Phytoremediation of TBT-contaminated Harbour Sediment: Draft report for the TBT CLEAN project

This sub-project of the TBT CLEAN project investigated the feasibility of growing plants on dredged harbour sediments and the influence of vegetation on TBT degradation. The toxicity of TBT to vascular plants was determined with the willow tree transpiration test. Compared to other species, the toxicity of TBT to willows was very low. In a field study from 2003, however, willows did not survive in fresh harbour sludge. A plausible reason is the salt content of the substrate: willows have a low salt tolerance. Besides, the structure of the soil resulting from the sludge was not supporting plant growth. It was therefore decided to lagoon the sediments before bringing plants out.
In a laboratory growth test, seeds of several plants were sown into fresh sediments, lagooned sediments, garden soil and in garden soil irrigated with salt water. Barley (Hordeum vulgare) performed best in these studies.
The sediment was characterized as clay loam/sandy clay loam with a high content of nutrients and neutral pH. Fresh sediments were highly saline, with electrical conductivities up to 14 mS/cm, but upon lagooning, the salinity dropped to moderate levels of 3.7 mS/cm.
In the outdoor growth test, fresh and lagooned sludge with high (around 33 mg/kg) and low TBT (about 3 mg/kg) content were used. Several plant species were growing excellently in this substrate, particularly barley (9 to 10 tons/ha dry weight) and sorghum (10 to 13 tons/ha dry weight). The TBT content had no negative influence on the growth of plants. Many species grew better on the highly contaminated sludge.
Samples were taken below vegetation and below unvegetated blanks in a depth of 5 – 15 cm and 50 cm and analyzed for TBT and its degradation products. Below barley, the degradation was significantly enhanced. However, resulting concentrations were far above the target value (0.007 mg/kg) of the OSPAR convention. No measurable uptake of TBT and metabolites was found for barley (corn), whereas TBT, DBT and MBT were taken up in the other two investigated crops, reed and clover/grass.
It was concluded that the dredged sediment is – after lagooning – a good substrate for plant growth. Although plants support the degradation of TBT, phytoremediation is a slow process which will take several years. The transfer of TBT into crops does not allow an agricultural use of the substrate. Non-food production, such as rape-seed for biodiesel or barley for alcohol, might be an alternative.