The interaction mechanism of gold(III) with the metal extractant PADA in DTAC micellar medium and applications to gold(III) extraction

The kinetics of binding of Au(III), initially present as AuCl4 -, to the azo-dye ligand pyridine-2-azo-p-dimethylaniline (PADA) in dodecyltrimethylammonium chloride (DTAC) micellar solution have been investigated as a preliminary study on gold micellar extraction and recovery. PADA (Figure 1) is endowed with excellent hydrophobic properties, which make it an ideal carrier for the transport of the metal ions from water to micelle, allowing the metal extraction process to be carried out. The kinetic study enables the mechanism of the binding reaction to be worked out under different investigated medium conditions, showing that in the presence of the DTAC micellar pseudo-phase the reaction is strongly accelerated (catalytic effect) compared to water. The results concur in suggesting that differently oxydrilated forms, originated from the starting AuCl4-, are reactive and, in DTAC at low pH, also the aquoform AuCl3(H2O) reacts with PADA, whereas the tetrachlorocomplex apparently does not react, except that in water at relatively low pH. The characteristics of the water/micelle system have been also exploited, for the purpose of extracting and recovering gold, by applying the micellar enhanced ultrafiltration (MEUF) procedure. Using MEUF the negative AuCl4- ion is extracted with yields near to 100% by DTAC, owing to direct adsorption on the micelle surface. The recovery step has been accomplished adding an electrolyte (NaCl) solution, which lowers the surface potential of the micelle, thus favoring the recovery process. Ammonia has been also added in order to convert the gold(III) chlorocomplexes into the ammonia complex, which is repelled from the positively charged DTAC surface and can be recovered. A recovery yield of 86% has been achieved, which provides a promising basis for the extraction of gold from water using the surfactant DTAC.