Bioresource Technology 81 (2002) 201±206 Uptake and recovery of gold ions from electroplating wastes using eggshell membrane Shin-ichi Ishikawa b, Kyozo Suyama a,*, Keizo Arihara b, Makoto Itoh b a b Department of Applied Bioorganic Chemistry, Division of Life Science, Graduate School of Agricultural Science, Tohoku University, Sendai 981-8555, Japan Faculty of Animal Science, School of Veterinary Medicine and Animal Sciences, Kitasato University, Towada, Aomori 034-8628, Japan Received 2 July 2000; received in revised form 28 June 2001; accepted 3 August 2001 Abstract The animal byproduct, hen eggshell membrane (ESM), was evaluated for its ability to sorb gold ions (dicyanoaurate(I) and tetrachloroaurate(III)) from solutions and electroplating wastewater. The gold uptake was dependent on pH, temperature and coions present in the solutions, with pH 3.0 being the optimum value. The equilibrium data followed the Langmuir isotherm model with maximum capacities of 147 mg Au(I)/g dry weight and 618 mg Au(III)/g, respectively. Desorption of sorbed gold(I) with 0.1 mol/l NaOH resulted in no changes of the biosorbent gold uptake capacity through ®ve consecutive sorption/desorption cycles. In column experiments, selective recovery of gold from electroplating wastewater containing various metal ions was noted. The anity of metal sorption was in the order Au > Ag > Co > Cu > Pb > Ni > Zn. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Eggshell membrane; Gold; Uptake; Recovery; Biosorption; Electroplating waste 1. Introduction With the growth of the electronics industry, the demand for gold has markedly increased from year-toyear. Gold recovery from secondary sources such as electronic scrap and waste electroplating solutions and from primary resources such as leach solutions is, therefore, an important technology. In recent years, attentions have been focused on the use of various industrial wastes, agricultural byproducts and biological materials as metal sorbents (Ajimal et al., 1996; Hasan et al., 2000; Low et al., 1999). Several studies suggest that the biosorbents can provide a coste€ective means of recovering gold from aqueous solutions (Hosea et al., 1986; Greene et al., 1986; Kuyucak and Volesky, 1989a; Matsumoto and Nishimura, 1992; Nakajima and Sakaguchi, 1993). It is actually expected that the biosorbents accumulate gold from secondary sources. Earlier investigations have shown that then eggshell membrane (ESM) is capable of binding various metal ions from aqueous solutions (Suyama et al., 1994; * Corresponding author. Tel.: +81-176-23-4371/+81-22-717-8818; fax: +81-176-23-8703/+81-22-717-8820. E-mail address: suyama@bios.tohoku.ac.jp (K. Suyama). Ishikawa and Suyama, 1998). The ESM (consisting of protein ®bers) resides between the egg white and the inner surface of the eggshell. The ESM is an intricate lattice network of stable and water-insoluble ®bers and has very high surface area with homogeneity (Suyama et al., 1994; Creger et al., 1976). The ESM is suggested to be the part where eggshell formation occurs, and it is thought to be a model for biomineralization (Carrino et al., 1996). Since the ESM materials are available in large quantities as a byproduct of egg industry, its potential utilization as a metal biosorbent is of interest. In this study, sorption of two types of gold ions, dicyanoaurate(I) and tetrachloroaurate(III) ion, by the ESM was examined. In addition, selective gold recovery from electroplating wastewater was also investigated in a ¯ow-through sorption column system. 2. Methods 2.1. Preparation of ESM ESM was mechanically stripped from the shell after immersion of the hen eggshell, which was obtained commercially, in 0.5 mol/l HCl overnight and then fur- 0960-8524/02/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 0 - 8 5 2 4 ( 0 1 ) 0 0 1 3 4 - 1 202 S.