Synthesis of St-g–PAA–PVSA adsorbent
The St-g–PAA–PVSA graft copolymer was prepared by gamma radiation for initiating the graft copolymerization of AA/VSA onto starch in the presence of NMBA as crosslinker. Figure 1 shows a schematic diagram for the synthesis processes as well as a suggested structure.
Structural analysis
FT-IR
FT-IR spectrum of starch and St-g–PAA–PVSA graft copolymer is shown in Fig. 2. A weak absorption band at 1368 cm− 1 is the characteristic absorbance band of –C–OH, O–C–H and C–C–H on the starch chain, the peak at 1645 and 1634 cm− 1 is ascribed to the δ(OH) bending of tightly band water present in starch and St-g-PAA-PVSA graft copolymer, respectively. The peak at 1459 cm− 1 indicates symmetric stretching of –COO − group and the band at 1634 cm− 1 indicates asymmetric stretching of –COO − group of AA. The absorption band at 1740 cm− 1, 2853–2924 cm− 1, and the strong and wideband at 3443 cm− 1 are assigned to the asymmetric stretching vibration of COOH group, –C–H, –OH, and –COOH group, respectively of starch, and AA, overlap each other. The absorption band at 1403 cm− 1 which is due to –SO3 and S = O vibration is typical of stretching vibration[10].
SEM
Figure 3 shows SEM images of St and St-g–PAA–PVSA graft copolymer. St micrographs show spherical or oval granules with smooth surfaces free of scratches or cracks. The SEM micrographs of starch-g-AA-VSA graft copolymer contains irregular morphology with different size granules and appeared as a porous surface with interconnected pores. This irregular morphology is caused by exposure to heat during the produce of the graft copolymer. Rough surfaces of the graft copolymer lead to enhancement of its surface area and increase the interaction between the adsorbent and the radionuclides and decrease the diffusion limitations in radionuclides adsorption[10].
TGA
Figure 4 shows the TGA and TDA of St and St-g–PAA–PVSA graft copolymer. St shows three-step thermal degradation. The first step occurs in the temperature range 81–172ºC with 11.82% weight loss due to dehydration and pyrolytic volatilization processes. The second and third stages are consecutive overlapping steps that occurred in the temperature range 275–490ºC with 72.53% weight loss. This is attributed to loss by depolymerization and oxidation of the organic matter. St-g–PAA–PVSA graft copolymer shows the fifth decomposition stage, the first stage continued up to the temperature of 200°C and count for about 7.15% weight loss. This corresponds to a loss of moisture. The second stage of degradation was observed from 200 to 290°C with weight loss of about 14.13%. It is mostly contributed to the decomposition of starch. In the third stage, the decomposition is characterized by a weight loss of 16.97% due to the chain scission of the PAA and PVSA molecules, which has a maximum temperature of 385°C. The fourth and fifth stages occurs with the temperature range 385–470°C with 21.7% weight loss due to the breaking up of covalent bonds. DTA curves released the TGA results; starch exhibited endothermic peaks at 142.38 ºC and exothermic at 300.92. However, St-g–PAA–PVSA showed endothermic decomposition at 156.90, 212.35, 313.50, 441.08, 525.12, and 567.05ºC, and exotherm peaks at 247.84, 405.70, 546.69, and 581.87 ºC. TGA-DTA studies revealed that the decomposition temperature of the graft copolymer was higher than that of native starch[5, 6, 8, 10].
Batch adsorption optimization
Effect of pH
Figure 5 shows how the pH values influence the adsorption of Co(II) and Eu(III) radionuclides on the St-g-PAA-PVSA graft copolymer. The adsorption capacity for both cations is very low at low pH values then increases with an increase in the pH values, clearly owing to the nature of the functional groups in the St-g-PAA-PVSA graft copolymer. At low pH, the surface charge of the graft copolymer will become positive and competition between H+ and Co(II) or Eu(III) ions for occupancy of the active sites was increased. Whereas at higher pH values the surface charge of the graft copolymer becomes negative, increasing radionuclides adsorption. The adsorption capacity of Eu (III) is higher than that of Co(II), this may be attributed to the adsorption capacity values are proportional to the ionic potential, i.e., Z/r of ions; along with other factors, especially in the case of Eu(III) (as an f-block element) whose coordination bonds are predominantly electrostatic. Consequently, it can be expected that the adsorption capacity of Eu3+ should be higher than that of Co2+[16–18].
Adsorption kinetics
Figure 6 shows the influence of the contact time on the adsorption of Co(II) and Eu(III) radionuclides. The results showed that adsorption has occurred rapidly. Then the adsorption nearly remains at a constant with increasing time, the adsorption was appeared to proceed rapidly when the numbers of available sites are larger than the number of adsorbed radionuclides. Lagergren’s pseudo-first-order, pseudo-second-order, and Intraparticle diffusion models were applied to examine the adsorption kinetic behavior of Co(II) and Eu(III) radionuclides. The linearized equations of the models can be expressed as in Table 1[10, 13, 16].
