Keeping up with the demand for energy has a downside: energy development, production, and use release pollutants that foul the environment. At ORNL we take an interdisciplinary approach to understanding and solving major environmental problems. Our teams have experts in biogeochemistry, environmental biotechnology, global environmental chemistry, ecosystem studies, geosciences, hydrology, social sciences, and environmental assessment. We also develop instruments for the sensitive detection and monitoring of radiation and chemicals.
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| One of our goals is to improve understanding of environmental processes, such as the ecological effects of elevated levels of atmospheric carbon dioxide and climate-related increases in temperature, precipitation, and times of drought. Another goal is to develop a scientific basis for implementing environmental technologies (monitoring, mitigation, and remediation). For example, we have developed two approaches to removing radioactive cesium-137 from highly radioactive waste to reduce waste disposal costs. |
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| Much of our research is possible only because of the existence of special natural areas at ORNL such as Walker Branch Watershed and the Oak Ridge National Environmental Research Park, a user facility. Other research makes use of the facilities of the Bioprocessing Research and Development Center as well as resources of the Center for Biotechnology and the Center for Global Environmental Studies. Through these resources, we are trying to keep up with the demand for new information and technologies to help preserve a clean environment. |
Advanced Liquid-free
Bioprocessing Technology
Brian Davison (left) and Chris C. Gable III (a student at the University of Tennessee at Knoxville) discuss ORNL’s new bioprocess that introduces reactants and recovers products in gaseous form. The process uses an immobilized enzyme to speed up the product-yielding reaction. Photograph by Tom Cerniglio.
For every kilogram of chemical produced in the United States, some 10 to 100 kilograms of waste by-products are generated. These by-products consist mostly of water, but this waste liquid requires treatment using energy-consuming processes before it can be released to our waterways—at a high cost to industry and ultimately the consumer.
Synthesis of chemicals may require several steps; furthermore, each step may use a different solvent and reaction vessel. Significant waste is generated from one reaction step to the next as solvents are switched (e.g., from hexane to water), and more waste is produced when the product is separated from the final solution.
Over the years, ORNL researchers have developed bioprocessing methods for making products in liquids (water), using various types of bioreactors. Traditionally, chemicals in a liquid, phase are transformed to useful products by microorganisms that are attached to support material through which the liquid passes.
Now, ORNL researchers have devised a revolutionary approach to bioprocessing that eliminates the use of liquids (including solvents and diluents), greatly reducing aqueous wastes from chemical production in bioreactors. The technology could save money because it uses less energy for waste treatment and eases the separation and recovery of products. It also may allow new enzymatic products to be obtained.
The approach both introduces reactants and recovers products in gaseous form. The process uses an immobilized enzyme (a protein extracted from living cells) to speed up the reaction that yields the product. Because all reactants are fast-moving gas molecules, they intermix with each other and the enzyme more readily than do liquid molecules. The recovery of products is also easier from the gas phase rather than from the dilute aqueous solution—typical of a traditional fermentation.
Just below or at its boiling point, a chemical compound can volatilize, producing a vapor in a gas stream. In the ORNL process, vapors of the starting material are pumped into a vertical column containing glass wool (like a tube filled with porous insulation) to which an enzyme is attached. The vapors diffuse through the column, where they come into contact with the enzyme. The enzyme catalyzes the reaction between the gaseous compounds, producing a gaseous product that flows out of the other end of the column, where it is collected. In one proof-of-principle experiment, ORNL chemical engineers used lipase, an enzyme extracted from a pig’s pancreas, and vapors of ethyl acetate and isoamyl alcohol as the reactants. The products were two gaseous species that later condensed into liquids—ethanol (alcohol) and isoamyl acetate (an aromatic ester known as banana oil because of its banana fragrance).
ORNL researchers hope to show that this new bioprocessing technology can be used for many different esterifications, chemical reactions that produce esters. An ester is an organic compound formed from an organic acid and an alcohol.
They have also shown that the new process can be used for oxidation reactions, which may have applications in waste treatment. In preliminary experiments, an enzyme extracted from mushrooms, polyphenoloxidase, converted phenol vapors reacting with oxygen in the air into quinone, an aromatic compound used in making dyes, tanning hides, and producing photographs.
