Facile Synthesis of Phytic Acid Impregnated Polyaniline for Enhanced

Article pubs.acs.org/jced

Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Facile Synthesis of Phytic Acid Impregnated Polyaniline for Enhanced U(VI) Adsorption Hao Lei,†,‡ Ning Pan,*,†,§ Xiaoqiang Wang,*,†,§ and Hao Zou†,§ Fundamental Science on Nuclear Wastes and Environmental Safety Laboratory, ‡School of National Defence Science and Technology, and §Sichuan Civil-Military Integration Institute, Southwest University of Science and Technology, Mianyang 621010, China

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ABSTRACT: A novel phytic acid impregnated polyaniline (PA-PANI) was synthesized by a in situ polymerization and supermolecular self-assembly method and has been used as an adsorbent for efficient capture of U(VI) from aqueous solution. The structure and morphology of the adsorbent were well characterized. The influence of different factors such as pH value, ionic strength, contact time, initial U(VI) concentration and temperature on the adsorption of U(VI) onto PA-PANI-3 were carried out by batch adsorption experiments. The results demonstrated an enhanced U(VI) adsorption capacity of 86.6 mg/g for PA-PANI-3, greatly exceeding that of polyaniline (PANI, 8.8 mg/g). The adsorption process follows the pseudo-secondorder kinetic and Langmuir isotherm models with the theoretical maximum adsorption capacity of 90.3 mg/g. According to the calculation from the Dubinin−Radushkevich isotherm model equation, it is suggested that the adsorption is mainly a chemical adsorption mechanism. U(VI) is adsorbed on PA-PANI-3 via the phosphoric acid, amine, and imine groups. The U(VI) can be desorbed from the U(VI) adsorbed PA-PANI-3 with a desorption efficiency of 90.8% by using 0.1 mol/L Na2CO3 solution. This study demonstrated that PA-PANI could be used as an efficient adsorbent for removal and recovery of U(VI) from aqueous solution.

1. INTRODUCTION Nowadays, energy crisis and environmental issue has become the crucial factor restricting the social development and economic growth. Uranium, has been considered as the dominant nuclear fuel to produce nuclear energy, but the toxicity of the heavy metal and radioactivity in nuclear waste that can result in groundwater contamination that threatens human health and ecosystem stability cannot be ignored.1,2 Therefore, to develop a method or material for efficient removal of U(VI) from radioactive wastewater is crucial. Several methods have been demonstrated to remove U(VI) from aqueous solution, for instance, chemical precipitation, ion exchange, adsorption, and solvent extraction.3−5 Among these methods, the adsorption method has been widely studied owing to its advantages of simple operation, high adsorption capacity, high selectivity, and ease of repetitive use.6−8 Various adsorbents have been reported for the elimination of U(VI), such as polymers (polyaniline, chitosan bearing α-aminophosphonate and polypyrrole),9−11 carbon material (graphene oxide, carbonaceous nanofibers),9,12 metal oxide (MnO2),13 and metal−organic frameworks.14 Considering those adsorbents, polymer materials have shown high adsorption capacity and selectivity for target ions © XXXX American Chemical Society

due to their easy chemical functionalization by introducing a large number of functional groups, for example, graphene oxide/polypyrrole composites.15 Polyaniline (PANI) is a polymer, which acts as host materials, and with its special backbone structure can be chemically functionalized with various functional groups for capturing target ions in aqueous solution.16 Additionally, the presence of a large number of amine and imine functional groups makes it possess strong affinity for radionuclides, such as Cs(I) (18.6 mg/g, pH = 3.0), Sr(II) (16.4 mg/g, pH = 3.0), Eu(III) (38.0 mg/g, pH = 3.0), and U(VI) (31.4 mg/g, pH = 3.0).9 Phytic acid (PA, C6H18O24P6), a nontoxic macromolecular organic compound, comes from distilling grain. This cyclic acidic molecule saturated with six dihydrogen phosphate molecules has a component of 12 hydroxyl groups which makes it possess strong affinity for multivalent metal cations (Cu(II), Cd(II), Pb(II),17,18 Hg(II) and Cr(VI)19). However, there is no report about choosing PA as a functional group to modify PANI to effectively improve its Received: August 4, 2018 Accepted: August 30, 2018

