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Dec 12, 2012 - College of Chemistry, Jilin University, Ximinzhu Street No. 938 ... extraction,2−4 filtration,5 ion exchange,6−8 liquid−liquid ex- traction,9−11 ...

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Adsorption of Au(III), Pd(II), and Pt(IV) from Aqueous Solution onto Graphene Oxide Li Liu, Shuxia Liu, Qiuping Zhang, Cui Li, Changli Bao,* Xiaoting Liu, and Pengfei Xiao College of Chemistry, Jilin University, Ximinzhu Street No. 938, Changchun 130026, P. R. China ABSTRACT: Graphene oxide (GO) was prepared and characterized by Fourier transform infrared spectrometry (FT-IR) and scanning electron micrographs (SEM). Batch adsorption studies were carried out to investigate the adsorption data, including the effects of pH, initial concentration, contact time, and temperature. The adsorption of Au(III), Pd(II), and Pt(IV) was optimum at pH 6.0. The adsorption isotherms all obeyed the Langmuir equation in the case of Au(III), Pd(II), and Pt(IV), and the maximum adsorption capacities were 108.342 mg·g−1, 80.775 mg·g−1, and 71.378 mg·g−1, respectively. The adsorption kinetics of Au(III), Pd(II), and Pt(IV) onto GO followed a pseudosecond-order kinetic model, indicating that the chemical adsorption was the rate-limiting step. Thermodynamic parameters such as Gibbs energy (ΔGo), enthalpy (ΔHo), and entropy (ΔSo) were calculated, indicating that the adsorption were spontaneous, endothermic, and feasible. The desorption studies showed that the best desorption reagents were 0.5 mol·dm−3 thiourea−0.5 mol·dm−3 HCl for Au(III) and 1.0 mol·dm−3 thiourea−0.5 mol·dm−3 HCl for both Pd(II) and Pt(IV).



INTRODUCTION Because of the specific physical and chemical properties of precious metals, they are widely used in many fields such as agriculture, biomedicine, jewelry, catalysts in chemical processes, and electrical and electronic industries. The increasing demand for precious metals is of great economical interest, and their sources are becoming more and more limited. Hence, more and more attention has been focused on the recovery of precious metals from aqueous solutions. At present, there are several available methods for the recovery of metal ions from aqueous solutions, such as electrolysis,1 solvent extraction,2−4 filtration,5 ion exchange,6−8 liquid−liquid extraction,9−11 precipitation,12−14 and adsorption.15−17 Each method has its own advantages and disadvantages. However, the recovery methods sometimes suffer from many problems such as highly toxic sludge, excessive time-consumption, and high costs. Comparatively, adsorption is a highly effective and economical way for the recovery of precious metal ions.18,19 In recent years, due to the environmental and economical point of view, considerable attention has been focused on the research of different kinds of low-cost and effective adsorbents for the recovery of precious metal ions, such as charcoal ash,16,20 zeolite,21 sepiolite,22 and biosorbents.23,24 Graphene is a new member of carbon materials with hexagonally, sp2-hybridized and one-atom-thick layer structure with interesting physical properties.25 It can be prepared from graphitea low-cost material. Since the discovery of graphene, graphene and its derivatives have attracted considerable attention due to their outstanding properties in various novel applications.26−28 In the family of graphene derivatives, graphene oxide (GO) is a single sheet form graphite and has © XXXX American Chemical Society

the ideal 2D structure with a monolayer of carbon atoms packed into a honeycomb crystal plane. Moreover, GO has many oxygen-containing hydrophilic functional groups on the graphitic backbone in the forms of hydroxyl, epoxide, carboxyl, and carbonyl groups. These oxygen groups which protrude from its layers can bind to metal ions by both electrostatic and coordinate approaches, especially the multivalent metal ions.29,30 It has been reported that GO represents great promise as a Cu2+ adsorbent.29 Actually, GO is an old material that has been known for more than 150 years.31 It is typically synthesized by reacting graphite powder with strong oxidizing agents such as KMnO4 in concentrated sulfuric acid, such as the well-known Hummer’s method.32 In the present study, the adsorption of Au(III), Pd(II), and Pt(IV) onto prepared GO was studied. Fourier transform infrared spectrometry (FT-IR) and scanning electron micrographs (SEM) were used to elucidate the functional groups and surface morphology of GO. The influences of experimental parameters such as pH, initial concentration, contact time, and temperature were investigated by batch adsorption studies. Desorption studies were conducted by several agents to regenerate the spent adsorbents.



