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Challenges in Environmental Chemical Science Chemical scientists and engineers have a particularly strong stake in the fu- ture of the environment. Many of the most important environmental threats are caused or at least perceived to be caused by release of undesirable chemicals into the air, water, or soil. In some cases, the source of such chemicals is natural, as in the highly publicized case of arsenic-contaminated groundwater in Bangladeshi and also in some parts of the United States.2 Chronic exposure to small amounts of arsenic in drinking water increases a person's risk of cancer and other diseases.3 High concentrations of arsenic found in the aquifers in Bangladesh and West Bengal pose serious threats to public health; estimates of population at risk run from 30,000,000 to a high of 80,000,000.4 In other in- stances, human activity has been the origin of the chemical release. Some of the most important cases are also the most ironic because the harmful effects on the environment were a direct consequence of a technological innovation that was intended to enhance environmental quality. Two compelling examples are pro- vided by DDT (Box 3-1) and chlorofluorocarbons (CFCs, Box 3-2~. iArsenic Contamination of Groundwater in Bangladesh; Kinniburgh, D. G.; Smedley, P. L., Eds.; Volume 1: Summary, BGS and DPHE, British Geological Survey Technical Report WC/OO/l9, Brit- ish Geological Survey: Keyworth, UK, 2001. 2Focazio, M. J.; Welch, A. H.; Watkins, S. A.; Helsel, D. R.; Horn, M. A., A Retrospective Analysis on the Occurrence of Arsenic in Ground-Water Resources of the United States and Limitations in Drinking-Water-Supply Characterizations: U.S. Geological Survey Water-Resources Investigation Report 99-4279, 1999; http://co.water. usgs.gov/trace/pubs/wrir-99-4279/. Arsenic in Drinking Water: 2001 Update, National Research Council, National Academy Press, Washington, D.C., 2001. 4Smith, A. H.; Lingas, E. O.; Rahman, M. Bulletin of the World Health Organization 2000, 78(9), 1093-1 103. 22
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CHALLENGES IN ENVIRONMENTAL CHEMICAL SCIENCE 23 How can environmental problems such as those associated with arsenic, DDT, and CFCs be solved? More specifically, how can the problems be re- solved, how can release of pollutants into the environment be reduced or stopped, and how can harmful materials be removed from the environment? The complex- ity of the issue is even greater if the question is extended to, Should efforts be made to remove the material from the environment? Ultimately, this leads to the question, How can such problems be prevented in the future?
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24 THE ENVIRONMENT
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CHALLENGES IN ENVIRONMENTAL CHEMICAL SCIENCE 25 If science and engineering are to be employed in solving and preventing problems, then scientists and engineers will need to greatly increase the level of understanding of the relevant issues. Society will need to develop a far more comprehensive knowledge of the environment, from a global scale down to the chemical reactions that take place in air, water, and soil and within living organ- isms. New and more powerful methods for detecting and analyzing chemical substances will be needed, and it will be essential to develop a systems approach to the complex network of interacting chemical, physical, and biological pro- cesses that must be monitored and evaluated. Only with this greatly expanded knowledge will it become possible to fully protect, restore, and preserve our envi- ronment. FUNDAMENTAL UNDERSTANDING Human Influence on the Natural Environment Evidence of anthropogenic influence on the natural environment is wide- spread: Examples include pesticide use (see Box 3-1), stratospheric ozone deple- tion (see Box 3-2), intercontinental transport of wind-borne dust and air pollut- ants, drinking water disinfection (see Box 3-3), and increasing levels of anthropogenic emissions into soils and groundwater. Intercontinental transport of wind-borne dust has been observed for many years,5 and satellites have tracked plumes of smoke from forest fires and biomass burning over thousands of kilometers.6 More than two generations ago, it be- came recognized that lead, principally associated with the use of tetraethyl lead as a motor fuel additive, was being distributed globally. Likewise, long-range trans- port of chemical pesticides, such as DDT and other organic chlorine compounds, and the associated ecological damage have been a subject of study for decades. In the 1980s, ozone pollution was identified as not just an urban problem, and trans- port of ozone across national boundaries became an international issue in North America, Europe, and most recently, Asia. Surface measurements have shown that ozone pollution from North America is easily detectable 3000 km downwind from the North American source region.7 Similar observations have been made of transport of Asian pollution across the Pacific.8 Long-range transport of poly- chlorinated biphenyls (PCBs) and dioxins has been extensively documented. Recently, polybrominated diphenyl ether (PBDE) chemicals used as flame retar- dants in consumer products appear to be contaminating pristine areas of the Arctic 5 Prospero, J. M.; Savoie, D. L. Nature 1989, 339, 687-689. 6 Wotawa, G.; Trainer, M. Science 2000, 288, 324-328. 7 Parrish, D. D.; Trainer, M.; Holloway, J. S.; Yee, J. E.; Warshawsky, M. S.; Fehsenfeld, F. C.; Forbes, G. L.; Moody, J. L. J. Geophys. Res. 1998,103, 13357-13376. ~ Jacob, D. J.; Logan, J. A.; Murti, P. P. Geophysical Research Letters 1999, 26, 2175-2178.