The ORNL process is limited currently to compounds that are volatile at 20 to 50°C. Higher temperatures and faster production rates may be possible as science harnesses enzymes that can tolerate higher temperatures and more extreme operating conditions (such as enzymes from thermophilic bacteria in hot vents under the sea). Then a wider range of starting materials could be used to make a wider range of products even more easily and more cheaply.
The research was supported by DOE’s Office of Industrial Technologies, Division of Biological and Chemical Technology Research.
Inverse Electrospraying May
Cleanse Polluted Water
Inverse electrospraying mixes liquids well, as shown in the images of a tracer dye in water. Top image: time = 0 - ; voltage = 0 V. Middle image: time = 0 + ; voltage = 4 kV. Bottom image: time = 1 second; voltage = 4kV.
Electrostatic spraying is used to paint cars and deposit herbicides on fruit trees. Such “electrospraying” is efficient because large drops are broken into a fine mist, providing a more uniform coating. In addition, the particles are attracted electrostatically to a grounded body, such as a car or tree.
Basically, electrospraying involves spraying an electrically conductive fluid into a nonconductive fluid, like water into air. It was once thought impossible to reverse this process and spray a nonconductive fluid into an electrically conductive fluid, like air into water. But ORNL researchers have proved that “inverse electrospraying” is possible; that it can be explained; and that it makes possible a lighter, simpler, more energy-efficient device for speeding up chemical reactions and cleansing polluted water.
Two years ago, our researchers set up an apparatus consisting of a pump, a glass column, and two electrodes immersed in water in the column. In this configuration, one electrode is at the top and the other is a vertical capillary electrode at the bottom of the water column through which another fluid (such as air) is introduced. After pumping air into the water column, the researchers increased the voltage from 0 to 4000 volts, creating a nonuniform electric field. After several experiments, they observed that the electric field actually pumped the air deep into the water, obviating the need for a mechanical pump. The explanation? Charge carriers are attracted to the oppositely charged electric field, causing the fluid to flow. This phenomenon, called electrohydrodynamic convection, creates a low pressure near the capillary tip, thus causing pumping.
As air bubbles passed through the nonuniform electric field, they were shattered into micron-sized bubbles. Thus, the electric field induced simultaneous pumping and spraying of air, forming microbubbles.
The liquid in this inverse electrospraying column appears red because of the pumping and spraying of dyed red kerosene microdroplets into water.
Inverse electrospraying was also used to pump and spray organic liquids, such as kerosene, into water. Our researchers found that this technique is very effective at mixing liquids. In this case, they used color tracers and fluorescent particles that would glow when laser light was shone on them. By making the flow visible, they observed excellent mixing of kerosene and water.
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| Images show the apparatus, with an inset providing a closeup of the electrified capillary through which a nonconducting fluid enters a conducting fluid in a water column. Here, after a voltage of 4 kV is applied, a spray of bubbles is observed in the column. The bubbles come from the ambient air that is simultaneously pumped and sprayed into the water. Photograph by Curtis Boles. |
Because inverse electrospraying mixes liquids well, the process could replace mechanical agitators, which are conventionally used for liquid mixing. Mechanical agitators convert electrical energy into mechanical energy, but inverse electrospraying uses electricity directly and thus is more energy efficient. In addition, a device based on inverse electrospraying would be simpler, lighter, and probably less expensive to build and operate than conventional mixing systems because it would require no mechanical devices such as pumps and agitators. Also, because it would have no moving parts, it would cause no vibrations.
Inverse electrospraying shows promise for removing organic chemicals and toxic metals from water. The Electric Power Research Institute, the research arm of the U.S. electric utility industry, is sponsoring an ORNL project to use inverse electrospraying to introduce ozone into water contaminated with toxic organic compounds such as polychlorinated biphenyls (PCBs), pesticides, and chemicals containing benzene. Ozone breaks these compounds down into simpler compounds that can be removed by other chemical or biological treatments. In our approach, ozone would be rapidly dispersed through the water in the form of extremely small bubbles. Because the ozone would be carried more quickly into the water, it would make contact with and break down organic contaminants more rapidly than would conventional methods of ozonation.
