Evaluation of Zeolitic Materials Synthetized from Coal Fly Ash as Potential Cd(II) Adsorbents from Aqueous Solutions

The aim of this research paper is to present a simple and efficient method to prepare zeolite materials from thermal power plant fly ash which is one of the most important waste resulted from power plants. The method of preparation of zeolite materials consists of alkaline activation followed by calcination at different temperatures. The zeolite materials prepared were studied by FT-IR spectroscopy, specific surface determination, X-ray fluorescence spectroscopy (XRF), scanning electron microscopy (SEM) and particle size composition. XRF data indicated that the zeolites synthesized are characterized by Si/Al ratio between 1.21 and 1.26, being mainly composed of Na-P1 zeolite. The optimum conditions of cadmium ions removal process by adsorption onto zeolite materials have been determined. It was concluded that increase of calcination temperature has as result increase of specific surface area and cosequently the increase of sorption capacity. An equilibrium, kinetic and thermodynamic study has been performed. The high value of correlation coefficient for the Langmuir isotherm reveals that the Cd(II) sorption onto zeolite materials occurs as a monolayer coverage of Cd(II) ions on homogenous zeolite surface. The pseudo-second-order kinetic model fits the experimental results of the Cd(II) sorption onto zeolite materials processes. As a results, the mechanism involved in Cd(II) sorption onto zeolite materials is based on chemical reactions. The thermodynamic results indicate that the Cd(II) adsorption process is more encouraging at higher temperatures.The results established that valuable NaP1 zeolite materials with high adsorption capacity can be prepared from thermal power plant fly ash through a simple method. The materials prepared can be utilized to remediate cadmium ions-bearing wastewater.

https://doi.org/10.37358/RC. 20.9.8323 from Merck, Germany). Dilutions of the 1000 mg Cd(II)/L stock solution at desired concentrations were prepared in 2% HCl suprapure solution. The solution's pH was modified by adding drops of HCl 32% suprapure, HCl 1N or NH4OH 0.1N analytical grade solutions.
The alkaline activation of thermal power plant was carried out on the Heidolph MR Hei-Standard Stirrer at a rotational speed of 500 rpm (rotations per min). The activated samples were calcinated on Caloris L1206 calcination furnance at different temperatures.
The zeolite materials prepared were characterized using modern analytical techniques such as X-ray diffraction, FT-IR analysis, specific surface determination, X-ray fluorescence spectroscopy (XRF), scanning electronmicroscopy (SEM) and particle size composition. FTIR spectra were recorded using a Fourier transformed spectrophotometer (Jasco FTIR 4100) by the use of the KBr pellet method. The specific surfaces (SBET) of the thermal power plant ash and zeolite samples were measured by adsorptiondesorption of N2 at -196 °C on an automatic adsorption system (Micromeritics ASAP 2020) using the Brunauer Emmett Teller method based on the partial pressure adsorption data (P/Po) in the range 0.05 -0.3. A Thermo Fisher Scientific X-ray fluorescence spectrometer (XRF) was used to determine the ash and zeolite materials chemical composition. The acquisition of zeolite materials micrographs was accomplished using a high-resolution electronic scanning microscope, Inspect F50 at 30KeV at various magnifications. The grain size composition of the ash and zeolite materials was determined using a RETSCH sieve system Studies on the Cd(II) ions retention were conducted with a GFL 3015 orbital shaker at a speed of 150 rpm. The pH of Cd(II) aqueous solutions was determined using an Agilent 3200P laboratory pH meter, and the Cd(II) ion content was determined by atomic absorption spectrometry using a ContrAA® 300 absorption spectrometer (Analytik Jena AG, Germany).
The retention capacity and the quantity of Cd(II) ions removed by zeolite materials at different times have been calculated using the equation: where: Qt -the adsorption/removal capacity expressed by the amount of Cd(II) retained on the adsorbent mass unit (mg/g); C0-the initial concentration of Cd(II) ions in solution (mg/L); Ct -the concentration of Cd(II) ions at time t (or at different pH values) (mg/L); Vthe volume of solution (L); m -the mass of adsorbent (samples of zeolite materials) (g). All experiments were performed in duplicate with an experimental error within 5% range.

