Sorption Characteristics and Separation of Rhenium Ions from

The preparation of a standard stock solution of ReVII (1.000 mg·mL–1) was as follows: 1.000 ..... The first linear portion included the sorption pe...
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Sorption Characteristics and Separation of Rhenium Ions from Aqueous Solutions Using Modified Nano-Al2O3 Lei Zhang,* Xiao Q. Jiang, Tian C. Xu, Li J. Yang, Yun Y. Zhang, and Hong J. Jin College of Chemistry, Liaoning University, Shenyang 110036, People's Republic of China S Supporting Information *

ABSTRACT: A novel adsorbent was prepared by NH4HCO3-modifying nanoalumina dioxide and was employed for the separation/preconcentration of ReVII ions from aqueous solution. It was found that ReVII ions could be adsorbed quantitatively (above 94%) on modified nano-Al2O3 in the pH range of 2.0−3.0, while only 8.3% of ReVII ions were adsorbed on unmodified nano-Al2O3. Effects of the pH, concentration of elution solution, and interfering ions on the recovery of ReVII were systematically investigated. Adsorption kinetics for ReVII was found to be very fast, and equilibrium was reached within 5 min following the pseudo-second-order model with observed rate constants (k2) of 14.44 g·mg−1·min−1 at 298 K. The overall rate process appeared to be influenced by both external mass transfer and intraparticle diffusion. The sorption data could be well interpreted by the Langmuir model with a maximum adsorption capacity of 1.94 mg·g−1 of ReVII on modified nano-Al2O3. Moreover, the thermodynamic parameters showed the spontaneous and endothermic nature of the adsorption process. Finally, modified nanoAl2O3 as the sorbent was successfully applied to the separation of ReVII from the ore samples with satisfactory results. separation of ReVII were optimized. The aim of this work was not only to understand the interaction between ReVII and modified nano-Al2O3 but also to develop a sensitive, rapid, and simple method for the application of modified nano-Al2O3 on separating trace amounts of rhenium.

1. INTRODUCTION Rhenium, which is a less common metal with special properties, was widely used in the petrochemical industry, aviation, medicine, metallurgy, etc. However, rhenium occurs mainly accompanying molybdenite and is usually concentrated into fumes during calcinations, so the recovery of rhenium from molybdenite calcine is of great environmental and economic significance because of its higher value than that of molybdenum. The similar chemical behaviors, small amounts of rhenium, and large amounts of molybdenum in aqueous solutions make the separation of rhenium from molybdenum quite difficult.1 A solvent extraction2 method is used more widely for the separation of rhenium from arsenic, tungsten, or molybdenum, compared with other methods such as precipitation, chlorination, adsorption, and ion exchange.3−6 In the rhenium extraction process, tri-n-octylamine, diisododecylamine,7 pyridine, Aliquat, tributyl phosphate, trioctylphosphine oxide, cyclohexane, ethyl xanthate, and mesityl oxide are frequently used.8,9 So, the extraction method is dangerous because of the use of a flammable organic phase, and it is slow and cannot be processed at high temperature because the solvents are generally volatile, which shows its disadvantages. So, a rapid, simple, and safe method for the separation of rhenium, such as adsorption, is needed. However, literature studies on the adsorption efficiency of nanoparticles for rhenium are very limited. Our experiment found that rhenium ions could not be selectively adsorbed from aqueous solution by some nanosorbents. So, the modification of adsorbents, which can improve the selectivity of nanoparticles, is usually required. In this paper, nano-Al2O3 as the sorbent was modified for the adsorption and separation of trace amounts of rhenium from samples, and the adsorption kinetics, isotherms, and thermodynamics of ReVII on modified nano-Al2O3 were systematically studied. The conditions for the maximum adsorption and © 2012 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Apparatus. A Cary 5000 UV−vis−near-IR spectrophotometer (Varian) was used for the determination of ReVII ions. An Agilent 7500c inductively coupled plasma mass spectrometer (Agilent) was used for to determine low concentrations of ReVII, MoVI, and WoVI ions. A Mettler Toledo 320-S pH meter Instrument (Shanghai Co. Ltd.) was used for pH measurements. A KQ-100 Controllable SerialUltrasonics apparatus (Kunshan Apparatus Company, China) was applied to disperse nano-Al2O3 in solution, operating at an ultrasonic frequency of 20−80 kHz and an output power between 0 and 50 W through manual adjustment. A model TDL80-2B centrifugal machine (Shanghai Anting Scientific Instrument Co., China) was used throughout. Fourier transform (FTIR) spectra of sorbents were measured using FTIR 5700 (Nicolet Co., USA). The sorbent powder X-ray diffraction (XRD) patterns were recorded on a Siemens D5000 diffractometer. Surface analysis was performed by an X-ray photoelectron microscope employing an ESCALAB250 surface analysis system (Thermo VG Co., Ltd.). The X-ray photoelectron microscope was equipped with a monochromatic Al Kα X-ray source generated at 15 kV and 10 mA; the pressure in the measurement chamber was about 5.5 × 10−8 mbar. Measurements of the Brunauer−Emmett−Teller (BET) surface Received: Revised: Accepted: Published: 5577