EXPERIMENTAL SECTION Materials and Reagents. Graphite powder was purchased from Tianjin Guangfu Fine Chemical Research Institute. Received: May 18, 2012 Accepted: December 12, 2012

A

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for one week. Finally, the product was centrifuged at 10 000 rpm for 2 h and dried at 333 K. Adsorption Studies. Adsorption experiments were carried out by the batch method. The pH value of the solutions with a volume of 10 cm3 was adjusted to the desired value with HCl, and the mass of adsorbent was 3.0 mg, stirring at room temperature (298 K) with a constant shaking rate of 160 rpm in a vibrator. The concentration of metal ions was determined by FAAS. The adsorption capacity was calculated according to eq 1

H2SO4 (w = 0.98), H2O2, HCl, and KMnO4 powder were obtained from Beijing Chemical Reagent Factory. The standard solutions of Au(III), Pd(II), and Pt(IV) were supplied by Beijing NCS Analytical Instruments Co. Ltd. Working standards were diluted by the standard solutions. All of the chemicals used in this study were used without further purification. Solutions used in the experiment were diluted by using ultrapure water. The descriptions of chemical samples are represented in Table 1. Table 1. Chemical Sample Details chemical name H2SO4 H2O2 HCl KMnO4 powder graphite powder Au(III) standard solution Pd(II) standard solution Pt(IV) standard solution a

source

grade

purification method

Beijing Chemical Reagent Factory Beijing Chemical Reagent Factory Beijing Chemical Reagent Factory Beijing Chemical Reagent Factory Tianjin Guangfu Fine Chemical Research Institute Beijing NCS Analytical Instruments Co. Ltd.

ARa

none

AR

none

AR

none

AR

none

CPb

none

AR

none

FAASc

Beijing NCS Analytical Instruments Co. Ltd.

AR

none

FAAS

Beijing NCS Analytical Instruments Co. Ltd.

AR

none

FAAS

Analytical grade. bChemical purity. spectrophotometer.

c

q=

analytical method

(C0 − Ce) V m

(1)

−1

where q (mg·g ) is the adsorption capacity of the adsorbent and C 0 and C e (mg·dm −3 ) are the initial and final concentrations of metal ions in aqueous solution, respectively. V (dm3) is the volume of the aqueous solutions, and m (g) is the mass of the adsorbent. Desorption Studies and Reuse. The desorption studies of the adsorbed metal ions from GO was carried out by HCl, thiourea, and thiourea−HCl solutions. The desorption percentage was calculated with eq 2 desorption(%) =

Ce′V ·100 qm

(2)

−1

where q (mg·g ) is the adsorption capacity. m (g) is the mass of the adsorbent, and V (L) is the volume of the aqueous solutions. Ce′ (mg·dm−3) is the concentration of Au(III), Pd(II), and Pt(IV) aqueous solutions after desorbed from the adsorbent.



Flame atomic adsorption

RESULTS AND DISCUSSION Characterization of GO. Graphite powder, the prepared GO, and GO adsorbed with metal ions were characterized by FT-IR, and the results are shown in Figure 1. As seen from Figure 1 a, there was no obvious peak in the FT-IR spectrum of graphite powder. However, the FT-IR spectrum of prepared GO had several peaks. The observed bonds of GO in Figure 1b were similar to previous reports in the literature,29 which