A

DOI: 10.1021/acs.jced.8b00688 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

Figure 1. Interaction between PA, PANI, and U(VI).

lands) using Cu Kα (λ = 1.540598 Å) irradiation at a scan rate of 0.03°/s in the 2θ range of 5−80°. Fourier transform infrared spectra of samples were recorded on a spectrum one autoima FT-IR spectrometer (FT-IR, PerkinElmer, America) in the range of 400−4000 cm−1 using the KBr pellets method. Thermal gravimetric analysis was conducted on a STA 8000 system thermal analyzer (TGA, PerkinElmer, USA) at a heating rate of 10 °C/min from 30 to 1000 °C under N2 atmosphere. The concentration of U(VI) in aqueous solution was analyzed by a Lambda 650 UV−vis absorption spectrophotometer (PerkinElmer, USA). 2.4. Adsorption Experiments. Batch adsorption experiments were employed to study the adsorption behavior of U(VI) on the adsorbent. Typically, 10.0 mg of the PA-PANI adsorbent was mixed with 10 mL of U(VI) solution with a desired pH value which was adjusted by 0.1 mol/L HNO3 or NaOH solutions. The mixture was then shaken at room temperature for 2 h to reach adsorption equilibrium. After the adsorption process, the aqueous solution was filtered by using a syringe filter (0.22 μm PES (polyether sulfone) membrane). The concentration of U(VI) in the residual aqueous solution or the initial U(VI) solution was measured by the UV−vis spectrophotometric method with arsenazo-III as the chromogenic agent which can complex with UO22+ to exhibit a new characteristic absorption peak at the wavelength of 652 nm.7,8,10 All the adsorption experiments were performed at least three times, and the average values ± standard deviation (SD) were given. The adsorption capacity (qe, mg/g) and the adsorption efficiency (%) of U(VI) adsorbed were calculated by using the following equations:

adsorption capacity for U(VI). In this study, we have synthesized the phytic acid impregnated polyaniline (PAPANI) through the use of a combination of in situ polymerization and supermolecular self-assembly method. This method was simple and repeatable. The objectives of this paper are (1) to synthesize and characterize PA-PANI-3; (2) to study the adsorption behaviors under different ambient conditions; (3) to study the adsorption kinetics, the adsorption isotherms and adsorption thermodynamics of U(VI) onto PA-PANI-3; (4) to evaluate the desorption efficiency.

2. EXPERIMENTAL SECTION 2.1. Materials. Phytic acid (70%) was purchased from the Aladdin Chemistry Co., Ltd. (Shanghai, China). Aniline, ammonium persulfate, ((NH4)2S2O8), sodium hydroxide (NaOH), hydrochloric acid (HCl), and all other chemicals were of analytical reagent grade and received without further purification. 2.2. Preparation of PA-PANI-3. The PA-PANI-3 with the weight ratio of PA to aniline monomer of 3 was synthesized by an in situ polymerization and supermolecular self-assembly method according to a previous literature.20 Briefly, 1.0 mL of aniline monomer (without distillation) and 1.0 mL of concentrated HCl were mixed with 20.0 mL of deionized water in a 50 mL beaker. The mixture was then stirred for 10 min, and 4.40 g of PA was added into the mixture. After this mixture was sonicated for another 20 min, 2.28 g of (NH4)2S2O8 was slowly added to induce and control the polymerization under vigorous magnetic stirring as it was maintained in an ice batch and kept at a temperature below 4 °C. Meanwhile, the PANI polymerization reaction and supermolecular self-assembly between PANI and PA occurred simultaneously. After stirring for 3 h, the mixture was filtered, and the solid was washed several times with deionized water and anhydrous ethanol (to remove excess reactant and impurities). Finally, the dark green PA-PANI-3 product was collected after drying in a vacuum oven at 60 °C for 12 h. For contrast, PANI was prepared under identical experiment conditions but without the addition of PA. The PA-PANI-x with a series of weight ratios (x = 1 and 5, x is the weight ratio of PA to aniline monomor) was also obtained. 2.3. Characterization. The surface morphology of samples were analyzed by an Ultra 55 field-emission scanning electron microscope (FE-SEM, Carl Zeiss NTS GmbH, Oberkochen, Germany) coupled with an energy dispersive spectrometer (EDS). X-ray diffraction patterns were collected on a X’Pert PRO X-ray diffraction analyzer (XRD, PANalytical, Nether-

