Recovery of gold(III) from an aqueous solution onto a durio zibethinus ...

Biochemical Engineering Journal 54 (2011) 124–131

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Recovery of gold(III) from an aqueous solution onto a durio zibethinus husk Mahani A.Z. Abidin a , Aishah A. Jalil a,∗ , Sugeng Triwahyono b , S. Hazirah Adam a , N.H. Nazirah Kamarudin a a b

Department of Chemical Engineering, Faculty of Chemical Engineering, Universiti Teknologi Malaysia, 81310 UTM Skudai, Johor, Malaysia Ibnu Sina Institute for Fundamental Science Studies, Universiti Teknologi Malaysia, 81310 UTM Skudai, Johor, Malaysia

a r t i c l e

i n f o

Article history: Received 26 November 2010 Received in revised form 8 February 2011 Accepted 12 February 2011 Available online 18 February 2011 Keywords: Recovery Gold(III) Durio zibethinus husk

a b s t r a c t The recovery of gold(III) ions from an aqueous solution onto a durio zibethinus husk (DZH) was examined after varying pH, contact time, adsorbent dosage, initial Au(III) concentration, and temperature. The functional groups of DZH were analyzed by FTIR and Au(III) recovery onto DZH was verified by FESEM–EDX and XRD analysis. Adsorption equilibrium isotherms and kinetics of the DZH were studied using Freundlich and Langmuir models, as well as pseudo first-order, second-order kinetic and intraparticle diffusion equations. The experimental data obtained with DZH fitted best to the Langmuir isotherm model and exhibited a maximum adsorption capacity (qmax ) of 1724 ␮mol g−1 . The data followed the pseudo second-order equation. The activation energy of the adsorption (Ea ) was estimated to be 38.5 kJ mol−1 . Thermodynamic parameters, such as changes in enthalpy, entropy and Gibbs free energy, showed that the adsorption is exothermic, spontaneous at low temperature, and is a chemisorption process. These results indicate that DZH adsorbs efficiently and could be used as a low-cost alternative for the adsorption of Au(III) in wastewater treatment. Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved.

1. Introduction Due to the excellent conductivity of electricity and also extremely resistant to corrosion, gold becomes indispensable material and its demand has increased annually worldwide, particularly for industrial purposes as well as medical applications and jewelry market. Hence, the development of appropriate technologies for gold uptake from waste solutions that are selective, economical and efficient is extremely important. To date, many techniques have been used, including ion exchange resins [1], precipitation [2], and solvent extraction [3]. However, these suffer from several drawbacks such as being expensive, toxic to the environment and ineffective. Over the last two decades, adsorption has been considered to be a promising technology for the removal and/or recovery of metal ions with the advantages of high efficiency and simple operation [4]. Several biomasses have been used for adsorptive recovery of gold including tannin, fungal biomass, alfalfa, various protein sources and fruit wastes [5–10]. However, locally available and abundant low-cost adsorbent materials are still necessary in order to recover gold from particularly electronic and electroplating factories wastewater.

∗ Corresponding author. Tel.: +60 7 5535581; fax: +60 7 5536165. E-mail addresses: [email protected], aishah [email protected] (A.A. Jalil).

Durian is a well-known and liked fruit in Southeast Asia. Its unique odor and taste make it desirable for food and flavor in a wide variety of sweet edibles such as candy, ice cream, and cake. Adored as the king of fruits, durian is cultivated in many areas in Malaysia, and about 400,000 tonnes are being produced per year [11]. The growth of cultivation techniques nowadays no longer makes durian a seasonal fruit but available throughout the year. One of the famous durian species found in a local area is durio zibethinus. Due to high consumption of it, massive amounts of its wastes are disposed, creating bad odors that draw insects. For this reason, finding uses for its waste, especially on a large scale, would be beneficial from an environmental and economic perspective. It has been reported that durio zibethinus husk (DZH) largely consists of holocellulose (73.5%), ␣-cellulose (60.5%) and hemicellulose (13.1%) [12]. This characteristic makes it attractive as a adsorbent for the removal of metal ions. To the best of our knowledge, there is no report on the recovery of precious metal ions using durian husk. Therefore, in the present work, the recovery of Au(III) from aqueous solution onto DZH was examined. Fourier transform infrared spectroscopy (FT-IR), Field-emission scanning electron microscopy–energy dispersive X-ray spectroscopy (FESEM–EDX) and X-ray diffraction (XRD) were used to elucidate the functional groups and surface morphology of the DZH and elemental composition before and after adsorption. The equilibrium, kinetics and thermodynamics of the adsorption were also studied in detail.

