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Breakthroughs Magazine

Special Report - Advanced Nanoscale Materials: Putting Science at Your Fingertips

Mercury—watch out!


An innovative sponge-like material that can "absorb" more than half its weight in contaminants from waste streams has been developed by scientists at Pacific Northwest National Laboratory.

Thiol-SAMMS, or Self-Assembled Monolayers on Mesoporous Supports, is a simple, inexpensive and easy-to-use tool that uses nanoscale technology to absorb mercury. "This technology will result in huge savings to users who are faced with costly disposal of mercury in the waste stream," said Shas Mattigod, who manages the project for PNNL. "Thiol-SAMMS is cost effective because it immobilizes the mercury, which allows it to be disposed of like ordinary waste."

Thiol-SAMMS integrates a nanoporous substrate with an innovative method for attaching monolayers—single layers of densely packed molecules—to the pore surfaces throughout the substrate. "It's like laying a carpet," Mattigod said. "Different carpet surfaces have different properties. A different type of monolayer can be applied to SAMMS depending on the contaminant you want to remove. This monolayer will seek out and adsorb specific contaminants."

Thiol-SAMMS also is fast and effective. Ninety-nine percent of thiol-SAMMS' mercury-absorbing action takes place in the first five minutes. "Thiol-SAMMS can absorb 60 percent of its weight in mercury, making it an efficient scavenger for mercury," Mattigod said. "There is no comparison with commercially available sorbents in terms of how fast it works."

Thiol-SAMMS has surpassed developers' expectations in several tests. For example, scientists tested thiol-SAMMS on 160 liters of waste solution containing about 11 parts per million of mercury or a total of 1.76 grams. Thiol-SAMMS reduced mercury concentration in the solution by 99.5 percent. "We estimate that it will cost about $200, including material, analysis and labor, to treat similar volumes of this waste solution," Mattigod said. "That would save $3,200 over more traditional disposal methods."

Thiol-SAMMS monolayer can be customized to search for a variety of contaminants. While thiol-SAMMS has been tailored to absorb mercury, silver, lead and cadmium, other SAMMS technology is being developed for removing toxic contaminants such as arsenic, chromium and radionuclides. SAMMS technology, including thiol-SAMMS, can be used in water and non-aqueous solutions.

Thiol-SAMMS is currently available in powder form for use on low-volume waste streams. PNNL researchers also are working on industrial applications for the technology. "In addition to the powder form, we're developing an engineered version that will be adaptable for small- to large-scale systems," said PNNL's Rick Skaggs, who coordinates the engineering effort.

Nanogetters stand guard

Not even stainless steel lasts forever. Scientists predict that after tens of thousands of years, stainless steel tanks that will be used to store nuclear waste at the Yucca Mountain Nuclear Waste Repository in Nevada will corrode and radioactive technetium and neptunium will leak into the environment.

Scientists at Pacific Northwest National Laboratory are looking at novel nanoporous materials called "nanogetters" to immobilize predicted leakage of risk-driver radionuclides—those radionuclides that will pose the greatest potential risk when they escape into the environment—at the Yucca Mountain repository.

The nanogetters concept is similar to previous generation Self-Assembled Monolayers on Mesoporous Supports (SAMMS) technology, in which single layers of densely packed molecules are attached to the pore surfaces of a mesoporous material. The molecules are chosen for their ability to bond with, and immobilize, the target substance. So the monolayer of these molecules will seek out and "absorb" specific radionuclides, such as technetium.

The nanogetters material eliminates the monolayer step. Instead of lining a mesoporous substrate with molecules, scientists build a nanoporous material, whose surface will actively absorb contaminants. "Nanogetters are more efficient and economical than current technology," said Shas Mattigod, PNNL project manager. "Because nanogetters have inherent 'sorbing' characteristics— there is no need to line them with adsorbing molecules."

PNNL scientists are using titanium and zirconium phosphates to create nanogetters, which contain thousands of uniform nano-sized pores, for attracting and absorbing specific contaminants, such as technetium. "Nanogetters will have as great a capacity and selectivity as previous generation materials, including thiol-SAMMS," Mattigod said.

If these nanogetters prove to be effective in sorbing selected radionuclides, they can be deployed for sequestering radionuclides released from waste packages at the Yucca Mountain repository.

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