3.Results and discussions
The thermal power was firstly washed with ultrapure water to remove the soluble materials. For this step, 300 g of ash were weighed, contacted with 1200 mL of ultrapure water and stirred at room temperature on a Heidolph MR Hei-Standard magnetic stirrer at a rotational speed of 500 rpm (rotations per minute). The washing was done for 48 h. After washing the thermal power plant ash was filtered and dried in the oven at 105°C.

The preparation of the zeolite materials
To prepare zeolite materials samples, the washed and dried ash was contacted with 3M NaOH solution and then calcinated at different temperatures. Thus, over 30 g of washed and dried ash was added 300 mL 3M NaOH and stirred at room temperature for 2 h, then at 100°C for 4 h. NaOH (3M) solution was selected as activation solution, since it presents higher conversion efficiency than KOH, under the same temperature. After cooling, the sample is filtered and calcinated at different temperatures in the oven for 8 h. The calcination temperatures were 150, 130, 110 and 90°C. After calcination, the zeolite samples prepared were subjected to analyzes in order to establish the physico-chemical and https://doi.org /10.37358/Rev.Chim.1949 Rev. Chim., 71 (9), 2020, 125-142 128 https://doi.org/10.37358/RC.20.9.8323 morphological characteristics. The process of preparing the zeolite materials is shown in the figure below.