1369-703X/$ – see front matter. Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.bej.2011.02.010

M.A.Z. Abidin et al. / Biochemical Engineering Journal 54 (2011) 124–131 Table 1 Band positions before and after Au(III) adsorption.

2. Methods 2.1. Preparation of durio zibethinus husk adsorbent

Suggested assignment

The collected durio zibethinus husks were washed thoroughly to remove debris and then cut into small pieces (1–2 cm). The samples were then oven dried at 100 ◦ C for 24 h and ground to 60-mesh size. 2.2. Adsorption experiments Samples of Au(III) solutions for the adsorption test were prepared using commercially available analytical grade HAuCl4 ·4H2 O salt (Germany, Merck) without further purification. Batch adsorption experiments were conducted by placing 0.005 g of durio zibethinus husk in a 20 mL reagent bottle containing 10 mL of a 10 mg L−1 Au solution. The pH of the samples was adjusted to the desired value with HCl or NaOH. The bottles were stirred at 300 rpm using a magnetic stirrer to reach equilibrium. An adsorption isotherm test was carried out by taking 10 mL of Au(III) solution of varying concentrations together with 10 mg of DZH. The kinetic of adsorption of Au(III) in DZH was studied at four different temperatures ranging from 30 to 45 ◦ C. After agitation for the required time, the solutions were filtered, and the filtrates were tested for Au(III) concentration using a Perkin Elmer model A Analyst 400 atomic absorption spectrophotometer. At different times, the amount of Au(III) adsorbed (qt , mg g−1 ) into the DZH was calculated from a mass balance using the following equation: qt =

(C0 − Ct )V M

125

–OH stretching –CH2 – symmetric stretching –C O stretching –OH bending vibrations –C–O stretching –C–O stretching

Band position (cm−1 ) Unloaded DZH

Au-loaded DZH

3422.09 2929.17 1745.31 1632.11 1250.08 1051.99

3422.09 2923.51 1736.82 1634.94 1241.59 1057.65

the active OH groups of phenolic group would play an important role in the adsorption of Au(III) ions [14]. The surface structure of DZH was analyzed using Field-emission scanning electron microscopy (FESEM) before and after Au(III) adsorption. Results are shown in Fig. 1. It was observed that the smoother surface of DZH before the adsorption became rough with irregularities and less sharp after the adsorption of small gold-like particles. The presence of Au on the surface of DZH was verified by energy dispersive X-ray spectroscopy (EDX) analysis. Table 2 shows the quantitative elemental analysis data of DZH taken after adsorption, indicating that 4.12% of Au was detected. The platinum is subjected to the coating material which was coated on the surface of the DZH before the analysis.

