Thermodynamics of the adsorption of palmitate and laurate soaps onto some metal ore surfaces in aqueous media

The thermodynamics of the adsorption of sodium-palmitate and sodium-laurate onto galena, hematite and cassiterite surfaces in aqueous media has been studied. The adsorption capacities of the soaps (adsorbates) were determined and the Langmuir, Freundlich and Temkin isotherm models were used to fit the experimental data. The Langmuir isotherm model was the best to describe the adsorption process with the highest correlation coefficient. Evaluation of the complete monolayer adsorption capacity at 29°C using the Langmuir equation showed 0.0506 mmol/g for galena, 0.0494 mmol/g for hematite, 0.0310 mmol/g for cassiterite (for sodium-palmitate adsorption) and 0.0802 mmol/g for galena, 0.0778 mmol/g for hematite, 0.0467 mmol/g for cassiterite (for sodium-laurate adsorption). These values are in the order: galena > hematite > cassiterite. The change in Gibbs free energy, ∆G, enthalpy, ∆H and entropy, ∆S of adsorption were computed and the values indicate that the adsorption processes are spontaneous, exothermic and favoured at low temperature suggesting physisorption as the predominant process.