Isothermal, kinetic and thermodynamic studies for solid-phase extraction of uranium (VI) via hydrazine-impregnated carbon-based material as efficient adsorbent

NUCLEAR CHEMISTRY, RADIOCHEMISTRY, RADIOPHARMACEUTICALS, NUCLEAR MEDICINE

Isothermal, kinetic and thermodynamic studies for solid-phase extraction of uranium (VI) via hydrazine-impregnated carbon-based material as efficient adsorbent

A. Morsy，

M.H. Taha，

Muhammad Saeed，

Amir Waseem，

Muhammad Asad Riaz，

M. M. Elmaadawy

Nuclear Science and Techniques

Vol.30, No.11

Article number 167

Published in print 01 Nov 2019

Available online 24 Oct 2019

DOI：10.1007/s41365-019-0686-z

32300

The current study describes the application of a new extraction method for efficient uranium adsorption via cost-effective hydrazine-impregnated activated carbon. Various experimental parameters such as time, adsorbent weight, temperature (^оC), and uranium concentration were thoroughly investigated. The synthesized adsorbent was characterized via X-ray diffraction (XRD), Fourier transformation infrared spectroscopy (FT-IR), Scanning electron microscopy (SEM) and Thermogravimetric analysis (TGA). The results showed 86% uranium extraction under optimized conditions (20% P₂O₅ at 25 °C, 120 min). The obtained findings fit well with thermodynamic and isothermal (Langmuir and Freundlich isotherms) models and pseudo second-order kinetics. In thermodynamic studies, the negative sign of ( $� G °)$ specified the spontaneity of process, the negative sign of $(� H °)$ revealed endothermicity and the positive sign of $(� S °)$ showed high randomness after adsorption.

UraniumAdsorptionPhosphoric acidHydrazineCarbon

1 Introduction

The demand for water-soluble fertilizers has gradually increased because of their purification and soluble capacity in water, which make them superior fertilizers that work as an injection of chemicals into an irrigation system. A precise and uncontaminated form of phosphoric acid, which is used as basic material to produce water-soluble chemical substances and many other precise forms of phosphates [1] is needed. The purity of phosphoric acid also terminates the movement of heavy and toxic metals, such as arsenic, cadmium [2], lead, and chromium into table water substances [3]. All radioactive compounds are lethal for human beings and harmful for natural equilibrium sources because of their emissive and toxic nature [4]. Furthermore, high levels of radioactive elements, such as uranium, in farming irrigation channels and waste water [5] can cause major health problems. Radioactive substances, such as uranium (VI) ions, have a poisonous nature, even at minimum concentrations and has been identified as a major health issue [6]. Thus, more studies are necessary to eliminate the present levels of uranium from waste water. To extract radioactive elements from phosphoric acid, a wet-process technique is an imperative approach. The available concentrations of radioactive elements and ions change within a particular location and in mineral fields (from 20 to 200 ppm).Taking into account the entire quantity of phosphate rock and uranium from these reserves is an optional mode to extract uranium from uranium mud [7]. Many processes have been examined to eliminate uranium from we-process phosphoric acid e.g., precipitation [8], ion-exchange [9], membrane separation [10], solid-liquid extraction [11], and liquid-liquid extraction [12]. The adsorption technique is one of the best techniques serving as a proficient, measured, and viable adsorbent process [13]. The adsorption method reveals that it is an elimination method and also indicates the enhancement and determined amount of radioactive substances [14]. There have been many studies on the adsorption of uranium on solids, such as,-activated carbon [15], Montmorillonite [16], kaolinite [17], zeolite [18], titanium nanotubes [19], hydrous titanium oxide [20], gibbsite [21], zirconium titanium oxide [22], graphene oxide based zeolitic Imidazolate framework [23], carbon framework composites [24], 3D layered doubled hydroxides (LDH) / graphene hybrid materials [25], and anion-exchanged resins (mainly for uranyl carbonate complexes)[26-28]. The bonding between nitrogen-oxygen donor ligands is an appealing part of the study with regard to coordination chemistry [29, 30]. The complex compound of substituted hydrazine-like hydrazides, hydrazones, and di-acyl-hydrazines are all considered compelling samples for bio-inorganic methods [31]. Much work has been conducted on complex keto-form hydrazides;-however, insufficient research has been performed on the de-protonated form. The keto-enol tautomerism phenomenon occurs in hydrazide compounds and also behave as like mono-negative bi-dentate or mono-negative tri-dentate [32]. Hydrazide ligand surrounds the central metal to promote the definite significant behavior of a specific metal atom, while, hydrazine ligand exhibits diverse behavior, yielding different types of large compounds with certain charged species of metal. Hydrazine molecules donate the single-electron pair and two-electron pairs to the central metal atom, for the formation of a complex linkage that has been the recognized mode in the literature [33], and the decaying behavior of hydrazides, even at minimum amount of heat, converts them into metal oxides. A wide-ranging applications of hydrazine compounds along with various charged species, is still under study [34]. Complex compounds, such as hydrazides, have many possible reactive atoms, such as, C=O, N-H, and NH₂. As a result, the synthesis and characterization of various large compounds have been carried out based on the shifting of electrons in an electric and or magnetic field [35-37]. There are many reported large compounds of hydrazine that show diverse behavior, such as metals and non-metals, when combined with many types of carboxylic acids [38-41]. Furthermore, better and more advanced quality semiconductors can be synthesized by the doping of hydrazide to remove polarity in solid devices. The entire process has been done with the assistance of the coordinated compound of zinc[18]. Application of these complexes can provide extraordinary elasticity in products. Hydrazides also present various chemical reactions [42]. The compounds of nitrate/azide/ perchlorates show variable behavior, and various suggested uses have been documented [43]. The synthesis of complexes such as nickel hydrazinium nitrate (NiHN) as a reactive compound has been newly published [44], and the possible applications of Ni-NH, which is a source of energy production, has been analyzed.