Measurements. The mass of adsorbent was measured by an ESJ110-4A electronic balance (Shenyang Longteng Electronic Co., China) with the accuracy of 0.1 mg. The concentrations of the metal ions were determined by the TAS-990 flam atomic adsorption spectrophotometer (PGeneral, China). FT-IR spectra were recorded on an Affinity-1 Fourier transform infrared spectrophotometer (Shimadzu, Japan). KBr pellets were used; the resolution was 4 cm−1 with 32 time scanning, and the scanning was performed in the range of (4000 to 400) cm−1. SEM measurements were carried out using a field emission scanning electron microanalyzer JSM6700F (JEOL, Japan) at 8.0 kV. The measurements of pH were performed by a digital pH meter (pHS-25C, Shanghai, China). A water-bathing constant temperature vibrator (SHABA, Shanghai, China) was used in the adsorption experiments. Preparation of GO. Graphite powder was used to prepared GO according to the well-known Hummer’s method with some modifications. Briefly, graphite powder (1.0000 g) and H2SO4 (23 cm3, w = 0.98) were put into a 250 cm3 flask under stirring for 30 min, and the flask was kept at a temperature below 293 K in an ice−water bath. KMnO4 (3.0000 g) was added slowly into the solution under stirring, and the flask was kept at 308 K for 2 h. Next, ultrapure water (46 cm3) was added with the temperature rising up to about 363 K quickly. Another sample of ultrapure water (140 cm3) and H2O2 (2.5 cm3) was added in sequence. After the filtration of the mixture, the residue was washed by HCl (w = 0.05) solution four times and ultrapure water five times. The product was dialyzed by ultrapure water

Figure 1. FT-IR spectra of (a) graphite powder, (b) GO, (c) Au(III), (d) Pd(II), and (e) Pt(IV) adsorbed onto GO, respectively. B

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indicated the preparation of GO was successful. The main characteristic bands of GO in the FT-IR spectrum were found at peaks of 3411 cm−1, 1727 cm−1, 1635 cm−1, and 1057 cm−1.33 The band at 3411 cm−1 resulted from the stretching vibrations of O−H bonds,34 and the bands at 1727 cm−1 were assigned to the stretching vibrations of CO in carboxylic acid and carbonyl moieties. The peaks at 1635 cm−1 and 1057 cm−1 corresponded to the skeletal vibrations of CC bonds and C− O in epoxy or alkoxy,35 respectively. In Figure 1c,d,e, the decreasing vibration intensity around 1720 cm−1 indicated the chemical interaction of Au(III), Pd(II), and Pt(IV) with the CO groups in carboxylic acid and carbonyl moieties of GO. Also, the decreasing vibration intensity around 1630 cm−1 and 1050 cm−1 indicated the chemical interaction of Au(III), Pd(II), and Pt(IV) with CC groups and CO groups in epoxy or alkoxy of GO. The comparison of the texture and morphology of GO before and after adsorption of Au(III), Pd(II), and Pt(IV) were performed by SEM, and the images were shown in Figure 2.

Figure 3. Effect of pH on Au(III), Pd(II), and Pt(IV) adsorption by 3.0 mg GO for 10 h at 298 K. ■, Au(III), initial concentrations: 60 mg·dm−3; ●, Pd(II), initial concentrations: 30 mg·dm−3; ▲, Pt(IV), initial concentrations: 30 mg·dm−3.

at the oxygen sites, because the oxygen atoms have a much higher affinity to metal ions than that of the skeleton carbon. There are two major interactions between GO and metals, namely, electrostatic interaction and coordination between metals and oxygen-containing groups on GO, especially the carboxyl groups. It is reported that GO remains negatively charged in a wide pH range from 2 to 11.36 Therefore, with the increase of pH, the negative charge associated with GO increases. At lower pH values, the major interaction may be coordination between GO and metals. Due to the high availability of chloride anions, the main species of Au(III), Pd(II), and Pt(IV) in lower pH values are AuCl4−, PdCl42−, and PtCl62−, respectively. Therefore, there is electrostatic repulsion between GO and metals, resulting in lower adsorption capacities. Meanwhile, at higher pH values, because of the lower availability of chloride anions, the ionic species of Au(III), Pd(II), and Pt(IV) are subject to hydration. As a result, the electrostatic interaction between GO and metals increases, leading the increase in adsorption capacity. Adsorption Isotherms. Batch adsorption experiments were conducted. Sample of 10 cm3 solutions with various initial concentrations ((10 to 90) mg·dm−3 in the case of Au(III), (10 to 50) mg·dm−3 in the case of Pd(II) and Pt(IV)) were shaken for 7 h at room temperature (298 K) with 3.0 mg of GO. Moreover, the solution pH value was adjusted to the desired value by using HCl. The Langmuir,37,38 Freundlich,39,40 and Temkin41 isotherm equations are used to interpret the adsorption experimental data. The Langmuir isotherm model is based on monolayer adsorption on the active sites of the adsorbent to evaluate the adsorption system. The expression of the Langmuir model is expressed by eq 3

Figure 2. SEM images of (a) GO, (b) Au(III), (c) Pd(II), and (d) Pt(IV) adsorbed onto GO, respectively (6000×).