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26 THE ENVIRONMENT even more rapidly than either PCBs or dioxins.9 The most comprehensive assess- ment to date of PBDEs in the breast milk of North American women indicates that the body burden in Americans and Canadians is the highest in the world, 40 times greater than the highest levels reported for women in Sweden.~° Fluori- nated organic compounds are globally distributed, environmentally persistent, and bioaccumulative.~i Water and Sediment Chemistry Environmental chemistry has progressed significantly over the past four to five decades from a science that was concerned primarily with measurements of trends in the distribution of problematic species in the environment to the more 9Ikonomou, M. G.; Rayne, S.; Addison, R. F. Environ. Sci. Technol. 2002, 36, 1886-1892. i°Betts, K. S. Environ. Sci. Technol. 2002, 36, 50A-52A. i~Giesy, J. P.; Kannon, K. Environ. Sci. Technol. 2002, 36, 147A-152A.
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CHALLENGES IN ENVIRONMENTAL CHEMICAL SCIENCE 27 modern approach that seeks to understand processes on a fundamental chemical and physical basis. Accordingly, our ability to understand environmental pro- cesses hinges on our ability to recognize the factors responsible for the complex biogeochemical interactions that are collectively acting to modify the systems. Analytical chemistry plays a huge role in providing us access to methodologies that will enlighten our abilities to recognize the responsible factors. Therein lies the future in environmental chemistry, especially if one can apply developing methodologies to solve new problems. There are some critical thrust areas that have to be developed further. First is the need to understand chemical processes at a molecular level of detail. We need to move from empirical observations that feed well into large-scale models of transport, circulation, biodegradation, and other processes, to developing a fundamental understanding of the biogeochemi- cal processes at a molecular level. It is only then that we can truly understand the factors governing such processes. We now have some very powerful analytical tools to do this, and we should strive to become proficient and knowledgeable in their application, especially with regard to solving some important environmental problems.
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28 THE ENVIRONMENT One problem that requires attention is developing a molecular-level under- standing of how carbon turns over in soils, sediments, and waters. Much of our understanding of how our planet will cope with rising CO2 levels and global warming trends is based on models that describe the behavior of carbon its location, form (e.g., as carbon dioxide, carbonate, or organic plant matter), and rate of conversion through biogeochemical cycles among various environmen- tal compartments. One huge variable in predicting outcomes from models is the molecular manner in which carbon turnover occurs. The input data are based on uptake and evolution of CO2 and nitrogenous components that provide only total rates. We really have little understanding of the factors that control turnover. Humic substances and their production from plant materials are important factors and we need to know more about the relevant biological and chemical processes down to the molecular level. Related to this is the nature of dissolved organic matter in natural waters a huge reactive and storage reservoir for carbon. We also need to better understand the biological and physicochemical interactions that occur in response to global greenhouse gas augmentations. We also must not fail to consider effects that climate change might have on environmental chemi- cal processes. The major goals of environmental bioinorganic chemistry are to elucidate the structures, mechanisms, and interactions of important "natural" metalloenzymes and metal-binding compounds in the environment and to assess their effects on major biogeochemical cycles such as those of carbon and nitrogen. By providing an understanding of key chemical processes in the biogeochemical cycles of elements, such a molecular approach to the study of global processes should help unravel the interdependence of life and geochemistry on planet Earth and their convolution through geologi- cal times. Francis Morel, Appendix D] Dissolved organic matter (DOM) in water leads to the binding and transport of organic and inorganic contaminants, produces undesirable by-products through reaction with disinfection agents, and mediates photochemical processes. DOM is also a major reactant in and product of biogeochemical processes and controls levels of dissolved oxygen, nitrogen, phosphorus, sulfur, numerous trace metals, and acidity. DOM can range in molecular weight from a few hundred to 100,000 Da, which is in the colloidal size range, and generally has similar characteristics to humic substances in soil. Moreover, contaminants of a bewildering array are i2Leenheer, I. A.; Croue, I.-P. Environ. Sci. Technol. 2002, 36, 19A-26A.
