Investigating the mechanism of Au(III) transport using a polymer inclusion membrane with dibutyl carbitol as a carrier

In this article, firstly, the separation and transfer of Au(III) from gold chloride solution was investigated using a polymer inclusion membrane (PIM) system containing dibutyl carbitol (DBC) as carrier molecules, dioctyl phthalate (DOP), bis(2-ethyl hexyl) adipate (dioctyl adipate) (DOA) and tris (2-ethyl hexyl) phosphate (T2EHP) as plasticizers and high molecular weight polyvinyl chloride (PVC) as the base polymer. Fourier transform infrared spectroscopy (FT-IR), and scanning electron microscope (SEM) were used to identify PIMs. Kinetic studies featuring a two-step reaction, with the first step being reversible, were conducted across various sections to investigate the processes of extraction, stripping, and transport. The effects of different stripping agents such as water, HCl, thiourea, sodium thiosulfate, and oxalic acid was investigated, and the best results were obtained with thiourea. Also, the mechanisms of Au(III) extraction and transport were studied. Under the conditions of source solution, tetrachloroaurate anionic complex ${\left(\mathrm{Au}{\mathrm{Cl}}_4\right)}^{-}$ with protonated DBC (${\left({\left(\mathrm{ROR}\right)}_2\mathrm{OH}\right)}_{\left(M\right)}^{+}$) was extracted by ion solvation mechanism. The validity of the proposed extraction mechanism was confirmed through the use of nuclear magnetic resonance (¹H NMR) spectroscopy. In order to make the membrane with the best composition and the highest efficiency, the effect of weight percentage of the carrier and plasticizer was investigated. There was an optimal value for the weight percentage of plasticizer (10–20% by weight) and carrier (40%), in which the highest flux and permeability in the membrane occurred. The effect of the viscosity of different plasticizers (DOP, DOA, and T2EHP) on the Au(III) flux was also investigated. It was found that the flux increases linearly with the reduction of plasticizer viscosity. Effect of membrane thickness on the permeability, initial flux, and efficiency of Au(III) transfer, were conducted with thickness of 120, 80, 45, and 30 μm. By reducing the thickness of the membrane, the transfer flux, permeability and percent of Au(III) transport, were improved from 12.9 × 10⁻³ $m{h}^{-1}$, 45.3 × 10⁻⁷ $\mathrm{mol}{m}^{-2}{s}^{-1}$ and 18.2% to 26.4 × 10⁻³ $m{h}^{-1}$, 83.5 × 10⁻⁷ $\mathrm{mol}{m}^{-2}{s}^{-1}$ and 37.3%, respectively. According to the results, a membrane with a combination of PVC:DBC:D2EHP with a weight percentage ratio of 50:40:10 with a thickness of 30 μm was suggested. Optimized results with the proposed membrane were obtained to be: extraction rate constant of 1.43 $h^{-1}$, extraction percent of 99.1%, stripping rate constant of 37.5 × 10⁻⁴ $h^{-1}$, stripping percent of 40.9%, and permeability of 27.0 × 10⁻³ $m{h}^{-1}$, initial flux of 82.1 × 10⁻⁷ $\mathrm{mol}{m}^{-2}{s}^{-1}$.