The images demonstrated that before adsorption the surface of GO appeared to have a smoother surface than after adsorption of Au(III), Pd(II), and Pt(IV). These changes shows the adsorption of metal ions on the surface of GO. Effect of pH. The pH value of solution is an important parameter for the adsorption of metal ions onto adsorbents. In this work, the pH values were adjusted to a range of 1.0 to 7.0. The initial concentrations of Au(III), Pd(II), and Pt(IV) were 60 mg·dm−3, 30 mg·dm−3, and 30 mg·dm−3, respectively. A sample of 3.0 mg of adsorbent was used, and the desired pH value was adjusted using HCl. Figure 3 showed the effect of pH on Au(III), Pd(II), and Pt(IV) adsorption by GO, respectively. The results indicated that the adsorption capacity of the metal ions onto GO increased significantly with the pH values increase and reached a maximum when pH was 6.0 for Au(III), Pd(II), and Pt(IV). Then the adsorption capacity decreased obviously. The optimum pH value 6.0 for Au(III), Pd(II), and Pt(IV) was chosen throughout the subsequent experiments. The adsorption mechanisms of metal ions onto GO suggest that the interaction of metal ions with GO most likely happens

qe =

qmbCe 1 + bCe

(3)

−1

where qe (mg·g ) is the adsorption capacity at equilibrium, qm (mg·g−1) is the theoretical saturation adsorption capacity for monolayer coverage, Ce (mg·dm−3) is the concentration of metal ions at equilibrium, and b (dm3·mg−1) is the Langmuir C

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constant related to the affinity of binding sites and is a measure of the energy of adsorption. The Freundlich isotherm assumes heterogeneous adsorption due to the diversity of the adsorption sites or the diverse nature of the metal ions adsorbed. The Freundlich adsorption equation is given as eq 4 qe = kCe1/ n

Table 2. Adsorption Isotherm Parameters for Au(III), Pd(II), and Pt(IV) onto 3.0 mg GO at 298 Ka isotherm Langumuir

Freundlich

(4)

where k and n are the Freundlich constants, related to the adsorption capacity of adsorbent and adsorption intensity, respectively. The Temkin isotherm considers the effects of the heat of adsorption that decreases linearly with coverage of the adsorbate and adsorbent interactions. The Temkin isotherm has been used in the form as eq 5 qe =

RT RT ln A + ln Ce b b

Temkin

parameter −1

qm (mg·g ) b (dm3·mg−1) R2 K N R2 A (dm3·g−1) B R2

Au(III)

Pd(II)

Pt(IV)

108.342 0.3939 0.9996 37.313 3.4724 0.8713 36.342 18.417 0.9332

80.775 0.4303 0.9989 30.473 3.3176 0.8934 27.586 15.355 0.9522

71.378 0.2695 0.9978 22.173 2.9752 0.8898 16.226 14.726 0.9476

a

Standard uncertainties u are u(m) = 0.2 mg, u(V) = 0.03 cm3, and u(T) = 1 K.

are between 1 and 10, suggesting that the adsorption is favorable at the studied conditions. The Temkin isotherm values A and B in Table 2 indicate that the heat of adsorption decreases linearly with coverage due to the adsorbent−adsorbate interactions and adsorption is characterized by a uniform distribution of the binding energies up to the maximum binding energy. Adsorption Kinetics. The kinetics of adsorption that describes the solute uptake rate governing the contact time of the adsorption reaction is one of the important characteristics that define the efficiency of adsorption. In this study, the kinetics of the adsorption process was studied by batch experiments at room temperature (298 K). A sample of 3.0 mg of GO was used in each experiment, and the metal initial concentrations were 50 mg·dm−3, 30 mg·dm−3, and 30 mg·dm−3 for Au(III), Pd(II), and Pt(IV), respectively. The adsorption kinetic data of Au(III), Pd(II), and Pt(IV) are analyzed in terms of pseudofirst-order,42 pseudosecond-order,43 and intraparticle diffusio44,45 kinetic equations. The pseudofirst-order kinetic model represented as eq 6