-i. Ishikawa et al. / Bioresource Technology 81 (2002) 201±206 ther in 0.1 mol/l NaOH for 1 h followed by rinsing with distilled deionized water 10 times. In the case of kinetics and column experiments, dried material was ground in the laboratory blender and sieved. Five di€erent fractions were collected: fraction 1 (#1, d ˆ 0:025±0:053 mm), fraction 2 (#2, d ˆ 0:053± 0:106 mm), fraction 3 (#3, d ˆ 0:106±0:250 mm), fraction 4 (#4, d ˆ 0:250±0:500 mm), and fraction 5 (#5, d ˆ 0:500±1:000 mm). The dry weight of samples was obtained after drying in vacuo to constant weight. Except for kinetics and column experiments, non-powdered ESM samples were used for experiments. 2.2. Chemicals and metal solutions Analytical grades of KAu CN†2 , HAuCl4 H2 O (Kanto Chemicals, Tokyo, Japan), NaCl, NaBr, NaI, NaCN, NaOH, and hydrochloric acid (37%) (Nacalai Tesque, Kyoto, Japan) were used. Gold atomic adsorption standard solution (1000 mg/l) was purchased from Nacalai Tesque. Waste electroplating solutions were acquired as follows: waste computers were disassembled, and then the gold-plated parts were collected. The collected parts were washed by methanol, and they were cyanized by dissolving with 0.05% (w/v) KCN containing 0.05% (w/v) CaO. The gold-dissolving solutions formed a precipitate, which was removed by ®ltration through a polyvinylidene ¯uoride (PVDF) 0.45 lm-pore-size membrane ®lter (Whatman, Clifton, NJ, USA). The concentration of gold and other metal ions was determined by an atomic absorption spectrophotometer (AES SAS-727, Seiko Instruments & Electronics, Chiba, Japan) or an inductively coupled plasma mass spectrometer (ELAN 6000 ICP-MS, Perkin Elmer, Norwalk, CT, USA). The sorption capacity (q) (mg/g) was calculated from the initial concentration (Ci ) (mg/l) and the ®nal (residual) concentration (Cf ) (mg/l) of the metal in solution according to the following formula: q ˆ V Ci Cf †=m; where V is the volume of the solution (l) and m is the dry mass of the biosorbent (g). 2.4. Desorption experiments The batch desorption experiments were performed as follows: ESM previously exposed to a gold-containing solution was pulled up from the solution with tweezers. The metal-loaded ESM was replaced in 10 ml of an eluent solution and mixed on the rotary shaker at 90 rpm for 1 h at 25 °C. It was known that the desorption reached equilibrium within 1 h from previous work (Ishikawa and Suyama, 1998). The gold concentration in the eluent solution was analyzed with the atomic absorption spectrophotometer. 3. Results 2.3. Sorption experiments 3.1. Gold sorption by ESM The synthetic gold solutions of desired concentrations were prepared by dissolving gold salts in deionized distilled water. Two procedures were utilized in experiments reported in this study. The ``batch'' experiments were performed in 100-ml bottles (0.025±0.035 g of ESM, with 50 ml of gold-bearing solution of known initial concentration) on a rotary shaker at 90 rpm at desired temperature. 1 mol/l HCl and NaOH were used for adjustment of solution pH toward the ®nal equilibrium. Separation of the eluent solution from the ESM materials was achieved by ®ltration through the Whatman membrane ®lter. For safety precautions, all experiments with cyanide-containing solutions were performed in a draft chamber to avoid exposure to HCN. In the ``column'' procedure, the crushed ESM powder-packed mini column (diameter 1.5 cm, length 12 cm) carried out an experimental scale of gold collection. The slurry of powdered ESM #3 was poured into a column, then a pH-adjusted solution containing metal ions was passed through the column with the ¯ow rate 1.0 ml/min and