Bioleaching an emerging field of biotechnology- “A remedy for e-waste surge”

Metals are an indispensable part of our lives. The fact can not be ignored that a complete human civilization has named on the metal discovered, employed, and ultimately consequent 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 into the 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’ employs the ability of acidophilic chemolithotrophic microorganisms to catalyze the oxidation reaction of mineral sulfides for extracting out metals in an aqueous medium (organic, alkaline, acidic).

The process has been established at an 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 (fulfills 20% of the world’s Copper demand).

The major group microorganisms used for bioleaching are acidophilic 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 the mineral surface to solubilize the metal into an acidic medium. Along with the depletion of primary metal sources, 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 millions of tonnes of waste among which China’s place is first while India is the 5th largest country in e-waste production.

The world is now looking for metal recovery from these urban mines by using a greener, economical and safer approach ‘bioleaching’ for e-waste treatment. Although the process is well established for the primary resources, major challenges 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 acidophilic 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.