Ten agricultural soils were spiked with 100 μg of Cu, Zn, and Pb g−1 of soil. The complexation of Pb by 10−4M DTPA was determined after 60 days of metal equilibrium.
A theoretical stability model of Fe, Mn, Cu, Zn, Ca, and Pb, between pH 1 and 10 was developed. It showed that CuL3− (55.4 to 63%), CuHL2− (0.1 to 9.4%), PbL3− (29.1 to 33.3%), PbHL2− (0.01 to 2.5%) and MnL3− (2.8 to 3.2%) as the predominant species between pH 6 and 9. Other species like CaL3−, CaHL2−, FeL2−, FeHL−, FeL3−, FeHL2−, ZnL3−, ZnHL2− and MnHL2− were less than 0.1% between pH 6 and 9.
In the experimental soils, DTPA-Pb complexes accounted for 12.3 to 21.2% of metal complexed after 3 days of DTPA equilibriation. DTPA-Cu complexes were the major complexed species in the soils ranging between 72.5 and 82.2% after 3 days equilibriation with 10−4M DTPA. The proportion of Zn complexed by DTPA (3.8 to 10.1%) was much greater than predicted by the theoretical model. On the contrary, Mn was not complexed in appreciable amounts by DTPA.