Based on the above information, the main aim of this study is the introduction of a new and cost-effective hydrazine-impregnated activated carbon that provides promising results for uranium adsorption from phosphoric acid solution. The newly prepared adsorbent was characterized by XRD, SEM, FT-IR, and TGA to identify the formation. Isothermal and kinetic measurements were also studied. Meanwhile, temperature-varying measurements were provided to explain through some temperature ranging graphic measurements.

2 Materials and Procedure

For this study, all analytical grades (high purity) reagents used were manufactured by Aldrich AG, Co., except hydrazine, which was supplied by Adwic, Co., charcoal collected from LOBA-Chemie. Co., and commercial phosphoric acid by di-hydrate process as collected from Abo Zaabal-Co., The chemical compositions are given in Table 1.The concentration of uranium was obtained by UV-VIS spectrophotometry [45].

Evaluation of composition in commercial phosphoric acid

Component	Concentration
P2O5	20%
CaO	0.10%
SiO2	0.66%
Fe2O3	1.40%
Cd	4 ppm
Co	3 ppm
Zn	102 ppm
U	50 ppm

3 Methods

3.1 Pre-treatment and solid phase preparation

For the extraction process (green acid), charcoal was first cooled at temperature (25-30 $℃)$ and then impregnated with clay particles for the removal of suspended solid. To eliminate solubilized organic material, it was treated with activated carbon (granular) and oxidized with H₂O₂. Hydrazine (0.5 mL) was impregnated into (1 g) of activated carbon using the previously reported method [46]. The impregnated solution (1000mL) was treated with charcoal (100 g) with constant stirring for 24 h and the prepared slurry was dried at 45 °C for 8 h. For uranium adsorption, modified charcoal (0.2 g) was mixed with 20 mL of di-hydrated phosphoric acid (commercial) at room temperature in a thermo stated shaker. The obtained values were used for further isothermal, kinetic and thermodynamical studies (Fig. 1).

Fig.1

Hypothetical diagram of uranium adsorption by Hydrazine-modified adsorbent

For elution of uranium, various reagents (H₂SO₄, acidified NaCl, and Na₂CO₃) were used; however, H₂SO₄ showed the best results because it tended to protonate the surface and led to the desorption of uranium by lowering the pH. Regarding the elution of uranium, 90% was eluted by using 0.1-M H₂SO₄ and reacted with 0.5 g of adsorbed material for 30 min at temperature (25-30 $℃)$ .

