ABSTRACT
Discarded, end-of-life, broken as well as obsolete electrical and electronic devices are referred to as electronic wastes (e-Wastes). The emerging problem with developed as well as developing nations is an obsession on electrical and electronic equipment; the unquenchable desire for latest devices and rapid advances in technology has resulted in the generation of huge amount of e-Waste globally. All the e-Wastes containprinted circuit boards (PCBs) as the main processing unit of the electronic devices. Ceramics, metals, and polymers are the main components of all the PCBs. PCBs are considered as a secondary source of several metals. However, the lion share of e-Waste is dumped as landfills or incinerated, which leads to environmental pollutions and harmful effects on human beings. In most of the countries, a larger portion of the e-Wastes arenot processed or recycled scientifically. To date, no standard eco-friendly technique is available for their safe disposal. In the past 20 years, attempts have been made for the recovery of metals from the e-Waste by using pyrometallurgical and hydrometallurgical technologies, but these processes also have their own limitations as they are cost-intensive and generate secondary pollutions. To solve some of the problems of pyro- and hydrometallurgical processes, "biohydrometallurgical" techniques are employed for the extraction and recovery of metals from the PCBs. Biohydrotechnology for e-Waste processing utilizes lixiviants generated by the potential of microorganisms, which solubilize metals from the PCBs at ambient conditions under much milder concentrations of the biogenerated lixiviants. Iron- and sulfur-oxidizing bacteria generate ferric sulfate and sulfuric acid, which leach out several metals in an aqueous phase. Cyanogenic microorganisms produce hydrocyanide, which is effectively used for extraction of gold and silver from the PCBs. Some fungi are also reported for metal solubilization from PCBs by generating organic acids. Microbially mediated technology is considered a novel, eco-friendly, and sustainable approach to solubilize metals from various types of PCBs. The main advantages of microbial technology are that it can work with dilute waste streams and work at ambient temperatures and pressures, it is cost-effective, it reduces environmental impacts, and it generates minimum secondary waste. Thus, this chapter explores and furnishes the understanding of the process and mechanisms of biohydrotechnology of extractions of the base and precious metals from the e-Waste PCBs
