Persimmon peel gel for the selective recovery of gold

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1 Hydrometallurgy 87 (2007) Persimmon peel gel for the selective recovery of gold Durga Parajuli, Hidetaka Kawakita, Katsutoshi Inoue, Keisuke Ohto, Kumiko Kajiyama Department of Applied Chemistry, Saga University, 1-Honjo, Saga , Japan Received 24 July 2006; received in revised form 29 November 2006; accepted 7 February 2007 Available online 16 March 2007 Abstract The use of persimmon peel gel for the recovery of Au(III) from aqueous chloride medium was investigated. By comparing with the adsorption of some other metal ions, the gel was found to be selective only for Au(III). The XRD analysis and the digital micrograph of the gel taken after adsorption supported the formation of gold particles during adsorption process. High selectivity and capacity of the gel for Au(III) is associated with the reduction of Au(III) to elemental form. Comparative study of reduction of Au(III) by typical tannin rich materials, green tea and oolong tea, and by various organic acids clarified the involvement of polyphenolic groups in the reduction of Au(III) during adsorption. Innovative use of this novel adsorption gel can fulfill the need of cost effective and environment friendly mean for the recovery of valuable metals Elsevier B.V. All rights reserved. Keywords: PP gel; Polyphenolics; Gold(III) 1. Introduction Corresponding author. Tel.: ; fax: address: inoue@elechem.chem.saga-u.ac.jp (K. Inoue). Gold is the most versatile metal possessing many unique properties. Apart from various traditional uses, its application in many technical purposes is now increasing. Meanwhile, a big proportion of gold is being wasted in the form of used electronic and electrical devices. Taking into account the declining resource of gold with its ever increasing applications, efforts should be made for its recovery and recycle from used appliances. For the purpose as such, a number of solvent extraction reagents, ion exchangers and sorbents have been developed. Some of them are commercially employed at present. However, the search for environmentally benign and cost effective method for gold recovery is continuing. Since the last few years research on the development of adsorption gels prepared from tannin is increasing. A great majority of the research works are concerned with the tannin extracted from persimmon (Matsuo et al., 1995; Nakajima and Baba, 2004; Nakajima et al., 2003; Nakajima and Sakaguchi, 1999; Nakano et al., 2001). There are two types of persimmon: sweet persimmons and astringent persimmons. Although the former are eaten as they are, the latter are rich in water soluble tannin which generates very astringent taste and, therefore, unsuitable for direct eating. These astringent persimmons are dried in open air after peeling. During drying, water soluble persimmon tannin is spontaneously crosslinked by condensation reaction between its hydroxyl groups making it water insoluble and diminishes astringent taste. Large amounts of dried persimmons thus produced are marketed and eaten all over X/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi: /j.hydromet

2 134 D. Parajuli et al. / Hydrometallurgy 87 (2007) Japan. This process generates a huge amount of persimmon peel waste. The major components of persimmon peel are lignocellulosics and tannin containing a large number of polyphenolic groups which have high affinity to some specified metal ions. Hence, this bio-waste can be utilized to develop low cost and environment friendly gel for metal extraction. In our previous works, we prepared crosslinked lignophenol gels, novel lignin derived adsorption gels which were found to perform reductive adsorption of Au(III) and exhibit selectivity only for Au (III) (Parajuli et al., 2005, 2006). Similar to lignin, persimmon peel is one of the many bio-products that can be innovatively employed for many productive purposes. In the present work, we prepared adsorption gel directly from the waste of peeled skin of astringent persimmon without extracting the tannin components different from the works mentioned above in order to lower the production cost of the gel by cutting off the cost of their extraction and investigated its adsorption behavior for some precious metals and base metals from hydrochloric acid medium. 