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Recovery of gold with Japanese cedar wood powder.

Gold is primarily obtained from its rich ores in nature. However, in most technologically advanced countries the so-called “urban mines,” formed by piles of obsolete electric and electronic devices, are attracting much attention as a rich source of gold and other precious metals. Although urban mines contain high concentrations of gold in comparison to natural ores, selective separation is a challenging task. Existing hydrometallurgical methods of gold recovery are solvent extraction and ion exchange. Despite being widely used on the industrial scale, these methods have serious problems, such as poor selectivity and environmental concerns that are associated with the use of organic solvents in excess and the end-of-use treatment of ion-exchange resins. To overcome these problems, biosorption is considered the best option. However, most biosorption materials studied were found to be highly effective in removing toxic heavy metals from weakly acidic to slightly basic wastewaters, with selectivity seldom being an important factor. Also, owing to selectivity and stability concerns, the use of biomasses—in their natural form—for recovering gold from multimetal mixtures in strongly acidic conditions is difficult. For this reason, some sort of chemical modification of the biomass is performed to achieve stability in strongly acidic media. Even being praised for their lessor non-hazardous origins, biosorbents have yet to find extensive application on the industrial scale. Expecting the use of available biomass for the recovery of gold from acidic waters, the sorption behavior of Japanese cedar (Cryptomeria japonica) wood powder (CWP) under a range of experimental conditions was studied, without performing any chemical treatment. CWP, used for the present study, was collected from a local saw mill. It was ground to various particle sizes (<0.1 to <0.8 mm) and tested for sorption of gold and some other metal ions in 0.1 to 6m HCl media. The influence of the temperature on the gold sorption capacity of CWP and the effect of the presence of other metal ions was also studied. All of the sorption experiments were performed at least twice so as to assure the consistency of the results. Concentrations of metals were measured by atomic absorption spectrometry (AAS; Shimadzu AA-6650). Initially the gold sorption ability of CWP (original particle size range ca. 2 mm) was studied by mixing with 1 mm Au solution in 1m HCl. No sorption was observed, even after 24 h of mixing at 30 8C. Then, CWP was ground to different sizes, ranging from 0.1 to 0.8 mm, and the Au sorption test was repeated. Sorption of Au was observed, as shown in Figure 1. At 4 h shaking time only about 20% sorption was observed regardless of the particle size. By extending the contact time to 12 h,