the e‚uent was analyzed. The column sorption experiments were performed at 25 °C. The e€ect of pore di€usion on the gold sorption rate was examined in batch kinetics experiments using different sizes of ESM materials. The resulting curves of gold uptake versus time for di€erent sizes of the biosorbent are presented in Fig. 1, demonstrating the rate di€erences between the samples. In the ®rst 30 min, the non-powdered ESM-gold ion system reached 45±65% of the total sorption and required more than 2 h to reach the ®nal sorption equilibrium. However, when the size of the ESM was smaller, the sorption dynamics of both types of gold was faster. Especially, the gold sorption by smaller particles (#2, d ˆ 0:053±0:106 mm) was very rapid, and the respective equilibria were reached within 5 min. Any di€erences in the amounts of the gold uptake at equilibrium among the samples could not be actually demonstrated for both types of gold. The pH of the solution is a very important factor in the gold sorption uptake by the ESM. The optimal sorption pH for both gold ions was pH 3. The gold uptake increased with increasing pH, to the maximum near pH 3, then decreased at higher pH values. The gold(I) uptake was signi®cantly dependent on pH and hardly observed above pH 7. 203 S.-i. Ishikawa et al. / Bioresource Technology 81 (2002) 201±206 3.2. E€ect of co-ions on gold sorption Fig. 1. E€ect of powdering of ESM and its particle size on the sorption kinetics of gold(I) and gold(III). The non-powdered ESM H; † and the ESM powder particles of sizes #2 ; † (small particles: d ˆ 0:053±0:106 mm) and #4 j; † (large particles: d ˆ 0:250±0:500 mm) were soaked in a solution (pH 3) containing 100 mg/l dicyanoaurate(I) or tetrachloroaurate(III) at 25 °C. The e€ect of the presence of co-ions in the solution on the gold uptake capacities of ESM was examined by adding respectively Cl , Br , I and CN anions, because dicyanoaurate(I) and tetrachloroaurate(III) were anionic gold complexes. Examinations were performed at pH 3.0 for di€erent concentrations of co-ions. These co-ions had an appreciable e€ect on the gold uptake capacities by ESM (Figs. 2(a) and (b)). A decrease in the gold uptake capacities was observed with increasing the concentration of co-ion. At high initial concentrations of 0.1 mol/l NaCl or NaCN, the gold(I) uptake capacity was suppressed by approximately 85% regardless of the species of co-ion. The gold(III) uptake capacity of ESM was depressed in the order Cl < Br < I < CN , increasing the concentration of co-ion from 0.01 to 0.1 mol/l.  The e€ect of temperature on the gold uptake by the ESM materials was also examined. The gold uptake capacities of the biosorbent were evaluated by plotting the sorption isotherms at pH 3 for di€erent temperatures.The initial concentration of gold in the solutions ranged from 100 to 500 mg/l. The dicyanoaurate(I) uptake tended to decrease when the temperature increased from 5 to 65 °C, whereas the tetrachloroaurate(III) uptake capacity increased with increasing temperature. A signi®cant temperature e€ect on the gold(III) uptake was observed at 65 °C. The highest experimentally observed gold uptake values of 137 mg Au(I)/g (at 5 °C and Cf ˆ 423 mg/l) and 477 mg Au(III)/g (at 65 °C and Cf ˆ 181 mg/l) still did not represent the full saturation values in the isotherms, and the respective qmax values were calculated from a reasonably well ®tting Langmuir model (Table 1). Table 1 also summarizes the sorption parameter, b, and the correlation coecients (r2 ) of the model (r2 : 1.0 ˆ perfect ®t). 