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CHALLENGES IN ENVIRONMENTAL CHEMICAL SCIENCE 29 being found in aquatic environments. These include detergents, disinfectants, in- sect repellents, fire retardants, plasticizers, reproductive hormone mimics, ste- roids, antibiotics, and numerous other prescription and nonprescription drugs.~3 Another important area is understanding the fate and transport of anthropo- genic chemicals in soils and sediments. Organic chemicals, including pharma- ceuticals, fertilizers, herbicides, and pesticides, are at the top of the list. It is important to develop ties among environmental chemists and engineers who model processes associated with fate and transport. Most models employ empiri- cally derived parameters for the kinds of interactions that accelerate or retard such processes. These models are often poorly developed for lack of a better understanding of molecular-level processes. We need to know how contaminants are hydrologically transported in a medium where they continually interact at the molecular level with DOM and with mineral, biological, and organic phases of soils and sediments. The occurrence and mobility of harmful chemical substances, whether of natural origin or anthropogenic contaminants, in the subsurface environ- ment pose both an intellectual and fundamental scientific challenge and practical concerns for the use and management of groundwater resources. The chemical sciences offer powerful approaches toward understanding and mitigating the problems of groundwater contamination. Society has benefited and will continue to benefit from this important application of chemistry to environmental problems. [Janet Hering, Appendix D] Another area that deserves attention is natural attenuation the diminution of pollutants in soil and groundwater by such natural processes as adsorption, dilution, dispersion, chemical or biological degradation, radioactive decay, and vaporization that take place without human intervention. This work has gained popularity in recent years, mainly because it is being used to justify corporate and governmental decisions. We need to evaluate the process in depth, again at the molecular level. The Earth system may well be capable of remediating itself, but we need to know the time scale and the outcome associated with such an approach. Chemistry plays a monumental role along with biology and environmental engi- neering in developing the crucial experiments to evaluate the fate of individual contaminants. Once the underlying science is well-understood, it may be possible to develop low-cost enhancements or acceleration of the natural processes. i3Enckson, B. E., Environ. Sci. Technol. 2002, 36, 141A-145A.
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30 THE ENVIRONMENT Chemistry has and should continue to play a leading role in developing an understanding of the toxicological effects of our industrialization effects re- sulting from petrochemicals, specialty chemicals, nuclear wastes, natural geo- chemical hazards, and, foremost, from the processing of our drinking water and foods. Developments in analytical methodologies and approaches can signifi- cantly extend our knowledge of the harmful effects of anthropogenic chemicals in the environment. Of specific concern and importance is the molecular-level relationship between bioavailability and the chemical speciation of various chemicals of environmental concern. Currently, there is great interest in under- standing how organisms utilize, in either a beneficial or a deleterious fashion, specific compounds in their surroundings. The specific form of the compound is crucial to understanding whether it is harmful, beneficial, or benign. The flurry of activity in big-inorganic chemistry is central to this issue, but the importance also extends to the speciation of organic compounds, including organometallic compounds. Not only does chemistry play an important role in characterizing the compounds of interest (a challenge for analytical chemists who often must do this at subpicomolar concentrations), but it is central to understanding the processes by which bioavailable forms become incorporated into cellular struc- . . lures In organisms. Gas-to-Particle Conversion and Combustion Aerosol Formation The major processes for creating atmospheric fine particles (diameter < 2.5 ,um) are combustion and gas-to-particle conversion (GPC). Whereas combustion particles are emitted directly to the atmosphere (primary aerosol), gas-to-particle conversion refers to the chemistry that leads to particulate matter by converting volatile gases into condensable substances under atmospheric conditions. Gas-to- particle conversion leads to an increase in the mass of preexisting particles and under some circumstances may lead to the creation of new particles. Particulate material produced by GPC is referred to as secondary aerosol. Understanding GPC entails identifying precursor gases, elucidating the chem- istry that converts them (in the gas phase, on surfaces, or in solution) into con- densable species, and determining the processes by which those species are then converted into particulate matter (e.g., by nucleation, condensation, or direct pro- duction on an existing particle). One can make a distinction between GPC pro- cesses that lead to new particles where gas-phase chemistry produces supersatu- rated conditions followed by nucleation into a single-component or, more likely, a multicomponent condensed phase and processes that chemically age a preexist- ing aerosol by heterogeneous or multiphase reactions and condensation. An understanding of GPC (both nucleation and growth) requires knowledge of which of the possible species participate in nucleation, their concentrations, and their thermodynamic and nucleation properties. For the atmosphere, one needs the concentrations not only of precursor gases but also of the oxidants, ozone,
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CHALLENGES IN ENVIRONMENTAL CHEMICAL SCIENCE 31 hydroxyl radicals, and nitrate radicals that initiate the process. Important gas pre- cursors that have been identified include sulfur species such as sulfur dioxide and dimethyl sulfide, volatile organic compounds (VOCs) such as aromatics in urban areas and monoterpenes in forested regions, and ammonia and nitric acid. Reac- tions of these species lead to low-volatility products such as sulfuric acid, ammo- nium sulfate, ammonium nitrate, and multifunctional organic compounds con- taining acid, carbonyl, hydroxyl, and nitrate groups. Formation of combustion particles also involves nucleation and condensa- tion of vapors, although the processes occur at elevated temperatures inside the combustion source and during cooling of the plume. Like secondary aerosols, combustion particles have a major semivolatile component composed of sulfates from sulfur dioxide oxidation and organic oxidation products, and of unburned fuel and oil as well. Furthermore, they contain a large non-volatile component consisting of soot, metals, and metal oxides. The most important problems involving fine particles involve their potential impacts on global climate and human health. Climate effects can occur through direct scattering and absorption of radiation and by altering cloud radiative prop- erties and lifetimes