Exothermic adsorption of chromate by goethite

Goethite is a common Fe oxyhydroxide coating soil particle surfaces, which has a high Cr(VI) adsorption capacity under acidic pH conditions. Batch equilibrium adsorption experiments with chromate concentrations of 0.1, 0.2, and 0.3 mM were performed using solutions with ionic strengths of 0.1, 0.05, and 0.01 M and pH values of 3-11 and at four temperatures between 10 and 75 degrees C. The results of these experiments show that the amount of chromate adsorbed decreases as the pH increases towards the zero-point-of-surface-charge of goethite (pHPZC 9.1), which is typical for anions. The chromate adsorption efficiency also depends on the ionic strength of the solution. The amount of chromate adsorbed decreases as the ionic strength increases, indicating that outer-sphere surface complexation affects the adsorption process. The data were interpreted using the charge distribution multisite surface complexation (CD-MUSIC) model framework. Variations in the complexation constants with temperature were fitted using the two-term van't Hoff equation to give partial molar enthalpy and entropy constants. The enthalpies were negative, indicating that chromate adsorption is exothermic, whereby outer sphere binding was found to be less exothermic (Delta(r)G(OS,298) -62.19 kJ mol(-1)) than inner-sphere binding (Delta(r)G(IS,298) -112.25 kJ mol(-1)). The adsorption strength, therefore, decreases as the temperature increases because of the exothermic nature of surface complexation.