In recent times, society has become more aware and concerned about the negative environmental impact associated with chemical industry. As such, there have been wider legislative interventions in order to ensure greater control and monitoring of chemical processes. These changes have led to increasing pressure on the chemical industry to continue to deliver high value products in an economical fashion, while minimising or eliminating the adverse environmental burden. This new challenge has driven the development of green chemistry for sustainable chemical processing.
Sustainable development has been defined by the United Nations as ‘…meeting the needs of the present without compromising the ability of future generations to meet their own needs’ [2]. Further, the United States Environmental Protection Agency (EPA) has extended this definition to give birth to green chemistry where the main goal is to ‘promote innovative chemical technologies that reduce or eliminate the use or generation of hazardous substances in the design, manufacture and use of chemical products’ [3]. Related to this is green engineering, which pertains to the design, commercialisation and the use of processes and products in an economical fashion to minimise pollution and risks to health and the environment. Utilising these concepts, 12 principles of green chemistry have been formulated [4], which are shown in figure 1.1.
Figure 1.1. 12 principles of green chemistry, after [4]. Copyright of OUP 1998.
Applying these green and sustainable principles has the potential to address the challenges facing chemical industry, in balancing the high value of products against the environmental burden. A common perception is that green chemistry is costly and/or can lead to costly products, thus reducing the profit for industry. However, this is not always true. While there may be initial investment needed for the development of green technologies, which can replace existing processes, such changes can also lead to reduced costs of production and products. Besides, considering the long-term benefits of adopting green chemistry, initial costs can be offset. Some of these points and the 12 principles will be discussed further in chapters 4 and 5 with specific relevance to nanomaterials.
1.1.2 Drivers for green approaches
One of the main issues leading to adverse environmental impact is the generation of waste in a chemical process. Depending on the technological, economical and legislative frameworks, the quantities of waste produced, in particular, in relation to the amount of product produced, vary from sector to sector. For example, some industrial sectors are technologically very advanced, such that they have developed ways to minimise waste, e.g. petroleum refining. On the other hand, in some other sectors, the cost of the product is significantly higher than the loss of value from waste or cost of treating waste, and hence waste minimisation is not given due importance (e.g. pharmaceuticals). There are also examples where pro-active legislation has driven the chemical industry to find innovative ways to minimise waste. Waste relates to inefficiencies in a given process, which leads to loss of valuable resources (e.g. substances and energy) and it can cause risks to the environment and health, which ultimately increases the process costs. Figure 1.2 illustrates the financial, environmental and societal origins of the costs associated with waste.
Figure 1.2. Costs associated with waste. Reproduced with permission of the Royal Society of Chemistry from [1].
If one considers the total costs for waste originating from all areas as shown in figure 1.2, it implies that in order to drive green chemistry, the process or product costs associated with a new green technology (N) should be lower than or at least equal to the costs of the existing (wasteful) processes (E),
i.e.N⩽E.(1.1)
Along similar lines, the profits from the new process should be higher or at least the same when compared to the existing process. The cost of green technology includes new production costs (P′) and the investment needed for developing new green technologies (G), which would reduce the waste and avoid the loss of the image of the business. On the other hand, continuing to operate using existing process includes current production costs (P) and incurs waste management costs (W) as well as costs associated with the loss of public image (I),
∴G+P′⩽P+W+I(1.2)
∴G⩽(P−P′)+(W+I).(1.3)
This is a simplistic approach to describe various scenarios and consequences. First, if the company does not wish to invest at all (G = 0), then it means that the new process costs can be higher than the existing process costs, but only by the sum of the costs of waste management and those associated with the loss of image,
i.e.∴(P′−P)=(W+I).(1.4)
Without any investment, it is not easy to obtain access to a new and sustainable process. Further, if the costs for managing the changes in the process (e.g. downtime, marketing, customer satisfaction, etc.) are considered, then in reality,
∴(P′−P)≪(W+I).(1.5)
This leaves very little incentive for developing a greener process and highlights the strong need for initial investment and the motivation for sustainability at all levels of the organisation. On the other hand, if a new process is developed such that there are no additional process costs (P − P′ = 0), then the investment needed would be of the order of the cost savings from waste reduction and maintenance of the image,
i.e.G⩽W+I.(1.6)
The production costs and the type and amount of waste generated are not just dependent on the chemical reaction. Other factors such as choice of solvent and downstream separation and purification processes play a major role. Therefore, developing greener approaches is not about simply operating at lower temperatures or using volatile solvents, for example. Sometimes it may be about balancing the priorities or perhaps radically changing the processes.
1.1.3 Estimating environmental impact
In order to make informed decisions about the need for green innovations for a given process, it is important to assess the environmental impact of that particular process. There are various methods and tools available to qualitatively and semi-quantitatively analyse the environmental impact of processes [1]. Selected methods are described below with their principles, use, advantages and limitations.
Environmental factor (E-factor) [5, 6], also known as waste-to-product ratio (equation (1.7)), is a simple measure for identifying the amounts of waste/by-products produced with respect to the mass of the product.
E-factor=massofwasteandby-product÷massofproducts.(1.7)
Sheldon and co-workers [5] have studied E-factors for various industries and reported great variations from one industrial
