Metal ions extraction and recovery by using micellar systems

This PhD thesis concerns a research on heavy metal ions extraction from aqueous solutions using surfactant based technologies. In particular surfactants dispersed into water matrices and able to form micelles have been investigated, as they form a pseudo-phase that could extract complexed metals with high efficiency. The research can be essentially divided in two parts. Actually, the first stage of the work has concerned kinetic and thermodynamic studies on complex formation between metal ions and ligands in the presence of micellar systems and for comparison, the same investigation has been performed in the absence of surfactant. The second part of the work was instead dedicated to developing a method for the extraction and recovery of metal ions from dilute aqueous solutions, based on micellar enhanced ultrafiltration process in the presence of an extractant (LM-MEUF) . The first part of the research is mainly related to the field of metal extraction optimization, actually knowledge of the equilibria and mechanisms of metal complex formation (and dissociation) reactions is of crucial importance in order to project efficient procedures for extraction and recovery of metals. Concerning the metals, palladium, gallium and cadmium have been taken into account. Palladium has been chosen as representative of precious metals, while cadmium represents very well the category of pollutant metals, being among the most hazardous heavy metals. Finally we found interesting to study the extraction process of a trivalent metal ions, as gallium, which meets increasing demand being more and more used in the fields of electronics and nuclear medicine. The anionic surfactant sodium-dodecyl-sulphate has been used in the extraction of cationic metals, i.e Ga(III) and Cd(II), whereas the cationic dodecyl-trimethylammonium chloride, suitable to attracting on the micellar surface negatively charged metal complexes, has been employed in the case of palladium which was kept in solution as tetrachloropalladate. As extracting agents two organic lipophilic ligands were chosen: pyridine -2 azo-p – dimethylaniline (PADA) and 8-hydroxyquinoline. The ligand PADA is able to form stable complexes with cadmium and palladium while 8-hydroxyquinoline was preferred for gallium extraction, since trivalent cations display higher affinity towards this ligand. This thesis shows that the HQ/SDS, PADA/DTAC and PADA/SDS systems, coupled with ultrafiltration techniques provide an excellent tool for gallium, palladium and cadmium extraction/separation/concentration respectively. Concerning the metal recovery cadmium can be easily recovered by a stripping reaction performed under mild conditions by using weakly acidic solutions of NaCl. Moreover the investigation on palladium extraction shows that a continuous process for removal and recovery of palladium from waste water could be planned and it would enhance the recovery yield.