through the action of particles as cloud condensation nuclei. The mechanisms by which particles impact human health, such as respiratory and cardiovascular function, are not yet fully understood, particularly in relation to particle size. Nevertheless, epidemiologic studies have been sufficiently convinc- ing to result in implementation by the EPA of new air quality standards for fine particulate matter. The properties of particles that determine their impacts on both global cli- mate and human health include number, concentration, size, composition, mass, and surface area. An aerosol may be chemically inhomogeneous from particle to particle, resulting from a mix of processes. Consequently, characterization of in- dividual atmospheric particles, rather than of bulk particulate matter, is most de- sirable for sorting out these details. In addition, there is a need to characterize the chemical structure within particles. Whether individual particles are chemically homogeneous or have organic coatings or inorganic incrustations, for example, will affect their radiative properties, atmospheric removal, heterogeneous reac- tions, and role as nuclei for cloud droplet formation and growth, as well as their health consequences. Although environmental chemists have traditionally flourished in the realm of atmospheric processes, some new discoveries have confounded explanation. For example, scientists have begun to recognize the important role of black car- bon (small particulate matter formed as a by-product in combustion of fossil fu- els) in greenhouse trend reversal. The absorption of sunlight by aerosols contain- ing black carbon can result in simultaneous warming of the atmosphere and cooling of the Earth's surface. However, evaluating the role of black carbon is difficult due to the lack of good measurement methodologies. The evolution of arctic smog during the summer season is an enigma not easily understood, but it
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32 THE ENVIRONMENT is clear that understanding the molecular-level chemistry and photochemistry in aerosols is crucial. The utilization of mass-independent isotopic measurements of atmospheric! hydrospheric, and geologic species has advanced understanding of a wide range of environmental processes. The future development of the utiliza- tion and understanding of this new technique clearly will have numerous applications that should, and will, be advanced. Issues in climate change, health, agriculture, biodiversity, and water quality all may be addressed. Simultaneous with the acquisition of new environmental insight will be the enhancement of the understanding offundamental chemical physics. [Mark Thiemens, Appendix D] Finally, there is an important role that environmental chemists can play in homeland security,~4 especially in areas such as radioactive contamination; de- liberate contamination of water, food, air, and soil; and detection of potentially harmful devices. It is likely that the future of our society depends on our ability to respond to the effects induced by acts of terror. Environmental chemists, employ- ing state-of-the-art analytical tools, can play a leading role in prevention, mitiga- tion, and prediction of harmful effects. Dual-use technologies will have applica- tions in monitoring environmental change, alerting society to homeland security threats, and characterizing dangerous agents. The integration of environmental measurements in a network could also identify when and where something un- usual occurs. Putting It All Together: Understanding Biogeochemical Cycles The real grand challenge is to understand fully the operation of biogeochemi- cal cycles and the implications of human use of chemical feedstocks. Most of today's environmental issues evolved from ignorance or disregard of the fates of the chemical by-products of human endeavors, and a full understanding of the complex interrelationships will be a necessary foundation for resolving the is- sues. This will be a difficult task, because the biogeochemical cycles cross a variety of boundaries. They involve the different scientific disciplines of chemis- try, biology, and geology; they cover water, soil, and air; and they include differ- ent scales from nanometers to kilometers. i4Challenges for the Chemical Sciences in the 21st Century: National Security & Homeland De- fense, National Research Council, The National Academies Press, Washington, D.C., 2002.
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CHALLENGES IN ENVIRONMENTAL CHEMICAL SCIENCE 39 Individual instruments will not be used to solve most environmental prob- lems. It will be necessary to develop suites of instruments that can be cross- calibrated and cross-correlated to show that the same part of the environment is being examined and that the measurements are taking place at the same time. It is likely that in situ and remote sensing will be used concurrently, while the result- ing data are evaluated in a model in real time. For example, impressive and ex- pensive mobile measurement systems were used in evaluating the problem of stratospheric ozone depletion by simultaneously measuring several different radi- cal species (halogen, hydrogen-oxygen, and nitrogen oxide) in real time, with 1- to 5-s resolution.~9 Similarly, in Mexico City a mobile laboratory was used for real-time measurements of fine particle and gas-phase species, looking at emis- sion sources, process studies, mapping, and understanding how one part of the city differs from another both in emissions and the chemical reactions taking place in the atmosphere.20 Additional analytical capabilities needed include the following: hers of samples. Instruments for remote sensing that are robust, portable, and miniaturized. High-throughput instrumentation for sampling and analyzing large num- · Instruments and methods that can measure single-particle composition in real time down to sizes of fresh nuclei (~1 nary). . Instruments and methods that can in near real time characterize more fully the speciated organic composition of secondary and combustion aerosols and that of the gas phase. In conjunction with laboratory studies, one may hope to use these techniques to elucidate the pathways and connect precursor volatile organic compounds to the nature of particulate matter. · Instruments and methods that can provide information on the composition and structure of particle surfaces. These would have to be real-time or near-real- time measurements because surfaces of collected particles would be prone to alteration. · Improved methods for collection of semivolatile compounds. State-of- the-art denuder systems are still prone to adsorption-desorption artifacts. Because of the high costs of particle mass spectrometers, most studies of organics will probably rely on particle collection with off-line analyses by gas chromatogra- phy-mass spectrometry (GC-MS), so reliable collection methods are important. · Instruments and methods for measuring atmospheric particle water con- tent, which is quite difficult because of the volatile nature of water. i9Brune, W. H.; Anderson, I. G.; Chan, K. R .1. Geophys. Res. -1989, 94(DJ4), 16,649-16,663. 20Jayne, J. T.; Leard, D. C.; Zhang, X.; Davidovits, P.; Smith, K. A.; Kolb, C. E.; Worsnop, D. R. Aerosol Sci. Technol. 2000, 33, 49-70.
