Selective Recovery of Gold by Novel Lignin-Based Adsorption Gels Durga Parajuli, † Chaitanya R. Adhikari, † Masayuki Kuriyama, † Hidetaka Kawakita, † Keisuke Ohto, † Katsutoshi Inoue,* ,† and Masamitsu Funaoka ‡ Department of Applied Chemistry, Saga UniVersity, 1-Honjo, Saga 840-8502, Japan, and School of Bioresources, Mie UniVersity, 1515 Kamihama, Tsu, Mie 514-8507, Japan Three different kinds of adsorption gels, viz., cross-linked lignophenol, cross-linked lignocatechol, and cross- linked lignopyrogallol, were prepared by the chemical modification of wood lignin. The adsorption behaviors of these gels for Au(III) along with some other metals were studied and compared to that of activated carbon. All three gels were found to be more selective for Au(III) than activated carbon with comparable adsorption capacities. Of the lignin gels, cross-linked lignophenol exhibited the highest selectivity for Au(III) and was found to be almost inert toward other metals tested. All three novel lignin gels as well as activated carbon were found to be efficient in reducing Au(III) to elemental gold, as indicated by XRD analysis of the sorbents taken after adsorption. However, a significant difference between the novel sorbents and activated carbon was found, i.e., the latter exhibited no selectivity among the metal ions tested, whereas the novel gels have a high selectivity to only Au(III). In addition, gold aggregates were visually observed in the case of the lignin gels and not in the case of activated carbon. This result provides a new approach for effective gold recovery. Introduction Millions of tons of spent electrical and electronic devices are discarded every year. More than half of these wastes consist of metals including a significant proportion of valuable metals or their compounds, which indicates not only the loss of huge amounts of resources but also the threat of environmental pollution. The high pace of technological change and competi- tive market strategies that encourage people to buy the latest models before their old appliances stop functioning have caused an alarming increase in electronic and electrical wastes. Along with other useful valuable metals, gold, which is mainly used in making gold-coated edge contacts on printed circuit boards, is also being wasted. In a rough estimate, the percentage composition of different metals by weight in a mobile phone, for example, is as follows: copper, 15%; iron, 3%; zinc, 1%; and less than 1% of a number of metals such as tin, palladium, and gold. 1 Although the portion of gold is very low compared to the other metals in one piece of a device, the amount of gold disposed in this form is much higher than the content in gold ore itself. 2 For a sustainable society and strong economy, it becomes necessary to recycle and reuse such precious metal resources in order not to waste them. The history of extraction of gold and its use is as old as human civilization. With technological advances, many methods of gold recovery have been formulated. 3 The most common processes at present are chloride leaching and cyanide leaching. Because of the toxicity associated with cyanide and its ineffectiveness in refractory ores and concentrates, cyanide leaching is not as common as chloride leaching. Some of the widely used chloride leaching methods are chlorination, electrolytic refining, and wet chemical processing. Of these, the most extensively employed method of gold extraction is wet chemical refining. Raw gold containing a number of other metals is first dissolved in hydrochloric acid in the presence of some oxidizing agent such as chlorine or nitric acid. The silver is separated as silver chloride precipitate, and the supernatant liquid is treated by means of a variety of chemical processes. Gold is separated by solvent extraction or ion exchange. 4,5 Dibutyl carbitol is commonly employed as a solvent extraction reagent. 6 Although this solvent has a high selectivity for gold, it is not completely satisfactory because of its water solubility, which is associated with some problems such as solvent loss and wastewater treatment. As mentioned above, either for safety or for effective extraction, hydrochloric acid is used along with other accessory chemicals. In this context, a more cost-effective and environ- mentally benign technique for selective gold recovery would be highly preferred. For the purpose of developing an environmentally benign and cost-effective process for the selective recovery of gold from a mixture of many metals, in a previous work, 7 we found a lignin- based adsorption gel, gel of lignophenol, that is efficient at recovering Au(III) as elemental gold. Our present work is focused on the development of various types of lignin gels, the structures of which are shown in Figure 1, their adsorption behavior, and a comparison of their efficiency and selectivity for Au(III) with wood-derived activated carbon. Because the structure of lignin is complex and irregular and consists of heterogeneous repeating units, the structures given in Figure 1 are only tentative. Experimental Section Reagents. Analytical-grade chloride salts of copper, iron, palladium, tin, and zinc were used to prepare the test solutions of the respective metals. Analytical-grade HAuCl 4 ‚4H 2 O and H 2 PtCl 6 ‚6H 2 O were used to prepare test solutions of gold and platinum, respectively. Preparation of Adsorption Gels. Lignophenol, lignocat- echol, and lignopyrogallol were prepared by immobilizing phenol, catechol, and pyrogallol, respectively, onto wood lignin. All of these lignin derivatives were prepared by a phase separation method. 8 As shown in their structures in Figure 1, phenol, catechol, and pyrogallol are bonded to the R-carbon of the aromatic nucleus. To avoid dissolution in aqueous solution, these lignin compounds were cross-linked with paraformalde- * To whom correspondence should be addressed. E-mail: inoue@elechem.chem.saga-u.ac.jp. Tel.: +81-952-28-8671. Fax: +81- 952-28-8591. † Saga University. ‡ Mie University. 8 Ind. Eng. Chem. Res. 2006, 45, 8-14 10.1021/ie050532u CCC: $33.50 © 2006 American Chemical Society Published on Web 12/01/2005