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Effect of pH and Redox on Predicted Heavy Metal-Chelate Equilibria in Soils¹

ABSTRACT

Metal-chelate stability diagrams were developed as a function of pH and redox to predict the behavior of DTPA, EDTA, CDTA, EGTA, and HEDTA in soils. The metals evaluated include H⁺, Fe³⁺, Fe²⁺, Al³⁺, Mn²⁺, Ca²⁺, Mg²⁺, Zn³⁺, Cu²⁺, Cd²⁺, Pb²⁺, and Ni²⁺. In acid systems, Ni²⁺ at 10^–5 to 10^–7M was the predominant ligand species for all chelating agents. Similar results were obtained in alkaline systems when NiCO₃ controlled Ni²⁺ solubility. In the absence of Ni²⁺, either Cd²⁺ or Pb²⁺ ligand species dominated at alkaline pH for all chelating agents. In acid systems, Cu, Pb, Fe, and Zn are the major chelated species.

In acid soils, the chelating agents EDTA, EGTA, and HEDTA deserve further study as extractants for metals and as reagents for determining metal ion activities through competitive equilibria. The results identify ligands which may be useful in determining various metal ion activities in soils. The DTPA soil test appears to have a sound theoretical basis for evaluating plant available Pb, Cd, and Ni in soils.

¹ Joint contribution from the Dep. of Agronomy, Colorado State Univ., Fort Collins, CO 80523 and Purdue Univ., West Lafayette, IN 47907. Published with the approval of the Directors of the Colorado State Univ. Exp. Stn., Scientific Series Paper No. 2387 and of the Purdue Univ. Agric. Exp. Stn., Journal Paper No. 7059. This work was conducted while the senior author was a Visiting Professor in the Department of Agronomy, Colorado State Univ. Presented before Div. A-5, American Society of Agronomy, Los Angeles, California, November 1977.

² Associate Professor of Agronomy, Purdue University and Professor of Agronomy, Colorado State University, respectively.

Received for publication February 21, 1978. Accepted for publication October 11, 1978.