Figure 1. Scheme for preparing zeolite materials
The characterization of thermal power plant as hand zeolite materials prepared The washed thermal power plant ash was characterized by determination of hygroscopic humidity, chemical composition and particle size composition. The thermal power plant ash has a 1.88% hygroscopic humidity determined using the drying method in the oven. The values of the hygroscopic humidity of zeolite material samples are presented in Table 1. It can be observed that Zeolite-90 sample has a higher water content compared to thermal power plant ash, and a decrease of the water content was shown with the increase of the calcination temperature.
The chemical composition of the thermal power plant ash is qualitatively similar to that of the natural materials in the composition of the earth, such as soils and shales. Oxide-like compounds of Si, Al, Fe and Ca account for about 90% of the composition of the thermal power plant ash. Other elements such as Mg, K, Na, Ti, S represent minor constituents and contribute in a very small percentage to the ash composition. In general, the power plant ash also contains other elements in the concentration of the order ppm, which represent about 1% of the ash composition. Ash is usually associated with various useful constituents such as Ca, Mg, Mn, Fe, Cu, Zn, B, S and P, but also toxic constituents such as Cr, Pb, Hg, Ni, V, As and Ba. The concentration of the trace elements in the ash is highly variable and depends on the coal (lignite) that has been used as raw material and on the combustion conditions and the efficiency of the emission control systems.
A Thermo Fisher Scientific X-ray fluorescence spectrometer (XRF) was used to determine the raw, washed ash chemical composition and zeolite materials prepared. The data obtained are presented in the Table 2. As shown in Table 2, the thermal power plant ash is a heterogeneous material having as main constituents SiO2 (43.15%), Al2O3 (27.14%), CaO (8.55%) Fe2O3 (6.23%), and MgO (2.20%). This composition of ash can lead to the conclusion that in this case the coal used in the power plant was of the pit coal type. There is also a change in the concentration of the components of the ash sample after washing.
By analyzing data regarding the composition of zeolite materials it can be seen that the alkaline hydrothermal treatment has as result the decrease of the SiO2 content and an increase of Na2O content. This can be due to the aluminosilicate glass dissolution. The ration between Si/Al in zeolite prepared varies between 1.21 and 1.26. According to the literature data these values for Si/Al are specific for NaP1 type zeolite [27]. It was also observed that the increase of temperature has as effect the decrease of SiO2 and Al2O3 content and the increase of Na2O content. Thus, the OH − in alkali medium notably provides the dissolution step of Si 4+ and Al 3+ in thermal power plant fly ash, while Na + in alkali solution influences the crystallization step of zeolite NaP1 [28].   To determine the particle size composition, 10 g of ash/zeolite material was sieved using a RETSCH sieve system and the data obtained are presented in Table 3. The ash fraction <200 μm has been used to alkaline treatment. It can be seen from the Table 3 that all the zeolite materials prepared have as high content the sieve fraction < 200 μm. Consequently, this particle size fraction was used in the adsorption experiments.
The pH of the washed thermal power plant ash was determined using an Agilent 3200P laboratory pH meter in slurry of 0.2 g ash and 25 mL of distilled water after a 24h contact time between the two phases. The value obtained is 7.81.
The raw thermal power plant ash and the washed thermal power plant ash were characterized by 5.43 m 2 /g and 4.81 m 2 /g values for specific surfaces (SBET). It can be observed a decrease of the SBET value after ash washing. The values of specific surfaces (SBET) for zeolite materials are presented compared to the value of SBET determined for washed ash in the Table 4. Data from Table 4 have revealed an increase of the value of specific surface (SBET) by transforming of the thermal power plant ash into zeolite materials. In addition, it was found that the increase of the temperature of thermal treatment has as result an increase of the specific surface. The high value of specific surface was determined for Zeolite-150. This value is higher with a factor of 8.5 compared to the value of SBET presented by thermal power plant ash.
FT-IR spectrometry was used to characterize the structural aspects of the thermal power plant ash and the synthesized zeolites. For this purpose, the FT-IR spectra presented in the figure below were recorded in the wavelength range 400 -4000 cm -1 .  [29].
The band occurring at 673 cm -1 is characteristic for T-O (where, T =Al, Si) symmetric stretching vibrations that correspond to quartz present in the fly ash and the band occurring at 1066 cm -1 is associated with T-O asymmetric stretching vibrations [29]. A very weak band at 773 cm -1 reveals presence of mullite in thermal power plant ash [30]. The sharp band found at 3684 cm -1 is characteristic for -OH groups from water molecules.
A very intense band that appears in the wavelength range 2900-2990 cm -1 indicates the presence of organic carbon in the ash structure [31].
The band that appears at wavelength 1393 cm -1 indicates the presence of carbonates, and the band at wavelength 1250 cm -1 indicates the presence of nitrates in the composition of the thermal power plant.
The transformation of the thermal power plant ash into zeolite materials is confirmed by the changes that occur in the FT-IR spectra of the zeolite materials. Antisymmetric stretch vibration bands common to zeolites generally occur in the region of 950-1250 cm -1 [26].
The very strong band, characteristic of the asymmetrical stretching vibrations of the T-O tetrahedrons from the thermal power plant ash that appears in the spectrum at wavelength 1066 cm -1 is shifted to smaller wavelengths (967 cm -1 in the case of zeolite prepared at 90°C, 960 cm -1 for zeolite prepared at 110°C, 959 cm -1 for zeolite prepared at 130°C, 963 cm -1 for zeolite prepared at 150°C). The shifting of this band and the decrease in intensity indicates the transformation of the thermal power plant ash into zeolite materials [310]. The shifting also indicates that the vitreous component of the thermal power plant ash reacted with the alkaline activator (NaOH) to form the zeolite materials [26].
Another important change observed in the FT-IR spectra of the prepared zeolite materials is the decrease in the intensity of the band characteristic of the organic matter and of the one characteristic of the water molecules, a decrease which is the more significant, as the calcination temperature is higher.

Adsorption study
The synthetic zeolitic materials have been used to treat aqueous solutions with Cd(II) ions content in order to determine their ability to remove the pollutants from heavy metal contaminated aqueous solutions. The optimum experimental conditions have been determined by batch tests.