qe(mg /g ) =

(C0 − Ce) × V m

adsorption efficiency(%) =

(1)

(C0 − Ce) × 100 C0

(2)

where C0 and Ce (mg/L) are the initial and equilibrium concentrations of U(VI) in the solution, respectively. V (L) is the volume of the solution, and m (mg) represents the mass of the adsorbent. 2.5. Desorption Experiments. The desorption efficiency of U(VI) adsorbed adsorbent were evaluated by choosing Na2CO 3, ethylenediaminetetraacetic acid disodium salt (EDTA-2Na), NaClO4, NaHCO3, and HCl as eluting agents. The desorption experiments were conducted immediately after the adsorption experiments. The U(VI) adsorbed adsorbent was separted from the solution by a centrifugal method, and the solid was washed with deionized water and dried in an B



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Figure 2. FE-SEM images of (a) PA-PANI-3, (b) PA-PANI-3-U(VI), and corresponding element mapping of phosphorus (c) and uranium (d) for PA-PANI-3-U(VI).

oven. Subsequently, the U(VI) adsorbed adsorbent was mixed with an equal volume of eluting agent (10 mL, 0.1 mol/L). The mixture was than shaken vigorously for 12 h to achieve desorption equilibrium. The concentration of U(VI) in the eluting liquid was measured by the spectrophotometric method. The desorption efficiency (%) was calculated by the following equations: desorption efficiency (%) =

Cf × 100 C0 − Ce

(3)

where Cf (mg/L) is the concentration of U(VI) in the eluting liquid. Figure 4. FT-IR spectra of PANI, PA-PANI-3, and PA- PANI-3U(VI).

3. RESULTS AND DISCUSSION 3.1. Characterization of Adsorbent. 3.1.1. SEM Study. The self-assembled material PA-PANI was produced immedi-

Figure 5. TGA data of PANI and PA-PANI-3 under nitrogen atmosphere. Figure 3. XRD patterms of PANI and PA-PANI-3.

and neutral PO oxygen atoms or −NH2/NH nirogen atoms which has lone pairs of electrons.21 Figure 1 shows the interaction between PA, PANI, and U(VI). The electrostatic attraction interaction, supermolecular interaction and coordination may have existed in the PA-PANI-U(VI). Figure 2panels a and b show the FE-SEM images of PAPANI-3 before and after the adsorption of U(VI). The PAPANI-3 has the regular coralliform structure.20 It can be clearly observed that PA-PANI-3-U(VI) has a similar surface morphology as compared to PA-PANI-3, suggesting U(VI) does not deeply destroy the structure of PA-PANI-3 and the

ately after the oxidation agent ((NH4)2S2O8) was added into the aniline monomer−HCl aqueous solution in the presence of PA. It is well-known that one molecule of PA has six molecules of phosphoric acid groups which would interact with PANI though electrostatic interaction as well as the N−H···O hydrogen bonding.17,20 Besides, the existence of phosphoric acid, amine and imine groups on PA-PANI provide more adsorption sites available to capature cationic radioactive ions in aqueous medium because of the negative charged P−O− C



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Figure 6. (a) Effect of solution pH on the adsorption of U(VI) onto PA-PANI-3 (C0 = 105 mg/L, t = 2 h, V = 10 mL, T = 298 K, and m = 10.5 mg). (b) PZC (pHf − pHi) plots of PA-PANI-3. (c) Effect of NaNO3 concentration on the adsorption of U(VI) onto PA-PANI-3 (pH = 3.0, C0 = 92 mg/L, t = 2 h, V = 10 mL, T = 298 K, and m = 10 mg). (d) Effect of the weight ratio of PA to aniline monomer on the adsorption of U(VI) onto PA-PANI-3 (pH = 3.0, C0 = 113 mg/L, t = 2 h, V = 10 mL, T = 298 K, and m = 10.5 mg).

