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 (CuE) and Na-pyrophosphate (CuP) extractable Cu in acid soils were studied by using diffusion cells in order to quantify Cu transport by measuring the variation of CuE and CuP 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, CuE, and CuP. After 30 d of diffusion, CuE and CuP 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 CuE and CuP 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 CuP in acid soils.