4 Mechanism of Adsorption

The proposed mechanism for uranium adsorption by using hydrazine-impregnated carbon was the interaction mechanism. The NH₂ group of hydrazine acts as a ligand toward uranium after bonding hydrazine with carboxyl group of activated carbon. This study confirms the adsorption of uranium (VI) from phosphoric acid solution through the interaction mechanism, as shown in Fig. 2.

Fig. 2

Suggested mechanism of interaction of U (VI) by hydrazine-impregnated activated carbon

4.1 Characterization Methods

FT-IR is a modern analytical technique that was used to analyze the produced vibrational bands in unmodified charcoal and hydrazine-impregnated charcoal. FT-IR data were recorded by using ATI MATTSON GENESIS Series-Model-960-M009in the range of (4000 cm^-1 to 400 cm^-1) (with a resolution of 1 cm^-1 and 15 scan) to obtain insight regarding the functional as well as fingerprint regions. The SEM, XRD, and TGA observations of unmodified and modified charcoal with hydrazine were analyzed by using Microscopy Model-JEM-2100 (JEOL, Japan), powder XRD method and modern Thermogram, respectively.

5 Result and Discussion

5.1 Powder XRD-analysis

Powder XRD pattern of unmodified and hydrazine-impregnated activated carbon is shown in Fig. 3. The pattern for uranium-adsorbed modified charcoal is compared with reference (JCPDS No. 00-006-0024). All the peaks are the same in unmodified and modified XRD spectra except the peak appearing at 2 $θ = 6.98$ with a basal spacing of 1.28 nm, which is shifted toward a lower 2 $θ$ value with an increase in basal spacing to 1.75 nm, owing to hydrazine impregnation.

Fig. 3

Powder XRD pattern of unmodified and hydrazine-impregnated activated carbon.

5.2 FT-IR Analysis

FT-IR analysis of unmodified charcoal, hydrazine-impregnated charcoal, and uranium-adsorbed modified charcoal are shown in Fig. 4a, b, respectively. In the FT-IR spectra of hydrazine-modified charcoal (Fig. 4a) the bands appear at 1594 cm^–1, corresponding to the N-H bending vibration. The bands appearing at 3450 cm^-1 shows the presence of the –OH stretch of water present in the interlayers, and the peak observed at 2830 cm^-1 is due to the sp³ –C-H stretch [47]. In the FT-IR spectra of uranium-adsorbed modified charcoal (Fig. 4b), the peak observed at 3458 cm^-1 is less intense than that of unmodified charcoal(Fig.4a),corresponding to the O–H stretch of water present in interlayers. In addition, the band appearing at 1594 cm^–1, belonging to the associated amide is shifted to 1581 cm^–1, and its intensity is also decreased. The peak observed in the region of 1500–1300 cm^-1 is due to bending vibrations, and the C-O stretch gives a peak at 1077 cm^-1. The peak appearing at 800–700 cm^-1 corresponds to the C-H in-plane bending vibration [48, 49]. All these peaks give confirmation to the formation of hydrazine-impregnated charcoal and adsorbed uranium-modified charcoal.

Fig. 4

FT-IR spectrum of unmodified charcoal (5), hydrazine-impregnated charcoal (a), and adsorbed uranium-modified charcoal (b)

5.3 Scanning electron microscopy-micrographs (SEM)

Morphological characterization studies of unmodified, hydrazine-impregnated-modified charcoal, and adsorbed uranium-modified charcoal were performed by using SEM, which are shown in Fig. 5a, b. Figure 5 shows that the unmodified charcoal is not more porous or agglomerated. The image in Fig. 5a before adsorption illustrates that the particles are more agglomerated and porous. However, in Fig. 5b, surface modification is clearly seen after the deposition of uranium onto prepared adsorbent. The above discussion clearly indicates the formation of modified charcoal and adsorption.

Fig. 5

Scanning electron micrographs of samples: unmodified charcoal (5), impregnated charcoal (a), and impregnated charcoal after the adsorption process of uranium (b).