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40 THE ENVIRONMENT · Instruments and methods for measuring particle pH. In light of recent studies indicating that acid-catalyzed particle-phase polymerization reactions may be important in secondary organic aerosol formation, ambient particle pH must be known to evaluate and model such reactions. · Identification of chemical tracers for the wide variety of organic combus- tion and secondary aerosol sources, which can be used to identify and quantify source contributions to ambient aerosol. For the health effects community, it would especially valuable to have simple indicators of sources, which need not be highly accurate but can be easily measured and used to correlate source contribu- tions with health criteria. Computing Tools and Applications The introduction last year of the Earth Simulator in Japan, which leapfrogged development of high-performance computing in this country, has provoked con- siderable discussion among academic and federal agencies. This computer is a general purpose, vector computer that is applicable to a wide range of problems in science and engineering, and it is more than a hundred times faster than the fastest computer accessible to nondefense scientists in the United States. David Dixon (Appendix D) described a high-performance computer with almost one- third of the computing power of the Earth Simulator that should be readily adapt- able to general environmental applications. With such higher-performance computers, accurate quantum mechanical cal- culations should be possible with heavy elements for which relativistic correc- tions are significant, accurate zero-point energies can be calculated to improve the accuracy of thermodynamic calculations dramatically, and rate equations can be used to search more accurately for transition states and to study interracial reactions (including those at the cellular interfaces of biological systems). The importance of improving accuracy in these basic calculations can hardly be over- estimated. As an example, consider an error of 0.2 kcal/mol in the activation energy to form the water dimer. When propagated during a calculation of nucle- ation phenomena the initial error could be expanded by 40 orders of magnitude, leading to an error of 10~2 in rate constants for nucleation. Progress using this class of computer requires teams of experts in computa- tional science, computational chemistry, applied mathematics, and software engi- neering to adapt and effectively use these tools. A major role for chemists is defining the problems to be tackled with such tools. Modeling tools many of which require development will be essential to traverse all relevant length and time scales, and computation will have to be coupled with measurement to con- firm models and make predictions. With success in this realm, modeling increas- ingly will replace experiments that are too difficult, too dangerous, or too expen- s~ve. Data storage and integration are major challenges for computer science and
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CHALLENGES IN ENVIRONMENTAL CHEMICAL SCIENCE 41 mathematics specialists. These challenges must be addressed before the sophisti- cated instrumentation described above can be employed with optimal effective- ness. Thus, except for demonstration units, few such instruments exist. Connec- tion to sophisticated computer networks and informatics processing are critical requirements for converting raw data into useful information. Modeling, Simulation, and Computational Chemistry What can theory and simulation accomplish? They greatly enhance our un- derstanding of known systems, providing qualitative and quantitative insights that can enable new experiments or new systems. Computational chemistry has the goal of providing quantitative results that can eliminate the need for experi- ments that are too difficult, dangerous, or expensive or can extend into temporal or spatial domains where the necessary experimental tools are not available. Cal- culations have become increasingly reliable, providing valuable methods for solv- ing Newton's laws of motion for molecular dynamics and Schroedinger's equa- tion for electronic motion. Extending these capabilities and solving the nuclear motion problem will be an important challenge in the coming decades. Progress will enable calculation of molecular structures and energetics, reaction equilibria, substitution effects, spectroscopic properties, rates of reactions, and reaction mechanisms. Computational chemistry can play a key role in advancing the scientific enterprise. It can provide the data input for many larger, more complex models and provide us with unique insights into molecular behavior so that we can design and construct new molecules for specific tasks. Computa- tional chemistry has become an established tool in the chemist's toolbox and is being used in broad areas of chemistry to replace experimental mea- surements and to provide us with improved understanding of molecular behavior. Computation will be the major tool that will enable us to cross the many temporal and spatial scales that characterize environmental sci- ence. [David Dixon, Appendix D] When applying computational chemistry to complex environmental prob- lems, major challenges are encountered in scale both in time and in space. This is illustrated in for atmospheric chemistry in Figure 3-1. Current methodologies 2iMany of the relevant issues are covered in one of the other reports in this series, Challenges for the Chemical Sciences in the 21st Century: Information and Communications, National Research Council, The National Academies Press, Washington, D.C., 2003.