Tiny bubbles are the key to removing trace amounts of heavy metals from water through a flotation method of separation. Inverse electrospraying can make microbubbles that collide with and attach to particles of heavy metals such as chromium, copper, nickel, and zinc. Because the microbubbles are buoyant, they carry the metal particles to an interface where they can be removed.
Inverse electrospraying may also have other applications. It could be used to make special materials that can be formed only from very small particles, such as ceramic precursors. It may be used to provide oxygen to aerobic bacteria that are introduced into oil by normal electrospraying to remove sulfur from the oil—a process being developed by ORNL in collaboration with several companies, including Texaco, Exxon, Chevron, and Energy Biosystems.
As information on inverse electrospraying becomes more widely dispersed, ideas on how to apply it should continue to flow.
The work was sponsored by DOE’s Environmental Management Science Program and Office of Energy Research, Basic Energy Sciences, Division of Chemical Sciences. The ozonation research is supported by the Electric Power Research Institute.
ORNL a Leader in Global Change
Hardwood forests on DOE’s Oak Ridge Reservation don’t get equal treatment. Thanks to ORNL’s unique facilities for performing some of the world’s largest ecological manipulations, white and chestnut oaks, red maples, and yellow poplar trees are exposed to more (or less) rain, higher levels of carbon dioxide (CO2) or higher temperatures than other trees in the natural forest environment. Combined with our computer modeling capabilities for understanding biological processes such as foliar photosynthesis, tissue respiration, growth, nutrient cycling, and decomposition, our “user facilities” are attracting many outside scientists to help us study the impacts of global climate change on forest ecosystems. ORNL is unequaled as a national resource for gaining a better understanding of how future climates can affect southern forests in the United States.
and Forest Studies
In the throughfall displacement experiment conducted in the Walker Branch Watershed on the Oak Ridge Reservation, troughs intercept a fraction of the normal precipitation in one area and divert it passively through gravity flow to another plot. As a result, one-third of the trees in the experiment receive normal rainfall, another third get one-third less rain and snow, and the final third receive one-third greater precipitation.
After three-and a-half years, oak, maple, gum, sourwood, and yellow poplar trees were not noticeably affected by the manipulated soil moisture levels. However, during the 1995 drought, 20% of the dogwood trees in this forest died on the dry plot and only 3% died on the wet plot. This finding suggests that, in a consistently drier future climate, the presence of dogwoods in southern forests would be dramatically reduced.
Another key conclusion of the throughfall displacement experiment is that daily to weekly rainfall patterns during the growing season are far more important than fluctuations in total annual precipitation for understanding the ecological impacts of moisture variations. Computer modeling experts attempting to assess the impacts of climate change on forest ecosystems must recognize that no rain in June when trees are actively growing fast is more significant than no rain in late summer when trees hardly grow at all. We continue to study the implications of dormant season changes in rainfall to identify impacts on forest nutrient cycling that might produce long-term change in forest productivity and composition.
In our open-topped chamber studies of tree growth responses to elevated CO2, we found little change in yellow poplar saplings but increased leaf area and more roots in white oak. A dozen similar experiments across the nation had widely different results. ORNL developed an analytical approach called the “growth per unit leaf area index” (the mass in grams of wood produced each year divided by leaf area in square meters) that shows the responses across different species and site conditions to be basically uniform. This approach enables extrapolation of individual tree species responses to predict the response of a whole mixed forest to elevated CO2.
We found that the growth of maple trees was reduced in chambers where the temperature was kept 4°C higher than ambient temperatures and that sugar maples (found no farther south than Chattanooga) were more severely affected than red maples (found as far south as the Gulf coast). In both species, the negative growth effects of temperature were offset by the positive growth effects of elevated CO2. This finding suggests that, for predicting tree growth in the future, the overall effect of both increased atmospheric CO2 concentrations and global warming must be considered.
