Proteins and Protein-Rich Biomass as Environmentally Friendly

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Environ. Sci. Technol. 2007, 41, 1359-1364

Proteins and Protein-Rich Biomass as Environmentally Friendly Adsorbents Selective for Precious Metal Ions TATSUO MARUYAMA,* HIRONARI MATSUSHITA, YUKIKO SHIMADA, ICHIRO KAMATA, MISA HANAKI, SAORI SONOKAWA, NORIHO KAMIYA, AND MASAHIRO GOTO* Department of Applied Chemistry, Graduate School of Engineering and Center for Future Chemistry, Kyushu University, 744 Moto-oka, Fukuoka 819-0395, Japan

Proteins exhibit specific interactions with various metal ions, which play important roles in a living cell. Here, we found that various proteins selectively adsorbed precious metal ions at a wide range of pH values. Studies on protein sequences and on synthesized peptides revealed that a histidine-containing sequence had specific interactions with precious metal ions (Au3+ and Pd2+). We then investigated a few types of protein-rich biomass as adsorbents for precious metal ions. In the presence of various transition metal ions, Au3+ and Pd2+ were also selectively adsorbed onto the biomass tested. The bound precious metal ions were recovered by aqua regia after charring the metalbound biomass. Finally, we demonstrated the successful recovery of Au3+ and Pd2+ from a metal refining solution and a metal plating waste using the biomass. We propose an environmentally friendly recycling system for precious metal ions using protein-rich biomass.

Introduction Gold and platinum group metals (e.g., Pd, Pt, and Rh) are of great importance in various industries due to their physical and chemical properties. Since the amounts of these precious metals and their mining are very limited throughout the world, the recovery of precious metals is in high demand (1). Industrial recycling techniques for the precious metals from wastes are currently hydro- or pyrometallurgical. Of the techniques employed, adsorption is the most cost-efficient and effective way of recycling precious metal ions (2). To date, a number of adsorption materials have been developed; for example, activated carbon, mineral materials, and ionexchange resins. In particular, various types of ion-exchange resins, which are made from petrochemical-dependent polymers, are designed for diverse complexes of metal ions and are widely employed in industry. In the past decade, carbon dioxide emission has become one of the major global concerns and petrochemicalindependent products have attracted much attention. Biomass, such as agricultural products and aquatic resources, does not emit extra carbon dioxide when burned. Biomass is therefore expected to be carbon-neutral and petrochemi* Address correspondance to either author. E-mail: tmarutcm@ mbox.nc.kyushu-u.ac.jp (T.M.); [email protected] (M.G.). 10.1021/es061664x CCC: $37.00 Published on Web 01/13/2007

 2007 American Chemical Society

cal-independent (3). Several research groups reported that various types of biomass (e.g., microorganisms, polyphenols, and polysaccharides) facilitate adsorption of several metal ions in aqueous solution (4-11). However, selective recovery of targeted metal ions from the aqueous solution, and study of the adsorption mechanism, have been challenging tasks, because biomass is an unpurified product composed of a wide variety of substances. This drawback also prevents any practical demonstration of using biomass for metal recycling from industrial waste containing various metal ions. Proteinrich biomass is produced as a byproduct in the food and agricultural industries, and is usually inexpensive (far less than 100 US$/kg). Owing to the high protein content (∼90%), biomass is an attractive source of peptides and amino acids (12). Proteins interact with metal ions, and these interactions play an important role in biological systems. There are, however, only limited studies on the interaction between proteins and precious metal ions (13-15). Craig et al. revealed, for the first time, strong interactions between ovalbumin and Au3+ (16). In 2000, the first crystal structure of an Auprotein complex was reported (17). These reports suggest that proteins are good candidates for being an adsorbent of precious metal ions. In the present study, we investigated and revealed strong and selective interactions between purified proteins and precious metal ions. Studies on synthesized peptides identified one of the adsorption sites of precious metal ions in proteins. Finally, we demonstrated that protein-rich biomass acts as an adsorbent selective for Au3+ and Pd2+ in industrial wastes.

