Copper Transport in Calcareous Soils: Miscible Displacement Experiments and Second‐Order Modeling

Abstract

Miscible displacement column experiments and batch adsorption were performed to assess Cu mobility and reactivity in calcareous soils. The strongest Cu retention was observed in the surface layer of a calcareous soil having 2.76% CaCO3 in comparison with the subsurface layer having 1.18% CaCO3. Measured Freundlich adsorption coefficients (Kf) were 5226.4 and 755.5 mL g−1 for the surface and subsurface soils, respectively. Breakthrough curves (BTCs) based on miscible displacement column experiments indicated strong Cu retention and limited mobility especially for the surface soil. Removal of carbonates from the surface soil resulted in extensive Cu mobility, with 87% of the applied Cu recovered in the effluent compared with only 27% when carbonates were not removed. Chemical extraction of the soil from the columns at different depths revealed that retained Cu was associated primarily with the carbonate and oxide fractions (53.9–69.2%). This amount decreased sharply to 9.6 to 20.6% when carbonates were removed from the surface soil. A linear model failed to predict concentration maxima and the extensive tailing of Cu BTCs. The use of a second‐order, two‐site (SOTS) model provided a good description of the BTCs, where arrival of Cu in the effluent, concentration maxima, and the slow release during leaching were well predicted. The use of a SOTS model, which accounts for kinetic reversible and irreversible retention mechanisms, is recommended for predicting Cu transport in calcareous soils.