A Rapid and Selective Isolation of Rhodium from Aqueous Solution

Article pubs.acs.org/jced

A Rapid and Selective Isolation of Rhodium from Aqueous Solution Using Nano-Al2O3 Lei Zhang,* Xin Li, Li-jun Yang, Ying Li, Huai-chun Chang, Xiao-jun Chu, Jing Zhang, Xin Wang, and Shuai An College of Chemistry, Liaoning University, Shenyang 110036, P. R. China ABSTRACT: Nano-Al2O3 was applied for the separation of Rh(III) ions from water solution. When pH value was within 5.0 to 9.0 and the concentration of Rh(III) was less than 50 mg·L−1, more than 90 % of Rh(III) ions were adsorbed by nano-Al2O3. Then the adsorbed Rh(III) ions were rapidly eluted when 2.0 mL of 0.3 mol·L−1 H2SO4 was added. The adsorption data corrected well with the Langmuir equation, and the saturated adsorption capacity was 5.52 mg·g−1 (25 ± 0.1 °C). The dynamics data indicate that the equilibrium time was about 2.5 min. The rate constant k2 = 2.714 g·mg−1·min−1 (25 °C) was gained according to the second-order-kinetic model. The overall rate process seemed to be affected by both film diffusion and interparticle diffusion, but it was mainly governed by interparticle diffusion. The average energy of sorption at ambient temperature was obtained to be 1.08 kJ·mol−1 through the Dubinin−Radushkevich (D-R) model. Moreover, the values of ΔH0 and ΔG0 for Rh(III) adsorption disclosed the spontaneous and endothermic nature of the process. The research of adsorption selectivity showed that 98 % of the Rh(III) ions could be adsorbed by nano-Al2O3 in the presence of Zn2+, Cu2+, Cd2+, Ni2+, Co2+, Fe3+, and Pb2+.

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INTRODUCTION The enrichment of rhodium has attracted considerable interest because of its economic interest and special use. Rhodium is widely used in chemical, automobile, and electronics industries because of its specific physicochemical properties. Since natural resources of rhodium metal are limited and its requirement in industry is growing, it is necessary to find out an efficient isolation method to recover pure rhodium metals from different secondary resources.1 Generally the natural abundance of Rh(III) is extremely small. The direct measurement of a trace amount is restricted with matrix interference and low sensitivity. For these reasons, the separation and enrichment of micro rhodium from samples is needed. Industrial technologies, for instance, pyrometallurgical and hydrometallurgical procedures, have been extensively used to retrieve noble metals from waste liquid.2 The hydrometallurgical methods, which include solvent extraction,3−7 ion exchange,8−10 and deoxidation of noble metal precipitated by reductant,11 have been used more frequently than the pyrometallurgical process. Generally speaking, solvent extraction played a vital role in the isolation and preconcentration of Rh(III). Solvent extraction methods of Rh(III) have been reported, such as TBP,12,13 N-n-oeytlnailine,14 and Kelex100.15 All of these methods are costly and time-consuming. Moreover, in the processes of precipitation and reduction, other chemical agents added generated a mass of secondary waste. So, it is necessary to find an effective and environmentally friendly method for the recovery of noble metals from waste liquid. The solid-phase extraction method has received increasing attention because of its high preconcentration factor and simple © 2012 American Chemical Society

procedure. Some attempts have been made toward the use of biosorbent materials.16−19 However, slow adsorption kinetics restrict the application of biosorbents in the rapid separation and preconcentration of Rh(III). Using nanoparticles as a sorbent for the isolation of trace elements is another research focus of solid-phase extraction because of its ability of rapid and highly efficient adsorption and applicability of combination with different detection techniques.20,21 However, literature on the adsorption of Rh(III) using nanoparticles is lacking. In this paper, nano-Al2O3 as sorbent was used for the isolation of a trace amount of Rh(III) ions from samples; the adsorption type, dynamics, and thermodynamics of Rh(III) ions on nano-Al2O3 were investigated in detail. The best conditions for recovery Rh(III) ions were studied and optimized. The object of the experiment was not only to know the interaction between Rh(III) ions and nanoAl2O3, but also to find an effective, rapid, and simple method for the separation of Rh(III) ions.

