Counter-ion effect on the kinetics of chromium (III) sorption by Amberlyst.15 in H+, Li+, Na+, Ca++, Al+++ forms

The counter-ion effect on the kinetics of Cr (III) sorption by macroporous strong acid exchanger Amberlyst.15 is investigated as a function of the nature of the counter-ions at different temperatures. The rate is observed to follow the order H+ > Li+ > Na+ > Ca++ > Al+++. The pH is observed to increase with time, during the sorption by Li+, Na+ and Ca++ forms of the exchanger and decrease during the Cr(III) sorption by the H+ form. In case of Al+++ form of the exchanger, however, the pH remains almost constant. Both the film and particle diffusion equations are applied to explain the kinetic data. The rate constant values obtained for the particle diffusion equation are observed to follow the same order as described above for the rate. The low activation energy values calculated from the Arrhenius equation confirm the diffusional nature of the exchange process. The thermodynamics parameters of activation are also calculated using the Eyring equation. The activation energy and enthalpy of activation are observed to increase while entropy of activations is observed to decrease from H+ to Na+ form of the exchanger. The free energy of activation is observed to be the same for H+, Li+, Na+, Ca++ and Al+++ forms of the exchanger. The thermodynamic parameters of activation reveal that the mechanism of Cr(III) sorption is the entropy driven ion exchange and is dependent upon the ionic radii and charge of the counter-ion initially present inside the resin. (C) 2010 Elsevier B.V. All rights reserved.