Cadmium Distribution within Model System Containing Kaolin and Humic Acids

Binding cadmium with humic acids (HA) taken from a sample of lake bottom sediment has been studied depending on the acid-base medium status parameter of the system (within the range of pH 3-10) and on the concentration of cadmium ions (5, 10 and 50 m mol/L). Ion-exchange separation of non-bound cadmium ions and its complexes with HA has been carried out using the method of ion exchange on sulphocationite with the subsequent analysis via the method of flame atomic absorption spectroscopy. Cadmium binding has been investigated for the system containing kaolin and HA, within the range of pH from 3 to 10. A comparative analysis has been performed for total cadmium binding in the systems containing kaolin and HA separately, as well as in the systems where they are present simultaneously with all other factors being constant. We took into account he fraction of metal bound with HA in the solution, in the sediment and on the surface of kaolin. It has been demonstrated that the additivity of the systems (the decrease of free metal concentration in a trinary system is equal to the sum of concentration decrease values in double systems under the same conditions) is observed within the pH range of 4.6-5.8. In other cases the binding of cadmium in a trinary system would be lower, than it is for the sum of double systems: first, due to HA deactivation owing to its adsorption onto the surface of kaolin (within acidic pH range), and then due to blocking the adsorption centres of kaolin (within neutral and alkaline pH ranges).