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42 1o3km 6- 5- 4- 1 km 3 2- 0 1- `/) 1 m 0- C~) - 1 - -2 - 1 mm -3- -8— 1 nm -9- THE ENVIRONMENT 8- / ~ cop' ;' Regional climate models Particle + gas measurements 1 A -10 1 1 — -12 -1 1 -10 -9 -8 1 ps 1 ns .~v __/ ~e ~ 3 -5 -4 -3 -2 -1 0 1 ms 1s Time Scale 1 2 3 4 5 6 7 8 9 hr day man yr century FIGURE 3-1 Dimensions of integration and the problem of scale as illustrated for atmo- spheric chemical calculation. do not connect the molecular scales with all of the subsequent scales above them. We lack the tools, theories, and methods to make the connections. Increasing the scale of the models will require increased computing power, and advances will rely on collaboration with mathematicians and computer scientists. These prob- lems of scale are analogous to those encountered in a chemical plant, where mod- eling and simulation must span the range from molecular processes and chemical kinetics through process optimization. Applications that range from climate mod- eling to bioinformatics and genomics also will require new developments in the analysis and manipulation of increasingly massive datasets. Many important environmental questions or problems will necessitate com- putational study if they are to be solved. Such problems include . the urban-to-regional migration of nitrate, sulfate, heavy metals, and or- ganic soot into the broader environment and the associated public health issues; · forecasting climate change and testing the forecast in a way that is acceptable to decision makers in science and in public policy; · ultraviolet dosage and its relationship to ozone depletion; · structure-toxicity relationships; · forecasting adaptation to global climate change; · chemical behavior and trends in oceans and estuaries; · molecular origins of toxicity (including gene-toxicant interactions);
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CHALLENGES IN ENVIRONMENTAL CHEMICAL SCIENCE 43 · integration of massive datasets dealing with many variables; · systems modeling of the environment, accounting for coupled transport processes and reactions in air, water, soil, and their interfaces; and · extrapolation of laboratory results over orders of magnitude in length and time. Each of these problems, many of which are linked, must be attacked with both observations and modeling. Consider for example the first three entries in the preceding list. The issue of global warming depends on carbon sources and sinks, and it ties back directly to the questions of nitrate, sulfate, heavy-metal, and organic-soot emissions from regional to global extent. The experimental requirements for addressing the three problems require high spatial resolution of fluxes, isotopes, and reactive intermediates. The details of molecular fluxes- how they interact with the boundary layer and how the boundary links into the free troposphere have not yet been addressed properly. An entirely new level of sophistication not only in experiments but also in modeling will be required for particles, aerosols, and the associated radiation field sets. New mid-IR laser-based instrumentation and use of long-duration bal- loons have helped make major advances in observations. The balloons can sit in the upper stratosphere and then be lowered to the lower stratosphere with power from fuel cells and solar panels. The modeling elements are equally important: it is necessary to test the model and its validity, and the model must link the mea- surements. The observations must be linked to trajectories, the trajectories must be initialized, and sources of specific chemicals must be identified along with the positions of those sources. Considerable progress has been made on observations and refinement of models to help explain low ozone loss at the mid-altitudes, the increase in UV dosage, the appearance of water vapor in the stratosphere, and possibly, of climate changes 50 million years ago. The future in advanced molecular modeling offers the opportunity to solve grand challenge problems in environmental science: · tuncramental advances in theory and computation will radically change the way we do science. Simulation science will become even more multidisci- plinary. Simulation and computation will fully come of age as the third branch of science, fulfilling the promise of the past 20 years. Simulation will be key to coupling multiple temporal and spatial scales while maintaining accuracy. New models will emerge that will completely replace the techniques that have been used so far. For example, new, fast methods will replace 50 years of traditional quantum chemistry approaches and we will have new salvation models. · The challenges will involve quantitative ah initio prediction of molecular- level chemistry of thermodynamics and kinetics with no empirical scaling, bridg- ing the gap from the molecular scale to the microscopic (nano- and biological)
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44 THE ENVIRONMENT length and time scales. The current deficiencies in theory, computers, and soft- ware will require computational chemists to develop radical new approaches. APPROACHES TO SOLUTIONS Pollution Prevention: Green Process Technology Green chemistry focuses on the design, at the molecular level, of manufac- turing processes and products that are environmentally benign reducing or eliminating the use of hazardous materials. A common definition of green chem- istry is "the design, development and implementation of chemical processes and products to reduce or eliminate substances hazardous to human health and the environment."22 Guided by a set of 12 principles,23 green chemistry offers the potential to develop technologies that could provide an important new approach to environmental protection through pollution prevention. We believe that the low viscosity of CO2, coupled with its excellent wetting properties, will enable whole new classes of thin-film coating operations that will at the same time be environmentally responsible. These are likely to be important, not just for microelectronics applications but also for bio- medical and nanotechnologyformulations. Even though there are still many technical and economic barriers to the total acceptance of these technolo- gies, we believe that environmental pressures as well as technical require- ments for pure component systems with high uniformity will over time help "dry" CO2-based processes play an increasingly important role in indus- trial environments. [Ruben Carbonell, Appendix D] As shown in Figure 3-2, green process technologies build on the input from multiple scientific disciplines. Many of the breakthroughs in green chemistry take place at the interfaces among these disciplines. Moreover, the contributions are not limited to the traditional interactions in which chemists, chemical engineers, physicists, and analytical chemists work together to develop a new process. The necessary collaboration may involve biologists, molecular biologists, and com- putational scientists as well. For the chemical industry to thrive in the United States we will need im- 22Anastas, P. T.; Warner, I. Green Chemistry Theory and Practice, Oxford University Press, Ox- ford, UK, 1998. 23Poliakoff, M.; Fitzpatrick, J. M.; Farren, T. R.; Anastas, P. T. Science 2002, 297, 807-810.