3.3. Desorption of gold-laden ESM The preceding experiments indicated that the dicyanoaurate(I) uptake by ESM was not observed at alkaline pH and that the tetrachloroaurate(III) uptake was inhibited by strong ligands. These suggest that desorption of gold-loaded ESM by complexation with competing ligands might be e€ective. Therefore, exploratory desorption tests for gold-loaded biosorbent were conducted under batch conditions. The gold-loaded ESM was contacted with NaOH or NaCN eluent for 1 h at di€erent concentrations (0±0.1 mol/l). Fig. 3 shows the e€ect of the NaOH concentration on the desorption of ESM-sorbed dicyanoaurate(I) and the NaCN concentration on the desorption of ESM-sorbed tetrachloroaurate(III). The results indicated that the desorption of gold from ESM was very e€ective using NaOH or NaCN. The NaOH solution (0.10 mol/l) was capable of desorbing more than 95% of the gold(I) sorbed to ESM, and the NaCN solution (0.10 mol/l) was capable of desorbing more than 92% of the gold(III). Table 1 Langmuir isotherm parameters (qmax and b) and the correlation coecients (r2 ) resulting from equilibrium sorption studies of dicyanoaurate(I) and tetrachloroaurate(III) by ESM Type of gold complex T (°C) Langmuir isotherm parameters qmax b 100† r2 Dicyanoaurate(I) 5 25 45 65 147 132 135 108 1.79 1.46 1.14 1.24 0.987 0.997 0.997 0.991 Tetrachloroaurate(III) 5 25 45 65 225 226 299 618 43.62 22.13 10.33 6.34 0.964 0.976 0.995 0.918 204 S.-i. Ishikawa et al. / Bioresource Technology 81 (2002) 201±206 Fig. 3. Desorption of ESM-sorbed gold(I) and gold(III). The ESM, initially sorbed with dicyanoaurate(I) (Ci ˆ 100 mg/l, for 3 h at 25 °C), was soaked in solutions containing di€erent concentrations of NaOH †, and the ESM sorbed with tetrachloroaurate(III) (at same conditions) was soaked in solutions containing di€erent concentrations of NaCN †.  Fig. 2. E€ect of co-ions on the sorption of gold(I) (a) and gold(III) (b) by ESM. The ESM was soaked in solutions (pH 3) of 100 mg/l dicyanoaurate(I) containing di€erent concentrations of NaCl † or NaCN †, and 100 mg/l tetrachloroaurate(III) containing di€erent †, NaBr †, NaI j†, or NaCN † for concentrations of NaCl 3 h at 25 °C, expressed as a percent of the uptake values in Fig. 1 (by the non-powdered ESM for 3 h).   The gold(I) sorption±desorption experiment was performed in ®ve consecutive cycles. The gold uptake capacity of ESM in the ®fth cycle was comparable to that in the ®rst cycle (results not shown). The loss in the dry weight of ESM was less than 5% during ®ve sorption±desorption cycles. 3.4. Recovery of gold and other metal ions from electroplating wastewater To determine the ability of ESM to remove various metal ions from the electroplating wastewater, an ESM column was used for the recovery of gold and other metal ions. Fig. 4 shows the elution patterns of the metal ions. The recovery order was Au 98%† > Ag 97%† > Co 94%† > Cu 17%† > Pb 15%† > Ni 4%† > Zn 3%†. Fig. 4. Recovery of gold and other metal ions from the electroplating wastewater by ESM column, expressed as a percent of initial metal concentration (Co †: 0.01, Ni †: 0.53, Cu j†: 5.26, Zn †: 0.56, Ag H†: 0.03, Au N†: 23.74, Pb †: 0.14 (mg/l)).  The ®gure indicates that the elution patterns are mainly classi®ed into the next three groups: ®rst, a group of the metal ions which are almost sorbed by ESM (Co, Ag and Au), second, the metal ions which are scarcely sorbed (Ni and Zn), and third, the metal ions which are sorbed for a time, and later desorbed (Cu and Pb). 4. Discussion An excellent potential for gold recovery was demonstrated by eggshell membrane, which consists of colla- S.