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thermodynamic parameters for the adsorption process were determined at each temperature. Kinetic experiments were performed using a series of 50 mL flasks containing 200 mg of modified nano-Al2O3 and 10.0 mL of a 20.0 mg·L−1 ReVII solution at pH 2.5 in a temperature range of 273−323 K. At certain intervals of reaction time, suitable aliquots were taken, and then the ReVII concentration was determined. The rate constants were calculated using a conventional rate expression. 2.4.3. Sample Preparation. Portions (1.000 g) of the certified reference material (GBW07285 molybdenum ore) were decomposed with aqua regia (30 mL), heated until the solution became transparent, and then continuously heated to 1 mL. After cooling to room temperature, 2 mL of a 9 mol·L−1 H2SO4 solution was added, and the samples were heated to emit white smoke for the removal of nitrogen oxide, finally diluted to 100 mL with water, and adjusted to pH 2.5.

area and pore distribution were performed using nitrogen adsorption−desorption isotherms on a Micromeritics (Norcross, GA). A Malvern Zetasizer Nano-ZS particle analyzer (Malvern, U.K.) was used to determine the ζ potential of the sorbents. 2.2. Materials and Reagents. Nano-γ-Al2O3, nano-SiO2, nano-TiO2, and nano-ZnO were purchased from Zhou Shan Nanometer Material Co., China, and their particle sizes were about 20−50 nm. Micron-Al2O3 was purchased from Shanghai Xinzhong Chemical Reagent Co., China. The certified reference material molybdenum ore (GBW07285) was provided by the State Technology Supervision Administration (China). The preparation of a standard stock solution of ReVII (1.000 mg·mL−1) was as follows: 1.000 g of a pure rhenium powder (99.999%) was added into 10 mL of a concentrated peroxide solution and heated to dissolve it. Then 30 mL of hydrochloric acid was added, and the solution was heated to remove excess H2O2 and finally diluted to a 1 L volumetric flask with doubledistilled water. All reagents, including a stannous chloride solution (20%), thiourea (10%), ammonium bicarbonate, hydrochloric acid, and sodium hydroxide, were of analytical reagent grade and were purchased from Shanghai Xinzhong Chemical Reagent Co., China. Double-distilled water was used throughout the experiments. Stock solutions of the various metal ions (mg·mL−1) were prepared with their nitrate or chloride salts (≥99.99%) and used to investigate the effects of interfering ions. 2.3. Modification of Sorbents. Ammonium bicarbonate is used as a modifier. Certain proportions of ammonium bicarbonate and nano-Al2O3 were mixed uniformly, squashed by 20 tons of pressure, and then calcined at 500 °C for 3 h. In order to obtain a high adsorption efficiency, the influences of different mixed ratios of nano-Al2O3 to ammonium bicarbonate from 1:2 to 1:14 were also considered. Finally, 1:6 of the solid/ solid ratio was adopted in the experiment. 2.4. Procedure. 2.4.1. Analytical Method. High concentrations of rhenium, molybdenum, and tungsten were determined by colorimetry using thiourea, thiocyanate, and phenylfluorone, respectively.10−12 2.4.2. Adsorption Studies. Adsorption experiments were carried out using a series of 50 mL flasks containing 200 mg of modified nano-Al2O3 and 10.0 mL of a 20.0 mg·L−1 ReVII solution (pH 2.5). If necessary, the pH of the solutions was adjusted by adding a HCl or NaOH solution before the addition of modified nano-Al2O3. After ultrasonic dispersion for 3 min, the solid/liquid phases were separated by centrifugation at 4000 rpm for 3 min. The suspensions were immediately analyzed for determination of the ReVII concentration. The adsorption percentage (Ads %) was calculated based on eq 1: Ads % =

