Simultaneous effect of pH, temperature, ionic strength, and initial concentration on the retention of Ni on illite

Adsorption of metals by clay minerals is a complex process involving different mechanisms, and controlled by different variables that can interact. The aim of this work was to study the simultaneous effect of pH, initial concentration, ionic strength, and temperature on the retention of Ni on illite by Response Surface Methodology. As fitting and analyzing response surfaces is greatly facilitated by the proper choice of experimental conditions, we previously obtained Ni adsorption isotherms at constant pH and adsorption edges as a function of pH. Adsorption isotherms at 15, 25 and 35 degreesC (pH 7.5) were performed to deduce thermodynamic parameters. Surface topography and atomic composition of illite samples doped with Ni were studied by scanning electron microscopy and X-ray fluorescence microanalysis. Below pH 7, ionic strength had the largest effect on Ni adsorption illite suggesting that ion exchange with H3O+ and Na(I) ions that saturated the exchange sites was the main retention mechanism; Al(III) ions whose concentration in solution increases with decreasing pH can also compete with Ni(H) ions. For pH values higher than 7, the amount of Ni retained increased with the initial concentration, pH and temperature; the effect of ionic strength on the amount of Ni(II) retained decreased with pH, being negligible at pH 8. Precipitation was nucleated by illite surfaces; as Ni percentage in illite surface increased, Si percentage decreased. These facts are compatible with a more specific process involving hydrolyzed species, where nickel can associate with illite as an inner sphere complex. (C) 2003 Published by Elsevier Science B.V.