Kinetics and thermodynamics of Cd (II) adsorption onto particle-sized fractions of yellow-brown soil]

A batch method was used to investigate the kinetics and thermodynamics of cadmium adsorption onto different particle-sized fractions (2 000 - 200 microm, 200 - 20 microm, 20 - 2 microm and 2 - 0 microm, which were named coarse sand, fine sand, silt and clay respectively according to international system) derived from yellow brown soil by using a novel technology combined with wet sieving, sedimentation-siphoning and centrifugation method. The results showed that the reaction of adsorption can be divided into two types: a fast reaction in the first 15 min and a slow reaction in the later reaction course. The amounts of Cd2+ adsorbed by the fast reaction exceed 95% of the adsorption capacity. As the temperature increased from 25 degrees C to 45 degrees C, the adsorption capacities of the four fractions increase by 4.86%-25.3%. First-order rate equation and pseudo second-order rate equations were applied to express adsorption kinetics. Adsorption processes for Cd2+ onto the four fractions were found to follow pseudo-second order type adsorption kinetics. The pseudo second-order rate constants exhibited that adsorption speed to reach equilibrium decreased with the increase of particle size. Intra-particle diffusion might be the major rate-limiting step. Thermodynamic parameters including deltaH0, deltaS0 and deltaG0 were also calculated from graphical interpretation of the experimental data. Positive deltaH0 values indicated the adsorption processes to be endothermic. Negative deltaG0 values implied that adsorption reaction was a spontaneous process.