Leaching of copper from contaminated soil following the application of EDTA. II. Intact core experiments and model testing

Abstract

In Part I, we proposed a model for EDTA-facilitated transport of copper, and we tested the robustness of this scheme by reference to experiments with various initial and boundary conditions on a repacked soil. Now we examine a quite different repacked soil, and consider intact cores of the same soil as in Part I.

Continuous leaching of an intact core of a contaminated orchard soil of volcanic origin under unsaturated conditions with excess 0.01 M EDTA (510 mm) reduced the copper concentration from 240 down to 80 μg/g in the top 25 mm. When a 480 μmol pulse of 0.001 M EDTA was applied to similar intact cores, 126 μmol of copper came out with immediate leaching, but only 18% of that was leached out from a core that was left for 1 month before leaching. The month's delay caused an increase in the amount of iron leached from 116 to 286 μmol. An experiment, in which daily pulses CuEDTA^2– were applied to a non-contaminated soil for 3 months, indicated that these changes were due to CuEDTA^2– slowly changing to Fe(III)EDTA^–, with the re-adsorption of the Cu²⁺ released.

Application of our model from Part I indicated that different rate constants are required for the chemical reactions for different soils, and also for the same volcanic soil when the pH is lower. But by changing just the dispersivity from 3 mm to 23 mm, the model with the same chemical parameters was able to simulate both the repacked columns and the intact core experiments using the volcanic soil.