(1)

where C0 and Ct are the liquid-phase concentrations of Au(III) at initial and any time (mg L−1 ), respectively. V (L) is the volume of Au(III) solution and M (g) is the mass of DZH used. 2.3. Analytical methods The infrared spectra of the DZH sample was obtained using a Fourier transform infrared spectrometer. The sample was prepared with KBr pellets and recorded within the range of 370–4000 cm−1 to identify the functional groups that were responsible for adsorption. The DZHs were examined before and after adsorption using a Field-emission scanning electron microscope equipped with an energy dispersive X-ray spectrometer. Before they were examined, the samples were coated in an Auto Fine Coater fitted with a Pt target. The surface components of the sample were examined by energy dispersive X-ray spectroscopy at 20 keV. The formation of the elemental gold on the surface of the DZH after the adsorption was verified by a Bruker Advanced D8 X-ray diffractometer. 3. Results and discussion 3.1. Characterization of durio zibethinus husk The functional groups and structures on the surface of the adsorbent strongly affect its metal adsorption capacity [13]. Functional groups in DZH were determined using FTIR spectroscopy in order to verify its binding mechanism with Au ions. In the FTIR spectrum of the sample, three major intense bands were observed around 3422, 1745, 1632 and 1052 cm−1 (Table 1). The wide band around 3422 cm−1 was assigned to the stretching an O–H group of macromolecule association [14]. The strong peak that appears at 1745 and 1632 cm−1 are due to the carbonyl group and O–H bending vibration of H2 O [14–16]. The band around 1052 cm−1 refers to C–O bonding of phenols [17]. From these results, it was supposed that

Fig. 1. FE-SEM images of DZH taken before (a) and after (b) adsorption of Au(III).

126

M.A.Z. Abidin et al. / Biochemical Engineering Journal 54 (2011) 124–131

Table 2 Quantitative elemental analysis data of DZH taken after adsorption of Au(III).

C O Mg Cl K Pt Au Total

Mass%

Atom%

57.27 20.37 0.38 5.09 2.98 9.79 4.12

75.11 20.06 0.25 2.26 1.20 0.79 0.33

100

100 80 Adsorption (%)

Element

100

60 40 20 0

3.2. Effect of contact time

0 Contact time between the adsorbate and adsorbent to achieve a steady-state equilibrium time is an important factor in adsorption since it depends on the nature of the system used. Fig. 2 shows the effect of contact time on adsorption of Au(III) ions into DZH. The adsorption rate increased rapidly during the first 30 min (82%), then remained constant with time, presumably due to the saturation of DZH surfaces with Au(III) ions as well as adsorption and desorption processes that occur after the saturation [18]. Finally, an equilibrium value of 91% was achieved after 180 min of contact time. 3.3. Effect of initial pH The initial pH of the solution is also a vital process parameter for controlling the adsorption. Fig. 3 shows the effect of the initial pH value in the range of pH 2–10 on the adsorption of 10 mg L−1 Au(III) solution onto DZH. Lower adsorption percentage was observed when the adsorption was carried out at lower pH. This could be due to the competition between Au3+ and excess H+ ions in the system, which leads to protonation of active sites prior to metal binding. As the pH increased, the total amount of protons and negatively charged surface area might become available, thus facilitating greater and gave 95% of maximum removal percentage at pH 4. However, on further increase of pH seems did not favor in this study since the hydrolyzed chlorogold complex may be formed which is precipitated and make the Au(III) ions unavailable for adsorption [5,19]. 3.4. Effect of adsorbent dosage

10

12

The effect of variation of the initial gold ion concentration was studied between 10 and 700 mg L−1 at an initial pH value of 4 with 5 mg of DZH. The results are illustrated in Fig. 5. It can clearly be observed that adsorption percentage of gold decreased while the gold uptake increased with increasing gold ion concentration. Equilibrium gold uptake was enhanced markedly as the initial concentration of Au(III) increased up to 200 mg L−1 . Afterward, it tended to saturate at higher Au(III) concentrations with 1650 ␮mol g−1 of maximum gold uptake at 700 mg L−1 . The existence of the uncovered surface may be responsible for the higher adsorption percentage at low concentration; however as the initial gold ion concentration increased the number of available adsorption sites decreased particularly after 50 mg L−1 , most probably due to the saturation of active sites on the DZH [4]. The system studied seems to favor treatment of lower concentrations of Au(III), since adsorption was almost complete at 10 mg L−1 initial concentration, which gave 99% adsorption capacity.

