Green Nanomaterials. Siddharth Patwardhan

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 sector to another (table 1.1).

Table 1.1. E-factors for various sectors. Adapted from [5].

Industry sector	Product capacity, tonnes	E-factor
Oil refining	106–108	∼0.1
Bulk chemicals	104–106	<1–5
Fine chemicals	102–104	5–50
Pharmaceuticals	10–103	25 to >100

      The main reasons for such variations appear to be related to the product value/profit margins, relevant legislations, cost of waste and market competition. The E-factor analysis of a process is simple and provides quick estimates of wastefulness, which can lead to waste minimisation campaigns. While E-factor is easy to use, it can provide misleading information in some cases. For example, consider the following reaction:

Reactant1+Reactant2⟶SolventAproduct+waste.(1.8)

      In reaction (1.8), when calculating the E-factor, if water is the waste, it will be treated as any other waste, although water is not inherently toxic or hazardous. In order words, E-factor does not take into account the nature and the actual impact of the waste or by-products and can treat waste on equal grounds despite significantly different environmental impacts.

      Environmental quotient (EQ) [5] has been introduced in order to address the weakness of E-factor. EQ is essentially a modified E-factor, which takes into account the environmental ‘unfriendliness’ quotient (Q) of the waste or by-products (equation (1.9)). Effective mass yield (EMY) is another similar metric (equation (1.10)), which also disregards benign substances used or produced. Therefore, both EQ and EMY help to distinguish between hazardous waste and non-hazardous waste. However, both metrics are susceptible to inconsistencies due to the vagueness around what is environmentally unfriendly or benign, leading to debate over what values to assign to individual substances.

EQ=E-factor×Q(1.9)

%EMY=100×(mass of products÷mass of non-benignmaterials used).(1.10)

      Life cycle analysis (LCA), is an extensive way of assessing the environmental impact and sustainability of a given process or product. LCA analyses the entire life cycle of a product, from the extraction of raw materials all the way to the fate of the product. In the context of nanomaterials, this has been extensively reviewed elsewhere [7]. This comprehensive analysis can overcome various pitfalls associated with the other metrics introduced above. However, performing LCA is laborious and time consuming, and it requires a large amount of process- and product-related data to be available. One important advantage of LCA is that for an alternative process or product, LCA can help differentiate between pollution/waste prevention and shifting pollution. For example, consider the reaction (1.8) shown above, and assume that the solvent A is a hazardous solvent. In order to remove the need for solvent A, an alternative reaction is available using a different precursor (equation (1.11)), where solvent B is benign:

Reactant3⟶SolventBproduct+waste.(1.11)

      On the face of this new reaction, it appears to be ‘green’ because the hazardous solvent has been replaced with a non-hazardous