Efficient removal of U(VI) from aqueous solutions by polyaniline/hydrogen-titanate nanobelt composites

Abstract

The organic–inorganic hybrid material of polyaniline/hydrogen-titanate nanobelt (PANI/H-TNB) composites was fabricated through a convenient oxidative polymerization approach. The characteristic results of X-ray powder diffraction and Fourier transformed infrared spectroscopy showed that aniline was successfully polymerized onto titanate surfaces. The adsorption capacity of PANI/H-TNB composites toward U(VI) was evaluated by batch experiments, and this adsorption process followed a pseudo-second order kinetics model and Langmuir model. The PANI/H-TNB composites showed a maximum adsorption amount of 216.82 mg g⁻¹ toward U(VI), which was higher that of PANI (48.75 mg g⁻¹) and many other adsorbents. The thermodynamic parameters calculated from the adsorption isotherms indicated a spontaneous and endothermic adsorption process. Higher adsorption capacities were observed after regeneration for six cycles. The PANI/H-TNB composites could be regarded as a potential adsorbent to remove U(VI) from wastewater.

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1. Introduction

The nuclear industry, especially the nuclear power program in China, has been developing rapidly recently. Large quantities of hazardous radionuclides have been generated and possibly discharged into natural water,¹ which results in serious environmental pollution. Uranium, which is one of the most predominant radionuclide contaminants, and produced in nuclear fuel production, ore mining, weapons manufacturing and research activities, is quite stable in soils and groundwater.^2,3 Under normal environmental conditions, uranium exists mainly as a uranyl cation UO₂²⁺ in aqueous solutions, which offers the possibility of removal by the transfer of U(VI) ions onto solid materials for effective separation and enrichment.⁴ Uranium contaminations, which are discharged into the environment, could cause a threat to human health and ecological systems due to their toxicity and radioactivity.^5,6 Thus it is necessary to develop advanced technologies to remove uranium from aqueous solutions before they are discharged into the environment.

Currently, various technologies have been proposed to remove U(VI) from wastewater, such as precipitation, membrane filtration, adsorption, ion exchange etc.^7–10 The adsorption technique is regarded as a feasible and environment-friendly method because of its low cost, simple design and operation.^11,12 However, the limited adsorption capacity restricts the practical application for the removal of U(VI) from aqueous solutions. Therefore, it is urgent to design new adsorbents with excellent stabilities and high adsorption capacities to remove U(VI) from aqueous wastes.¹³

In recent years, nanocomposites based on organic–inorganic hybrids for adsorbents have been attracting widespread concern due to their potential adsorption capacity and unique functionality. For example, Gao et al.¹⁴ reported that a novel organic–inorganic hybrid of polyaniline/titanium phosphate holds a good adsorption capacity toward Re(VII). Pérez et al.¹⁵ found that the synthesis of organic–inorganic interpenetrated hybrids based on cationic polymers and hydrous zirconium oxide composites exhibited excellent adsorption capacity for arsenic from wastewater. Wang et al.¹⁶ found that an organic–inorganic hybrid of polyaniline/α-zirconium phosphate performed good adsorption behavior for the removal of organic pollutants from aqueous solutions. However, the investigation of the organic–inorganic hybrid materials for adsorption is still required for practical utility. Polyaniline (PANI) is an easily produced and low cost polymer with excellent chemical stability and adjustable electrical conductivity.^17–20 With its primary and secondary amino functional groups, PANI is expected to exhibit a strong affinity to heavy metal ions in water.^14,21 Titanates have a unique microstructure that can produce a high adsorption capacity via ionic exchanges.²² We combined organic PANI with inorganic hydrogen titanate nanobelts to obtain PANI/hydrogen titanate nanobelt (PANI/H-TNB) composites, which may retain the best properties of PANI and the adsorption capacity of titanate.²³ Therefore, it would be of great interest to synthesize this organic–inorganic hybrid material of PANI/H-TNB composites for wastewater treatment.

In this work, organic–inorganic hybrid materials of PANI/H-TNB composites were synthesized by the in situ polymerization of aniline onto the surface of titanate. The PANI/H-TNB composites were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared (FT-IR) spectroscopy, powder X-ray diffraction (XRD), and zeta potentials. Batch experiments were adopted to assess the removal performance of PANI/H-TNB composites toward U(VI) under various conditions. The influencing factors such as contact time, ionic strength, solid content, pH value, were investigated. Besides, the repeated practicability was also carried out to estimate the possible practical applications.