3.1.4. TGA Analysis. Thermal stability of PANI and PAPANI-3 were measured by TGA. As shown in Figure 5, there is a three-step weight loss of PANI between 30 and 1000 °C under N2 atmosphere. The slight weight loss (12.43%) from 30 to 120 °C can be attributed to the loss of adsorbed water. The second weight loss (9.51%) occurs from 120 to 280 °C, which can be assigned to the degradation of PANI, and the weight loss at above 280 °C can be owing to decomposition of PANI.6,9 A comparison of the weight loss of PA-PANI-3 (58.72%) with that of PANI (63.58%) at 1000 °C shows that PA-PANI-3 has better thermal stability, which can be ascribed to the existence of a strong interaction between PA and the protonated imine of PANI chains. 3.2. Adsorption of U(VI) by PA-PANI. 3.2.1. Effect of Solution pH and Ionic Strength. The solution pH plays an important role in the adsorption of U(VI) from aqueous solution as a result of the extent of protonated adsorbent and different uranium species in solution under different pH values. Figure 6a shows the effect of solution pH on the adsorption of U(VI) onto PA-PANI-3. It can be seen that the adsorption capacity of U(VI) by PA-PANI-3 was remarkably increased from 30.3 to 79.0 mg/g when the solution pH value is increasd from 1.0 to 3.0, and slightly decreases (from 79.0 to 73.9 mg/ g) when the solution pH value is in the range of 3.0−4.0. The value of the point of zero charge (PZC) was measured via the initial pH (pHi) and final pH (pHf) method according to a previous report;1 the data was given in Figure 6b. The value of PA-PANI-3 was calculated as 2.0. On the basis of the results mentioned above, we can speculate that the lower adsorption capacity at pH less than 2.0 (PZC) can be attributed to the protonated phosphate groups which would result in the electrostatic repulsion between U(VI) cations and positively charged PA-PANI-3. When the pH is greater than 2.0 (PZC), the PA-PANI-3 is negatively charged, which has a strong electrostatic attraction for positively charged U(VI).12 In

PA-PANI-3 represents a good stability in the U(VI) aqueous solution. Additionaly, the results of SEM-element mapping indicates that both of the phosphorus and uranium elements are uniformly distributed on the surface of PA-PANI-3-U(VI), implying the presence of PA and the success of adsorbing U(VI) from aqueous solution. 3.1.2. XRD Analysis. Figure 3 shows the XRD patterns of PANI and PA-PANI-3. The typical XRD pattern of PANI presents two broad diffraction peaks centered at 2θ = 20.19°and 25.21°,22 which can be assigned to the periodicity parallel and perpendicular to the PANI chains, respectively. Compared to PANI, the XRD pattern of PA-PANI-3 has the two characteristic diffraction peaks but the peak width becomes narrow, implying that PA has a mild effect on the crystal structure of PANI. 3.1.3. FT-IR Analysis. To confirm the functionalization of PA on PANI, the FT-IR spectra analysis were applied. As shown in Figure 4, the main characteristic peaks of PANI are 1627 (stretching vibration of the quinonoid rings), 1465 (C C deformations stretching of benzenoid rings), 1300 (C−N stretching of the benzenoid ring), 1103 (bending vibration mode of the quinonoid rings), and 798 cm−1 (the out-of-plane deformation of C−H in the para-disubstituted benzene ring).9,16,17,23 The FT-IR spectrum of PA-PANI-3 exhibits these characteristic peaks of PANI but the intensity of the peak located at 850−1200 cm−1 was increased, which originates from the phosphate group and protoned −NH− groups.20 Compared to that of PA-PANI-3, another peak appears at 1384 cm−1 in the FT-IR spectrum of PA-PANI-3-U(VI). These peaks are respectively attributed to the adsorption of NO3−.24 The results of FT-IR analysis indicates the successful functionalization of PA on PANI to obtain PA-PANI-3, and U(VI) was successfully adsorbed on it. D