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CHALLENGES IN ENVIRONMENTAL CHEMICAL SCIENCE 45 FIGURE 3-2 Sources of green process technologies. Influence diagram showing the in- formation flow from scientific disciplines to green chemistry and engineering and then to green chemistry process technology and then environmentally benign products and pro- cesses. proved efficiency and better ways of meeting regulatory hurdles. Otherwise eco- nomic decisions will be made to move our manufacturing offshore which may not be a desirable outcome. One approach is to invest in better processes, and in many cases, in green process technologies. This is beginning to happen, with examples such as those described in Chapter 2. A recent report from the RAND Science and Technology Policy Institute lists four major barriers to the development and implementation of new green technologies.24 Finding ways to overcome these barriers will be a significant challenge to chemists and chemical engineers as they pursue their R&D agenda in the environmental arena: neenng; · Need for additional research, technology development, or process engi- 24Lempert, R. J.; Norling, P.; Pernin, C.; Resetar, S.; Mahnovski, S. Next Generation Environmen- tal Technologies: Benefits and Barriers, RAND, Arlington, VA, 2003; http://www.rand.org/publica- tions/MR/MR1682/.
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46 THE ENVIRONMENT · Need to surmount infrastructure and integration barriers; · Need to make the up-front investment; and · Regulatory barriers. The RAND report provides detailed analyses for a variety of case studies on next-generation technologies that demonstrate significant contributions by chem- ists and chemical engineers. Examples include · Water purification: development of technologies such as new chemical methods, membrane technology, and ultraviolet irradiation could greatly reduce the quantities of chlorine from present levels; . Liquid and supercritical CO2 as reaction solvent: development of new processes could reduce the use of halogenated and other organic solvents; · Depolymerization of polymers to monomers: conversion of polymers to the corresponding monomers can provide an alternative to recycling of the poly- mer, reduce landfill burden, and provide a new source of monomer with lower consumption of new raw materials; . Biobased processes: the use of renewable feedstocks and biocatalytic pro- cesses reduce waste and greenhouse gas emissions while providing greater en- ^~- . orgy Decency; . New routes to hydrogen peroxide: new methods for direct synthesis of hydrogen peroxide (from hydrogen and oxygen) in a controlled, safe manner could provide a lower cost oxidant that reduces the use of chlorine. For example, in situ generation of hydrogen peroxide can be used to produce propylene oxide in place of the chlorohydrin route; and · Dimethyl carbonate: new methods for synthesis and use of dimethyl car- bonate could greatly reduce the use of highly toxic feedstocks such as phosgene; other waste streams (such as HC1) would be reduced as well. Remediation Soil and groundwater contaminated with hazardous materials create special challenges for chemists and chemical engineers. Determination of the composi- tion and mobility of the contaminants, and the risks they pose to humans and the environment, often requires specialized analytical techniques. In some cases the hazardous nature of the contaminants may be reduced by natural attenuation due to chemical or biological activity in the soil, and a better understanding of the mechanism of attenuation can help to predict or accelerate the rate of hazard reduction. When remediation of the site is deemed necessary, cleanup or contain- ment procedures must be tailored to the specific characteristics of the site. The nation has a contamination legacy that results from practices by both the government and the private sector. Waste chemicals were dumped into trenches and waterways, contaminating hundreds of millions of tons of soil and water. It
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CHALLENGES IN ENVIRONMENTAL CHEMICAL SCIENCE 47 will also be necessary to dispose of the enormous radioisotope burden in tank wastes in the DOE complex. DOE is spending billions of dollars annually on cleanup of contaminated nuclear sites, but current methodologies may not be adequate to complete the task, even over a 50-year time span.25 Extensive re- search efforts in biogeochemistry and reactive transport will be needed, based on the best ideas of geologists, chemists, chemical engineers, and microbiologists. Catalysis and catalytic processes accountfor nearly 20% of the U.S. gross domestic product and nearly 20% of all industrial products. Chemical transformations in industry take a cheap feedstock (usually some type of hydrocarbons and convert it into a higher-value product by rearranging the carbon atoms and adding functional groups to the compound. About 5 quads per year are used in the production of the top 50 chemicals in the United States and catalytic routes account for the production of 30 of these chemicals, consuming 3 quads. Improved catalysts can increase efficiency leading to reduced energy requirements, while increasing product selectiv- ity and concomitantly decreasing wastes and emissions. A process yield improvement of only 10% would save 0.23 quad per year! In addition, pro- duction of the top 50 chemicals leads to almost 21 billion pounds of CO2 emitted to the atmosphere per year. Improved catalysts can help