C I − Ca × 100 Ci

3. RESULTS AND DISCUSSION 3.1. Selection of Sorbents. Five kinds of sorbents were chosen to test their adsorbability of ReVII ions. In order to make their adsorption efficiency for ReVII ions as high as possible, the adsorption conditions (solution pH, sorbent dosage, etc.) of each sorbent were optimized. It could be seen from Table 1 Table 1. Effect of Different Nanomaterials on the Adsorption of ReVII Ions (m = 0.2 g; CRe = 20 mg·L−1) sorbent

ReVII/Ads %

nano-SiO2 nano-ZnO nano-TiO2 micron- Al2O3 nano-γ-Al2O3 modified nano-γ-Al2O3

3.27 2.13 3.54 3.79 8.32 94.6

that ReVII ions could not be effectively adsorbed by the original nanoparticle sorbents, so a highly effective sorbent with modification is needed. In order to quantitatively separate ReVII ions from the molybdenum ore sample, nano-Al2O3 was selected and modified. Modified nano-Al2O3 can quantitatively absorb ReVII ions; the adsorption percent of ReVII was greatly improved and exceeded 94% (Table 1). 3.2. Characterization of Sorbents. The FTIR spectra of unmodified and modified nano-Al2O3 are compared in Figure 1a. For modified nano-Al2O3, the hydroxide and Al−O−Al group peaks broaden obviously (3650−3200 and 880−550 nm), which means that the content of hydroxyl oxygen increases after modification. The FTIR spectrum indicated that many hydroxyl groups were introduced onto the surface of nano-Al2O3 in the alkali treatment process: hydroxyl groups, which can provide a large number of chemical adsorption sites, can increase the adsorption capacity of nano-Al2O3. X-ray photoelectron spectroscopy (XPS) measurement was performed to characterize the surface compositions of modified and unmodified Al2O3 (shown in Figure 1b). It is interesting to increase the oxygen content for modified nano-Al2O3 compared to unmodified nano-Al2O3. On the basis of the results obtained above, it is inferred that large amounts of OH groups were added into nano-Al2O3 by a modified method. The XRD patterns of unmodified and modified nano-Al 2 O 3 are investigated.

(1)

where Ci and Ca are the initial and final concentrations of ReVII in the solution phase, respectively. Adsorption isotherm studies were carried out with certain concentrations of ReVII, the amount of sorbent was kept constant (200 mg), and the experimental temperatures were controlled at 273, 298 and 323 K, respectively. The 5578

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Figure 1. (a) FTIR spectra of unmodified and modified nano-Al2O3. (b) XPS spectra for unmodified and modified nano-Al2O3. (c) XRD pattern of a nano-Al2O3 powder. (d) Pore-size distributions of modified nano-Al2O3. Inset image: BJH pore-size distribution curve.

Figure 2. (a) Effect of the pH on the adsorption efficiency of different concentrations of ReVII on modified nano-Al2O3 (m = 0.2 g). (b) Effect of the pH on the adsorption of ReVII, MoVI, and WoVI on modified nano-Al2O3 (m = 0.2 g; CRe = CMo = CWo = 10 mg·L−1).

desorption isotherms and Barrett−Joyner−Halenda (BJH) pore-size distribution curves (inset) of the sorbents, suggesting the existence of macroporous structures in the modified sorbent. After modification, the surface areas and pore volumes all increased. The pore-size distribution, calculated from the isotherm using the BJH model, indicates that a wide distribution of macropores appeared around 120 nm for modified nano-Al2O3, except for 10 nm for modified nano-

