Atmospheric and Environmental Chemistry1
Some Challenges for Chemists and Chemical Engineers
Chemists and chemical engineers want to understand the chemical composition and behavior of the earth, its rivers, lakes, and oceans, and its atmosphere. What are the complex interactions among these systems that occur naturally, how are they influenced by human activity, and what will be the chemical and physical consequences that affect our environment? To what extent can the knowledge and capabilities of chemists and chemical engineers be used to understand and prevent or correct problems, either by direct chemical intervention or by guiding changes in human behavior? As a society, we want to be assured that the products we use are safe for us and our environment—and to be sure that the methods of their production do not harm us, our children, or our environment. The safety of our environment is not a local issue. Concerns about global climate change make it clear that chemists and chemical engineers will want to help—and will need to help—in addressing these important scientific issues on a worldwide basis.
Systems in place—ranging from laws and policies of government regulatory bodies to voluntary programs of the industry—are intended to anticipate, detect, and prevent unacceptable risks to the public, both now and in the future. The ability of chemists and chemical engineers to meet these challenges, as contributors to and beneficiaries of the chemical enterprise, requires that they utilize the best science available today and aggressively advance that science in the future. This ability will also be enhanced by educational efforts—not only efforts by the scientific community to increase public understanding of science and technology, but also changes in the ways that engineers and scientists are educated. Greater understanding of the societal implications of their work by scientists and engineers will enhance our stewardship of this planet.
PROGRESS TO DATE
Geochemists have made major progress in learning about the chemistry of the earth and its components, including rivers, lakes, and oceans. Much of this involves such fundamental theories as thermodynamics, but on a scale much larger than the molecular level that has been the past focus of the chemical sciences. In the last decade, the chemistry of the atmosphere has also been elucidated in much more detail. A field called earth systems engineering is emerging, and it will require further development, with the chemical sciences and engineering as a crucial component. This field will address matters such as global warming, carbon sequestration, and environmentally benign manufacturing. It will also address new analytical, computational, and assessment techniques through which global-scale interactions in complex systems can be better understood and optimized. Earth systems engineering is made even more intricate and involved by its international character. The factual, quantitative, and analytical input that can be supplied by chemical scientists will be of great value in this endeavor.
Chemists and chemical engineers have identified the processes that convert ordinary oxygen molecules (O2) into ozone (O3) in the high altitude ozone layer of the stratosphere, under the influence of ultraviolet light from the sun. Furthermore, chemists and chemical engineers have elucidated, through both experimentation and computational modeling, the processes by which extremely stable anthropogenic (human-generated) gases such as chlorofluorocarbon refrigerants cause degradation and depletion of the ozone layer. The ozone layer plays an important protective role for life on earth by blocking very-high-energy ultraviolet light; consequently, this fundamental atmospheric chemistry has important practical consequences. Similar understanding of the interaction of anthropogenic materials with water and with the minerals of the earth is also being developed by geochemists.
The chlorofluorocarbon effect on the ozone layer illustrates another chemical concern—the special problem that can arise when materials released into the environment are able to act as catalysts. If every chlorine atom generated in the upper atmosphere simply destroyed one ozone molecule, the effect would be minimal. But chemists have elucidated the catalytic cycle by which each chlorine atom destroys thousands of ozone molecules. It is particularly important for chemists to study and understand which substances can have such catalytic effects— and to learn how to prevent the release of such substances into the environment.
The interaction of gases such as carbon dioxide (CO2) with earth and water has also been investigated. Carbon dioxide is in most respects a harmless molecule—the product of human, animal, and plant respiration and the starting material for the growth of plants by photosynthesis. It is also produced by burning carbonaceous fuels for energy, and in converting limestone to lime for cement production. It is now becoming clear that too much carbon dioxide in the atmosphere can contribute to what is called the greenhouse effect. The temperature of the earth’s surface is governed by what happens to the energy in incident sunlight—how much of it is reflected back into space versus how much is retained by conversion into thermal energy, and how much of that is reemitted back into space as infrared radiation. There is a delicate balance among these processes, and a change in that balance can affect the overall temperature of the earth. Greenhouse gases absorb some of this infrared radiation and prevent its transmission back into space. Other gases, particularly water vapor, also contribute to the greenhouse effect, but carbon dioxide is of particular importance because CO2 levels correlate with human activity. For this reason there has been extensive debate about the extent to which global climate change is anthropogenic.
