Microbial leaching of heavy metals from e-waste: opportunities and challenges

In the present scenario, environmental pollution due to toxic and recalcitrant materials remains a global threat. In 2012, the world's cities produced 1.3 billion tons of waste, and the World Bank estimates wastes to be 2.2 billion by 2025. The decreasing costs and increasing availability of electronic equipment like mobile phones, television sets, computers, and their accessories results in easy purchase by the public. This leads to changing outdated equipment or their spare units at a rapid rate, and it results in the emergence of a new stream of waste known as electronic waste (e-waste). E-wastes are considered dangerous and toxic to the environment as well as living organisms, depending on their condition and density. Apart from this, precious metals like gold, silver, copper, selenium, and mercury present in e-wastes can be reused, refurbished, or recycled. Recovering precious metals from e-waste can be made cost effective and economically viable. According to the Environmental Protection Agency, “Experts estimate that recycling one million cell phones can recover about 24kg (50 lb) of gold, 250kg (550 lb) of silver, 9kg (20 lb) of palladium, and more than 9000kg (20,000 lb) of copper.” After collection, e-waste is dismantled, and individual components are tested and isolated. Hazardous materials are separated and are disposed of safely without creating any harm to the environment. Then precious metals like gold, copper, silver, palladium, etc., are extracted from e-waste and can be reused. Even though there are different methods available for the extraction of heavy metals from e-waste, it warrants researchers to show more interest in bioleaching/biometallurgic approaches. Moreover, the purity of base and precious metals in e-wastes is about 10 times higher than the ores. Hence, if potential e-waste recycling methods are developed and implemented, it would change the problem of pollution into a profitable metal resource. In comparison to the traditional metallurgical methods, bioleaching is less energy intensive and is environmentally friendly. The process of bioleaching utilizes the potential of different microorganisms to extract heavy metals from any source, including e-wastes, in an eco-friendly manner. There are several strains of bacteria, fungi, and actinobacteria from both terrestrial and aquatic ecosystems. Several extremophiles have been reported to remove/leaching of heavy metals from e-wastes. In this chapter, we are going to describe the current status of the impacts of e-wastes on the environment, various strategies for heavy metal removal from e-wastes, significance of bioleaching approach, and its present challenges and future perspectives.