The presence of mercury in the environment is a persistent and increasing problem. Mercury is widely used in industrial applications because it conducts electricity, is liquid at room temperature, can be alloyed with almost all common metals, and is easily vaporized or frozen. As a result, mercury finds its way into the manufacture of many products including batteries, paints, and electrical switches, and is used as a catalyst in producing urethane and polyvinyl chloride.
Major man-made sources of mercury pollution include incinerators, fossil fuel plants, and municipal sewage systems. Natural sources include volcanoes, forest fires, and dust. The EPA estimated that in 1989 approximately 643,000 kg of mercury was discarded as municipal solid waste, with 84% of it landfilled. Household batteries were the single largest source.
The EPA recently released the eight-volume
Report to Congress on Mercury
as part of the requirements of the Clean Air Act Amendments of 1990. For human health, the report considered three forms of mercury--elemental, inorganic, and methylmercury--and found that the toxic effects of mercury depend on the chemical form. For example, inhaled mercury vapor--mainly a hazard to workers exposed on the job (manufacturing fluorescent lights, batteries, and latex paint, for example)--damages the nervous system. Inorganic mercury compounds are not as highly toxic because they are not well adsorbed and do not easily penetrate the blood-brain or placental barriers.
Organic mercury (for example, methylmercury) in fish and seafood is a common and very toxic form of exposure. Through the process of methylation, bacteria in water convert inorganic mercury to the organic form, which is then concentrated in the marine food chain.
Methylmercury is particularly toxic to the developing fetus and small children. Since the 1950s, methylmercury poisoning has attracted attention because of neurological damage among the Japanese resulting from eating seafood from Minamata Bay and, later, Niigata's Agano River that was contaminated with mercury-laden industrial wastes. Similarly, serious cases of poisoning occurred in rural Iraq as a result of eating grain treated with methylmercury-based pesticides.
Mercury may be toxic only at high doses. The EPA report data also prompted a moderate degree of concern that inorganic mercury can act as a human germ cell mutagen, and a high degree of concern for human germ cell mutagenicity associated with methylmercury.
Adsorbing Technology
In a new approach to the problem of mercury contamination, scientists at Pacific Northwest National Laboratory (PNNL) in Richland, Washington, have been working on an adsorbing technology that may provide the ability to remove and concentrate mercury--and many other hazardous substances--from a liquid waste stream. The PNNL team, including Jun Liu, Xiangdong Feng, Glen Fryxell, Li-Qiong Wang, and Meiling Gong, has developed a method of coating mesoporous silica with monolayers of tris(methoxy)mercaptopropylsilane (TMMPS) that bond with heavy metals, which can then be removed with a hydrochloric acid flush.
Mesoporous materials resemble classic microporous zeolites (aluminum silicate minerals or their corresponding synthetic compounds) that are commonly used as catalysts and adsorption media. The porous nature of these zeolites means that they can be used in many filtration and ion-exchange systems. However, one limitation on applications has been that zeolites have pore sizes below 1.3 nanometers (nm). On the other hand, mesoporous materials have pore sizes that can be adjusted from 2 to 15 nm, allowing larger inorganic particles and potentially even biological materials to pass through channels in the media. The pores create a very large surface area of approximately 1,500 m
2
per gram of silica. TMMPS was first developed in 1992 by researchers at Mobil Research and Development Corporation in Fairfax, Virginia. Other mesoporous materials include alumina, zirconia, titania, niobia, tantalum oxide, and manganese oxide.
Preparing the mesoporous silica involves mixing ceramic precursors in a surfactant solution and reacting the agents at temperatures below 150°C. In principle, surfactants form ordered micellar phases, the most common of which consists of rod-like micelles packed in hexagonal arrays. A continuous ceramic phase is formed after the precursors bind to the head groups of the surfactant molecules and condense together. The surfactant molecules are removed by applying heat.
Recipe for a silica sponge
. Researchers are using a fairly simple process to create a mesoporous material that can adsorb metals 30 to 10,000 times more effectively than current commercial filters.
