Heavy Metal Removal from Wastewaters by Biosorption: Mechanisms and Modeling

Abstract

There is an unprecedented global demand for metals and other valuable resources due to surge in demand for technology, the increasing rate of technology development and the expansion of developing economies. At the same time, the growing global waste streams generated by this demand are becoming increasingly problematic. These waste streams include electronic waste, wastewaters from diverse industrial and mining activities, construction and demolition debris, metal- lurgical slags from smelting operations, industrial sludges, dusts and residues from metal extraction and refining, spent catalysts as well as many others (Fig. 1). Paradoxically, the source of problems can also be the locus of the solution. The source of waste is often the source of the expertise to recover the valuable recyclable materials from complex products that have reached their end of useful lives. Although several metal removal technologies based on physical, chemical and biological processes have been successfully implemented in full-scale opera- tion, metal recovery from wastes, which is beneficial for economic and environ- mental reasons, is still limited due to challenges arising from downstream processing. For instance, bioleaching of metals from their ores is a well-established technology with a number of full-scale applications. Conversely, bioleaching of electronic wastes to recover metals, which is a highly promising technology with low environmental impact and high cost-effectiveness, is a technology that is still in its infancy. This book presents sustainable technologies for heavy metal removal and recov- ery from mining and metallurgical wastes, construction and demolition wastes, spent catalysts and electronic wastes (Fig. 2). In Chap. 1, the focus is on life cycle assessment (LCA), which is a method that can be used to evaluate the suitability of various technologies. LCA is applied to a bioleaching process to recover metal from electronic waste, an increasingly signif- icant waste stream. Adsorption technology as described in Chap. 2 can also be used for the removal of metals from contaminated waste streams and other liquid and gas phase toxic pollutants. Awide range of adsorbents, including chitosan, fly ash, used rubber tyre, wood char, rice husk and alumina silicates, have been evaluated for the removal of heavy metals from water. These can be effective and economical, as well as environmentally friendly, as they can be derived from renewable resources. Metal recovery from industrial and mining wastewaters can also be performed using sulphate-reducing bioreactors as described in Chap. 3. Various substrate options, i.e. the carbon source, and bioreactor configurations are available. How- ever, metal recovery is hampered because metals precipitate partly in the biomass. There is future potential to explore the efficient use of substrates and intelligent control of process conditions for the recovery of metals in bioreactors. Biological sulphate reduction can also be used for the treatment of construction and demolition debris (Chap. 4). This debris contains high metal and sulphate concentrations, which can create environmental problems due to odours and health impacts due to hydrogen sulphide gas generation, especially at landfill sites. In order to reuse this debris, both sulphate and heavy metals have to be removed. Both chemical and biological processes can be used. Although chemical sulphate removal processes perform well, chemicals such as barium and lead compounds have to be added. Biological sulphate reduction is an environmentally friendly and sustainable option. Metals are precipitated as metal sulphides, and excess sulphide can be recovered as elemental sulphur or sulphuric acid. In Chap. 5, another important aspect of metal remediation that is related to lead and zinc metallurgical slag mineralogy and weathering is discussed. Predicting the environmental impact requires an understanding of minerals at microscopic scales as well as mineral-water interactions. This requires detailed characterisation of slag mineralogy and surface area, as well as performing dissolution tests. However, the usual short-term tests do not have the capacity to predict tens to hundreds of years of reactivity of these metallurgical wastes. From the view point of recovering base metals from metallurgical slags, it is recommended to manipulate and combine conditions for both chemical and biological leaching for successful heterotrophic leaching of metals. Two other important environmental issues, such as the growing demand for metals and environmental impacts caused by metallurgical wastes, have been addressed in Chap. 6 with a focus on the extraction and recovery of heavy metals. Wastes from ferrous and non-ferrous metallic industries are a potentially important resource for metal extraction. Sludges, dusts and other wastes generated by metal- lurgical industries still contain significant amounts of valuable base and heavy metals, precious metals like gold and silver as well as rare earth elements, depending on the nature of the mining site and composition of primary ores used. It is possible to use various hydrometallurgical and bio-hydrometallurgical leaching processes for the extraction of metals from these wastes. A combination of knowl- edge on the mineralogical composition of waste with various leaching and metal recovery processes will help to use these metallurgical wastes as potential second- ary sources of metals. In Chap. 7, a similar case study is presented in the form of recovery of molybdenum from spent catalysts, since spent catalysts are generated in large quantities as solid waste on a yearly basis. Consequently, from both an ecological and an economical viewpoint, metal recovery from spent catalysts is very impor- tant. It is possible to recover molybdenum using chemical leaching, which offers yields exceeding 90%. Although bioleaching offers a more cost-efficient, simpler and more environmentally friendly process, it has long leaching cycles of approx- imately 20 days and low extraction efficiencies of molybdenum. In Chap. 8, recovery of metals from electronic waste, the fastest-growing segment of solid waste, has been presented as an important secondary source of metals. Copper is the predominant metal by weight, along with substantial amounts of other base metals and precious metals. Therefore, the composition of leachate solutions from electronic waste is very complex, and thus novel strategies are required. Biotechnological metal recovery techniques enable environmentally friendly and cost-effective processes and are expected to play a significant role in sustainable development. With individual chapters of this book focussing on applications and limitations of different technologies, it is intended that this book will serve as a useful resource for chemical engineers, environmental engineers, mining engineers, biotechnolo- gists, graduate students and researchers in these areas. We also hope that by illustrating increasing numbers of case studies of metal removal and recovery from complex wastes, the expertise and knowledge necessary for sustainable metal remediation will be developed and enhanced. Ultimately, we hope to see engineers, chemists and biotechnologists playing leading roles in realising the full cycle of metal extraction, refining and, finally, recycling and recovery. We wish to express our appreciation to the multidisciplinary team of authors for discussion and communications and above all for their scientific contribution to this book. We are very grateful to Prof. Eric Lichtfouse (French National Institute for Agricultural Research, INRA, France) for providing many perceptive editorial comments and accepting this book to be a part of the book series on Environmental Chemistry for a Sustainable World. Thanks to Ms. Judith Terpos from Springer (the Netherlands) and her production team for supporting us constantly during the editorial process. We firmly believe that the information contained in this book will enhance the skills of the readers, while it will also deepen their fundamental knowledge on resource recovery from wastes. We hope you like reading this book.