| Land application of biosolids results in heavy metal accumulation in surface soils. For environmental safety, the high concentrations of heavy metals should be removed. Four studies were done to determine if chelate-facilitated phytoremediation could enhance extraction of metals from biosolids-treated soil. The purpose of the first experiment was to determine the availability to sunflower (Helianthus annuus L.) and hybrid poplar ( Populus deltoides Marsh. × P. nigra L.) of sevenheavy metals (Cd, Cu, Fe, Mn, Ni, Pb, Zn)) in field soil injected with biosolids since 1976 and treated with the tetrasodium salt of ethylenediamine-tetraacetic acid (EDTA) at the following rates: 0, 0.5, 1, and 2 g EDTA salt per kg soil. Sunflower was grown at two densities: 20,000 and 60,000 plants/ha; poplar was grown at 10,000 plants/ha. The EDTA did not affect uptake by poplar of the seven heavy metals. For sunflower, the 1.0 g/kg rate of chelate addition resulted in maximal removal of the three toxic heavy metals. The sunflower at the higher density removed more of the toxic heavy metals than sunflower at the lower density. The second study followed the leaching of heavy metals in soil with biosolids after solubilization with the tetrasodium salt of EDTA. Soil from the 25-yr-old biosolids farm was placed in long columns (105 cm long; 39 cm in diam.) with or without poplar in a greenhouse. Without EDTA, concentrations of the seven heavy metals in the leachate from columns with or without plants were low or below detection limits. With or without plants, EDTA mobilized all heavy metals except Mn and increased their concentration in drainage water. The third study compared the availability of heavy metals in composted biosolids with soil containing (non-composted) biosolids, after solubilization with the tetrasodium salt of EDTA. Poplar seedlings were planted in columns containing the two media in a greenhouse. In general, EDTA did not affect concentrations of the seven heavy metals in roots, stems, or leaves of poplar grown in either medium. The fourth study aimed to determine if the hormone, auxin, known to promote root growth, would increase root growth in soil with heavy metals from biosolids, when the tetrasodium salt of EDTA was added. Sunflower grew in pots containing either soil from a biosolids farm or composted biosolids. Indole-3-acetic acid (IAA) at 6 mg/L increased root growth of plants grown in the soil with biosolids, but only when no EDTA was present. For roots grown in the compost, IAA had no effect on weight of roots and EDTA reduced root weight. |
