3.1. Characterization of Adsorbent
This section investigated the characteristics of the MFA adsorbent. A comprehensive analysis from the SEM, EDX, INAA, XRD, FTIR, and BET surface area points of view is presented.
The surface properties of the adsorbent were examined through the SEM technique. Additionally, the SEM image of fly ash is represented in Figure 1
in order to provide a better visualization and to determine if the treatment with NaOH contributed to the modification of the surface of the fly ash material. The SEM image of the MFA adsorbent compared with unmodified fly ash is illustrated in Figure 1
. In general, fly ash particles are predominantly spherical in shape with a relatively smooth surface texture. According to Figure 1
, after NaOH treatment, a structure with crystals in the form of a few elongated rods is clearly displayed by the MFA material. It can be highlighted that the activation time of 168 h has a significant influence on the morphology.
An EDX mapping image was used to find the elements present in the MFA adsorbent.
The EDX evaluation related in Table 2
shows that oxygen (33.85 wt%), silica (18.89 wt%), and aluminum (18.33 wt%) are the basic elements of the MFA adsorbent. The presence of magnesium, potassium, calcium, titanium, and iron is detected, but the quantities are lower than 4 wt%.
The element sodium is incorporated into the MFA material by 4.35 wt% higher compared to fly ash due to the hydrothermal treatment with 2 M of NaOH solution [9
Instrumental neutron activation analysis (INAA) is a method used to determine the qualitative and quantitative analyses of macroelements, microelements, and rare elements in a material.
The advantages of this analysis are that:
Through this analysis, more than 30 elements in a sample can be analyzed, even if the elements are found in low levels;
It does not require a large quantity of the sample.
The determination of the trace elements of the MFA material was performed by the INAA method and the results are presented in Table 3
. This type of analysis is performed in order to establish the trace elements from the adsorbent, especially the content of U, Cs, Ba, Eu, Cd, and Cr [9
The synthesized material contains higher amounts of Mn (4.46 × 102 ppm), Ba (2.25 × 103 ppm), and Sr (1.39 × 103 ppm).
The crystallographic information concerning the MFA was determined by XRD analyses. The X-ray diffraction pattern is presented in Figure 2
. The identification of peaks within the 2θ angle range of 5° to 70° was performed based on data related to peak positions and intensities presented in the literature by Treacy and Higgins (2007) [37
As can be seen from Figure 2
, the most intense phase was identified as quartz (Q). In addition, the XRD analysis confirmed the presence of M (mullite) [38
]. As was reported in previous studies, the diffraction peaks of quartz and mullite could not be completely dissolved during the treatment [23
Analcime (A) is presented at 2 theta degrees of 33.38° and 42.64°. Clinotobermorite (CT) and chabazite (Cha) are found at 29.72° and 31.1°, respectively. Moreover, the element feldspar (F) is detected at 21° in the synthesized material.
The analysis concerning the FTIR spectra is presented in Figure 3
. The FTIR spectra analysis was used to determine the functional groups present in the synthesized adsorbent. For the analysis, the sample was mixed with KBr and measured within the wavenumber range of 4000–400 cm−1
The FTIR spectrum possesses similar bands to unmodified fly ash, such as 458 cm−1 (O–Si–O or Si–O–Si), 567 cm−1 (Al–O–Si and Si–O–Si), 794 cm−1 (Si–O), 1040 cm−1, 1653 cm−1, 2364 cm−1, and 3440 cm−1 (-OH and H–O–H, respectively).
The BET method is used in the study of surfaces in order to determine the areas of porous solids through the physical adsorption of gas molecules. It is used to identify the surface area and the pore sizes of the adsorbent before it is used.
The textural properties of MFA were evaluated using the N2
adsorption–desorption isotherms at 77 K (Figure 4
). According to the IUPAC classification, the material presents a type IV isotherm with a small plateau at relatively high pressures, with the H3 type hysteresis loop. Consequently, the results obtained are typical of mesoporous materials.