6 TGA

Thermogravimetric analysis (TGA) was studied by using the thermo-balance (TGA), heating rate (30 °C/min), and temperature range (40–1000 °C) in a helium environment to quantify the amount of mass loss (%) in unmodified and modified material, shown in Fig. 6a, b and Table 2, respectively. This study shows that in unmodified form the following changes occur: water evolution at below 150 °C and de-hydroxylation of –OH group of water between 400–600 °C. However, in modified form, the following transitions occur: organic compounds such as hydrazine evaporation at 110 °C, water evolution below 150 °C, -OH de-hydroxylation between 400–550 °C. Thus, TGA information also justifies the formation of modified adsorbent for efficient adsorption of uranium (VI).

TGA mass loss (%)

Sample	Water evolution (%)	De-hydroxylation of modified adsorbent (%)	Decomposition of organic substances (%)
Hydrazine-impregnated activated carbon	6.99%	6.21%	5.88%

Fig. 6

TGA and DTG thermographs of unmodified and hydrazine-impregnated adsorbent

7 Optimization

The amount of uranium adsorbed (q_e) by using hydrazine-impregnated charcoal was determined by measuring the difference between the initial concentration and equilibrium concentration, represented as Eq. (1):

q_{e} = (C_{o} - C_{e}) \times \frac{V}{m},

(1)

where C_e and C_o correspond to the equilibrium and initial concentration in mg/L respectively, m is the mass of adsorbent, and V corresponds to the volume of solution (L). The removal percentage efficiency and distribution coefficient (K_d) of concerned ions were determined by following Eq. (2) and (3), respectively.

Uranium adsorption (%) = \frac{C_{o} - C_{e}}{C_{o}} \times 100,

(2)

K_{d} = \frac{C_{o} - C_{e}}{C_{o}} \times \frac{V}{m} .

(3)

7.1 Effect of phosphoric acid concentration on uranium adsorption

To investigate the effect of phosphoric acid concentration, acid concentration was changed (20–40%) while maintaining the other parameters constant (time 2 h, modified adsorbent 0.20 g, solution 20 mL at 300 rpm, and room temperature). The decrease in adsorption efficiency with increase in the amount of solution concentration is shown in Fig. 7a. The observed data reveal that the adsorption of uranium ions decreases by increasing the concentration of phosphoric acid, achieving maximum efficiency for 0.20 g adsorbent, due to the availability of specific adsorbent sites for 20% solution [50].

Fig. 7

Effect of different parameters on adsorption efficiency: (a) phosphoric acid concentration, (b) time, (c) temperature, (d) adsorbent weight, and (e) uranium concentration

7.2 Effect of contact time on uranium adsorption

The adsorption efficiency (%) for the removal of uranium ions from P₂O₅ solution was investigated as a function of time by varying the time (5–480 min) and maintaining other parameters constant (P₂O₅: 20% (50 ppm); solution: 20 mL; adsorbent: 0.20g; 300 rpm; room temperature), as shown in Fig. 7b. The study of contact time reveals that the adsorption efficiency increases by increasing the time by 5–120 min; however, after that, the adsorption efficiency is constant, and no significant change is indicated in the plotted graph [51].

7.3 Temperature effect on uranium adsorption

The outcome of the temperature effect on uranium elimination from P₂O₅ solution was investigated by varying the temperature (25, 40, 50, 60, and 70 °C),while maintaining the other parameters constant (P₂O₅ 20% (50 ppm), solution 20 mL, adsorbent 0.20 g, and at 300 rpm), and observed data is plotted in Fig. 7c. Regarding the temperature effect on adsorption efficiency (%), an ineffective increase in adsorption of uranium by modified charcoal occurs at higher temperatures, indicating that the adsorption of U(VI) ions is an endothermic process and is also favored at room temperature.

7.4 Adsorbent weight effect on uranium adsorption

To investigate the important parameter (effect of modified adsorbent weight) and keep the other parameters constant (time 2 h, solution 20 mL at 300 rpm, and room temperature), the observed data were plotted in Fig. 7d, revealing that the adsorption efficiency increases by increasing the amount of hydrazine-impregnated modified charcoal (adsorbent) because of the availability of a larger surface area of modified adsorbent. By increasing the amount of adsorbent, more activated sites are available for adsorption[52]. However, the adsorption process is less when the weight of adsorbent material is decreased.