Experimental Section Competitive Adsorption of Metal Ions to Proteins. Metalion standard solutions of Au3+, Pd2+, and Pt4+ used were hydrochloric solution. Those of Cu2+ Ni2+ and Zn2+ were nitric solution. The metal-ion standard solutions (1000 ppm) were mixed and diluted with 0.1 M HCl to prepare an aqueous solution containing six different metal ions (Au3+, Pd2+, Pt4+, Cu2+ Ni2+, and Zn2+). Lysozyme (from hen egg white), bovine serum albumin (BSA), and ovalbumin were used as purified model proteins. Proteins (2 mg) were dissolved in the solution (10 mL) containing the metal ions, followed by gentle stirring for 1 h at room temperature. The solutions were filtered through an ultrafiltration membrane with a molecular cutoff of 10 kDa (Amicon Ultra-4, Millipore). The filtrate was subjected to inductively coupled plasma (ICP)-atomic emission spectrometry (Optima 3100 RL; Perkin-Elmer) to determine the concentration of unadsorbed metal ions. The adsorption rate (R) of metal ions was calculated by the following equation:

R ) 100 × [(initial metal ion concentration) (metal ion concentration in the filtrate)]/ [initial metal ion concentration] A solution containing six different metal ions without proteins was filtered through the ultrafiltration membrane, and no adsorption of metal ions to the filtration membrane was verified. The plots with error bars in all figures are based on the results in triplicate. Error bars represent standard deviations. Adsorption Isotherm and Effect of pH. To study the adsorption isotherm for Au3+ and Pd2+, protein concentrations (lysozyme, BSA, and ovalbumin) were varied from 0 to 1 g/L. The adsorption experiments were carried out using an aqueous solution containing Au3+ or Pd2+ (10 ppm) at pH 4. VOL. 41, NO. 4, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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After filtration as described above, the concentration of unadsorbed Au3+ and Pd2+ in the filtrates was determined by atomic absorbance spectrophotometry (AA6700; Shimadzu, Kyoto) using flame. The effect of pH on Au3+ and Pd2+ adsorption to proteins was investigated by varying the pH of the metal ion solutions. Au3+ and Pd2+ solutions (10 ppm finally) at various pH values were prepared by mixing 0.1 M HCl containing the metal ion (10 ppm) and 0.1 M NaOH containing the metal ion (10 ppm). In the case of pH 1, the metal ions solutions were prepared only by 0.1 M HCl. The concentration of unadsorbed metal ions was determined as described above. Desorption of Au3+ from proteins by thiourea was conducted as follows. Thiourea (2 mM) was added to the Au3+-protein solution and the solution was gently stirred for 1 h at room temperature, followed by ultrafiltration. The concentration of unadsorbed Au3+ in the filtrate was determined by atomic absorbance spectrophotometry. Peptide Synthesis on a Solid Phase and Adsorption of Metal Ions to Peptides. To identify one of the binding sites in proteins, we looked at the amino acid sequence of lysozyme. Lysozyme contains His-Gly-Leu in its amino acid sequence and this sequence is a candidate for the Au- or Pd-binding site (17). Various tripeptides (N-Ac-X-Gly-Leu) were synthesized on Merrifield resin based on the wellestablished 9-fluorenylmethoxycarbonyl (Fmoc) chemistry (18). The N termini of the tripeptides were acetylated using acetic anhydride. Cys-containing peptide was reduced by dithiothreitol (10 mM) overnight and washed with 0.1 N HCl. The N-acetyl tripeptides immobilized on the resin were subjected to adsorption. The N-acetyl tripeptides (0.7 µmol) immobilized on the resin were added to 10 ppm metal ion solutions (5 mL, pH 4.0) containing 0.5 M NaClO4. After gentle stirring for 1 h, the solutions were centrifuged. Supernatants were subjected to atomic absorbance spectrophotometry to determine unadsorbed metal ions in solution. Adsorption of Metal Ions to Protein-Rich Biomass. Protein-rich biomass tested here was soybean protein (Fuji Oil Co., Osaka, Japan), chicken egg-shell membrane (Q.P. Corporation, Tokyo, Japan) and seasoning pollack-roe membrane (Yamaya Communications, Fukuoka, Japan). Since most of waste solutions containing metal ions are acidic, the adsorption of metal ions to biomass was studied in 0.1 M HCl. Each biomass (10 mg) was added to 5 mL of 0.1 M HCl solution containing six different metal ions (Au3+, Pd2+, Pt4+, Cu2+, Ni2+, and Zn2+; 10 ppm each). After gentle stirring for 1 h at room temperature, the solution was filtered and the filtrate was subjected to ICP-atomic emission spectrometry to determine unadsorbed metal ions in solution. In the case of soybean protein, prior to filtration, 0.1 M sodium acetate solution was added to adjust the pH to 4. Soybean protein was insoluble around pH 4. The precipitated soybean protein was removed by filtration and the filtrate was subjected to ICP-atomic emission spectrometry. Practical Recovery of Au3+ and Pd2+ from Actual Copper Refining Solution and Metal Plating Wastewater. Copper refining solution containing various metal ions was kindly provided by a non-ferrous metal refining company in Japan and metal plating wastes containing Au3+ or Pd2+ were kindly provided by Muromachi Technos (Omuta, Japan). Prior to adsorption experiments, all the sample solutions were filtered through microfiltration membranes with a pore size of 0.8 µm. Egg-shell membrane (300 mg) was added to the filtrate (5 mL), followed by gentle stirring for 1 h. After adsorption, the mixture was filtered and the filtrate was subjected to atomic absorbance spectrometry to measure unadsorbed metal ions. The egg-shell membrane adsorbing metal ions was charred at 600 °C for 2 h. The resultant ash was added to aqua regia (20 mL) and mixed for 3 h. After filtration, the 1360

