T MAY soon be possible to clean up oil
spills that contaminate the soil in and
around petrol pumps, thanks to the
efforts of Dutch researchers. Koen
Weytingh and his colleagues from the
Technical University of Delft and De
Ruiter Milieutechnologie in Halfweg, ,he Netherlands, have developed a technique that uses sound wave s to break up oil droplets present between grains of
soil (New Scientist, Vol 146, No 1980).
The method commonly used for
cleaning contaminated soil involves digging it and replacing it - which might
require demolition of the building
housing the petrol pump, if the soil
underneath it needs cleaning.
Other methods include pumping out the groundwater or
use of oil-digesting bacteria,
but these techniques remove
only the hydrocarbons which
are more soluble in ground
water. The heavier hydrocarbons remain trapped in large
droplets between soil grains .
Weytingh and his colleagues have been trying to
use sound waves to break up
droplets containing the heavier fractions of hydrocarbons,
so that they could be easily washed out.
The researchers passed sound waves, at
several frequencies, through 2 columns
of wet sand contaminated with diesel
fuel and then compared the amount of
oil that could be removed from each
column by pumping water through the
2 sand beds, until no more soil emerged.
They found that after 5 days, 71 per
cent of the fuel from the column
exposed to sound waves was removed,
while it took I I days to remove only 40
per cent of the fuel from the untreated
column. The sound waves make the oil
droplets vibrate at their natural frequency until they disintegrate into
smaller droplets, and once freed from
the soil, these can easily be washed off.
According to Weytingh, high frequency sound vibrations tend to be
damped by the groundwater, thus limiting their usefulness. But low frequency
sounds were found to be effective, he
says. Following the success in laboratory, the Dutch researchers are now trying
to incorporate sound waves in large
scale clean up systems.