The continual and unprecedented consumer demand for the electrical and electronic equipments (EEEs) coupled with their accelerated product obsolescence rate due to the rapid advances in technology has led to increasing generation of waste electrical and electronic equipment (WEEE) worldwide. The WEEE, commonly known as electronic waste (e-waste), generation is so massive that it constitutes about 8% of municipal solid waste (MSW) and is the fastest growing waste stream in the world. Further, the e-waste problem in emerging economies like India is compounded by the trans-boundary export of e-waste in considerable quantities by some developed and industrialized countries despite of the Basel Convention prohibition 1992. It has been reported that worldwide generation of e-waste was around 44.7 Mt in the year 2016 and is predicted to increase at growth rate of 4-5% per annum. A large portion of e-waste remains unaccounted and are deprived of treatment through appropriate recycling chains and methods. They are often found to managed by the informal sector using rudimentary approaches which not only cause environmental pollution but also lead to serious health hazards to the workforce employed. Out of total e-waste generated, only a small share of e-waste is collected formally by the take-back system while the rest is disposed off to waste bins which finally reach to incinerators, landfills and open dump-yards. Leachates from landfills potentially transport toxic substances into soil and water whilst combustion in incinerators can introduce toxic gases into the atmosphere. Further, employed workers along with the residents living around the landfills or incineration plants are directly subjected to dust, toxic gaseous emissions and exposure to leachates Printed circuit boards (PCBs) make the foundation of EEEs. The dramatic increase in the amount of waste PCBs with the increase in e-waste generation is indispensable. PCBs have complex and heterogeneous matrix with significantly high content of metals including base metals, toxic metals as well as precious metals in quantities even higher than their natural deposits. This high metallic content contributes perpetually and comprehensively to the economic vitality of recycling of metals from PCBs. In addition to recovery of metals for economic development, recycling of metals from waste PCBs is also an important aspect of waste treatment to avert environmental pollution. Conventional approaches for recovery of metals from e-waste, viz. pyrometallurgical and hydrometallurgical techniques are rapid and efficient, but cause secondary pollution and are economically unviable. Limitations of the conventional techniques have led to a shift towards biometallurgical technique involving microbiological leaching or bioleaching of metals from e-waste in eco-friendly manner. In comparison to conventional technologies bioleaching based processes offer a number of advantages such as low operation costs, environmental compatibility, relatively low energy consumption and easy technology for metals extraction. However, biometallurgical technique is the most time consuming, expertise intensive and slow process as compared to the other two metallurgical techniques. Taking into account the serious problem of e-waste, the present project aims at the development of an eco-friendly technique for faster and efficient extraction of metals from e-waste without any environmental and health hazards in line with the Government's Clean India Mission and Make in India initiatives for the country's economic development. For hydrometallurgical extraction safer, non-toxic chemicals like natural lemon juice containing citric acid as well as commercial grade citric acid and synthetic chelators EDTA have replaced traditional hazardous chemical reagents. Further, a hybrid bioleaching process comprising of hydrometallurgy and biometallurgy involving combination of safer, non-toxic ligand in the form of natural lemon juice containing citric acid as an active natural chelator and bioleaching microbes is attempted so that both will complement each other for efficient and improved metals extraction from waste PCBs.
