True grit could help in clean-up
15 Jan 2000
Tiny granule of ceramic could hold the key to the clean-up of contaminated sites, according to researchers at the Pacific Northwest National Laboratory in Washington State. Known as SAMMS - self-assembled monolayers on mesoporous supports - the grains could also aid in the recovery of precious metals from waste streams.
SAMMS integrates two different technologies - a porous ceramic, originally developed by Mobil as a catalyst support, whose pore size and density can be controlled; and a method for attaching dense monolayers of molecules to the surfaces of the pores. These molecules are tailored so that they will selectively bind various metal ions, which could be toxic, such as mercury or lead, or valuable, such as silver.
Each grain of the SAMMS ceramic - a type of silicate - is five to 15 microns in diameter, and honeycombed with highly-ordered cylindrical pores. According to research director Jun Liu, a tablespoon of this material contains a surface area similar to a football pitch. The monolayer coating, attached by a process developed by PNL chemist Glen Fryxell, optimises the density of the functional material without blocking off the pores.
`I think the most exciting thing about SAMMS is that we not only have the ability to change pore size but also to custom design the molecular species so that the molecules can recognise certain species and reject others,' comments Liu. `That kind of molecular recognition has tremendous potential.'
The first use for the SAMMS system is likely to be in removing mercury contamination from soil and water. On releasing the SAMMS into water, the team claims, it quickly grabs and immobilises the metal ions, reducing the concentration to far below drinking water standards. The strong bonding from the monolayer chemical, combined with the small diameter of the pores, prevents the metal ions from escaping back into solution. `The SAMMS material has demonstrated the highest metal-loading capacity reported by anyone so far,' comments Liu; moreover, the bonding of the metal ions is so strong that the granules form a very effective and safe waste storage medium.
Other possible uses include applications in the mining and metal finishing industries, where it could be used to clean processing water and recover valuable metals in waste streams, says commercialisation manager Nick Lombardo. The materials work well in water, non-aqueous and gas-phase streams, so the possibilities are as huge as the particles are small.
* Other work at Pacific Northwest includes the use of an enzyme isolated from spinach leaves to neutralise explosives. The nitroreductase enzymes, which spinach plants use to extract nitrogen from the soil, have the ability to digest explosives such as TNT, reducing them to harmless compounds. Principal investigator Manish Shah explains that the US has 500 000t of explosives stockpiled; enzyme treatment is a safe, easy to use, environmentally friendly and mobile way of rendering these materials safe: the alternative is incineration.