80

80

20

8

3.5. Effect of initial gold ion concentration

100

40

6 pH

of DZH between 0.5 and 3 g L−1 (Fig. 4). The adsorption capacity increased with decreasing adsorbent dosage and the highest gold uptake (84.5 ␮mol g−1 ) was achieved when using 0.5 g L−1 of DZH. As the amount of adsorbent decreased, the total space of DZH exposed also decrease. Hence, more Au(III) ions were adsorbed onto the surface per gram unit of DZH, which resulted in a higher gold uptake.

100

60

4

Fig. 3. Adsorption behavior of Au(III) in various pH (initial concentration: 10 mg L−1 , contact time: 3 h, mass dosage: 0.005 g, at 30 ◦ C).

Gold uptake (μmol/g)

Adsorption (%)

The effect of DZH dose on gold ion adsorption was studied for a 10 mg L−1 initial Au(III) ion concentration by varying the dose

2

60 40 20 0

0 0

60

120 180 Time (min)

240

Fig. 2. Effect of contact time on the adsorption of Au(III) onto DZH (initial concentration: 10 mg L−1 , mass dosage: 0.01 g, pH 2, at 30 ◦ C).

0

0.01

0.02

0.03

Dose of DZH (g) Fig. 4. Effect of adsorbent dosage on adsorption of Au(III) (contact time: 3 h, initial concentration: 10 mg L−1 , pH 2 at 30 ◦ C).


127

1800 100

1200

80 Gold uptake Adsorption

60

900 600

Gold uptake (μmol/g)

Adsorption (%)

1500

40 300 0

20 0

100

200

300

400

500

600

700

Initial gold ion concentration (mg/L) Fig. 5. Effect of initial concentration on the adsorption of Au(III) onto DZH (mass dosage: 0.005 g, pH 4 at 30 ◦ C).

Fig. 7. XRD pattern of DZH taken after adsorption of Au(III).

while hydroxyl groups of cellulose are oxidized to carbonyl groups: 3.6. Adsorption mechanism

R–OH → R O + H+ + e−

Various spectroscopic methods of analysis have been used to study the mechanisms of gold adsorption and reduction processes. Most of the mechanism involved the reduction of ionic gold to gold metallic [20]. Fig. 6 shows the FTIR spectra of unloaded and Au-loaded DZH. A broad absorption peak was observed for both unloaded and Au-loaded DZH at 3422 cm−1 , which may assigned to OH stretching vibration in hydroxyl groups. Some peaks shown in Table 1, which may attributed to certain functional groups were shifted after the adsorption, signifying the feasible participation of the functional groups on the surface of the DZH. The –CH2 – symmetric stretching at 2929 shifted to 2923 cm−1 and carbonyl group stretching at 1745 shifted to 1736 cm−1 [15]. The peaks at 1632 and 1051 cm−1 which can be correspond to hydrogen bonded carbonyl group and –C–O stretching in cellulose were also shifted to 1634 and 1057 cm−1 , respectively. Hence, the mechanism of the DZH on Au(III) can be presumed to be the same with the adsorption of Au3+ onto tannin gel adsorbent [5]. As the DZH was added into the gold solution, the below redox reaction supposed to take place, where chlorogold complex is reduced to Au(0): AuCl4 − + 3e− → Au0 + 4Cl−

(3)

The generation of Au(0) on the DZH after the adsorption was elucidated by XRD diffractogram shown in Fig. 7. The four peaks at 2 = 38.1◦ , 44.3◦ , 64.4◦ and 77.4◦ , which certainly belong to elemental gold verifying the reduction of Au(III) to Au(0). This suggests Au(III) underwent subsequent reduction after being adsorbed onto DZH surface [21]. Furthermore, the increase of the intensity of the adsorption bands at 1736 cm−1 in Fig. 6 may also explain the oxidation of hydroxyl groups of cellulose to the carbonyl groups. 3.7. Adsorption isotherms In this study, two commonly used models, the Langmuir [22] and Freundlich [23] isotherms, were applied to understand the DZH–Au(III) interaction. The Langmuir adsorption model is based on the assumption that the maximum adsorption corresponds to a saturated monolayer of solute molecules on the adsorbent surface. The equation is given as: qe =