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FIGURE 1. Competitive adsorption of metal ions to typical purified proteins. Concentration of metal ions was 10 ppm each and protein concentration was 0.2 g/L. The solution contained 0.1 M HCl. Adsorption experiments were carried out at 25 °C for 1 h. concentration of targeted metal ions in the solution was measured by atomic absorbance spectrophotometry. Based on the atomic absorbance analysis, the recovery rate of precious metal ions was calculated as a relative value to the initial amount of the metal ions in the refining solution or the plating wastes.

Results and Discussion Competitive Adsorption of Metal Ions to Proteins. There have been many studies on adsorption of metal ions using biomaterials but very few have reported selective adsorption of precious metal ions in the presence of different metal ions (19-21). We first investigated competitive adsorption of precious metal ions to typical purified proteins in the presence of various metal ions (Au3+, Pd2+, Pt4+, Cu2+, Ni2+, and Zn2+). All the metal ions tested were at 10 ppm each. Figure 1 shows competitive adsorption of metal ions using 0.2 g/L proteins. Interestingly, all the proteins tested selectively adsorbed Pd2+ and Au3+ to varying degrees, while little adsorption of Pt4+, Cu2+, Ni2+, and Zn2+ was observed. Although many papers have revealed interactions between amino acid residues (e.g., His) and various transition metal ions (e.g., Cu2+ and Zn2+), there is, so far, no report on the selective adsorption of precious metal ions to proteins. We believe that the present study describes for the first time that Pd2+ and Au3+ were selectively adsorbed to proteins. It should be noted that proteins selectively adsorbed Pd2+ and Au3+ in an acidic aqueous solution containing 0.1 M HCl, because in many cases, metal ion wastes are in acidic conditions. Adsorption Isotherms of Precious Metal Ions to Proteins. The adsorption isotherms of precious metal ions (Au3+ and Pd2+, 10 ppm each) to the purified proteins [lysozyme, bovine serum albumin (BSA) and ovalbumin] were examined. Figure 2 shows the adsorption isotherms for Au3+ and Pd2+ to the proteins at 25 °C. Although the adsorption amounts of Au3+ and Pd2+ per protein decreased with protein concentration in Figure 2, the total adsorbed amounts increased with increasing the protein concentrations. All of Au3+ and Pd2+ were finally adsorbed at a protein concentration of 0.6 g/L. The maximal adsorption capacity of the proteins for the precious metal ions was calculated at the presence of excess metal ions (less than 50% adsorption rate, Table 1), based on the concentrations of unadsorbed metal ions determined by atomic absorbance spectroscopy, when the protein concentrations were set at 0.05 g/L, except Au-binding to lysozyme (0.1 g/L), and the concentrations of metal ions

FIGURE 2. Isotherms for Au3+ and Pd2+ adsorption to typical purified proteins. Adsorption experiments were carried out using 10 mL aqueous solution containing 10 ppm metal ions at 25 °C and pH 4 for 1 h.

