Abstract
Biological technologies for recycling rare metals, which are essential for high-tech products, have attracted much attention because they could prove to be more environmentally friendly and energy-saving than other methods. We have developed biological recycling technologies by cell surface engineering for the selective recovery of toxic heavy metal ions and rare metal ions from aqueous wastes. In this study, we aimed to construct a unique biological technique to recover rare metals ‘in solid’ form by reducing rare metal ions, leading to a practical next-generation recovery system. Sulfate-reducing bacteria (SRB) can reduce Pt(II) to Pt(0), and hydrogenases of SRB contribute to the reduction. Therefore, we constructed yeasts displaying their hydrogenases on the ‘cell membrane’, and reduction experiments were performed under anaerobic conditions without any electron donors. As a result, hydrogenase-displaying yeasts produced black precipitates in PtCl4 2− solution. Based on X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) observations, the constructed yeasts were found to successfully produce the precipitates of Pt(0) through the reduction of Pt(II). Interestingly, the precipitates of Pt(0) were formed as nanoparticles, suitable for industrial usage.
Highlights
Rare metals are essential for high-tech products such as cell phones and automobiles, leading to ever-growing demands today
To confirm the cell membrane display of the hydrogenases, immunofluorescent labeling of the transformed yeasts and wild-type yeast was performed with or without cell wall digestion, which improves the accessibility of the antibody to the membrane-displayed proteins (Fig. 2)
According to the result of X-ray photoelectron spectroscopy (XPS) analysis (Fig. 4), the black precipitates produced by hydrogenase-displaying yeasts included Pt(0) with binding energies of 70.7– 71.3 eV, suggesting that the hydrogenases displayed on yeast cell membrane contributed to the Pt(II) reduction to Pt(0)
Summary
Rare metals are essential for high-tech products such as cell phones and automobiles, leading to ever-growing demands today. A stable supply of rare metals is difficult on a global scale because rare metals are expensive and localized, and their abundances are extremely low. Technologies for recovering rare metals from waste products, industrial wastewater, and sea are required. Various physical, chemical, and biological methods have been studied to prepare metallic forms for the recycling of rare metals (Du et al 2008; Fredrickson et al 2008; Cueto et al 2011). Physical and chemical methods have some disadvantages. They involve toxic solvents and consume high energy.
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