Use of general purpose adsorption isotherms for heavy metal clay mineral interactions

General purpose adsorption isotherm equations were fitted to the sorption data of lead and cadmium on both unmodified and Ca-saturated kaolinite and montmorillonite. Langmuir-Freundlich, Redlich-Petersen, Toth, Dubinin-Radushkevich, competitive Langmuir, and some modified forms of these isotherms were fitted to experimental data, and their goodnesses of fit are compared. The adjustable parameters of the Langmuir-and Freundlich-based isotherms were estimated by nonlinear least-squares analysis. The application of these two isotherms which allow for the effect of surface heterogeneity for both multiple and single desorbing ions during the sorption process was also studied. It was concluded that Redlich-Petersen, Toth, and original and modified forms of Dubinin-Radushkevich isotherms perform superior to the others both in high and lour concentration regions for both unmodified and Ca-saturated clays. Competitive Langmuir adsorption isotherm for Ca-saturated clays fitted web for Pb adsorption, while a change of slope was observed for Cd adsorption. (C) 1998 Academic Press

Effect of pH in an aqueous medium on the surface area, pore size distribution, density, and porosity of montmorillonite

Surface area, pore volume distribution, and porosity of montmorillonite are determined after being exposed to aqueous solutions with various pH values. For measurement, after each pH treatment the clay samples were freeze-dried in order to keep the structure of the clay same as that in the aqueous solution. Surface area and pore size distribution measurements were performed on an unmodified and four pH (2.5, 4.0, 5.5, 9.0)-adjusted clay samples using N-2 and CO2 as adsorbates. The surface area measurements at lower pH are lower than those measured at higher pH due to the replacement of Na+ ions from interlayers at lower pH which may be followed by the reduction of repulsive forces. As the pH of the montmorillonite increases to 9.0, the surface area increases almost seven times compared to that at pH 2.5. Pore size distribution was determined by CO2 adsorption at 273 K for micropores (500 Angstrom). Since N-2 molecules undergo a molecular sieving at 77 K CO2 gas at 273 K was used to measure the micropore surface area and gave higher surface area measurements than those measured by N-2. Surface area values measured by CO2 adsorption ranged between 30 and 200 m(2)/g for montmorillonite while those measured by N-2 adsorption were 13.7-70 m(2)/g in the pH range studied. (C) 1999 Academic Press