2. Materials and method

2.1. Materials

Titania P₂₅ (TiO₂; ca. 80% anatase and 20% rutile) was used as the titanium source. All other chemicals were purchased from Sinopharm Chemical Reagent Co. Ltd at an analytically pure grade and used directly without further treatment.

2.2. Synthesis of the PANI/H-TNB composites

Titanate nanobelts were prepared according to a previous study.²⁴ P₂₅ powder (1.0 g) was added into 5 mol L⁻¹ NaOH (80 mL) aqueous solution, which was then placed in a 100 mL airtight steel container. The mixture was heated to and maintained at 200 °C for 96 h. After cooling to room temperature, the solid was obtained by filtration and further washed with deionized water, and washed with dilute HCl to neutralize it. After that, the product was dried under vacuum at 60 °C for 8 h to obtained the desired H-TNB.

The PANI/H-TNB composites were prepared via a self-assembly method, as detailed in a previous study.²⁵ Firstly, H-TNB (80 mg), aniline (6 mmol) and phytic acid (PA, 1.8 mmol) were added into 50 mL of deionized water with magnetic stirring for 1 h at room temperature. Then, 30 mL of 0.2 mol L⁻¹ ammonium peroxydisulfate ((NH₄)₂S₂O₈, APS), precooled with an ice bath for 5 min, was added into the above mixture. The reaction was performed in the ice-water bath and stirred vigorously for about 20 h. The PANI/H-TNB composites were obtained by filtration and further washed with deionized water and ethanol for several times, and finally dried at 60 °C under vacuum for 24 h.

2.3. Material characterization

The PANI/H-TNB composites were characterized by SEM, TEM, XRD, FT-IR, and zeta potentials. The SEM images were obtained using a JEOL JSM-6330F instrument operated at the beam energy of 15.0 kV and the TEM images were recorded with a JEOL-2010 microscope. The XRD patterns were obtained with Cu Kα radiation (λ = 1.5406 Å) of 10–70° with a step size of 0.02° and a count time of 8 s. The FTIR spectra were recorded in pressed KBr pellets (Aldrich, 99%, analytical reagent) over a range from 400 to 4000 cm⁻¹. The zeta potentials of the PANI/H-TNB composites were measured on a Malvern ZEN2600 Zetasizer.

2.4. Adsorption experiments

Adsorption experiments were carried out in polyethylene centrifuge tubes with 0.3 g L⁻¹ PANI/H-TNB composite and 20 mg L⁻¹ U(VI) solutions. A background electrolyte solution of 0.1 mol L⁻¹ NaNO₃ was prepared to achieve the desired ionic strength. A negligible volume of 0.1–1.0 mol L⁻¹ HNO₃ or NaOH solution was added into the suspension solutions to adjust the pH to the desired values. To achieve the adsorption equilibrium, the tubes were placed on an oscillator and kept shaking for 24 h, then the solutions were centrifuged at 8000 rpm for 15 min. The concentration of U(VI) was measured by a Dichlorophosphonoazo III Spectrophotometer (V-1600 Mapada Shanghai) at the wavelength of 669 nm. The amount of U(VI) adsorbed on PANI/H-TNB can be calculated from the difference between the initial (C₀) and the equilibrium concentration (C_e) of the U(VI) solutions. The adsorption was expressed in terms of adsorption percentage (%) and distribution coefficient (K_d), which were calculated from the following equations:where C₀ (mg L⁻¹) is the initial concentration, C_e (mg L⁻¹) is the equilibrium concentration of U(VI) after adsorption, m (g) is the mass of PANI/H-TNB, and V (L) represents the volume of the suspension. The relative errors of the data were less than ±5%.

