Binding of Pd(II) to Pada in water/micellar system: Complex formation, kinetics in water and DTAC solution

The kinetics of binding of Pd(II), initially present as PdCl42−, to the azo-dye ligand pyridine-2-azo-p-dimethylaniline (PADA) in dodecyltrimethylammonium chloride (DTAC) micellar solution has been investigated as a preliminary study on palladium micellar extraction and recovery. For comparison, the same investigation has been performed in water. Palladium reacts with PADA forming a strong 1:1 complex both in water and in DTAC micellar solution. Addition of DTAC, at concentrations just above the critical micellar concentration, results in a large enhancement of the rate of complex formation. The analysis of the kinetic behaviour reveals that several reaction paths are present. In water, under the experimental hydrogen ion concentration range investigated, reaction steps involving the protonated ligand forms PADAH+ and PADAH2++ are important and the reacting palladium species are PdCl42− and Pd(H2O)Cl3−. In DTAC micellar solution, PADA reacts in its unprotonated form and a new reaction path appears involving the hydroxo species PdCl3OH2− which is about 10-times faster than the other reaction paths. The large catalytic effect produced by DTAC micellar solution provides a promising basis for the extraction of palladium from water using the PADA/DTAC system.