The diffraction peaks of the particles matched well with the diffraction data from the JCPDS card, and nearly no change of the structure of nano-Al2O3 was observed before and after modification in Figure 1c. A nitrogen adsorption−desorption study was performed to determine the specific surface area of the sorbents. The measured BET surface areas are 219.87 and 204.77 m2·g−1 for modified and unmodified nano-Al2O3, respectively. Figure 1d presents the nitrogen adsorption− 5579

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Al2O3, so modified nano-Al2O3 shows high adsorption efficiency and excellent selectivity for rhenium. 3.3. Effect of the Solution Acidity. The standard solutions with different concentrations (5−30 mg·L−1) of ReVII were applied to test the sorption behavior at different pH values. The result is shown in Figure 2a. When the concentration of ReVII was less than 20 mg·L−1, the adsorption percentage of ReVII was found to be above 90% in the pH range of 1.5−3.0. Figure 2b shows the effect of the pH on the adsorption (Ads %) of ReO4−, MoO4−, and WO4−. It can be found that ReO4− can be quantitatively adsorbed onto modified nano-Al2O3 within a pH range of 1.5−3.0 and also MoO4− and WO4− species within a pH range of 2.0−8.0. So, pH 2.5 was selected for the preconcentration of ReO4−, and pH 8.0 was selected for the separation of MoO4− and WO4−. The ζ potential was measured in 0.001 mol·L−1 NaCl solutions in order to observe the surface charges of the modified nano-Al2O3 suspensions. The point of zero charge (pzc) for modified nano-Al2O3 was obtained as 4.2 (pHpzc = 8.0−9.0 for unmodified nano-Al2O3). At pH < pHpzc, the modified nano-Al2O3 surface is positively charged, whereas at pH > pHpzc, the modified nano-Al2O3 surface is negatively charged:

Scheme 1. Formation of a Hydrogen-Bond Scheme of Modified Al2O3 toward ReO4−

of 10 mL of 20 mg·L−1 ReVII solutions (pH 2.5). It was found that the adsorption percentage increased with the increasing amount of modified nano-Al2O3, and when the amount exceeded 200.0 mg, the adsorption percentage reached 95% with no obvious change. Therefore, 200.0 mg of the sorbent was the optimum amount for the adsorption of ReVII. 3.5. Optimization of the Elution Conditions and Enrichment Factor. Figure 2a shows that the adsorption of ReVII obviously decreased when pH > 3.5. The elution experiment was carried out by adding various concentrations of NaOH to the solutions after adsorption. At the optimum concentration of the eluent (0.5 mol·L−1 NaOH), the effect of the eluent volume on desorption of the analytes was studied. It was found that the elution percentage was above 99% using 3.0 mL of 0.5 mol·L−1 NaOH. In order to study the effect of the sample volume on the recoveries of ReVII ions, ReVII was enriched from volumes of 10, 20, 30, and 50 mL of sample solutions containing 200 μg of ReVII by the procedure mentioned above. A quantitative recovery (≥95%) was still obtained for the sample volume of 30 mL for ReVII ions. In the experiment, 30 mL of the sample solution was adopted and the enrichment factor was 10 with 3.0 mL of 0.5 mol·L−1 NaOH elution. 3.6. Chemical-Reaction-Based Kinetic Modeling. It is important to be able to predict the rate at which the target analytes are removed from aqueous solutions in order to design a separation/preconcentration experiment. Figure 3 depicts variation of the adsorption amount with the adsorption time at

Al−OH(surf) + H+ → Al−OH 2+(surf), pH < pH pzc Al − OH(surf) + OH− → Al − O−(surf) + H 2O, pH > pH pzc

The possible reasons of ReVII adsorption on modified nanoAl2O3 are as follows: the electrostatic attraction may play an important role in the adsorption of ReVII on modified nanoAl2O3. For ReVII ions, in strong acid media, most of the ReVII ions exist as HReO4, and at higher pH values, more ReVII ions were in the form of ReO4−. In the pH range of 2.0−3.0, the surface of nano-Al2O3 carries positive charges, which will enhance its electrostatic attraction with ReO4−. As a result, the process of sorption takes place more easily, so the adsorption percentage of ReVII was higher. Adsorption of ReVII through electrostatic attraction (at pH 2.0−3.0): Al−OH 2+(surf) + ReO4 − ⇔ Al−OH 2+(surf)···ReO4 −