-i. Ishikawa et al. / Bioresource Technology 81 (2002) 201±206 gen-like proteins (Arias et al., 1991, 1997). It is known that the ESM proteins have a high content of histidine, cystein and proline compared to cuticle or matrix proteins, however, the membrane proteins are relatively low in glycine (Stadelman and Cotterill, 1977). Gold(I) and gold(III) are classi®ed as ``soft'' metal ions according to the hard and soft principle of acids and bases (Pearson, 1963). As to a counter-ion of the gold complex, a cyanide ion, being classi®ed as a soft base, coordinates very strongly with the gold ion. The dicyanoaurate(I) ion, Au CN†2 , is the most stable complex ion formed with gold(I), and the stability constant, K, has been estimated to be 1039 (Puddephatt and Vittal, 1994). A chloride ion, which is classi®ed as a borderline base, does not coordinate so strongly with the gold(III) ion. Consequently, the tetrachloroaurate(III) ion, AuCl4 , is not stable in the solution and easily dissociates (Kuyucak and Volesky, 1989a). X-ray absorption spectroscopy experiments on the alga Chlorella vulgaris have provided signi®cant insight into the nature of the binding of the gold complexes dicyanoaurate(I) and tetrachloroaurate(III) (Watkins II et al., 1987). These results indicate that chemical reactions, not only simple electrostatic interactions but also ligand-change reactions, are involved in the binding of the gold to the algae. The ESM sorption systems may be the same reaction. The ESM contains amines, amides and carboxylic surface functional groups, which are protonated or deprotonated, depending on the pH of the aqueous solution. Generally, they have positively charged sites, in which mainly ±NH‡ 3 groups form as a result of acidi®cation. Below pH 3, the Au CN†2 uptake by the ESM is inhibited with increasing concentration of co-ion, Cl , of pH adjustment. The characteristics of the dicyanoaurate(I) sorption pro®le suggest that ionic interactions possibly involve electrostatic interactions between the negatively charged dicyanoaurate(I) complex and the positively charged ESM ligands. In increasing the concentration of the chloride ion in addition to HCl, AuCl4 became more stable (Kuyucak and Volesky, 1989a), and the sorption of gold(III) ion by ESM was depressed. Tetrachloroaurate(III) is also believed to react with the lysine, histidine, and arginine side chains of ovalbumin (Craig et al., 1954). The mechanism of this reaction is thought to involve the initial formation of an ion pair between negatively charged tetrachloroaurate(III) and positively charged nitrogenous groups, followed by elimination of the chloride. The dependence of the gold uptake rate on the biosorbent size may be an indication, however, that di€usion prevails in controlling the reaction rate. The insensitivity of the metal uptake process to external bulk 205 suspension mixing would point to the interparticle diffusional limitations in the transport of ionic species. A Langmuir sorption model was used to evaluate the sorption behavior of the material. It served to estimate the maximum metal uptake values where they could not be reached in the experiments. Its constant b can serve as an indicator of the isotherm which re¯ects quantitatively the ``anity'' between the sorbent and the sorbate. The model, which is based on assumptions of surface adsorption (Langmuir, 1919), showed good ®t for the present case of biosorption. In cases of a more complex metal uptake apparently involving metabolically driven bioaccumulation and deposition of the metal, the model might not ®t the experimental data so well (Atkins, 1990). The highest gold(I) uptake was observed at lower temperature, the highest gold(III) uptake at higher temperature. According to the relationship between the distribution coecient, Kd (the gold uptake capacity (q)/ the residual gold concentration (Cf )), and absolute temperature (T) (Nakajima and Sakaguchi, 1993), the enthalpy of the gold(I) sorption by ESM was estimated to be )4.8 kJ/mol, which indicated that