Mechanisms of gold recovery from aqueous solutions using a novel tannin gel adsorbent synthesized from natural condensed tannin

ARTICLE IN PRESS

Water Research 39 (2005) 4281–4286 www.elsevier.com/locate/watres

Mechanisms of gold recovery from aqueous solutions using a novel tannin gel adsorbent synthesized from natural condensed tannin Takeshi Ogata, Yoshio Nakano Department of Environmental Chemistry and Engineering, Interdisciplinary Graduate School of Science and Technology, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan Received 12 July 2004; received in revised form 13 May 2005; accepted 16 June 2005

Abstract We report a novel recovery system for gold (Au), which is one of the precious metals contained in electronic scrap, utilizing tannin gel particles. Tannin gel particles were prepared by a process of cross-linking of condensed tannin (wattle tannin), which is a ubiquitous and inexpensive natural material having many hydroxyl groups. The adsorption mechanism of gold onto tannin gel particles was elucidated: the adsorption of gold takes place through the reduction of trivalent gold ions to metallic gold on the surface of tannin gel particles, which is accompanied by the simultaneous oxidization of the hydroxyl groups of tannin gel. Additionally, the adsorption capacity of gold was found to be extremely high, 8000 mg-Au/g-dry gel. The outstanding characteristics of tannin gel particles for gold offers the possibility of efficient recovery of other precious metals. r 2005 Elsevier Ltd. All rights reserved. Keywords: Tannin; Gel; Recovery; Gold; Redox; Biosorption

1. Introduction Increasing demand for gold due to its vast applications in the fields of electronics and so on has required its recovery from electronic scrap. Cellular phones, one of the electronic devices, contain 200 g-Au per ton-scrap which is much higher than the content of gold in gold ores, only 5–30 g-Au per ton-ore. Therefore, the scrap from electronic devices containing gold, such as cellular phones and personal computers, is expected to be a resource superior to gold ores (Shibata and Okuda, 2002). Corresponding author. Tel.: +81 45 924 5432; fax: +81 45 924 5441. E-mail address: [email protected] (Y. Nakano).

Hydroprocessing has been widely utilized for the recovery of gold from electronic scrap and includes (i) adsorption by ion exchange resin (Sa´nchez et al., 2000; Gomes et al., 2001), (ii) solvent extraction (Martı´ nez et al., 1996; Akita et al., 1996), and (iii) gold precipitate reduced by reagents. However, these current recovery processes require much labor and time at high cost. Furthermore, a large amount of secondary wastes is generated, resulting from the addition of chemical agents for precipitation and reduction in the processes. Therefore, there is a need to develop a low-cost system, generating little secondary waste, to recover gold from scrap. There has lately been a growing interest in biosorption, which is a property of certain types of inactive, dead, microbial biomass materials to bind

0043-1354/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2005.06.036

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R3 OH R2 B-ring HO

particles of 125–250 mm in diameter. They were washed successively with distilled water and HNO3 solution (0.05 mol/dm3) to remove unreacted substances, and finally rinsed with distilled water again. 2.2. Experimental procedure for gold recovery

O OH A-ring OH R1

Fig. 1. Estimated chemical structure of condensed tannin molecule. A-ring: R1 ¼ OH, R2 ¼ H, phloroglucinolic; R1 ¼ R2 ¼ H, resorcinolic; R1 ¼ H, R2 ¼ OH, pyrogallolic. B-ring: R3 ¼ H, catecholic; R3 ¼ OH, pyrogallolic.

and concentrate metal ions from aqueous solutions (Volesky and Holan, 1995; Romero-Gonza´lez et al., 2003). These biomass materials are relatively inexpensive and available in large quantities worldwide. Condensed tannin is well known to be an inexpensive and ubiquitous natural polymer and has many hydroxyl groups, as shown in Fig. 1 (Hemingway et al., 1989). The tannin has high affinities for heavy metals such as cadmium, cobalt and uranium. When tannin is used as an adsorbent, however, it must be immobilized because the tannin molecules are soluble in water. It has been reported that insolubilized tannin has the ability to remove several metals such as uranium (Sakaguchi and Nakajima, 1994), lead (Zhan et al., 2001), chromium (Nakano et al., 2000, 2001a) and silver (Nakano et al., 2001b). The tannin gel is expected to be useful as an adsorbent for the recovery of such precious metals. The objective of the present investigation is to elucidate the mechanism of gold adsorption onto the tannin gel particles to develop a novel recovery system for gold, which is simple and efficient, compared with the current recovery systems.

