Removal of 2,4-dichlorophenoxy acetic acid from aqueous solutions by partially characterized organophilic sepiolite:: thermodynamic and kinetic calculations

The adsorption of 2,4-dichlorophenoxyacetic acid (2,4-D) on organophilic sepiolite (dodecylammonium sepiolite, DAS) was Studied as a function of solution concentration and temperature. The observed adsorption rates were found to be equal to the first-order kinetics. The rate constants were calculated for temperatures ranging between 25 and 40 degreesC at constant concentration. The adsorption energies, E. and adsorption capacity, q(m), for 2,4-D adsorption on organophilic sepiolite was estimated using the Dubinin-Radushkevic equation. Thermodynamic parameters (Deltag(a), Deltah(a), Deltas(a)) were determined by a new approximation from the isotherm of 2,4-D adsorption on DAS. Also, DeltaS(0) and DeltaH(0) values were calculated from the van't Hoff equation. These isotherms were modeled according to the Freundlich and Dubinin-Radushkevic adsorption equations. The amount of adsorption of this herbicide on organophilic sepiolite was found to be dependent on the relative energies of adsorbent-adsorbate, ad sorbate-sol vent, and adsorbate-adsorbate interaction. (C) 2004 Elsevier Inc. All rights reserved.