Ionic liquids have been seen, and promoted, as green solvents, and this aspect has been a driving force for their application in many fields to promote the sustainability of processes and minimize their environmental impact. Yet surprisingly, their application into environmental engineering, i.e. addressing water, soil and air treatment, or pollution prevention, has been much more restricted, with the major exception being CO₂ capture from gaseous effluents. However, despite the enormous body of knowledge on the design of ionic liquids for CO₂ capture, there has not been a major project that has scaled up ionic liquids for CO₂ capture much beyond the laboratory stage. This book intends to be a call to action towards the application of ionic liquids for environmental applications. In terms of gaseous effluents’ treatment, they could be used far more, reaching other applications than just CO₂ capture. In particular, the use of ionic liquids has been changing the landscape of sensing for gases allowing for the development of simpler, more accurate and cheaper sensors for many atmospheric pollutants, as highlighted by Zeng and co-worker in Chapter 1. However, considering that most of the world’s population breathes air that exceeds air quality limits, the use of ionic liquids for the actual removal of air contaminants, such as volatile, semi-volatile and very volatile organic compounds, and volatile non-organic compounds is still in its infancy. A critical discussion of the strategies and results obtained so far, typically still referring to laboratorial gas models, is described by Carvalho et al. in Chapter 2. Coming back to CO₂ capture, a critical analysis of the real technical and economic demands of large-scale CO₂ capture processes using ionic liquids and their derivatives, along with the urgency with which such processes must be deployed is offered by Bara in Chapter 3.

Clean water scarcity in both developing and industrialized nations has already been linked to major public health concerns. The urgent need to develop innovative and sustainable water solutions, without further stressing the environment or endangering human health, was included in the United Nations’ 2030 Agenda for Sustainable Development. Ionic liquids’ outstanding solvation and physicochemical properties can play a crucial role in facilitating the current analytical approaches through the development of high-throughput technologies affording a faster and more sensitive analysis of organic pollutants, both persistent and those of emerging concern, in aqueous streams, as highlighted by Pino and co-workers in Chapter 4, and metals, as discussed by Soylak et al. in Chapter 5. The development of ionic liquids as extracting agents has steadily increased within both academia and the industrial community. Instead of the initial trial-and-error approach, the mechanisms of extraction are being studied using computational tools, so that task-specific Ionic liquids can be confidently designed and synthetized for the extraction of micropollutants such as pesticides and metals, as highlighted in Chapters 6 and 7 by Banerjee and Jirsa’s research groups, respectively.

From a different perspective and recognizing the important role that ionic liquids could play in reaction, separation and purification industrial processes, the treatment of generated industrial effluents is vital to enable their implementation from both an economic and environmental point of view. If, on the one hand, the removal of ionic liquids from effluents by sorption and regeneration strategies seems to be a viable alternative for wastewaters, as discussed by Palomar et al. in Chapter 8, on the other hand, the efficiency of abatement oxidation processes is being tested in the degradation of ionic liquids, as described by Mohedano et al. in Chapter 9.

Finally, the application of ionic liquids in a well-established technology, crude oil production, is reviewed and discussed by Gardas and co-workers in Chapter 10. Ionic liquids can be advantageously used as environmentally benign demulsifiers to overcome the formation of oil–water emulsions and aggregates, which are major challenges associated with crude oil extraction and processing. Ionic liquids’ properties such as aggregation and micellization behavior, toxicological profile, and surface, interface and transport properties are discussed in detail to explore their potential in advanced oil dispersion.

Surprisingly, given the breadth of the applications here described, attempts at using ionic liquids for these purposes have been lagging, not due to the poor performance of ionic liquids, because, as shown in this book, they have tremendous potential in this field, but for a strange lack of interest from the Environmental Engineering community. We can only hope that having identified this opportunity, this book will contribute by the example of the works here described to accelerate and promote the development of this field.

João Coutinho and Isabel Marrucho