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1.0 mol/L (as shown in Figure 6c), suggesting that the adsorption behavior is mainly dominated by the inner-sphere surface complexation mechanism.26 3.3.2. Effect of the Weight Ratio of PA to Aniline Monomer. By varying the initial weight ratio of PA to aniline monomer, the results of the adsorption test for PA-PANI-x (x = 0, 1, 3, and 5) is shown in Figure 6d. The adsorption capacities of PA-PANI-1, PA-PANI-3, and PA-PANI-5 are 50.0, 86.6, and 90.3 mg/g, respectively, which is higher than that of PANI (8.8 mg/g), indicating that the introduction of PA onto PANI efficiently enhanced the capturing of U(VI). When the additional amount of PA was beyond 3, the U(VI) adsorption capacity was slightly increasd. Hence, PA-PANI-3 was used for the study of the adsorption mechanism. 3.3.3. Effect of Contact Time and Adsorption Kinetics Study. Figure 7a shows the effect of contact time on the adsorption of U(VI) onto PA-PANI-3. It is observed that the adsorption capacity sharply increases from 0 to 36.1 mg/g with an increase of contact time from 0 to 40 min, and then reaches adsorption equilibrium within 1 h with a value of approximately 42.8 mg/g. The rapid adsorption process can be attributed to the strong complexation interaction between U(VI) and the phosphate functional groups on the PA-PANI3. To obtain the adsorption process mechanism of U(VI) onto PA-PANI-3, two adsorption kinetics models, namely, pseudofirst-order and pseudo-second-order kinetic models were used to fit the data.2,25,27 The relevant equations are expressed as follows:

Table 1. Parameters of the Pseudo-first-order and the Pseudo-second-order Kinetic Models of U(VI) Adsorption by PA-PANI-3 pseudo-first-order model

pseudo-second-order model

qe1.cal (mg/g)

R2

k2 (g/(mg/min))

qe2.cal (mg/g)

R2

0.032

39.6

0.979

0.0011

55.6

0.997

(4)

t 1 1 = + t 2 qt qe k 2qe

(5)

where qt (mg/g) and qe are the adsorption capcities of U(VI) at contact time t and at equilibrium time. k1 (1/min) and k2 (g/(mg/min)) are the rate constants of the pseudo-first-order and the pseudo-second-order kinetic models, respectively. The parameters calculated from the pseudo-first-order and the pseudo-second-order kinetic models are listed in Table 1. The adsorption process can be well described by the pseudosecond-order kinetic model equation according to the correlation coefficient (R2 = 0.997), implying that the adsorption is chemisorption.2,28 And the calculated equilibrium adsorption capacity qe.cal is 55.6 mg/g which is consistent with the experimental value (qe.exp = 49.4 mg/g). 3.3.4. Effect of Initial U(VI) Concentration and Adsorption Isotherms Study. The effect of initial U(VI) concentration on the adsorption of U(VI) onto PANI and PA-PANI-x (x = 1, 3, and 5) and the PA-PANI-3 at four diffierent temperature (303, 313, 323, and 333 K) is shown in Figures 8a and 9a, respectively. With the increase of concentration, the adsorption capacity of U(VI) is increased which is due to the occupation of most of the vacant available sorption sites that causes the equilibrium adsorption capacity at a high initial U(VI) concentration to improve.29 A higher adsorption capacity of U(VI) at higher temperature for PA-PANI-3 suggests that the adsorption process of U(VI) on PA-PANI-3 is endothermic. Furthermore, three adsorption isotherms, such as Langmuir, Freundlich, and Dubinin−Radushkevich (D-R) isotherm were applied to fit the data to obtain the adsorption method and mechanism.