2. Experimental 2.1. Materials used Analytical grade chloride salts of copper, iron, palladium, tin, and zinc were used to prepare the test solution of respective metals. Analytical grade HAuCl 4 4H 2 O and H 2 PtCl 6 6H 2 O were used to prepare gold and platinum solutions, respectively. Analytical grade ascorbic acid, oxalic acid, and gallic acid were used to prepare solutions of required concentration Preparation of persimmon peel gel Raw persimmon peel was crushed into fine pieces and mixed with concentrated sulphuric acid. The mixture was stirred for 24 h at 100 C in order to enhance the condensation reaction for crosslinking. The product was filtered, neutralized with sodium bicarbonate solution, washed with distilled water, dried in convention oven for 24 h, and finally crushed to get powder. The dried rigid mass of crosslinked persimmon peel gel is abbreviated as PP gel hereafter Preparation of tea extracts 500 mg of Japanese green tea or Chinese oolong tea was taken in a beaker, added 200 ml of water and boiled for 2 h. The mixture was left to settle and the supernatant liquid was filtered to get about 50 ml aqueous solution of tea extract Batch wise tests using PP gel Adsorption behavior of the PP gel was tested batchwise. 0.2 mm of individual metal solutions were prepared in varying concentration of hydrochloric acid. 10 ml of the metal solution was mixed together with 10 mg of the gel and shaken for 24 h at 30 C to attain the equilibrium. The % adsorption for each metal ion was calculated according to Eq. (1), where C i is the initial concentration of metal ion and C e stands for the equilibrium concentration measured after adsorption on PP gel. k Adsorption ¼ C i C e C i 100 ð1þ 2.5. Kinetic studies Kinetics of adsorption of Au(III) on PP gel was studied at various temperatures (10 C 50 C) by taking 10 ml of 2 mm Au(III) solution prepared in 0.1 M HCl together with 10 mg PP gel Comparative study of Au(III) recovery For the study of the recovery of Au(III) by using green tea and oolong tea, 2 ml of tea extracts was mixed with 10 ml of 1 mm Au(III) solution prepared in 0.1 M HCl and shaken at 30 C for 0 72 h. After filtration the concentration of Au(III) in the filtrate was measured. Similar experiment was carried out by using oxalic acid, gallic acid, and ascorbic acid solutions. In this case, the concentrations of Au(III), HCl and organic acid in 10 ml test mixture were maintained at 1 mm, 0.1 M, and 0.5 mm, respectively. Under the same experimental condition, adsorption test of Au(III) on raw (uncrosslinked) persimmon peel was also carried out by taking 20 mg of crushed material. Recovery of Au(III) was calculated according to Eq. (2) where C i is the initial concentration of metal ion and C e stands for the equilibrium concentration. k Recovery ¼ C i C t C i 100 ð2þ 2.7. Redox analysis In order to examine the oxidation-reduction property of PP gel, redox potential of the Au(III) PP gel and Au (III) organic acids systems were measured at different

3 D. Parajuli et al. / Hydrometallurgy 87 (2007) time intervals at 30 C by using Orion model 9180BN triode ORP electrode Measurement methodology Metal concentration before and after the adsorption was measured by using Shimadzu model AA-6650 atomic absorption spectrophotometer. The initial and equilibrium concentration of hydrochloric acid was measured by titration with sodium hydroxide solution using phenolphthalein indicator. The X-ray diffraction spectrum was recorded using a Rigaku type RINT-8829 X-ray diffractometer. KEYENCE model VHX/VH series micro photographer was used to take the digital micrographs. 3. Results and discussion 3.1. Adsorption behavior of metal ions on PP gel Fig. 1 shows % adsorption of Au(III), Cu(II), Fe(III), Pd(II), Pt(IV), Sn(IV), and Zn(II) on PP gel at varying hydrochloric acid concentrations. Highest selectivity was observed for Au(III) whereas the gel was found to exhibit only weak extractability for other metal ions. The % adsorption of Au(III) was nearly independent of hydrochloric acid concentration. Although some extractability was observed for Pt(IV), it was much lower in comparison to Au(III). From this result it is obvious that the PP gel has very high selectivity to Au(III) and is expected to uptake Au(III) away from a number of coexisting precious or base metals tested so far Adsorption isotherm of Au(III) on PP gel As the PP gel was