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Adsorption of Pt(IV) and Pd(II) by calcined dried aluminum hydroxide gel from aqueous solution system
G Model
JECE 125 1–7 Journal of Environmental Chemical Engineering xxx (2013) xxx–xxx
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Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece 1 2 3 4 5
Adsorption of Pt(IV) and Pd(II) by calcined dried aluminum hydroxide gel from aqueous solution system Q1 Fumihiko
Ogata, Naohito Kawasaki *
Faculty of Pharmacy, Kinki University, 3-4-1 Kowakae, Higashi-Osaka, Osaka 577-8502, Japan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 26 June 2013 Accepted 7 August 2013
In this study, calcined dried aluminum hydroxide gels (Gs) were prepared at 300–1000 8C (G300–G1000) as adsorbents. The properties of the adsorbents (i.e., XRD patterns, SEM images, specific surface areas, and number of hydroxyl groups) were investigated. The adsorption of Pt(IV) and Pd(II) onto Gs at different temperatures was evaluated. Calcination transformed virgin G into g- and a-alumina. G600 had the largest specific surface area (107.2 m2/g) and number of hydroxyl groups (1.12 mmol/g) of the Gs. G600 also adsorbed the greatest amount of Pt(IV) and Pd(II). The amount of Pt(IV) and Pd(II) adsorbed was more closely related to the number of hydroxyl groups than the specific surface area. The adsorption mechanism of Pt(IV) and Pd(II) onto G600 mainly involved ion exchanges. The optimal pH for the adsorption of Pt(IV) and Pd(II) onto G600 was 5.0, which suggests that [PtCl5(OH)]2 and [PdCl3(OH)]2 species are suitable for adsorption. Pt(IV) and Pd(II) compete with chloride ions for adsorption sites on G600 in the aqueous solution system. The adsorption of Pt(IV) and Pd(II) onto G600 reached equilibrium within 24 h. The experimental data fit the pseudo-second-order model (correlation coefficient: 0.986– 0.995) better than the pseudo-first-order model (correlation coefficient: 0.879–0.973). Moreover, the Weber–Morris plot also was evaluated. The adsorption isotherms of Pt(IV) and Pd(II) onto G600 fit the Freundlich and Langmuir models, respectively. Thus, G600 is useful for the adsorption of Pt(IV) and Pd(II) in aqueous solution. ß 2013 Published by Elsevier Ltd.
Keywords: Platinum Palladium Adsorption Dried aluminum hydroxide gel
6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
Introduction Platinum group metals (PGMs), e.g., platinum and palladium, are precious metals that are widely used in industry because of their specific physical and chemical properties. PGMs are also of interest due to their high value and catalytic properties [1]. Among PGMs, about 34% of Pt and 55% of Pd are consumed in the production of auto catalysts [2]. The catalysts containing Pt and/or Pd have the capability to oxidize carbon monoxide and hydrocarbons [3]. Generally, an overall recovery higher than 95% is necessary for a process to be cost-effective [2]. The two most important reasons and motivations for precious metal separation are the economical impact of losing these metals and their Q2 environmental concerns. The presence of PGMs in environment may cause severe primary skin and eye irritations [4]. Natural resources of platinum and palladium group metals are limited, and because of their extensive use, their demand is increasing. Low rate of production of these metals due to their low concentration in
* Corresponding author. Tel.: +81 6 6730 5880x5556; fax: +81 6 6721 2505. E-mail addresses: [email protected] (F. Ogata), [email protected] (N. Kawasaki).
related ores and their high costs of production from naturally occurring supplies has made precious metals’ recovery from aqueous solution and spent catalyst a viable and cost-effective alternative of their preparation [5]. Many recent studies have focused on the extraction and separation of precious metals because of both the increasing industrial need for these metals and their limited resources [6]. Separation of these metal ions is still problematic and, because of their complex chemistry and the overlapping properties, represents a real challenge [7,8]. The effective recovery of PGMs from both natural ore and industrial waste is important for full utilization of resources. Conventional methods can be used for the adsorption of PGMs from solution [9–14]; among these methods, adsorption plays a particularly important role in the elimination of metal ions from aqueous solutions to minimize water pollution [15]. Different kinds of adsorbents are applied in adsorption process for metal separation from dilute solutions, such as carbon adsorbents, biosorbents and polymeric adsorbents. Adsorbents should be available at low cost and easy recycle [5]. Modified chitosan, bayberry tannin, and biomass had also been reported. Dried aluminum hydroxide gel (G), which has hydroxyl groups on its surface, is widely used as an antacid. The component of G is more than 50% Al2O3. It can adsorb phosphate, arsenic, and
2213-3437/$ – see front matter ß 2013 Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.jece.2013.08.011
Please cite this article in press as: F. Ogata, N. Kawasaki, Adsorption of Pt(IV) and Pd(II) by calcined dried aluminum hydroxide gel from aqueous solution system, J. Environ. Chem. Eng. (2013), http://dx.doi.org/10.1016/j.jece.2013.08.011
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JECE 125 1–7 2
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Nomenclature q C0 Ce V M qe qt k1 k2 t ki K 1/n a Ws
–the amount of Pt(IV) or Pd(II) adsorbed (mg/g); –the concentration of Pt(IV) or Pd(II) before adsorption (mg/L); –the concentration of Pt(IV) or Pd(II) after adsorption (mg/L); –the volume of solvent (L); –the mass of adsorbent (g); –the amount adsorbed on the adsorbent (mg/g) at equilibrium; –the amount adsorbed on the adsorbent (mg/g) at time t (h); –the pseudo-first-order rate constant (h1); –the pseudo-second-order rate constant (g/mg h); –time (h); –the intraparticle diffusion constant (mg/g min0.5); –the Freundlich capacity factor; –the Freundlich intensity parameter; –the constant related to the affinity to the binding sites during the adsorption (L/mg); –is the maximum monolayer adsorption capacity (mg/g).
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chromium ions, and the amount adsorbed depends on the number of hydroxyl groups on G [16]. Moreover, the adsorption of perrhenate ions, fluoride ions, and quinidine gluconate onto G has also been reported [17,18]. However, G has not yet been reported to adsorb platinum and palladium. Since G is an aluminum compound, it is possible to desorb the adsorbate using a desorption solution. If adsorption of platinum and palladium onto G were possible, this method would be useful for the recovery of PGMs. Moreover, the adsorption of Pt(IV) and Pd(II) complexes onto alumina for the purpose of the preparation of dispersed metal catalysts has been investigated [19–23]. It has been suggested that the adsorption of Pt(IV) and Pd(II) onto alumina can take place by the exchange of one chloride ion in the coordination sphere of PtCl62 and PdCl42 complexes in solution with OH or H2O. It has also been reported that the solution pH and chloride ion concentration influence the adsorption of metal complexes [15]. In this work, G was used to adsorb platinum and palladium from aqueous solutions by batch sorption. The effects of contact time, chloride ion concentration, and pH of the aqueous solution on the efficiency of platinum and palladium adsorption were also investigated. Finally, the adsorption isotherms, adsorption rate, and mechanism of adsorption were investigated.
69
Material and methods
70
Materials
71 72 73 74 75 76 77 78 79 80 81
Dried aluminum hydroxide gel was obtained in the form of a white amorphous powder (Tomita Pharmaceutical Co., Ltd.). G (20 g) was placed in a magnetic crucible and heated to the target temperature over 2 h in a muffle furnace. The temperature was maintained at the desired temperature, i.e., 300–1000 8C, for 2 h to form G300–G1000, respectively [16]. XRD analysis was performed using a RINT2100V diffractometer (Rigaku, Japan). Electron microscopy was carried out using a scanning electron microscope (JSM-5200; JEOL, Japan), and the specific surface area of the G was measured using a specific surface analyzer (NOVA4200e; Yuasa Ionics, Japan). The number of
hydroxyl groups was calculated based on the number of adsorbed fluoride ions [24].