3. Results and discussion

3.1. Characterization

The surface morphologies of the PANI and PANI/H-TNB composites were characterized by SEM (Fig. 1a and b) and TEM techniques (Fig. 1c and d). Fig. 1a shows that PANI formed as piles of nanofibers, aggregated loosely and arranged in a disordered structure. These disordered structured composites were considered to have a potential capacity for water treatment including the removal of heavy metal ions.²⁶ Compared to the PANI nanotubes, the PANI/H-TNB composites had a wrinkled and crumpled structure, and were much more rough, indicating PANI covered on the surfaces of nanobelts (Fig. 1b). Fig. 1c shows PANI nanostructures that are composed of nanowires and nanotubes with rough surfaces, which means the surface area of such kinds of PANI nanowires/tubes is larger. Fig. 1d demonstrated that the PANI/H-TNB composites had an obvious core–shell structure.

Fig. 1 SEM images of bare PANI nanorods (a) and PANI/H-TNB (b); TEM images of bare PANI nanorods (c) and PANI/H-TNB (d).

The as-prepared H-TNB, PANI, and PANI/H-TNB composites were characterized by XRD (Fig. 2). The main peaks at 2θ = 11.78, 24.93, 28.24, 33.63, 36.12, 43.45 and 48.36° corresponded to the (200), (110), (310), (112), (312), (204) and (020) planes of the H-TNB, respectively. The XRD pattern of H-TNB was in good agreement with the standard diffraction data (JCPDS no. 44-0131).²⁷ Two new broad peaks of PANI/H-TNB composites were observed at 2θ = 20.46 and 25.82°, which was almost the same as those of pure PANI nanorods.²⁸ The results suggested that the PANI was successfully grafted onto the hydrogen titanate.

Fig. 2 XRD patterns of H-TNB, PANI and PANI/H-TNB.

Fig. 3 shows the FTIR spectra of H-TNB, PANI and PANI/H-TNB. In the FTIR spectrum of PANI, the characteristic bands at 1572, 1495, 1300 and 1127 cm⁻¹ were attributed to the stretching vibrations of C–N bonds in N=Q=N, N–B–N, B–NH–B and B–NH⁺=Q (Q, quinoid ring; B, benzenoid ring), respectively.²⁹ The characteristic peaks at 658 and 480 cm⁻¹ correspond to Ti–O–Ti asymmetric and symmetric stretching modes, respectively, in the H-TNB spectrum. As for the PANI/H-TNB composites, similar peaks at 1572, 1495 and 1300 cm⁻¹ were observed from PANI, indicating the successful modification of PANI onto H-TNB to form PANI/H-TNB.

Fig. 3 FTIR spectra of H-TNB, PANI and PANI/H-TNB.

As shown in Fig. 4a, the presence of U 4f in PANI/H-TNB–U obviously demonstrated the adsorption of U(VI) onto the PANI/H-TNB surface after the adsorption equilibrium.³⁰ For the PANI/H-TNB sample after the reaction (Fig. 4b), the XPS spectrum of U 4f_5/2 and U 4f_7/2 clearly displayed peaks at 393.2 and 382.4 eV, respectively.^31,32 The adsorbed U was only detected in the oxidation state of U(VI) (E_b = 382.2 ± 0.3 and 393.1 ± 0.3 eV), and no reduced state of U(IV) (E_b = 380.4 ± 0.3 and 391.3 ± 0.3 eV) was detected, suggesting no chemical reduction was occurring.

Fig. 4 The XPS survey for the PANI/H-TNB after adsorption (a), U 4f (b).

Fig. 5 shows the zeta-potential values of PANI and PANI/H-TNB as a function of pH values. The zeta potential of PANI was observed at about 8.3. The zeta potential of the PANI/H-TNB composites increased with decreasing pH values, which can be explained by the protonation of the amino functional groups of PANI on the PANI/H-TNB surfaces. The electrostatic point of PANI/H-TNB is measured to be about 5.8, suggesting a positively charged surface at pH < 5.8 and a negatively charged surface at pH > 5.8.

Fig. 5 The zeta potentials of PANI and PANI/H-TNB, T = 293 K, m/V = 0.3 g L⁻¹.

3.2. Effect of contact time

Fig. 6 shows the adsorption kinetics of U(VI) on the PANI/H-TNB composites. A relatively fast adsorption was observed within the first 5 h, and this reached equilibrium after 5 h. To ensure adsorption equilibrium, a contact time of 24 h was selected for further experiments. This relatively fast adsorption equilibrium indicated that chemisorption and inner-sphere surface complexation were the main interaction mechanisms of U(VI) with the composites.³³

Fig. 6 Adsorption kinetics of U(VI) on PANI/H-TNB surface. T = 293 K, m/V = 0.3 g L⁻¹, C₀ = 20 mg L⁻¹, I = 0.01 mol L⁻¹ NaNO₃, pH = 5.0 ± 0.1.