TABLE 1. Adsorbed Precious Metal Ions/Protein Moleculea Au3+ adsorbent

M.W.

pI

Pd2+

mg/ mol/ mg/ mol/ g-protein mol-protein g-protein mol-protein

ovalbumin 44 287 4.6 36 ( 5 BSA 66 267 4.6 40 ( 2 lysozyme 14 307 10.7 19 ( 2

15 ( 2 91 ( 7 25 ( 1 104 ( 3 2.6 ( 0.2 104 ( 15

38 ( 3 65 ( 2 14 ( 2

a Adsorption conditions were 10 ppm metal ions and pH 4 at room temperature for 1 h. The adsorption capacities were calculated from the triplicate results where the protein concentrations were set at 0.05 g/L, except Au-binding to lysozyme (0.1 g/L) (meaning excess amounts of metal ions, See Figure 2).

were 10 ppm. All the proteins tested here showed high adsorption capacity for Au3+ and Pd2+. For example, BSA adsorbed 25 ( 1 Au ions and 65 ( 2 Pd ions per protein molecule, and lysozyme adsorbed 2.6 ( 0.2 Au ions and 14 ( 2 Pd ions per protein molecule. The adsorption capacity varied greatly with the type of protein. At a high adsorption rate (over 80%), the adsorbed metal ions per protein molecule decreased probably due to the adsorption equilibrium. In general, electrostatic adsorption of metal ions to adsorbents is influenced by pH because forms of metal ions and their ionic charges depend on pH. The effect of pH on the adsorption of Au3+ and Pd2+ to proteins was studied (Figure 3). Both Au3+ and Pd2+adsorption was affected by pH. Au3+ adsorption decreased above pH 6 but Pd2+ adsorption decreased below pH 3. There was little difference in the pH effect among the proteins. Taking account of the pI diversity of proteins tested (Table 1), the surface charge of the proteins was not so important for the adsorption of these precious metal ions. The present study employed [AuCl4]- and [PdCl4]2- as Au and Pd ions. The Cl ligand in these complexes can be replaced by OH- (16). The stability of these tetrachloro complexes is influenced by pH and Clconcentration. The pH effects on adsorption were probably attributed to the stability of the metal complexes. Ionic strength of solution is also a significant factor for electrostatic adsorption. Since addition of NaCl influences the stability of [AuCl4]- due to Cl-, we used sodium

FIGURE 3. Effect of pH on Au3+ and Pd2+ adsorption to typical purified proteins. The adsorption was carried out using 1.0 g/L protein and 10 ppm metal ion solution at 25 °C for 1 h. perchlorate for the investigation of ionic strength. Varying the concentration of sodium perchlorate from 0 to 1 M to control the ionic strength of the solution, we did not observe any notable difference in the Au3+ and Pd2+ adsorption to the proteins (See Supporting Information, Figure S1). The effects of pH and ionic strength suggest that there was a major mechanism for the adsorption of these precious metal ions to proteins other than electrostatic interaction. Investigations on the Precious Metal Ion-Protein Adsorption Mechanism. Au3+ and Pd2+ are likely to be reduced to produce nanoparticles. It is possible that the interaction between the proteins and precious metal ions produced the nanoparticles. A gold or a palladium nanoparticle solution generally has a visible color depending on the nanoparticle size (22, 23). In the present study, we did not observe any change in color of the solution when mixing precious metal ions and proteins, indicating that there was no production of nanoparticles. Another possibility to be clarified is the production of fine particles that were larger than nanoparticles and were precipitated. We tried desorption of the precious metal ions from proteins using thiourea. Thiourea reduces Au3+ to Au+ and forms a stable 2:1 complex in aqueous solution (24). Addition of excess thiourea (2 mM) to the Au3+-protein complex resulted in >95% desorption of Au ions from proteins. Au ions desorbed from proteins could be passed through an ultrafiltration membrane with a molecular cutoff of 10 kDa. This result means that Au3+ was not reduced to form fine particles or precipitates by its interaction with proteins. The reason for no production of nanoparticles in the present system is considered to be the absence of a reducing agent. Although there are many reports describing the production of nanoparticles using biomolecules or biological systems, most of them employed additional reducing agents or reducing conditions in a living system. Of the tested proteins, lysozyme adsorbed only 2.7 Au3+ ions per protein molecule. We tried to identify the Au3+- or Pd2+-binding site of the precious metal ions in lysozyme. Several reports have demonstrated a strong interaction between a histidine residue and Au3+ or Pd2+, and have succeeded in the crystallization of Au3+- and Pd2+-peptide complexes (25-27). Lysozyme has only one histidine residue (H15) in its amino acid sequence (28). To demonstrate that the peptide sequence containing a His residue in lysozyme VOL. 41, NO. 4, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 5. Competitive adsorption of metal ions to protein-rich biomass. Adsorption experiments were carried out using 10 mg biomass and 0.1 M HCl solution (5 mL) containing Au3+, Pd2+, Pt4+, Cu2+, Ni2+, and Zn2+ (10 ppm each) at room temperature for 1 h. In the case of soybean protein, after adsorption of metal ions, the solution pH was adjusted to 4 in order to precipitate soybean protein. FIGURE 4. (a) Adsorption of precious metal ions to N-acetyl tripeptides immobilized on polymer resin. Adsorption experiments were carried out using 10 ppm metal ions (5 mL), tripeptide (0.7 µmol) in the presence of 0.5 M NaClO4 at pH 4 for 1 h. (b) Competitive adsorption of metal ions to tripeptide (N-acetyl His-Gly-Leu). Metal ion solution contained six different metal ions at 10 ppm each. The tripeptide (0.7 µmol/5 mL) was used for adsorption in the presence of 0.5 M NaClO4 at pH 4 for 1 h. facilitates the adsorption of precious metal ions, we synthesized a lysozyme-derived tripeptide (N-acetyl-His-GlyLeu-resin) as a model peptide and various analogous tripeptides (N-acetyl-X-Gly-Leu-resin) on the solid phase. As a result, only His-containing peptide exhibited remarkable binding affinity for Au3+ and Pd2+, while the other tripeptides showed very low binding affinity for these metal ions (Figure 4a). Surprisingly, Cys-containing peptide did adsorb little amount of these precious metal ions. It should be noted that prior to the adsorption experiment the Cys-containing peptide used here was reduced by dithiothreitol to have a free thiol group. Zou et al. also reported no binding of Au+ to the thiol group of Cys in a protein (17). And without reduction by dithiothreitol we did not also observe any adsorption of these precious metal ions. So the His-containing sequence is the first candidate for the Au3+- and Pd2+-binding site of in lysozyme. Using the N-acetyl-His-Gly-Leu peptide, competitive adsorption of metal ions was investigated in the presence of various metal ions (Au3+, Pd2+, Pt4+, Cu2+, Ni2+, and Zn2+). We observed selective binding of Au3+ and Pd2+ to the tripeptide (Figure 4b). Most of the Au3+ and Pd2+ bound to the tripeptide were readily desorbed from the tripeptide by the addition of an excess amount of thiourea (data not shown). These results with the tripeptides agree well with those of the proteins, strongly suggesting that the Hiscontaining peptide was one of the binding sites of Au3+ and Pd2+ in proteins. Gardea-Torresdey’s and Volesky’s groups also investigated the mechanisms of metal-ion binding to biomass and suggested the importance of nitrogen atoms in the biomass (5, 7, 9). In our present system, a nitrogen atom in His is also thought to play a key role for the binding. 1362