82 83
Amount of Pt(IV) or Pd(II) adsorbed onto virgin G and Gs calcined at different temperatures
84 85
Adsorbent (0.02 g, virgin G and G300–G1000) was added to 50 mL of Pt(IV) or Pd(II) solution (10 mg/L). The suspensions were shaken at 100 rpm for 24 h at 25 8C. The sample was then filtered through a 0.45 mm membrane filter, and the filtrate was analyzed using an inductively coupled plasma-atomic emission spectrometer (ICP-AES, Shimadzu). The amount of Pt(IV) or Pd(II) adsorbed in single solution was calculated using Eq. (1), as follows:
86 87 88 89 90 91 92
q ¼ ðC 0 C e ÞV=M;
(1)
where q is the amount of Pt(IV) or Pd(II) adsorbed (mg/g), C0 is the concentration of Pt(IV) or Pd(II) before adsorption (mg/L), Ce is the concentration of Pt(IV) or Pd(II) after adsorption (mg/L), V is the volume of solvent (L), and M is the mass of adsorbent (g).
95 94 93 96 97 98
Amount of Pt(IV) or Pd(II) adsorbed onto G600
99
Effect of the solution pH G600 (0.02 g) was added to 50 mL of Pt(IV) or Pd(II) solution (10 mg/L) with pH values of 3–8, which were adjusted using a sodium hydroxide solution.
100 101 102 103
Saturated amount adsorbed G600 (0.02 g) was added to 50 mL of Pt(IV) or Pd(II) solution at 50 mg/L.
104 105 106
Effect of chloride-ion concentration G600 (0.02 g) was added to 50 mL of Pt(IV) or Pd(II) solution (10 mg/L) with different chloride-ion concentrations (i.e., 0.01 or 0.04 mol/L).
107 108 109 110
Adsorption isotherms G600 (0.02 g) was added to 50 mL of Pt(IV) or Pd(II) solutions at different initial concentrations (0.5–20 mg/L). The suspensions (2.3.1–2.3.4) were shaken at 100 rpm for 24 h at 25 8C. The samples were filtered through 0.45 mm membrane filters, and the amount adsorbed was then measured using the method described above.
111 112 113 114 115 116 117
Effect of contact time on the adsorption of Pt(IV) and Pd(II) onto G600
118
G600 (0.02 g) was added to 50 mL of Pt(IV) or Pd(II) solution (10 mg/L). The suspensions were shaken at 100 rpm for 0.5–48 h at 25 8C. The sample was filtered through a 0.45 mm membrane filter, and the amount adsorbed was then measured using the method described above.
119 120 121 122 123
Results and discussion
124
Properties of virgin and calcined G
125
The results of the XRD analyses of the Gs are shown in Fig. 1 and confirm that virgin G and G300 have similar structures while G400–G700 have amorphous structures [16]. The structures of G800–G1000 were significantly different from that of virgin G. The results of the XRD analyses indicate that G900 and G1000 have gand a-type structures and that calcining virgin G above 600 8C transforms its structure into the g-type. The 2u value of about 928 and 1368 indicated that g-alumina was produced from Virgin G. Moreover, the 2u value of about 1028 and 1238 suggested that a-alumina was produced from Virgin G. The specific surface area
126 127 128 129 130 131 132 133 134 135
Please cite this article in press as: F. Ogata, N. Kawasaki, Adsorption of Pt(IV) and Pd(II) by calcined dried aluminum hydroxide gel from aqueous solution system, J. Environ. Chem. Eng. (2013), http://dx.doi.org/10.1016/j.jece.2013.08.011
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JECE 125 1–7 F. Ogata, N. Kawasaki / Journal of Environmental Chemical Engineering xxx (2013) xxx–xxx
3
1.1 Specific surfaec area (m2/g)
105
Fig. 1. XRD of virgin G and Gs calcined at different temperatures (~) g-alumina and (*) a-alumina.
1
100
and amount of hydroxyl groups of the virgin G and Gs that were calcined at different temperatures are presented in Fig. 2. Virgin G had a specific surface area of 94.9 m2/g and a hydroxyl group density of 1.00 mmol/g. The specific surface areas of virgin G and the calcined Gs were not significantly different. G600 had the largest specific surface area of 107.2 m2/g. The densities of hydroxyl groups of virgin G and G300–G700 were also quite similar; however, that of G800–G1000 decreased with increasing calcination temperature. G600 also had the highest density of hydroxyl groups (1.12 mmol/g). SEM images of virgin G and the calcined Gs are shown in Fig. 3: The images indicate that the particles did not distinguishably change during the calcining process and only the crystal type was affected by calcination. Moreover, it is clearly observed the sphere and had a particle size distribution of around 100 mm.
151 152
Amount of Pt(IV) and Pd(II) adsorbed onto virgin G and Gs calcined at different temperatures
153 154 155 156 157 158 159 160 161 162
The amount of Pt(IV) and Pd(II) adsorbed onto the Gs is shown in Fig. 4; G600 adsorbed the most Pt(IV) and Pd(II). To evaluate the relationship between the amount adsorbed and adsorbent properties, we plotted the amount adsorbed against the specific surface area and density of surface hydroxyl groups of the adsorbents. The correlation coefficients between the amount adsorbed and specific surface area ranged from 0.142 to 0.270 (data not shown), which indicates that the specific surface area is not related to the adsorption of Pt(IV) and Pd(II). In contrast, the correlation coefficients between the amount adsorbed and density
0.8
95
0.7 90 0.6
85
136 137 138 139 140 141 142 143 144 145 146 147 148 149 150
0.9
Density of hydroxyl group (mmol/g)
1.2
110
Virgin G G300 G400 G500 G600 G700 G800 G900 G1000
0.5
Fig. 2. Specific surface area and density of hydroxyl group of virgin G and Gs calcined at different temperatures.
of surface hydroxyl groups ranged from 0.974 to 0.975 (Fig. 5), which suggests that the density of surface hydroxyl groups is closely related to the adsorption of Pt(IV) and Pd(II). Moreover, Ogata et al. (2013) reported that the adsorption mechanism of Pt(IV) and Pd(II) onto calcined gibbsite (aluminum hydroxide) is related to the number of surface hydroxyl groups [25]. Therefore, the mechanism of Pt(IV) and Pd(II) adsorption onto G600 is likely similar to that for calcined gibbsite. At saturation, 23.9 and 23.4 mg/g of Pt(IV) and Pd(II), respectively, can be adsorbed onto G600. Previous studies reported that the maximum amounts of Pt(IV) that can be adsorbed onto modified chitosan and Fe3O4 nanoparticles are 43.1 and 13.3 mg/g, respectively, while those of Pd(II) are in the range of 11.0–29.3 mg/ g [26,27]. Comparison of the Pt(IV) and Pd(II) adsorption capacities of the various adsorbent is listed in Table 1 [28–30]. Amount of Pt(IV) and Pd(II) adsorbed onto bayberry tannin immobilized collagen fiber membrane, PA-Lignin, and amberlite IRC718 was greater than that onto G600. But, the price of G600 was low compared to other adsorbents. Moreover, the amount adsorbed onto G600 is greater than that onto calcined gibbsite (aluminum compound), which indicates that the crystal structure of G600 is well suited for the adsorption of Pt(IV) and Pd(II) [25]. The adsorption of Pt(IV) and Pd(II) onto virgin G has not been reported. As described above, G600 would be useful for the adsorption of Pt(IV) and Pd(II) from aqueous solutions.
Fig. 3. SEM images of virgin G and Gs calcined at different temperatures.