At pH < 2.0, there is a balance reaction: H+ + ReO4 − ⇔ HReO4

The main chemical species of ReVII is HReO4, so the adsorption percentage of ReVII was lower. At pH > pHpzc, the adsorption percentage of ReVII decreased, which may be mainly attributed to the formation of Al−O− and ReO4−, both of which are negatively charged and which repel each other. Besides electrostatic adsorption, the formation of hydrogen bonds may also contribute to the adsorption reaction occurring on the surface of modified nano-Al2O3. Scheme 1 shows the ReO4− species adsorbed onto the modified nano-Al2O3 surface through the formation of hydrogen bonds. 3.4. Optimal Amount of Sorbent. In order to attain the optimal amount of modified nano-Al2O3 for the adsorption of ReVII ions, the adsorption experiments were carried out by adding 50.0−500.0 mg of the modified nano-Al2O3 to a series

Figure 3. Curves of the adsorption capacity of ReVII on modified nanoAl2O3 versus time at different temperatures (200 mg of modified nanoAl2O3; CReVII = 20.0 mg·L−1; pH 2.5). 5580

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different temperatures. The obtained curves reflect that the adsorption can reach equilibrium in 5 min with increasing adsorption capacity. The ReVII adsorption kinetics was systematically studied by pseudo-first-order, pseudo-second-order, Weber−Morris, Boyd, and external mass-transfer models, which are applied to fit the experimental kinetic data. The kinetic constants estimated by the above-mentioned models were statistically significant at a 95% confidence level. 3.6.1. Pseudo-First-Order Model. The pseudo-first-order equation is given as eq 2:13 log(q1 − qt ) = log q1 −

k1t 2.303

(2)

where q1 and qt are the amounts of ReVII adsorbed on the sorbent (mg·g−1) at equilibrium and at time t, respectively, and k1 is the rate constant of the first-order adsorption (min−1). The values of k1 for ReVII adsorption on modified nano-Al2O3 were calculated from the plot of log(q1 − qt) against t. 3.6.2. Pseudo-Second-Order Model. The pseudo-secondorder model is represented as eq 3:13 t 1 t = + 2 qt q2 k 2q2

Figure 4. Intraparticle diffusion plots for the sorption of ReVII at three different temperatures (200 mg of modified nano-Al2O3; CReVII = 20.0 mg·L−1; pH 2.5).

3.6.4. Boyd Plot. Adsorption kinetic data were further analyzed by the Boyd model.20

(3)

Bt = −ln(1 − F ) − 0.4977

where F = qt/qe; qt and qe are the amounts of Re adsorbed on modified nano-Al2O3 (mg·g−1) at time t (min) and at equilibrium time (min), respectively: plots of Bt versus time t. However, the plots did not pass through the origin, confirming the involvement of external mass transfer in the entire adsorption process. This result again confirmed the ratecontrolling mechanism of adsorption stated in Weber−Morris kinetic model studies.21 The calculated B values were used to calculate the effective diffusion coefficients Di (cm2·s−1) using eq 6:

where k2 is the rate constant of the second-order adsorption (g·mg−1·min−1). The straight-line plots of t/qt versus t have been tested to obtain rate parameters. From Figure 3, it is clear that the adsorption of ReVII could be better described by the pseudo-second-order model based on the correlation coefficients. Furthermore, it was possible to calculate the activation energy (Ea) for the adsorption employing the Arrhenius equation based on the obtained rate constants. A value of Ea for ReVII adsorption on modified nano-Al2O3 was obtained as 7.52 kJ·mol−1. 3.6.3. Weber−Morris Model. In order to determine the actual rate-controlling step involved in the ReVII sorption process, regression analysis was carried out for the graph plotted between the amount of ReVII adsorbed (q) and the square root of time (t1/2), employing the Weber−Morris eq 4:

qt = Kdt 1/2 + I

(5) VII

B = π 2Di /r 2

(6)