7.5 Uranium concentration effect on adsorption

By using hydrazine-impregnated modified charcoal, the influence of uranium ions was investigated by changing its concentration (50–300 ppm), keeping other parameters are fixed; results for 0.20 g of hydrazine-impregnated charcoal, a solution of 20 mL, and an optimum temperature of 25 °C are shown in Fig. 7e. The percentage efficiency of the removal of uranium from P₂O₅ solution is decreased by increasing the initial concentration of uranium ions in solution due to the greater mobility of uranium ions in solution, which have less interaction between adsorbent and concerned ions [53]. The maximum adsorption capacity (q_e = 5.5 mg/g) was achieved at 50 mg/L uranium solution, as indicated by present study.

7.6 Adsorption kinetic modeling

The kinetic activity for the adsorption of uranium (VI) ions using hydrazine-impregnated charcoal was analyzed from pseudo-first- and second-order intra-particle models, and the Elovich model [54], which are represented in Eq. (4)–(7), respectively.

\log (q_{e} - q_{t}) = \log q_{e} - \frac{K_{1}}{2.303} t,

(4)

\frac{t}{q_{t}} = \frac{1}{K_{2} q_{e}^{2}} + \frac{1}{q_{e}} t,

(5)

q_{t} = K_{F} t^{0.5},

(7)

q_{t} = α + β \ln t,

(6)

where q_e and q_t in (mg/g) correspond to the equilibrium adsorption capacity and at time (t). K₁ (min^-1) and K₂ (g/(mg min^-1) represent the rate constants of the pseudo-first- and pseudo-second-order kinetic models, α and β correspond to the rate constant of the Elovich model, and K_F is the rate constant of intra-particle model.

Table 3 illustrates the observed values from the kinetic study. The correlation coefficient (R²) of the pseudo-second-order kinetic model (0.99) attributes the adsorption of uranium onto hydrazine-impregnated charcoal following chemisorption behavior, as shown in Fig. 8. The q_e (calculated) and q_e (experimental) values have a close resemblance. In addition, the rate-determining step follows the chemisorption behavior [55].

The determined parameters of kinetic models with linear regression coefficients (R²)

Pseudo-first order				Pseudo-second order			Elovich kinetic model			Morris-Weber model
q_(exp)(mg/g)	q_(calc)(mg/g)	K₁(min^-1)	R²	q_(exp)(mg/g)	K₂(g/mg min^-1)	R²	α (mg/min)	β (mg/g)	R²	K_F (min^-1)	R²
4.30	2.42	0.01	0.95	4.51	0.016	0.99	6.81	1.92	0.90	0.11	0.80

Fig. 8

Pseudo-second-order plot for uranium sorption from phosphoric acid onto hydrazine-impregnated activated carbon

7.7 Adsorption isotherms

For the selection of the adsorbent and equilibrium methods for adsorption, the isotherm models play a crucial role. These isotherm models are very significant for adsorption as they provide information regarding the nature of adsorption (homogeneous and heterogeneous)[56]. Langmuir (1918) and Freundlich (1906) models were applied to investigate the homogeneous and heterogeneous characteristic, respectively. The linear equations of the Langmuir isotherm and Freundlich isotherm models are shown below in Eq. (8) and (9), respectively:

\frac{C_{e}}{q_{e}} = \frac{1}{b q_{\max}} + \frac{C_{e}}{q_{\max}},

(8)

\ln q_{e} = \ln K_{F} + \frac{1}{n} \ln C_{e},

(9)

where q_e represents the adsorbed amount of uranium (mg/g), C_e represents the equilibrium concentration of uranium (mg/L), q_max and b correspond to the monolayer capacity of adsorption and the Langmuir constant, respectively, and K_F and n represents the Freundlich constants correlated with the adsorption capacity and intensity, respectively. Figure 9 and Table 4 show a linear plot of the Langmuir and Freundlich isotherms model and correlation coefficient (R²). It is observed for the collected data that the uranium adsorption by using hydrazine-impregnated charcoal followed Langmuir model more closely than the Freundlich isotherm model, which is indicative of the homogeneous nature of adsorption. The maximum adsorption capacity for the adsorption of uranium by using modified adsorbent was 5.913 mg/g, which indicates that a high amount of uranium was adsorbed, with improved results over the previous study. The value of n (3.61) is greater than 1, which is indicative of favorable adsorption of uranium on hydrazine-impregnated charcoal. The Langmuir model can also be expressed by using dimensionless parameter R_L, which is given in the following Eq. (9a).

