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 (α  -FeOOH). Batch equilibrium sorption experiments were carried out at 25 °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 (R2⩾0.891)(R2⩾0.891) and Freundlich (R2⩾0.990)(R2⩾0.990) isotherms and the following parameters were derived from triplicate experiments: Kf=1.903±0.030mg/g and 1/n=0.728±0.0191/n=0.728±0.019 for the Freundlich model and b=0.021±0.003L/mg and Qmax=61±8mg/g for the Langmuir model. The thermodynamic parameters determined were the equilibrium constant, Keq=1.323±0.045Keq=1.323±0.045, and the Gibb's free energy, ΔG0=−0.692±0.083kJ/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±0.73.9±0.7; n=30n=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.

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