(2)

qm bCe 1 + bCe

(4)

where Ce is the concentration of Au(III) solution at equilibrium (mg L−1 ), qe and qm are the equilibrium and maximum amount of Au(III) adsorbed per unit weight of adsorbent at equilibrium (␮mol g−1 ), and b is the Langmuir equilibrium constant, which is related to the energy of adsorption. The Langmuir constants, b and qm , can be determined from the intercept and slope of the linear plot of Ce /qe versus Ce (Fig. 8a) and are presented in Table 3. The essential characteristics of the Langmuir isotherm can be expressed in terms of a dimensionless constant separation factor RL [24], which is given by Eq. (5): RL =

1 1 + bC0

(5)

Table 3 Coefficient of Langmuir and Freundlich isotherm. Temperature (◦ C)

Langmuir constant qm (␮mol g−1 )

Fig. 6. FTIR spectra of DZH before and after adsorption of Au(III).

30 35 40 45

1724 1389 1064 862

Freundlich constant

b (L ␮mol−1 )

R2

1/n

KF

R2

0.021 0.019 0.019 0.015

0.996 0.995 0.995 0.993

0.232 0.256 0.256 0.307

374 254 196 114

0.990 0.989 0.990 0.977

128


(a)

30°C 35°C 40°C 45°C

0.8

Ce/q e (g/L)

been reported that when 1 < n < 10, the adsorption is favorable, and the higher the n value, the stronger the adsorption intensity [24]. Table 3 shows that the values of the Freundlich exponent n are in the range of 3.26–4.31, indicating that the adsorption process is favorable. Comparing the correlation coefficient values (R2 ), it was observed that the Langmuir isotherm model was more suitable for the experimental data than was the Freundlich isotherm at all temperatures studied. This data suggests that the adsorption of Au(III) on DZH occurs as a monolayer coverage during the chelating interaction [25]. A similar result was reported for adsorption of Au(III) on eggshell membrane [26]. DZH has a relatively large adsorption capacity (1724 ␮mol g−1 ), suggesting that it could be a promising material for the adsorption of Au(III) from aqueous solution.

1

0.6

0.4

0.2

0 0

100

200

300

400

500

600

700

3.8. Adsorption kinetics

Ce (μmol/L) In this study, three models were used to investigate the details of the Au(III) adsorption process onto DZH: the Largergren pseudofirst order model [27], the Ho pseudo-second order model [28] and the intraparticle diffusion model [29]. The Lagergren pseudo-first order model is based on the assumption that the rate of change of solute uptake over time is directly proportional to the difference in saturation concentration and the amount of solid uptake over time:

(b) 3.25

log q e

3.05

2.85

2.65

dqt = k1 (qe − qt ) dt

30°C 35°C 40°C 45°C

2.45

Integrating Eq. (8) and noting that qt = 0 at t = 0 gave the following equation:

2.25 0.5

1

1.5

2

2.5

3

log Ce Fig. 8. Langmuir (a) and Freundlich (b) adsorption isotherm of Au(III) on DZH.

where C0 (mg L−1 ) is the highest initial concentration of adsorbent. The parameter RL indicates the nature of shape of the isotherm in accordance with the following: RL > 1 0 < RL < 1 RL = 0 RL = 1

Unfavorable adsorption Favorable adsorption Irreversible adsorption Linear adsorption

1/n

(6)