(5)

where B = RT/b, R (8.3145 J·mol−1·K−1) is the gas constant, T (K) is the temperature, and b (J·mol−1) is the Temkin constant related to heat of adsorption. A (dm3·g−1) is the equilibrium binding constant, qe (mg·g−1) is the adsorption capacity at equilibrium, and Ce (mg·dm−3) is the concentration of metal ions at equilibrium. A plot of qe versus ln Ce enables one to determine the constants, A and B. The adsorption data of Au(III), Pd(II)m and Pt(IV) onto GO are shown in Figure 4. Table 2 displays the adsorption

log(qe − qt ) = log qe −

k1 t 2.303

(6)

−1

where qe and qt (mg·g ) are the capacities of Au(III) and Pd(II) adsorbed at equilibrium and time t (h). k1 (min−1) is the pseudofirst-order rate constant. The qe and rate constant can be calculated by plotting the log (qe − qt) vs t. The pseudosecond-order equation after transformation into linear form can be written as eq 7 t 1 t = + 2 qt qe k 2qe (7)

Figure 4. Effect of initial concentrations on Au(III), Pd(II), and Pt(IV) adsorption by 3.0 mg GO for 7 h at 298 K. ■, Au(III), initial concentrations: (10 to 90) mg·dm−3; ●, Pd(II), initial concentrations: (10 to 50) mg·dm−3; ▲, Pt(IV), initial concentrations: (10 to 50) mg·dm−3.

where qe and qt (mg·g−1) are the capacity of Au(III) and Pd(II) adsorbed at equilibrium and time t (h), respectively. k2 (g·mg−1·min−1) is the pseudosecond-order rate constant. Additionally, h (mg·g−1·min−1) stands for original adsorption rate which can be defined as h = k2qe2. The intraparticle diffusion kinetic model can be written as eq 8

isotherms parameters for Au(III), Pd(II), and Pt(IV) onto GO at 298 K. It can be seen from Table 2 that the Langmuir model shows the best fit (R2 > 0.99) compared to other isotherm models under the experimental conditions, implying a monolayer adsorption and a favorable adsorption. For the Langmuir isotherm, the maximum adsorption capacity qm for the adsorption of Au(III), Pd(II), and Pt(IV) onto GO are 108.342 mg·g−1, 80.775 mg·g−1, and 71.378 mg·g−1, respectively. For the Freundlich isotherm, the n values

qt = k idt 0.5

(8) −1

−0.5

where kid (mg·g ·min ) is the intraparticle diffusion rate constant, which can be obtained from the slope of the plot qt vs t0.5. Figure 5 shows the effect of contact time on Au(III), Pd(II), and Pt(IV) adsorption by 3.0 mg GO at 298 K. It can be seen D

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range of adsorption may be chemical adsorption. Therefore, the rate-limiting step for the adsorption of Au(III), Pd(II), and Pt(IV) onto GO may be chemical adsorption, in agreement with the results by other groups.47 Additionally, the coefficients of the intraparticle diffusion kinetic model given in Table 3 and the intraparticle diffusion model obtained in Figure 6 show that it is multilinearity with

Figure 5. Effect of contact time on Au(III), Pd(II), and Pt(IV) adsorption by 3.0 mg GO at 298 K. ■, Au(III), initial concentration 50 mg·dm−3 and contact time 7 h; ●, Pd(II), initial concentration 30 mg·dm−3 and contact time 300 min; ▲, Pt(IV), initial concentration 30 mg·dm−3 and contact time 240 min.