where r is the particle radius. The average Di values were estimated at about 2.034 × 10−9, 2.599 × 10−9, and 3.357 × 10−9 cm2·s−1 at 273, 298, and 323 K, respectively. If film diffusion (external mass transfer of adsorbate across the liquid film to the adsorbent exterior surface) is to be the rate-controlling step, the value of Di should be in the range of 10−6−10−8 cm2·s−1, while if intraparticle diffusion is the ratelimiting step, the Di value should be in the range of 10−11− 10−13 cm2·s−1.22 However, in the experiment, the Di values were in neither of the two ranges but the range between them (10−8−10−11 cm2·s−1), so both film and intraparticle diffusion may play an important role in the given adsorption system. 3.6.5. External Mass-Transfer Model. In order to further confirm the involvement of external mass transfer, the external mass-transfer model, represented by eq 7,23 was applied to the first linear portion of the plots of the Weber−Morris model (Figure 4)

(4)

Kd is the intraparticle diffusion rate constant (mg·g−1·min−1/2). Values of I give an idea about the thickness of the boundary layer; i.e., the larger the intercept, the greater the boundary layer effect will be.14 Figure 4 shows qt versus t1/2 plots for the sorption of ReVII ions at different temperatures. Piecewise linear regression of the data showed that qt versus t1/2 plots had three distinct regions. The first linear portion included the sorption period of 0−60 s, which represents external mass transfer. The second linear portion included the sorption period of 60−130 s, representing intraparticle diffusion. The third linear portion included the time period of 130−270 s, which indicated adsorption−desorption equilibrium. However, the curves do not cross the origin (I ≠ 0 in all of the test conditions), suggesting that intraparticle diffusion was not the only ratecontrolling step15−18 and external mass transfer was simultaneously occurring. However, the ratio of the time taken by external mass transfer to intraparticle diffusion was about 1:1. So, the overall ReVII adsorption process was jointly controlled by external mass transfer and intraparticle diffusion.19

⎡ d(Ct /Ci) ⎤ ⎢ ⎥ = −kES ⎣ dt ⎦t = 0

(7)

where Ci and Ct respectively represent the concentration in the beginning and at time t (mg·L−1). The rate constant of the external mass-transfer model, kES (min−1), was calculated by the slope of the plot of Ct/Ci versus time (Figure 5). The results showed that ReVII adsorption data could be interpreted by the 5581

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where qm is the maximum monolayer adsorption (mg·g−1), Ce is the equilibrium concentration of ReVII, qe is the amount of ReVII adsorbed per unit weight of modified nano-Al2O3 at equilibrium concentration (mg·g−1), and b is the Langmuir constant related to the affinity of the binding sites (mg·L −1). Furthermore, the widely used empirical Freundlich equation based on sorption on a heterogeneous surface is given by eq 9: log qe = log KF +

1 log Ce n

(9)

where KF and n are Freundlich constants indicating the sorption capacity (mg·g−1) and intensity, respectively. The Langmuir and Freundlich isothermal constants were determined from the plots of Ce/qe versus Ce, log qe versus log Ce, respectively, at 273, 298, and 323 K. The obtained isothermal constants and correlation coefficients are presented in Table 2. The constants estimated by the Langmuir and Freundlich isotherm models were statistically significant at a 95% confidence level. It is found that the adsorption of ReVII on modified nano-Al2O3 correlated well (r > 0.999) with the Langmuir equation compared to the Freundlich equation (r > 0.980) in the studied concentration range. With a discussion of the shape of the isotherm, we can predict whether the studied adsorption system is favorable or unfavorable.24 RL, which is a dimensionless constant referred to as the separation factor or equilibrium parameter, can express the essential feature of the Langmuir isotherms and can be calculated using eq 10:

Figure 5. Plot of Ct/Ci versus time t for calculating the external masstransfer rate constant for ReVII sorption at different temperatures (200 mg of modified nano-Al2O3; CReVII = 20.0 mg·L−1; pH 2.5).

external mass-transfer model, which correlates well with the rate-controlling mechanism explained above. 3.7. Adsorption Isotherms. Figure 6 depicts the equilibrium adsorption amount of ReVII on modified nano-

