Saturday, 15 July 2006

EDTA-Assisted Phytoremediation of Heavy Metals in Biosolids: Comparison of Composted Biosolids and Soil with Injected Biosolids.

M. B. Kirkham, Kansas State Univ, 2004 Throckmorton Hall, Dept of Agronomy, Manhattan, KS 66506 and M. Stanley Liphadzi, Water Research Commission, Private Bag X03, Gezina, Pretoria, South Africa.

Biosolids (wastewater residuals) can be applied to land either as a liquid or as composted material. With either method of disposal, heavy metals in the biosolids are added to soil. Chelated-facilitated phytoremediation might remove the metals. Yet no studies compare the availability of heavy metals in composted biosolids with soil containing applied (non-composted) biosolids, after solubilization with chelates. The objective of this work was to determine the effect of the tetrasodium salt of EthyleneDiamine-Tetraacetic Acid (EDTA) on uptake by hybrid poplar (Populus deltoides Marsh. x P. nigra L.) of heavy metals in composted biosolids and in soil that had received injected (incorporated as liquid) biosolids. The composted biosolids came from the Topeka, Kansas, Wastewater Treatment Plant. The biosolids are digested anaerobically and then placed in piles that compost over time until they meet regulatory requirements that allow them to be placed on land (called “Class A” biosolids by the U.S. Environmental Protection Agency). The soil with injected biosolids came from the top 50 cm of soil at the 25-year old Biosolids Farm of Manhattan, Kansas. The soil at the site is a Haynie very fine sandy loam (coarse-silty, mixed, superactive, calcareous, mesic Mollic Udifluvents), and the biosolids are injected at the 15-to-20 cm depth. The soil with biosolids was mixed before the experiment began. Poplar seedlings were planted in the two media, which were in columns (c. 120 cm long and 18 cm in diameter) in a greenhouse. After 133 days of growth, the chelate was added at a rate of 1 g per kg media. The plants grew in the two media for another 162 days for a total growth period of about 10 months. At harvest, Cd, Cu, Fe, Mn, Ni, Pb, and Zn were determined in leaves, stems, and roots. Total and extractable concentrations of these seven heavy metals were determined in the media. EDTA did not affect concentrations of the seven heavy metals in roots, stems, and leaves of poplar plants grown in composted biosolids or in soil with biosolids, except in two cases: Cd in leaves of plants grown in composted biosolids with EDTA was lower than Cd in leaves of plants grown in composted biosolids without EDTA; and Fe in roots of plants grown in the biosolids-treated soil with EDTA was lower than Fe in roots of plants grown in biosolids-treated soil without EDTA. Except for Ni, total and extractable concentrations of the heavy metals were higher in the composted biosolids than in the soil with biosolids, and EDTA had no effect on these concentrations. Total concentrations of Ni in the two media were similar, but extractable concentrations of Ni in the compost were higher than those in the soil with biosolids. The results indicated that the chelate had little impact on phytoremediation of heavy metals in the composted biosolids or the biosolids-treated soil. The compost was highly saline, which appeared to inhibit the shoot growth of poplar for the first 9 months of the experiment, until the salts leached out of the columns. The fact that the poplar roots in the compost remained alive during the period of no shoot growth showed that they are tolerant to highly saline media.

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