"We then attach special organic molecules to the silica to make it functional since it is in a powder form and doesn't normally have any binding capabilities," says Liu. He and his colleagues coat the silica with TMMPS to create this functionalized monolayer. The layer contains thiol groups that bind strongly to heavy metals. The binding works because the TMMPS molecules covalently bond to the silica while the thiol groups on the opposite end are free to bond with the metals.
The adsorptive power of the material is measured in terms of the distribution coefficient (
K
d
)--the amount of adsorbed metal per gram of adsorbing material divided by the metal concentration left behind. The functionalized monolayers have shown a
K
d
as high as 100,000,000 for mercury, compared to 10,000 for other resin material. The group has loaded over 700 mg of mercury for each gram of the adsorbing material. Washing the mixture with hydrochloric acid removes all of the mercury, while the regenerated materials retain nearly half of the original loading capacity and are effective for several regeneration and reuse cycles. Potentially, the mercury-loaded material could be disposed of as a permanent waste form because of its long-term durability and the fact that the relatively small pore size would minimize the ability of bacteria to put the mercury back in solution.
The PNNL team is not the only group working on mesoporous silica for mercury removal, Liu adds. In an independent study, Thomas Pinnavaia, a researcher and professor of chemistry at Michigan State University in East Lansing, obtained a mercury loading of 310 mg per gram of adsorbing material.
Liu says that his group's mesoporous silica is undergoing tests at several locations. The Department of Energy (DOE) is particularly interested in addressing mercury-contaminated wastes at its Savannah River site in South Carolina. Major Thompson, a senior advisory scientist in the chemical and hydrogen technology section of the Savannah River Technology Center in Aiken, says that initial tests using a powder form of the material are very promising. "We're in the second year of the project, and will soon have an engineered form of the silica that is suitable for testing in an ion-exchange column," he says. In batch mode, Liu's group has tested solutions that mimicked waste water conditions at the site. Ionic and elemental mercury concentrations were reduced to 10 parts per trillion, which is well below the EPA limit for drinking water of 1 part per billion (ppb). Silver and lead also were reduced to negligible concentrations.
If the technology proves itself, Thompson says that the mesoporous silica would be used to process mercury-contaminated oils and several million gallons of contaminated water a day resulting from site cleanup processes. Mercury contamination in this water ranges from 1-5 parts per million (ppm) from an effluent facility to 60-600 ppm from vitrification recycling processes. Thompson expects that large quantities of the material will be relatively inexpensive to produce, and certainly less expensive than the resins that are currently used. The DOE is not expecting to regenerate the material, but rather to use it as a final form of waste disposal.
Dental Dilemma
The PNNL group has been developing other ideas for turning the mesoporous silica into a commercially useful form. "This technology has been around for a few years and has generated lots of potential interest," says Liu.
One of the first targets is the dentist's office, where dental amalgam (usually an alloy of mercury and silver) is discharged directly into the municipal waste water system. Liu and his colleagues have been working with researchers at the American Dental Association (ADA) in Chicago, Illinois, to develop a filter that could reduce mercury concentrations in waste water.
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Putting teeth in the solution
. Mesoporous silica may be an answer to the problem of disposing of mercury in dental amalgams.
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Chet Siew, director of toxicology research for the ADA, says, "Mercury amalgam has been used in dental restoration for over 100 years. The waste water may contain small amounts of solid or dissolved mercury. Solids can be collected by a particulate sieve, and we've given dentists guidelines for a simple trap. However, minute concentrations of soluble [mercury] may be discharged down the sewer. While there is no evidence that these minute concentrations represent a significant environmental problem, we've looked for means to remove it, and remain open-minded."
Magnet for metals
. Mesoporous silica, made up of tunnels lined with special organic molecules, can adsorb mercury and other heavy metals.
Siew says that the PNNL technology is very promising because it is specific to mercury, and it is reusable--that is, after the material is saturated, it can be flushed with acid and then reused. It also looks to be cost-effective because of the material's large surface area. The mesoporous silica mixture could be used where solid waste is collected and at the point where water is discharged. It could exist in the form of a filter or a cartridge.
