Technology News - October 27, 2004

Novel nanomaterial strips contaminants from waste streams

A unique chemically modified nanoporous ceramic can remove contaminants from all types of waste streams faster and at a significantly lower cost than conventional technologies such as ion exchange resins and activated carbon filters, according to researchers at the Pacific Northwest National Laboratory (PNNL). This nanosponge could be used in a wide range of environmental applications, including drinking-water purification, wastewater treatment, site remediation, and waste stabilization.

The technology, which is known as self-assembled monolayers on mesoporous supports (SAMMS), could prove particularly important in meeting new regulations that will require ever-lower discharge limits, says Shas Mattigod, a geochemist at PNNL. For example, SAMMS could be used at coal-fired power plants to treat mercury and other toxic byproducts captured through water-based scrubbing processes that currently wind up in fly-ash ponds.

The nanosponges are constructed from porous, silica-based ceramics, referred to as mesoporous supports, by flowing compounds inside that stick to the internal surface of the material and self-assemble in a molecular layer. The self-assembling molecules are then coated with a moiety that captures the targeted pollutant. For example, one of the first SAMMS was designed to strip mercury from aqueous and viscous liquid waste streams. And, with 99% of the mercury-absorbing action taking place in the first five minutes, there’s no comparison with commercially available sorbents in terms of how fast it works, Mattigod says.

This capture speed means shorter contact times with the nanosponge than with resins or activated carbon, which translate into lower material and capital equipment costs. Additionally, because the mercury is immobilized, the contaminant-loaded SAMMS can be disposed of like ordinary waste; this also leads to huge cost savings. By means of the U.S. EPA’s toxicity characteristic leaching procedure, which is widely used to judge how much contaminant will leach out of a material, the PNNL researchers found that, unlike resins or activated carbon filters, SAMMS leach mercury at levels well below the agency’s regulatory limits.

Preliminary comparisons of removal and disposal costs show that for a waste stream containing roughly 10 parts per million (ppm) mercury, SAMMS is about 3 times cheaper than an ion exchange resin designed to remove mercury and 90 times cheaper than activated carbon, according to Mattigod.

However, it is also possible to regenerate SAMMS. According to the PNNL scientists, the mercury can be removed without stripping out the monolayer. In this case, you end up with a liquid effluent of highly concentrated mercury.

By tuning the chemistry of the self-assembling monolayer, Mattigod and his colleagues have found that they can customize the material to sequester a variety of contaminants, including other toxic heavy metals such as cadmium and lead; anions such as chromate, arsenate, and selenite; and actinides.

The new technology gets at the limits of conventional sorbents in a number of ways. For one, other sorbents have a much lower loading capacity and slower absorption time; therefore, they produce a larger quantity of contaminated materials that require disposal.

“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,” explains Glen Fryxell, a synthetic chemist at PNNL. SAMMS, on the other hand, has a rigid ceramic backbone where the pores are always open and all the binding sites available. Consequently, “Diffusion into and out of the pores is a continuous ongoing process, so you don’t have any of the kinetic limitations that you run into as a result of the solvent swelling with polymers,” Fryxell notes.

Additionally, the high surface area of the SAMMS materials (roughly 1000 square meters per gram) allows for an extremely high density of binding sites; this ability dramatically lowers the amount of garbage produced. Five grams of SAMMS powder, for example, have the same surface area as a football field, according to Mattigod, and the binding molecules fully cover the available surface. Moreover, because the surface chemistry is tailored to trap a specific group of contaminants, selectivity is much higher than for conventional sorbents, again resulting in a smaller amount of material being used.

The researchers have tested SAMMS on several waste streams containing various levels of mercury, including lab waste and scrubber effluent. The material even handled viscous liquids by treating mixed-waste vacuum pump oils. In each case, Mattigod says, mercury levels were reduced to well below EPA thresholds for land disposal.

In tests at Oak Ridge National Laboratory (ORNL), “SAMMS far exceeded our expectations,” says Tom Klasson, previously a biochemical engineer at ORNL and now with the U.S. Department of Agriculture. He performed both bench-scale and large-scale treatment demonstrations on vacuum pump oils, in which SAMMS achieved mercury decontamination to well below the target goal of 0.2 ppm. The technology “proved very effective in removing other metals as well,” including cadmium, lead, and chromium, Klasson adds.

An added benefit is the stability offered by the SAMMS material. The pore size, roughly 6 nanometers, of the mesoporous supports is far smaller than the microorganisms that could get in and release the bound materials to the environment, Fryxell notes. “They simply can’t get into these little bitty pores to access the mercury and metabolize it into more mobile and toxic forms,” he says. Contrast that with resins and other sorbent forms where the functionalities are external, allowing easy access to microbial attacks.

The PNNL researchers are currently developing several engineered forms of SAMMS for commercial deployment in membranes and fiber-type materials. So far, they’ve received inquiries from a variety of industries. Those interested include dental practitioners who use mercury in amalgam fillings, drinking-water companies that make water filters, and oil and gas companies whose waste waters often contain mercury contaminants. —KRIS CHRISTEN