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Effect of Redox Potential on the Stability of Zinc and Copper Chelates in Flooded Soils¹

ABSTRACT

Stability of Zn and Cu chelates of EDTA and DTPA was studied under controlled redox potential-pH soil conditions. The metal chelates ZnEDTA, ZnDTPA, CuEDTA, and CuDTPA were reacted with soil suspensions incubated at six different redox potentials (–200, –100, 0, +100, +300, and +500 mV) and at pH 7 for reaction periods varying from 2 to 192 hours.

The redox potential has a very significant effect on the stability of all four metal chelates studied. The percentage of added Zn/Cu that remained in soil solution decreased with decreasing redox potential from +500 to –200 mV. Even though Zn and Cu chelates were stable at higher redox potentials, their stability decreased with time, apparently due to physical adsorption and microbial decomposition of metal-chelate complex. At lower redox potentials, Zn and Cu chelates were found to be highly unstable. This instability was found to be mainly due to chemical fixation of added Zn and Cu, not due to physical/microbial decomposition of metal-chelate complex. Iron, Mn, Ca, and Mg were found to be forming metal-chelate complexes with EDTA and DTPA ligand molecules once Zn and Cu were removed from their metal-chelate complexes.

¹ Contribution from the Laboratory of Flooded Soils and Sediments, Agronomy Dep., Louisiana Agric. Exp. Stn., Louisiana State Univ., Baton Rouge, LA 70803.

² Research Assistant and Professor, respectively.

Received for publication October 4, 1976. Accepted for publication February 23, 1977.