Diffusion-induced changes on exchangeable and organic bound copper fractions in acid soil samples enriched with copper

Transport-induced changes in the concentrations of NH₄Cl (Cu_E) and Na-pyrophosphate (Cu_P) extractable Cu in acid soils were studied by using diffusion cells in order to quantify Cu transport by measuring the variation of Cu_E and Cu_P in a half-cell with no copper added (receiving half-cell). An empirical approach based on the Freundlich equation was used to describe the relationship between Cu in the soil water extract, Cu_E, and Cu_P. After 30 d of diffusion, Cu_E and Cu_P in the receiving half-cells increased from 0.2 to 2.6% and from 4.1 to 24%, respectively, relative to their hypothetical concentrations after infinite time. Copper transport was found to be influenced by the nature of the soil and its moisture content.Diffusion was modeled in order to calculate the increase in Cu_E and Cu_P in the receiving half-cell, which were assumed to be in equilibrium with Cu in the soil water extract. Nonlinear retardation in the diffusive transport model was implemented by using the Freundlich equation to obtain nonlinear effective diffusion coefficients for each Cu fraction. The results of the simulations were consistent with their experimental counterparts, which suggests that the moisture content and retardation used in the model provide reasonable estimations of the increase in Cu_P in acid soils.