Application des modèles de Langmuir et Freundlich aux isothermes d'adsorption des métaux lourds par l'argile purifiée

Abstract
Bentonite, which consist essentially of clay minerals belonging to the smectite group, have a wide range of chemical and industrial uses. The structure chemical composition, exchangeable-ion type and small crystal size of smectite are responsible for several properties, including a large chemically active surface area, a high cation-exchange capacity and interlamellar surface having usual hydratation characteristics. A sample collected from Zaghouan (North East Tunisia, North Africa) is studied through some physico-chemical methods. Results from X-ray diffraction, chemical analysis, infrared spectroscopy, thermogravimetric analysis (TGA) and differential thermal analysis (DTA), cation exchange capacities, specific and total surfaces, confirm the general smectite character of the sample. The adsorption capacity of this clay was tested out using three metallic ions (Pb²⁺, Zn²⁺, Ni²⁺). The results showed that, in all cases, adsorption can be illustrated by Freundlich or Langmuir isotherms. However, for 10^-3M Pb²⁺ the low value of the correlation coefficient (R²) indicated that the experimental data for the adsorption didn't fit to any linear form of the Langmuir equation. Metal adsorbed onto Zaghouan clay varied in the decreasing order PbPb²⁺ > Zn²⁺ > Ni²⁺ and fitted in satisfactorily with the uptake capacity. For Pb²⁺ the amount of adsorbed ions remained higher than the CEC (cation exchange capacity) of the clay fraction. This result may be due to adsorption of hydroxy lead complex in addition to sorption of bivalent lead form which explains the high amount of Pb²⁺ removed from aqueous solution.