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However, lysozyme, BSA and ovalbumin have one, 17 and seven His residues, respectively. The number of His residues does not account for the adsorbed precious metal ions listed in Table 1. Moreover, more Pd2+ than Au3+ ions were bound to proteins. There may be another adsorption mechanisms for Pd2+ adsorption different from that of Au3+. Further investigations on other binding sites are required. Protein-Rich Biomass as Adsorbents Selective for Precious Metal Ions. From a practical viewpoint, an adsorbent for metal ions should be inexpensive. We adopted proteinrich biomass as an adsorbent instead of the purified proteins tested in the above investigations. Since protein-rich biomass has many attractive features, e.g., its abundance, low cost, and zero-emission of CO2, the efficient use of biomass in various industries is desired throughout the world. Here, we employed soybean protein, chicken egg-shell membrane, and seasoning pollack-roe membrane as protein-rich biomass, which are produced in huge amounts as byproducts in the Japanese food industry. The protein contents of soybean protein, chicken egg-shell membrane and seasoning pollack-roe membrane are approximately 91, 90, and 64%, respectively. Figure 5 shows the competitive adsorption of metal ions to the protein-rich biomass. Under the present experimental conditions, all the biomass tested adsorbed almost 100% of Au3+ and Pd2+ (5 mg/g-biomass), but did not adsorb Cu2+ or Ni2+ to such an extent. In the case of Pt4+ adsorption, there were differences among the types of biomass. Egg-shell membrane effectively adsorbed Pt4+, while soybean protein adsorbed only a small amount of Pt4+. Pt4+ adsorption to the egg-shell membrane was probably due to electrostatic interaction, because varying the ionic strength using NaClO4 reduced adsorption (Supporting Information Figure S2). The adsorption selectivity of protein-rich biomass observed in Figure 5 was very similar to that of purified proteins (Figure 1). These results allow us to conclude that protein-rich biomass, as well as purified proteins, works as an adsorbent selective for precious metal ions. Recycling of Precious Metal Ions in Metal Refining Solution and Industrial Waste Using Protein-Rich Biomass.