2. Materials and methods

All adsorption experiments were carried out in a batch system. The stock solution of gold was prepared from hydrogen tetrachloroaurate (III) tetrahydrate (Wako Co. Ltd., Japan). The pH, pCl and ionic strength of the solution were adjusted by adding HCl, NaOH and NaCl solutions. Tannin gel particles (50 mg, dry basis) were added to the gold solution (100 ppm on the basis of gold, 500 mL), and the solution was then stirred at 180 rpm. A small amount of the solution was taken out several times at intervals. The concentration of the gold solution was determined by an inductively coupled plasma spectrometer (Shimadzu, ICPS-8100 type). Ultraviolet–visible (UV–VIS) spectra of the gold solution were observed by a UV–VIS spectrophotometer (JASCO, V550). The X-ray diffraction (XRD) pattern and Fourier transform infrared (FT-IR) spectra of the tannin gel particles (before and after adsorption of gold) were measured by a powder X-ray diffractometer (Rigaku, RINT-Ultima type) and an FT-IR spectrophotometer (JASCO, FT-IR 550 type), respectively.

3. Results and discussion 3.1. Adsorption behavior of gold on tannin gel particles It is known that the chemical morphology of chlorogold complexes is an important factor affecting the adsorption behavior of gold in aqueous solution. The relative concentration of each chlorogold complex is dependent on the chloride ion and hydrogen ion concentration as well as the temperature. The molar fractions of these gold species based on various conditions in this study were calculated as a function of pH with the equilibrium constants at 0.01 mol/dm3 of chloride ion concentration and are shown in Fig. 2. The equilibrium constants (Silen, 1964) are as follows: AuðOHÞ4  þ Hþ þ Cl Ð AuClðOHÞ3  þ H2 O;

K1 ¼ 108:51 ;

AuClðOHÞ3  þ Hþ þ Cl Ð AuCl2 ðOHÞ2  þ H2 O;

K2 ¼ 108:06 ;

AuCl2 ðOHÞ2  þ Hþ þ Cl Ð AuCl3 ðOHÞ þ H2 O;

K3 ¼ 107:00 ;

2.1. Preparation of tannin gel particles Tannin gel particles were prepared by means of the method proposed by Nakano et al. (2001a). Wattle tannin powder (28 g) was dissolved in 50 mL of NaOH solution (0.225 mol/dm3) at room temperature, and 6 mL of formaldehyde (37 wt%) was then added as a cross-linking agent. After gelation at 353 K for 12 h, the tannin gel obtained was crushed and sieved to produce

AuCl3 ðOHÞ þ Hþ þ Cl Ð AuCl4  þ H2 O;

K4 ¼ 106:07 :

It is evident that the predominant complex of gold is AuCl 4 at low pH below 3. Increasing solution pH causes the hydrolysis reaction of AuCl 4 to proceed, and

ARTICLE IN PRESS T. Ogata, Y. Nakano / Water Research 39 (2005) 4281–4286

thus hydrolyzed chlorogold complexes such as AuCl3(OH) appear in the aqueous chloride solution. In order to investigate the influence of the initial pH on the adsorption behavior of gold, the adsorption experiments were carried out in the range of 2.0–3.8 initial pH at the same initial chloride ion concentration (0.01 mol/dm3) and at 293 K as conditions shown in Fig. 2, where the predominant complex of gold is AuCl 4. The initial pH dependency of the amount of gold adsorbed onto the tannin gel particles is shown in Fig. 3. In this initial pH range of 2.0–3.8, the entire amount of gold was adsorbed perfectly by tannin gel particles, and the pH of the solution was observed to decrease rapidly with adsorption of the chlorogold complexes. In addition, the gold recovery from the aqueous chloride solution by tannin gel particles was nearly independent of the initial pH. The lack of dependence on the initial

1

Molar fraction [–]

0.8

− AuCl4

0.6 AuCl3 (OH)

0.4

− AuCl2 (OH)2

0.2 0 0

−

1

2

3

4

5

pH Fig. 2. Molar fractions of chlorogold complexes calculated with equilibrium constants at pCl 2.0 and 293 K.