Calculated data from isotherm models for adsorption of uranium (VI)

Adsorbent	Langmuir isotherm model						Freundlich isotherm model
	q_m (mg/g)	Exp.q_m(mg/g)	K_L	R²	R_L		n_F	K_F(mg/g)	R²
					C _o
Hydrazine-impregnated activated carbon	^5.91	^5.50	^0.07	^0.99	50	0.23	3.61	1.366	0.94
					100	0.13
					300	0.05

Fig. 9

Langmuir adsorption isotherms for uranium (VI)

R_{L} = \frac{1}{1 + K_{L} \cdot C_{o}}

(9a)

The R_L constant points out the favorability of uranium adsorption onto modified material in which C_o corresponds to the initial concentration of uranium. The R_L assessment reveals the nature of uranium adsorption as either unfavorable (R_L> 1), favorable (0 < R_L < 1), or irreversible (R_L = 0). The R_L values for uranium adsorption are observed to be in the range of 0.05–0.23, which is lower than 1, showing favorable adsorption.

7.8 Adsorption thermodynamic studies

To investigate spontaneity (∆G^o), enthalpy (∆H^o), and entropy (∆S^o), thermodynamical parameters were applied on the adsorption of uranium at equilibrium by changing the temperature. Regarding the thermodynamic study, the distribution coefficient is correlated with the change in enthalpy and entropy at equilibrium and temperature as (Eq. (10)):

\log K_{d} = - \frac{Δ S °}{R} - \frac{Δ H °}{R T} .

(10)

where log K_d correspond the distribution coefficient (cm³/g), ∆S^o and ∆H^o indicate the standard entropy and enthalpy, T is the temperature (K), and R represents the gas constant in (kJ/mol∙ K). The Gibbs free energy (∆G^o) was determined by using standard Eq.(11) as given

Δ G ° = Δ H ° - T Δ S ° .

(11)

The calculated parameters from thermodynamic study are listed in Table 5. It is observed that the uranium adsorption onto hydrazine-impregnated charcoal is an endothermic process due to the positive values of ∆H^o. In addition, the calculated value of ∆G^o is negative, which corresponds to the spontaneous adsorption process, and a more positive value of ∆S^o (standard entropy) indicates the randomness behavior of adsorbent after adsorption of uranium.

Calculated data from thermodynamic study for Uranium (VI) adsorption

∆G^o(kJ/mol)					∆H ^o (kJ/mol)	∆S ^o (J/(mol∙K))
T=25℃	T=40℃	T=50℃	T=60℃	T=70℃
-11.03±0.2	-11.67±0.2	-12.11±0.2	-12.55±0.3	-12.95±0.4	1.72±0.11	42.82±0.24

8 Conclusion

A new extraction and cost-effective method has been developed via hydrazine-impregnated activated carbon for studying uranium adsorption from phosphoric acid solution. The removal mechanism of uranium by modified activated carbon was suggested owing to the interaction with NH₂ group, which acts as a ligand toward uranium adsorption. The results of the present study showed promising results (86% uranium) under optimized operating conditions using 20% P₂O₅ solution at 25 $℃$ for 2 h. The current study showed the direct dependence of adsorption efficiency with time, temperature and mass of adsorbent and is indirectly related to the concentrations of phosphoric acid and uranium. Adsorption kinetics follows a pseudo-second-order-kinetic model (rate constant 0.016 g/mg.min^-1), which demonstrates the behavior of chemisorption. The isothermal results follow the Langmuir model more so than the Freundlich model, with a maximum uranium adsorption capacity of 5.913 (mg/g), predicting homogeneity in the system. However, the present data fit very well with the isothermal and thermodynamic studies.

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