SCIENCE/TECHNOLOGY
Microbially Based Treatment Process Removes Toxic Metals, Radionuclides • Anaerobic bacteria and citric acid used to stabilize, precipitate, and reduce mass of wastes in soil, sediment, sludge icrobiologists at Brookhaven National Laboratory, Upton, N.Y., have developed a comprehensive process for the removal of metals and radionuclides from contaminated soils, sediments, and sludges. A new twist in hazardous-waste treatment, the process uses microbes to stabilize and reduce the mass of wastes. And according to A. Joseph Francis, microbiology group leader at Brookhaven's department of applied science, the process has significant commercial potential because of its wide applicability and because it doesn't generate the secondary waste streams usually found in existing processes. The process depends on citric acid, explains Francis. A naturally occurring complexing agent, citric acid is used to extract toxic metals such as cadmium, nickel, lead, and zinc and the radionuclides of elements such as cobalt, strontium, thorium, and uranium from solid wastes. The citric acid forms water-soluble metal-citrate complexes and may involve, among others, bidentate, tridentate, binuclear, or polynuclear complex species. The extract containing the complexes is subjected to bacterial and/or photochemical degradation. The citric acid is degraded to environmentally acceptable compounds. The metals and radionuclides are precipitated from solution and eventually may be recovered in concentrated form. Uranium, however, forms a binuclear complex with citric acid and is not biodegraded. With exposure to light, though, the uranium rapidly degrades to an insoluble, stable polymeric form of the element.
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The citric acid approach causes very little environmental damage to soil, thus treated soil can eventually be returned to normal use. In addition, Francis says, the process provides a means for recovering important metals in a concentrated form for reuse and recycling. This, he adds, can be done economically and in a way that is environmentally safe. The solutions containing radionuclides and toxic metal wastes may be treated with anaerobic bacteria such as Clostridia to release a large fraction of the waste solids into solution and to con-
Process has commercial potential because it is widely applicable and doesn't produce secondary waste streams
vert the radionuclides and toxic metals to a more concentrated form. At the same time there is a corresponding reduction in the mass and volume of the toxic and radioactive materials. Francis says that in this new approach the unique capabilities of anaerobic bacteria solubilize and /or precipitate radionuclides. The nonhazardous materials in the solid phase are easily removed. The remobilized radionuclides and toxic metals are stabilized by precipitation reactions and redistributed with the stable mineralogical fractions of the waste. Previously, the presence of naturally occurring chelating agents in the treatment of nuclear and toxic metal wastes caused concern because of the potential for mobile ions to migrate from the waste site. Citric acid was one of the principal chelating agents. But in this
process, citrate ions form stable complexes with many toxic metals and radionuclides. The biodegradation of these complexes, precipitating metal ions as insoluble hydroxides, oxides, or other salts, should hinder migration. In very recent work, Francis has studied the biodegradation of the citrate complexes of iron(III), iron(II), cadmium, copper, nickel, lead, and uranium. Several of these complexes were not readily biodegraded by bacteria. This was not because of any toxicity to the bacteria but because of the chemical nature of the complex. However, the ability of the anaerobic bacteria to immobilize most of the hazardous materials is a new step in environmental bioremediation. The potential of biological organisms in treating very hazardous wastes has been known for some time. However, the knowledge has been rather fractionated and only recently has an organized screening of candidate organisms been undertaken. The dissolution of metals and radionuclides by autotrophs under aerobic conditions is an example; most of the data come from experience in ore leaching. These microbial processes are being exploited commercially to extract copper and uranium from ores and to recover certain metals from wastes. In general, the solubilization for the leaching of metals from ores occurs as a result of the activities of iron and sulfuroxidizing bacteria, such as Thiobacillus ferrooxidans and T. thiooxidans. The inorganic components in coal and nuclear wastes may also undergo chemical and biochemical reactions that solubilize minerals. Direct bacterial attack involves an enzymatic attack on the components of the mineral that are susceptible to oxidation. In the case of indirect leaching, it is the oxidation of soluble ferrous iron to soluble ferric iron, an oxidizing agent that reacts with other metals and transforms them into soluble, oxidized forms in sulfuric acid solution. The treatment of coal wastes has become a major problem in many places, JULY 27, 1992 C&EN 3 5
SCIENCE/TECHNOLOGY