pH can be explained by conversion of the predominant chlorogold complexes to AuCl 4 due to the decreasing pH of the solution. Fig. 4 shows the UV–VIS spectra of gold in 10 ppm HAuCl4 solution at initial pH 2.0 and initial pCl 2.0 over the temperature range of 293–333 K. The positions of the two principal AuCl 4 bands are reported to be located at 228 and 312 nm (Leontidis et al., 2002). The chemical morphology in aqueous chloride solution is nearly independent of temperature over the temperature range of 293–333 K, because of little change due to temperature in the UV–VIS spectra. The effect of reaction temperature on the adsorption amount of gold was investigated over the temperature range of 293–333 K at the same initial pH (2.0) and initial pCl (2.0) and is shown in Fig. 5. Tannin gel particles adsorbed all of the gold from the solution over this temperature range. The adsorption rate of gold increased with increasing temperature, and at 333 K all of the gold was adsorbed within 4 h. Considering that the chemical morphology of the chlorogold complexes is little changed by temperature as shown in Fig. 4, this implies that the difference in the adsorption rate at various temperatures is attributable not only to the morphology of the chlorogold complexes but also to other factors accompanied by some chemical reactions. In order to determine the rate constant k of gold adsorption in the temperature range of 293–333 K, ln([Au]/[Au]initial) was plotted against time, as shown in Fig. 6(a). The plots gave a straight line and the k values, which were obtained from the slopes of these straight lines, are shown in Fig. 6(a). The activation energy Ea was calculated by the Arrhenius equation: ln k ¼ ln A  ðE a =RTÞ,

1200

4

800

Adsorption amount Initial pH 2.0 Initial pH 3.0 Initial pH 3.8

3

600

pH

Amount of Au adsorbed onto tannin gel [mg /g-dry gel]

1000

2

400

Time history of pH Initial pH 2.0 Initial pH 3.0 Initial pH 3.8

200 0

0

20

40 60 Time [h]

4283

80

1 100

Fig. 3. Initial pH dependency of adsorption amount of gold at initial pCl 2.0 and 293 K.

(1)

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0

2

Abs. [–]

In ([Au]/[Au]initial)

293K 298K 313K 333K

1.5

1

0.5

-0.2

300 Wavelength [nm]

293 298 313 333

1.48 2.06 3.73 5.94

2.5

3

-0.3

-0.4

0 200

Temp. k [K] [S−1]

-0.1

400

0.5

0

1

Fig. 4. UV–VIS spectra of gold in 10 ppm HAuCl4 solution at initial pH 2.0, initial pCl 2.0 in the temperature range of 293–333 K.

1.5

2

3.5

Time × 10−3 [S]

(a) -7

-8 In k [S−1]

Amount of Au adsorbed onto tannin gel [mg /g-dry gel]

1200 1000

-9

800 Ea = 27.7 KJ / mol

600 -10 2.5

293K 298K 313K 333K

400 200 0 0

10

20 30 Time [h]

40

50

4

1000 / T [K−1]

Fig. 6. (a) Determination of the rate constants k of pseudofirstorder reactions for the adsorption of gold in the temperature range of 293–333 K. (b) Arrhenius plot of ln k vs. 1000/T to evaluate activation energy Ea.

Fig. 5. Temperature dependency of adsorption amount of gold at initial pH 2.0 and initial pCl 2.0.

10000 Amount of Au adsorbed onto tannin gel [mg/g-dry gel]

where A is the frequency factor and R is the ideal gas constant. The Ea for gold adsorption on tannin gel particles obtained from the slope of the line plotting ln k vs. 1000/T was about 27.7 kJ/mol (Fig. 6(b)). All of the gold was adsorbed onto the tannin gel particles (50 mg dry gel) in 100 ppm and 500 mL HAuCl4 solution, as shown in the previous section. To examine the maximum adsorption capacity for gold, a small amount of tannin gel particles (5 mg dry gel) was added into the aqueous HAuCl4 (100 ppm and 500 mL) solution at the initial pH 2.0, initial pCl 2.0 and 333 K. Fig. 7 shows the time history of the amount of gold adsorbed. Based on the result, the tannin gel particles were found to have an extremely high adsorption capacity, 8000 mg of gold per 1 g of tannin gel particles on a dry basis. The maximum adsorption capacity is extremely high compared with any other adsorbent (alfalfa, which is a biomass material, 40 mg-Au/g-alfalfa (Gardea-Torresdey et al., 2000); aluminum hydroxide, 1.85 mg-Au/g-Al (Yokoyama et al., 2001).

3.5

3

(b)

8000 6000 4000 Maximum adsorption capacity, about 8000 mg-Au/g-dry gel

2000 0 0

100

200 Time [h]

300

400

Fig. 7. Maximum adsorption capacity of gold at initial pH 2.0, initial pCl 2.0 and 333 K.