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EXPERIMENTAL SECTION Apparatus. XSERIES 2 inductively coupled plasma−mass spectrometry (ICP-MS; Thermo Scientific) was used for the determination of Rh(III) and other metal ions. A KQ-00 SerialUltrasonics instrument (Kunshan Apparatus Company) was used for dispersing nano-Al2O3 in aqueous solution, and a Malvern Zetasizer Nano-ZS particle analyzer (Malvern) was Received: March 1, 2012 Accepted: August 29, 2012 Published: September 7, 2012 2647

dx.doi.org/10.1021/je300365p | J. Chem. Eng. Data 2012, 57, 2647−2653

Journal of Chemical & Engineering Data

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determined. The rate constants were computed according to the rate equation. Adsorption Thermodynamics. Adsorption isotherm experiments were conducted with different initial concentrations of Rh(III) [(0.5 to 20.0) mg·L−1], and a constant amount of sorbent (10.0 mg) was added at the temperatures of (273, 298, and 323) K. Finally, ΔH0, ΔS0, and ΔG0 of the sorption procedure were obtained, respectively.

used to measure the zeta potential and particle size of sorbents. A model TDL80-2B centrifugal machine (Shanghai Anting Scientific Instrument Co.) was used throughout the experiment. Materials and Reagents. Nano-Al2O3 (γ), micron-Al2O3, nano-TiO2 (anatase type), nano-TiO2 (rutile type), and nanoSiO2 were purchased from Beijing Nachen Nano-Material Co., and the particle size and specific surface area of each particle are listed in Table 1. A 1.000 mg·L−1 standard stock solution of Rh(III) was obtained by dissolving 2.104 g of K3[RhCl6] in 0.01 mol·L−1 HCl and then diluting to a 500 mL brown flask volumetric flask; the standard solution of Rh(III) was diluted to a certain concentration in the procedure and stored in the refrigerator. All reagents, including HCl, H2SO4, and NaOH, were of analytical reagent grade and were purchased from Sinopharm Chemical Reagent Co., Ltd. Doubly distilled water was used during the test. The standard stock solutions of all of the other metal ions were obtained by dissolving their chlorid or nitrate salts (≥ 99.99 %) and used to investigate the adsorption selectivity. General Adsorption and Desorption Procedure. The sorption experiments were performed in a series of 50 mL Erlenmeyer flasks containing 300 mg of nano-Al2O3 and 10.0 mL of certain concentration Rh(III) solution at pH 6.0. A proper volume of concentrated NaOH or HCl solutions was used for the adjustment of pH of the solution after adding nano-Al2O3 when necessary. The solid−liquid phases were isolated by centrifuging at 4000 rpm. The adsorption ratio (Ads %) was calculated based on eq 1: Ads % =

(C i − Ca) ·100 Ci

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RESULTS AND DISCUSSION Selection of Sorbents. Four general sorbents (listed in Table 1) were chosen to test their adsorbability of Rh(III). The Table 1. Comparison of Rh(III) Adsorption Behavior on Different Sorbents (CRh(III): 5.0 mg·L−1; Temperature: 20 ± 0.1 °C)

(1)

(qe /ce)i (qe /ce)j

specific surface area

nm

m2·g−1

Ads %

nano-Al2O3 (γ) micron-Al2O3 nano-SiO2 nano-TiO2 (rutile type) nano-TiO2 (anatase type)

