Chemical Speciation and Phytotoxicity of Heavy Metals in Sewage Sludge for the Germination of Chinese Cabbage Seeds

Single extractions with distilled water and diethylene-triamine-pentaacetic acid (DTPA), and a sequential extraction procedure were conducted in this study to determine the chemical speciation of Cd, Cr, Cu, Ni, Pb and Zn in four municipal sewage sludges and subsequently evaluate the phytotoxicity of the metals by a seed germination test of Chinese cabbage (Brassica chinensis L.). Analytical results indicated that a high percentage of organic carbon was found in all the sludges, and thus beneficial to application on agricultural land. Total concentrations of the heavy metals were much lower than the pollutant concentration limits for land application of sewage sludge in the USA and Europe. However, Cu and Zn were the most abundant elements in the sewage sludges in this study. In most cases, the water extractable contents of the heavy metals in the sludges were undetectable. However, the heavy metal contents with DTPA extraction were much higher than those with water extraction, respectively. Additionally, the Fe-Mn oxide fraction of Zn was the dominant solid phase, but Zn was the most mobile metal in all the sludges. Lead and Cr were concentrated in residual fractions, while Cu showed the organic fraction as being dominant. The seed germination test of Chinese cabbage showed no significant (p<0.05) inhibition by the sludge extracts with different dilution ratios, but the original sludge extracts inhibited the root growth of Chinese cabbage. Copper, Ni and Zn might cause toxicity to plant growth, as demonstrated in this study. _____________________________________________________________________________________________________________