Investigations into the kinetics and thermodynamics of Sb(III) adsorption on goethite (α-FeOOH)

This study reports thermodynamic and kinetic data of Sb(III) adsorption from single metal solutions onto synthetic aqueous goethite (alpha-FeOOH). Batch equilibrium sorption experiments were carried out at 25 degrees C over a Sb:Fe molar range of 0.005-0.05 and using a goethite concentration of 0.44 g Fe/L. Experimental data were successfully modelled using Langmuir (R-2 >= 0.891) and Freundlich (R-2 >= 0.990) isotherms and the following parameters were derived from triplicate experiments: K-f = 1.903 +/- 0.030 mg/g and 1/n = 0.728 +/- 0.019 for the Freundlich model and b = 0.021 +/- 0.003 L/mg and Q(max) = 61 8 mg/g for the Langmuir model. The thermodynamic parameters determined were the equilibrium constant, K-eq = 1.323 +/- 0.045, and the Gibb's free energy, Delta G(0) = -0.692 +/- 0.083 kJ/mol. The sorption process is very fast. At a Sb:Fe molar ratio of 0.05, 40-50% of the added Sb is adsorbed within 15 min and a steady state is achieved. The experimental data also suggest that desorption can occur within 24 h of reaction due to the oxidation of Sb(III) on the goethite surface. Finally, calculated pH of the aqueous solution using MINTEQ2 agrees well with the measured pH (3.9 +/- 0.7; n = 30). At pH 4, the dominant Sb species in solution are Sb(OH)3 and HSbO2 which both likely adsorb as inner sphere complexes to the positively charged goethite surface. (c) 2006 Elsevier Inc. All rights reserved.