10 to 15 800 to 1000 20 to 50 10 to 15 10 to 15

180 ± 10 20 ± 10 160 ± 20 30 ± 10 210 ± 10

98.9 56.8 45.0 63.5 70.0

sorption conditions (sorbent dosage, solution pH value) of each sorbent were optimized to make sure that their adsorption ratio can reach the respective highest value. The results in Table 1 showed that nano-Al2O3 was the best sorbent for Rh(III). Therefore nano-Al2O3 was used as a sorbent for Rh(III) ions in this paper. Usually, adsorbability depends on adsorbent’s characteristics: texture (surface area, pore distributions), surface chemistry (surface functional groups), and mineral matter content. It also depends on adsorbate’s characteristics: molecular weight, molecular size, solubility, and hydrophilicity. Nano-Al2O3 with proper texture and surface chemistry properties is more appropriate for the adsorption of Rh(III), compared to other nanoparticles. Effect of the Amount of Sorbent. To obtain the optimum addition amount of nano-Al2O3 for the sorption of Rh(III) ions, sorption tests were conducted by adding (50.0 to 400.0) mg of the nano-Al2O3 to a series of Erlenmeyer flasks containing 10 mL of 8 mg·L−1 Rh(III) solutions (pH 6.0). When the amount of nano-Al2O3 exceeded 300 mg, the sorption ratio of Rh(III) on nano-Al2O3 approached 100 %. Therefore, 300.0 mg of the sorbent was the optimum addition amount for the sorption of Rh(III). Effect of pH. To investigate the effect of pH on the sorption of Rh(III) onto sorbent, the pH values of solutions were changed within the range of 2.0 to 9.0. The results are shown in Figure 1. When the concentration of Rh(III) was below 50 mg·L−1, the sorption ratio of Rh(III) was more than 90 % in the pH range 5.0 to 9.0. To find the surface electric charge of nano-Al 2 O 3 suspensions, the zeta potential (ξ) was measured in the medium in 0.001 mol·L−1 NaCl solutions. The zeta potential and particle size of nano-Al2O3 are shown in Figure 2. The zero charge potential of nano-Al2O3 obtained was 9.0. Around the zero charge potential the particle size of nano-Al2O3 was the largest because of the weakest electrostatic repulsion and the strongest aggregation effect in nano-Al2O3.

Ci is the initial concentration of Rh(III) in the liquid phase; Ca is the final concentration of Rh(III) in the liquid phase. To check any adsorption of rhodium on the Erlenmeyer flask surface, control experiments were performed without adsorbent. The results showed that no rhodium was adsorbed on the Erlenmeyer flask wall. The adsorbed Rh(III) ions were quantitatively eluted with 2.0 mL of 0.3 mol·L−1 H2SO4. Adsorption Selectivity. The selective adsorptions of Rh(III) were conducted using a multielement solution including Pb2+, Ni2+, Co2+, Cu2+, Fe3+, Zn2+, and Cd2+, respectively, obtained from metal salts. The mixture solution consists of 4.85 × 10−5 mol·L−1 Rh(III) and 0.02 mol·L−1 Pb2+, Cu2+, Zn2+, Ni2+, Co2+, Cd2+, and Fe3+, respectively. The adsorption was carried out, and the adsorption selectivity eq 2 was measured by ICP-MS analyses. Seli = log

particle size sorbent

(2)

where Seli is the adsorption selectivity, Ce is the equilibrium concentration of Rh(III) (mg·L−1), and qe is the quantity of Rh(III) adsorbed per unit mass of nano-Al2O3 (mg·g−1). The index i pertains to Rh(III), and index j refers to the metals other than Rh(III) in the mixed system. Adsorption Dynamics. Dynamics experiments were conducted using a series of 50 mL Erlenmeyer flasks containing 10 mg of nano-Al2O3 and 10.0 mL of 8.0 mg·L−1 Rh(III) solution at pH 6.0 at a constant temperature [(273, 298, and 323) K]. At certain intervals of reaction time, suitable aliquots were taken, and then the concentration of Rh(III) was 2648



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Table 2. Selection of Desorption Agent desorption agent 0.5 1.0 2.0 0.3 0.5 1.0 2.0 0.1 0.3 0.5 0.7

M M M M M M M M M M M

desorption efficiency (%)

HCl HCl HCl HNO3 H3PO4 H3PO4 H3PO4 H2SO4 H2SO4 H2SO4 H2SO4

42.4 31.1 30.2 65.8 60.4 43.0 26.1 47.5 95.4 63.4 48.1

and the adsorption capacity of Rh(III) was (5.52, 5.46, 5.40, 5.33, and 5.29 mg·g−1. These results showed that the adsorbability of nano-Al2O3 did not deteriorate significantly after recycling five times, and the elution efficiencies were still above 90 %. Therefore, nano-Al2O3 had good reutilization and could be used in the recovery of Rh(III). Adsorption Kinetic Model. Figure 3 depicted the change of sorption capacity with sorption time. The sorption equilibration time of Rh(III) on the surface of nano-Al2O3 was about 2.5 min.

