Availability and plant uptake of heavy metals in EDTA-assisted phytoremediation of soil and composted biosolids

Abstract

Heavy metals should be removed in soil for the safety of the environment, and phytoremediation, which is the use of plants to remove contaminants from the environment, can be useful in rehabilitating polluted sites. Chelating agents such as ethylenediamine tetraacetic acid (EDTA) have been used in different situations in phytoremediation to enhance the extraction of heavy metals by plants from soil. The objectives of the study were (1) to assess the availability of heavy metals in soil injected with biosolids and in composted biosolids with or without EDTA amendment, (2) to determine the efficacy of EDTA on uptake of heavy metals by sunflower plants from soil with biosolids and composted biosolids, and (3) to investigate whether EDTA applied in the previous season can reduce growth of the next crop. A laboratory and two greenhouse experiments were conducted at Kansas State University. In the laboratory, five EDTA doses were applied into the flasks with 5 g of either soil injected with biosolids (soil) or composted biosolids (compost). The soil and EDTA solution in flasks were shaken by electronic shaker for 4 h. The solution was decanted 2 h after shaking and then tested for the concentration of heavy metals by inductively coupled plasma emission spectrometry (ICP-ES). In the greenhouse, sunflower ( Helianthus annuus) plants were grown in soil with biosolids and in the composted biosolids. At the flowering stage, EDTA salt at 0, 1.0, and 2.0 g/kg soil or compost was applied. Plant organs were separated and dried at the end of the trial. The concentrations of heavy metals (Cd, Cu, Fe, Mn, Ni, Pb, and Zn) were analyzed using ICP-ES. After harvesting the first greenhouse experiment and extraction of the roots, the pots were refilled with the same soil or compost and five sunflower seeds were sown to establish a second greenhouse experiment. The emergence of seedlings and plant growth was determined. In the laboratory experiment, heavy metal concentration in the solution was increased by EDTA in both soil with biosolids and the composted biosolids. The solution from composted biosolids which received EDTA had a higher concentration of heavy metals than solution from soil injected with biosolids. EDTA increased the accumulation of Cd and Ni in the roots of plants grown in the composted biosolids. Essential heavy metal (Cu, Fe, Mn, Zn) concentrations were increased in the organs of the plants grown in the composted biosolids where EDTA at 2.0 g/kg soil was applied. The concentrations of all seven heavy metals in the plant organs were not affected by EDTA in soil injected with biosolids. The high EDTA dose of 2.0 g/kg soil reduced seedling emergence and height of the plants grown in the composted biosolids.