Clean Water Advocacy - Newsroom - AMSA in the News
Water Environment & Technology
Copyright 2005 Water Environment Federation
January 1, 2005
Volume 17; Issue 1
Ceramic 'Sponge' Absorbs Mercury
Christen, Kris
As states and communities contemplate ever more stringent mercury limits, a
unique, nanoporous ceramic material could help wastewater treatment plants meet
new limits by removing mercury at the source.
Researchers at the Pacific Northwest National Laboratory (PNNL; Richland,
Wash.), who developed the technology, have found that this new sponge-like
material can remove mercury and other toxic contaminants from hydrophilic and
hydrophobic wastestreams faster and much cheaper than conventional technologies,
such as ion-exchange resins and activated carbon filters. They currently are
developing several engineered forms for commercial use in membranes and fibers
that could be packed into fixed-bed columns.
"This technology will result in huge savings to users who are faced with costly
disposal of mercury in the wastestream," said Shas Mattigod, a geochemist at
PNNL. The nano material absorbs 99% of the mercury within 5 minutes, he said,
and because it immobilizes the mercury, it can be disposed in ordinary
landfills.
The synthetic material, made up of self-assembled monolayers on mesoporous
supports (SAMMS), was designed specifically to strip mercury from wastestreams.
But by tuning the chemistry inside the mesoporous supports, Mattigod and his
colleagues found that they can customize SAMMS to scavenge for other
contaminants, including other heavy metals, such as cadmium and lead; anions,
such as chromate, arsenate, and selenite; and actinides. So, SAMMS could be used
in a wide range of environmental applications, from drinking water and
wastewater treatment to site remediation to waste stabilization, Mattigod said.
Mercury Concerns Growing
Mercury is a toxic, persistent pollutant that bioaccumulates in the food chain,
and 44 states currently issue fish consumption advisories because of mercury
contamination, according to the U.S. Environmental Protection Agency (EPA).
Also, many of the waters on Clean Water Act sec. 303(d) impaired waters lists
are there because of mercury pollution. Most of this pollution is caused by
atmospheric deposition from local, state, regional, national, and global
sources.
The effects of mercury exposure on human health and the environment are driving
a number of efforts to reduce its presence - including increasingly stringent
mercury effluent limits in National Pollutant Discharge Elimination System
(NPDES) permits, according to a 2002 report, Mercury Source Control & Pollution
Prevention Program Evaluation: Final Report, by the Association of Metropolitan
Sewerage Agencies (AMSA; Washington, D.C.). And as mercury becomes more widely
monitored, the number of facilities with mercury effluent limits is "likely to
significantly increase," AMSA predicted.
Because dental offices are often the largest commercial source of influent
mercury, "communities look at these facilities pretty early on," said Chris
Hornback, AMSA's director of regulatory affairs. Mercury is an ingredient in the
amalgam that dentists use and flush down the drain as they fill their patients'
cavities. These operations are "controllable because they're a commercial source
that pretreatment programs have the ability to regulate," Hornback noted.
AMSA currently is studying mercury levels entering and leaving POTWs to find out
how much difference dental amalgam separators can make when installed at dental
offices. "We're trying to see if communities that require this equipment at
dental offices are able to, first of all, decrease the amount of mercury coming
into their treatment plants, and secondly, come closer to meeting these very
stringent mercury limits," Hornback said.
Nano Advantages
The new PNNL technology could play a role here, as well as with other
wastestreams, because it performs better than conventional sorbents in several
ways, said Glen Fryxell, a synthetic chemist at PNNL. For one, he noted, other
sorbents have a much lower loading capacity and slower absorption time, which
means a large quantity of contaminated materials to dispose.
"Various ion-exchange resins are commonly polystyrene based, and polymer
solvents swell, meaning that you only have a small fraction of your binding
sites available at any given time," Fryxell explained. SAMMS, on the other hand,
has a rigid ceramic backbone, where the pores are always open and all the
binding sites are available. "Diffusion into and out of the pores is a
continuous, ongoing process," he noted, "so you don't have any of the kinetic
limitations that you run into as a result of the solvent swelling with
polymers."
Also, the high surface area of SAMMS materials (roughly 1000 m2/g) allows for an
extremely high density of binding sites, which dramatically lowers the amount of
garbage produced. Five grams of SAMMS powder, for example, has the same surface
area as a football field, Mattigod noted, and the binding molecules fully cover
the available surface. Moreover, he said, because the surface chemistry is
tailored for specific contaminants, selectivity is much higher than with
conventional sorbents, so less material is wasted.
The researchers have tested SAMMS on several wastestreams containing various
levels of mercury, including lab waste, scrubber effluent, and mixed-waste
vacuum pump oils. In each case, Mattigod said, mercury levels were reduced well
below EPA thresholds for land disposal.
In tests at Oak Ridge National Laboratory (ORNL), "SAMMS far exceeded our
exportations," said Tom Klasson, a former ORNL biochemical engineer who now
works at the U.S. Department of Agriculture. He performed both benchscale and
large-scale treatment demonstrations on vacuum pump oils, where SAMMS cut
mercury levels well below the target goal of 0.2 parts per million. The
technology "also proved very effective in removing other metals as well,"
including cadmium, lead, and chromium, Klasson said.
Another benefit is the material's stability. The pore size of the mesoporous
supports (roughly 6 nanometers) is far smaller than the microorganisms that
could release the bound materials to the environment, Fryxell noted. "They
simply can't get into these little bitty pores to access the mercury and
metabolize it into more mobile and toxic forms," he said. Contrast that to
resins and other sorbent forms, where the functionalities are external, allowing
easy access to microbial attacks, he noted.
PNNL researchers expect to commercialize SAMMS within a year. So far, they've
received inquiries from various industries, including dental practitioners,
water filtration companies, and oil and gas companies.