Please cite this article in press as: F. Ogata, N. Kawasaki, Adsorption of Pt(IV) and Pd(II) by calcined dried aluminum hydroxide gel from aqueous solution system, J. Environ. Chem. Eng. (2013), http://dx.doi.org/10.1016/j.jece.2013.08.011
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JECE 125 1–7 F. Ogata, N. Kawasaki / Journal of Environmental Chemical Engineering xxx (2013) xxx–xxx
4
Table 1 Comparison of the Pt(IV) and Pd(II) adsorption capacities of the various adsorbent.
10
Adsorption capacity (mg/g)
Adsorbent
Amount adsorbed (mg/g)
8
30 -Nitro-4-amino azobenzene modified chitosan Fe3O4 nano-particles Bayberry tannin immobilized collagen fiber membrane PA-Lignin Amberlite IRC718 G600
6
4
2
0 Virgin G G300 G400 G500 G600 G700 G800 G900 G1000
188
Fig. 4. Amount of Pt(IV) and Pd(II) adsorbed onto virgin G and Gs calcined at different temperatures.
0.07
Amount adsorbed (mmol/g)
0.06 0.05 0.04 0.03
y = -0.011 + 0.05x r= 0.974 y = -0.050 + 0.10x r= 0.975
0.02
: Pt(IV) : Pd(II)
0.01 0 0.5
0.6
0.7
0.8
0.9
1
1.1
1.2
Amount of hydroxyl groups (mmol/g) Fig. 5. Relationship between amount of Pt(IV) or Pd(II) adsorbed and density of hydroxyl groups.
Pt(IV)
Pd(II)
43.1
29.3
[26]
13.3 45.8
11.0 33.4
[27] [28]
66.3 42.9 23.9
58.8 40.4 23.4
[29] [30] This work
Effect of pH on the adsorption of Pt(IV) and Pd(II) onto G600 Since the adsorption of ions is usually influenced by the pH of the solution, it is important to assess the adsorption behavior of Pt(IV) and Pd(II) onto G600 in solutions with different pH values. The results of the adsorption of Pt(IV) and Pd(II) onto G600 in solutions with different pH values are shown in Fig. 6: The optimal pH for the adsorption of Pt(IV) and Pd(II) is 5.0. The speciation of platinum and palladium altered with the pH values; [PtCl5(OH)]2 and [PdCl3(OH)]2 species were present at a pH of 5.0, which suggests that these ions are suitable for adsorption [31,32]. Pd(II) precipitated when it is used at pH > 5.0, which indicated that palladium hydroxide was generated. However, this phenomenon was not observed with Pt(IV) solution. With increasing pH, the negative charge associated with G600 increases, which suggests that adsorption of Pt(IV) and Pt(II) was not easily occurred. On the other hand, with decreasing pH, the positive charge associated with G600 increased. Moreover, [PtCl5(OH)]2 and [PdCl3(OH)]2 species decreased at lower pH, and then adsorption capability of Pt(IV) and Pd(II) onto G600 was lower. The adsorption mechanism of Pt(IV) and Pd(II) onto g-Al2O3 has been reported to involve ion exchange or coordinative bond formation on the g-Al2O3 surface [33]. The component of G600 was similar to g-Al2O3. In this study, the amount of Pt(IV) and Pd(II) adsorbed is related to the number of hydroxyl groups on G600 (the correlation coefficients are 0.762– 0.908, as shown in Fig. 5), which suggests that the adsorption mechanism of Pt(IV) and Pd(II) onto G600 mainly involves ion exchange. (Fig. 7).
25
[PdCl3(OH)]2− [PtCl5(OH)]2−
Amount adsorbed (mg/g)
189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214
: Pt(IV) : Pd(II)
References
20
[PtCl5(OH)]2−
OH
OH
: Pt(IV) : Pd(II)
OH
[PdCl3(OH)]2−
OH
Al – O – Al – O – Al – O – Al
15
G600 surface
10 OH[PdCl3(OH)]2−
5
OH-
0
[PtCl5(OH)]
OH[PdCl3(OH)]
OH[PtCl5(OH)]2−
Al – O – Al – O – Al – O – Al
0
2
4 6 8 Final pH in solution
10
12
Fig. 6. Amount of Pt(IV) and Pd(II) adsorbed onto G600 at different pH condition.
G600 surface
Fig. 7. Schematic drawing of Pt(IV) and Pd(II) adsorption onto G600.
Please cite this article in press as: F. Ogata, N. Kawasaki, Adsorption of Pt(IV) and Pd(II) by calcined dried aluminum hydroxide gel from aqueous solution system, J. Environ. Chem. Eng. (2013), http://dx.doi.org/10.1016/j.jece.2013.08.011
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JECE 125 1–7 F. Ogata, N. Kawasaki / Journal of Environmental Chemical Engineering xxx (2013) xxx–xxx
Table 2 Kinetic parameters for adsorption of Pt(IV) and Pd(II) onto G600 using pseudo-firstorder and pseudo-second-order models.
20 Amount adsorbed (mg/g)
Samples
15 Pt(IV) Pd(II)
Pt(IV) Pd(II)
Pseudo-second-order model
qe(mg/g)
r
k2 (g/mg/h)
qe (mg/g)
r
0.07 0.06
15.62 5.94
0.973 0.879
0.02 0.05
20.00 9.09
0.995 0.986
within 24 h. The amount of Pt(IV) and Pd(II) adsorbed onto G600 was 20.3 and 8.2 mg/g, respectively. The experimental results suggest that the amount adsorbed increased with increasing contact time. (Correlation coefficient of fitting curves of Pt(IV) and Pd(II) was 0.989 and 0.929, respectively.) In order to elucidate the kinetic mechanism that controls the adsorption process, pseudofirst-order and pseudo-second-order models were employed to interpret the experimental data [36,37], and the correlation was examined to determine the adsorption mechanism of the metal ions onto the solid phase [38]. The pseudo-first-order rate model is expressed as
5
0.01
0.04
Concentration of chloride ion (mol/L) Fig. 8. Effect of chloride ion on adsorption of Pt(IV) and Pd(II).
215
Effect of chloride ions on the adsorption of Pt(IV) and Pd(II) onto G600
216 217 218 219 220 221 222 223 224 225 226 227
The adsorption of Pt(IV) and Pd(II) in solutions including chloride ions was influenced by the concentration of chloride ions and number of hydroxyl groups on alumina [34]. Therefore, we investigated the effect of chloride ions on the adsorption of Pt(IV) and Pd(II) onto G600 (Fig. 8). The amount of Pt(IV) and Pd(II) adsorbed onto G600 in a solution containing 0.01 mol/L chloride ions was greater than that in a solution containing 0.04 mol/L chloride ions, which indicates that Pt(IV) and Pd(II) compete with chloride ions for adsorption sites on G600 (i.e., hydroxyl groups of G600). Uheida et al. and Spieker et al. reported that chloride ions in solution compete with Pt(IV) and Pd(II) for adsorption [27,35]. Similar trends were observed in this study.
228
Effect of contact time on the adsorption of Pt(IV) and Pd(II) onto G600
229 230
The effect of contact time on the adsorption of Pt(IV) and Pd(II) onto G600 is shown in Fig. 9. Adsorption equilibrium was reached
lnðqe qt Þ ¼ ln qe k1 t;
t=qt ¼ 1=k2 q2e þ t=qe ;
5
r= 0.989 (Pt(IV))
y = 0.24 + 0.11x r= 0.986 (Pd(II))
3 t/qt
20
15
10
r= 0.929 (Pd(II))
5
: Pt(IV) : Pd(II)
0 10
20
30
40
50
2 y = 0.12+ 0.05x r= 0.995 (Pt(IV))
1
0
0
10
20
30
40
50
t (h)
Elapsed time (hr) Fig. 9. Adsorption rate of Pt(IV) and Pd(II) onto G600.