Figure 1. Influence of the solution acidity on the sorption percentage of Rh(III) onto nano-Al2O3 (nano-Al2O3: 300 mg; temperature: 20 ± 0.1 °C).

Figure 2. Zeta potential and particle size of nano-Al2O3 in the various acid media.

At pH < pHpzc, the nano-Al2O3 surface carries positive charges, while at pH > pHpzc the nano-Al2O3 surface is negatively charged. The electrostatic attraction may play an important role on the sorption of Rh(III) onto nano-Al2O3. In acid media, Rh(III) ions often exist as [RhCl6]3−. In neutral or alkaline media, hydroxide, hydroxoaquochloride, and polynuclear complexes are formed, and even precipitate appeared.22 At pH < pHpzc, the nano-Al2O3 surface is positively charged, which will enhance its electrostatic attraction with [RhCl6]3−. For this reason, the adsorption progressed more easily, so the sorption of Rh(III) was more efficient. But when pH < 3, there is an equilibrium reaction: 3H+ + [RhCl6]3− ↔ H3[RhCl6]. The main chemical speciation of Rh(III) is H3 [RhCl6],22 so the sorption ratio of Rh(III) was lower. The sorption ratio of Rh(III) was higher than 95 % in the pH range of 5.0 to 9.0 when the initial concentration of Rh(III) was less than 15 mg·L−1. Therefore pH 6.0 was chosen for the experiment. Desorption and Reusability. The elution efficiency of Rh(III) was investigated by different concentrations of HNO3, HCl, H3PO4, and H2SO4 solutions. The results listed in Table 2, 2.0 mL of 0.3 mol·L−1 H2SO4 solution, could effectively desorb the Rh(III) (> 95 %) from sorbent material. Also the nano-Al2O3 could be reused after elution with 0.3 mol·L−1 H2SO4 solution. Nano-Al2O3 was recycled five times,

Figure 3. Influence of time on the sorption of Rh(III) onto the nanosorbent (nano-Al2O3: 10 mg; CRh(III): 8 mg·L−1; pH: 6.0).

First-order-kinetic, second-order-kinetic, Weber−Morris kinetic, and Richenberg models were used to study the sorption procedure of nano-Al2O3 for Rh(III). First-Order-Kinetic Model. The first-order-kinetic formula is given as: log(q1 − qt ) = log q1 −

k1t 2.303

(3) −1

qt is the quantity of Rh(III) sorbed on the nano-Al2O3 (mg·g ) at time t; q1 is the quantity of Rh(III) sorbed on the nanoAl2O3 (mg·g−1) at adsorption equilibrium time; k1 is the rate constant of the first-order-kinetic model (min−1). The values of k1 for Rh(III) sorption onto nano-Al2O3 were calculated from the plot of log(q1 − qt) against t. Second-Order-Kinetic Model. The second-order-kinetic model is represented as: 2649



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Table 3. Dynamic Parameters for Rh(III) Sorption on Nano-Al2O3 at (273, 298, and 323) K pseudofirst-order model

pseudosecond-order model

T/K

k1/min−1

q1/(mg·g−1)

r

k2/(g·mg−1·min−1)

q2/(mg·g−1)

r

h

273 298 323

0.014 0.056 0.036

2.105 5.220 4.185

0.927 0.961 0.928

0.478 2.714 10.525

1.297 1.463 1.508

0.999 0.997 0.997

2.223 35.723 256.64

Figure 4. Curves of −ln(1 − F) and Bt against t for Rh(III) sorption onto nano-Al2O3 at (273, 298, and 323) K.

t 1 t = + 2 qt q2 k 2q2

The intraparticle diffusion may be the rate-dominating step25 because the lines almost passed through the origin. Weber−Morris Kinetic Model. Sorption kinetic data were again used to confirm whether intraparticle diffusion is the ratelimiting step and to obtain rate parameters for intraparticle diffusion. For such purpose, the Weber−Morris equation was applied:

(4)

where k2 is the rate constant of the second-order-kinetic sorption (g·mg−1·min−1). The rate parameters could be obtained from the linear plots of t/qt versus t. The batch kinetic data correlated well with the second-orderkinetic model. Table 3 shows the results of the kinetic parameters. It could be seen from the correlation coefficients that the sorption of Rh(III) onto nano-Al2O3 was best described by the second-order-kinetic model. The second-pseudo rate constants were used for calculating the initial adsorption rate h (mg·g−1·min−1),23 given by:

h = k 2q2 2

qt = Kdt 1/2 + I

(8)

I is the thickness of boundary layer (values of intercept I are proportional to the extent of the boundary layer); Kd is the rate constant for intraparticle diffusion.26 Lines of qt against t1/2 are shown in Figure 5, and the values of Kd and I are listed in Table 4.

