The influence of temperature on the adsorption of cadmium(II) and cobalt(II) on goethite

The adsorption of Cd(II) and Co(TI) onto goethite was measured at five temp eratures between 10 and 70 degrees C. For both cations the amount adsorbed at any given pH increased as the temperature was increased. Cd(II) adsorbed at a slightly lower pH at each temperature than Co(II). Adsorption isother ms at pH 7.00 for Cd(II) could be fitted closely by a simple Langmuir model , but a two-site Langmuir model was needed for Co(II). Potentiometric titra tions of goethite suspensions in the presence and absence of added cation c ould be modeled closely by a constant-capacitance surface complexation mode l that assumed the adsorption reactions M2+ + SOH reversible arrow SOM+ + H + and M2+ + SOH + H2O reversible arrow SOMOH + 2H(+), where M represents Cd or Co. This model also fitted the experimental data from the adsorption ed ge and adsorption isotherm experiments. Thermodynamic parameters estimated from both Langmuir and surface complexation models showed that the adsorpti on of both metals was endothermic. Values obtained for the adsorption entha lpies from both modeling schemes were similar for both cations. Estimates o f the adsorption entropies were model-dependent: Langmuir parameters yielde d positive entropies, while some of the surface complexation parameters gen erated negative adsorption entropies. (C) 1999 Academic Press.