244 242 243 245 246 247
(3)
where k2 is the pseudo-second-order rate constant (g/mg h). Plotting t/qt versus t results in a straight line, which enables calculation of the values of qe and the rate constant, k2. Table 2 lists the rate constants obtained from the models. The correlation coefficient (r) for the pseudo-second-order adsorption model was the highest (>0.986), and the adsorption capacities calculated by the model are close to those determined experimentally. However, the correlation coefficient for the pseudo-firstorder model is unsatisfactory. Thus, the pseudo-second-order adsorption model should be used to describe the rates of the adsorption Pt(IV) and Pd(II) onto G600 (Fig. 10).
4
231 232 233 234 235 236 237 238 239 240 241
(2)
where qe and qt are the amounts of Pt(IV) and Pd(II) adsorbed on the adsorbent (mg/g) at equilibrium and at time t (h), respectively, and k1 is the rate constant (h1). The pseudo-second-order rate model is given as
25
Amount adsorbed (mg/g)
Pseudo-first-order model k1 (1/h)
10
0
0
5
Fig. 10. Pseudo-second-order kinetic model for the adsorption of Pt(IV) and Pd(II) onto G600.
Please cite this article in press as: F. Ogata, N. Kawasaki, Adsorption of Pt(IV) and Pd(II) by calcined dried aluminum hydroxide gel from aqueous solution system, J. Environ. Chem. Eng. (2013), http://dx.doi.org/10.1016/j.jece.2013.08.011
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JECE 125 1–7 F. Ogata, N. Kawasaki / Journal of Environmental Chemical Engineering xxx (2013) xxx–xxx
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Table 3 Freundlich and Langmuir constants for adsorption of Pt(IV) and Pd(II) onto G600.
25
Samples
Amount adsorbed (mg/g)
20 Pt(IV) Pd(II)
10
: Pt(IV) : Pd(II)
0 0
10
20
30
40
50
60
Time (min 0.5) Fig. 11. Weber–Morris plots for adsorption of Pt(IV) and Pd(II) onto G600.
The adsorption transport from the solution phase to the surface of the adsorbent particles occurs in several steps. The overall adsorption process may be controlled either by one or more steps, e.g., film or external diffusion, pore diffusion, surface diffusion and adsorption on the pore surface, or a combination of more than one step [39]. The kinetic data were analyzed using the equation proposed by Weber and Morris, as follows: [40] qt ¼ ki t 1=2 ;
(4)
270 Thus, the intraparticle diffusion constant ki (mg/g min0.5), can 269 268 271 be obtained from the slope of the qt (uptake at any time, mg/g) 272 versus the square root of time. If this plot passes through the origin, 273 then intraparticle diffusion is the rate controlling step. Fig. 11 274 Q3 shows the plots of qt versus t1/2, with multilinearity cleary 275 observed in the case of G600, which implies the process involves 276 more than one kinetic stage (or sorption rates). The G600 exhibited 277 three stages. The first was attributed to the sorption of Pt(IV) or
30 Amount adsorbed (mg/g)
Langmuir constants
log K
1/n
R
Ws (mg/g)
a (L/mg)
r
2.9 3.1
0.41 0.33
0.974 0.785
18.1 15.5
<0.01 0.04
0.702 0.994
15
5
261 262 263 264 265 266 267
Freundlich constants
25
20 15 10 : Pt(IV) : Pd(II)
5 0
0
5
10
15
20
Equilibrium concentration (mg/L) Fig. 12. Adsorption isotherms of Pt(IV) and Pd(II) onto G600 in a single solution system.
Pd(II) over the surface of the G600, and; hence, was the fastest sorption stage. The second, ascribed to intraparticle diffusion, was a delay process. The third stage may be regarded as the diffusion through smaller pores, which is followed by the establishment of equilibrium [41]. The calculated intraparticle diffusion constants (ki) for Pt(IV) and Pd(II) were 0.16–0.66 mg/g min0.5 and 0.09– 0.61 mg/g min0.5, respectively.
278 279 280 281 282 283 284
Isotherms of the adsorption Pt(IV) and Pd(II) onto G600
285
Isotherms of the adsorption of Pt(IV) and Pd(II) onto G600 are shown in Fig. 12. The amounts of Pt(IV) and Pd(II) adsorbed onto G600 are 25.5 and 21.9 mg/g, respectively. The experimental data were analyzed via the Freundlich isotherm with multiple-layer adsorption and uniform energy and Langmuir isotherm models based on monolayer adsorption on the active sites of the adsorbent in order to evaluate the adsorption system. The Freundlich isotherm equation can be expressed in the linear form as follows [42]:
286 287 288 289 290 291 292 293 294
log q ¼ log K þ ð1=nÞlog C e ;
(5)
where q is the amount adsorbed (mg/g), Ce is the equilibrium concentration (mg/L), and K and 1/n represent the Freundlich capacity factor and Freundlich intensity parameter, respectively. The Langmuir equation is given as follows: q ¼ aW s C e =ð1 þ aW s Þ;
297 296 295 298 299 300
(6)
where a is a constant related to the affinity to the binding sites during the adsorption (L/mg) and Ws (mg/g) is the maximum monolayer adsorption capacity. The Freundlich and Langmuir parameters for the adsorption of Pt(IV) and Pd(II) onto G600 are listed in Table 3: The correlation coefficients of the Freundlich equation for the adsorption of Pt(IV) and Pd(II) onto G600 are 0.974 and 0.785, respectively, and those of the Langmuir model for Pt(IV) and Pd(II) onto G600 are 0.702 and 0.994, respectively. The experimental data on the adsorption of Pt(IV) or Pd(II) onto G600 were well described by the Freundlich and Langmuir models, respectively, which indicated that the adsorption was thought to be monolayer adsorption onto the surface of the G600. The value Ws are greater for Pt(IV) than for Pd(II), which indicates that the amount of adsorbed Pt(IV) was greater. Moreover, when the Freundlich constant 1/n is in the 0.1– 0.5, the adsorption can occur easily. On the other hand, if 1/n > 2, adsorption is considered to be difficult [43]. The Freundlich constant 1/n for Pt(IV) and Pd(II) has values of 0.41 and 0.33, respectively. These results show that Pt(IV) and Pd(II) were easily adsorbed onto G600.
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Conclusions
323
Dried aluminum hydroxide gels calcined at different temperatures were prepared. G600, which was calcined at 600 8C, had the highest specific surface area (107.2 m2/g) and density of hydroxyl groups (1.12 mmol/g). Moreover, the amount of Pt(IV) (23.9 mg/g) and Pd(II) (23.4 mg/g) adsorbed onto G600 was also greater than onto another adsorbents (e.g. Fe3O4 nano-particles). The
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G Model
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adsorption mechanism of Pt(IV) and Pd(II) was more closely related to the density of hydroxyl groups (Correlation coefficient: 0.142–0.270) than the specific surface area (Correlation coefficient: 0.974–0.975). The optimal pH for the adsorption of Pt(IV) and Pd(II) onto G600 is 5.0. [PtCl5(OH)]2 or [PdCl3(OH)]2 species were present at a pH of 5.0. These results suggest that adsorption mechanism of Pt(IV) and Pd(II) onto G600 mainly ionexchange with the number of hydroxyl groups onto G600. Moreover, the results obtained in this study indicate that the solution pH and molecular species are very important for adsorption of Pt(IV) and Pd(II) from aqueous solution. The amount adsorbed was influenced by the presence of chloride ions in solution, which indicates that Pt(IV) and Pd(II) are in competition with the chloride ions for adsorption sites. Adsorption equilibrium of Pt(IV) and Pd(II) was reached within 24 h. The experimental data was fitted to the pseudo-second-order model. The adsorption isotherm data for Pt(IV) and Pd(II) onto G600 are well described by the Freundlich and Langmuir models, respectively, which indicated that the adsorption was thought to be monolayer adsorption onto the surface of the G600. These result reveal to be useful for adsorption of Pt(IV) and Pd(II) onto G600 from aqueous solution.
