CHANGES IN OPERATIONAL FRACTIONS OF TRACE-METALS IN 2 SOILS DURING 2-YEARS OF REACTION-TIME FOLLOWING SEWAGE-SLUDGE TREATMENT

Two soils amended with a sewage sludge were incubated at two ambient t emperatures (15 degrees C and 25 degrees C) under greenhouse condition s for two years. Representative samples of the sewage sludge and perio dically collected soil samples were subjected to an operationally-defi ned sequential extraction procedure. This procedure fractionated metal s in three broad fractions through the use of CH3COOH, NH2OH . HCl and H2O2 in sequence. In the sludge, the greatest percentages (63-98%) of the five metals studied (Cd, Cu, Ni, Pb and Zn) were extracted by H2O 2/NH4OAc. The application of sludge td soils substantially increased t he contents of metals in all the three fractions. Cadmium, Ni and Zn i n the sludge-amended soils were predominantly in the CH3COOH extractab le fraction while Cu and ph were extracted primarily by H2O2/NH4OAc. T he contents of Cd, Cu, Pb and Zn extractable by CH3COOH increased in s amples collected over a period of two years while that of Ni did not s how any significant change. Metal concentrations extractable by NH2OH . HCl remained more or less constant over the residual period. Metal c ontents extracted in H2O2/NH4OAc decreased considerably during the fir st year before stabilising for the rest of the study period. The diffe rent incubation temperatures had no obvious effect on CH3COOH and NH(N )2OH . HCl extractable metal fractions. However, the observed decline in metal contents, over time, extracted by H2O2/NH4OAc was faster in s amples collected from the warm environment (25 degrees C) than those f rom cooler conditions (15 degrees C). The observed trend of increasing metal concentrations in CH3COOH- extractable fraction and decrease in the H2O2/NH4OAc fraction coincided with decreasing soil organic matte r status due to microbial oxidation of sludge-added organic matter. Th e importance of trace metal fractionation and transformation between f ractions are discussed.