsorbent (mg/g), that is the amount of adsorbed metals was taken as the difference in concentration between the amount added initially and that remaining in solution after equilibra-  (Oren & Kaya, 2006;Hong et al., 2019), C o and C e are the initial and remaining concentrations of the heavy metal in the liquid phase (mg/L) at time t, V is the volume of aqueous solutions (L), and m is the mass of the adsorbent (g).
The distribution coefficient (binding energy) K d has been used to indicate the adsorption affinity of a solid adsorbent toward a solute (Naiya et al., 2009). Was obtained from the following equation :   3 where C e (mg/L) is the equilibrium concentration of Cd 2+ ions.

Results and discussion
Zeolite and kaolin characterization Chemical properties From Table 1, zeolite and kaolin vary in the alkalinity degree. Synthetic zeolite-Y and kaolin showed higher alkalinity (pH 10.23 and 8.18), respectively. The materials salinity was noticed with different degrees, kaolin is moderately saline and synthetic zeolite-Y is non-saline (Hardie & Doyle, 2012).
The cation exchange capacity (CEC) results revealed that the obtained CEC for synthetic zeolite-Y was higher than kaolin 401.09 meq/100g clay and 50.30 meq/100g clay, respectively.

Chemical and mineralogical analysis
The chemical composition (in wt. %) of kaolin before and after calcination and synthesis zeolite was listed in Table 1. The synthesis zeolite from kaolin formed in two steps, the first step is the metakaoliniziation and the second is zeolization. The volatile matter of the clay developed and di hydroxylation takes place in the course of calcination at 800 ○ C/6 h. The X-ray pattern of (unheated) kaolin is shown in (Figure 3a), the XRD pattern of raw material (kaolin) it contains kaolinite (K), Na-montmorillonite (Mo) and quartz as major impurities (Qz). The metakaolinization of kaolin by heating for 6 h at 800 °C resembled all the kaolinite peaks expect for the peaks due to the admixed impurities. Figure  2b shows that the characteristic peaks of kaolinite disappear during the calcination step and reduce in peaks intensity of quartz. The crystallinity of the zeolite-Y increase with increasing the time at 2d of metakaolinite. Na OH (1.0 M) at 90 °C/2d) used to produce the synthetic zeolite-Y. Figure 3c shows the peaks of zeolite-Y with small peaks of zeolite-P. The zeolite Y in pure phase was crystallized at 90 o C at a time of crystallization of 48 hours, and this result is agreement with (Ayoola et al., 2017;Belaabed et al., 2017). Zeolites formation was observed in (Figure 4c). The chemical formula of zeolite Na-Y is: Na 60 H 32 (Si 100 Al 92 O 384 ) zeolite-P is: Na 6 Al 6 Si 10 O 32 ꞏ12 H 2 O Figure 4 shows (SEM) micrographs of raw material (4a), metakaolinite under hydrothermal conditions with Na OH (4b) and synthetic zeolite -Y (4c).

Efficiency of synthetic zeolite for removing Cd 2+ ions
Zeolites were characterized by their higher capabilities for removing heavy metal ions because of their large specific surface area, channels and cavities, and their high number of negatively charged bonding sites. Besides, their ability to close fissures and cracks, which may form paths for leachates.

Optimum contact time
The results of the optimum contact time were given in Figure 4 of the removal efficiency of Cd 2+ ions using zeolite-y at room temperature. It was noted that the maximum amount adsorbed (q t mg/g) on synthetic zeolite was fast in the first 10 minutes and the percentage of removal efficiency reached 99.93 % as shown in Figure 6. The fast adsorption at the initial uptake was probably due to the interaction between Cd 2+ ions (adsorbate) in the solution and active sites those available on the zeolite-y (adsorbent) surface take place. The equilibrium of adsorption occurs when all active sites on the surface of Zeolite were blocked by Cd 2+ ions from the solution (Mehdizadeh et al., 2014). This may be attributed to the fact that SEZ can act as an ion exchanger. The adsorption capacity depends on the dose of adsorbents, contact time and pH (Jamil et al., 2010).