Figure 7. (a) Effect of contact time on the adsorption of U(VI) onto PA-PANI-3, and the data fitted by (b) pseudo-first-order model and (c) pseudo-second-order kinetic model (pH = 3.0, C0 = 86 mg/L, V = 10 mL, T = 298 K, and m = 10 mg).

k1 (1/ min)

ln(qe − qt) = ln qe − k1t

addition, because the uranium can be easily hydrolyzed in aqueous medium at a higher pH value,25 a solution pH value of 3.0 was chosen in the later study. The effect of ionic strength on the adsorption of U(VI) onto PA-PANI-3 was studied. We can found that the ionic strength has no distinct effect on the adsorption of U(VI) onto PAPANI-3 with an increase of NaNO3 concentration from 0 to E



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Figure 8. (a) Effect of initial U(VI) concentration on the adsorption of U(VI) onto PANI, PA-PANI-1, PA-PANI-3, and PA-PANI-5, and the data fitted by (b) the Langmuir isotherm model, (c) the Freundlich isotherm model, and (d) D-R isotherm model. (pH = 3.0, t = 2 h, V = 10 mL, T = 298 K, and m = 10.0 mg).

Figure 9. (a) Effect of initial U(VI) concentration on the adsorption of U(VI) onto PA-PANI-3 at 308, 318, 328, and 338 K, and the data fitted by (b) the Langmuir isotherm model, (c) the Freundlich isotherm model, and (d) D-R isotherm model. (pH = 3.0, t = 2 h, V = 10 mL, T = 298 K, and m = 10.0 mg).

where qm (mg/g) and KL (L/mg) are the theoretical maximum adsorption capacity and Langmuir adsorption equilibrium constant that is related to the adsorption energy, respectively. The Freundlich isotherm is based on the assumption that the adsorbent has heterogeneous surfaces and the coverage of adsorbent occurs in a multilayer adsorption.14 The model can be expressed by the following linear equation:

The Langmuir isotherm is based on the assumption that the adsorbent has identical adsorption sites, there is no interaction between the adsorbates, and the coverage of adsorbent occurs in a monolayer adsorption.14 The model can be expressed by the following linear equation: Ce C 1 = e + qe qm KLqm

(6) F



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and the relevant linear curves are shown in Figure 8b−d and Figure 9b−d. The adsorption of U(VI) onto the four adsorbents (PANI, PA-PANI-1, PA-PANI-3, and PA-PANI5) obeys the Langmuir isotherm model (R2 > 0.99) by comparing the R2 parameters of Langmuir and Freundlich isotherm models, which suggested that the adsorption of U(VI) onto the adsorbents follows the monolayer adsorption manner.14 The theoretical maximum adsorption capacity are about 35.4, 88.1, 90.3, and 131.9 mg/g for PANI, PA-PANI-1, PA-PANI-3, and PA-PANI-5, respectively. Also, the theoretical maximum adsorption capacity is about 87.0, 106.7, 107.3, and 114.3 mg/g for PA-PANI-3 at 308, 318, 328, and 338 K, respectively. These results demonstrated that the adsorption capacity of PA-PANI-3 is higher than that of pure PANI and some other adsorbents, for example, the calcon carboxylic acid grafted polyacrylamide (18.8 mg/g, pH = 5.5),1 porous magnetic Ni0.6Fe2.4O4 (57.7 mg/g, pH = 5.0),2 activated carbon (25.5 mg/g, pH = 3.0), graphene oxide (86.2 mg/g, pH = 3.0),9 reduced graphene oxide (74.1 mg/g, pH = 4.0), polypyrrole (87.7 mg/g, pH = 5.0),11 but is lower than some reported adsorbents, such as the PA-based adsorbent (PA/ PANI/FeOOH, 555.8 mg/g, pH = 8.0)25 and Zeolitic Imidazolate Framework-67 (1683.8 mg/g, pH = 4.0).30 Therefore, it is desired to improve the adsorptive capacity of this adsorbent by choosing suitable substrate material in which PA groups can act as a functional group in the future. According to the fitting results of the D-R isotherm model, the value of E was calculated about 9.8−13.0 kJ/mol for PAPANI-x (x = 0, 1, 3, and 5) and 12.3−20.3 kJ/mol for PAPANI-3 at the four differnet temperatures, the values of which are >8 kJ/mol, indicating the adsorption process is mainly chemical adsorption mechanism.2,10,31 3.3.5. Effect of Temperature and Thermodynamic Parameters Study. The effect of temperature on the adsorption of U(VI) onto PA-PANI-3 is shown in Figure 10. It can be seen that the adsoption capacity of U(VI) onto PAPANI-3 increased with the rise of temperature which indicated that the adsorption process is endothermic. Moreover, the adsorption thermodynamics have been studied. The thermodynamic parameters including enthalpy change (ΔH, kJ/mol), entropy change (ΔS, J/mol/K), and Gibbs free energy (ΔG, kJ/mol) are calculated by the following equations:24,25,27,32