RL =

1 1 + bC i

(10) −1

VII

where Ci is the initial Re concentration (mg·L ) and b is the Langmuir adsorption equilibrium constant (L·mg−1). The calculated RL values are also listed in Table 2. In the present investigation, the equilibrium parameter RL was found to be in the range from 0 to 1; hence, the sorption process was quite favorable, and the employed adsorbent exhibited a good potential for the sorption of ReVII. Finally, also the Dubinin−Radushkevich (D−R) isotherm was tested in its linearized form eq 11:25

Figure 6. Isotherm of ReVII adsorption on modified nano-Al2O3 at different temperatures (200 mg of modified nano-Al2O3; the initial ReVII concentration range was 20.0−250.0 mg·L−1; pH 2.5).

ln qe = ln qm − K ε2

where qe and qm have the same meaning as previously indicated and K is the parameter related to the adsorption energy. ε, the adsorption potential, is defined by Polanyi as the free-energy change required to move a molecule from the bulk solution to the adsorption area. The Polanyi potential varies with the concentration according to eq 12:

VII

Al2O3 as a function of the equilibrium concentration of Re , and increased adsorption is observed for ReVII until saturation is attained. The equilibrium sorption isotherm is often described by the Langmuir model eq 8: Ce C 1 = e + qe qm bqm

(11)

⎛ 1⎞ ε = RT ln⎜1 + ⎟ Ce ⎠ ⎝

(8)

(12)

Table 2. Langmuir, Freundlich, and D−R Isotherm Constants and Correlationa Langmuir

a

−1

−1

Freundlich −1

D−R −1

T (K)

qm (mg·g )

b (L·mg )

r

RL

KF (mg·g )

n

r

qm (mg·g )

E (kJ·mol−1)

r

273 298 323

1.752 1.854 1.931

0.115 0.117 0.117

0.999 0.999 0.999

0.303−0.028 0.300−0.028 0.299−0.028

0.877 0.906 0.945

8.033 7.726 7.766

0.976 0.980 0.979

1.124 1.261 1.386

5.73 6.71 7.24

0.996 0.996 0.998

Data indicate that model parameters are statistically significant (t test) at a 95% confidence level. 5582

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Table 4. Analytical Results of MoVI and ReVII Ions in Standard Reference Material (Mean ± Standard Deviation; n = 7)a (m = 0.2 g; CRe = 20 mg·L−1)

where R is the ideal gas constant and T is the temperature (K). A linear correlation is obtained by plotting ln qe versus ε2, indicating that ReVII adsorption also obeys the D−R equation (shown Table 2). The adsorption energy for ReVII adsorption can be calculated by eq 13:

sample GBW07285

E = ( −2K )−1/2

(13)

According to eq 16, ΔH° and ΔS° parameters can be calculated from the slope and intercept of the plots of ln Kc versus 1/T, respectively. The thermodynamic parameters are summarized in Table 3. The positive ΔH° and ΔG° showed the endothermic and spontaneous nature of the sorption process. Table 3. Thermodynamic Parameters for the Adsorption of ReVII on Modified Nano-Al2O3 (m = 0.2 g; CRe = 20 mg·L−1)a

273 K

298 K

323 K

100

2.2452

0.0336

−9.167

−10.006

−10.846

ReVII

ReVII/recovery (%)

synthetic-1 synthetic-1 synthetic-2 synthetic-3

20 50 1000 5000

20 50 1000 5000

4 5 5 10

98.29 98.43 94.30 93.70

■

ΔG° (kJ·mol−1) ΔS° (kJ·mol−1·K−1)

WO42−

4. CONCLUSION In this paper, the adsorption behavior of ReVII ions on modified nano-Al2O3 was investigated. The experimental results indicate that modified nano-Al2O3 can effectively separate ReVII ions from aqueous solutions in the pH range of 2.0−3.0 with an adsorption percentage above 94%. Kinetic studies suggest that equilibrium is achieved within 5 min and pseudo-second-order is followed. Diffusion-based kinetic models suggest that both intraparticle and external mass-transfer processes play important roles in the adsorption of ReVII ions by modified nanoAl2O3. The adsorption isotherms could be well fitted by the Langmuir adsorption isotherm equations, with a maximum adsorption capacity of 1.94 mg·g−1 (293 K) of ReVII on modified nano-Al2O3. The thermodynamic parameters imply that the adsorption is a spontaneous and endothermic process. The method using modified nano-Al2O3 to separate and determine ReVII was successfully applied on the geological samples with complex matrixes. The results obtained from this work suggest that modified nano-Al2O3 is an economical, viable, and highly efficient adsorbent media, which has a potential application for separating ReVII from aqueous solutions.