Recent estimates indicate that the level of carbon dioxide in the atmosphere has increased by a third since the beginning of the industrial age, and that it contributes significantly to global warming. Other major contributors include methane, tropospheric ozone, and nitrous oxide. Methane is the principal component of natural gas, but it is also produced by other sources such as rice paddies and farm animals. Tropospheric ozone is generated naturally and by the sunlight-
induced reactions of combustion by-products, and nitrous oxide is formed in microbial reactions, in part from nitrogen-containing fertilizers, and as a by-product of some chemical processes. Carbon dioxide and methane—generated by human activity—appear to be making the greatest contributions to global climate change. Both gases are more abundant in the atmosphere than at any time during the preceding 400,000 years.2 Solution of this inherently chemical problem will require major contributions by chemists and chemical engineers.
At one point it was assumed that the earth, its oceans and rivers, and its atmosphere were so vast or self-cleansing that we could discharge anything into them without damage to our planet. We now know this is not true. Currently, we must deal with toxic waste dumps, with smog, with acid rain that kills forests, and with pollution of rivers and the ocean by chemical discharges. How did this happen?
Part of the problem is simply the slow accumulation of materials released into the environment. Any single activity may seem small and harmless, but all such activities add up. The combustion of fossil fuels has long been recognized as a major source of air pollution. For example, burning petroleum components in gasoline and diesel engines of vehicles can lead to air pollution by emission of unburned hydrocarbons and sulfur and nitrogen oxides produced during the combustion process. When this was realized, fuels were refined differently to reduce sulfur content and vehicles were fitted with catalytic converters, designed by chemists and chemical engineers, to remove the hydrocarbons and nitrogen oxides from vehicle exhaust. The catalytic converter was introduced in the U.S. automotive industry and is now used worldwide, an excellent example of the beneficial global effects of chemical science and technology. Fuel efficiency was also increased, so less fossil fuel had to be burned per mile traveled. This has provided major improvements, but the real solution may come from a change to other forms of energy production for transportation, as described in Chapter 10.
The chemical process industries make a huge contribution to civilization, but they have the potential for environmental pollution. During the early growth of the industry, many chemical companies built their plants in river valleys. The plants used the river water and subsequently returned it to the river with water-soluble by-products, while they discharged gaseous by-products into the air. Water and air pollution were serious problems near these plants. In recent decades, chemical manufacturing has undergone a revolution. Almost all chemical manufacturers in the United States, and increasingly worldwide, subscribe to a program called Responsible Care.3 In brief, it involves a pledge by the manufacturers to make only products that are harmless to the environment and to its living occupants, and by processes that are also environmentally and biologically benign.
Climate Change Science: An Analysis of Some Key Questions, National Research Council, National Academy Press, Washington, D.C., 2001.
Another important initiative is called green chemistry,4 developed as part of efforts to reduce pollution at the source; it is defined as “the design of chemical products and processes that reduce or eliminate the use and generation of hazardous substances.”5 Originated in the United States (where prestigious awards for accomplishments in this area are given annually), it is increasingly becoming a worldwide program.
Benign processes help make chemical plants into good neighbors, and their operators into good citizens, but the products themselves are now being examined with a new perspective. For example, DDT (invented by Paul Müller, who received a Nobel Prize in 1948 for this invention) was an effective insecticide that greatly reduced the occurrence of insect-borne diseases such as malaria. However, its widespread use caused problems, because it is too stable. DDT persists in the environment and enters the food chain, a phenomenon that led to the nearly complete elimination of DDT use after it was shown to interfere with bird reproduction. At one point it was believed that persistence was a good thing, since the insecticide would keep on working, but it is now recognized that persistent chemicals can accumulate in the environment and lead to new or unexpected problems.
A similar situation is found with the herbicides that help make agriculture more productive by controlling weeds. Advantages are now recognized for herbicides having limited persistence, lasting long enough to do the job and then harmlessly disappearing.
Persistence was also the unforeseen problem with the role of chlorofluorocarbons (CFCs) in degrading the earth’s ozone layer. CFCs were invented to replace toxic and dangerous gases (such as sulfur dioxide) that had been used as the working fluid for compressors in early refrigerators and air conditioners. Indeed, CFCs are so unreactive under normal conditions that they are quite harmless to humans and other living things. However, they are so stable that they diffuse throughout the atmosphere. When they reach the stratosphere—where the ozone layer is found—the greater intensity of high-energy radiation from the sun finally causes slow decomposition of the CFCs. This decomposition process produces chlorine atoms that catalyze the destruction of ozone in a chemical sequence that is now well understood. For elucidating these chemical processes in the stratosphere, Paul Crutzen, Mario Molina, and F. Sherwood Rowland received a Nobel Prize in 1995.