from Figure 5 that the adsorption capacity of metal ions increases with the increasing adsorption time and the adsorption capacity reaches the equilibrium around 6 h, 2 h, and 40 min in case of Au(II), Pd(II), and Pt(IV), respectively. It might due to the availability of vacant sites that saturated with the increasing adsorption time. The corresponding kinetic parameters are listed in Table 3. As shown in Table 3, the R2 values of pseudosecond-order model for the adsorption of Au(II), Pd(II), and Pt(IV) are higher than that of pseudofirst-order model. Also, the qe values obtained by the pseudosecond-order equation are 110.011 mg·g−1, 75.019 mg·g−1, and 57.013 mg·g−1, which are close to the values obtained by the experiment. However, the calculated qe values obtained by the pseudofirst-order equation are not in agreement with the experimental qe values, suggesting that the adsorption of Au(II), Pd(II), and Pt(IV) does not follow the pseudofirst-order kinetics. Based on the higher R2 values and the agreement of the qe values with the experimental values, the adsorption kinetics of Au(III), Pd(II), and Pt(IV) can be welldescribed by the pseudosecond-order model. Moreover, the pseudosecond-order kinetics based on the assumption that the rate-limiting step may be chemical adsorption and the adsorption behavior may involve valence forces through sharing or exchange of electrons between adsorbent and adsorbate.46 Hence, it is more likely to predict the behavior over the whole

Figure 6. Intraparticle diffusion model on Au(III), Pd(II), and Pt(IV) adsorption by 3.0 mg of GO at 298 K. ■, Au(III), initial concentration 50 mg·dm−3 and contact time 7 h; ●, Pd(II), initial concentration 30 mg·dm−3 and contact time 300 min; ▲, Pt(IV), initial concentration 30 mg·dm−3 and contact time 240 min.

three different stages of adsorption, suggesting the adsorption mechanism is quite complex. The first adsorption stage is the initial curved adsorption relating to the external surface adsorption. The second adsorption stage describes the gradual adsorption stage. The final adsorption is attributed to the equilibrium stage. The kinetic results manifest that the intraparticle diffusion may not be the rate-limiting step. The adsorption kinetics of Au(III), Pd(II), and Pt(IV) can be well-described by the pseudosecond-order model, suggesting that the chemical adsorption may be the rate-limiting step. Thermodynamic Studies. The effect of temperature for the adsorption of metal ions onto 3.0 mg of GO was performed at (303, 313, and 323) K. The initial concentrations with desired pH were (50, 40, and 40) mg·dm−3 for Au(III), Pd(II), and Pt(IV), respectively. The solutions were shaken for 7 h.

Table 3. Coefficients of Pseudofirst-Order and Pseudosecond-Order Kinetic Parameters and Intraparticle Diffusion Modela pseudofirst-order kinetic

pseudosecond-order kinetic

intraparticle diffusion

qe,exp

k1

qe(cal)

k2

h

qe(cal)

metal

mg·g−1

min−1

mg·g−1

R2

g·mg−1·min−1

mg·g−1·min−1

mg·g−1

R2

mg·g−1·min−0.5

R2

Au(III) Pd(II) Pt(IV)

98.645 67.077 55.333

0.01126 0.02522 0.05504

78.842 55.162 30.371

0.9847 0.9844 0.9055

0.000197 0.000506 0.003350

2.3867 2.8463 10.8897

110.011 75.019 57.013

0.9987 0.9948 0.9988

0.0091 0.0133 0.0175

0.9292 0.7863 0.5056

kid

Standard uncertainties u are u(m) = 0.2 mg, u(V) = 0.03 cm3, u(c0, Au) = 0.1 mg·dm−3, u(c0, Pd) = 0.1 mg·dm−3, u(c0, Pt) = 0.1 mg·dm−3, u(ce, Au) = 0.2 mg·dm−3, u(ce, Pd) = 0.1 mg·dm−3, u(ce, Pt) = 0.1 mg·dm−3 and u(T) = 1 K, and the combined expanded uncertainties Uc are Uc(c0 − ce, Au) = 0.2 mg·dm−3, Uc(c0 − ce, Pd) = 0.1 mg·dm−3, Uc(c0 − ce, Pt) = 0.1 mg·dm−3, Uc(qe,exp, Au) = 0.032 mg·g−1, Uc(qe,exp, Pd) = 0.030 mg·g−1, and Uc(qe,exp, Pt) = 0.030 mg·g−1.