Adsorption kinetic studies
To design a convenient adsorption process, one should have enough information about the rate at which adsorption occurs. Thus, the rate of constants for the adsorption of Cd 2+ ions from aqueous solution onto SEZ was determined in Figure 6 using the three kinetic models. They can be used for examination of the controlling mechanisms of the adsorption process, such as chemical reaction, diffusion control, and mass transfer onto an adsorbents namely pseudofirst-order, pseudo-second-order and Intra-particle diffusion kinetic models (equations 3, 4 and 5) (Yildiz, 2017;Franus, et al., 2019;Kwakye-Awuah et al., 2019). The comparison between the three kinetic models are presented in Table 3. The theoretical q e-cal values calculated by the pseudosecond-order model were found to closer to the experimental q e -exp values and the correlation coefficients R 2 were higher for the Cd 2+ ion. Therefore, the second-order kinetics model described the adsorption of Cd 2+ ions onto synthetic zeolite.

Pseudo-first-order equation (PFO)
The pseudo-first-order equation was represented by the following equation (Lagergren, 1898) Log (q e -q t ) = logq e -[(k 1 /(2.303 )*t] where: q e and q t (mg/g) are the concentration of metal ions in the adsorbent at equilibrium and at time t respectively, and k 1 is the pseudo-first-order rate constant (min -1 ) k 1 and q e-cal can be determined from the slope and intercept of the linear plot of log(q e -q t ) versus t as Figure 7.

Pseudo-second-order equation (PSO)
To model the sorption kinetics, a pseudo-second-order rate equation was employed (Ho & McKay, 1998). The pseudo-second-order rate equation was represented as: t / q t = 1 /( k 2 q e 2 ) + (1 / q e ) t where k 2 is the rate constant of the pseudo-second-order equation, q t (mg/g) is the amount of Cd 2+ sorbed on different adsorbents materials at time t (min) and q e is the equilibrium sorption capacity (mg/g). k 2 and theoretical q e-cal values were obtained from the intercept and slope of the plot of t/q t versus t as Figure 8.
And the initial sorption rate h at various initial concentrations is related in the following equation h = k 2 q 2 e (5) where: h (mg / g min) is the initial sorption rate.

Intra-particle diffusion model (IPD)
The diffusion model analysis can be represented using the intra-particle model. This kinetic model was proposed by (Zewail & Yousef, 2015;Ali, 2018). The linearize equation is expressed as in equation (6).
where: K id denotes a constant of intra-particle diffusion (mg/(g‧min 0.5 ), t 0.5 is the square root of the time and C (mg/g) is the intercept relative to the thickness of the boundary layer (Fakhri, 2017).When q t versus t 0.5 as Figure 8, the graph is plotted, the linearity of the curve indicates that intra-particle diffusion takes place within the bio-sorption system. The main rate-limiting step for the adsorption was the intraparticle diffusion process. On the other hand, if the relationship q t vs. t 0.5 does not pass through the origin or is non-linear, the adsorption process is accompanied by other mechanisms (Franus, et al.,   shows that the plot of t/q t versus t is a straight line. The values of qe and intercept k 2 were calculated from the slope and intercept and were tabulated in Table 3. The experimentally obtained q e-exp value was found to be in good agreement with the q e-cal values calculated from kinetic experiments (Chen & Shi, 2017;Fakhri, 2017). According to the (PSO) model, high correlation coefficients (R 2 ) for 10 mg/L of Cd 2+ (R 2 = 1.00) onto SEZ. These results suggested that the sorption kinetics of Cd 2+ onto SEZ adsorbent follows a PSO process and that the overall rate constant of Cd 2+ ion was controlled by the chemisorption process (Fahmy et al., 2016).

Adsorption Studies Cadmium Sorption Behavior
Adsorption isotherms were presented in Figure 10. The amount of adsorbed metals increased with an increase in the initial adsorbate concentration from 10 to 30 mg/L. The maximum adsorption capacity value of Cd 2+ obtained from the Langmuir equation. Table 4 shows the maximum sorption capacity (Q m ) which represents monolayer coverage of sorbent with sorbate as shown in Figure 10. According to (Naiya et al., 2009) at lower concentrations, all of the Cd 2+ ion in the solution would interact with the binding sites and it was favor almost 100 % adsorption. At higher concentrations, more Cd 2+ ions were left un-adsorbed in the solution. The Cd 2+ ion adsorption was referring to different mechanisms of ion exchange as well as to the adsorption process. During the ion exchange process, the Cd 2+ ion had to be moved through the pores of the adsorbent mass, but also through the channels of the lattice, they have to be replaced exchangeable cations due to the saturation of the binding sites. It was observed that adsorption onto this material is practically linear in solutions containing one metal (Mishra & Patel, 2009). Figure 11 shows that the free adsorption sites reduced by increasing the initial concentration of Cd 2+ ions, therefore, the best initial concentration of solid adsorbents was 10mg/L, this is consistent with (Naiya et al., 2009;Chen et al., 2011;Al-Rudaini, 2017).  Figure 12 represents the function of Cd 2+ ions concentration. The values of distribution coefficient (binding energy) K d was calculated by equation (3) and decreased with the increasing concentration of Cd 2+ ions. All of the Cd 2 + ions in the solution will interact with the binding sites at lower concentrations and thus facilitate more than 95 % adsorption (Shaheen et al., 2012). At high concentrations, due to the saturation of the binding sites ions were left unabsorbed in the solution. This indicates that the energetically less favorable sites become involved with increasing ion concentration in aqueous solution. The Cd 2+ ions adsorption was attributed to different mechanisms of ion exchange as well as to the adsorption process. During the ion exchange process, the Cd 2+ ions had to be moved through the pores of the adsorbent mass, but also through the channels of the lattice, they have to be replaced exchangeable cations (Naiya et al., 2009).

