The kinetics of gold(III) extraction by pyridine-2-azo‑p‑dimethylaniline in water and in micellar systems

Abstract

The kinetics of reaction between AuCl4-, and the azo-dye pyridine-2-azo-pdimethylaniline (PADA) have been investigated in water and in the presence of DTAC micelles, using classical spectrophotometry and the stopped-flow technique. PADA reacts with different chloro/hydroxo gold(III) complexes, in turn formed as the pH and Cl-concentration were changed, according to a network of parallel paths. In aqueous solution, at low pH values, a fast step is observed which is ascribed to the ligand induced expulsion of a labile water molecule from the reacting species Au(H2O)Cl3 which forms at low pH values. At higher values of pH, the reaction is much slower because in the key step PADA has to replace the more inert Clions in the gold coordination shell. In the presence of DTAC a remarkable catalytic effect is observed, owing to the absorption and attraction of the reactants on the micelle surface. Moreover, DTAC favors the formation of aquochloro aurates, thus inducing a change in the gold(III) speciation compared to that in water. The analysis of the data suggests that the aquo species Au(H2O)Cl3 and Au(H2O)2Cl2 + play a major role in the reaction mechanism.

The mechanism of the reaction between Au(III) and PADA in sodium dodecylsulphate

“…The extent of retention is independent of the solution pH. [8], is totally different from the spectrum in water. In contrast, the shape of the spectrum in SDS does not differ from that of the spectrum recorded in water.…”

“…The commercially available ligand PADA ( Figure 1) is totally adsorbed on a variety of surfactants, including SDS [4] and has proven to be an excellent metal extractor, being able to transfer metal ions from water to micellar phases with yields approaching 100% [5][6][7] Moreover, we have found that PADA is able to complex Au(III) with great affinity even if the coordination shell of the metal is occupied, as in AuCl4 - [8,9]. In this case, a molecule of PADA replaces two chloride ions forming a strong chelate.…”

“…The stock solutions of these reactants were prepared by dissolving appropriate amounts of the relevant solid material in water. The gold content was determined by direct UV absorption spectrometry exploiting the absorption band of AuCl4ion centred at 233 nm whose maximum absorption is proportional to the gold concentration [8,9]. Water, purified by pumping deionized water through a Milli-Q apparatus, was used to prepare the solutions and also as the reaction medium.…”

Gold(III) extraction and recovery and gold(III)/copper(II) separation using micelles

“…Note that Au(III) absorbs in the UV [31], hence, the UV-vis spectrophotometric technique could have been used to quantify the gold content. However, spectrophotometry becomes unsuitable for low Au(III) concentrations ( = 7.1 × 10 3 M −1 cm −1 at 233 nm).…”

Counterion‐Induced Cationic Flexible Nanoparticle Catalytic of Piperidinolysis of Phenyl Salicylate

“…Thus, under such conditions, the rate of hydrolysis is negligible compared with rate of piperidinolysis of PSa -. It is widely known that the ability of surfactant to form nanoparticle aggregates such as micelles could create different media in reaction phases that might affect the rate of the reactions [39,40]. A pseudophase micellar (PM) model can be used to explain the effect of TTABr micelles on the rate of nucleophilic reaction between Pip and PSa - [41] as 4) and (5) with kinetic parameters (k 0 , h, K X/S and k 0 , X k cat , K X/S ) listed in Table 1, at ½4-FBzNa] op 0 ¼ 0:0046 (filled circle), 0.0093 (filled triangle) and 0.0131 (filled diamond) 4) and (5) with kinetic parameters (k 0 , h, K X/S and k 0 , X k cat , K X/S ) listed in Table 1 [26].…”