a

E

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The results are shown in Figure 7. It is obvious that the adsorption capacity increased with the temperature ranging

Table 4. Thermodynamic Parameters for Au(III), Pd(II), and Pt(IV) onto 3.0 mg of GOa ΔGo

temperature metal

K

Au(III)

303 313 323 303 313 323 303 313 323

Pd(II)

Pt(IV)

−1

J·mol

−916.94 −4017.35 −6906.79 −375.49 −1950.99 −3929.73 −167.85 −1209.59 −2136.87

ΔHo

ΔSo

−1

J·mol−1·K−1

R2

89.849

299.668

0.9995

53.388

177.231

0.9953

29.682

98.573

0.9988

kJ·mol

a

Standard uncertainties u are u(m) = 0.2 mg, u(V) = 0.03 cm3, u(c0, Au) = 0.1 mg·dm−3, u(c0, Pd) = 0.1 mg·dm−3, u(c0, Pt) = 0.1 mg·dm−3, u(ce, Au) = 0.2 mg·dm−3, u(ce, Pd) = 0.1 mg·dm−3, u(ce, Pt) = 0.1 mg·dm−3, and u(T) = 1 K, and the combined expanded uncertainties Uc are Uc(ΔGo, Au, 303 K) = 0.01 J·mol−1, Uc(ΔGo, Au, 313 K) = 0.02 J·mol−1, Uc(ΔGo, Au, 323 K) = 0.06 J·mol−1, Uc(ΔGo, Pd, 303 K) = 0.01 J·mol−1, Uc(ΔGo, Pd, 313 K) = 0.01 J·mol−1, Uc(ΔGo, Pd, 323 K) = 0.01 J·mol−1, Uc(ΔGo, Pt, 303 K) = 0.01 J·mol−1, Uc(ΔGo, Pt, 313 K) = 0.01 J·mol−1, and Uc(ΔGo, Pt, 323 K) = 0.01 J·mol−1.

Figure 7. Effect of temperature on Au(III), Pd(II), and Pt(IV) adsorption by 3.0 mg of GO for 7 h. ■, Au(III), initial concentration 50 mg·dm−3; ●, Pd(II), initial concentration 40 mg·dm−3; ▲, Pt(IV), initial concentration 40 mg·dm−3.

with ultrapure water. Then 10 cm3 of desorption agents were added and shaken for 6 h, and the concentrations were determined by FAAS. Table 5 presents the desorption results, Table 5. Desorption Data of Au(III), Pd(II), and Pt(IV)a

from (303 to 323) K, and this may be due to the increase in collision frequency between GO and metal ions. The data obtained by the effect of temperature studies was used to estimate thermodynamic parameters such as Gibbs energy (ΔGo), enthalpy (ΔHo), and entropy (ΔSo), which are determined by eqs 9 to 11:48 Kc =

CAe Ce

desorption (%) desorption agent 0.3 0.5 1.0 1.5

(9)

o

ΔG = −RT ln Kc o

(10)

0.2 0.5 1.0 1.5

o

ΔS ΔH ln Kc = − (11) R RT −1 −1 where R (8.3145 J·mol ·K ) is the ideal gas constant, T (K) is the absolute temperature, Kc is the thermodynamic equilibrium constant, and CAe and Ce (mg·dm−3) are the equilibrium concentrations of Au(III), Pd(II), and Pt(IV) on the adsorbent and in solution, respectively. The values of ΔHo (kJ·mol−1) and ΔSo (J·mol−1·K−1) are calculated from the slope and intercept of the van't Hoff linear plots of ln K vs 1/T by using eq 11. The results are presented in Table 4. The negative values of ΔGo obtained at all of the experimental temperatures indicated the spontaneous nature of the adsorption process, and they increased with the temperature increase, indicating a higher adsorption capacity at higher temperatures. The values of ΔHo were positive, indicating that the adsorption of Au(III), Pd(II), and Pt(IV) onto GO was endothermic. The positive values of ΔSo mean an irregular increase of the randomness at the GO/solution interface during the adsorption process. Desorption Studies and Reuse. The desorption percentage of adsorbed Au(III), Pd(II), and Pt(IV) onto 3.0 mg of GO was studied by various concentrations of 10 cm3 thiourea, HCl, and thiourea−HCl solutions at room temperature. A sample of 3.0 mg of GO adsorbed with 10 cm3 metal ions was washed

