Metals are an indispensible part of our lives. The fact can not be ignored that a complete human civilization has named on the metal discovered, employed and ultimately cosequent the transition of ages. The metal-bearing mineral ores have been mined from the earth’s crust and being treated by various physical, thermal and chemical methods. The biological treatment of these primary resources came in picture in the 19th century when the high grade ores (ores with maximum metal values) were depleted due to the extensive mining and the conventional processes which need high energy and economic inputs became uneconomical for the low-grade ores.
The biological metal extraction or ‘bio-hydrometallurgy’ or ‘bioleaching’ employes the ability of acidophilic chemolithotrophic microorganisms to catalyze the oxidation reaction of mineral sulphides for extracting out metals in an aqueous medium (organic, alkaline, acidic). The process has been established at industrial scale as an applied and developed field of ‘Biotechnology’ for low-grade secondary copper sulfide (CuS) ores, pre-treatment of the refractory gold ores and chalcopyrite (fufills 20% of the world’s Copper demand). The major group microorganisms used for bioleaching are acidophillic chemolithotrophic microorganisms, the iron (Fe) and sulfur (S)-oxidizing microorganisms that utilize ferrous (Fe 2+ ) and sulfur (S) as their energy source (as an electron donor for ETS during aerobic respiration) and oxidize them into ferric (Fe 3+ ) and sulfuric acid (H2SO4 ) respectively. Both Fe 3+ and the acid are lixiviants in the bioleaching operations which attacks on the mineral surface to solubilize the metal into acidic medium. Along with the depletion of primary metal soruces there is a surge of metal containing wastes among which electronic waste is the most hazardous and rapidly growing municipal solid waste.
The waste contains a vast variety of metals including base metals (Cu, Ni, Zn, Fe), precious metals (Ag, Au, PGM) and rare earth metals and are termed as urban mines; the secondary source of metals. Many countries worldwide including China, Japan, the USA etc., are generating milions of tonnes of the waste among which China’s place is first while India is the 5 th largest country in e-waste production. The world is now looking for metal recovery from these urbon mines by using a more greener, economical and safer approach ‘bioleaching’ for e-waste treatment. Althogh the process is well established for the primary resources, major challages in the employment of the process for e-waste treatment at large scale is that the waste is alkaline in nature thus toxic for the acidophillic microorganisms, the metal content in the waste is hazardous for the growth of the microbial culture. Besides these challenges, the method has huge efficiency in sustainable development, waste recycling and regeneration of the lost revenue along with management of the waste.