3.2. Mechanism of reaction between gold and tannin gel particles The tannin gel particles were collected for XRD analysis after 8 h of adsorption to determine the chemical form of gold adsorbed onto the tannin gel

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Table 1 Assignments of FT-IR absorption bands for tannin gel particles before and after adsorption

a b c d

Trans. [%]

Fig. 8. XRD pattern of gold adsorbed onto the tannin gel particles at initial pH 2.0, initial pCl 2.0 and 333 K. Vertical lines are the peaks of metallic gold.

c a

d

b

3500

3000

2500

2000

Assignment

3600–3000 1720–1710 1620–1610 1460–1440 1300–1100

O–H stretching CQO stretching CQC stretching CQC stretching C–H bending

1300–1100 cm1 (d) are ascribed to the chemical structure of the condensed tannin molecule shown in Fig. 1. In the case of the FT-IR spectrum after adsorption, however, the intensity of the band at 1720–1710 cm1 (b: carbonyl groups) increased. This result suggests that the hydroxyl groups of the tannin gel particles are oxidized to carbonyl groups. Both the results of XRD analysis and the FT-IR spectra indicate that the redox reaction takes place between the tannin gel particles and chlorogold. Because the predominant chlorogold complex is AuCl 4 under the conditions of this study, as shown in Fig. 2, the reduction reaction of the chlorogold complex can be expressed as AuCl4  þ 3e ! Au0 þ 4Cl :

Before adsorption After adsorption 4000

Wave number (cm1)

1500

1000

500

Wavenumber [cm–1] Fig. 9. FT-IR spectra of the tannin gel particles before adsorption and after adsorption at initial pH 2.0, initial pCl 2.0 and 333 K.

particles. Fig. 8 shows the XRD pattern of gold adsorbed at initial pH 2.0, initial pCl 2.0 and 333 K. In the diffraction pattern, several peaks are clearly observed at 2y ¼ 38:1, 44.4, 64.6, 77.5, 81.6 and 98.01 which are in good agreement with metallic gold peaks (lines in Fig. 8). The particle size of gold adsorbed onto tannin gel particles is 191 A˚ (without considering crystal strain). This fact indicates that the trivalent gold ions in aqueous chloride solution can be reduced to metallic gold on the tannin polymer gel particles during the adsorption. The FT-IR spectra of the virgin tannin gel particles and the tannin gel particles with metallic gold collected after 24 h are shown in Fig. 9 with the assignments of the FT-IR absorption bands (Table 1). The bands of both tannin gel particles before and after the adsorption of gold at 3600–3000 (a), 1620–1610 (c), 1460–1440 (c), and

(2)

From the results of FT-IR spectra, hydroxyl groups of tannin gel particles are oxidized to carbonyl groups, which is expressed as R2OH ! RQO þ Hþ þ e :

(3)

Based on Eqs. (2) and (3), the overall reaction is AuCl4  þ 3R2OH ! Au0 þ 3RQO þ 3Hþ þ 4Cl : (4) In this redox reaction, the decrease in pH due to the hydrogen ions that were released from hydroxyl groups was observed experimentally, as shown in Fig. 3. To confirm the stoichiometric relation of the redox reaction (Eq. (4)), the relationship between the amount of generated hydrogen ions and the amount of adsorbed gold is plotted in Fig. 10. The amount of gold adsorbed is directly proportional to the amount of hydrogen ions generated. The slope suggests that the gold adsorbed and the hydrogen ions generated are in the ratio of 1 to 3, which supports the proposed redox reaction (Eq. (4)).

4. Conclusions In order to develop a novel and effective recovery system for gold utilizing tannin gel particles, the mechanism of gold adsorption onto the tannin gel

ARTICLE IN PRESS T. Ogata, Y. Nakano / Water Research 39 (2005) 4281–4286

4286 0.5

Au adsorbed [mmol]

0.4

∆Au = 0.353∆H+

0.3 0.2 0.1 0

∆Au: ∆H+ = 1:3 0

0.2

0.4

0.6

0.8

1

H+ generated [mmol]

Fig. 10. Relationship between generated hydrogen ions and amount of Au adsorbed onto the tannin gel particles.

particles was clarified. The adsorption of gold was found to take place through the reduction of trivalent gold ions to metallic gold on the surface of the tannin polymer composing the gel particles, which is accompanied by oxidization of the hydroxyl groups of the tannin gel to carbonyl groups. In addition, tannin gel has an extremely high adsorption capacity, 8000 mg-Au/g-dry gel. The overall reaction was found to be as described in Eq. (4). The results suggest that tannin gel is very useful as an adsorbent in a novel recovery system for gold. The recovery system for gold utilizing tannin gel particles is quite an effective way to achieve a series of unit operations (extraction, reduction, adsorption, solid–liquid separation) simultaneously onto the tannin gel network chains, and there is no need to add chemical agents for those unit operations. Therefore, this system shows good promise as a recovery method for gold because of little generation of secondary waste as well as its simplicity.

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