Influence of pH
The pH of the solution/wastewater is one the most important operational parameters in the removal process due to the fact that it can affect the characteristics of adsorbent such as the concentration of the counter ions on the functional groups, the degree of ionization of the adsorbate, the solubility of the metal ions and the nature of the heavy metal species present in the solution [32]. It is well known that at higher pH values, most of the heavy metal ions precipitate as hydroxides and the adsorption of heavy metal ions will compete with precipitation. Consequently, the adsorption of Cd(II) ions onto zeolite materials prepared at pH value ranging from 2.5 to 7.5 was studied as shown in Figure 5. From Figure 5, it can be concluded that an increase in pH corresponds to an increase in adsorption, reaching the maximum adsorption capacity at a pH of about 5.91 (this pH value corresponds to the pH value of the initial 100 mg/L Cd(II) solution). The lower adsorption of heavy metal at acidic pH can be explained by the fact that at lower pH values, where the concentration of H3O + is high, the competition by the negative sites on the zeolite surface is enhanced and the Cd(II) ions sorption is reduced accordingly [31]. Moreover, at low pH value the zeolite crystals will dissolve and they will be hard to be removed from treated water. A slow decrease of the adsorption capacity has been registered at pH value higher than 5.91. The optimum pH value experimentally determined is 5.91 for all zeolite materials prepared.
The results regarding the effect of the pH onto zeolite materials adsorption capacity are in agreement with the results previously reported by other authors [32].

Effect of the contact time
Another important operational parameter is the contact time required to reach the adsorption equilibrium. The adsorption of Cd(II) onto zeolite materials, as a function of contact time was examined at 20±2°C (room temperature) by varying the contact time from 5 to 420 min. The results are displayed in Figure 6  From Figure 6, it can be observed that the adsorption of the Cd(II) ions is increased from the beginning and then kept increasing gradually until the equilibrium is reached and then it remained constant. The removal rate of Cd(II) is faster at the initial stage because initially, all the zeolite sites are vacant and the Cd(II) concentration gradient is high [32]. The time to reach the equilibrium is 240 minutes for all four zeolite materials demonstrating that they have a strong ability for binding Cd(II) ions. Thus, a contact time of 240 min was found to be sufficient to achieve uptake equilibrium, and the contact time was fixed at 240 min for the following batch experiments.
The adsorption capacity of zeolite materials increases with the increase of the temperature of the thermal treatment. Thus, the lowest adsorption capacity (127.75 mg Cd(II)/g adsorbent) was determined for zeolite material prepared at 90°C and the highest adsorption capacity (234.85 mg Cd(II)/g adsorbent) was established for zeolite material prepared at 150°C.

Effect of adsorbent dose
The removal of Cd(II) on synthetized zeolite materials was investigated on various amounts of adsorbents from 0.01 to 0.15 g. The results obtained are illustrated on Figure 7.  Figure 7 shows that the maximum adsorption capacity was determined for an adsorbent dose equal to 0.01 g/50 mL. Increasing of the dose of sorbent did not lead to an increase in the adsorption capacity, which could be attributed to saturation of the binding sites [31].

Adsorption isotherm
The experimental data were fitted with Langmuir and Freundlich isotherms which are two of the most commonly used equilibrium models [32].
According to the Langmuir isotherm model the uptake occurs on homogeneous surface by monolayer sorption without interaction between sorbed molecules [32]. The linear form of Langmuir's isotherm model is presented as follows: Qe represents the amount of Cd(II) ions retained at equilibrium (mg/g), Qmax is the maximum adsorption capacity (mg/g) [32]. The Freundlich isotherm is an empirical mathematical equation (3) engaged to describe heterogeneous systems [32]. The isotherm experimental data, which were collected at different initial Cd(II) ion concentrations ranging from 5 to 100 mg/L, were plotted between Ce/Qe versus Ce (Figure 8). Freundlich isothermal plots are presented in Figure 9. The Langmuir and Freundlich constants and the statistical fits of the Cd(II) sorption data to these equations are listed in Table 5. Regression values (R 2 ) shown in Table 6 indicated that the sorption data for Cd(II) ions removal fitted well the Langmuir isotherm. Meanwhile, the high value of correlation coefficient (R 2 > 0.9892) for the Langmuir isotherm predicts the monolayer coverage of Cd(II) ions on zeolite particles [33].
A dimensionless separation factor (RL) defined as follows represents one of the fundamental characteristics of the Langmuir equation: where: C0 is the highest initial Cd(II) ions concentration (mg/L), KL is the Langmuir's adsorption constant (L/mg) [33]. The RL value can be used to predict if the Langmuir model describes favorable the sorption process studied. Thus, the RL value implies the adsorption to be unfavorable (RL > 1), linear (RL= 1), favorable (0 < RL< 1) or irreversible (RL= 0) [33]. For zeolite materials tested, the values of RL range from 0.0198 to 0.0542 for Cd(II) ( Table 6). According to these values it can be said that the adsorption of Cd(II) ions onto zeolite materials prepared is considered favorable.
The results of the isothermal modelling of the sorption of Cd(II) ions on zeolite materials follows the Langmuir isotherm model that stated that the removal occurs on homogeneous surface by monolayer sorption without interaction between sorbed ions [33].