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References
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JECE 125 1–7 Journal of Environmental Chemical Engineering xxx (2013) xxx–xxx
Contents lists available at ScienceDirect
Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece 1 2 3 4 5
Adsorption of Pt(IV) and Pd(II) by calcined dried aluminum hydroxide gel from aqueous solution system Q1 Fumihiko
Ogata, Naohito Kawasaki *
Faculty of Pharmacy, Kinki University, 3-4-1 Kowakae, Higashi-Osaka, Osaka 577-8502, Japan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 26 June 2013 Accepted 7 August 2013
In this study, calcined dried aluminum hydroxide gels (Gs) were prepared at 300–1000 8C (G300–G1000) as adsorbents. The properties of the adsorbents (i.e., XRD patterns, SEM images, specific surface areas, and number of hydroxyl groups) were investigated. The adsorption of Pt(IV) and Pd(II) onto Gs at different temperatures was evaluated. Calcination transformed virgin G into g- and a-alumina. G600 had the largest specific surface area (107.2 m2/g) and number of hydroxyl groups (1.12 mmol/g) of the Gs. G600 also adsorbed the greatest amount of Pt(IV) and Pd(II). The amount of Pt(IV) and Pd(II) adsorbed was more closely related to the number of hydroxyl groups than the specific surface area. The adsorption mechanism of Pt(IV) and Pd(II) onto G600 mainly involved ion exchanges. The optimal pH for the adsorption of Pt(IV) and Pd(II) onto G600 was 5.0, which suggests that [PtCl5(OH)]2 and [PdCl3(OH)]2 species are suitable for adsorption. Pt(IV) and Pd(II) compete with chloride ions for adsorption sites on G600 in the aqueous solution system. The adsorption of Pt(IV) and Pd(II) onto G600 reached equilibrium within 24 h. The experimental data fit the pseudo-second-order model (correlation coefficient: 0.986– 0.995) better than the pseudo-first-order model (correlation coefficient: 0.879–0.973). Moreover, the Weber–Morris plot also was evaluated. The adsorption isotherms of Pt(IV) and Pd(II) onto G600 fit the Freundlich and Langmuir models, respectively. Thus, G600 is useful for the adsorption of Pt(IV) and Pd(II) in aqueous solution. ß 2013 Published by Elsevier Ltd.
Keywords: Platinum Palladium Adsorption Dried aluminum hydroxide gel
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Introduction Platinum group metals (PGMs), e.g., platinum and palladium, are precious metals that are widely used in industry because of their specific physical and chemical properties. PGMs are also of interest due to their high value and catalytic properties [1]. Among PGMs, about 34% of Pt and 55% of Pd are consumed in the production of auto catalysts [2]. The catalysts containing Pt and/or Pd have the capability to oxidize carbon monoxide and hydrocarbons [3]. Generally, an overall recovery higher than 95% is necessary for a process to be cost-effective [2]. The two most important reasons and motivations for precious metal separation are the economical impact of losing these metals and their Q2 environmental concerns. The presence of PGMs in environment may cause severe primary skin and eye irritations [4]. Natural resources of platinum and palladium group metals are limited, and because of their extensive use, their demand is increasing. Low rate of production of these metals due to their low concentration in
* Corresponding author. Tel.: +81 6 6730 5880x5556; fax: +81 6 6721 2505. E-mail addresses: [email protected] (F. Ogata), [email protected] (N. Kawasaki).
related ores and their high costs of production from naturally occurring supplies has made precious metals’ recovery from aqueous solution and spent catalyst a viable and cost-effective alternative of their preparation [5]. Many recent studies have focused on the extraction and separation of precious metals because of both the increasing industrial need for these metals and their limited resources [6]. Separation of these metal ions is still problematic and, because of their complex chemistry and the overlapping properties, represents a real challenge [7,8]. The effective recovery of PGMs from both natural ore and industrial waste is important for full utilization of resources. Conventional methods can be used for the adsorption of PGMs from solution [9–14]; among these methods, adsorption plays a particularly important role in the elimination of metal ions from aqueous solutions to minimize water pollution [15]. Different kinds of adsorbents are applied in adsorption process for metal separation from dilute solutions, such as carbon adsorbents, biosorbents and polymeric adsorbents. Adsorbents should be available at low cost and easy recycle [5]. Modified chitosan, bayberry tannin, and biomass had also been reported. Dried aluminum hydroxide gel (G), which has hydroxyl groups on its surface, is widely used as an antacid. The component of G is more than 50% Al2O3. It can adsorb phosphate, arsenic, and
2213-3437/$ – see front matter ß 2013 Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.jece.2013.08.011
Please cite this article in press as: F. Ogata, N. Kawasaki, Adsorption of Pt(IV) and Pd(II) by calcined dried aluminum hydroxide gel from aqueous solution system, J. Environ. Chem. Eng. (2013), http://dx.doi.org/10.1016/j.jece.2013.08.011
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G Model
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Nomenclature q C0 Ce V M qe qt k1 k2 t ki K 1/n a Ws
–the amount of Pt(IV) or Pd(II) adsorbed (mg/g); –the concentration of Pt(IV) or Pd(II) before adsorption (mg/L); –the concentration of Pt(IV) or Pd(II) after adsorption (mg/L); –the volume of solvent (L); –the mass of adsorbent (g); –the amount adsorbed on the adsorbent (mg/g) at equilibrium; –the amount adsorbed on the adsorbent (mg/g) at time t (h); –the pseudo-first-order rate constant (h1); –the pseudo-second-order rate constant (g/mg h); –time (h); –the intraparticle diffusion constant (mg/g min0.5); –the Freundlich capacity factor; –the Freundlich intensity parameter; –the constant related to the affinity to the binding sites during the adsorption (L/mg); –is the maximum monolayer adsorption capacity (mg/g).
47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68
chromium ions, and the amount adsorbed depends on the number of hydroxyl groups on G [16]. Moreover, the adsorption of perrhenate ions, fluoride ions, and quinidine gluconate onto G has also been reported [17,18]. However, G has not yet been reported to adsorb platinum and palladium. Since G is an aluminum compound, it is possible to desorb the adsorbate using a desorption solution. If adsorption of platinum and palladium onto G were possible, this method would be useful for the recovery of PGMs. Moreover, the adsorption of Pt(IV) and Pd(II) complexes onto alumina for the purpose of the preparation of dispersed metal catalysts has been investigated [19–23]. It has been suggested that the adsorption of Pt(IV) and Pd(II) onto alumina can take place by the exchange of one chloride ion in the coordination sphere of PtCl62 and PdCl42 complexes in solution with OH or H2O. It has also been reported that the solution pH and chloride ion concentration influence the adsorption of metal complexes [15]. In this work, G was used to adsorb platinum and palladium from aqueous solutions by batch sorption. The effects of contact time, chloride ion concentration, and pH of the aqueous solution on the efficiency of platinum and palladium adsorption were also investigated. Finally, the adsorption isotherms, adsorption rate, and mechanism of adsorption were investigated.
69
Material and methods
70
Materials
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Dried aluminum hydroxide gel was obtained in the form of a white amorphous powder (Tomita Pharmaceutical Co., Ltd.). G (20 g) was placed in a magnetic crucible and heated to the target temperature over 2 h in a muffle furnace. The temperature was maintained at the desired temperature, i.e., 300–1000 8C, for 2 h to form G300–G1000, respectively [16]. XRD analysis was performed using a RINT2100V diffractometer (Rigaku, Japan). Electron microscopy was carried out using a scanning electron microscope (JSM-5200; JEOL, Japan), and the specific surface area of the G was measured using a specific surface analyzer (NOVA4200e; Yuasa Ionics, Japan). The number of
hydroxyl groups was calculated based on the number of adsorbed fluoride ions [24].