Technique identifies bacteria that degrade wastes b the project. "Once the automatic Researchers at Oak Ridge NaI plate reader indicates which [bactional Laboratory (ORNL) have I teria] samples are most effective devised a method to quickly -$ for that particular contaminant, identify bacteria that will be | we can study them in more demost effective at metabolizing | tail, and we can eliminate the particular wastes in soil and waf bacteria that aren't effective." ter. * According to Palumbo, the According to ORNL, tests have 5 technique has made it possible to shown that the laboratory's rapid g screen about 400 isolates in one screening process is up to 10 I week. Current methods allow times more efficient than current 1 screening of only about 40 or 50 methods used to identify bacteria £ isolates a week. that degrade volatile contaminants such as trichloroethylene. | Another important aspect of Studies of such bacteria are con°" the screening process, Palumbo sidered increasingly important as says, is isolate enrichment, in efforts to restore and clean up which bacteria are grown in a sohazardous waste sites continue to lution containing soil, for examexpand. The new method arose ple. "This process allows the from a need by ORNL to screen good bacteria to better reveal some 5000 bacteria samples subthemselves," he says. "We can inmitted to the lab through the DeTechnician inoculates bacteria into a plate troduce a specific contaminant partment of Energy's subsurface into the soil sample and then see that holds 96 cultures science program. which bacteria prove to be the best at metabolizing it." The new technique employs a According to Janet Strong-Gunderrectangular plastic plate about the bacteria are capable of metabolizing size of a standard index card. The the contaminant, the dye is activated son, a postdoctoral student working plate is lined with rows of half-inch- and becomes visible in graduated with Palumbo at ORNL, the fact that deep cavities, or "wells," into which shades of purple, depending on the the bacteria being studied occur natrate of metabolism. The plate is then urally in soil is a critical advantage. samples of bacteria are inoculated. The plate is then placed in a sealed put into a device that scans the wells "Because these microbes come from chamber along with a contamination and produces a numerical indication the ground and are not genetically engineered," she says, "they can be source, such as a container of chloro- of the level of color change. "The darker, the better," says Tony placed directly on a contaminated form. The contaminant is allowed to vaporize and enter the wells. The Palumbo, an ORNL microbial ecolo- area or in the groundwater." wells contain a special dye: If the gist who is principal researcher on James Krieger
not only in mining regions but also where large quantities of coal are used to generate power. The more common types of coal waste have large amounts of pyrites and low levels of metals and residual carbon. However, sometimes the reverse characteristics are present. In the more common case, natural autotrophic bacteria under aerobic conditions solubilize various amounts of contaminants such as arsenic, chromium, manganese, copper, nickel, lead, and zinc from the filter cakes of tailing fines. The extraction of these contaminants is aided by the addition of nitrogen and phosphorus nutrients for the microorganisms. The predominant mechanism for the dissolution of metals from coal wastes under aerobic conditions is bacterial oxidation of the pyrite. Anaerobically, some bacteria can oxidize sulfur with the coupled reduction of ferric to ferrous iron as an alternative to the reduction of oxygen. No anaerobic 36
JULY 27,1992 C&EN
iron-oxidizing bacteria have yet been found. Use of biological processes in the geochemistry of uranium has been studied extensively, and recovery of uranium from ores is feasible and economical. T. ferrooxidans plays a primary role in this chemistry. This bacillus can directly reduce compounds of uranium to its hexavalent form without using ferrousferric complexes as electron carriers. A particularly worrisome contaminant in many wastes is lead. Lead may be present in nuclear and coal wastes and is released to the atmosphere from fossil fuel combustion and smelters. The particular lead species formed is usually not known but is believed to be an oxidized form, possibly lead oxide. The species is quite insoluble in water. The life cycle of lead in wastes is not fully understood, but it appears to be highly dependent on local conditions. In one study, conducted at Brook-
haven, Clostridia found in coal-cleaning wastes solubilized a significant amount of lead oxide and some lead sulfate but not elemental lead, lead sulfide, or galena. Dissolution of the lead oxide was the result of the production of organic acids and a lowering of the pH in the growth medium. The solubilized metal was available to the organism at the biological level. The use of microbial processes to decontaminate solid wastes has not been fully exploited, in large measure because the very complex interactions between the organic, inorganic, and radiochemical species and their further interactions with a great variety of microbes are not comprehensively understood. The work at Brookhaven suggests that the understanding is improving and that microbiological and biochemical processes for decontamination will be increasingly useful in the future. Joseph Haggin