Adsorption isotherms
The adsorption isotherms are very important for describing the adsorption mechanism of metal ions onto the adsorbent and plays an important role in the identification and evaluation of the maximum capacity onto adsorbents, three adsorption isotherms models namely Langmuir, Freundlich and Harkins-Jura isotherms models (Yuna, 2016;Erdogan, 2019). For Langmuir adsorption isotherm model is an example of a favorable isotherm and is applicable under an assumption: a uniform surface of the solid, the absence of interaction between the different adsorbed molecules, adsorption in a monolayer and adsorption occurs on a structurally homogeneous adsorbent (Mishra & Patel, 2009). This model is presented by equation (7) (Langmuir, 1918).

Langmuir adsorption isotherm
q e = Q m k L C e /(1+k L C e ) where: q e (mg/g) is the adsorbed amount of metal ions, C e (mg/L) is the equilibrium concentration of metal ions in the solution, Q m is the monolayer adsorption capacity, is the amount of metal ions at complete monolayer coverage (mg/g), K L is the energy of sorption constant. The linear form of Langmuir isotherm equation (8) is given by: C e /q e = (1/ (K L Q m )) + (C e /Q m ) The Langmuir parameters can be used to predict the affinity between the sorbate and the sorbent using the dimensionless separation factor R L (Rani & Sasidhar, 2012;Aly et al., 2014;Franus et al., 2019).
where: C o the initial concentration (mg/L) of Cd 2+ , k L is the dimensionless Langmuir parameter constant (L/mg). The value of the equilibrium parameter (R L ) indicates the type of isotherm to be irreversible (R L = 0), favorable (0 < R L < 1), linear (R L = 1) or unfavorable (linear) (R L > 1) (Rahman et al., 2010).
According to (Aly et al., 2014), the values of R L are less than 1 and greater than 0 indicating the favorable sorption Cd 2+ onto the different adsorbents. While the value of R L for synthetic zeolite-Y is close to zero (0.019-0.007) as shown in Table 3 and Figure 13 indicating adsorption characteristic is irreversible.

Freundlich adsorption isotherm
The Freundlich expression is a heterogeneous surfacebased, analytical equation. Freundlich isotherm can be given by equation (10) (Freundlich & Hatfield, 1926) q e = k F C e 1/n (10) where: q e (mg/g) is the adsorbed amount of metal ions, C e (mg/L) is the equilibrium concentration of metal ions in the solution, K F is the constants related to the adsorption capacity, n is an empirical parameter related to the intensity of adsorption, which varies with the heterogeneity of the adsorbent. For values in the range 0 < 1/n < 1, adsorption is favorable; the linear form of the Freundlich isotherm equation (11) is given by: Log q e = logK F + 1/n log C e (11)

