3.1. Synthesis and characterization of Cd-ZIF nano-adsorbent
The FTIR spectrum of Cd-ZIF nano-adsorbent is shown in Fig. 1(a).
The peak of 1171 cm-1 can be related to the stretching of C-N functional group. C=N stretching is appeared at 1608 cm-1. The peaks in the range of 600 cm-1 to 1500 cm-1 can be related to the stretching and bending states of the imidazole ring [46]. The peak at the wavelength of 482 cm-1 can be attributed to the stretching vibration of Cd-N. The peak around of 3109cm-1 may be related to the C-H stretching in the imidazole ring. Therefore, it can be concluded from the interpretation of the FT-IR spectrum that the imidazole ring is present in the structure of the nano-adsorbent.
Fig. 1(b) shows the FSEM image of the synthesized Cd-ZIF nano-absorbent. According to the image, the synthesized adsorbent particles have a non-homogeneous and agglomerate. It shows that the adsorbent has a spherical crystal structure with the particle size of 22-30 nm. EDS was used to identify the elemental composition. The weight of the elements C, N, O and Cd are 27.72, 17.70, 25.66, and 28.92, respectively. It confirms the synthesized Cd-ZIF nano-adsorbent.
The XRD pattern of Cd-ZIF is shown in Fig. 1(c). As can be seen, Cd-ZIF is accurately synthesized that pattern indicates the presence of peaks with strong diffraction intensity at 19.21˚, 29.51˚, 35.26˚, 48.96˚, and 67.16˚. Moreover, the comparison between the XRD patterns and the elemental distributions confirms the presence of Cd+2 ions on the Cd-ZIF adsorbent.
Nitrogen adsorption-desorption isotherm obtained for nano-absorbent at −196 ˚C is presented in Fig. 1(d). The mesoporous volume, the pore volume, and BET surface are 1.3901 cm3/g, 0.35126 cm3/g, and 6.0503 cm2/g, respectively. Langmuir surface area is 9.1133 cm2/g and the average pore diameter is 23.22 nm.
3.2. Adsorption studies
3.2.1. The effect of pH, adsorbent dosage, and temperature on adsorption
Before the effect of pH on the adsorption of SY onto the adsorbent is investigated, the point of zero charge (pHPZC) is determined. The pHPZC is a parameter related to the phenomenon of surface adsorption, which describes the conditions where the electric charge density of the surface is zero. Fig. 2(a) shows the pHPZC of Cd-ZIF adsorbent that has been determined as 7.0. Also, the below this pH, due to protonation of functional groups, Cd-ZIF acquires a positive charge and above this pH there is negative charge on Cd-ZIF surface. Anionic dyes are favored to adsorb at pH < pHPZC where the surface becomes positively charged.
Also, Fig. 2(b) shows the effect of pH values on the SY removal by 20 mg of the nano-adsorbent at 25 ˚C, C0=10 mg L-1, and 60 min contact time. According to the results of the pHPZC parameter, the nano-adsorbent surface has a positive charge at pH < pHPZC. As the system's pH increases, the number of positively charged sites is increasing, and the number of negatively charged sites is falling. Therefore, in this range, the electrostatic attraction is created between the cationic nano-adsorbent and the anionic dye and the adsorption process is very strong in acidic pH. So, the maximum color removal percentage has been obtained at pH=5 with the value of 79%, which is chosen as the optimal pH for the next steps.
The effect of dye removal with Cd-ZIF dosage is shown in Fig. 2(c). This figure has been obtained in the conditions of pH = 5, 25 ˚C, C0=10 mg L-1, and 60 min contact time. The SY removal increases from 57 to 83% with increasing the nano-adsorbent from 5 to 30 mg. The addition of the SY removal with increasing amount of the adsorbent can be attributed to the increase in the number of active adsorbent sites. Although, the SY removal has a negligible change from 20 to 30 mg of adsorbent. However, increasing the number of adsorbents causes adsorption sites to overlap or aggregate and takes place decreasing in the adsorption capacity.
Fig. 2(d) shows the effect of temperature changes on the SY removal by 30 mg of the Cd-ZIF at pH = 5, C0=10 mg L-1, and 60 min contact time. The results show that the optimum adsorption temperature is 40 ˚C. As can be seen, the adsorption has increased up to 40 ˚C and then decreased with the addition temperature. The SY removal at 40 ˚C has been obtained 91.54%. Also, the SY adsorption is an endothermic process. Therefore, the interaction between the dye molecules and the active nano-adsorbent sites increases with temperature rise. This fact can be attributed to the increased dispersion and mobility of the SY molecules in the solution and the availability of more the active surface sites for adsorption on the adsorbent surface.