In logarithmic form, Eq. (6) can be represented as: log qe = log KF +

1 log Ce n

log(qe − qt ) = log qe −

k1 t 2.303

(9)

where qt is the amount of Au(III) adsorbed per unit of adsorbent (␮mol g−1 ) at time t, k1 is the pseudo-first order rate constant (min−1 ), and t is the contact time (min). The adsorption rate constant (k1 ) was calculated from the plot of log (qe − qt ) versus t (Fig. 9a). The Ho pseudo-second order model is presented as: dqt = k2 (qe − qt )2 dt

(10)

and when qt = 0 at t = 0, Eq. (10) can be integrated into the following equation:

In this study, all the values of RL were in the range of 0.06 and 0.87 for the initial gold concentration range from 10 to 700 mg L−1 indicating favorable adsorption of gold onto DZH. The Freundlich isotherm is an empirical equation that assumes that the adsorption surface becomes heterogeneous during the course of the adsorption process. The heterogeneity arises from the presence of different functional groups on the surface and from the various adsorbent–adsorbate interactions. The Freundlich isotherm is expressed by the following empirical equation: qe = KF Ce

(8)

(7)

where KF and n are Freundlich adsorption isotherm constants, which indicate the extent of the adsorption and the degree of nonlinearity between the solution concentration and the adsorption, respectively. The values of KF and n can be calculated from the intercept and slope of the linear plot between log qe and log Ce (Fig. 8b) and are listed in Table 3. In general, as the KF value increases, the adsorption capacity of the adsorbent for a given adsorbate also increases. It has

t 1 t = + qe qt k2 q2e

(11)

where k2 is the pseudo-second order rate constant (g ␮mol−1 min−1 ). The values of qe and k2 can be obtained from the linear plot of t/qt versus t (Fig. 9b). According to Eq. (9), the plot of ln (qe − qt ) versus t, and Eq. (11), the plot of t/qt versus t should each give a straight line. Table 4 shows both rate constants (k1 and k2 ) and the corresponding linear regression correlation coefficient values (R2 ) for both models. The highest values of R2 were observed with the pseudo-second order model (R2 ≥ 0.991 for all temperatures). The theoretical qe values obtained from this model were also closer to the experimental qe,exp values compared to the values from the pseudo-first order model. These results indicate that the pseudo-second order kinetic model gave a better correlation for the adsorption of Au(III) on DZH compared to the pseudo-first order model. Similar results have been observed in the adsorption of Au(III) onto modified chitosans [30–32]. Generally, in adsorption systems the diffusion of solute or sorbate onto and within the sorbent particle proceed through four stages: (1) transport of sorbate through the bulk liquid to the boundary layer of fixed film of liquid surrounding the adsorbent; (2) diffusion of the material from external liquid film boundary layer to external surface sites; (3) migration of sorbate within the


129

Table 4 Coefficients of pseudo-first and second-order adsorption kinetic models. qe,exp (␮mol g−1 )

T (◦ C)

Pseudo-first order qe (␮mol g

30 35 40 45

650 468 360 240

−1

Pseudo-second order −2

)

k1 (×10

1191 570 500 368

min

−1

)

1.68 1.66 1.64 1.42

R

qe (␮mol g−1 )

k2 (×10−5 g/␮mol min)

R2

0.970 0.938 0.962 0.941

725 543 469 344

4.01 3.53 2.31 2.08

0.998 0.999 0.991 0.995

2

pores of the sorbent by intraparticle diffusion and (4) attachment of adsorbate to adsorbent at available adsorption sites [33]. In this study, the kinetic results were analyzed using the Weber and Morris intraparticle diffusion model: +C

(12)

where kid is the intraparticle diffusion rate constant (mg g−1 min−1/2 ) and C is the slope that represents the thickness of the boundary layer. According to this model, a plot of the amount of Au(III) adsorbed (qt ) versus the square root of time (t1/2 ) should be linear, and if these lines pass through the origin, then the intraparticle diffusion is the only rate-controlling step [34]. However, as shown in Fig. 10, the plots are not linear over the whole time range and, instead, can be separated into two-linear curves, illustrating that two stages were involved in the adsorption process. A well agitation of batch system might reduce