Table 2. Adsorption Isotherm Parameters of U(VI) Adsorption by PANI, PA-PANI-1, PA-PANI-3, and PAPANI-5 parameter Langmuir qm KL R2 Freundlich KF (mg/g(L/ mg)1/n) 1/n R2 D-R qDR

units

PANI

PAPANI-1

PAPANI-3

PAPANI-5

(mg/g) (L/mg)

35.4 0.017 0.994

88.1 0.013 0.991

90.3 0.058 0.997

131.9 0.027 0.992

3.12 0.41 0.955

4.10 0.52 0.978

17.8 0.30 0.856

10.7 0.47 0.916

0.0014

0.00092

0.0021

0.00043

0.0053 9.8 0.987

0.0030 13.0 0.888

0.0045 10.5 0.939

0.0043 10.8 0.969

(mol/g) (mol2/ kJ2) (kJ/mol)

B Ea R2

ln qe = ln KF +

1 ln Ce n

(7)

1/n

where KF (mg/g(L/mg) ) and n are Freundlich constants that are related to the adsorption capacity and adsorption intensity of the adsorbent, respectively. The D-R isotherm is based on the assumption that the adsorbent has a nonuniform adsorption energy that obeys Gaussian distribution.1,25 The model can be expressed by following linear equation: ij 1 yzz ε = RT lnjjj1 + z, j Ce zz{ k

ln qe = ln qD − R − Bε 2 , E=

1 2B

(8)

where B (mol2/kJ2) is the D-R constant depending adsorption mean free energy, ε is the Polanyi potential, qD‑R (mol/g) is the D-R adsorption capacity, R and T represent the ideal gas constant (0.008314 kJ/mol/K) and absolute tempreture (K), respectively, and E (kJ/mol) is the free energy change which is required to transfer one mole of ion from the infinity in the solution to the adsorbent surface. The fitting results calculated from the Langmuir, Freundlich, and D-R isotherm models are summarized in Tables 2 and 3,

Kd =

q Co − Ce V = e Ce m Ce

(9)

Table 3. Adsorption Isotherm Parameters of U(VI) Adsorption by PA-PANI-3 at 308, 318, 328, and 338 K units Langmuir qm KL R2 Freundlich KF 1/n R2 D-R qDR B Ea R2

T = 308 K

T = 318 K

T = 328 K

T = 338 K

(mg/g) (L/mg)

87.0 0.029 0.992

106.7 0.34 0.999

107.3 0.45 0.999

114.3 0.42 0.995

(mg/g(L/mg)1/n)

48.7 0.17 0.793

53.9 0.15 0.912

58.4 0.15 0.828

12.7 0.34 0.997

(mol/g) (mol2/kJ2) (kJ/mol)

0.00089 0.0033 12.3 0.997

0.00077 0.0015 18.4 0.839

0.00073 0.0013 20.0 0.950

0.00076 0.0012 20.3 0.866

G



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Figure 10. (a) Effect of temperature on the adsorption of U(VI) onto PA-PANI-3, (b) relationship between ln Kd and 1/T (pH = 3.0, C0 = 84 mg/ L, t = 2 h, V = 10 mL, and m = 10 mg).