found to be selective only for Au (III), test of adsorption isotherm was carried out for Au Fig. 1. Plot of % adsorption of different metal ions on PP gel as a function of hydrochloric acid concentration. Initial concentration of metal ions=0.2 mm, weight of adsorbent=10 mg, shaking time=30 h, temperature=30 C. Fig. 2. Adsorption isotherm of Au(III) on PP gel. [HCl]=0.1 M, weight of adsorbent=10 mg, shaking time=30 h, temperature=30 C. (III) ion. Fig. 2 shows the adsorption isotherm of Au(III) at HCl concentration of 0.1 M. From this figure, it is clear that the adsorption increases with increasing gold concentration in low concentration region and, after tending to approach a constant value (=4.5 mol/kg dry gel), it again increases with further increase in concentration in higher concentration region, which is a typical BET type adsorption isotherm based on multilayer adsorption model. According to this model, the energy of the second adsorption layer formation is lower than that of the first and hence, it is inferred that, after the formation of the first layer, the formation of the second layer starts with ease. This result is very interesting due to the occurrence of multilayer adsorption which is not a common phenomenon in typical chemical adsorption. In addition, the above mentioned result shows the feasibility of selective uptake of Au(III) from low to high concentration by using PP gel. In order to understand the above mentioned anomalous result, X-ray diffraction analysis of the gel after adsorption of Au(III) was performed. As shown in Fig. 3, sharp peaks were observed around the 2θ values of 38, 44, 64, and 77 which are assigned as typical values of elemental gold. This result confirms that the adsorption of Au(III) by PP gel is accompanied by the formation of elemental gold, which was further confirmed by the observation of beautiful gold particles in the digital microphotograph as shown in Fig. 4. It is evident from this figure that the gold particles are separated from gel particles, from which it appears promising to recover gold as elemental gold particles even from very low concentration level in acidic medium. In contrast to many commercial processes involving ion exchange or solvent extraction steps, the application of this gel avoids the use of any additional reducing agents for the recovery of Au(III) in elemental form and has very high adsorption capacity. Since the gel is selective only to Au(III), the gold particles

4 136 D. Parajuli et al. / Hydrometallurgy 87 (2007) Fig. 3. XRD pattern of PP gel taken after the adsorption of Au(III). obtained by using PP gel would be undoubtedly free from any impurities. Besides, the gel is environmentally benign because of its natural origin Comparison with other polyphenolic materials From the above discussion it is clear that the PP gel selectively adsorbs only Au(III) and the adsorption is accompanied by the reduction to Au(0). A similar adsorption phenomenon was reported in our previous work of lignophenol gels that explains the role of polyphenolic groups in the reduction of Au(III) (Parajuli et al., 2005, 2006). Similar to lignophenol, the tannin of persimmon peel consists of a number of phenolic and polyphenolic functional groups as shown in Fig. 5 (Matsuo and Ito, 1978). Also, tannin gel prepared from mimosa tannin is reported to shown the similar property during Au(III) adsorption (Ogata and Nakano; 2005). Hence, in order to understand the reductive adsorption of Au(III) on PP gel, Au(III) reduction tests were carried out using extracts of Japanese green tea and Chinese oolong tea that are rich in tannin content, as well as raw persimmon peel, the feed material, for comparison. Fig. 6 shows the comparison of % recovery of Au(III) from 1 mm solution against shaking time among the Fig. 4. Gold aggregates distinctly observed in the digital micrograph of PP gel taken after the adsorption of Au(III). Fig. 5. Schematic structure of persimmon tannin. tested adsorbents and tea extracts. Although in the case of PP gel, 100% recovery has been achieved within 20 h, tea extracts have shown parallel result whereas comparatively poor result was observed for uncrosslinked persimmon peel even though the mass of uncrosslinked persimmon peel taken is twice of that of PP gel. This difference suggests the necessity of activation of raw persimmon peel for better