Chelate-Assisted Heavy Metal Removal by Sunflower to Improve Soil with Sludge

Abstract

Crops are used to improve land through phytoremediation. When heavy metals are the pollutants, chelates are added to increase their solubility for uptake and removal. The U.S. Environmental Protection Agency recommends recycling of sewage sludge back to land, which results in heavy metal accumulations at sludge farms. However, almost no information is available concerning the efficacy of chelates applied to soil with sludge for heavy-metal removal. The objective of this work was to compare heavy metal uptake by sunflower (Helianthus annuus L.) grown in composted sludge or in soil from a sludge farm, when the chelate ethylenediamine-tetraacetic acid (EDTA) was added to facilitate uptake of the heavy metals. Plants were grown in pots in a greenhouse. Fifty-four days after planting, the tetrasodium salt of EDTA was applied at rates of 0, 1, and 2 g EDTA salt per kg medium (composted sludge or soil with sludge). At harvest, 68 days after planting, three non-essential (Cd, Ni, Pb) and four essential heavy metals (Cu, Fe, Mn, Zn) were measured in roots, stems, and leaves. At 2 g EDTA salt/kg, EDTA increased uptake of Cd, Ni, Pb, Cu, Fe, and Zn by roots of sunflower grown in the composted sludge. The EDTA did not increase concentrations of the heavy metals in any plant part of the sunflowers grown in the soil with sludge or in the aboveground portions of the sunflower grown in the composted sludge. With no EDTA, stems and leaves of plants grown in the composted sludge had higher-than-normal concentrations of Cd, Ni, and Pb. With no EDTA, the bioaccumulation coefficients (concentration of metal in the plant/concentration of metal in the medium) for Cd, Ni, and Pb in leaves of sunflower grown in the composted sludge were 2.0, 1.4, and 0.4, respectively; for stems, the bioaccumulation coefficients were 2.9, 2.2, and 0.7, respectively. With no EDTA, the bioaccumulation coefficients for Cd, Ni, and Pb in leaves of sunflower grown in the soil with sludge were 2.3,0.4, and 0.4, respectively; for stems, they were 0,0, and 0, respectively. (The concentrations of Cd, Ni, and Pb in stems of sunflower grown in the soil with sludge were below detection limits.) The higher-than-one bioaccumulation coefficients for Cd and Ni in the leaves and stems of sunflower grown in the composted sludge with no EDTA indicate that natural chelating agents were solubilizing these non-essential heavy metals and making them bioavailable.