(5)

The values of the initial adsorption rate h are also listed in Table 3. With the increase of temperature, the initial adsorption rate increased in a certain sorption system. The activation energy (Ea) could also be calculated based on the obtained rate constants presented in Table 3 using the Arrhenius formula. A value of Ea for Rh(III) sorption onto nano-Al2O3 was obtained to be 3.32 kJ·mol−1. Richenberg Model. Usually the rate-dominating step in adsorption or ion exchange is either particle diffusion or film diffusion. To prove the rate-dominating step, the following formulas were used to analyze the sorption kinetic data.24,25 For the film diffusion:

R t = −ln(1 − F )

(6)

For the interparticle diffusion: Bt = −ln(1 − F ) − 0.4977

(7)

Figure 5. Weber−Morris plots for Rh(III) ion sorption onto nanoAl2O3 at (273, 298, and 323) K.

where F = qt/qe; qe is the quantity of Rh(III) sorbed on nanoAl2O3 (mg·g−1) at balance time; R is the rate constant of film diffusion; B = π2Di/r2; r is the radius of the particle; Di is the intrapartical diffusion coefficient. Linear correlations were obtained from −ln(1 − F) versus t (Figure 4a). Since the lines did not pass through the origin, the film diffusion was not involved in the sorption kinetics of Rh(III) onto nano-Al2O3. Figure 4b depicts the linear correlation between Bt versus t.

These plots showed that two regions were distinctly observed: the first linear part was the film diffusion; the second linear part represented the intraparticle diffusion. Moreover, the lines did not pass through the origin, indicating the exits of film diffusion. The variety of the intercepts indicated that the 2650



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Table 4. Estimated Parameters of the Weber−Morris Kinetic Model for the Sorption of Rh(III) at (273, 298, and 323) K initial linear portion

second linear portion

T/K

Kd1/(mg·g−1·min−1/2)

I1

R1

Kd2/(mg·g−1·min−1/2)

I2

R2

273 298 323

0.159 0.257 0.318

0.258 0.660 1.537

0.994 0.985 0.996

0.062 0.151 0.775

1.147 1.725 3.652

0.969 0.959 0.946

The maximum saturated adsorption capacity of Rh(III) ions on nano-Al2O3 was (2.81, 5.52, and 6.94) mg·g−1 at (273, 298, and 323) K, respectively. The aim of discussing the shape of the isotherm was to estimate whether an adsorption method is favorable or not.27 RL, a dimensionless constant referred to as the equilibrium parameter, can express the essential feature of Langmuir isotherms. The following equation is applied to calculate RL:

intraparticle diffusion played a main role in rate control, but film diffusion (external mass transfer) could not be ignored. Adsorption Isotherm and Adsorption Capacity. Adsorption isotherms represent how Rh(III) ions interact with the nano-Al2O3. Figure 6 depicts the equilibrium sorption quantity of Rh(III) onto nano-Al2O3 increased with the initial concentration of Rh(III) until saturation is attained.

RL =

1 1 + bC i

(11)

Ci is the initial Rh(III) concentration (mg·L−1); b is the Langmuir adsorption equilibrium constant (L·mg−1). The calculated RL values are listed in Table 5, and the results showed that RL was in the range of 0 to 1, which indicated that the adsorption procedure was quite favorable, and the nanoAl2O3 performed a good adsorption capacity for the adsorption of Rh(III). Finally, the D-R isotherm28 model was represented by the equation: ln qe = ln qm − Kε 2 Figure 6. Curves of qe against Ce for Rh(III) ion sorption onto nanoAl2O3 at (273, 298, and 323) K [nano-Al2O3: 10 mg; pH: 6.0; CRh(III): (0.5 to 20.0 mg·L−1)].