In order to analyze the interaction mechanisms in detail, the adsorption kinetics of U(VI) on PANI/H-TNB composites were fitted by pseudo-first order and pseudo-second-order kinetic models. The pseudo-first-order³⁴ and the pseudo-second-order³⁵ can be given in equations:

ln(q_e − q_t) = ln q_e − k₁t (3)

where q_e and q_t (mg g⁻¹) are the amount of U(VI) adsorbed at equilibrium and at time t, respectively.

The parameters of the adsorption kinetics were summarized in Table 1. A higher correlation coefficient (R²) was observed for the pseudo-second-order model (0.999) than the pseudo-first-order model (0.885), indicating a pseudo-second-order adsorption process in this case. This result further corroborated the dominant chemisorption or strong surface complexation mechanism for the adsorption of U(VI) on PANI/H-TNB composites.

Table 1 Kinetic model parameters for the adsorption of on PANI/H-TNB composites

C0 (mg L^−1)	Pseudo-first-order kinetic model			Pseudo-second-order kinetic model
	qe (mg g^−1)	k1 (h^−1)	R^2	qe (mg g^−1)	k2 (g mg^−1 h^−1)	R^2
20	101.42	1.44	0.885	65.79	0.05	0.999

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3.3. The effect of solid content amount

The effect of adsorbent content is shown in Fig. 7. Increased adsorption efficiencies were obtained with increased amount of PANI/H-TNB composite. This phenomenon was expected, and can be explained by the fact that the increasing content of PANI/H-TNB provided more available functional groups, which led to the available sites for the binding of U(VI). The distribution coefficient (K_d) is also shown in Fig. 7, and slowly decreases with the increase of PANI/H-TNB content. With a low adsorbent content, the surface active sites were entirely exposed for adsorption and the surface was highly saturated, leading to a high distribution coefficient. However, the competition among the adsorption sites increased at high adsorbent concentrations, which could decrease the K_d values of U(VI). Similar adsorption behaviors of U(VI) were reported by Song et al.³⁶ and Zhao et al.³⁷

Fig. 7 Effect of adsorbent content on U(VI) adsorption on the PANI/H-TNB surface, C₀ = 20 mg L⁻¹, I = 0.01 mol L⁻¹ NaNO₃, pH = 5.0 ± 0.1.

3.4. Effects of pH and ionic strength

The effect of pH values toward U(VI) adsorption was investigated and plotted in Fig. 8b. The adsorption efficiency increased with increasing pH values from 2.0 to 5.0, retained a high level at pH 5.0–6.0, and then decreased at higher pH values. This phenomenon was in accordance with the adsorption behavior of U(VI) on phosphate-functionalized graphene oxide,³⁸ which was dependent on the species of U(VI) (Fig. 8a) and the surface properties of PANI/H-TNB composites at different pH values.³⁹ As illustrated in Fig. 8a, U(VI) mainly existed as UO₂²⁺ at pH < 5, and then predominant species such as UO₂(OH)⁺, (UO₂)₄(OH)₇⁺, (UO₂)₃(OH)₅⁺ were formed at pH 5.0–7.0. Negatively charged species, such as UO₂(OH)₃⁻, UO₂(OH)₄²⁻ and (UO₂)₃(OH)₇⁻ appeared and dominated at pH > 7.0. As shown in Fig. 5, the isoelectric point of the PANI/H-TNB composite was calculated to be 5.8. Therefore, the electrostatic repulsion between the positively charged PANI/H-TNB surfaces and UO₂²⁺ led to the low adsorption efficiencies at pH < 5. As the pH value increased, the surface of PANI/H-TNB composites was gradually negatively charged, resulting in increased adsorption efficiencies. At pH = 5.0–6.0, the electrostatic attractions between the negatively charged PANI/H-TNB surfaces and positively charged U(VI) species dominated, leading to the highest adsorption efficiencies. At pH > 7.0, negatively charged U(VI) appeared, promoting the electron repulsion with the negatively charged PANI/H-TNB surfaces, resulting in decreased adsorption efficiencies.

