Adsorption kinetics and mechanism of maxilon blue 5G dye on sepiolite from aqueous solutions

The use of sepiolite for the removal of maxilon blue 5G from aqueous solutions at different contact times, stirring speeds, initial dye concentrations, pHs, ionic strengths and temperatures was investigated. The adsorption process attained equilibrium within 60 min, which was an economically favorable requisite, in addition to the local abundance of the raw material. The extent of dye removal increased with increasing initial dye concentration, contact time, pH and temperature, and decreased with increasing ionic strength. On the other hand, stirring speed had no important effect on adsorption process. The adsorption process was found to be endothermic and physical in nature. Four kinetic models, the pseudo-first- and second-order equations, Elovich equation and the intraparticle diffusion models were selected to follow the adsorption process. Kinetic parameters, rate constants, equilibrium adsorption capacities and related correlation coefficients for each kinetic model were calculated and discussed. It was shown that the adsorption of maxilon blue 5G onto sepiolite could be described by the pseudo-second-order equation. The diffusion coefficients were calculated and found to be in the range of 3.625 x 10(-8) to 12.100 x 10(-9) cm(2)/s. Various thermodynamic activation parameters such as enthalpy of sorption Delta H*, Gibbs energy change Delta G* and entropy Delta S* were estimated. The positive value of Delta H* and Delta G* shows the sorption process is endothermic and not spontaneous. The negative value of entropy Delta S* shows the decreased randomness at the solid-liquid interface during the sorption of dye ions onto sepiolite particles. In addition, an activation energy of sorption has also been determined based on the pseudo-second-order rate constants. The activation energy of system (E-a) was calculated as 19.25 kJ/mol. Sepiolite, an inexpensive and easily available material, could be an alternative for more costly adsorbents used for dye removal in wastewater treatment processes. (C) 2007 Elsevier B.V. All rights reserved.