While the high stability and persistence of CFCs provided a major advantage for such applications as refrigeration and air conditioning, escape of the gases into the air has resulted in unacceptable changes in the upper atmosphere. The solution to the problem is to invent new relatives of CFCs that are adequate coolants but are less environmentally persistent. Chemists have indeed created such new substances, which are now replacing CFCs.
A common definition of green chemistry, which clearly encompasses considerable chemical engineering as well, is “the design, development and implementation of chemical processes and products to reduce or eliminate substances hazardous to human health and the environment,” (P. T. Anastas and J. Warner, Green Chemistry Theory and Practice, Oxford University Press, Oxford, 1998). A more recent article expands this definition to twelve principles (M. Poliakoff, J. M. Fitzpatrick, T.R. Farren and P.T. Anastas, Science, 297, 807-810 (2002).
Many of the problems that affect our environment are the result of unexpected effects that accompany an entirely reasonable intention. The effect of CFCs on the ozone layer is such a case. Chlorofluorocarbons seemed to be perfect— they were chemically stable and noncorrosive, and they were not at all toxic—as replacements for the earlier toxic refrigerants. But then the ozone effect was discovered. Similarly, DDT seemed to be a perfect insecticide—controlling mosquitoes and other insects that transmit disease—until its effect on birds was discovered. A more recent example is methyl tertiary-butyl ether (MTBE), a fuel additive that improves the performance of gasoline in automotive engines. Unfortunately, spillage and leakage from underground gasoline tanks has allowed MTBE to enter the ground water. This has led to concern about the biological effects of MTBE and a probable end to its use.
To prevent or at least minimize such problems, we must better understand the environment at all levels, including the fundamental chemical processes that affect it. We have learned the lesson that when assessing the fate of new products in the environment, we should not underestimate the potential of these to appear in unexpected places. The recognition, avoidance, or solution of complex environmental problems requires the expertise of a variety of science and engineering disciplines. Only then will it be possible to produce realistic evaluations of how new compounds will be distributed and will act in the ecosystem. In addition to chemistry and biochemistry, fields such as solution thermodynamics and transport phenomena in which many chemical engineers work, as well as earth sciences and environmental engineering, have crucial contributions to make.
Another environmental problem is the contamination of soil by heavy metals or organics. These soil-based contaminants can produce health hazards by release of volatile substances, contamination of groundwater, or accumulation of heavy metals by plants growing in the soil. Incineration has been used to treat soils heavily contaminated by organic materials, but this approach is expensive and may result in other environmental problems. The potential toxicity of contaminated soils is governed by a variety of factors; the factors that affect bioavailability of soil-bound contaminants or the movement of contaminants in subsurface fluids are not well understood. Close cooperation of chemists and chemical engineers will be needed to gain the necessary understanding.
One promising form of treatment is bioremediation. Microbes in the soil are capable of converting organic chemicals to other compounds, and in the ideal case to CO2 and H2O, and some microbes have been found to convert heavy metals to complexes with reduced toxicity. Microbes can be harnessed through addition of nutrients, other chemicals, or specific microbes to enhance beneficial microbially mediated chemical reactions. However, the complex chemistry and biology of subsurface soil systems makes it difficult to achieve predictable responses. A much deeper knowledge of the interaction of the chemistry of the soil, fluid movement, and microbial physiology is an important challenge to making
bioremediation a fully practical technology. (see Chapter 7 for discussion of related matters).
A half-century ago, people were only beginning to understand the extent to which human activity could affect the environment, often in very negative ways. Many of the early problems of pollution were the result of chemical processes, and “chemistry” received the blame. But “chemistry” has also provided solutions, and dramatic improvements have occurred. No longer do industrial plants belch foul smoke into the atmosphere, and no longer do chemical plants discharge brown or orange sludge into nearby streams and rivers. These improvements have been implemented by chemists and chemical engineers, and they have been implemented in ways that often have provided economic benefits to the United States.