Thiourea thiourea thiourea thiourea thiourea Thiourea−HCl mol·dm−3 thiourea−0.5 mol·dm−3 mol·dm−3 thiourea−0.5 mol·dm−3 mol·dm−3 thiourea−0.5 mol·dm−3 mol·dm−3 thiourea−0.5 mol·dm−3 mol·dm−3 mol·dm−3 mol·dm−3 mol·dm−3

HCl HCl HCl HCl

Au(III)

Pd(II)

Pt(IV)

28.79 29.50 35.22 31.01

35.05 44.70 75.86 71.85

45.51 58.33 90.38 80.77

41.56 95.45 60.32 58.05

51.32 67.28 94.64 71.99

70.20 86.75 99.37 93.38

a Standard uncertainties u are u(m) = 0.2 mg, u(V) = 0.03 cm3, u(c0, Au) = 0.1 mg·dm−3, u(c0, Pd) = 0.1 mg·dm−3, u(c0, Pt) = 0.1 mg·dm−3, u(ce, Au) = 0.2 mg·dm−3, u(ce, Pd) = 0.1 mg·dm−3, u(ce, Pt) = 0.1 mg·dm−3, and u(T) = 1 K.

showing that the highest desorption percentage reaches to 95.45 % in the case of Au(III), 94.64 % in the case of Pd(II), and 99.37 % in the case of Pt(IV) when 0.5 mol·dm−3 thiourea−0.5 mol·dm−3 HCl, 1.0 mol·dm−3 thiourea−0.5 mol·dm−3 HCl, 1.0 mol·dm−3 thiourea−0.5 mol·dm−3 HCl are used as desorption agents for Au(III), Pd(II), and Pt(IV), respectively. Table 6 shows the adsorption and desorption process for three cycles, demonstrating that the adsorption capacities of Au(III), Pd(II), and Pt(IV) onto GO are not significantly changed. Therefore, GO has high reusability and can be well used for the recovery of Au(III), Pd(II), and Pt(IV).



CONCLUSIONS The adsorption of Au(III), Pd(II), and Pt(IV) was investigated in this study. It was demonstrated that GO was efficient in F

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Table 6. Adsorption and Desorption Recyclea Au(III)

Pd(II)

Pt(IV)

cycle

q/(mg·g−1)

desorption (%)

q/(mg·g−1)

desorption (%)

q/(mg·g−1)

desorption (%)

1 2 3

99.667 98.530 97.010

95.45 91.42 89.33

70.333 68.256 66.630

94.64 92.93 87.56

59.216 53.374 51.652

99.37 94.32 91.14

Standard uncertainties u are u(m) = 0.2 mg, u(V) = 0.03 cm3, u(c0, Au) = 0.1 mg·dm−3, u(c0, Pd) = 0.1 mg·dm−3, u(c0, Pt) = 0.1 mg·dm−3, u(ce, Au) = 0.2 mg·dm−3, u(ce, Pd) = 0.1 mg·dm−3, u(ce, Pt) = 0.1 mg·dm−3, and u(T) = 1 K, and the combined expanded uncertainties Uc are Uc(q, Au) = 0.034 mg·g−1, Uc(q, Pd) = 0.031 mg·g−1, and Uc(q, Pt) = 0.031 mg·g−1. a

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adsorption for Au(III), Pd(II), and Pt(IV) from aqueous solutions. The adsorption process was dependent on the pH values, initial concentration, and contact time. The desorption studies showed that the desorption process could be influenced by the kind of eluents and GO can be well used repeatedly. Hence, maybe GO is a promising adsorbent for the determination and recovery of Au(III), Pd(II), and Pt(IV) in the future.



AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] Notes

The authors declare no competing financial interest.



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H

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