The data given in the Table 2 indicated the experimental kinetic data of adsorption of Co(II) and Eu(III) radionuclides were closer and have high correlation coefficient (R2) to those calculated from the pseudo-second-order model. The chemisorption of Co(II) and Eu(III) is assumed in these studies.
Table 1
The linearized equations of the used models
Models
|
Equations
|
Parameters
|
Lagergren's pseudo first-order model
|
\(\text{l}\text{o}\text{g} ({\text{q}}_{\text{e}}-{\text{q}}_{\text{t}}) =\text{log}{\text{q}}_{\text{e}}- \frac{{\text{k}}_{1}\text{t}}{2.303}\)
|
qe (mg/g) is the amount of absorbed radionuclides
at equilibrium, qt (mg/g) is the amount of absorbed radionuclides at t, t is the time, and k1 is the Lagergren’s rate constant (min− 1). k1 is determined from the slope of the plots of log(qe-qt) versus t
|
Lagergren's Pseudo-second-order model
|
\(\frac{\text{t}}{{\text{q}}_{\text{t}}} =\frac{1}{{\text{k}}_{2}{\text{q}}_{\text{e}}^{2}}+ \frac{\text{t}}{{\text{q}}_{\text{e}}}\)
|
qe and qt is mentioned previously. K2 (g mg− 1 min− 1) is the pseudo-second order rate constant. K2 were calculated from the slope and intercept of the plot t/qt versus t
|
Intraparticle diffusion
|
\({\text{q}}_{\text{t}} ={\text{k}}_{\text{i}}{\text{t}}^{0.5}+{\text{C}}_{\text{i}}\)
|
Ci is the boundary-layer thickness and Ki (mg g− 1 min0.5) is the intra-particle diffusion rate constant. Ki and Ci were calculated from the slope and intercept of the plot qt versus t0.5
|
Langmuir isotherm model
|
\(\frac{{\text{C}}_{\text{e}}}{{\text{q}}_{\text{e}}}=\frac{1}{{\text{K}}_{\text{L} }{\text{q}}_{\text{m}}}+ \frac{{\text{C}}_{\text{e}}}{{\text{q}}_{\text{m}}}\)
|
Ce is the radionuclides concentration at equilibrium in solution (mg/L), qm is the maximum capacity of monolayer coverage (mg/g), and KL is Langmuir constant (L/g). qm and Kc values are calculated from the slope and the intercept of the linear plot of Ce/qe against Ce.
|
Freundlich isotherm model
|
\(\text{log}{\text{q}}_{\text{e}}=\text{log}{\text{K}}_{\text{f}}+ \frac{1}{\text{n}}\text{log}{\text{C}}_{\text{e}}\)
|
n represents a constant of the adsorption
intensity. Kf and n are the Freundlich constants and counted from the intercept and the slope of the linear plot.
|
Temkin model
|
\({\text{q}}_{\text{e}}=\frac{\text{R}\text{T}}{{\text{b}}_{\text{T}}}\text{l}\text{n}{\text{k}}_{\text{T}}+ \frac{\text{R}\text{T}}{{\text{b}}_{\text{T}}}\text{l}\text{n}{\text{C}}_{\text{e}}\)
|
KT is equilibrium binding constant (L g− 1), b is related to heat of adsorption (kJ mol− 1), R is the universal gas constant (8.314 J mol− 1 K− 1), and T is the absolute temperature (K). KT and bT are calculated from the slope and intercept of the qe versus ln Ce plot.
|
Dubinin–Radushkevich model
|
\(\text{log}{\text{q}}_{\text{e}}=\text{ln}{\text{x}}_{\text{m}}+ \frac{1}{\text{n}}{{\beta }{\epsilon }}^{2}\)
|
Xm is the theoretical saturation capacity (mg g− 1), β is a constant related to mean free energy of adsorption per mole of the adsorbate (mol2/kJ2), and ε is the Polanyi potential which is related to equilibrium
|
Thermodynamic study
|
\({\text{l}\text{n}\text{K}}_{\text{L}}=\frac{\varDelta \text{S}^\circ }{\text{R}}-\frac{\varDelta \text{H}^\circ }{\text{R}\text{T}}\)
\(\varDelta \text{G}^\circ =\varDelta \text{H}^\circ -\text{T}\varDelta \text{S}^\circ\)
|
Changes in enthalpy (ΔH, kJ mol− 1) and entropy (ΔS, kJ mol− 1) were calculated from the slop and intercept of the Van’t Hoff plot.
|
Table 2
Adsorption kinetic parameters
Parameters
|
Pseudo-First-Order
|
Pseudo-Seond-Order
|
Intraparticle diffusion model
|
Exp.
|
k1
(min− 1)
|
qe
(mg/g)
|
R2
|
k2
(g mg− 1min− 1)
|
qe
(mg/g)
|
R2
|
K
(mg g− 1 min0.5)
|
Ci
(mg/g)
|
R2
|
qmax
(mg/g)
|
60Co
|
2.44E-03
|
18.580
|
0.270
|
4.50E-04
|
95.147
|
0.998
|
0.338
|
-10.429
|
0.467
|
93.422
|
152+154Eu
|
3.02E-03
|
62.400
|
0.881
|
8.75E-05
|
132.275
|
0.952
|
2.894
|
38.629
|
0.528
|
123.587
|
Adsorption isotherms
The adsorption isotherm is helping in understanding the interaction between the graft copolymer and the radionuclides in the adsorption process. Figure 7 shows that the adsorption capacity of 60Co and 152+154Eu radionuclides increases by increasing the initial concentration of radionuclides. It is maybe a result of an increase of driving force for mass transfer at high initial concentration.