K is the parameter related to the adsorption energy; ε is the adsorption potential.

⎛ 1⎞ ε = RT ln⎜1 + ⎟ Ce ⎠ ⎝

The Langmuir isotherm model is often used to describe the sorption isotherms: Ce C 1 = e + qe qm bqm

(12)

(13)

T is the temperature (K); R is the ideal gas constant. A linear correlation is gained by plotting ln qe versus ε2, indicating that Rh(III) adsorption also correlates with the D-R model. The formula of adsorption energy of Rh(III) is as follows:

(9)

b is the Langmuir constant related to the affinity of binding sites (L·mg−1); Ce is the equilibrium concentration of Rh(III) (mg·L−1); qm is the maximum monolayer adsorption capacity (mg·g−1); qe is the quantity of Rh(III) adsorbed per unit of nano-Al2O3 (mg·g−1). The Freundlich model is expressed as: 1 log qe = log KF + log Ce (10) n

E = ( −2K )−1/2

(14)

The sorption energy was (0.790, 1.084, and 2.074) kJ·mol−1 at (273, 298, and 323) K, respectively, which suggested that the adsorption of Rh(III) onto nano-Al2O3 was a physical process,29 that is, < 8 kJ·mol−1. Thermodynamic Studies. The sorption behaviors of various concentrations of Rh(III) on nano-Al2O3 were critically studied at various temperatures. ΔH0, ΔS0, and ΔG0 were calculated using the following formulas:

where KF and n are the characteristic constants related to the strength of the adsorptive bond. According to eqs 9 and 10, Langmuir and Freundlich isothermal constants were calculated, and the results are shown in Table 5. It suggested that the sorption of Rh(III) onto nano-Al2O3 was well-explained by the Langmuir isotherm model under a certain concentration range.

ΔG 0 = −RT ln K C

(15)

Table 5. Sorption Isotherm Constants of Rh(III) onto Nano-Al2O3 at (273, 298, and 323) K Langmuir

Freundlich

D-R

T/K

qm/(mg·g−1)

b/(L·mg−1)

r

RL

KF/(mg·g−1)

n

r

qm/(mg·g−1)

E/kJ·mol−1

r

273 298 323

2.81 5.52 6.94

0.42 0.44 1.36

0.996 0.996 0.999

0.106 to 0.704 0.102 to 0.694 0.035 to 0.424

0.57 3.01 11.77

0.47 0.48 0.42

0.973 0.968 0.954

2.39 4.35 6.30

0.79 1.08 2.07

0.972 0.947 0.982

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Cu2+−Cd2+−Zn2+−Co2+, and Rh3+−Pb2+−Cu2+−Ni2+−Cd2+− Zn2+−Co2+−Fe3+, respectively. It indicated that the nano-Al2O3 exhibited good adsorption selectivity for Rh(III).30 These findings suggest that nano-Al2O3 can be applied in the separation and preconcentration of Rh(III) from a multicomponent solution system. Rh(III) Separation from Multicomponent Solutions. Nano-Al2O3 was further studied for the separation of Rh(III) from a six-component metal solution system (Rh3+−Pb2+− Cu2+−Cd2+−Zn2+−Co2+) and an eight-component metal solution system (Rh3+−Pb2+−Cu2+−Ni2+−Cd2+−Zn2+−Co2+− Fe3+). The experiments were carried out at pH 6.0, and normal temperature and the adsorption percentage of the metals were obtained from the concentration change of the metals before and after adsorption. The results of the test are depicted in Figure 7. Nano-Al2O3 displayed good selectivity with a selectivity coefficient of 1.9 for rhodium from the six-component solution containing 4.85 × 10−5 mol·L−1 Rh3+, 0.02 mol·L−1 Pb2+, 0.02 mol·L−1 Co2+, 0.02 mol·L−1 Cu2+, 0.02 mol·L−1 Cd2+, and 0.02 mol·L−1 Zn2+ (Figure 7a), and about 95 % of rhodium was separated. Using nano-Al2O3, nearly 98 % of the targeted rhodium in the solution was successfully adsorbed from eightcomponent (4.85 × 10−5 mol·L−1 Rh3+, 0.02 mol·L−1 Pb2+, 0.02 mol·L−1 Co2+, 0.02 mol·L−1 Cu2+, 0.02 mol·L−1 Cd2+, 0.02 mol·L−1 Zn2+, 0.02 mol·L−1 Ni2+, and 0.02 mol·L−1 Fe3+) solution with a selectivity coefficient of 2.7 (Figure 7b).

