EDTA-assisted heavy-metal uptake by poplar and sunflower grown at a long-term sewage-sludge farm

Abstract

Little information is available concerning the efficacy of chelates applied to biosolids (sewage-sludge)-treated soil for heavy-metal removal. The purpose of the experiment was to determine the availability to sunflower (Helianthus annuus L.) and hybrid poplar (Populus deltoides Marsh. × P. nigra L.) seedlings, of non-essential (Cd, Ni, Pb) and essential heavy metals (Cu, Fe, Mn, Zn) in field soil injected with biosolids since 1976 and treated with ethylenediamine-tetraacetic acid (EDTA) in 2001. Sunflower was grown at two densities, 20 000 and 60 000 plants/ha, and poplar at 10 000 plants/ha. The tetrasodium salt of EDTA was applied at rates of 0, 0.5, 1, and 2 g EDTA salt per kg surface (25-cm depth) soil. The EDTA did not affect uptake by poplar of the three non-essential (Cd, Ni, Pb) and four essential (Cu, Fe, Mn, Zn) heavy metals. For sunflower, the 1.0 g/kg rate of chelate addition resulted in maximal removal of the three non-essential heavy metals (Cd, Ni, Pb). Uptake of the essential heavy metals by sunflower was little affected by the EDTA. At the 20 000 plants/ha density, leaves of sunflower grown with 1.0 g EDTA Na4ċ2H2O per kg soil accumulated more Cd, Ni, and Pb than leaves of sunflower grown without the EDTA salt. At this density, concentrations of Cd in leaves of sunflower without EDTA and with 1.0 g/kg EDTA salt were 2.2 and 6.5 μg/g, respectively; for Ni, they were 6.7 and 19.2 μg/g, respectively; and for Pb, they were 15.6 and 46.9 μg/g, respectively. At the 60 000 plants/ha density, stems of sunflower grown with 1.0 g EDTA Na4ċ2H2O per kg soil accumulated more Cd, Ni, and Pb than stems of sunflower grown without the EDTA salt. At this density, concentrations of Cd in stems of sunflower without EDTA and with 1.0 g/kg EDTA salt were 0.6 and 4.6 μg/g, respectively; for Ni, they were 1.7 and 17.6 μg/g, respectively; and for Pb, they were 5.2 and 42.8 μg/g, respectively. Removal of the non-essential heavy metals by sunflower was greater at the higher plant density (60 000 plants/ha) compared to the lower one (20 000 plants/ha).