Fig. 8 Effect of pH and ionic strength on U(VI) adsorption on PANI/H-TNB T = 293 K, m/V = 0.3 g L⁻¹, C₀ = 20 mg L⁻¹.

The effect of ionic strength was also measured at three different concentrations, i.e. 0.001, 0.01 and 0.1 M NaNO₃ solution, as shown in Fig. 8b, which indicated an independent ionic strength effect toward U(VI) adsorption within the measured pH values, suggesting a dominant inner-surface complexation mechanism rather than an outer-sphere surface complexation or ion exchange mechanism.^8,40

3.5. Effect of coexisting ions

Various components simultaneously exist in aqueous solutions. To research the influence of coexisting ions on U(VI) adsorption, the adsorption of U(VI) on the PANI/H-TNB composites was carried out in 0.01 mol L⁻¹ solutions, such as NaClO₄, NaNO₃, NaCl, Na₂SO₄, KNO₃, Ca(NO₃)₂ and Mg(NO₃)₂, as a function of pH values. The effect of the cationic ions was illustrated in Fig. 9a by comparing four cationic solutions, i.e., NaNO₃, KNO₃, Ca(NO₃)₂ and Mg(NO₃)₂. No obvious adsorption effects were observed. Positively charged cations may alter the surface property of the PANI/H-TNB composites. However, one can see herein that the adsorption of U(VI) on the PANI/H-TNB composites was not influenced obviously in the presence of three different cations, indicating that the uptake of U(VI) was mainly controlled by inner-sphere surface complexation.

Fig. 9 Effect of coexisting electrolyte cations (a) and anions (b) on U(VI) adsorption to PANI/H-TNB, T = 293 K, m/V = 0.3 g L⁻¹, C₀ = 20 mg L⁻¹.

Meanwhile, the anionic ions effects are shown in Fig. 9b, obtained by measuring in four different solutions, i.e., NaCl, NaNO₃, Na₂SO₄ and NaClO₄. An obvious anionic ions effect was observed. The highest adsorption capacity was obtained in 0.01 M NaClO₄ solution, as compared with NaCl, NaNO₃ and Na₂SO₄ solutions, which was consistent with previous studies.^8,41 This phenomenon indicated that the negatively charged anions with high chemical affinity and strong complexation ability towards both metal ions and solid surfaces, could obviously influence the mobility of the metal ions in aqueous solutions. The Cl⁻, NO₃⁻ and SO₄²⁻ ions could form soluble complexes with U(VI) (e.g., UO₂Cl⁺, UO₂NO₃⁺, UO₂(SO₄)₂²⁻ species) whereas the ClO₄⁻ could not form stable complexes, hence leading to the decrease of U(VI) adsorption on PANI/H-TNB composites.

3.6. Effect of soil humic/fulvic acid

The influence of HA/FA on U(VI) adsorption onto PANI/H-TNB composites as a function of pH is shown in Fig. 10. Slightly positive effects were observed for both HA and FA at pH < 6.0, while negative effects were found at pH > 6.0. At low pH values, the negatively charged HA/FA could be readily adsorbed on the positively charged surfaces of PANI/H-TNB composites. This adsorption will partially neutralize the positively charged PANI/H-TNB surfaces, promoting the adsorption of positively charged U(VI) species. Thus, positive effects were obtained. However, at high pH values, the PANI/H-TNB surfaces were getting negatively charged, which made it difficult to adsorb the negatively charged HA/FA. On the contrary, the free HA/FA molecules will interacted with U(VI) to form complexes of HA/FA–U(VI) in aqueous solution, resulting in an overall decrease in U(VI) adsorption. It was very interesting to note that the influence of FA on U(VI) adsorption to PANI/H-TNB was similar to that of U(VI) adsorption in the presence of HA. Both FA and HA have a macromolecular structure^42,43 and could be described as aggregates of aromatic molecules (mostly phenolic) carrying a large number of functional groups. The samples of HA and FA were obtained from the same material and they possessed similar functional groups such as carboxyl and phenolic groups. These similar functional groups might explain the resemblance of the adsorption curve of U(VI) on PANI/H-TNB in the presence of HA/FA.

Fig. 10 Effect of HA/FA on the adsorption of U(VI) on PANI/H-TNB as a function of pH, m/V = 0.3 g L⁻¹, C₀ = 20 mg L⁻¹, I = 0.01 mol L⁻¹ NaNO₃, T = 293 K.

