Determination of the rate control step of chlorinated anilines in ligand exchange reactions on solid phase by using breakthrough technique

Kinetic studies of chlorinated anilines as ligands on the cobalt(II)-carboxylated diaminoethyl-sporopollenin (CDAE-sporopollenin) have been performed using continuous column runs. Adsorption rate measurements were carried out by using a UV–visible spectrophotometer, and rate control step of the reaction was determined. The adsorption of chlorinated anilines from solutions of different initial concentrations onto resin was measured as a function of time at 25 °C. The initial ligand concentrations ranged from 6.0×10−4 to 2.5×10−3 mol l−1. The results concerning the influence of the external solution concentration of the ligands upon the fractional attainment of equilibrium prove that the diffusion of the ligands into the resin beads plays an important role in the mechanism of the exchange reaction, although the exchange rate is chemical in its ultimate mechanism. A very fast initial adsorption resulting in an almost instantaneous attainment of the bulk of the equilibrium adsorption was followed immediately by a very slow rate of adsorption. In the case of 4- and 2,5-dichloroaniline, in all of the concentration range, the graphs of −ln[1−U(α)] versus t were a straight line, which suggests that fractional attainment of equilibrium adsorption under film diffusion control is observed. On the contrary, 2- and 3-chloroaniline, the graphs were not a straight line and have underwent adsorption under particle diffusion control.