Clearing widespread and persistent pollutants from great swathes of land is an almost impossible task, compounded by chemical changes induced by weathering. One practical solution is phytoremediation, which has been successfully employed to transfer DDT from contaminated soils to pumpkin and zucchini plants.

It has long been recognised that some chemicals persist in the environment for many years, moving into the food chain and accumulating in the fatty tissue of birds, animals and humans. Many of these persistent organic pollutants (POPs) have adverse health effects, prompting the United Nations to draw up the Stockholm Convention, restricting the use of various POPs. Many nations have signed up to the Convention, bringing pressure on the non-compliants to join up.

One of the major POPs is DDT, introduced during the Second World War as an insecticide to control insects that carry disease, such as mosquitoes (malaria and yellow fever) and fleas or lice (typhus). Later, its use was extended to a broad spectrum insecticide. In the air, DDT breaks down rapidly, with a half-life of just two days but once it hits earth, it is a different story. Due to its strong hydrophobicity, the half-life is anything up to 15 years, depending upon the type of soil.

Environmental DDT is an anthropogenic problem, but we have not yet devised an adequate solution for our own mistakes. There is no easy way to remove DDT and its breakdown products from the vast areas of land that are contaminated. Once it reaches the food chain, according to the Agency for Toxic Substances and Disease Registry, it can affect the nervous system of mammals, including humans, and may prevent women from breastfeeding infants. On the other hand, DDT-exposed workers did not show increased incidence of cancer, although it can induce liver cancer in some animals.

One possible solution to widespread contamination is the emerging technology of phytoremediation, by which chemicals are removed from soil, water or other complex areas like industrial waste by specially selected plants. This technique has been adapted by Kenneth Reimer and colleagues from the Environmental Sciences Group at the Royal Military College of Canada at Kingston for removing DDT and its two metabolites DDE and DDD from contaminated soil. Technical DDT, the kind used in commercial pesticides, consists of 77% of 4,4'-DDT {1,1,1-trichloro-2,2-bis(4-chlorophenyl)ethane}, 15% of its isomer 2,4-DDT, and smaller amounts of other related compounds. In soil, 4,4'-DDT degrades to DDE {1,1-dichloro-2,2-bis(4-chlorophenyl)ethylene} and DDD {1,1-dichloro-2,2-bis(4-chlorophenyl)ethane}, and 2,4-DDT forms the equivalent compounds. There are health concerns over all of these chlorinated compounds.

The Reimer team tested zucchini, tall fescue, alfalfa, rye grass and pumpkin as phytoremediators, growing them from seed on soil from a known contaminated site at Kittigazuit in the Northwest Territories and reported their findings in Environ. Sci. Technol. 2004, The amounts of DDT/DDE/DDD (total DDT) in the soil were estimated by GC/ECD, following extraction into dichloromethane and clean up by Florisil chromatography. This is standard technique for highly chlorinated compounds, and the method detection limit was 10 ng/g (10 ppb), equivalent to one drop of water in an Olympic size swimming pool.

Similarly, the amounts of chlorinated compounds taken up by the plants were extracted in dichloromethane after drying and grinding, followed by gel permeation chromatography to separate the lipids from the pesticides, then Florisil chromatography. GC/ECD was again used, with detection limits ranging from 1-50 ng/g, depending on sample size.

All five plants removed DDT/DDE/DDT from the soil, but the best were clearly zucchini and pumpkin, both members of the Cucurbita family. From soil treated with 3700 ng/g total DDT, pumpkin accumulated 1519 ng in the roots and 57,536 ng in the shoots. The equivalent figures for Zucchini were 2043 and 35,277 ng. Their success was attributed to a high transpiration volume that induced a larger movement of plant sap, large above-ground biomass and the particular composition of the root exudates.

So, these plants have great potential for the phytoremediation of DDT-contaminated areas, with the added bonus that they act as ready-made storage containers for the contaminant during shipment and subsequent treatment.

Related links:

• United Nations Environment Programme on POPs, including the Stockholm Convention
• Agency for Toxic Substances and Disease Registry on DDT, DDE and DDD
• The US Agricultural Research Service on the efforts of its own scientists in phytoremediation
• Royal Military College of Canada, Kinston, Ontario
• Environ. Sci. Technol. 2004: Uptake of weathered DDT in vascular plants: potential for phytoremediation (Article in Press)

Article by Steve Down