Extractability of zinc, cadmium, and nickel in soils amended with EDTA

Synthetic chelating agents are produced in large quantities for use in many industrial applications. Certain chelates, such as ethylenediaminetetraacetic acid (EDTA), are persistent in the environment. The presence of EDTA in soil may alter the mobility and transport of Zn, Cd, and Ni in soils because of the formation of water soluble chelates, thus increasing the potential for metal pollution of natural waters. Mobility of metals is related to their extractability. To investigate metal extractability affected by EDTA, Zn, Cd, and Ni were added to a portion of eight Georgia topsoil samples at rates of 75.9, 1.62, and 4.30 mg kg(-1), respectively. Both natural and metal-amended soils were treated with Na-2-EDTA at rates of 0, 1.0, and 2.0 g kg(-1). After 5 months of incubation, soil samples were extracted with Mehlich-1, DTPA (diethylenetriamine-pentaacetic acid) and 1 M Mg(NO3)(2), the latter of which extracts the exchangeable form of metals. Results showed that Zn and Ni in Mehlich-1 and DTPA extractions increased with increasing rates of EDTA. The increase for Cd was not as great as for Zn or Ni. Similar changes were found for the Mg(NO3)(2) extraction. As a percentage of total metal concentration, the Mehlich-1 and DTPA extractable Zn was greater than Ni in the natural soils, and the order for the metal-amended soils was Cd > Zn > Ni. The results also suggested that EDTA significantly elevated the extractability of Zn and Ni in both natural and metal-amended soils, but it did not affect the extractability of Cd in the metal-amended soils. The order of mobility based on extractability was: Cd > Zn > Ni for metals added to soils, but when EDTA was present, added Ni was more extractable than Zn or Cd.