Effect of pH and Redox on Predicted Heavy Metal‐Chelate Equilibria in Soils

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.