the gold(I) sorption by ESM involved an exothermic reaction. On the other hand, the enthalpy of the gold(III) sorption was +23.1 kJ/mol, which indicated that the gold(III) sorption involved an endothermic reaction. Physical sorption reactions are normally exothermic, and the enthalpy change in the physical sorption is usually smaller than 25 kJ/mol (Atkins, 1990). The fact would support that the sorption of dicyanoaurate(I) by ESM may proceed via a physical adsorption, such as electrostatic interactions. The gold(III) sorption by ESM may proceed via a more complex chemical reaction involving the dissociation of Au3‡ from tetrachloroaurate(III) as previously described. An entropy charge of the dissociation of tetrachloroaurate(III) ion in an aqueous solution is assumed to be positive (Nakajima and Sakaguchi, 1993), because the mole number of ions in the solution increases after dissociation AuCl4 ! Au3‡ ‡ 4Cl †. As DG dissociation†ˆDH dissociation† T DS dissociation†, thus the dissociation of tetrachloroaurate(III) ion can be endothermic when T DS dissociation† > DH dissociation†. All anions which were examined for their co-ion e€ect on the gold sorption markedly a€ected the gold uptake by ESM. In case of the gold(I) sorption, their co-ions can interact the positively charged surfaces of the material. The uptake capacity of tetrachloroaurate(III) by ESM was depressed in the order Cl < Br < I < CN . This order is consistent with the order of ligand strengths toward gold(III) and suggests that AuCl4 becomes a more stable complex like AuBr4 ; AuI4 or Au CN†4 (Greene et al., 1986). 206 S.-i. Ishikawa et al. / Bioresource Technology 81 (2002) 201±206 Reportedly, the elution of gold sequestered on a brown marine alga, Sargassum natans, was used by a solution of 0.1 mol/l thiourea with 0.02 mol/l ferric ammonium sulfate; however, the optimum time for desorption was determined to be as long as 17 h (Kuyucak and Volesky, 1989b). Since the desorption on ESM reaches equilibrium within 1 h, the ESM system has an advantage for practical use. The present method of recovering the gold sorbed by sorbents such as ion-exchange resins is by drying and incineration of the resins, because the high anity of Au CN†2 to the resin makes it extremely dicult to elute the gold. This process has two major drawbacks: ®rst the potential for substantial loss of gold and second the environmental hazards encountered due to the incineration of the resin. The ESM material is a very stable protein against the alkaline solutions and can be repeatedly used for gold recovery. The AMT-BIOCLAIMe metal recovery agent granules have been examined for use for recovery of gold from jewelry manufacturers' wastewater (Brierley et al., 1986). A column containing the biomass removed more than 99% of the gold, silver and copper for the ®rst 2 l of solutions of mixed-metals and cyanide-complexed metals. However, after treating 4 l, only 58% of the gold was removed, and the selective recovery of metals was not observed. Unlike the AMT-BIOCLAIMe, the ESM material was capable of accumulating gold, silver and cobalt selectively from the electroplating wastewater containing various metal ions. Reportedly, the surfaces of biosorbent play an important role in the selective removal of metal ions (Forster, 1983). The selectivity in the ESM system would depend on the ESM surface charge and ionic radii of the species of metal ions. These experiments suggest that the applications of ESM biosorption column systems have a potential for the recovery and re®ning of gold. The present work presents new data characterizing the performance of a new biosorbent material which e€ectively recoveries gold from aqueous solutions. The results obtained indicate the high