The Interaction Mechanisms of Gold(III) with the Metal Extractant PADA in Water/Micelle Media and Applications to Gold(III) Extraction

This PhD thesis reports the results of studies on extraction and recovery of gold and gold/copper separation from aqueous solutions using surfactant based technologies and on the thermodynamic and kinetic aspects of the reactions involved. Ionic surfactants able to form micelles in water matrices have been investigated, as they form a pseudo-phase which can be able to extract metals with high efficiency. The surfactants employed are sodium dodecylsulfate (SDS) and dodecyltrimethylammonium chloride (DTAC). The research can be essentially divided in three parts. In the first part (Chapter 3) the changes of the physico-chemical properties of the medium induced by the addition of the surfactant are investigated. Actually, the presence of micelles can induce changes of the medium pH, and the thermodynamics and kinetics of reactions eventually occurring on the micelle surface will be undoubtedly affected. Among these reactions, those involving ligands able to complex metals and carry them from the aqueous medium to the micelle surface are of special relevance for extraction purposes. Such reactions are pH dependent; hence, the first study has been focused to establish how much the pH on the micelle surface differs from that of the aqueous pseudo-phase and, as a consequence, to find out the shift of equilibrium constants induced by addition of micelles. The second part includes kinetic investigations of the binding of the selected precious metal to PADA (pyridine- 2 azo-p–dimethylaniline) in water, in DTAC and in SDS which are described in Chapters 4, 5, and 6 respectively. It has been found that PADA, besides forming stable complexes with gold(III), is endowed with excellent hydrophobic properties which make it an ideal carrier for the transport of the metals from water to micelle, making it possible metal extraction also in cases where the electrostatic attraction does not work anymore or acts in the contrary direction, as in the case of gold tetrachloroaurate extraction by SDS. The kinetic study enables the mechanism of the binding reaction to be worked out under the different investigated medium conditions. The analysis of the results shows that the starting reactant AuCl4− hydrolyzes to give differently oxydrilated forms (AuCl3OH−, AuCl2(OH)2−) and, in DTAC at low pH, also the aquoform AuCl3(H2O). These species are the reactive ones, whereas the tetrachlorocomplex apparently does not react with PADA, except that in water at relatively low pH values. The third part of the work (Chapter 7) has been devoted to developing methods for the extraction and recovery of gold from dilute aqueous solutions and for the separation of the components of mixtures of gold and copper. These methods are based, on one hand, on the direct transfer of the metals to the micelle and subsequent separation of the micelle-metal aggregate from the aqueous matrix by ultrafiltration. This procedure is denoted as “Micellar Enhanced Ultra Filtration (MEUF). On the other hand, a method where a ligand is added to the solution (to improve the efficiency of the extraction procedure or to impart selectivity to the system) has been employed. In this case the ultrafiltration technique is denoted as “Ligand Modified Micellar Enhanced Ultrafiltration “(LM-MEUF). The methods devised in the present study for separation of the elements present in mixtures of gold and copper are based on the electrostatic attraction of one of the species and on repulsion of the other by a given micelle. In this work, the negatively charged species AuCl4- is attracted by the positively charged DTAC micelles while simultaneously the repulsion of Cu2+ does occur. On the same principle is based the Au/Cu separation based on Cu2+ attraction by negatively charged SDS micelles and repulsion of AuCl4-. The systems considered in this thesis have been subject to the following studies: • Thermodynamic study of the equilibria of ligand protonation in the presence of surfactant. • Study of the ligand partition between the aqueous and micellar phases. • Spectral observations of the metal/ligand interaction, in aqueous solution and in the presence of micelles. • Kinetic studies of the mechanism of metal/ligand complexation, in aqueous solution and in the presence of DTAC and SDS micelles. • Developing of an ultra-filtration process to separate the micellar pseudo phase, containing the extracted metal, from the aqueous phase, i. e setting a LM-MEUF separation process. • Separation of gold and copper from their mixtures by micellar ultrafiltration. • Study of the metal stripping by means of acids and salts addition for metal recovery purposes.