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with 20 or 30 mg of the ligand as indicated. The glass tube was plugged with glass wool at both ends and also between the ligand and the filter paper. The glass tube was mounted inside a stainless steel extraction vessel, tightened into the heating block and statically extracted using unmodified SF-C02 for 30 min at 400 atm, with temperature varied from 60 to 120°C. The extraction cell was then vented into a collection vessel containing 5 ml of DMSO or MIBK for 15 min (dynamic extraction). Calixarene reagents C2, C3 and C4 are soluble in MIBK while C1 is soluble in DMSO. Having established the optimum temperature for each ligand, the pressure was then varied between 200 and 400 atm. Collected extracts were analysed by flame atomic absorption spectrometry (FAAS). The percentage gold extracted was determined by direct comparison with collecting solutions spiked with standard Au (III) solution. The extraction was also studied as a function of the amount of water added to the spiked Au(III) on the filter papers. The filter papers were allowed 30 min to air dry after the metal spike and were subsequently each spiked with different amounts of water (0 - 40 ml): Extractions were then carried out under optimum conditions, as described above. Finally extractions were carried out on mg samples of solid Au(O) placed in the extraction cell instead of spiked filter paper. Au(0) can be obtained in mg quantities from the standard Au(III) solution by reduction and heating or by the addition of hydroxylamine hydrochloride. The effect of adding microlitre volumes of standard Fe(III) in HCl solution as an oxidant onto the reduced gold solid was followed by FAAS analysis of collected extracts. RESULTS AND DISCUSSION Macrocyclic ligands have received particular attention, as host molecules, since the pioneering work of Pedersen on the synthesis and metal extraction properties of crown ethers. His work began a revolution in macrocyclic ligand and receptor design which continues today as newer host molecules, such as the calixarenes, with unique selectivities of binding and mechanisms of release are produced. The concepts that the cavity size and shape in these macrocycles could be tailor-made and fine-tuned to suit the selected metal cation diameter and that donor atom choice determines the cation selectivity, have caught the imagination of many chemists over the last thirty years. In particular separation scientists have been quick to demonstrate selective metal extractions, using techniques of liquid-liquid extraction, solid phase and membrane extraction. Our laboratory has previously reported the selective supercritical fluid extraction of Fe(III) from metal mixtures using fluorinated molecular baskets and linear monohydroxamate reagents (22, 23). The schematic diagram shown below illustrates how such macrocyclic reagents, when sufficiently soluble in supercritical CO2, can be used to permeate through a solid matrix, sequester the targeted metal and deposit it in a collection vessel. This paper examines the use of new fluorinated calixarenes, aptly named 'molecular baskets' for the selective supercritical fluid extraction of gold. 56 <:00' Cold Bulletin 1999,32(2) Sample Matrix in Extraction Cell CO2 Figure 3 Schematic diagram ofSFE ofAu ions using macrocyclic extractants Initially, the solubilities of the reagents were determined in unmodified SF-C02 using the method of weight loss described in the experimental section, at 60°C and 200 atm and also at 60°C and 300 atm. The presence of a fluorinated side chain greatly improves the solubility of the calixarenes. For C2 - C4, measured solubilities (rnmol/L) were 0.12, 0.13, 0.74 at 60°C, 200 atm increasing to 2.31, 0.29 and 1.96 at 60°C, 300 atm, respectively. The selective extraction of Fe(III) in unmodified SF- CO2 from a mixture with other metal ions spiked on cellulose paper, has been demonstrated previously using C1. Here, using Au(III) spiked onto paper, with the pressure set at 400 atm, extractions were carried out with the temperature varied between 60 and 120°C. The extract using C1 was collected in DMSO and the extracts involving the other reagents C2, C3 and C4 were collected in MIBK. Collected extracts were analysed at the ppm level for Au(III) by FAAS (Figure 4). No gold extraction was recorded for C1 under the above conditions. This result was expected for C1. Apart from the calixarene thiol C3, the synthesized calixarenes are expected to form charged complexes with Au(III). The extraction of gold especially at the levels obtained for C4 is thus surprising and as suggested by Wai, the involvement of counter ions is suspected. MR Ligando in Supercritical Extracted Metal Ions as MR complexes Targeted Au(III) Ions 0 Diverse Metal Ions • •PDF Image | Carrying Gold in Supercritical C02
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