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| This aerial view shows the towers and vent pipes of ORNL’s novel Free Air CO2 Enrichment system for introducing additional amounts of carbon dioxide to a small sweetgum tree stand on the Oak Ridge Reservation. If the trees soak up the additional CO2, the effects may include changes in the stand’s biological processes and soil composition. Photograph by Curtis Boles. |
ORNL researchers seek to determine if forests could slow down the onset of global warming by soaking up additional CO2 concentrations injected into the air by industrial activities. Some of this extra carbon from the air could end up in the soil following the decomposition of leaf litter and roots. Such a “soil carbon signal” may be seen in 10 years as a result of a new ORNL manipulation of a small sweetgum tree stand using the novel Free Air CO2 Enrichment (FACE) technology. We have characterized this site and are now studying changes in biological processes and soil composition as FACE vent pipes suspended from a tower blow out extra CO2 for the wind to carry to the trees. Eventually, we may be able to predict reliably whether the world’s forests are equal to the task of removing some of the excess atmospheric CO2 attributed to human activities.
The research was funded by DOE’s Office of Health and Environmental Research, the National Science Foundation, and ORNL’s Laboratory Directed Research and Development Program.
ORNL Process May Help Treat
Imagine a bottle of oil and vinegar in which the oil floats on a “vinegar” containing dissolved contaminants. Dissolved in the oil are special molecules that act like Pac-Man. When the oil and vinegar are mixed temporarily by vigorous shaking of the bottle, the Pac-Man molecules snatch the contaminant molecules from the drops of the vinegar and transfer them to the oil drops for final removal. A similar “solvent extraction” process is the basis of a practical system being designed by ORNL for the 55 million gallons of radioactive waste in the Department of Energy’s 177 tanks (67 of which are leaking) in Hanford, Washington.
Hanford Tank Waste
DOE is committed to cleaning up these tanks at the lowest possible cost. The waste will be vitrified as low-level glass logs that are safe enough to store on the Hanford site and as high-level glass logs that must be permanently isolated in a geologic repository.
The Hanford tank wastes contain fission products from reprocessing spent reactor fuel to recover plutonium for nuclear weapons production that started during World War II. The fission products are in an alkaline solution of sodium nitrate formed by adding sodium hydroxide to the nitric acid used for reprocessing. Although this treatment made the waste less corrosive to the storage tanks, it also created a need for new separations technologies for ultimate cleanup. The fission products targeted by ORNL’s separation process are long-lived, mobile technetium-99 and highly radioactive strontium-90 and cesium-137, both of which have half-lives of about 30 years. Removal and concentration of these fission products would minimize the volume of high-level waste that must be vitrified, resulting in tremendous cost savings.
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| This Pac-Manlike crown ether molecule captures a radioactive cesium ion (yellow ball) from liquid waste because its cavity was sculpted to the right size and shape to trap this particular ion. Images by Jeff C. Bryan. |
In the proposed process, the fission products would be extracted from the alkaline sodium nitrate waste solution into the “oil” phase—kerosene containing the extractants. The Pac-Man-like molecules in the oil are “crown ethers”—large, cyclic, designer molecules that each have a cavity sculpted to the right size and shape in which to trap a specific ion. A commercially available crown ether is used in the ORNL process to snatch strontium-90 ions or positively charged sodium ions that attract (and extract) technetium in the form of the negatively charged pertechnetate ion.
To seize cesium from the waste stream, ORNL researchers focused on a new, highly selective macrocycle whose basic framework was designed in France. Because the Hanford tanks contain a million sodium ions for every cesium ion, the French molecule was appealing because it can recognize and grab one cesium “needle” in every haystack of sodium ions. Although this molecule has the high selectivity needed, it still could not function adequately in a solvent extraction process adaptable for Hanford tank-waste separations. To overcome this problem, the ORNL researchers modified both the crown ether and the solvent system to develop an effective solvent extraction process for removing cesium from radioactive waste. They also took advantage of the high concentrations of sodium and nitrate ions, which drive the solvent extraction reactions.
The process also uses water mixed with a little nitric acid to strip the fission products from the kerosene solvent. As a result, the solvent can be recycled many times and the fission products can ultimately be encapsulated in high-level glass canisters.
Technetium removal has been successfully tested on a batch scale on Hanford waste and waste from ORNL’s own Melton Valley Storage Tanks. Tests on cesium removal from Hanford waste are being conducted in 1997. ORNL’s goal is to develop a combined solvent extraction process using centrifugal contactors in which two crown ethers remove and concentrate the three fission products simultaneously. Such a process should prove attractive to Hanford and DOE because it leaves the waste otherwise unchanged, produces little secondary waste, provides a concentrated stream of fission products for vitrification, and is compact, cheap, safe, reliable, and fast.