adsorption performance. In the case of tea extracts, the change in Au(III) concentration is equivalent to the amount of Au (III) reduced to elemental form. According to Gammons et al., Au(III) in HCl medium, AuCl 4, cannot be reduced in aqueous solutions because the equilibrium constant, K, for the equation: Au(s) +3H + +4Cl +3/4 O 2 (g)= AuCl 4 +2/3 H 2 Ois at 25 C, which is quite large (Gammons et al., 1997). Hence, from the similar behaviors of PP gel and tea extracts, it is clear that the reduction of Au(III) is accompanied by the exchange of ligands between AuCl 4 and polyphenolic groups of substrates followed by reduction of Au(III) to elemental form. In addition, very high loading capacity of PP gel for Au(III) is possibly due to the release of adsorbed Au (III) from the gel body in elemental form and use of vacant sites by Au(III) in the solution phase that makes an adsorption [of Au(III)] reduction [to Au(0)] release [of Au(0)] adsorption cycle. As mentioned earlier, PP gel is furnished with several polyphenolic groups that are responsible for the reduction of Au(III). To make this explanation more evident, one

5 D. Parajuli et al. / Hydrometallurgy 87 (2007) Fig. 6. Comparative study of Au(III) recovery by using PP gel, raw persimmon peel, oolong tea solution and green tea solution at 30 C. Initial metal concentration=1 mm, shaking time=0 30 h, [HCl] =0.1 M, wt. of PP gel=10 mg, wt. of raw persimmon peel=20 mg, volume of tea solution=2 ml, total volume of solution=10 ml. more comparative kinetic experiment was conducted by using three different organic acids working as reducing agents, namely gallic acid (pyrogallol group), oxalic acid, and ascorbic acid. Fig. 7a shows the time variation of % recovery of Au(III) by using different organic acids and PP gel. Comparatively faster kinetics was observed for PP gel, which is the combined function of adsorption and reduction of Au(III). The behavior of gallic acid is comparable with the gel whereas those observed for oxalic acid and ascorbic acids is very poor. Analogous to gallic acid, persimmon peel consists of many pyrogallol groups together with other phenolic and polyphenolic groups. These pyrogallol groups reinforce the involvement of the other phenolic and polyphenolic groups in the reduction of Au(III) to Au(0) during adsorption on PP gel. Adsorption of Au(III) on PP gel and subsequent reduction to elemental form and the reduction of Au(III) by gallic acid can be explained by observing the change in ORP (emf) of the system. Fig. 7b shows the time variation of ORP in Au(III) acid or Au(III) gel system. An almost constant potential was observed for oxalic and ascorbic acid Au(III) mixtures which is correlated with almost constant concentration of Au(III) in the aqueous solution within the given time interval. As expected from the result shown in Fig. 7a, a sharp decrease in ORP was observed for gallic acid and PP gel, indicating the gradual reduction of Au(III) to Au(0). Also, from the above explanation, it is clear that the reduction of Au(III) on PP gel during adsorption is caused by the activity of polyphenolic groups present in the gel molecule Kinetics of adsorption The adsorption kinetics of Au(III) on PP gel was studied by using 2 mm Au(III) solution in 0.1 M HCl medium. The experiment was conducted at various temperatures: 10 C, 30 C, 40 C, and 50 C. Fig. 8a shows the plots of time variation of Au(III) concentration based on pseudo-first order kinetics. Two distinct phases are observed in each case, the first phase represents exchange of ligands between polyphenolic groups of the gel and AuCl 4 leading to adsorption and the second, most probably, corresponds to the reduction phase. In the case of first phase, the plots corresponding to different temperatures lie on proportional straight lines fitting with the pseudo-first order kinetics. From the slopes of these lines, the rate constants at 10, 30, 40, and 50 C were calculated as , , , and h 1, respectively. From the relationship between rate constant and temperature, the Arrhenius plot was obtained as shown in Fig. 8b from Fig. 7. (a) Comparison of Au(III) adsorption on PP gel, and recovery by gallic acid, ascorbic acid, and oxalic acid at 30 C. (b) Time variation of ORP in these systems. Initial metal concentration=1 mm, volume of solution=10 ml, wt. of gel=10 mg, concentration of acids=0.5 mm.

