1Department of Chemistry, University of Athens, 15771, Greece
2Department of Soil Science and Agricultural Chemistry, Szent István University, H-2103, Hungary
3National Agricultural Research Foundation of Greece, Soil Science Institute of Athens, 14123 Lycovrissi, Attiki, Greece
ABSTRACT
The adsorption of Cu by bentonite and by soils high in iron oxides, and the effect of pH were the subject of this investigation. These processes were studied in the presence of 0.01 M K2SO4 under different initial concentrations of Cu (12–20 mg.cm-3 of bentonite or soil solution) and pH (4.0–8.0). Parameters characteristic of the isotherm models recorded for each pH value were used to compare the copper sorption processes examined at various pH values. In the course of this work the Langmuir isotherm model was employed. Using the parameters of these separate models, a model suitable for the description of the pH dependence of sorption was elaborated. This model, which is a logistic function containing an additive member, provides a good description of the pH dependence of both the adsorption maximum and the steepness of the initial section of the adsorption isotherm. Models using either the adsorption maximum or the initial steepness proved to be equally suitable for the description of the copper adsorption taking place on bentonite. In soil a linear correlation was found at all pH values between the quantity of adsorbed copper and the concentration of the equilibrium solution. These straight lines can be regarded as the section of the Langmuir equations, which can be approximated with an initial straight line. The slope of the straight lines equals to the product of the adsorption maximum and the energetic constant. With the help of the model it is possible to calculate the pH value at which copper adsorption changes to the greatest extent as the result of changes in pH. In the cases examined, this value was around pH 7, which correlates well with the fact that the copper ion forms adsorbed on the surface exhibit the most intensive change at a pH of around 7.
Keywords: adsorption, model, copper, pH, bentonite, soil.