The research is sponsored by DOE’s Office of Environmental Management, Office of Science and Technology, Efficient Separations and Processing Crosscutting Program.
Cesium Removal Demonstrated at ORNL
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| This aerial view shows six Melton Valley Storage Tanks (below crane, which points to ORNL’s Hydrofracture Facility). Photograph by Curtis Boles. |
Radioactive cesium-137 is a really bad actor in ORNL’s mixed waste. Because the isotope emits the bulk of the waste’s radioactivity as high-energy gamma rays, the waste must be shielded in lead or concrete. Currently, most of the cesium-containing liquid wastes are generated by ORNL’s Radiochemical Engineering and Development Center and are concentrated in the Bethel Valley Evaporator, which also handles all liquid radioactive waste from other past and present operations. The liquid mixed waste is stored on site in eight stainless-steel Melton Valley Storage Tanks in an underground concrete vault.
Other bad actors in ORNL’s waste are toxic metals that must be disposed of as hazardous waste according to the Resource Conservation and Recovery Act (RCRA). These metals include arsenic, barium, cadmium, chromium, mercury, lead, nickel, selenium, silver, and thallium. Because some of the ORNL radioactive liquid waste contains some of the RCRA metals at levels above legal limits, they are also classified as mixed waste—hazardous waste mixed with radioactive waste.
If cesium-137 could be removed from ORNL’s mixed waste, the remaining compounds in a solution of sodium and potassium nitrate could be combined with grout, solidified as concrete monoliths, and disposed of on site as low-level waste. Such separations would reduce ORNL’s waste disposal costs.
To help solve this problem, ORNL researchers have developed a method to remove both cesium-137 and some of the RCRA metals from the mixed waste, leaving low-level waste behind for disposal at a reduced cost. They have developed and tested a laboratory-scale ion-exchange continuous-flow system, scaled it up, and proved it works in the Cesium Removal Demonstration Project. The key to the system’s success is a commercial preparation of crystalline silicotitanate (CST) manufactured by UOP’s Molecular Sieves Division under a DOE program also involving Texas A&M University and Sandia National Laboratories. ORNL researchers, who had tested eight different cesium sorbents, were the first to show in a continuous hot-cell operation using real waste liquid that CST can effectively remove cesium-137 from radioactive liquid waste.
In the ion-exchange process, the liquid waste flows down a column through CST crystals, which contain sodium ions as a result of the manufacturing process. CST takes up and holds both cesium and some of the RCRA metals while releasing the sodium ions to the liquid waste passing through. CST prefers cesium and some of the other metals to sodium because they fit better in CST’s crystal structure. The final waste-loaded CST product is a batch of dried granular material resembling white sand. A full-scale demonstration of the CST successfully processed 15% of the ORNL waste inventory. The first batch loaded with radioactive cesium from the Melton Valley Storage Tanks was sent to DOE’s Savannah River Plant to be vitrified into glass logs. The remaining batches are being stored on site until they can be shipped to DOE’s permanent disposal site (probably the Nevada Test Site).
Waste-loaded CSTs have passed several toxicity characteristic leaching procedure (TCLP) tests required by the Environmental Protection Agency. When the leach testing solution was contacted with the CST test batches, none of the RCRA metals were leached out at levels above the leach testing limits. As long as TCLP tests verify that the toxic metals are permanently trapped in each CST batch, DOE will accept it for long-term geologic isolation (unless it contains transuranic elements, in which case its destiny is likely to be DOE’s Waste Isolation Pilot Plant in New Mexico).
The CST process is also being considered for cesium removal at DOE’s Hanford, Idaho, and Savannah River sites. It has been estimated that the CST process would save Hanford $500 million in reduced waste disposal costs. CST also takes up strontium-90 (an emitter of beta radiation), but it’s not the ideal sorbent for the task. Research is under way to find a better strontium sorbent to effectively remove another bad actor from DOE waste.
The research was sponsored by DOE’s Office of Waste Management and Office of Science and Technology.
