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ACS Sustainable Chemistry & Engineering assessments of chemical processes.45 On the other hand, emphasis is currently placed more on the global warming potential (GWP) of the CO2 generated in the burning of fuels and the manufacture of chemical products. In the context of green chemistry and sustainable development, it is very important to establish if energy is derived from fossil resources or renewable biomass. Thus, the seventh principle of Green Chemistry is “A raw material or feedstock should be renewable rather than depleting wherever technically and economically practicable.” Environmental Impact of Waste. The prevention of waste generation, thereby eliminating its environmental impact, continues to be the underpinning tenet of green chemistry. It has been further strengthened and consolidated by the more recent emergence of the concept of a waste-free circular economy (see below). However, a shortcoming of mass- efficiency metrics is that all types of waste are assigned the same weighting. The original intention of the E factor and other mass-based metrics was to draw attention to the inefficiency of many batch chemical processes and the fact that this is the direct cause of the generation of copious amounts of waste, especially in the fine chemical and pharmaceutical industries. Nonetheless, not only the mass but also the environmental burden of the waste needs to be considered. One kilogram of sodium chloride does not have the same impact on the environment as 1 kg of a chromium(VI) compound or 1 kg of dichloromethane solvent. This was recognized when the E factor was introduced in 1992, and the term “environmental quotient”, EQ, where Q is an arbitrarily assigned unfriendliness multiplier or weighting factor, was introduced shortly there- after.46 In the teaching of green chemistry at the high school level in The Netherlands, these are now referred to as the E factor and the Q factor.47 For example, one could arbitrarily assign a Q value of 1 to NaCl and, say, 100−1000 to a heavy metal salt, such as chromium, depending on its threshold limit value (TLV), ease of recycling, etc. Although arbitrary assignment of Q values is debatable, it was clear that (monetary) values could be assigned to waste streams. [One reason for choosing Q to represent the “unfriendliness multiplier” was that the pronunciation of EQ is the same as that of ECU (the European Currency Unit) which subsequently became known as the euro.] Much attention has been devoted, therefore, over the past two decades to quantifying Q. This generally involves completing a score sheet based on safety, health, and environment (SHE) hazards of the raw materials (input) and waste (output). We note, in this context, that any impact score assigned to waste would presumably include the raw materials, solvents, etc., as the waste will inevitably contain some of these materials. An early example of the development of methodologies for quantifying EQ, during the process design stage, is provided by the work of Heinzle and co-workers.48 This involved combining mass-loss indices (MLI) with ecological and economic weighting factors to yield environmental (EI) and economic indices (CI). Environmental weighting factors were based on classification schemes related to environmental laws and regulations, and economic indices were based on raw material and waste treatment costs but could also include equipment and operating costs. Subsequently, Eissen and Metzger49 developed the simple and easy-to-use EATOS (Environmental Assessment Tool for Organic Synthesis) software for measuring the potential environmental impact of various routes to a target molecule. G Perspective Interestingly, for the input, that is the mass of all raw materials, including solvents, catalysts, and auxiliary chemicals used in workup, they used mass intensity, which they called the mass index, S1−, and for the output they used the E factor. They also noted that it would be good to include energy use but that the necessary data are not usually available for bench scale reactions. They developed a relatively simple method, using readily available data, to assess the potential environmental impact, PEI.kg−1, of each substance in both the input and output, Qinput and Qoutput, respectively, by assigning a score of 1−10. For the input, this was based on risk phrases (R-phrases) used to designate toxic and hazardous substances.50 For the output (waste), the score was based on potential eco- toxicological and human toxicological effects such as persistence, bioaccumulation, and ecotoxicity. The outcome is equivalent to EQ in that it constitutes an integration of the amount of waste with quantifiable environmental indicators based on the nature of the waste. It also introduced an economic componentthe cost of raw materialsinto the assessment. Similarly, van Aken and co-workers51 introduced a semi- quantitative post-synthesis tool, the EcoScale, for evaluating both economic and environmental impact factors of organic syntheses on a bench scale. As with EATOS, it is based on assigning a range of penalty points to six parametersin this case yield, cost, safety hazards, technical setup, reaction conditions, and ease of downstream processingand subtract- ing the sum of all penalty points from 100 to give the Ecoscale value of a particular synthesis. Life Cycle Assessment. Life Cycle Assessment (LCA)52−56 was specifically designed for assessing the environmental impact of a product, in all stages of its “life”, that is, from raw materials acquisition through production and use to end-of-life treatment and disposal or recycling (cradle-to- grave). In addition to determining mass and energy balances LCA is concerned with the evaluation of quantifiable environmental impact categories, such as global warming potential, ozone depletion, acidification, eutrophication, smog formation, and human- and eco-toxicity as described by the ISO 14040:2006 standard.57 It is used as a decision-making tool by both industry and governmental organizations. LCA is an iterative process consisting of four phases: (i) definition of the goal and scope, (ii) inventory analysis (LCI), (iii) impact assessment (LCIA), and (iv) interpretation. The scope includes defining system boundaries and level of detail and can differ considerably depending on the goal of the study. The data collection phase (LCI) concerns the documentation of all the energy and material input and output flows within the boundaries of the study and the collection of data relating to the various environmental impact categories for the chemical substances involved from, for example, the LCI database Ecoinvent.58,59 The assessment phase (LCIA) is conducted with the data collected in the LCI phase. Environmental impact categories are assigned to the mass and energy flows and quantified in terms of representative units, for example, carbon dioxide equivalents for global warming potential of emissions. Particular categories are assigned a weighting based on their importance in the context of the goal and scope of the study. A number of LCIA methods have been described.60 One of the most up-to- date examples is ReCiPe which consists of 18 impact categories related to human- and eco-toxicity, climate change, ozone depletion, smog formation, acidification and eutrophication, DOI: 10.1021/acssuschemeng.7b03505 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXXPDF Image | Metrics of Green Chemistry and Sustainability
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