Adsorption of Fe(III) from water by natural and acid activated clays: Studies on equilibrium isotherm, kinetics and thermodynamics of interactions

The present work investigates the removal of Fe(III) ions from an aqueous solution by kaolinite, montmorillonite and their acid activated forms. The specific surface areas of kaolinite, acid activated kaolinite, montmorillonite and acid activated montmorillonite were 3.8, 15.6, 19.8 and 52.3 m2/g respectively whereas the cation exchange capacity (CEC) was measured as 11.3, 12.2, 153.0, and 341.0 meq/100 g for four clay adsorbents respectively. Adsorption increased with pH till Fe(III) became insoluble at pH > 4.0. The kinetics of the interactions is not certain, but the second order kinetics (k2 = 4.7 × 10− 2 to 7.4 × 10− 2 g mg− 1 min− 1) appears to give a better description. Langmuir and Freundlich isotherms were applied and isotherm coefficients were computed. The Langmuir monolayer capacity of the clay adsorbents was from 11.2 to 30.0 mg g− 1. The process was exothermic with Δ H in the range of −27.6 to −42.2 kJ mol− 1accompanied by decrease in entropy (Δ S = −86.6 to −131.8 J mol− 1 K− 1) and decrease in Gibbs energy. The results have shown that kaolinite, montmorillonite and their acid activated forms could be used as adsorbents for separation of Fe(III) from aqueous solution. Acid activation enhanced the adsorption capacity compared to the untreated clay minerals.