Harkins-Jura adsorption isotherm
Harkins-Jura isotherm explains multi-layer adsorption in connection with the presence of a heterogeneous pore distribution and as explained in (Harkins & Jura, 1944), the linear form of the Harkins-Jura isotherm equation (12)  Adsorption isotherms describe the interaction between Cd 2+ and porous adsorbents. The parameters obtained from the three isotherm models were represented in Table 4. The comparison between the experimental adsorption results with the isotherm models (Freundlich, Langmuir, and Hrkins-Jura models) on Cd 2+ ions using SEZ sorbent is illustrated in Figure (13 a, b, and c), respectively.
From this study it can be seen that the value of linear correlation coefficient (R 2 ) is located in (0.965) demonstrating that the experimental data fitted well with the Langmuir isotherm model on SEZ. The Langmuir maximum sorption capacity Q m was found at a high value (14.006 g/g) for the SEZ. This result reveals that SEZ has great potential for Cd 2+ ions removal from contaminated solutions; this finding indicates that the Cd adsorption on the porous adsorbent sample follows the Langmuir monolayer adsorption. As summarized in Table 4.
From the Freundlich isotherm constant can be used to calculate the adsorption capacity and intensity of the reaction and to explore the favorability of a sorption process the values of n lies between one and ten this indicates a favorable sorption process (Dada et al., 2012). From the data in Table 4, the value of 1/n < 1 indicating that the sorption of Cd 2+ ions onto the adsorbent is favorable at the studied conditions (Yildiz, 2017;Chen & Shi, 2017). The heterogeneity factor value 1/n near to zero that conform heterogeneous distribution of adsorption sites on the adsorbent surface. The R 2 value is 0.867 for the SEZ. In Harkin-Jura isotherm, R 2 value is closer to unity for the Langmuir isotherm model than that for the Harkins-Jura isotherm models for Cd 2+ ions adsorption.
k F and A are the parameters for maximum sorption capacity for the Freundlich and Harkins-Jura equations. From the results the nature of metal ions adsorption on the adsorbent is more compatible with the Langmuir isotherm model; this indicates the occurrence of homogeneous and singlelayered adsorption on the used sorbent. The applicability of the three isotherm's models for the present data nearly followed the order: Langmuir > Harkins-Jura > Freundlich.
These results reveal that the Cd 2+ ions adsorption on SEZ is against the rule of multilayer adsorption (Erdogan, 2019). The sorption capacity followed the trend of CEC values; however, the synthetic zeolite-Y has a high value of CEC (401.09 meq/100g clay) (Singh et al., 2000).  Harkins-Jura (c) isotherms for the adsorption of Cd 2+ onto SEZ According to (Aly et al., 2014), the values of R L were shown in Figure 14 are less than one and greater than zero indicating the favorable sorption Cd 2+ onto adsorbent. While the value of R L for synthetic zeolite-Y is close to zero (0.019-0.007) indicating adsorption characteristic is irreversible. The aqueous solution pH seems to be an important factor for controlling the adsorption mechanism and cadmium ions removal on aluminosilicates (Fahmy et al., 2016). During the sorption experiments, pH in equilibrium solutions was recorded at different initial metal concentrations. This is shown in Figure 15 for Cd 2+ ions. The figure revealed that the case of synthetic zeolite-Y adsorbent pH increases gradually with an increase in the amount removed. According to (Chen et al., 2011) an increase of the solution pH induces an increase in the adsorption of Cd 2+ ions, with increasing pH the number of negatively charged: SOgroups and the hydrolysis of Cd 2+ ions increases, which leads to an increase of Cd 2+ ions adsorption. By increasing pH, the number of H + ions reduced and then the competition between H + ions and metal ions for adsorption sites decreased. Therefore, the adsorption of Cd 2+ ions into synthetic zeolite-Y increased. Cd 2+ ions may form complexes with OH − . However, metal ions have a significant impact on the removal efficiency of zeolite and the selectivity of the metal ions by zeolite was also influenced by the character of metal complex predominating at a common pH value. As known, zeolite has hydroxyl groups which exist on the surface (Si-OH and Al-OH); therefore, degree of ionization depends on the pH values, and the acid/base reaction occurring between the hydroxyl groups on the surface with Cd 2+ ions from solution (Covarrubias et al., 2005).

Conclusions
In this study, batch adsorption experiments for the removal of Cd 2+ from aqueous solutions were carried out using Egyptian synthetic zeolite-Y from Hagul area as low-cost adsorbents. The adsorption characteristics were examined at initial ion concentrations, contact time initial pH of Cd 2+ solution, the obtained results can be summarized as follows: -It was observed that the best time for synthetic zeolite-Y was 10 minutes, in which the substance was saturated with the element and it reached for stability at 20 min and the removal efficiency reached 99.926 %. -The experimental data were better described by the pseudo-second-order model as evident from the correlation coefficient values (R 2 ). -The Langmuir isotherm model was found to be better fitted than the Harkins-Jure and Freundlich isotherm models and the monolayer adsorption capacity for Cd 2+ onto SEZ was 14.006 mg/g. Synthetic zeolite-Y can be considered a good alternative adsorbent for removing metal ions from wastewater and contaminated soil immobilization when considering the abundance of low-cost raw materials used for its preparation.