Adsorption kinetics
The kinetics of sorption of Cd(II) ions by zeolite materials was studied for its possible transforming from lab scale experiments to pilot remediation of metal-bearing industrial wastewater. Consequently, the kinetic behavior of the Cd(II) ion removal by different zeolite prepared samples has been examined, using the pseudo-first, the pseudo-second order kinetics, and intraparticle diffusion kinetic models.
The pseudo first-order kinetic model assumes that the rate of adsorption is proportional to the number of free active centers on the surface of the adsorbent/zeolite materials [34,35]. This model is indicated by the linear Lagergren equation (5): where: k1 is the rate constant of the pseudo first-order sorption process (min -1 ) and Qe, Qt are the equilibrium sorption capacity and, respectively, the amount of Cd(II) ions retained on adsorbent at time t (mg/g) [36].
The kinetic model of pseudo second-order sorption is expressed using Ho equation [37]: where: k2 represents the rate constant of the pseudo second-order sorption process of (g/mg/min); Qe, Qt are the equilibrium sorption capacity and, respectively, the amount of Cd(II) ions retained on adsorbent at time t (mg/g).
The pseudo second-order model reveals that the main mechanism involved in the process is chemisorption.
The intraparticulate diffusion kinetic model is mathematically expressed by the equation (7) submitted in 1962 by Weber and Morris [38].
where ki is the intraparticulate diffusion rate constant (mg/g·min 0.5 ), and C (mg/g) is the intercept which gives indications about the thickness of the diffusion layer.
The constants of these models were found through a regression analysis over the acquired experimental data. The plots showing the match between the experimental data and the models are presented in the Figures 8-10 and the regression analysis results are presented in the Table 7. pH value = 5.91; temperature = 20±2°C) Figure 13 reveals that for all zeolite tested materials, it is possible to draw two lines from the graphical representation of Qt versus t 0.5 which can lead to the conclusion that the studied adsorption process could take place in two steps: the external diffusion followed by the intraparticle diffusion. Due to the fact that the lines do not pass through the origin, it can be concluded that the adsorption process of the Cd(II) ions on the zeolite prepared materials can't be modeled using the kinetic diffusion intraparticle model. The results presented in Table 6 show that, for Cd(II) sorption process the R 2 values (R 2 ranging from 0.9165 to 0.9765) for pseudo-first order kinetic model were much lower than those obtained using the pseudo-second-order kinetic model (R 2 is in the range from 0.9988 to 0.9999). Thus, the pseudosecond-order kinetic model explains better the Cd(II) sorption onto zeolite materials processes. Consequently, the mechanism involved in Cd(II) sorption onto zeolite materials is based on chemical reactions.