In this study, Langmuir, Freundlich, Temkin, and Dubinin–Radushkevich isotherm models were investigated to analysis of the equilibrium data of sorption of Co(II) and Eu(III) radionuclides onto St-g-PAA-PVSA graft copolymer. The linear equations are given in Table 1[10, 11, 16]. The parameters of the studied isotherm models were summarized and are listed in Table 3. The high correlation coefficient (R2) value indicated that the experimental data for the adsorption of 152+154Eu, and 60Co radionuclides by St-g-PAA-PVSA graft copolymer follow the Langmuir model better than other models. According to the Langmuir model, adsorption happens by monolayer adsorption onto a homogenous surface.
Table 3
Adsorption isotherm parameters
Parameters
|
Langmuir Isotherm:
|
Frundlich Isotherm:
|
Dubinin- Radushkevich Isotherm:
|
Temkin Istherm
|
Exp.
|
qmax (mg/g)
|
KL (L/g)
|
R2
|
1/n
|
kf
|
R2
|
E (kJ/mol)
|
B
(mol2/kJ2)
|
qmax
(mg/g)
|
Ln KT
(L/g)
|
bt
kJ/mole
|
qmax (mg/g)
|
60Co
|
105.374
|
0.175
|
0.999
|
0.238
|
28.493
|
0.853
|
6.72E + 02
|
1.11E-06
|
35.303
|
1.802
|
174.710
|
103.980
|
152+154Eu
|
206.185
|
9.00E-02
|
0.993
|
0.4759
|
22.050
|
0.700
|
7.71E + 02
|
8.41E-07
|
0.54104
|
1.559
|
0.094
|
198.770
|
Thermodynamic studies
The effect of temperature on the adsorption of cobalt and europium onto St-g–PAA–PVSA graft copolymer was determined by varying the temperature between 20 and 70°C. Figure 8 shows that the capacities slightly increase with the increasing temperature this may be attributed to decreases the viscosity of the solution and correspondingly increases the diffusion rate of the radionuclides within the pores of the graft copolymer by increasing temperature.
Thermodynamic parameters are given in Table 1[19, 20, 20, 21] and the values of ΔH°, ΔS°, and ΔG° are given in Table 4. The positive value of ΔH° indicated that the adsorption was endothermic. The positive value of ΔSo for Co(II) radionuclides indicated that the randomness is increased at the solid–solute interface, while the negative value of ΔSo for Eu(III) radionuclides assumed is the arrangement of Eu(III) radionuclides is shaped more ordered onto the surface of the graft copolymer after adsorption. The negative value of ΔGo indicated the feasibility of the reaction and spontaneous nature of the adsorption at a given temperature.
Table 4
Thermodynamics adsorption data
Radionuclides
|
T (K)
|
ΔGo (kJ/mol)
|
ΔHo (kJ/mol)
|
ΔSo (KJ/mol.K)
|
60Co
|
303
|
-0.0091
|
1.24718E-05
|
3.01816E-05
|
313
|
-0.0094
|
323
|
-0.0097
|
333
|
-0.001
|
152+154Eu
|
303
|
-9.788
|
-30.928
|
-0.0698
|
313
|
-9.089
|
323
|
-8.392
|
333
|
-7.693
|
343
|
-6.996
|
Desorption study- batch method
Figure 9 illustrates the influence of desorbing agent concentrations (0.001–0.1 M of HCl, NaCl, MgCl2 and AlCl3) on the desorption percentage of cobalt(II) and europium(III). the data in the figure show that the desorbing agent concentration played a significant role in the desorption of cobalt(II) and europium(III). Increasing the desorbing agent concentration, concentration resulted in an increase in the desorption percentage of cobalt(II) and europium(III) radionuclides. The maximum desorption percentage of cobalt(II) ions about 59.60 and 54.72% is achieved by AlCl3 and HCl, respectively, at concentrations ≥ 0.1 M. while The maximum desorption percentage of europium(III) ions about 63.72 and 63.05% is achieved by AlCl3 and HCl, respectively, at concentrations ≥ 0.1 M. This desorption results proposed that the radionuclides are adsorbed onto the active sites of the graft copolymer. While the fraction remained onto the adsorbent, 45 − 40% of adsorbate could not be desorbed, clarified that insignificant amount of radionuclides are adsorbed onto the internal adsorption sites[22, 23].