This chapter has focused on the chemistry of the earth and its immediate atmosphere, but the concept of “environment” need not stop with the earth’s atmosphere. Molecular chemistry also goes on in space, and it is an area of interesting chemical science. Large, although extremely diffuse, clouds of molecules exist between the stars, and such clouds are believed to be the origins of stars. So far, the chemistry in the clouds has been investigated not by direct sampling, but instead by observing the light that is emitted from them. Observations of such light in the microwave range have led to the identification of many remarkable substances—and of the likely transformations that they undergo. Chemists have synthesized some of these unusual substances here on earth so their microwave radiation can be compared with that from space, for identification of the space molecules. In addition, computational chemistry has reached the stage at which it is possible to predict the microwave radiation that some molecules would emit, again enabling earth-bound scientists to obtain evidence for the structures of materials that exist only in outer space.
CHALLENGES AND OPPORTUNITIES FOR THE FUTURE
Considerable progress has been made in understanding the environment and the chemical processes that affect the environment. However, the preceding discussion clearly shows that many challenges remain. Chemical scientists must learn how to make useful substances that have limited persistence—and will generate only completely harmless products when they degrade.
A full understanding will be needed of the complex chemistry by which the atmosphere and the earth interact, including the dependence of global climate on carbon dioxide concentrations in the atmosphere. Is there a way to deal with the carbon dioxide produced by burning coal and other hydrocarbon fuels so that it causes no problem? Chemical scientists will need to investigate effective ways to trap CO2 that would otherwise build up in the atmosphere. Alternatively, it will be necessary to find ways to reduce the generation of carbon dioxide. As human
activity continues to destroy forests, will this affect the carbon dioxide cycle and levels, and what can be done about that?
Today we are witnessing a “green” revolution as it pertains to manufacturing industries’ implementation of carbon dioxide as a replacement for their dependency on water and organic solvent usage. A major impetus for this conversion is driven by a concern for our environment—to reduce a company’s footprint on our planet by reducing their usage of solvents and water. In addition, because of the low heat of vaporization of supercritical CO2 (sCO2) relative to water and organic solvents, corporate motivations also include the reduction of energy usage associated with using CO2-based processes relative to conventional solvents and water. However, beyond pollution prevention and energy efficiency issues, sCO2 is finding increasing appeal because of increased performance attributes associated with its inertness to many chemistries, to its exceedingly low surface tension and viscosity, and to its adjustable solvent quality due to its compressibility, especially in the supercritical state. Its critical temperature is conveniently located at 31°C, and it is both nontoxic and inexpensive. Recently, sCO2 has been or is in the process of being commercialized in:
These processes are enabled by breakthroughs in many areas. A fundamental understanding is emerging of the rational design of surfactants for sCO2—molecules that reversibly self-assemble as the density of compressible sCO2 is adjusted. Such surfactants enable the stabilization of polymer colloids in sCO2 for heterogeneous polymerizations and for the emulsification of numerous sCO2-insoluble substances including proteins, water, and catalysts to name a few. Creative engineering unit operations have been designed that allow for continuous reactions, automated reaction/separation schemes, and novel membrane-based separations. And finally, creative combinations of chemistry (novel compounds such as sCO2-soluble functional polymers) with sCO2-based applications have enabled entirely new processes.
Future commercial use of CO2 will surely include such diverse applications as thin-film deposition for microelectronics using recently developed pressurized spin-coating and free meniscus coating instruments; in separations of value-added products from fermentation broths in biotechnology fields taking advantage of the immiscibility of CO2 with water; and as the solvent in a broad range of synthesis including transition metal and enzymatic catalysis. All of these applications will lead to sustainable manufacturing methods that are not only ecologically preferable, but in themselves are enabled by working with a unique solvent that has the density of common liquids but the transport properties of a gas.
Humans affect the environment through simple living activities—they generate waste. Ordinary waste sites, in which people place trash and garbage, generate methane by microbiological action, and methane is one of the most potent greenhouse gases. Is this a significant problem, and if so, what can be done to prevent or solve it? Will it be possible to recycle waste that we currently bury, perhaps ameliorating the problem of waste sites? And if so, could we recover some of the chemicals and energy that were used to manufacture the discarded materials? Chemists and chemical engineers will need to devise the ways in which materials that are harvested from the earth can be recycled, not just discarded or burned. One of the approaches is bioremediation, in which microorganisms, perhaps genetically modified, are used to deal with the waste. This approach has already been used to some extent in dealing with oil spills. The ultimate goal is sustainability: using and recycling materials so that we do not simply exhaust the substances we have inherited, thus leaving less for our descendants.