In terms of the BET analysis, the surface areas of the FA and MFA samples differ. BET investigation reveals that the surface of the MFA adsorbent (calculated from the BET equation) increased by 1.24 times compared to the fly ash surface. The activation of the fly ash with an alkaline solution for 7 days of contact time leads to an increase in pore volume (0.091 cm3/g vs. 0.024 cm3/g for fly ash). The pores of solid materials are classified into three categories: micropores (d < 2 nm), mesopores (2 nm < d < 50 nm), and macropores (d > 50 nm). The average pore volume of 1.594 nm indicates that the prepared material is microporous.
Through thermal analysis, the thermal properties of the synthesized adsorbent are established. Figure 5
presents the thermal analysis data of the MFA adsorbent. In order to determine the mass losses, the analysis was performed in a N2
atmosphere at temperatures between 20 and 900 °C, with a heating speed of 10 °C/min. The initial mass of the sample was 4.9710 mg.
Analyzing the results presented in Figure 5
, it can be seen that the loss on ignition (LOI) and the differential thermal gravimetry (DTG) for the MFA adsorbent takes place in four stages:
At 43.90 °C and 82.55 °C, when the loss of moisture occurs (1.81 and 2.22%, respectively);
At 500.45 °C, when the loss of crystallization water takes place (2.32%);
Between 778.72 and 900 °C, due to the decarbonation of the structure (3.81%).
The thermogravimetric analysis showed that the sample has a total mass loss of 10.16%.
The results obtained in the first stage of the study reveal the capacity of the material as an adsorbent for heavy metals from aqueous solutions, in this case referring to copper ions.
Therefore, after the basic characterization, the synthesized material was employed as an adsorbent for Cu(II) ions from aqueous solution. The batch adsorption experiments were carried out at different pH values. In addition, different amounts of adsorbent and various concentrations of copper ions at different time intervals were measured.
3.3. Adsorption Kinetic Study
A kinetic model is a mathematical representation of the rate at which a physical or chemical process takes place [43
]. The kinetic study indicates the adsorption rate and the efficiency of the adsorbent. In addition, through the obtained data, the mechanism that takes place can be established.
In the present study, the data obtained were applied to three kinetic models: a pseudo-first order kinetic model (PFO), a pseudo-second order kinetic model (PSO), and an intraparticle diffusion model. This was done in order to evaluate the experimental data obtained at initial Cu(II) concentrations of 300, 500, and 700 mg/L.
The PFO model assumes that the adsorption takes place only at some specific sites. The adsorption kinetics described by the PSO model suppose that the rate limiting step is a chemisorption process. The intraparticle diffusion model [44
] assumes that:
The transport of adsorbate from the bulk solution to the outer surface of the adsorbent is by molecular diffusion;
Internal diffusion, the transport of adsorbate from the particle’s surface into an interior site, takes place;
The adsorption of the solute particles from the active sites into the interior surface of the pores occurs.
To calculate the kinetic parameters, the equations presented in Table 4
were used [45
where (mg/g) is the amount of Cu(II) ions adsorbed at time t, (mg/g) is the amount of Cu(II) ions adsorbed at equilibrium, is the pseudo-first order rate constant (1/min), is the pseudo-second order rate constant (g/mg min), and is the intraparticle diffusion rate constant.
The plots of the data were created for the three initial Cu(II) concentrations of 300, 500, and 700 mg/L (Figure 9
). The kinetic parameters are shown in Table 5
Analyzing Table 5
, it can be seen that the experimental data could not be predicted by the PFO model.
On the other hand, the data confirm that the intraparticle diffusion model does not fit the experimental data. The R2 value for the intraparticle diffusion model was lower compared to the PSO model. In addition, another indicator that supports this statement is that the plots did not pass through the origin.
The high correlation coefficient value, R2, of 0.9999, and the agreement between qe cal and qexp validate the fact that Cu(II) adsorption processes followed the PSO model, which indicated the chemisorption mechanism.
It must be noted that it is difficult to compare the adsorption capacities of some adsorbents in different working adsorption conditions due to factors such as the pH, copper initial concentration, adsorbent doses, and type of synthesis (one of the most important parameters). Taking into account this plausible observation, at the end of the research, a comparison with other materials presented in the literature was shown in order to establish the efficiency of the adsorbent (Table 6
The data presented in Table 6
show that the MFA adsorbent has a good adsorption capacity, and this fact could imply that the adsorbent can be used for the treatment of wastewater-containing Cu(II) ions.