sorptive capacity of the new biosorbent. The key parameters described serve as a basis for the sorption process evaluation and design, o€ering the possibility of an e€ective, simple, and relatively accurate sorption process scaleup. References Ajimal, M., Khan, R.A., Siddiqui, B.A., 1996. Studies on removal and recovery of Cr(VI) from electroplating wastes. Water Res. 20, 231± 261. Arias, J.L., Fernandez, M.S., Dennis, J.E., Caplan, A.I., 1991. Collagen of the chicken eggshell membranes. Connect. Tissue Res. 26, 37±45. Arias, J.L., Nakamura, O., Fernandez, M.S., Wu, J.J., Knigge, P., Eyre, D.R., Caplan, A.I., 1997. Role of X collagen on experimental mineralization of eggshell membranes. Connect. Tissue Res. 36, 21±33. Atkins, P.W., 1990. Physical Chemistry, 4th ed. Oxford University Press, Oxford. Brierley, J.A., Brierley, C.L., Goyak, G.M., 1986. AMT-Bioclaime: A new wastewater treatment and metal recovery technology. Biohydrometallurgy, 291±304. Carrino, D.A., Dennis, J.E., Wu, T.M., Arias, J.L., Fernandez, M.S., Rodriguez, J.P., Fink, D.J., Heuer, A.H., Caplan, A.I., 1996. The avian eggshell extracellular matrix as a model for biomineralization. Connect. Tissue Res. 35, 325±329. Craig, J.P., Garrett, A.G., Williams, H.B., 1954. The ovalbuminchloroauric acid reaction. J. Am. Chem. Soc. 76, 1570±1575. Creger, C.R., Phillips, H., Scott, J.T., 1976. Formation of an egg shell. Poult. Sci. 55, 1717±1723. Forster, C.F., 1983. The size of activated sludge ¯ocs in relation to their surface characteristics. Environ. Technol. Lett. 4, 329± 334. Greene, B., Hosea, M., McPherson, R., Henzl, M., Alexander, M.D., Darnall, D.W., 1986. Interaction of gold(I) and gold(III) complexes with algal biomass. Environ. Sci. Technol. 20, 627±632. Hasan, S., Hashim, M.A., Gupta, B.S., 2000. Adsorption of Ni SO4 † on Malaysian rubber±wood ash. Bioresour. Technol. 72, 153±158. Hosea, M., Greene, B., McPherson, R., Henzl, M., Alexander, M.D., Darnall, D.W., 1986. Accumulation of elemental gold on the alga Chlorella vulgaris. Inorg. Chim. Acta 123, 161±165. Ishikawa, S., Suyama, K., 1998. Recovery and re®ning of Au by goldcyanide ion biosorption using animal ®brous proteins. Appl. Biochem. Biotechnol. 70/72, 719±728. Kuyucak, N., Volesky, B., 1989a. Accumulation of gold by algal biosorbent. Biorecovery 1, 189±204. Kuyucak, N., Volesky, B., 1989b. The elution of gold sequestered on a natural biosorbent. Biorecovery 1, 205±218. Langmuir, I., 1919. The adsorption of gases on plane surfaces of gas, mica and platinum. J. Am. Chem. Soc. 40, 1361±1403. Low, K.S., Lee, C.K., Ng, A.Y., 1999. Column study on the sorption of Cr(VI) using quaternized rice hulls. Bioresour. Technol. 68, 205± 208. Matsumoto, K., Nishimura, Y., 1992. Recovery by Aspergillus oryzae of gold from waste water from gold plating. Nippon Nogei Kagaku Kaishi 66, 1765±1770. Nakajima, A., Sakaguchi, T., 1993. Uptake and recovery of gold by immobilized persimmon tannin. J. Chem. Technol. Biotechnol. 57, 321±326. Pearson, R.G., 1963. Hard and soft acids and bases. J. Am. Chem. Soc. 85, 3533±3539. Puddephatt, R.J., Vittal, J.J., 1994. Gold: Inorganic & coordination chemistry. In: King, R.B. (Ed.), Encyclopedia of Inorganic chemistry, vol. 3. Wiley, Chichester, pp. 1320±1331. Stadelman, W.J., Cotterill, O.J., 1977. Egg Science and Technology, 2nd ed. The Avi Publishing Company, Westport, Connecticut. Suyama, K., Fukazawa, Y., Umetsu, Y., 1994. A new biomaterial, hen egg shell membrane, to eliminate heavy metal ion from their dilute waste solution. Appl. Biochem. Biotechnol. 45±46, 871±879. Watkins II, J.W., Elder, R.C., Greene, B., Darnall, D.W., 1987. Determination of gold binding in an alga biomass using EXAFS and XANES spectroscopies. Inorg. Chem. 26, 1147±1151.