The mechanism of the reaction of gold(III) ion with pyridine-2-azo-p-dimethylaniline in water and SDS

The gold extraction and recovery from its scraps and waste fluids has attracted considerable attention because of increasing industrial demand of gold in electronics and devices, in biomedicine and catalysis. In recent years the research on metal extraction based on micellar system indicates that ligand modified micellar enhanced ultrafiltration (LM-MEUF) can be a promising green technique for the separation, extraction and recovery of metal-ions. Previous studies from our laboratory on gold extraction with sodium dodecyl sulphate (SDS) have been demonstrated that the use a complexing ligand, as pyridine-2-azo-p-dimethylaniline (PADA), greatly enhances gold(III) extraction yields. Furthermore, in order to understand the fundamental aspects of process, the kinetics of complex formation reaction between gold(III), initially present as AuCl4-, and PADA have been investigated in water, and in mixtures of water and SDS using classical spectrophotometry and stopped-flow technique. The tetrachloroaurate anion in aqueous solution undergoes multiple hydrolysis forming several chlorogold(III) species. Our results show that the equilibria represented by equation (1) play a very important role in the kinetics of gold(III)/PADA system. Actually, it has been found that the reaction rates depend in a rather complex way on the acidity and chloride ion concentration. The analysis of the kinetic behavior reveals role of the surface charge of the micellar system on the binding reaction. In effect, in the presence of anionic micelles (SDS), an “anticatalytic effect” has been observed that should be ascribed to repulsion of the negatively charged gold(III) complexes by the negative SDS micelles