Thermodynamic study
The thermodynamic parameters such as change in Gibbs free energy (ΔG°), change in entropy (ΔS°) and change in enthalpy (ΔH°) for Cd(II) adsorption process on zeolite materials have been determined from Van't Hoff equation (8).
where: kC is the equilibrium sorption constant depending on the temperature, R is the universal gas constant (8.314 J·mol −1 ·K −1 ) and T (degrees Kelvin) is the absolute temperature.
The slope and intercept of the plot of ln(Qe/Ce) versus 1/T were involved in calculation of ΔH° and ΔS°.
The standard free energy of sorption (ΔG°) was determined using equation (9): and the values of kC were expressed according to partition theory: The values of ΔG° were calculated from the experimental results achieved at three different temperatures (293, 303 and 313 K) from equation (11): The negative values of ΔG° show that the adsorption process is spontaneous and thermodynamically stable [39]. The positive value of ΔS° reveals that the adsorbent has more affinity towards heavy metal ions and there is increased randomness at solid-liquid interface during the adsorption process [39]. The negative value of ΔH° indicates that the adsorption process is exothermic in nature [39].
Batch experiments were performed at 20, 30 and 40 °C with 100 mg/L Cd(II) initial concentration solutions to predict the nature of Cd(II) adsorption onto Zeolite-90, Zeolite-110, Zeolite-130 and Zeolite-150 and the energetic changes occurring during the process. The thermodynamic parameters such as the standard Gibbs free energy (ΔG 0 ), standard enthalpy changes (ΔH 0 ), and standard entropy changes (ΔS 0 ) were estimated from the plots lnKC versus 1/T ( Figure 11). The slope and intercept of the linear regression were employed to calculate ΔS 0 and ΔH 0 , respectively.  The positive values of ΔH 0 and ΔS 0 ( Table 8) show an endothermic sorption process and increasing randomness at the solid-liquid interface during the adsorption of Cd(II) onto zeolites [39]. A positive value of ΔS 0 implies that the Cd(II) adsorption process involves a dissociative mechanism [39]. These kinds of adsorption processes are not encouraging at high temperatures [39]. The Gibbs standard energy determined based on Eq. 11 decreased with the temperature for all zeolite samples. The negative ΔG 0 values (Table 7) indicate that Cd(II) adsorption is spontaneous and thermodynamically favorable [39]. A decrease in the value of ΔG 0 with an increase in temperature (Table 7) indicates that the Cd(II) adsorption process is more encouraging at higher temperatures. This could be possible because the mobility of Cd(II) ions in the solution increase with increase in temperature and that the affinity of Cd(II) on the zeolite surface is higher at high temperatures [39].

4.Conclusions
The synthesis of zeolite materials from fly ash an abundant waste from thermal power plant was performed in order to prepare efficient adsorbents for Cd(II) removal from wastewater. Analyses by means of FT-IR, XRF, and BET surface area measurement indicated that the zeolites synthesized are https://doi.org/10.37358/RC.20.9.8323 characterized by Si/Al ratio between 1.21 and 1.26, being mainly composed of Na-P1 zeolite. The increase of the temperature of thermal treatment has as result an increase of the specific surface of zeolite materials.
The adsorption of Cd(II) ions was found to be dependent on pH, adsorbent dose, initial concentration, and contact time. The adsorbed amount of Cd(II) increased with increasing pH, initial concentration, and contact time till the reaching the equilibrium. The increase of the adsorbent dose has as result a decrease of the adsorption capacity.
Langmuir isotherm displayed a better fitting model than Freundlich isotherm. This indicates that the Cd(II) adsorption occurs on homogeneous zeolite's surface by monolayer sorption. The lowest value of the maximum adsorption capacity determined from Langmuir equation was recorded for Zeolite-90 sample (126.58 mg/g) and the highest value was calculated for Zeolite-150 sample (243.90 mg/g). Thus, the increase of the temperature of the fly ash thermal treatment process has as a consequence the increase of the adsorption capacity of the zeolite material prepared. The experimental values of the adsorption capacity are close to maximum adsorption capacity calculated values from Langmuir equation.
The kinetic modelling reveals that the pseudo-second-order model can be used to describe the Cd(II) sorption onto zeolite materials process. The rate constants for Cd(II) sorption onto zeolite materials were found to be in the range from 4.5421·10 -4 to 1.1978·10 -3 g/mg·min at 20±2 o C.
The results demonstrate that valuable zeolite materials with high adsorption capacity can be prepared from thermal power plant fly ash through a simple method. The materials prepared can be applied to remediate heavy metals-bearing wastewater.