reduce this carbon burden on the atmosphere. As new products become ever more sophisticated, the need to quickly develop new catalysts grows rapidly in importance. A fundamental understanding of chemical transformations is needed to enable scientists to address the grand challenge of the precise control of molecular processes by using catalysts. [David Dixon, Appendix D] Factors that must be considered in developing a remediation strategy include the chemical nature, quantity, and location of the contaminants; the permeability of the soil and how soil interacts with contaminants; and how various cleanup or containment methods may impact workers, the community, and remediation costs. For shallow sites, it may be preferred to remove the contaminated soil and incinerate it or wash it ex situ. For deeper sites with porous soils it may be pos- sible to flush out the contaminants with surfactants or solvents and treat the haz- ardous materials at the surface. If the contaminants are volatile, it may be pos- sible to heat the soil and/or pump air or steam into the soil and capture the vaporized chemicals at the surface. In some cases, treatment chemicals may be 25See D. Dixon in Appendix D.
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48 THE ENVIRONMENT injected into the soil to react with the contaminants and produce a nonhazardous product. In other cases it may be preferable to immobilize the contaminants in situ by injecting a material that tightly binds to them or by heating the soil to form a virtually impermeable glass. In some cases an impermeable cap may be put over the contaminated site to mitigate the problem. Innovative methods are under development to reduce remediation costs and to deal with particularly difficult sites. For example, electro-osmosis is being developed to move and treat contaminants in deep, low-porosity soils. Bio- remediation is being developed to treat soils with microbes, and phytoremediation is being developed to utilize plants to remediate surface contamination. Novel chemical treatments are being developed to convert difficult contaminants, such as chlorinated hydrocarbons, to benign by-products. Permeable reaction barriers are being developed to trap or convert chemicals that pass through the barrier via natural migration or by electric field or pressure gradients. Improved geotextiles, landfill liners, and materials to contain radioactive wastes are being developed to enhance waste containment effectiveness. Many challenges still remain to cost-effectively treat contaminants such as radioactive materials, and inert, tightly bound chemicals such as PCBs. The chemical community will play a major role in developing solutions to these and other complex contamination problems. In some cases as with radioactive ma- terials it may not be feasible to destroy the waste, so long-term storage must be considered as an alternative. For such situations, chemists and chemical engi- neers will play an important role in developing safe and reliable approaches to containment of the waste (e.g., with improved materials for containers or in situ barriers that could limit migration of pollutants). Cost-effectiveness A major challenge to the chemists and chemical engineers in developing solutions to environmental problems is that of cost. Any solution that is proposed for a problem ultimately must be both technically and economically feasible. If cost precludes its implementation, then it is not an actual solution. Regulations sometimes require action that is accompanied by increased cost, but voluntary implementation of major change is unlikely without an accompanying economic advantage. INTERFACES AND INFRASTRUCTURE Many examples of collaborative work were discussed up during the work- shop (Appendix G). These efforts have made substantial contributions to the de- velopment of environmental science and to improvements in the environment. In many ways environmental studies are inherently multidisciplinary as illustrated by Table G-2 in Appendix G. Finding ways to facilitate and enable such cross-
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CHALLENGES IN ENVIRONMENTAL CHEMICAL SCIENCE 49 disciplinary work will constitute a significant challenge, but it is one worth ad- dressing because chemistry and chemical engineering will continue to contribute to fully understanding and solving environmental problems. Workshop participants enumerated a variety of problems that currently in- hibit effective collaborations, ranging from difficulty in communication across disciplines and the need for more cross-disciplinary educational programs to ad- ministrative barriers that inhibit research teams. Interdisciplinary activity in the academic environment would be enhanced by a reward structure that better rec- ognizes the value of such collaborative efforts. Enhancements in the infrastructure for education and research will be essen- tial to future environmental progress. There was considerable sentiment among workshop participants that the current disciplinary structure of academic depart- ments and funding agencies inhibits advances in the environmental arena. Com- munication and collaboration among federal funding agencies will also contrib- ute to future progress. Research investment will need to address interdisciplinary activities, the need for development of new instruments, improved computational capabilities, and shared user facilities that may be too expensive for individual institutions.
Representative terms from entire chapter: