Green chemistry

Definition:Green chemistry is focused on finding new, more environmentally friendly approaches to chemical reactions or techniques. Green chemists strive to design more efficient ways to utilize resources, reducing the use of nonrenewable energy resources and decreasing waste. Green chemistry also involves reducing the use of products that are harmful to human health and the environment, for example, the replacement of a carcinogenic chemical with a safer alternative. Green chemistry is of growing importance as society increasingly searches for ways to balance environmental stewardship with the quality-of-life benefits of advanced technology.

Basic Principles

The foundations for green chemistry were laid in the 1970s, which were characterized by an unprecedented push for increased environmental awareness and protection. However, it was not until the early 1990s that the field of green chemistry truly came into being. Paul Anastas—known as the father of green chemistry—coined the term while working as a chemist at the United States Environmental Protection Agency (EPA). Anastas and John C. Warner later developed the Twelve Principles of Green Chemistry, which define the field and continue to be used as a basis for green chemistry initiatives. Green chemistry is a thriving field; universities have established green chemistry academic programs, and corporations have placed increased emphasis on green chemistry as they strive to increase the efficiency of their manufacturing processes and comply with increasingly stringent regulations in the United States and abroad concerning acceptable pollution levels. Finally, green chemists are also highly employable in such government agencies as the EPA as researchers and consultants.

Although the terms “green” and “environmental” are used interchangeably in many contexts, green chemistry and environmental chemistry are different fields. As described above, green chemistry is about reengineering chemical processes to minimize harm to people and the environment; in contrast, environmental chemistry is the study of chemistry pertaining to the environment. The latter would include studying the iron cycles in a lake or the variations in organic content in soil with depth. Using climate change as an example, an environmental chemist would seek to better understand the reactions occurring in the atmosphere that are depleting the ozone layer, whereas a green chemist would invent a new method for the large-scale manufacturing of plastics that minimizes the release of toxic gas.

Core Concepts

Green chemists rely on a combination of chemistry and cost-benefit analysis to improve the environmental friendliness of chemical processes. Concepts from both of these fields that are especially relevant to green chemistry are introduced below.

Life-Cycle Analysis. The most critical (and often overlooked) aspect of analyzing greenness, be it the greenness of a process or a product, is to consider its “cradle-to-grave” impact, also known as life-cycle analysis. For example, consider the issue of plastic bags. Legislation has been introduced in various cities, states, and countries to eliminate their use or penalize their users. At first glance, this approach might seem unquestionably green—replacing plastic bags with paper or reusable bags would reduce litter. However, a thorough life-cycle analysis paints a different picture. Instead of only considering the item disposal, life-cycle analysis considers the greenness of the material, the manufacturing of the product, its distribution, use, and disposal in quantitative terms whenever possible. In this case, the manufacturing of plastic bags creates 70 percent fewer emissions than that of paper bags or compostable reusable bags, demonstrating the complexities of this issue. Furthermore, evidence has shown that nine out of every ten people reuse plastic bags; a plastic bag ban in Ireland increased the sale of trash bags by 400 percent. Life-cycle analysis is a fundamental tool used by green chemists when making decisions about how to best minimize environmental impact.

Atom Efficiency. Atom efficiency is a key concept in green chemistry. In simple terms, atom efficiency refers to the degree to which the different compounds that go into a synthesis come out in the product rather than as waste. Atoms are the basis of this measurement because chemical reactions involve rearranging the chemical bonds between atoms, making atoms the conserved quantity. Higher atom efficiencies indicate reactions with less waste.

Catalysis. Much of green chemistry entails the use of catalysts, substances that speed up a reaction but are either unaltered or regenerated over its course. Thus, a simplified expression of a catalyzed reaction is reactants + catalyst products + catalyst. Because catalysts are renewable by definition, they embody the very spirit of green chemistry. Though some catalysts are quite toxic, an efficiently designed catalysis reaction requires a very small amount of toxic catalyst, which is often more environmentally friendly than the use (and subsequent discarding) of a large quantity of a less toxic substance. Although catalysts are regenerated in the primary chemical reaction, they tend to become unusable over time due to reaction with their environment. In addition to searching for new catalysts, research in this area includes extending a catalyst’s lifetime and increasing its surface area, improving its efficiency.

Solvents. A solvent is the substance that provides a favorable environment for the chemicals of interest. For example, water is the solvent in saltwater; it provides the salt with an environment in which it can dissolve. Water is a common solvent and is biologically benign. However, many other common solvents are quite toxic, such as chloroform. Solvents are generally used in large volumes, especially in large-scale chemical manufacturing, and then discarded. In addition to being toxic to the environment, hazardous solvents are also a danger to the people exposed to them: researchers, employees at chemical manufacturing plants, and end users. A thriving area of green chemistry entails finding ways to synthesize a given product using mild solvents (e.g., replacing chloroform with water) and using less solvent overall.

Renewable Materials. In addition to finding ways to use renewable resources, such as wood or solar energy, instead of nonrenewables, such as coal, green chemists also look for ways to utilize industrial and agricultural waste when possible. For example, the inorganic silicates formed from the ashes of biomass combustion can be used as a nontoxic fire-retardant binder. Starch is a common agricultural byproduct with many applications, including in carpet adhesive. Starch occurs naturally in potatoes, corn, rice, and wheat, among other staple foods, and can be obtained as waste from food processing.

Twelve Principles of Green Chemistry. The foundation of this field, set down by Paul Anastas, consists of twelve principles for reducing harm to the environment and human health. A complete description of these principles exceeds the scope of this work, and the reader is encouraged to consult the Further Reading for additional information. The basis of the principles can be divided into two elements: reduce, reuse, and recycle (the three Rs) and safety.

Of the three Rs, Anastas’s principles are heavily focused on “reduce.” For example, the first principle reads, “It is better to prevent waste than to treat or clean up waste after it is formed,” emphasizing the reduction of waste, and many of the other principles focus on streamlining reactions in terms of minimizing the use of energy and materials that do not end up in the final product. Examples of “reuse” are found in the seventh and ninth principles, which encourage the use of renewable resources and catalysts, respectively. “Recycle” is not heavily emphasized in the principles, as the disposal of the product is somewhat outside the realm of green chemistry, which tends to focus on the creation of the product instead.

Regarding safety, these principles advocate for the use of materials that minimize toxicity and the risk for explosions, fire, etc. The third principle states, “Wherever practicable, synthetic methodologies should be designed to use and generate substances that possess little or no toxicity to human health and the environment,” and the twelfth principle reads, “Substances . . . used in a chemical process should be chosen to minimize potential for chemical accidents, including releases, explosions, and fires.” Furthermore, the tenth principle suggests that products be designed such that whatever they break down into is also nontoxic.

Applications Past and Present

Energy. Green chemistry is fundamental in developing renewable energy to the point where it will completely replace oil, coal, and natural gas. Green chemistry is involved in optimizing solar cells, ensuring that they capture as much energy from the sun as possible, and creating solar cells in interesting forms, such as bendable films and nearly transparent window films. In terms of biomass, green chemistry is used to identify the best biomass feedstock, conversion technique, and processing parameters. After energy is captured, it must be stored until use. To this end, green chemists are devising green storage devices, such as a catalyst that can use the captured energy to split water into hydrogen and oxygen, which are stored separately. When the energy is needed, the hydrogen and oxygen are recombined, releasing energy.

Plastics. Plastics are everywhere: furniture, electronics, safety equipment, toys, storage, cars, and clothing, just to name some everyday examples. Plastics are also integral to medicine, manufacturing, and food processing. The ubiquity of these materials, their often-hazardous syntheses, and the health and environmental risks posed by their degradation make them strong candidates for green chemistry research. Researchers have attempted to make biodegradable plastics using organic catalysts, to reduce the use of hazardous compounds in the manufacturing of polyurethane, and to design syntheses that produce plastic from biological sources. In addition to lessening the environmental damage caused by plastics and their production, green chemistry also aims to identify and circumvent health hazards posed by long-term exposure to plastics. For example, bisphenol A (BPA) made the news when its abilities to mimic estrogen came to light. BPA is used to produce certain types of plastic used in food storage; it can remain in the finished product and leak into food. After public pressure, many companies have removed BPA from their products. However, its replacement, bisphenol B, may be just as dangerous. Green chemistry attempts to assess these hazards and find suitable replacements.

Wastewater Treatment. Water pollution has many sources: the chlorine added to water to reduce waterborne disease; sewage; and industrial waste, to name a few. Chlorine is particularly harmful; it reacts with other compounds to form carcinogens, which are then consumed by humans and by plants and animals that are later consumed by humans. Green chemistry can be used to remove some of this pollution, especially when applied before discharge. Many of the traditional treatment processes generate large amounts of toxic waste due to their use of hazardous chemicals to remove water contaminants. Green approaches to this issue focus on the use of microbes, less toxic chemicals, and chemicals that degrade rapidly into nontoxic components.

Agriculture. One of the biggest challenges in modern agriculture is pest control. The acres of well-maintained, fertilized, homogenous fields of crops are perfect feeding grounds for pests, and the plentiful food source and lack of predators leads to unprecedentedly large pest populations. The application of pesticides to combat these infestations poses a human health risk and an environmental hazard in terms of runoff and drift (wind-induced dispersion of sprayed pesticides). Furthermore, any given pesticide is usually only of limited application—insects quickly adapt to pesticides, rendering them useless after a certain amount of use. Scientists attempting to remedy this situation rely heavily on green chemistry and green chemical engineering. An example of the successful application of green chemistry in this field was Dow AgroSciences’s development of spinosad, an insecticide that is produced by the decay of an organism, is effective in low doses, and degrades in sunlight quickly enough that it does not linger in the environment.

Pharmaceuticals. In addition to designing new drugs, pharmaceutical companies have much to gain from devising more efficient syntheses. As the patent expired on 4-isobutylacetophenone, which was used to produce the painkiller ibuprofen, its makers were motivated to find a new, less wasteful and toxic synthesis. The original synthesis produced large amounts of aluminum-contaminated wastewater and acidic gases and required cyanide and elemental phosphorus, which are extremely toxic. This process required six steps, and less than 40 percent of the atoms that were used as reactants ended up in the products (the rest being waste). At first glance, the newly devised synthesis sounds more toxic: it uses hydrofluoric acid, which is extremely dangerous and can cause death even when spilled on relatively small patches of skin. However, this synthesis uses hydrofluoric acid as a catalyst, meaning that it is recovered and repeatedly reused. Thus, considering the overall danger to humans and the environment, the newer synthetic procedure was a dramatic improvement upon the original. In addition, the newer synthesis required only three steps and 80 percent of the atoms used as reactants appeared in the final products, a near-doubling of the efficiency of resource use.

Cosmetics and Personal Care Products. The cosmetics industry, which has long relied on petroleum products to produce moisturizers and cleansers, switched to palm oils in response to increasing economic pressure. Although this change satisfies the principle of using renewables whenever possible, the increase use of palm oils has led to various ecologically harmful practices, such as deforestation to create fields for palm crops and the carbon dioxide generation that accompanies modern agricultural practices. To overcome these issues, scientists have been studying ways to produce the key components of cosmetics in more environmentally friendly ways. For example, the paraffins found in lipstick wax may be able to be obtained from waste wheat straw, and enzymes may be able to create esters, which are used keep mixtures of oily and watery substances, such as lotions, from separating. From a health perspective, increased understanding of the dangers of certain compounds found in pesticides, health products, and food packaging has led to their removal. For instance, some formulations of Johnson & Johnson’s baby shampoo initially contained formaldehyde, a known carcinogen. The applications of green chemistry in this area serve to both increase the sustainability of manufacturing practices and decrease health risks.

Social Context and Future Prospects

Interest in green chemistry is growing as society at large becomes more concerned with pollution and its long-term effects on our health. As evidenced by climate change and dwindling natural resources, many current lifestyles and industrial practices are unsustainable. In addition, people are becoming more aware of the negative effects of long-term exposure to some now-common synthetic materials, such as certain types of plastic. Although the benefits gained by the use of these materials may still outweigh the negative health effects, consumer pressure gives companies that can devise alternatives to these materials a marketplace advantage.

Green chemistry also has interesting economic ramifications for the producers of renewable resources. For example, the use of coconut husks in car parts would increase the price of coconut husks (currently considered waste), benefiting farmers in the equatorial regions in which they are grown. Furthermore, the reappropriation of waste into industrial feedstock also lessens the issue of how to dispose of this waste. In the example of coconut husks, Ghanaian farmers have traditionally disposed of coconut husks in a large pile. However, the coconut husks pose a health hazard in that they collect water, creating a breeding ground for mosquitoes, the transmitters of malaria to humans. Many other countries suffer from overflowing landfills, making waste reduction vital.

The long-term goal of green chemistry has been best expressed by its founder, Paul Anastas: “We’ll know that green chemistry is successful when the term ‘green chemistry’ disappears because it’s simply the way we do chemistry.” Ideally, the principles of green chemistry will become seamlessly embedded in the design of new chemical processes. However, even if green chemistry becomes absorbed into chemistry at large, there will always be a need for researchers and consultants who specialize in assessing and improving the environmental friendliness of chemical reactions and products.

Bibliography

Anastas, Paul T., and John C. Warner. Green Chemistry: Theory and Practice. New York: Oxford UP, 2000. Print.

“Basics of Green Chemistry.” US Environmental Protection Agency, 2 May 2024, www.epa.gov/greenchemistry/basics-green-chemistry. Accessed 28 Aug. 2024.

“What Is Green Chemistry?” American Chemical Society, www.acs.org/greenchemistry/what-is-green-chemistry.html. Accessed 28 Aug. 2024.

Jacobs, Jeremy P. “‘Green Chemistry’ Guru Charting New Course for EPA Science.” New York Times. New York Times, 20 June 2011. Web. 25 Oct. 2012.

Lancaster, Mike. Green Chemistry: An Introductory Text. Cambridge: RSC, 2010. Print.

Matlack, Albert. Introduction to Green Chemistry. 2nd ed. New York: CRC, 2010.