Gauri
Shelar-Lohar
ab and
Satyawati
Joshi
*a
aDepartment of Chemistry, Savitribai Phule Pune University, Pune, Maharashtra, India. E-mail: id-ssjoshi@chem.unipune.ac.in; jayapune@gmail.com
bDepartment of Chemistry, Fergusson College, Shivajinagar, Pune, Maharashtra, India
First published on 13th December 2019
Uranium and thorium ions were selectively removed from aqueous solution using synthesized gum ghatti grafted poly(acrylamide) gum-g-poly(AAm) composite. A gamma radiation induced free radical copolymerization technique was used to synthesize the copolymer composite of gum-g-poly(AAm). Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TG), X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) were used to characterize the graft copolymer gum-g-poly(AAm). The adsorption of uranium ions and thorium ions using the gum-g-poly(AAm) copolymer composites has been investigated in batch mode. The adsorptive characteristics were investigated by varying the pH, concentration and time for both ions. The adsorption method depends on the pH of each metal ion, and the highest adsorption percentage was achieved at pH 6.0. The adsorption statistics were justified by isotherm, kinetic and thermodynamic models. The Langmuir adsorption model was revealed to be the best fitted monolayer arrangement, with a maximum adsorption capacity of 367.65 mg g−1 for the uranium ions and 125.95 mg g−1 for the thorium ions. The adsorption of metal ions occurred by the ion exchange process, which was specified through the rate controlling step with a best-fitted pseudo-second order kinetic rate model. Thermodynamic analysis shows that the ΔH and ΔS values for the uranium ions and thorium ions were positive. The negative ΔG values decreased with an increase in temperature, suggesting that the metal ion adsorption process was endothermic and spontaneous in behaviour.
Subsequently, it becomes imperative to reduce the concentrations of these radioactive elements from industrial effluents before they are released to nature. Numerous methods have been utilized for radioactive element removal such as adsorption, ion exchange, biodegradation, photocatalysis, flocculation and coagulation.5 Adsorption as a selective separation method for the efficient recovery of uranium and thorium is most desirable for environment protection and for nuclear energy.6 The efforts are diverted more on optimizing the surface properties. The affinity of uranium and thorium ions can be enhanced by surface modification with novel functional groups.7,8
Numerous studies have been carried out on various low-cost adsorbents such as activated sludge,9 seaweed,10 starch,11 cellulose12 and gum.13 Polysaccharide-based polymers have been mostly and effectively used for heavy metal ion adsorption from wastewater.14–16 Gum ghatti is biocompatible, and is naturally and abundantly available. Gum ghatti is acquired as exudates of the Anogeissus latifolia tree. Anogeissus latifolia is a type of small-to-average sized tree local to India, and is found in Western Ghats. They have numerous benefits over conventional adsorbents. The grafted polymer composites of gum ghatti with vinyl monomer possess good mechanical and physical properties over the ungrafted composites.17,18
For graft copolymer synthesis, acrylamide is a potential monomer. Acrylamide is water-soluble and can furnish the grafted copolymer chains with an amide group. The average molecular weight of gum ghatti is 8.94 × 107 g mol−1. The main composition of gum ghatti is sugar in the form of L-arabinose, D-galactose, D-mannose, D-xylose and D-glucuronic acid in a 48:
29
:
10
:
5
:
10 molar proportion. It has alternate 4-O-substituted and 2-O-substituted α-D-mannopyranose segments with chains of 1 → 6 connected β-D-galactopyranose segments as the side chains, which are most persistently the single L-arabinofuranose unit.19–21
Recently, a few studies on the removal of radioactive waste with different adsorbents were performed. Impregnated cellulosic beads synthesized by the chemical precipitation method were employed for the removal of toxic U(VI) ions.22 Using the plasma initiation method, gelatin-modified attapulgite was synthesized for the uptake of uranium.23 The superabsorbent grafted copolymer composite of poly(methacrylic acid) with cellulose/bentonite synthesised by the chemical initiation method (using potassium per sulphate as the initiator) was used for the recovery of thorium(IV).24
The present work reports the synthesis of gum ghatti grafted copolymer with acrylonitrile by the gamma irradiation induced method. The gamma irradiation route has several advantages over other synthesis routes. No chemical initiator is required to initiate the polymerization, and there are no side products. In addition, one can control the reaction by altering the radiation dose. The synthesis was carried out under ambient conditions. FTIR, TGA, FESEM, XRD and BET analysis were used to characterize the synthesized gum-g-poly(AAm) composite. This study is focused on the adsorption behaviour of synthesized gum-g-poly(AAm) for the selective adsorption of uranium and thorium ions. The relationships between the adsorbents' behaviour and their competence, as well as the adsorption mechanism, are evaluated by equilibrium adsorption isotherms, and by kinetic and thermodynamic assessment.