The future production of chemicals will require continued awareness of possible unforeseen consequences, particularly with substances that persist in the environment. This will require investigation of how these substances interact with the environment, and it will necessitate the invention of new substances. As noted above, new and safe refrigerants will be needed that lack the persistence of CFCs. The world’s food supply will depend on the discovery of new insecticides that do not harm unintended targets in any way and other agricultural chemicals such as herbicides and fertilizers that are neither harmful nor overly persistent. Other sectors of the chemical industry will rely on the invention of new and selective catalysts that enable the manufacture of useful products, including polymers, without producing unwanted waste products and without using excessive energy.
The existence of waste from past activities has created problems that will demand the attention of chemists and chemical engineers. Environmental cleanup —of toxic wastes, of contaminated groundwater, of radioactive waste—is a daunt-
ing challenge. Only by developing effective ways to clean up contaminated sites— and by developing processes by which such contamination can be prevented in the future—will it be possible to take full advantage of the technological opportunities that science has provided over the last century.
There are serious questions about the effect of continued growth of the human population. We often focus on the matter of adequate food production, but large populations also affect the environment and our need for energy. Chemists can furnish the tools to help deal with the political and social questions. Some population growth occurs not because it is intentional, but because adequate methods for birth control are unavailable to people in poor countries. Thus one challenge for chemists is to develop better methods that would be safe, effective, and inexpensive and would enable all people to pursue their own decisions regarding population growth.
Small amounts of some contaminants can be serious. When we burn coal, not only do we produce carbon dioxide, the major product, but we can liberate small amounts of mercury and larger amounts of sulfur dioxide. The mercury can form toxins that harm fish as well as humans, while sulfur dioxide can produce acid rain that destroys forests and water supplies. How can these contaminants be most effectively removed or dealt with? And how can we generate the energy that we need without releasing such by-products?
Environmentally responsible methods to manufacture useful products and to generate energy still need the largest contributions from chemists and chemical engineers. As we reexamine all human activities with a fresh eye, to see their environmental impact, the continuing challenge will be to invent new ways to achieve what society needs. Understanding the environmental effects of what we do is the first step, a job for basic chemical science. Inventing ways to improve what we do while still meeting human needs is a job for applied chemistry and chemical engineering. The opportunities for our science to improve the human condition will continue to pose exciting and important challenges.
Important educational challenges also confront the chemical sciences. Continuing emphasis on science education by government, foundations, industry, and educational institutions at all levels is essential. It is also essential to engage the professional practitioners in the chemical sciences—chemists and chemical engineers engaged in the broad spectrum of activities that characterize these fields— in education. Public outreach programs of professional societies like ACS and AIChE are important components, but more effort is needed both at the grass-roots community level and in the improvement of the visibility of the chemical sciences through mass media. Greater emphasis in science and engineering education on the human aspects of the scientific endeavor is needed. Chemists and chemical engineers must also continue their own educations. This will improve their ability to contribute to a broader public understanding of science and technology, and it will enhance the contributions they can make to society.
We have come a long way. But there is still a long way to go. If we are to provide a favorable legacy for future generations, chemists and chemical engineers will need to develop effective ways to clean up existing waste and find ways to prevent the generation of waste in the future. And most importantly, they will need to develop a system that is fully sustainable—that will safely provide the energy, chemicals, materials, and manufactured products needed by society while neither irreversibly depleting the earth’s scarce raw materials nor contaminating the earth with unhealthy by-products.
WHY ALL THIS IS IMPORTANT
Human activity, including manufacturing, can damage our environment if we are not thoughtful and careful. We do not want to live on a planet where the air and water are dangerous to health, or where life is not possible for various plants and animals. In the past society did not worry enough about such questions, but now we have learned to take them seriously. Fortunately, it has turned out that environmentally benign chemical manufacturing, using the principles of Responsible Care, is both environmentally useful and economically acceptable. In fact, many companies have discovered that good environmental practices are actually cost effective. In any case, the challenge is to learn how to continue to develop a modern civilization without causing environmental damage.
The consequences of human action on the environment are not always completely foreseeable, so there is also the challenge to recognize the likelihood and magnitude of those consequences that we can foresee, and to recognize patterns of particular vulnerability. To meet this challenge it is essential that we continue to cultivate a strong base of fundamental knowledge in the chemical and biological sciences—for we will need those tools not only to create new technologies, but also to anticipate their consequences. We must approach the creation of science and technology for human advancement with intelligence, knowledge, and reason. That is, after all, what makes the human animal special.