of the eluting agent on the desorption of U(VI) from PAPANI-3-U(VI) are studied. It is found that the desorption efficiency can reach 90.8% when 0.1 mol/L Na2CO3 solution was used as the eluting agent, suggesting that the adsorption of U(VI) onto PA-PANI-3 is reversible. The highest desorption efficiency may be due to the strong complexation between UO22+ and CO32−.33

Table 4. Thermodynamic Parameters for the Adsorption of U(VI) onto PA-PANI-3 T (K)

ΔH (kJ/mol)

ΔS (J/mol/K)

ΔG (kJ/mol)

298 308 318 338

15.8

121.7

−20.5 −21.8 −23.0 −25.4

4. CONCLUSIONS A self-assembly material, phytic acid-impregnated polyaniline (PA-PANI), was successfully synthesized by a facile method. When PA was added to aniline monomer at a weight ratio of 3 in synthesis, the obtained PA-PANI-3 displayed higher adsorption capacity (86.6 mg/g) than that of PANI (8.8 mg/g). The optimum conditions of the adsorption of U(VI) by PA-PANI-3 was at a pH of 3.0 and contact time of 1 h. The adsorption process was dependent on ionic strength. The adsorption behaviors could be well described by the pseudosecond-order kinetic and Langmuir isotherm models. The maximum adsorption capacity can be as high as 90.3 mg/g. The adsorption behaviors of U(VI) onto PA-PANI-3 followed the monlayer adsorption, and the adsorption mechanism was mainly chemical adsorption due to the phosphate, amine, and imine groups of PA-PANI-3 forming strong interaction (coordination and hydrogen bonding interaction) with uranyl ions. Desorption experiment studies indicated that U(VI) can be recovered. This adsorbent has found potential for practical application in remediation of radioactive wastewater.

Figure 11. Effect of eluting agent on the desorption of U(VI) from PA-PANI-3-U(VI) (pH = 3.0, C0 = 30 mg/L, t = 2 h, V = 10 mL, T = 298 K, m = 10 mg, C(eluting agent) = 0.1 mol/L).

ln Kd =

ΔS ΔH − R RT

ΔG = ΔH − T ΔS

(10)

■

(11)

where Kd (mL/g) is the distribution coefficient. T (K) and R (8.314J/mol/K) are the absolute temperature and the ideal gas constant, respectively. The values of ΔH and ΔS are calculated from the slope and intercept of the linear curve of ln Kd vs 1/T, respectively. The results of thermodynamic parameters are shown in Table 4. The positive ΔH (15.8 kJ/mol) and ΔS (121.7 J/ mol/K) suggest that the adsorption of U(VI) onto PA-PANI-3 is endothermic in nature and the randomness at the solid/ solution interface increases during the adsorption process.11 The ΔG values are calculated as approximately −20.5, −21.8, −23.0, and −25.4 kJ/mol at 298, 308, 318, and 338 K, respectively. The negative values of ΔG imply that the adsorption is a spontaneous process and at high temperature favors a more negative value of ΔG.2,25 3.4. Effect of Eluting Agent. Evaluation of the recovery of U(VI) from the U(VI) adsorbed adsorbent is important for uranium resources recycle. As shown in Figure 11, the effects

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*Tel.: +86-816-6089872. Fax: +86-816-6089672. E-mail: [email protected]. *E-mail: [email protected]. ORCID

Ning Pan: 0000-0001-6524-3159 Funding

The authors are grateful for the financial support from the National Natural Science Foundation of China (21806133), the Doctor Research Foundation of Southwest University of Science and Technology (Grant 16zx7156), and the Longshan academic talent research supporting program of Southwest University of Science and Technology (17LZX531, 18LZXT03). Funding from the Fundamental Science on Nuclear Wastes and Environmental Safety Laboratory (Grant No. 16kfhk01) is also acknowledged. H



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The authors declare no competing financial interest.

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