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The desorption percent was calculated by the following equation:
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The TG curves of pure gum ghatti and the gum-g-poly(AAm) composite are illustrated in Fig. 2. The TG curve of gum ghatti showed a two-step degradation. The weight loss of gum ghatti comprises the consequent stages: an initial 10.6% weight loss observed in the temperature range 55 to 170 °C is due to dehydration; the second weight loss starts from 170 to 420 °C with a 55.9% weight loss for the complete thermal decomposition of gum ghatti. Conversely, gum-g-poly(AAm) shows three distinct weight losses: the first weight loss between 55 to 180 °C with a 9.2% loss in weight is due to the loss of moisture or the degradation of the ungrafted gum chain; the second stage ranging from 215 to 310 °C with a 19.5% weight loss is due to the depolymerisation of the backbone polymeric chain, and the last degradation commenced from 310 to 510 °C with a 37.8% weight loss is attributed to the complete degradation of gum-g-poly(AAm). It also confirms that the decomposition temperature of gum-g-poly(AAm) in the grafted polymer is much higher than that of pure gum ghatti. The total weight losses of gum ghatti and gum-g-poly(AAm) are 74.2% and 66.5%, respectively. Thus, the thermal stability of the backbone polymer (gum ghatti) was enhanced noticeably by grafting with AAm.
Gum ghatti and gum-g-poly(AAm) composite were analysed by FESEM microscopy, and exposed substantial information in regards to their surface morphology. As it appears in the FESEM micrograph (Fig. 3), gum ghatti exhibits a less uneven surface. By grafting the monomer acrylamide with gum ghatti, the surface becomes irregular and crosslinked, and contains more pores with enhanced monomeric units and reduced backbone contents. Thus, these data reveal that the grafting procedure was done successfully.
To understand the adsorption behaviour, different isotherm models such as the Langmuir, Freundlich, and Temkin isotherms have been employed to ensure the adsorption performance.29Fig. 6 shows the adsorption behaviour of all adsorption isotherm models. The Langmuir model implies that the metal ions are distributed from the aqueous to the solid phase at equilibrium, and also showed the related monolayer arrangement of the adsorbate onto the adsorbent. This is restricted to the identical sites of adsorption for constant adsorption energy, and there is no interaction between the adsorbed molecules and the adjacent binding sites of the adsorbate. In addition, the Freundlich isotherm signifies that the adsorption process is predominantly heterogeneous. According to the Freundlich isotherm theory, the ratio of the amount of solute adsorbed onto a given mass of adsorbent to the concentration of the solute is not constant at different concentrations in the solution. A smaller value of the Freundlich constant implies that the adsorption of the adsorbate onto the adsorbent is easy.30 The Temkin adsorption isotherm model and adsorbent–adsorbate interactions provide information on the impacts of interaction between the adsorbent and adsorbate on the adsorption process and heat of adsorption (ΔHads), signifying the adsorption process as a physical adsorption or chemical adsorption. This model assumes that the adsorbent has uniform binding energy sites, and the heat of adsorption of all molecules in a layer decreases linearly due to the adsorbate–adsorbent interactions. The mathematical linear formulae of all isotherm models are given in Table 1.