The extensive characterization of waste printed circuit boards (PCBs) in terms of its metal resources and toxicity characterization leading to the development of a novel and efficient hybrid bioleaching technique for improved metals recovery from electronic waste (e-waste) has been attempted in the study. Owing to faster extraction of metals by chemical leaching, safer, green and natural chelator in form of lemon juice containing citric acid as an active constituent have been combined with the bioleaching process to enhance the metal extractability of the process. This work is an initiative taken up to contribute to Make in India mission for development of eco-friendly, cost effective hybrid bioleaching technique for metal recovery from e-waste, further, the work will also help in management of e-waste aiding in Clean India Mission. The study was conducted in three parts. The first part dealt with comprehensive characterization including chemical characterization in terms of content of 33 general, precious and rare earth metals (REMs) along with toxicity assessment of PCBs in terms of metals leachability due to indiscriminate dumping of end-of-life electrical and electronic equipment (EEE). This is obligatory for the prioritization of the target metals to be recovered for sustainable resource circulation and conservation. The second part of the study is focused on evaluating the recovery of metals by chemical leaching using safer chelators such as commercial citric acid, lemon juice containing natural citric acid as an active chelator and synthetic ethylenediaminetetraacetic acid (EDTA) along with conventional and hybrid bioleaching. The conventional bioleaching process exploiting the pure and mixed culture of bacteria belonging to two distinct classes of alphaproteobacteria and gammaproteobacteria viz., Acidiphilium acidophilum and Acidithiobacillus ferrooxidans have been assessed for recovery of metals from e-waste under variable pulp density. The pure and mixed bacterial cultures with the supplementation of 5 to 15% of lemon juice containing citric acid as an active constituent and natural tetradentate chelating agent were used to simulate hybrid bioleaching process. Further, the role of various parameters such as pH, redox potential (Eh), ferrous concentration and bacterial exopolymeric substances (EPS) on bioleaching of metals were also evaluated. The third part of the study dealt with assessment of the effect of acclimatization of bioleaching bacteria on the extraction of metals under both conventional and hybrid bioleaching approach followed by extraction of selected bioleached metals by fractional chemical precipitation. Comprehensive characterization of PCBs of end-of-life EEEs revealed presence of high metals content. Chemical leaching using natural lemon juice demonstrated comparable metal extractability to synthetic chelators. Under conventional approach, the highest dissolution of metals was achieved by the mixed culture of bioleaching bacteria. The pronounced effect of the presence of natural lemon juice in bioleaching medium with the mixed bacterial culture was observed to enhance metal solubilization under the hybrid bioleaching condition. Further, the acclimatization of bacterial cultures with PCB comminution fines markedly improved bioleaching of metals under both the conventional and hybrid conditions. At a pulp density of 7.5 g/L with 10% (v/v) lemon juice containing 0.2 M citric acid an improved metal extraction efficiency with 99% each of Cu and Zn, 83% Pb, 81% Ni, 53% Ag, 46% Au, 37% Sc, 33% each of La and Ce, 32% Nd was observed from comminuted PCBs using acclimatized mixed bacterial consortium under the hybrid condition at 18 days of bioleaching. Further, more than 99% of the selected base metals, Cu, Zn, Pb and Ni were recovered from the bioleached liquor by chemical precipitation. Thus, owing to the comparably improved metal recovery, hybrid bioleaching technique followed by chemical precipitation can be deliberated as a complete solution for metal recycling for resource conservation from e-waste.
Link: In order to meet the environmental norms for hazardous waste disposal and conservation of natural resources around the world, there is a growing concern for the safe recycling of e-waste. Management of e-waste stream is a challenging task considering its rapid generation worldwide coupled with complex composition. The hierarchy of e-waste management recommends the reuse and/or remanufactures the whole end-of-life EEE followed by recovery of materials by recycling techniques. Considering the depth of the problem, the present innovation is directed towards the development of a novel, safe, eco-friendly and efficient metallurgical technique for faster metal recovery from e-waste. The unique features of this innovation are:
(a) Majority of hydrometallurgical recovery of metals from e-waste use strong reagents, hazardous chemicals, toxic acids and base and thereby create secondary pollution. In the present innovation, safer, non-toxic chemicals like natural lemon juice containing citric acid as well as commercial grade citric acid and synthetic chelators EDTA have replaced traditional hazardous chemical reagents for faster and efficient hydrometallurgical recovery of metals from e-waste without any secondary pollution.
(b) There is no systematic study examining hybrid bioleaching comprising hydrometallurgy and biometallurgy together involving combination of safer chelators and bioleaching microbes with optimization of various process parameters. The present innovation led to development of an eco-friendly hybrid bioleaching process with optimized process-related parameters for enhanced and improved metals extraction from waste PCBs present in e-waste stream.
(c) As majority of the e-waste generated in the country is being managed in informal sector using a rudimentary approach, it is envisaged that development of this hybrid bioleaching technology will provide an alternative to the metallurgical sector with no environmental and health hazards. Development of an eco-friendly technique for faster and efficient extraction of metals from e-waste without any environmental and health hazards is in line with the Government's Clean India Mission and Make in India initiatives for the country's economic development.
Problem Scale: Worldwide
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