KC is the distribution coefficient; T is the system temperature (K); R is the ideal gas constant (8.314 J·mol−1·K−1). The KC value was calculated using the following fomula: q KC = e Ce (16) The values of ΔH0 and ΔS0 for the sorption were calculated from the following formula: ln K C = −

ΔH 0 ΔS 0 + RT R

(17)

Table 6 gives a list of values of ΔH0, ΔS0, and ΔG0. The negative ΔG0 and positive ΔH0 proved the spontaneous and endothermic nature of the adsorption process. Table 6. Thermodynamic Constants for the Sorption of Rh(III) onto Nano-Al2O3 ΔH0

C0 mg·L 8.0

−1

−1

kJ·mol

ΔG0/(KJ·mol−1)

ΔS0 −1

−1

kJ·mol ·K

10.300

0.035

273 K

298 K

323 K

−0.043

−0.131

−3.152

Adsorption Selectivity. The sorption selectivity of nanoAl2O3 for Rh(III) at pH 6.0 was studied, and the results were shown in Table 7. It was very obvious that Rh(III) was readily Table 7. Sorption Selectivity of Nano-Al2O3 for Rh(III) from Mixed Metal Ion Solution system Rh3+−Zn2+ Rh3+−Ni2+ Rh3+−Cd2+ Rh3+−Ir4+ Rh3+−Co2+ Rh3+−Fe3+ Rh3+−Pb2+ Rh3+−Cu2+ Rh3+−Pb2+−Cu2+−Cd2+− Zn2+−Co2+ Rh3+−Pb2+−Cu2+−Ni2+− Cd2+−Zn2+−Co2+−Fe3+

Rh capacity (mg·g−1)

selectivity

ads. of Rh(III) (%)

5.39 5.43 5.48 5.35 5.52 5.52 5.52 5.52 5.41

1.7 2.4 3.4 1.1 ∞ ∞ ∞ ∞ 1.9

98.8 99.6 99.7 97.3 99.0 99.9 99.3 94.2 97.0

5.46

2.7

98.0

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CONCLUSION

It was found that the sorption of Rh(III) onto nano-Al2O3 depended on the initial rhodium concentration, solution pH, contact time, and system temperature. The data suggested that the adsorption reactions showed good agreement with the second-order-kinetic, Langmuir, D-R, and intraparticle diffusion models. A sample of 0.3 mol·L−1 H2SO4 was applied for eluting Rh(III) from the nano-Al2O3, and the adsorbent could be recycled up to five times without obvious reduction in the adsorption ratio for Rh(III). The sorption process was spontaneous and endothermic and had good affinity properties according to the thermodynamic data. Competition sorption research showed that nano-Al2O3 performed good selectivity toward targeted Rh(III) ions in the presence of other metal ions. The results obtained in this work suggest that nano-Al2O3 has a potential application as adsorbent media for the recovery of rhodium from industrial procedure and waste liquid because

adsorbed by the nano-Al2O3 from the mixed solution containing Rh3+−Pb2+, Rh3+−Cu2+, Rh3+−Ni2+, Rh3+−Cd2+, Rh3+−Zn2+, Rh3+−Ir4+, Rh3+−Co2+, Rh3+−Fe3+, Rh3+−Pb2+−

Figure 7. Separation of Rh(III) ions from a multicomponent metal solution system. 2652



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of its high adsorption capacity and remarkable adsorption selectivity for rhodium.

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AUTHOR INFORMATION

Corresponding Author

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

This project was supported by NSFC (51178212), Liaoning Provincial Natural Science Funds (No. 201102082), and Natural Science Foundation of 211 Project for Innovative Talents Training, Liaoning University. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the aid of co-workers and school associates who assisted in this work.

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REFERENCES

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A Rapid and Selective Isolation of Rhodium from Aqueous Solution

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