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Fig. 6 (a) Langmuir adsorption isotherm, (b) Freundlich adsorption isotherm, and (c) Temkin adsorption isotherm. |
Adsorption isotherm models | Linear equation forms | Parameter description | Values for uranium ions | Values for thorium ions | |
---|---|---|---|---|---|
Langmuir isotherm model31 |
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q max = adsorption capacity at equilibrium, (mg g−1) | 367.65 | 125.95 | |
b = bonding energy of adsorption, (L mg−1) | 1.253 × 10−1 | 1.145 ×10−1 | |||
R L = equilibrium parameter | 0.138 | 0.148 | |||
r 2 | 0.997 | 0.998 | |||
Freundlich isotherm model32 |
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K f = strength of the adsorptive bond, (L g−1) | 57.111 | 95.583 | |
n = the adsorption intensity, (g L−1) | 2.833 | 5.139 | |||
r 2 | 0.926 | 0.952 | |||
Temkin isotherm model33 |
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b = Temkin constant related to heat of the adsorption, (J mol−1) | 20.470 | 26.986 | |
a = equilibrium binding constant, (L g−1) | 0.168 | 0.154 | |||
r 2 | 0.984 | 0.971 | |||
Pseudo-first order kinetic model35 | ln(qe − qt) = ln![]() |
k 1 = rate constant (min−1) | 1.38 ×10 −2 | 2.04 ×10 −2 | |
q e = adsorption capacity, (mg g−1) | 210.81 | 358.252 | |||
r 2 | 0.968 | 0.988 | |||
Pseudo-second-order kinetic model36 |
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k 2 = rate constant, (g mg−1 min−1) | 1.095 × 10 −4 | 3.436 × 10 −4 | |
q e = adsorption capacity, (mg g−1) | 353.35 | 172.41 | |||
r 2 | 0.998 | 0.993 | |||
Weber Morris model37 | q t = Kt0.5 + C | K = intra-particle diffusion rate constant, (mg g−1 min1/2) | Step 1 | 27.528 | 0.262 |
Step 2 | 12.642 | 0.087 | |||
Step 3 | 0.592 | 0.019 | |||
C = intercept, (mg g−1) | Step 1 | 72.384 | 3.845 | ||
Step 2 | 218.227 | 5.759 | |||
Step 3 | 319.084 | 6.888 | |||
r 2 | Step 1 | 0.986 | 0.996 | ||
Step 2 | 0.945 | 0.936 | |||
Step 3 | 0.998 | 0.979 |
With r2 of 0.997 and 0.998 for the uranium and thorium ions, respectively, the Langmuir data is better fitted than the other two adsorption isotherm models. As indicated by the Langmuir isotherm adsorption model, the adsorption takes place by monolayer adsorption. The adsorption limit was reached to maximum evaluation with 367.65 mg g−1 uranium ions and 125.95 mg g−1 thorium ions. The significance of the Langmuir adsorption isotherm was verified by the estimations of RL, which is in the range of 0–1 (Table 1) for the adsorption. The equilibrium binding constant (a) (L g−1) and the Temkin constant (b) (related to the heat of the adsorption (J mol−1)) were determined for both metal ions from the Temkin plot, which showed that the adsorption process followed physisorption. Furthermore, a greater value of qm for the uranium ions than thorium ions was due to the higher binding capacity of the uranium ion with the gum-g-poly(AAm) composite. The thermal motion may interfere with the adsorption of ions and being selective for adsorption. The adsorption capacities for both metal ions are different, and this may be due to a difference in the ionic radius. With increasing ionic radius, the steric crowding on the adsorption surface will also increase; thus, a saturation limit of adsorption is rapidly attained.
From these adsorption isotherm studies, it was observed that the adsorption capacity values for gum-g-poly(AAm) for the uptake of uranium ions and thorium ions were different. The adsorption of metal ions depends on various characteristic properties of the metal ions, such as the ionic radius, hydration energy, electronegativity and solubility. The ionic radii, hydration energy and electronegativity values for the uranium ion are 0.97 Å, −3958 kJ mol−1 and 1.38 eV; whereas for the thorium ions, the values are 1.19 Å, −3332 kJ mol−1 and 1.30 eV, respectively. With decreasing cationic radii, the hydration energy will increase since it ideally adsorbs the metal ions faster. In the case of the uranium ion, it is available in the form of uranyl ions (UO22+) in aqueous medium. Due to the presence of oxygen, it may show an increase in the ionic size, which leads to a decrease in the hydration energy compared to the uranium ions. Under an aqueous condition, thorium is available in the Th4+ form. Therefore, in the present study, it was observed that the adsorption capacity of the uranium ion is more. This difference in the adsorption capacity is not only related to the metal ion properties, but is also due to the different properties of the adsorbent, such as the presence of the functional groups, the surface area and surface morphology.
