Thermodynamics of copper desorption from soils as affected by citrate and succinate

Abstract

Desorption of Cu and low molecular weight dissolved organics are the primary factors that impact fate and transport of Cu in soils. To improve predictions of the toxicity and threat from Cu contaminated soil, it is critical that time-dependent desorption behaviour be understood. In this paper, the effect of organic ligands citrate and succinate on the kinetics of Cu desorption from contaminated soils varying widely in soil characteristics was investigated at three different temperatures. The results showed that the first order equation adequately described the kinetics of Cu desorption from clay and sandy soils under isothermal conditions. The reaction rate constant (k<sub>d</sub>) values of the first order kinetic equation for Cu desorption increased consistently with temperature, indicating faster release of Cu at higher temperatures. The Cu desorption rate from the studied soils at all three temperatures was as follows: citric > succinic. The E<sub>a</sub>values indicates that Cu desorption from the studied soils in the presence of two organic ligands is a diffusion controlled reaction. The negative values of ΔH* suggest that the desorption reactions are not energy consuming process. The higher negative values of (ΔH*) for Cu desorption from the studied soils in the presence of succinic ligand indicate that the heat energy required to overcome the Cu desorption barrier was greater than that for Cu desorption in the presence of citric ligand. Computation of the free energy of activation (ΔG*) yielded values ranging for 87 to 87.9 kJ/mol. The largest value represents ΔG* for Cu desorption for clay soil in the presence of succinic acid while the lowest value represents ΔG* for Cu desorption for sandy soil in the presence of citric acid. The information in this study is quite necessary to construct full functioning models that will help scientists to better understand mobility and bioavailability of metals in soils.