82 83
Amount of Pt(IV) or Pd(II) adsorbed onto virgin G and Gs calcined at different temperatures
84 85
Adsorbent (0.02 g, virgin G and G300–G1000) was added to 50 mL of Pt(IV) or Pd(II) solution (10 mg/L). The suspensions were shaken at 100 rpm for 24 h at 25 8C. The sample was then filtered through a 0.45 mm membrane filter, and the filtrate was analyzed using an inductively coupled plasma-atomic emission spectrometer (ICP-AES, Shimadzu). The amount of Pt(IV) or Pd(II) adsorbed in single solution was calculated using Eq. (1), as follows:
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q ¼ ðC 0 C e ÞV=M;
(1)
where q is the amount of Pt(IV) or Pd(II) adsorbed (mg/g), C0 is the concentration of Pt(IV) or Pd(II) before adsorption (mg/L), Ce is the concentration of Pt(IV) or Pd(II) after adsorption (mg/L), V is the volume of solvent (L), and M is the mass of adsorbent (g).
95 94 93 96 97 98
Amount of Pt(IV) or Pd(II) adsorbed onto G600
99
Effect of the solution pH G600 (0.02 g) was added to 50 mL of Pt(IV) or Pd(II) solution (10 mg/L) with pH values of 3–8, which were adjusted using a sodium hydroxide solution.
100 101 102 103
Saturated amount adsorbed G600 (0.02 g) was added to 50 mL of Pt(IV) or Pd(II) solution at 50 mg/L.
104 105 106
Effect of chloride-ion concentration G600 (0.02 g) was added to 50 mL of Pt(IV) or Pd(II) solution (10 mg/L) with different chloride-ion concentrations (i.e., 0.01 or 0.04 mol/L).
107 108 109 110
Adsorption isotherms G600 (0.02 g) was added to 50 mL of Pt(IV) or Pd(II) solutions at different initial concentrations (0.5–20 mg/L). The suspensions (2.3.1–2.3.4) were shaken at 100 rpm for 24 h at 25 8C. The samples were filtered through 0.45 mm membrane filters, and the amount adsorbed was then measured using the method described above.
111 112 113 114 115 116 117
Effect of contact time on the adsorption of Pt(IV) and Pd(II) onto G600
118
G600 (0.02 g) was added to 50 mL of Pt(IV) or Pd(II) solution (10 mg/L). The suspensions were shaken at 100 rpm for 0.5–48 h at 25 8C. The sample was filtered through a 0.45 mm membrane filter, and the amount adsorbed was then measured using the method described above.
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Results and discussion
124
Properties of virgin and calcined G
125
The results of the XRD analyses of the Gs are shown in Fig. 1 and confirm that virgin G and G300 have similar structures while G400–G700 have amorphous structures [16]. The structures of G800–G1000 were significantly different from that of virgin G. The results of the XRD analyses indicate that G900 and G1000 have gand a-type structures and that calcining virgin G above 600 8C transforms its structure into the g-type. The 2u value of about 928 and 1368 indicated that g-alumina was produced from Virgin G. Moreover, the 2u value of about 1028 and 1238 suggested that a-alumina was produced from Virgin G. The specific surface area
126 127 128 129 130 131 132 133 134 135
Please cite this article in press as: F. Ogata, N. Kawasaki, Adsorption of Pt(IV) and Pd(II) by calcined dried aluminum hydroxide gel from aqueous solution system, J. Environ. Chem. Eng. (2013), http://dx.doi.org/10.1016/j.jece.2013.08.011
G Model
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3
1.1 Specific surfaec area (m2/g)
105
Fig. 1. XRD of virgin G and Gs calcined at different temperatures (~) g-alumina and (*) a-alumina.
1
100
and amount of hydroxyl groups of the virgin G and Gs that were calcined at different temperatures are presented in Fig. 2. Virgin G had a specific surface area of 94.9 m2/g and a hydroxyl group density of 1.00 mmol/g. The specific surface areas of virgin G and the calcined Gs were not significantly different. G600 had the largest specific surface area of 107.2 m2/g. The densities of hydroxyl groups of virgin G and G300–G700 were also quite similar; however, that of G800–G1000 decreased with increasing calcination temperature. G600 also had the highest density of hydroxyl groups (1.12 mmol/g). SEM images of virgin G and the calcined Gs are shown in Fig. 3: The images indicate that the particles did not distinguishably change during the calcining process and only the crystal type was affected by calcination. Moreover, it is clearly observed the sphere and had a particle size distribution of around 100 mm.
151 152
Amount of Pt(IV) and Pd(II) adsorbed onto virgin G and Gs calcined at different temperatures
153 154 155 156 157 158 159 160 161 162
The amount of Pt(IV) and Pd(II) adsorbed onto the Gs is shown in Fig. 4; G600 adsorbed the most Pt(IV) and Pd(II). To evaluate the relationship between the amount adsorbed and adsorbent properties, we plotted the amount adsorbed against the specific surface area and density of surface hydroxyl groups of the adsorbents. The correlation coefficients between the amount adsorbed and specific surface area ranged from 0.142 to 0.270 (data not shown), which indicates that the specific surface area is not related to the adsorption of Pt(IV) and Pd(II). In contrast, the correlation coefficients between the amount adsorbed and density
0.8
95
0.7 90 0.6
85
136 137 138 139 140 141 142 143 144 145 146 147 148 149 150
0.9
Density of hydroxyl group (mmol/g)
1.2
110
Virgin G G300 G400 G500 G600 G700 G800 G900 G1000
0.5
Fig. 2. Specific surface area and density of hydroxyl group of virgin G and Gs calcined at different temperatures.
of surface hydroxyl groups ranged from 0.974 to 0.975 (Fig. 5), which suggests that the density of surface hydroxyl groups is closely related to the adsorption of Pt(IV) and Pd(II). Moreover, Ogata et al. (2013) reported that the adsorption mechanism of Pt(IV) and Pd(II) onto calcined gibbsite (aluminum hydroxide) is related to the number of surface hydroxyl groups [25]. Therefore, the mechanism of Pt(IV) and Pd(II) adsorption onto G600 is likely similar to that for calcined gibbsite. At saturation, 23.9 and 23.4 mg/g of Pt(IV) and Pd(II), respectively, can be adsorbed onto G600. Previous studies reported that the maximum amounts of Pt(IV) that can be adsorbed onto modified chitosan and Fe3O4 nanoparticles are 43.1 and 13.3 mg/g, respectively, while those of Pd(II) are in the range of 11.0–29.3 mg/ g [26,27]. Comparison of the Pt(IV) and Pd(II) adsorption capacities of the various adsorbent is listed in Table 1 [28–30]. Amount of Pt(IV) and Pd(II) adsorbed onto bayberry tannin immobilized collagen fiber membrane, PA-Lignin, and amberlite IRC718 was greater than that onto G600. But, the price of G600 was low compared to other adsorbents. Moreover, the amount adsorbed onto G600 is greater than that onto calcined gibbsite (aluminum compound), which indicates that the crystal structure of G600 is well suited for the adsorption of Pt(IV) and Pd(II) [25]. The adsorption of Pt(IV) and Pd(II) onto virgin G has not been reported. As described above, G600 would be useful for the adsorption of Pt(IV) and Pd(II) from aqueous solutions.
Fig. 3. SEM images of virgin G and Gs calcined at different temperatures.
Please cite this article in press as: F. Ogata, N. Kawasaki, Adsorption of Pt(IV) and Pd(II) by calcined dried aluminum hydroxide gel from aqueous solution system, J. Environ. Chem. Eng. (2013), http://dx.doi.org/10.1016/j.jece.2013.08.011
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4
Table 1 Comparison of the Pt(IV) and Pd(II) adsorption capacities of the various adsorbent.