(a)

3 30°C 2.5

35°C

log (q e-q t )

40°C 2

45°C

1.5 1

0 0

60

120

180

240

300

t (min)

300 mg/L

1200

700 mg/L

900 600 300 0 3

6

9

12 t 1/2 (min1/2 )

15

18

21

Fig. 10. Intraparticle diffusion model.

the boundary layer surrounding the particle, thus the first stage of diffusion process could be neglected [35]. In addition, the stage (4) process usually occurred rapidly and is not considered to be the rate limiting steps in the sorption process. Therefore, the first line may represent the diffusion of the solute from the external to the surface of the DZH and the second line corresponds to the intraparticle diffusion of Au(III) ions within the pores. These kinetic results confirmed that the process is chemisorptions while the intraparticle diffusion is not only the rate determining step [36].

Fig. 11 shows the effect of temperature on gold uptake of five different initial concentrations (10, 50, 100, 300 and 700 mg L−1 ) of Au(III) solutions in the temperature range of 30–45 ◦ C. The gold uptake per unit weight of adsorbent (␮mol g−1 ) is observed to decrease with increasing temperature for all concentrations of Au

1.6

1800 30°C

1.4

35°C 1.2

40°C 45°C

1

10 mg/L 50 mg/L 100 mg/L 300 mg/L 700 mg/L

1500 Gold uptke (μmol/g)

t/q t (min·g μmol-1)

100 mg/L

3.9. Thermodynamics studies

0.5

(b)

50 mg/L 1500

q t (μmol/g)

qt = kid t

1/2

1800

0.8 0.6 0.4

1200 900 600 300

0.2 0 0

60

120

180

240

300

360

t (min)

0 25

30

35 Temperature

Fig. 9. Pseudo-first-order (a) and pseudo-second-order (b) kinetic plots for the adsorption of Au(III) onto DZH at various temperatures (initial concentration: 10 mg L−1 , adsorbent dosage: 0.5 g l−1 , initial pH 4).

40

45

(oC)

Fig. 11. Effect of temperature on the adsorption of Au(III) onto DZH (mass dosage: 0.005 g at pH 4).

130


Table 5 Thermodynamics parameters for the sorption of Au(III) at different temperatures. T (K)

Kc

G◦ (kJ mol−1 )

H◦ (kJ mol−1 )

S◦ (kJ mol−1 K−1 )

Ea (kJ mol−1 )

303 308 313 318

142.2 90.0 59.1 27.1

−12.5 −11.5 −10.6 −8.73

−86.2

−0.243

−38.5

adsorption system at equilibrium due to the interaction between active sites and metal ions [30]. The rate constant k2 at the different temperatures (30, 35, 40, and 45 ◦ C) listed in Table 4 was used to estimate the activation energy of the adsorption of Au(III) on DZH. The magnitude of activation energy also gives information as to whether the type of adsorption is mainly physical or chemical [41]. Assume the correlation among the rate constant k2 , temperature T, and activation energy Ea follows the Arrhenius equation:

6 5

ln Kc

4 y = 10373x - 29.208 3 2

ln k2 = ln A −

1 3.1

3.15

3.2

3.25

3.3

3.35

1/T x 103 (K-1) Fig. 12. Plot of ln Kc versus 1/T for estimation of thermodynamic parameters for the adsorption of Au(III) onto DZH.

ions in the test solution studied, indicating an exothermic nature [37]. Thermodynamic parameters such as the changes in the standard free energy (G◦ ), enthalpy (H◦ ) and entropy (S◦ ) can be calculated using the following equations: G◦ = −RT ln Kc

(13)

G◦ = H ◦ − S ◦ T

(14)

Kc is the equilibrium constant of the adsorption, which was obtained from Eq. (15): Kc =

Ce (adsorbent) Ce (solution)