The adsorption of uranium and thorium ions was calculated and is illustrated in Fig. 7. The adsorption rates extended rapidly within the initial time because of the significant available adsorption sites and enhanced concentration gradient. Due to the occupied adsorption sites and decreasing concentration gradient, the adsorption rate decreased and reached equilibrium.34 Equilibrium was achieved within 2 h for uranium ion uptake and 3 h for thorium ion uptake by the gum-g-poly(AAm) composites. Four adsorption kinetic models (pseudo-first order, pseudo-second order, intraparticle diffusion, and Elovich models) were studied and are shown in Fig. 7. Their mathematical expressions are shown in Table 1.
Fig. 8 shows all kinetics models for uranium and thorium adsorption. According to the regression coefficients (r2) of the straight fitting, the pseudo-second order model was revealed best fitted for the uranium ions and thorium ions with r2 = 0.998 and r2 = 0.996, respectively. The pseudo-second order adsorption gives information about the interaction between the adsorbate and adsorbent. This model describes the rate of adsorption of the number of active/binding sites occupied on the adsorbent surface as being proportional to the square of the number of available sites on the adsorbent at equilibrium. The Weber–Morris model illustrates that, in many adsorption cases, the metal ion uptake varies proportionally with t1/2. As seen from Fig. 8(c), for both metal ions, the intraparticle diffusion plots show three-step processes. It describes the boundary layer diffusion of the adsorbate to the adsorbent. The Weber–Morris plots for both metal ions do not pass through the origin, which shows that intraparticle diffusion processes are controlled to some extent by the boundary layer diffusion. The first step indicates that the diffusion of the metal ions occurs through the solution to the external surface of the gum-g-poly(AAm), or may be through the boundary layer diffusion of the adsorbent molecule. The second step diffusion describes the diffusion of the adsorbent into the mesopores of the adsorbate particles. From the third step diffusion, it was observed that diffusion occurs through the micropores of the adsorbate, which exhibits the lowest slope relating to the rate-limiting step in the adsorption process. This may be influenced by various factors such as the size of the adsorbate molecule, concentration of the metal ions, affinity of the metal ions to adsorb, diffusion coefficient of the metal ions within the bulk phase, and the pore size distribution to the adsorbate.
![]() | (4) |
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The change in the Gibbs free energy was calculated from the equation:
ΔG = ΔH − TΔS | (6) |
The obtained values of ΔH and ΔS calculated from the plot of lnKdvs. T−1 are listed in Table 2. The positive values for ΔH and ΔS indicate that the adsorption process is endothermic and there will be a small amendment within the structural appearance of the adsorbate surface, increasing the randomness at the solid–solution interface. Negative ΔG values were decreased with increasing temperature, which suggest a spontaneous adsorption of both ions.
Parameter | ΔH (kJ mol−1) | ΔS (kJ mol−1 K−1) | ΔG (kJ mol−1) | |||
---|---|---|---|---|---|---|
303 K | 313 K | 323 K | 333 K | |||
Values for uranium ions | 85.48 | 0.284 | −0.568 | −3.408 | −6.248 | −9.088 |
Values for thorium ions | 112.19 | 0.377 | −2.041 | −5.811 | −9.581 | −13.351 |
To further assess the reusability, 0.1 N HCl was used as a desorption eluent. After three adsorption and desorption cycles (Fig. 10(b)), the adsorption capacity of gum-g-poly(AAm) was decreased from 91.1% to 85.0% and 89.8% to 80% for the removal of uranium and thorium ions, respectively. This shows that gum-g-poly(AAm) can be used effectively after the regeneration of metal ions.
Metal ions | Adsorbents | Maximum adsorption capacity (mg g−1) |
---|---|---|
Uranium ions | Gum-g-poly(AAm) composite (present work) | 367.65 |
Layered double oxide/carbon dot nanocomposites40 | 354.2 | |
MAO-chitosan beads41 | 117.65 | |
Polyaniline (PANI) modified bentonite7 | 14.1 | |
TMP-g-AO8 | 35.37 | |
Thorium ions | Gum-g-poly(AAm) composite (present work) | 125.95 |
PVA/Fe3O4/SiO2/APTES nanohybrid adsorbent42 | 62.5 | |
Tannin modified poly(glycidyl methacrylate) grafted zirconium oxide densified cellulose (TMPGZDC)43 | 96.69 |
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