Rare metal recovery using metal biotechnology
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APEC-VC Article “Biometal”

Rare metal recovery using metal biotechnology

Ike Michihiko
Division of Sustainable Energy and Environmental Engineering
Graduate School of Engineering Osaka University

 The use of rare metals, including rare earth, which is the general name for metals such as indium, molybdenum and selenium, is sharply increasing in leading-edge IT, space and nuclear industries. It is called the vitamin of industries. Rare metals are extremely limited in recoverable reserves at mines. Furthermore, since current mining and smelting are difficult from technical and economical standpoints, there are some concerns about depletion. Country-level measures to ensure and store the rare metals have been implemented. As part of this, technologies to use relatively abundant metals as alternatives to scarce elements are moving forward. In addition, the development of technologies to recycle metals from waste products as typified by the recovery of noble metals from mobile phone circuit boards is also being advanced. However, although there are large amounts of rare metals contained in sewage and non-separated waste, which is the bottom of the resource flow, the development of technologies to recover and recycle them has not yet moved forward. In the sense of industry sustainability it is predicted that the depletion of rare metals will be a serious problem in the future.
 Physical and chemical methods have been applied to the recovery and recycling of rare metals. However, for sewage and waste containing target materials mixed into a complicated physical matrix at low concentrations and low content, (which cannot be categorized as anything else), a practical technology has not yet been established because of the problems of cost and energy consumption. Consequently, a new resource recovery technology at a low environmental load, which can minimize energy and the use of resources with sufficient economic potential, is expected to arrive.
 Taking advantage of not only the resource and energy saving of biotechnology but also its high economy and environmental suitability, a “metal biotechnology” is suddenly drawing attention as a key technology to establish the sustainable relationship between metals and humans that the physical and chemical methods can not do (1).
 Generally, it is well known that biological action itself is involved in the composition and decomposition of microorganisms. The biological action, however, catalyzes a variety of reactions to metals, which are inorganic substances. For instance, some microorganisms change their physical and chemical properties through oxidation-reduction of metals as an electron donor or acceptor for energy transduction. Additionally, some microorganisms effectively ingest and enrich metals at a low concentration as nutritive components, while others control ingestion of toxic metals or actively emit them. Metal biotechnology can apply conversion of the chemical form of metals by these biological actions, the changes between three phases (solid-liquid-gas), and the increase and decrease in solubility and adsorption to a recovery and recycling of metals from sewage and waste and to the related technologies. Shown in the Table are major samples of the actions of various microorganisms that can be used for metal biotechnology.
 The recovery of metals with metal biotechnology can be said to be a future technology except for one practical application currently being used as an economical mining technology using iron-oxidizing and sulfur-oxidizing bacteria in South America. The following various technologies aiming at practical application have been studied and developed: a technology to reduce, solidify and recover soluble selenium at a low concentration in the smelting plant discharge through biomineralization; a technology to convert soluble palladium into high-value nanoparticle as material resources and to recover it; a technology to adsorb molybdenum onto recombinant yeast cells with a metal-binding protein surface display and to concentrate it; a technology to recover gold from printed circuit boards, taking advantage of cyanide-producing bacteria (1).
 If the study and development of high-demanded metal biotechnology in rare metal recovery advances, we can expect its application to the environment-adaptable industrial manufacturing technology (White biotechnology) by using scientific findings and new biotic catalysts and biomaterials obtained in this process. Smelting and processing of metals is generally a high environmental-load process because of its use of strong acids or high-temperature. However, if this process can be alternated with an environmentally friendly bio-process and achieve greening, this will contribute greatly to the creation of sustainable industry. Metal biotechnology is expected to be one of the technology architectures that create an enriched recycling-oriented society with safety and security because it can be utilized in industrial fields as well as in agricultural fields. (e.g. It can be used in the bleeding of crops which contain a good balance of trace metals at a healthy level.)

References

(1) Supervisor: Ueda, Ike “Metal-biotechnology for Environmental Conservation and Resource Recycling – The key Technology for Safe and Sustainable Use of Metal Elements, CMC Publishing (2009)

Table Microbial processes that are expected to be applied to metal biotechnology
Bioleaching Action of metal extraction in the solid phase into the liquid phase
(e.g.) mineral dissolution through sulfuric acid produced by sulfur-oxidizing bacteria; mineral dissolution through organic acid produced by sugar fermentation; dissolution of iron compounds caused by ferric iron reduction and etc. 
Biomineralization Solidification and recovery of metals in the liquid phase by mineralization
(e.g.) solidification by reduction of metal-oxide ions (Se, Te etc.) to elemental state; formation of oxide mats by iron oxidation and manganese oxidation and adsorption and insolubilization of other metals and etc.
Biovolatilization Removal and recovery of metals in the liquid and solid phases by gasification
(e.g.) reduction-volatilization of Hg ions by Hg-reductase to elemental Hg; volatilization of methylated Se and As and etc.
Biosorption Removal and recovery of metals by adsorption onto cell surface and secretions
(e.g.) adsorption onto the cell surfaces of bacteria, yeasts, fungi or alga; adsorption onto extracellular polysaccharide (biopolymers) and etc.

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