Chloride ion concentrations as well contribute to the change of the reacting species of Au(III) according to the equilibrium reaction:

107 -Conclusions-

CONCLUSIONS

A study of the complex formation between gold(III) and the ligand PADA (pyridine-2-azo-p-dimethylaniline) has been performed in pure water, and in the micellar systems water/SDS where the SDS micelles are negatively charged, and water/DTAC were the DTAC micelles are positively charged. The binding process formation is rather complex since several hydroxochlorocomplexes of Au(III) are formed in quite narrow ranges of pH and chloride concentrations which make rather complex the features of the binding process.

The behavior of reacting system is very sensitive to the micelle charge. In the presence of the DTAC pseudo-phase the reaction is strongly accelerated (catalytic effect) compared to water whereas in the presence of SDS the reaction is largely decelerated.

Chloride ion concentrations as well contribute to the change of the reacting species of Au(III) according to the equilibrium reaction:

Cl

+ Au(OH)

Cl

AuCl(OH)

Cl

+ OH

The above equation, beside explaining the chloride dependence of the reaction also enlighten the dependence of the reaction features on [OH

] (expressed as a dependence on pH). The features reflect changes of the gold(III) species reacting with PADA, while the ligand itself will react as a neutral molecule or a protonated molecule depending on pH and on the kind of micelle present in solution. Compared to the acidity level in water, the pH in SDS is lower while the in DTAC is higher. This fact reflects in the pK

of weak acids present in solution, including PADA. The pK

change occurring when transferring PADA from water to SDS and to DTAC has been evaluated in this thesis.

Finally the characteristics of the water/micelle systems have been exploited for

the purpose of extracting and recovering gold by applying both the micellar

108

-Conclusions-

enhanced ultrafiltration (MEUF) and the ligand modified micellar enhanced ultrafiltration (LM-MEUF) procedures. Using MEUF the negative AuCl

ion is extracted with yields near to 100% by DTAC owing to direct adsorption on the micelle surface. The water/DTAC system also allows a quantitative separation of AuCl

from Cu

since the copper ions stay in the aqueous pseudo phase while gold(III) is completely extracted on the DTAC surface.

Concerning the water/SDS system, MEUF application results in the extraction of gold(III), although with a modest yield of about 30%. The ability of SDS to extract gold present as AuCl

, despite the negative charges present on the micelle surface, can be explained assuming that SO

groups constituting the head of the surfactant display some binding ability towards gold(III). On LM- MEUF application, the yield of extraction is raised to 100% by virtue of the binding affinity of PADA for gold(III) (which transfers the negative charged gold chloro-complexes in to a positively charged Au-PADA chelate) and the hydrophobic characteristics of the extractant.

Once the extraction procedure has been applied, the second step has been that of recovering the metal from the micelle to which it is bound either in form of chloro complex or in form of chelate with PADA. The recovery step is accomplished adding an electrolyte solution (as NaCl or HCl or their mixtures) which lower the surface potential of the micelle, thus favoring the transfer of the metal from the micelle to the recovery compartment. In the case of DTAC, ammonia has also been added as a stripping agent in order to convert the gold(III) chloro complexes into positively charged gold(III) ammonia complex.

The latter is repelled from the DTAC surface and can be recovered. On adding first NaCl and then NH

as stripping solutions, a recovery yield of 86% has been achieved.

The obtained results seem to promise that the procedures developed in this

thesis will lead to a satisfactory method for gold extraction, although future

experiments are needed to improve the recovery yield.