Soil polluted by organic toxins can be blasted clean with ultrasound, say researchers in Australia. The method may prove to be more effective at cleaning up contamination from oil refineries, power stations and aluminium factories than existing methods.

The new clean-up method was inspired by the mining industry, which uses ultrasound to process some minerals. Researchers at CSIRO Industrial Physics have shown it can also destroy the toxic or carcinogenic persistent organic pollutants (POPs) that commonly contaminate industrial land. POPs include PCBs and DDT, and can spread in water and air and accumulate in the food chain.

Cleaning them up is difficult. Incineration can produce toxic breakdown products, while chemical treatment methods can require huge amounts of energy or involve substances almost as toxic as those being cleaned up – risking dangerous leakages. Researcher Andrea Sosa Pintos says: "None of these provide a complete or cost-effective solution. And some of them can be time-consuming."

Sosa Pintos and colleagues developed a prototype system that works more effectively, she says. Contaminated soil is mixed with water and passed through a chamber that blasts the mixture with ultrasound. The team tested their system on sand spiked with pollutants as well as samples collected from industrial sites and claim that it destroys up to 97 per cent of contaminants in just a few minutes.

Hot shockwaves

Sound waves travel through water as a series of high pressure waves with low pressure areas in between. The low pressure causes the water to boil and form microscopic bubbles. The high pressure then forces the bubbles to collapse, generating a shockwave that produces localised temperature flashes of more than 4000°C and pressures of about 1000 atmospheres. That is more than enough to break down any complex molecules in the water, Sosa Pintos says.

In a slurry mixture, these bubbles form around the edges and surfaces of solid suspended particles – exactly where pollutants tend to end up. "Since these chemicals are hydrophobic, they are readily absorbed onto the surface of soil particles, so the energy released is selectively directed towards them," explains Sosa Pintos.

Dodging side effects

The current pilot plant can treat between one-quarter and half a tonne of soil per day. "We are working to improve it to be able to treat larger quantities," Sosa Pintos told New Scientist. The team say that in a couple of years they could have a completed mobile plant that could be taken where needed on the back of a truck.

"The compounds they tested are the greatest risk for soil contamination," says Kirk Semple, on expert on the way bacteria and other organisms can clean up pollutants, based at Lancaster University in the UK. "There's a desire to find new technologies for remediation because the ones we have produce environmental side effects," he says.

But Semple cautions that the prototype still needs to be tested at more sites. "The media used in the tests is a little simplistic," he explains. "Soil is a very complex environment." As well as having a mixture of different grain sizes, organic material in soil might alter the ultrasound's effect, he says.