(16)

ΔH° (kJ·mol−1)

sample

MoO42−

a Data indicate that model parameters are statistically significant (t test) at 95% confidence level.

where Ce is the equilibrium concentration of ReVII and qe is the amount of ReVII adsorbed per unit weight of modified nanoAl2O3 at equilibrium concentration (mg·g−1), respectively. The enthalpy change (ΔH°) and entropy change (ΔS°) of adsorption were estimated from eq 16:

C0 (mg·L−1)

51.7 ± 1.1 0.0312 ± 3.7

spike (μg·mL−1)

−1

ΔH ° ΔS° + RT R

48.8 ± 0.6 0.0304 ± 4.2

Mo ReVII

Table 5. Analytical Results of the Synthetic Samples (n = 7)a

where R is the universal gas constant (8.314 J·mol ·K ), T is the temperature (K), and Kc is the distribution coefficient. The Gibbs free-energy change of adsorption (ΔG°) was calculated using ln Kc values for different temperatures. The Kc value was calculated using eq 15: q Kc = e Ce (15)

ln Kc = −

reference (mg·g−1)

Data indicate that model parameters are statistically significant (t test) at a 95% confidence level.

(14) −1

found (mg·g−1)

VI

a

The values of the adsorption energy were evaluated as 5.73, 6.71, and 7.24 kJ·mol−1 at 273, 298, and 323 K, respectively. The range of energy is less than 8 kJ·mol−1; the adsorption process is accompanied by physisorption.26 3.8. Thermodynamic Studies. Adsorption thermodynamics of ReVII were investigated with different concentrations of ReVII at 273, 298, and 323 K, respectively. The thermodynamic parameters were calculated from eq 14: ΔG° = −RT ln Kc

element

ASSOCIATED CONTENT

S Supporting Information *

Detailed estimated kinetic parameters of external mass transfer and intraparticle diffusion. This material is available free of charge via the Internet at http://pubs.acs.org.

a Data indicate that model parameters are statistically significant (t test) at a 95% confidence level.

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AUTHOR INFORMATION

Corresponding Author

3.9. Sample Analysis. According to the recommended procedure, the method was applied to the determination of the content of rhenium in synthetic samples and standard reference material (GBW07285 molybdenum ore) to verify its validity. The analytical results are shown in Table 4, suggesting a good agreement between the determined values and the certified or reference values. This method was also applied to the determination of synthetic samples. Table 5 shows the analytical results along with the recoveries for the spiked samples. Good recoveries were found (93−98%).

*Tel: +86-24-62207809. Fax: +86-24-62202380. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was supported by National Natural Science Foundation of China (Grant 51178212), Natural Science Foundation of Liaoning Province, China (Grant 201102082), and Foundation of 211 Project for Innovative Talents Training, 5583

dx.doi.org/10.1021/ie300008v | Ind. Eng. Chem. Res. 2012, 51, 5577−5584

Industrial & Engineering Chemistry Research

Article

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Liaoning University. The authors also thank their colleagues and other students who participated in this work.

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NOMENCLATURE qe, qt, qm = adsorption at equilibrium, at time t, and at maximum monolayer, respectively k1, k2 = rate constants of first- and second-order adsorption, respectively Ea = activation energy h = initial adsorption rate Kd = intraparticle diffusion rate constant r = particle radius Ct, Ce = concentrations at time t and at equilibrium, respectively kES = rate constant of the external mass-transfer model KF = sorption capacity of Freundlich constants n = sorption intensity of Freundlich constants b = Langmuir adsorption equilibrium constant RL = equilibrium parameter K = parameter of the adsorption energy ε = adsorption potential R = gas constant T = temperature Kc = distribution coefficient ΔG° = Gibbs free-energy change of adsorption ΔH° = enthalpy change ΔS° = entropy change

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dx.doi.org/10.1021/ie300008v | Ind. Eng. Chem. Res. 2012, 51, 5577−5584