(15)

where Ce (adsorbent) and Ce (solution) are the equilibrium concentration of the dye ions on adsorbent and in solution, respectively [38]. Eqs. (13) and (14) then give the van’t Hoff equation as: ln Kc =

S ◦ H ◦ − R RT

(16)

As shown in Fig. 12, the plot of ln Kc versus 1/T gives a straight line with a slope of H◦ (kJ mol−1 ) and an intercept of S◦ (kJ mol−1 K−1 ). The values of these thermodynamic parameters (studied at four different temperatures) are listed in Table 5. The increase in G◦ values with increasing temperature indicates a decrease in the feasibility and spontaneity of the adsorption at higher temperatures. The negative H◦ value indicates that Au3+ adsorption is an exothermic process. Generally, the change in adsorption enthalpy for physisorption is between 20 and 40 kJ mol−1 , but chemisorption is in the range of 80–400 kJ mol−1 [34]. This is due to the forces of attraction between the molecules of the adsorbate and the adsorbent in physisorption are of the weak dispersion interaction type; however are very strong chemical bond in chemisorptions [39]. Therefore, this Au3+ adsorption may classify to a chemisorption process rather than a physical adsorption process since the H◦ was 86 kJ mol−1 . The negative values of S◦ suggests that there is decreased randomness at the solid–solution interface during the adsorption of Au(III) in aqueous solution on DZH [40]. Also, the negative value of S◦ suggests a highly ordered

Ea R

1 T

(17)

where R is the gas constant (8.314 J mol−1 K−1 ). The slope of plot of ln k2 versus 1/T gave the value of the activation energy (Ea ), which was 38.5 kJ mol−1 (Table 5). This value suggests that the adsorption of Au(III) onto DZH is a chemical adsorption, which was confirmed from the fact that the magnitude of activation energy for chemical adsorption is usually between 8.4 and 83.7 kJ mol−1 [41]. The negative value of Ea suggests that a decrease in temperature favors adsorption. These results are also consistent with the value of enthalpy from Table 5, which indicates a chemisorption process that is exothermic in nature. 4. Conclusion The results obtained herein show that durio zibethinus husk can be used as an effective adsorbent for adsorption of Au(III) from aqueous solution. Its high affinity toward Au(III) binding at low pH and ambient temperature will work well in the conditions of industrial waste effluents. The Langmuir adsorption isotherm was found to provide the best fit to the experimental data, with a maximum adsorption capacity of 1724 ␮mol g−1 . The kinetics of sorption can be described by a pseudo second-order model. Thermodynamic parameters obtained indicated that the adsorption process is chemisorption and exothermic in nature. The use of this type of biomass waste, which is abundant, cheap and does not require any chemical pretreatment, would be profitable in recovering gold(III) ions from local e-waste. Acknowledgement Our gratitude goes to Ministry of Science, Technology and Innovation Malaysia for their financial supports under the Fundamental Research Grant (No. 78326). We are also grateful to the Hitachi Scholarship Foundation for their support and advice. References [1] J.L. Cortina, A. Warshawsky, N. Kahana, V. Kampel, C.H. Sampaio, R.M. Kautzmann, Kinetics of goldcyanide extraction using ion-exchange resins containing piperazine functionality, React. Funct. Polym. 54 (2003) 25–35. [2] P.F. Sorensen, Gold recovery from carbon-in-pulp eluates by precipitation with a mineral acid. III. The acid precipitation step in applications, Hydrometallurgy 21 (2) (1988) 249–254. [3] M.G. Saˇınchez-Loredo, M. Grote, Carboxyl-substituted derivatives of S-decyl dithizone as solvent extractants for precious metal ions, Solv. Extr. Ion Exch. 18 (1) (2000) 55–76. [4] W.S.W. Ngah, M.A.K.M. Hanafiah, Adsorption of copper on rubber (Hevea brasiliensis) leaf powder: kinetic, equilibrium and thermodynamic studies, Biochem. Eng. J. 39 (2008) 521–530.

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