3.7. Adsorption isotherms

The adsorption isotherms of U(VI) on PANI/H-TNB, H-TNB and PANI were displayed in Fig. 11. Higher adsorption capacities were observed for PANI/H-TNB composites than that of PANI and H-TNB due to the more active binding sites on PANI/H-TNB surface. It could also be seen that the uptake of U(VI) on PANI/H-TNB composites increased with increasing solution concentrations. In order to quantify the adsorption data and understand the adsorption mechanism better, the experimental data were simulated by the Langmuir⁴⁴ and Freundlich models.⁴⁵

Fig. 11 Adsorption isotherms of U(VI) ions on PANI/H-TNB, H-TNB and PANI. m/V = 0.3 g L⁻¹, pH = 5.0 ± 0.1, I = 0.01 mol L⁻¹ NaNO₃. Symbols denote experimental data, the solid lines represent Langmuir model simulation, and the dotted lines represent the Freundlich model.

The Langmuir model is a theoretical model for the monolayer adsorption process. Its form can be described as follows

and it can be converted into a linear form

where q_max (mg g⁻¹) is the maximum capacity of U(VI) adsorbed on PANI/H-TNB and L (L mg⁻¹) is the Langmuir adsorption isotherm models constant.

The Freundlich isotherm model allowed for several types of adsorption on the solid and fitted the adsorption onto heterogeneity surface. This model can be defined as follows

and it can also be converted into a linear form as

k_f (mg¹⁻ⁿ Lⁿ g⁻¹) is the Freundlich adsorption coefficient related to the adsorption capacity, and n represents the degree of dependence of adsorption with equilibrium concentration.

The fitted linear lines with the Langmuir and Freundlich models were plotted in Fig. 12a and b, respectively. The relative parameters calculated from the Langmuir and Freundlich models are displayed in Table 2. A comparison of the correlation coefficients (R²) between the Langmuir and Freundlich models clearly indicated that the Langmuir model was better than the Freundlich model, indicating a monolayer adsorption process. The q_max value calculated from the Langmuir model was 216.82 mg g⁻¹ at 293 K, which is higher than that of other adsorbents, as listed in Table 3, indicating a potential promising adsorbent of PANI/H-TNB in radioactive wastewater treatment.

Fig. 12 Langmuir (a) and Freundlich (b) simulation for the adsorption isotherms of U(VI) on PANI/H-TNB at three different temperatures, m/V = 0.3 g L⁻¹, pH = 5.0 ± 0.1, I = 0.01 mol L⁻¹ NaNO₃.

Table 2 The parameters of Langmuir and Freundlich models simulations, and the adsorption isotherms of U(VI) on PANI/H-TNB composites

Sorbents	Langmuir				Freundlich
	T (K)	L (L mg^−1)	qmax (mg g^−1)	R^2	n	kf (mg^1−n L^n g^−1)	R^2
PANI	293	0.02	48.75	0.997	0.63	1.95	0.980
H-TNB	293	0.03	116.73	0.992	0.56	8.03	0.956
PANI/H-TNB	293	0.13	216.82	0.985	0.35	49.61	0.964
	313	0.29	243.77	0.996	0.29	80.25	0.915
	333	0.80	263.02	0.982	0.24	116.79	0.903

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Table 3 Comparison of the maximum adsorption capacity of U(VI) with other adsorbents

Adsorbent	Experiment condition	Adsorption capacity (mg g^−1)	Reference
PANI–CMK-3	T = 298 K, pH = 7.0	118.30	46
PANI@GO	T = 298 K, pH = 3.0	245.14	29
Amidoxime modified Fe3O4@SiO2	T = 298 K, pH = 5.0	105.00	8
Amidoxime modified bentonite	T = 298 K, pH = 4.0	33.30	47
Fe3O4@agarose microsphere	T = 293 K, pH = 5.2	273.94	48
Fe3O4@TiO2	T = 298 K, pH = 6.0	91.10	49
PANI/H-TNB	T = 293 K, pH = 5.0	216.82	In this work

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3.8. Thermodynamic parameters

The adsorption amount of the PANI/H-TNB composites increased with increasing temperatures, as shown in Fig. 11, indicating an endothermic adsorption process. The thermodynamic parameters (ΔG⁰, ΔS⁰ and ΔH⁰) can be determined from the temperature-dependent adsorption isotherms. The values of thermodynamic parameters are obtained from the following equations:

ΔG⁰ = ΔH⁰ − TΔS⁰ (9)

where C_e (mg L⁻¹) is the equilibrium concentration, R (8.314 J mol⁻¹ K⁻¹) is the universal constant and q_e (mg g⁻¹) is equilibrium adsorption capacity of U(VI).