10
Adsorption capacity (mg/g)
Adsorbent
Amount adsorbed (mg/g)
8
30 -Nitro-4-amino azobenzene modified chitosan Fe3O4 nano-particles Bayberry tannin immobilized collagen fiber membrane PA-Lignin Amberlite IRC718 G600
6
4
2
0 Virgin G G300 G400 G500 G600 G700 G800 G900 G1000
188
Fig. 4. Amount of Pt(IV) and Pd(II) adsorbed onto virgin G and Gs calcined at different temperatures.
0.07
Amount adsorbed (mmol/g)
0.06 0.05 0.04 0.03
y = -0.011 + 0.05x r= 0.974 y = -0.050 + 0.10x r= 0.975
0.02
: Pt(IV) : Pd(II)
0.01 0 0.5
0.6
0.7
0.8
0.9
1
1.1
1.2
Amount of hydroxyl groups (mmol/g) Fig. 5. Relationship between amount of Pt(IV) or Pd(II) adsorbed and density of hydroxyl groups.
Pt(IV)
Pd(II)
43.1
29.3
[26]
13.3 45.8
11.0 33.4
[27] [28]
66.3 42.9 23.9
58.8 40.4 23.4
[29] [30] This work
Effect of pH on the adsorption of Pt(IV) and Pd(II) onto G600 Since the adsorption of ions is usually influenced by the pH of the solution, it is important to assess the adsorption behavior of Pt(IV) and Pd(II) onto G600 in solutions with different pH values. The results of the adsorption of Pt(IV) and Pd(II) onto G600 in solutions with different pH values are shown in Fig. 6: The optimal pH for the adsorption of Pt(IV) and Pd(II) is 5.0. The speciation of platinum and palladium altered with the pH values; [PtCl5(OH)]2 and [PdCl3(OH)]2 species were present at a pH of 5.0, which suggests that these ions are suitable for adsorption [31,32]. Pd(II) precipitated when it is used at pH > 5.0, which indicated that palladium hydroxide was generated. However, this phenomenon was not observed with Pt(IV) solution. With increasing pH, the negative charge associated with G600 increases, which suggests that adsorption of Pt(IV) and Pt(II) was not easily occurred. On the other hand, with decreasing pH, the positive charge associated with G600 increased. Moreover, [PtCl5(OH)]2 and [PdCl3(OH)]2 species decreased at lower pH, and then adsorption capability of Pt(IV) and Pd(II) onto G600 was lower. The adsorption mechanism of Pt(IV) and Pd(II) onto g-Al2O3 has been reported to involve ion exchange or coordinative bond formation on the g-Al2O3 surface [33]. The component of G600 was similar to g-Al2O3. In this study, the amount of Pt(IV) and Pd(II) adsorbed is related to the number of hydroxyl groups on G600 (the correlation coefficients are 0.762– 0.908, as shown in Fig. 5), which suggests that the adsorption mechanism of Pt(IV) and Pd(II) onto G600 mainly involves ion exchange. (Fig. 7).
25
[PdCl3(OH)]2− [PtCl5(OH)]2−
Amount adsorbed (mg/g)
189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214
: Pt(IV) : Pd(II)
References
20
[PtCl5(OH)]2−
OH
OH
: Pt(IV) : Pd(II)
OH
[PdCl3(OH)]2−
OH
Al – O – Al – O – Al – O – Al
15
G600 surface
10 OH[PdCl3(OH)]2−
5
OH-
0
[PtCl5(OH)]
OH[PdCl3(OH)]
OH[PtCl5(OH)]2−
Al – O – Al – O – Al – O – Al
0
2
4 6 8 Final pH in solution
10
12
Fig. 6. Amount of Pt(IV) and Pd(II) adsorbed onto G600 at different pH condition.
G600 surface
Fig. 7. Schematic drawing of Pt(IV) and Pd(II) adsorption onto G600.
Please cite this article in press as: F. Ogata, N. Kawasaki, Adsorption of Pt(IV) and Pd(II) by calcined dried aluminum hydroxide gel from aqueous solution system, J. Environ. Chem. Eng. (2013), http://dx.doi.org/10.1016/j.jece.2013.08.011
G Model
JECE 125 1–7 F. Ogata, N. Kawasaki / Journal of Environmental Chemical Engineering xxx (2013) xxx–xxx
Table 2 Kinetic parameters for adsorption of Pt(IV) and Pd(II) onto G600 using pseudo-firstorder and pseudo-second-order models.
20 Amount adsorbed (mg/g)
Samples
15 Pt(IV) Pd(II)
Pt(IV) Pd(II)
Pseudo-second-order model
qe(mg/g)
r
k2 (g/mg/h)
qe (mg/g)
r
0.07 0.06
15.62 5.94
0.973 0.879
0.02 0.05
20.00 9.09
0.995 0.986
within 24 h. The amount of Pt(IV) and Pd(II) adsorbed onto G600 was 20.3 and 8.2 mg/g, respectively. The experimental results suggest that the amount adsorbed increased with increasing contact time. (Correlation coefficient of fitting curves of Pt(IV) and Pd(II) was 0.989 and 0.929, respectively.) In order to elucidate the kinetic mechanism that controls the adsorption process, pseudofirst-order and pseudo-second-order models were employed to interpret the experimental data [36,37], and the correlation was examined to determine the adsorption mechanism of the metal ions onto the solid phase [38]. The pseudo-first-order rate model is expressed as
5
0.01
0.04
Concentration of chloride ion (mol/L) Fig. 8. Effect of chloride ion on adsorption of Pt(IV) and Pd(II).
215
Effect of chloride ions on the adsorption of Pt(IV) and Pd(II) onto G600
216 217 218 219 220 221 222 223 224 225 226 227
The adsorption of Pt(IV) and Pd(II) in solutions including chloride ions was influenced by the concentration of chloride ions and number of hydroxyl groups on alumina [34]. Therefore, we investigated the effect of chloride ions on the adsorption of Pt(IV) and Pd(II) onto G600 (Fig. 8). The amount of Pt(IV) and Pd(II) adsorbed onto G600 in a solution containing 0.01 mol/L chloride ions was greater than that in a solution containing 0.04 mol/L chloride ions, which indicates that Pt(IV) and Pd(II) compete with chloride ions for adsorption sites on G600 (i.e., hydroxyl groups of G600). Uheida et al. and Spieker et al. reported that chloride ions in solution compete with Pt(IV) and Pd(II) for adsorption [27,35]. Similar trends were observed in this study.
228
Effect of contact time on the adsorption of Pt(IV) and Pd(II) onto G600
229 230
The effect of contact time on the adsorption of Pt(IV) and Pd(II) onto G600 is shown in Fig. 9. Adsorption equilibrium was reached
lnðqe qt Þ ¼ ln qe k1 t;
t=qt ¼ 1=k2 q2e þ t=qe ;
5
r= 0.989 (Pt(IV))
y = 0.24 + 0.11x r= 0.986 (Pd(II))
3 t/qt
20
15
10
r= 0.929 (Pd(II))
5
: Pt(IV) : Pd(II)
0 10
20
30
40
50
2 y = 0.12+ 0.05x r= 0.995 (Pt(IV))
1
0
0
10
20
30
40
50
t (h)
Elapsed time (hr) Fig. 9. Adsorption rate of Pt(IV) and Pd(II) onto G600.
244 242 243 245 246 247
(3)
where k2 is the pseudo-second-order rate constant (g/mg h). Plotting t/qt versus t results in a straight line, which enables calculation of the values of qe and the rate constant, k2. Table 2 lists the rate constants obtained from the models. The correlation coefficient (r) for the pseudo-second-order adsorption model was the highest (>0.986), and the adsorption capacities calculated by the model are close to those determined experimentally. However, the correlation coefficient for the pseudo-firstorder model is unsatisfactory. Thus, the pseudo-second-order adsorption model should be used to describe the rates of the adsorption Pt(IV) and Pd(II) onto G600 (Fig. 10).