3.2.2. The effect of the SY concentration and adsorption isotherm models
The equilibrium studies have been obtained in the different dye concentrations for 30 mg of the adsorbent, pH = 5, T=40 ˚C, and 60 min contact time. Fig. 3(a) shows the results and the maximum dye removal has obtained 97% at 160 mg L-1.
Langmuir [12,13] and Freundlich [14] isotherms of the dye adsorption data were used at the different concentrations, the optimal pH, and temperature. The linear forms of the models are expressed in Eq. 3 and 4, respectively:
The symbols in the equations are:
Ce (mg L-1): the concentration of SY at equilibrium, qe (mg g-1): the adsorption capacity at equilibrium, qmax (mg g-1 or µg mg-1): the maximum adsorption capacity of adsorbent, KL (L mg-1): Langmuir constant, n & KF ((mg g-1) (L mg-1)1/n): Freundlich isotherm constants for adsorption capacity and intensity.
Fig. 3(b) and 3(c) show the results and the calculated parameters from the isotherms models are given in Table 1.
Table 1. The R2 values and the parameters obtained for different isotherm models
Model
|
R2
|
Parameters
|
Langmuir
|
0.9746
|
KL=0.1927
qmax=34.77 (µg mg-1)
|
Freundlich
|
0.9963
|
KF=1.011
|
According to the R2 values, Freundlich model confirms that it is suitable for fitting the data of the Cd-ZIF adsorbent and SY adsorption. This model is used for the heterogeneous adsorbent surface [15]. It is presumed that the stronger binding sites are first occupied, and that the binding strength decreases with the increasing occupancy of the site [16]. Therefore, the Cd-ZIF adsorbent has heterogeneous surface.
3.2.3. The effect of contact time and the adsorption kinetic studies
In the time range of 0.5-60 min, the contact time was investigated at 40 ˚C, pH=5, C0=160 mg L-1, and 30 mg of nano-adsorbent. Fig. 4(a) shows that the maximum SY removal occurs in the first 20 minutes (95.5%) with the high speed because the empty sites on the surface of the nano-adsorbent are accessible to the dye.
According to the kinetic model equations below, the pseudo-first -order (PFO) [17] and pseudo-second-order (PSO) [18] kinetic models were used to determine the adsorption kinetics of tested dye onto Cd-ZIF to match the experimental results. The applied equations are expressed in Eq. 5 and 6, respectively:
The symbols in the equations are:
qe (mg g-1): the adsorption capacity at equilibrium, qt (mg g-1): the adsorption capacity at t time, k1 (min-1): PFO rate constant, k2 (g mg-1 min-1): PSO adsorption rate constant.
Fig. 4(b) and 4(c) show the results and the calculated parameters from the kinetic models are given in Table 2.
Table 2. The R2 values and the parameters obtained for different kinetic models
Model
|
R2
|
Parameters
|
PFO
|
0.9834
|
k1=0.2226
qe=27.14 (µg mg-1)
|
PSO
|
0.9994
|
k2=0.1036
qe=36.75 (µg mg-1)
|
Comparing the kinetic models, the R2 value of the pseudo-second-order kinetic model (0.9994) is slightly higher than those of the pseudo-first-order kinetic model (0.9834). The correlation coefficients confirm that the pseudo-second-order kinetic model is in good agreement with the experimental results. In this adsorption process, the rate-limiting step may be chemisorption involving strong forces through sharing or exchange of electrons between the adsorbent and the adsorbent [19]. The adsorption mechanism may be explained by the electrostatic interactions between the negatively charged dye ion on the Cd-ZIF surface and the positively charged sites. The SY contains anionic dye (SO3-). The dye is dissociated to the sulfite anion (SO3-) in aqueous solution. The sulfite anion may bind to the group (H+) at base pH. However, in a strong acidic environment, Cd-ZIF showed high adsorption capacity for the SY. The increased efficiency in a heavy acidic environment resulted from the higher charged Cd-ZIF (pH< pHPZC) and the increased yield of H+ facilitated by increased concentration of H+. The results showed that Cd-ZIF can be applied to adsorb anionic dye as a highly efficient Adsorbent.
3.2.4. The results of adsorption-desorption cycles
Regeneration of the adsorbent is an important property for practical applications. Fig. 5 shows the reusability of the adsorbent in consecutive cycles of dye absorption under optimal conditions at 40 ˚C, pH=5, the initial concentration C0=160 mg L-1, 30 mg of Cd-ZIF, and t= 60 min. According to the 2.2.4. (desorption experiments) section, the nano-adsorbent was washed with 0.05, 0.1, and 0.2 M NaOH and then washed with distilled water several times. After that, the second step of the SY removal percentage was obtained 92%, 96%, and 93%, respectively. Therefore, 0.1 M NaOH was used for the initial washing.
After seven cycles testing, the SY removal has changed from 97% to 94. %. This result shows the Cd-ZIF has good characteristic of reusability.