In tests conducted by the ADA and the PNNL, mesoporous silica with functionalized monolayers reduced the concentration of mercury in dental waste water to 25 ppb. The current EPA limit for mercury in discharged waste water is 200 ppb. As additional bonuses, the silica showed a wide working pH range (3-9) and resistance to infiltration by microbes.
The mesoporous silica technology appears to be more efficient than the filtration technology currently in place at the U.S. Navy's Great Lakes Naval Research Institute in Chicago, Illinois, according to Siew. "There are lots of technologies available. The institute uses a system based on sulfur ions, which have an affinity for mercury, but you still have the waste in a bulky form." He notes that the components of such a system last only a few years or are expensive.
There are no figures available for how much soluble mercury waste is generated by dentists, but Siew says there are about 170,000 dentists practicing in the United States, and most of them are in general practice. The quantity of amalgam waste produced depends on how busy each office is. However, Siew stresses that dental amalgam is very insoluble, and the amount of soluble mercury actually released into the environment from this solid waste will likely be very small.
In the coming months, Siew says, the ADA plans to verify the results obtained so far and to gather more data. If the material still looks promising, especially in terms of cost-effectiveness, then the ADA will encourage other researchers to engineer a cleaning system based on the technology.
Seeking Selectivity
The work at the PNNL is one of several promising techniques, according to Steven Strauss, a professor of chemistry at Colorado State University in Fort Collins who has worked on extracting mercury from similar material. Says Strauss, "There are many approaches and no one knows yet which ones will work best. It's been possible to remove contaminants with similar technology for some time, but as materials become more selective there is a greater focus on recovery and reusability." He observes that the PNNL work treats the modified silica with acid so that the mercury is recovered from the extractant in an aqueous solution but in a much smaller volume than the primary waste stream. It may still be necessary to reduce the volume further for ultimate disposal; Strauss thinks it is vital to obtain the smallest volume of secondary waste possible. He adds, "If you can recover mercury in a commercially viable form, you could sell it instead of burying it." Strauss and colleague Peter Dorhout have developed lithium-intercalated metal sulfides to remove mercury, lead, and silver. In their redox-recyclable extraction and recovery process, over 94% of the ion-exchanged mercury is recovered in a cold trap as metallic mercury.
Meanwhile, Liu says that he and his colleagues are developing other functional groups to replace the mercury-specific monolayer. Their goals are targeting radioactive elements in tank wastes at the DOE's Hanford site in Washington State and removing copper, nickel, cobalt, and other metals and anions. In addition, a number of funded projects are underway to investigate the technology as a way of cleaning up heavy metals in the paper and pulp industry, as a dielectric film in the semiconductor industry, and to disperse catalysts in the chemical industry. Mesoporous silica may even find use as an energy storage material for automobiles and for the controlled and sustained release of pheromones in biological pesticides in agriculture. It's a potentially vast agenda, but, as Liu states, "We want to do small things first and gradually expand."
W. Conard Holton
ATSDR toxicological profile for mercury (update) [draft for public comment]. Atlanta, GA:U.S. Department of Health and Human Services, Public Health Service, Agency for Toxic Substances and Disease Registry, 1998.
Feng X, Fryxell GE, Wang L-Q, Kim AY, Liu J, Kemner KM. Functionalized monolayers on ordered mesoporous supports. Science 276:923-926 (1997).
Liu J, Feng X, Fryxell GE, Wang L-Q, Kim AY, Gong M. Hybrid mesoporous materials with functionalized monolayers. Adv Mater (in press).
Miaw C-L, Chou H-N, Gruninger SE, Liu J, Siew C. A novel method for removing mercury from dental wastewater. Am Assoc Dental Res (in press).
Schoeny R. Use of genetic toxicology data in U.S. EPA risk assessment: the mercury study report as an example. Environ Health Perspect 104(suppl 3):663-673 (1996).
U.S. Environmental Protection Agency. Mercury health effects update--health issue assessment. EPA-600/8-84-019F. Washington, DC:Environmental Protection Agency, 1984.
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Last Update: February 5, 1998