EFFECTS OF TIME AND TEMPERATURE ON THE BIOAVAILABILITY OF CD AND PB FROM SLUDGE-AMENDED SOILS

A pot experiment was conducted to compare the behaviour and bioavailability of Cd and Pb from two soils mixed with sewage sludge at three rates (0, 50 and 150 t ha-1) and maintained at two contrasting ambient temperatures (15-degrees-C and 25-degrees-C) over a period of one year following the treatments. Ryegrass (Lolium perenne) accumulated Cd and Pb in the sewage sludge treated soils, although accumulation was significantly lower in the soils treated at the high rate (150 t ha-1) compared to the low rate (50 t ha-1). Ryegrass grown in the warm environment (25-degrees-C) accumulated significantly higher levels of Cd and Pb than that grown in cooler conditions (15-degrees-C). Samples of the soils spiked with nitrate salts of Cd and Pb at equivalent rates of metal loading resulted in the ryegrass accumulating much higher levels of both the metals than on the sludge treated soils. Metal uptake by the ryegrass from the sludge treatments increased over successive harvests while that from metal salt treatments decreased. The observed trend of increasing plant metal uptake over time coincided with a trend of decreasing pH in the sludge treatments. However, the concentrations of Cd and Pb extracted by DTPA failed to predict the changes in plant metal uptake. The importance of sewage sludge as both a source and a sink of pollutant metals and the trend of increasing bioavailability over time shown by this experiment are discussed.