Novel Tracers Map ORNL’s Underground
Silicon beads in groundwater collected from monitoring wells emit a green and yellow glow under ultraviolet light, indicating that they have been tagged with DNA fragments. The injection well from which each bead started through the groundwater system can be determined by identifying the specific DNA sequence with which each bead is tagged.
An environmental researcher uncaps an injection well on the Oak Ridge Reservation and pours in a container of what appears to be plain water. The clear solution actually contains trillions of microscopic silicon beads tentacled with DNA fragments. As stand-ins for different radioactive contaminants, their job is to show how contaminants in groundwater move through a subterranean maze of broken rock and soil.
As they drift down a groundwater current, some of the minuscule beads lodge against fracture walls or become trapped in dead-end fissures. The survivors seep calmly along until they near an intersecting fracture; the current accelerates, and soon they’re navigating whitewater: a veritable tidal wave surges through the junction, shooting half the spheres upward into an ascending stream. There they careen into another fleet of beads flying their own DNA flags. This fracture soon empties into yet another, and spheres from both fleets finally sweep into a monitoring well to be taken into custody for analysis.
Those flag-flying silicon vessels represent a major advance in tracing the movement of radioactive contaminants through the reservation’s tortuous underground environment. One of several novel types of tracers developed at ORNL, they hold great promise for aiding DOE’s environmental restoration efforts, as well as for guarding against pollution of water supplies in other settings.
Tracers are “surrogate contaminants,” harmless substances that mimic the behavior and physical and chemical makeup of specific toxic counterparts, such as radioactive waste or hazardous chemicals. They can indicate the origin and movement of contaminants by showing which sources do—and which sources don’t—feed into a contaminated area.
The DNA tracers are silicon hydroxide beads, a fraction of a micron in diameter, tagged with fragments, each containing 20 pieces of the DNA constituents A, C, T, and G. The number of DNA tags available to label tracers is thus 420, more than a trillion. Therein lies a key strength of the technology: It allows production of virtually unlimited numbers of tracers that behave identically but that are easily distinguished because each DNA tag naturally pairs with complementary DNA fragments of a known sequence. Until the DNA tracers were developed, there were not enough different tracers to map all the intersecting groundwater pathways underlying the Reservation that various radioisotopes could take.
Groundwater under the reservation traverses fractured bedrock that transports it rapidly and porous shale that absorbs and then releases it slowly. Think of the subsurface as a huge sponge with a web of cracks. Water runs through the cracks quickly, perhaps hundreds of meters per day, but seeps slowly through the sponge (matrix) itself, a couple meters per year at best. Travel time depends on the type of tracer and its interaction with the matrix. So several types of tracers are used to map the same area: reactive tracers mimic contaminants that attach onto matrix material; diffusive tracers mimic those that diffuse into the matrix; and colloidal tracers (the silicon beads) mimic those that travel the fractures rapidly because they are too big to diffuse.
In the case of our groundwater-surfing beads, the time it takes a tracer to migrate from the particular injection point to the monitoring well indicates how fast the contaminants it mimics would move through that area. When tracers introduced into different fractures in a zone turn up in the same monitoring well, researchers know that those fractures connect underground and that their contents, including contaminants, mingle. Other types of tracers indicate to what extent the radioactive counterparts of the tracers will attach to soil or diffuse into the shale and leach out slowly over years.
In addition to the DNA tags, ORNL uses other novel tracers, such as ice- nucleating bacteria, nonreactive gases, and rare-earth elements to map, model, and measure groundwater transport of radioactive contaminants. Their use has led to a new understanding of the direction and speed of radionuclide transport and the effectiveness of remediation methods such as pumping and treating groundwater.
The ORNL team is working on refinements to make the new tracers sufficiently economical for use in small-budget projects. The tracers promise to be valuable for detecting contaminant pathways in many settings. For example, tracers introduced at various points in a contaminated drainage system could pinpoint from whence a pollutant is coming. Environmental researchers now have an armada of informants to send down into the dark to shed light on subterranean waterways.
The research was sponsored by ORNL’s Laboratory Directed Research and Development Program, ORNL’s Groundwater Program Office, and DOE’s Office of Technology Development and Office of Science and Technology.
