Metal ion extraction in surfactant solution: Ni2+(aq) and Cd2+(aq) with the ligands PADA and PAR in SDS micellar systems

There is much interest in industrial chemistry in the selective extraction and concentration of toxic metals from aqueous process streams and liquid wastes. In this paper, the use and scope of two simple ligands pyridine-2-azo-p-dimethylaniline (PADA) and 4-(2-Pyridylazo) resorcinol (PAR) in micellar solution are explored in relation to nickel and cadmium ion extraction. The thermodynamics and kinetics of the complexation reactions of Ni2+(aq) and Cd2+ (aq) with PADA and PAR are discussed; also the stripping reaction, to re-form the metal ion, which is achieved on addition of H+(aq) is investigated. A dramatic enhancement of metal ion extraction ability by the ligands is observed for both systems, the increase in the apparent rate and equilibrium constants for complex formation being of the order of 100. The stripping reaction is rapid for both metal ions. The mechanisms of the various reactions are discussed.