The values of average standard enthalpy change (ΔH⁰) and standard entropy change (ΔS⁰) were derived from the slope and y-intercept of the plots of ln k₀ versus 1/T (Fig. 13). The relative parameters calculated from the thermodynamic equations were illustrated in Table 4. The positive values of ΔH⁰ confirmed an endothermic adsorption process, which can be explained by the competitive dissolution of U(VI) in water. The hydration sheaths of the U(VI) were supposed to be destroyed before adsorption on the PANI/H-TNB surfaces, and the energy required for this dehydration process exceeded the exothermicity of the ions attached to the surface, resulting in an endothermic adsorption process. Thus a higher temperature was favored. The negative ΔG⁰ value suggested a spontaneous adsorption process. The value of ΔG⁰ becoming more negative with the increasing temperature indicated that the adsorption process was more favorable at a higher temperature. The positive value of ΔS⁰ indicated the high chemical affinity of PANI/H-TNB towards U(VI) ions as well as some structure changes on the adsorbents.⁵⁰

Fig. 13 Plots of ln k₀ versus 1/T for U(VI) adsorption on PANI/H-TNB. m/V = 0.3 g L⁻¹, pH = 5.0 ± 0.1, I = 0.01 mol L⁻¹ NaNO₃.

Table 4 Thermodynamic parameters for U(VI) adsorption on PANI/H-TNB composites

C0 (mg L^−1)	ΔH^0 (kJ mol^−1)	ΔS^0 (J mol^−1 K^−1)	ΔG^0 (kJ mol^−1)
			293 K	313 K	333 K
20	30.77	134.75	−8.71	−11.41	−14.10
40	44.34	172.21	−6.09	−9.56	−13.01
50	47.19	178.56	−5.13	−8.70	−12.27

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3.9. Regeneration and reversibility

Taking into consideration minimizing wastewater treatment cost and adsorbent secondary pollution, the reusability of PANI/H-TNB composites was also studied as a crucial factor for its potential application in the removal and recovery of U(VI). Uranium(VI)-loaded adsorbent was rinsed with 0.5 mol L⁻¹ HCl, followed by deionized water, and dried at 60 °C for reuse. Fig. 14 illustrated adsorption capacities after each recycle. Specifically, the adsorption capacity decreased from 65.15 mg g⁻¹ to 56.41 mg g⁻¹ after 6 cycles. The excellent regeneration capacity indicated that the PANI/H-TNB composites had a potential application prospect as an effective absorbent for removal of U(VI) from large volumes of aqueous solutions.

Fig. 14 Recycling of PANI/H-TNB for the removal of U(VI). C₀ = 20 mg L⁻¹, pH = 5.0 ± 0.1, m/V = 0.3 g L⁻¹, I = 0.01 mol L⁻¹ NaNO₃ and T = 293 K.

4. Conclusions

In this work, batch experiments were employed to study the adsorption of U(VI) onto PANI/H-TNB composites from aqueous solutions as a function of various experimental conditions such as contact time, solid content, pH value, foreign ions, and temperature. The adsorption process was strongly dependent on pH values, but ionic strength independent, suggesting an inner-sphere surface complexion mechanism. The adsorption process was observed to follow pseudo-second order kinetics and the adsorption isotherms could be fitted well by the Langmuir model. Thermodynamic data revealed a spontaneous and endothermic adsorption process. Moreover, the sorption capacity remained at a high level even after 6 cycles. The results in this study indicated that the PANI/H-TNB composites could be used as a suitable absorbent for the removal of uranium(VI) from contaminated wastewater.

Acknowledgements

The authors acknowledge the financial support from Special Scientific Research Fund of Public Welfare Profession of China (201509074) and the National Natural Science Foundation of China (21575079).

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