TABLE 2. Recycling of Precious Metal Ions from Copper Refining Solution and Metal Plating Wastes Using Chicken Egg-shell Membrane targeted metal ion

initial concentration (g/L)

concentration after adsorption (g/L)

adsorption (%)

recovery in aqua regia (%)b

yieldc (g metal/kg biomass)

sample solution

pH

Cu refining solutiond

-0.3

Au Pd Pt Cu Pb

82 3.77 21 0.81 0.023

47 3.4 20 0.80 0.02

43 10 5 1 13

10 17 0.3 n.d.a n.d.a

165 11 1 n.d.a n.d.a

Au plating wastee

6.2

Au

5.3

1.5

70

34

30

10.3

Pd

100

6

94

>99

2030

0.7

Pt

5.7

0.86

85

>99

110

Pd plating

wastef

Pt plating wasteg a

n.d.: Not determined. b Recovery means the percentage of metal ion recovered in the aqua regia compared to the initial metal ion concentration in the sample solution. c Yield represents the total amount of the targeted metal adsorbed to the biomass, based on final recovery in aqua regia. d Cu refining solution contained 2 M HCl other than above metal ions. e Gold cyanide solution containing Ni2+ and Cu2+ f Palladium was dissolved as [Pd(NH3)4]Cl2. g Pt was dissolved in sulfuric acid.

The above investigations describe selective adsorption of Au3+ and Pd2+ from a metal ion solution prepared in a laboratory, but not from a practical industrial sample solution. Finally, we investigated the recovery of Au3+ and Pd2+ from practical solutions containing metal ions produced in an industrial setting. There were fairly large amounts of Au3+ and Pd2+ involved in a copper refining solution and metal plating wastes. Adsorption experiments were carried out in a similar manner to the above investigations. As an adsorbent, egg-shell membrane was used because of its high performance for selective adsorption and its insolubility in an aqueous solution at a variety of pH values. After adsorption, the biomass adsorbing metal ions were charred, and the bound metal ions were redissolved in aqua regia for recovery. Table 2 summarizes the results of the adsorption and recovery of precious metal ions from practical sample solutions using egg-shell membrane. Since the copper refining solution was produced after copper refinement, other metal ions, except Cu2+, existed at relatively high concentrations. Egg-shell membrane adsorbed Au3+ in the copper refining solution, while the biomass hardly adsorbed Pt and Cu ions. This sample solution contained many metal ions and HCl (2 M). Therefore, the high ionic strength would have prevented the adsorption of Pt ions. The adsorption ratio of Pd ions was relatively low, which can be explained by its low initial concentration and by the insufficient amount of biomass compared to the metal ions. Indeed, the adsorption ratio increased as the amount of biomass increased (data not shown). Based on the redissolved Au ions by aqua regia, 165 g Au per kg biomass was finally recovered. This recycling procedure would be suitable for potential applications, because of its simplicity and of no other specific materials needed. The adsorption study on metal plating wastes also succeeded in the recovery of precious metal ions. Au, Pd, and Pt ions were recovered from plating wastes using biomass. Interestingly, most of the Pd ions were recycled from the [Pd(NH3)4]2+ solution, which is an inert complex. This means that protein-rich biomass adsorbs Pd ions whether Pd ions are in a cationic or anionic complex. The present study demonstrates the selective adsorption of Au3+ and Pd2+ to various proteins in the presence of transition metal ions. Adsorption involves more than one mechanism but one of these is due to the complex between the precious metal ions and a His-containing peptide. Furthermore, we succeeded in recycling Au and Pd ions from an actual copper refining solution and metal plating wastes using protein-rich biomass, which will contribute to an environmentally friendly recycling system for precious metal ions.

Acknowledgments This study was financially supported by Kieikai Research Foundation. We thank Q.P. Corporation, Fuji Oil, and Yamaya Communications for providing protein-rich biomass. We thank Dr. M. Waki for his valuable advice on peptide synthesis. We thank Muromachi Technos for providing metal plating waste containing precious metal ions.

Supporting Information Available Effect of ionic strength on the Au3+ and Pd2+ adsorption to Lysozyme (Figure S1) and effect of ionic strength on the Pt2+ adsorption to egg-shell membrane (Figure S2). This material is available free of charge via the Internet at http:// pubs.acs.org.

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Received for review July 13, 2006. Revised manuscript received November 29, 2006. Accepted December 4, 2006. ES061664X