6 138 D. Parajuli et al. / Hydrometallurgy 87 (2007) Fig. 8. (a) Pseudo-first order plot in the adsorption of Au(III) on PP gel at various temperature. (b) Corresponding Arrhenius plot. Initial metal concentration=2 mm, wt. of gel=10 mg, volume of Au(III) solution=10 ml, shaking time=0 72 h, temperature range=10 50 C. which the activation energy, E a, was calculated as 81.7 kj/mol. The result concludes that the adsorption and subsequent reduction of Au(III) on PP gel is an endothermic process following first order adsorption kinetics. During the kinetic study, the time variation of ORP of the reaction system was also observed as shown in Fig. 9a. Because of slow rate of reduction of Au(III) at 10 C, a negligible change in potential was observed whereas a sudden decrease was recorded at 50 C. This result confirms faster reduction of Au(III) to Au(0) at higher temperature. Fig. 9b shows a comparison between the time variation in ORP and concentration of Au(III) at 40 C. As Au(III) concentration decreases, the ORP of the system also decreases and the process appears to be endothermic because sharp decrease in ORP is observed at higher temperatures. A number of factors can be responsible for the reduction of Au(III) to element gold during adsorption on PP gel. At first, the behavior of Au(III) ion in aqueous medium should be taken into consideration. The standard reduction potential of Au(III) ion is V, and that of AuCl 4 is 1.0 V (Raubenheimer and Cronje, 1999). These values are quite higher than that of Pt(IV), Pd(II), and all the base metals employed in the present study. Both, free auric ion and auric chloride anion are reported to behave like oxidizing agent in aqueous medium. In the present case, the persimmon peel molecules are realized to have some oxidizing tendency attributable to persimmon tannin. As shown in Fig. 5, the primary structure of persimmon tannin consists of complicated flavonoid framework in which a large number of hydroxyl groups are bonded to the aromatic rings majority of which are catechol and pyrogallol groups. Because the aromatic rings are electron rich, the polyphenolics are considered to exhibit high oxidizing tendency in aqueous medium. During redox analysis the response of different phenolics is different. For this reason, comparison of ORP of PP gel (complex phenolics) Au(III) system was not carried out with that of gallic acid, a simple Fig. 9. (a) Variation of ORP during adsorption of Au(III) on PP gel at different temperatures. (b) Correlation of Au(III) concentration with corresponding ORP. Reaction conditions are the same as in Fig. 8.

7 D. Parajuli et al. / Hydrometallurgy 87 (2007) phenolics. Hence, from this comparative study we found that a mutual redox system established by the influence of Au(III) on PP gel or vice-versa leads to the reductive adsorption of Au(III) which results the high capacity and selectivity of PP gel for the ion. Innovation of PP gel from a widely available biomass that exhibits selectivity only for Au(III) and comprises high capacity as well is a promising progress in gold recovery. Acknowledgement This study was supported by the Industrial Technology Research Grant Program in 2006 (KS ) from New Energy and Industrial Technology Development Organization (NEDO) in Japan. References Gammons, C.H., Yu, Y., Williams-Jones, A.E., The disproportionation of gold (I) chloride complexes at 25 to 200 C. Geochim. Cosmochem. Acta, 61 (10), Matsuo, T., Ito, S., On mechanisms of removing astringency in persimmon fruits by carbon dioxide treatment. Part II. The chemical structure of kaki-tannin from immature fruit of the persimmon (Diopyros kaki L.). Agric. Biol. Chem. 42 (9), Matsuo, T., Kamigama, N., Saitoh, M., Adsorption characteristics of immobilized kaki-tannin and industrial application. Acta Hortic. 398, 285 (postharvest physiology of fruits). Nakajima, A., Baba, Y., Mechanism of hexavalent chromium adsorption by persimmon tannin gel. Water Res. 38 (12), Nakajima, A., Sakaguchi, T., Recovery of uranium from uranium refining waste water by using immobilized persimmon tannin. J. Radioanal. Nucl. Chem. 242 (3), 623. Nakajima, A., Ohe, K., Baba, Y., Kijima, T., Mechanism of gold adsorption by persimmon tannin gel. Anal. Sci. 19 (7), Nakano, Y., Takeshita, K., Tsutsumi, T., Adsorption mechanism of hexavalent chromium by redox within condensed-tannin gel. Water Res. 35 (2), 496. Parajuli, D., Inoue, K., Kuriyama, M., Funaoka, M., Makino, K., Reductive adsorption of gold (III) by crosslinked Lignophenol. Chem. Lett. 34 (1), 34. Parajuli, D., Adhikari, C.R., Kuriyama, M., Kawakita, H., Ohto, K., Inoue, K., Funaoka, M., Selective recovery of gold by novel lignin-based adsorption gels. Ind. Eng. Chem. Res. 45 (1), 8. Ogata, T., Nakano, Y., Mechanisms of gold recovery from aqueous solutions using a novel tannin gel adsorbent synthesized from natural condensed tannin. Water Res. 39 (18), Raubenheimer, H.G., Cronje, S., Gold halides, pseudohalides and related compounds. In: Schmidbaur, H. (Ed.), Gold Progress in Chemistry, Biochemistry and Technology. John Wiley & Sons, Chichester, p. 557.