4
231 232 233 234 235 236 237 238 239 240 241
(2)
where qe and qt are the amounts of Pt(IV) and Pd(II) adsorbed on the adsorbent (mg/g) at equilibrium and at time t (h), respectively, and k1 is the rate constant (h1). The pseudo-second-order rate model is given as
25
Amount adsorbed (mg/g)
Pseudo-first-order model k1 (1/h)
10
0
0
5
Fig. 10. Pseudo-second-order kinetic model for the adsorption of Pt(IV) and Pd(II) onto G600.
Please cite this article in press as: F. Ogata, N. Kawasaki, Adsorption of Pt(IV) and Pd(II) by calcined dried aluminum hydroxide gel from aqueous solution system, J. Environ. Chem. Eng. (2013), http://dx.doi.org/10.1016/j.jece.2013.08.011
250 249 248 251 252 253 254 255 256 257 258 259 260
G Model
JECE 125 1–7 F. Ogata, N. Kawasaki / Journal of Environmental Chemical Engineering xxx (2013) xxx–xxx
6
Table 3 Freundlich and Langmuir constants for adsorption of Pt(IV) and Pd(II) onto G600.
25
Samples
Amount adsorbed (mg/g)
20 Pt(IV) Pd(II)
10
: Pt(IV) : Pd(II)
0 0
10
20
30
40
50
60
Time (min 0.5) Fig. 11. Weber–Morris plots for adsorption of Pt(IV) and Pd(II) onto G600.
The adsorption transport from the solution phase to the surface of the adsorbent particles occurs in several steps. The overall adsorption process may be controlled either by one or more steps, e.g., film or external diffusion, pore diffusion, surface diffusion and adsorption on the pore surface, or a combination of more than one step [39]. The kinetic data were analyzed using the equation proposed by Weber and Morris, as follows: [40] qt ¼ ki t 1=2 ;
(4)
270 Thus, the intraparticle diffusion constant ki (mg/g min0.5), can 269 268 271 be obtained from the slope of the qt (uptake at any time, mg/g) 272 versus the square root of time. If this plot passes through the origin, 273 then intraparticle diffusion is the rate controlling step. Fig. 11 274 Q3 shows the plots of qt versus t1/2, with multilinearity cleary 275 observed in the case of G600, which implies the process involves 276 more than one kinetic stage (or sorption rates). The G600 exhibited 277 three stages. The first was attributed to the sorption of Pt(IV) or
30 Amount adsorbed (mg/g)
Langmuir constants
log K
1/n
R
Ws (mg/g)
a (L/mg)
r
2.9 3.1
0.41 0.33
0.974 0.785
18.1 15.5
<0.01 0.04
0.702 0.994
15
5
261 262 263 264 265 266 267
Freundlich constants
25
20 15 10 : Pt(IV) : Pd(II)
5 0
0
5
10
15
20
Equilibrium concentration (mg/L) Fig. 12. Adsorption isotherms of Pt(IV) and Pd(II) onto G600 in a single solution system.
Pd(II) over the surface of the G600, and; hence, was the fastest sorption stage. The second, ascribed to intraparticle diffusion, was a delay process. The third stage may be regarded as the diffusion through smaller pores, which is followed by the establishment of equilibrium [41]. The calculated intraparticle diffusion constants (ki) for Pt(IV) and Pd(II) were 0.16–0.66 mg/g min0.5 and 0.09– 0.61 mg/g min0.5, respectively.
278 279 280 281 282 283 284
Isotherms of the adsorption Pt(IV) and Pd(II) onto G600
285
Isotherms of the adsorption of Pt(IV) and Pd(II) onto G600 are shown in Fig. 12. The amounts of Pt(IV) and Pd(II) adsorbed onto G600 are 25.5 and 21.9 mg/g, respectively. The experimental data were analyzed via the Freundlich isotherm with multiple-layer adsorption and uniform energy and Langmuir isotherm models based on monolayer adsorption on the active sites of the adsorbent in order to evaluate the adsorption system. The Freundlich isotherm equation can be expressed in the linear form as follows [42]:
286 287 288 289 290 291 292 293 294
log q ¼ log K þ ð1=nÞlog C e ;
(5)
where q is the amount adsorbed (mg/g), Ce is the equilibrium concentration (mg/L), and K and 1/n represent the Freundlich capacity factor and Freundlich intensity parameter, respectively. The Langmuir equation is given as follows: q ¼ aW s C e =ð1 þ aW s Þ;
297 296 295 298 299 300
(6)
where a is a constant related to the affinity to the binding sites during the adsorption (L/mg) and Ws (mg/g) is the maximum monolayer adsorption capacity. The Freundlich and Langmuir parameters for the adsorption of Pt(IV) and Pd(II) onto G600 are listed in Table 3: The correlation coefficients of the Freundlich equation for the adsorption of Pt(IV) and Pd(II) onto G600 are 0.974 and 0.785, respectively, and those of the Langmuir model for Pt(IV) and Pd(II) onto G600 are 0.702 and 0.994, respectively. The experimental data on the adsorption of Pt(IV) or Pd(II) onto G600 were well described by the Freundlich and Langmuir models, respectively, which indicated that the adsorption was thought to be monolayer adsorption onto the surface of the G600. The value Ws are greater for Pt(IV) than for Pd(II), which indicates that the amount of adsorbed Pt(IV) was greater. Moreover, when the Freundlich constant 1/n is in the 0.1– 0.5, the adsorption can occur easily. On the other hand, if 1/n > 2, adsorption is considered to be difficult [43]. The Freundlich constant 1/n for Pt(IV) and Pd(II) has values of 0.41 and 0.33, respectively. These results show that Pt(IV) and Pd(II) were easily adsorbed onto G600.
303 302 301 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322
Conclusions
323
Dried aluminum hydroxide gels calcined at different temperatures were prepared. G600, which was calcined at 600 8C, had the highest specific surface area (107.2 m2/g) and density of hydroxyl groups (1.12 mmol/g). Moreover, the amount of Pt(IV) (23.9 mg/g) and Pd(II) (23.4 mg/g) adsorbed onto G600 was also greater than onto another adsorbents (e.g. Fe3O4 nano-particles). The
324 325 326 327 328 329
Please cite this article in press as: F. Ogata, N. Kawasaki, Adsorption of Pt(IV) and Pd(II) by calcined dried aluminum hydroxide gel from aqueous solution system, J. Environ. Chem. Eng. (2013), http://dx.doi.org/10.1016/j.jece.2013.08.011
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330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350
adsorption mechanism of Pt(IV) and Pd(II) was more closely related to the density of hydroxyl groups (Correlation coefficient: 0.142–0.270) than the specific surface area (Correlation coefficient: 0.974–0.975). The optimal pH for the adsorption of Pt(IV) and Pd(II) onto G600 is 5.0. [PtCl5(OH)]2 or [PdCl3(OH)]2 species were present at a pH of 5.0. These results suggest that adsorption mechanism of Pt(IV) and Pd(II) onto G600 mainly ionexchange with the number of hydroxyl groups onto G600. Moreover, the results obtained in this study indicate that the solution pH and molecular species are very important for adsorption of Pt(IV) and Pd(II) from aqueous solution. The amount adsorbed was influenced by the presence of chloride ions in solution, which indicates that Pt(IV) and Pd(II) are in competition with the chloride ions for adsorption sites. Adsorption equilibrium of Pt(IV) and Pd(II) was reached within 24 h. The experimental data was fitted to the pseudo-second-order model. The adsorption isotherm data for Pt(IV) and Pd(II) onto G600 are well described by the Freundlich and Langmuir models, respectively, which indicated that the adsorption was thought to be monolayer adsorption onto the surface of the G600. These result reveal to be useful for adsorption of Pt(IV) and Pd(II) onto G600 from aqueous solution.
351
References
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