Radioactive waste? Just add water

Researchers at Los Alamos National Laboratory are developing an environmentally friendly method for using supercritical carbon dioxide and water to remove radioactive particles and hazardous metals from waste.

A research team at Los Alamos National Laboratory is developing the promise of an environmentally friendly method for using supercritical carbon dioxide and water to remove radioactive particles and hazardous metals from mixtures of waste.

In early tests, the technique removed virtually all of the contaminants from exposed materials. The researchers believe the technique ultimately could be used to extract contaminants from containers and other types of refuse generated by workers in laboratories and industrial plants. That would result in the need to dispose only of small volumes of contaminants rather than using water in bulk for cleaning or putting the vast volumes of contaminated work materials into the waste stream.

An advance over previous research in the field, the Los Alamos team reportedly worked with a microemulsion comprised of supercritical carbon dioxide and water, which was modified with the addition of a polyether.

By itself, supercritical carbon dioxide, which is CO2 under pressure and at a certain temperature, is an effective solvent for a number of materials except metal ions, said Los Alamos research team leader Mark McCleskey. Carbon dioxide is abundant and low in cost. It is also environmentally benign because it is inert, non-toxic and non-flammable. But the only way previously known to employ it in metal ion extraction was by combining it with a molecule known to combine with certain kinds of metals. The effectiveness of this method, however, is said to be limited.

Water could be a desirable transport medium in a supercritical solvent system, McCleskey said, except that water does not stabilise in carbon dioxide. Water by itself tends to trap on the surface of solid materials and in low volumes cannot effectively penetrate deep into the pores to get at all of the contaminants.

To test their theory that small amounts of water could be employed in metal-contaminant remediation, the team worked with a supercritical carbon dioxide microemulsion modified with the addition of an inert polyether known to stabilise water. They used the emulsion to extract copper and europium from filter paper, wood, cement and activated carbon. Copper and europium were selected for extraction because these elements are spectroscopically active and would yield information on the microemulsion’s effects.

After applying the microemulsion to the materials, the team achieved 98 percent contaminant recovery. ‘We found that the metals targeted for extraction concentrated in the nanodroplets of water in the microemulsion, allowing us to separate the metals from the contaminated materials easily,’ McCleskey said. ‘In addition, the properties of this microemulsion allow penetration even into small pores of contaminated materials usually not accessible to bulk water.’

Microemulsions are especially advantageous for extracting metals from waste, he said, because the amount of water required is proportional to the amount of contaminant being removed, not to the amount of waste to be cleaned. ‘The result is that grams of contaminants can be captured with just a few millilitres of water.’

To remove the captured contaminant from the microemulsion, the team adjusted the carbon dioxide pressure and added water. This phased separation of contaminant from carbon dioxide has the potential of providing an easy recycle system for the microemulsion. One test showed that 81 percent of a microemulsion’s initial capacity was retained for its second application.

If further research produces consistent results, McCleskey believes the technique has potential to become an environmentally friendly method for remediating hazardous wastes.