Chapter 6
Extracting Gold in Supercritical CO : Fluorinated Molecular Baskets and Thiourea Ligands for Au 2
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Jeremy D. Glennon , Josephine Treacy, Anne Μ. O'Keeffe, Mark O'Connell, Conor C. McSweeney, Andrew Walker, and Stephen J. Harris Analytical Chemistry and the Supercritical Fluid Centre, Department of Chemistry, University College Cork, Cork, Ireland Corresponding author: email:
[email protected] *
Selective ligands for precious and heavy metal complexation and extraction in sc-CO , capable of replacing environmentally unfriendly processes using solvents or hazardous chemicals, are being researched in our supercritical fluid centre at UCC. In particular, the design and synthesis of a series of fluorinated calixarene ligands, aptly named molecular baskets, has led to demonstrations of their extractive power for gold(III). Further research on a series of new 3,5-di(trifluoromethyl)-phenyl thiourea derivatives proved remarkably successful for the solubilisation and extraction of gold in sc-CO . 2
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© 2003 American Chemical Society
Gopalan et al.; Supercritical Carbon Dioxide ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
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68 Gold is a metal that continues to be precious in the market place, in exploration, in medicine and in science and technology (1). Its unique chemical properties ensure that it continues to be researched and applied in many established and in many new fields, including the treatment of arthritis (2), as advanced biocompatible materials in medicine (3,4), in the nanoengineering of optical properties as gold nanoshells (5) and in biosensing (6). This plethora of applications demands efficient methods of extraction and analysis of gold in a variety of matrices. Chromatographic, spectroscopic and atomic absorption spectrometric methods have received particular attention, particularly for application in gold exploration and mining. For example, ioninteraction reversed-phase chromatography (7,8) was used to analyse aurocyanide in liquors drawn from the carbon in pulp process and capillary zone electrophoresis (CZE) has been used in the determination of gold (I) and silver (I) cyanide complexes in alkaline cyanide solution (9). Gold is also frequently determined using spot tests and spectrophotometrically, as for example using the chelating reagent 2,3-dichloro-6-(3-carboxy-2-hydrox-l-napthazo)quinoxaline, to yield a yellow brown complex (λ = 575 nm) (10). However, atomic absorption spectrometry has proven to be an extremely useful technique for the determination of gold in geological materials, especially following an effective gold extraction step. ιηΒΧ
Meier described a method for the dissolution of gold from geological material into hydrobromic acid and bromine (II). The gold was extracted into methyl isobutyl ketone and determined using an atomic absorption spectrometer equipped with a graphite furnace atomiser. Reddi et al. reported on an acid decomposition procedure at room temperature where sample decomposition is effected by treatment with hydrochloric acid and bromine for 24 hr (12). From the resulting bromine extract the noble metal is co-precipitated with tellurium. The precipitated tellurium metal is collected in toluene. The noble metal, together with tellurium, is then re-extracted into aqua regia for determination by atomic absorption spectrometry. Several other authors have reported the use of HBr-Br and aqua regia for the extraction of gold from geological material prior to analysis by atomic absorption (13-15). 2
Once in solution, more advanced extraction approaches can be used including the use of free or immobilised macrocyclic extractant carriers. Derivatives of 18-crown-6 were used by Yakshin and coworkers (16) in the extraction of trace quantities of gold from HC1, while, Fang and Fu (17) used benzo-15-crown-5 in the presence of potassium chloride to extract A u . Trom et al(18) studied the solvent extraction and transfer through bulk liquid membranes of gold and silver cyanide complexes using dicyclohexano-18-crown-6. The same group investigated the solvent extraction and the transport through a supported liquid membrane of metal cyanide complex salts of gold (I) and silver (I) by macrocyclic extractant carriers (19). Bradshaw and co-workers (20,21) have developed silica gel bonded thiamacrocycles, which have shown high selectivity for A u . Miller et al. (22) studied the solvent extraction of gold from alkaline cyanide solution by alkyl phosphorous esters. A method was also developed for the rapid and selective extraction of A u with 2-mercapto-benzothiazole into chloroform (23). 3+
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Gopalan et al.; Supercritical Carbon Dioxide ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
69 Supercritical Fluid Extraction (SFE)
The use of supercritical fluids as alternatives to organic solvents is revolutionising a huge number of important science areas (24). Scientific applications vary from established processes, such as the decaffeination of coffee and the extraction and synthesis of active compounds, to the destruction of toxic waste in supercritical water, the production of nanoparticles and new materials, to novel emerging clean technologies for chemical reactions and extraction. Supercritical C 0 has so far been the most widely used because of its convenient critical temperature, cheapness, non-explosive character and nontoxicity. Unmodified sc-C0 can be used to extract large organic solute molecules even if they have some polar character; the addition of small amounts of modifiers such as the lower alcohols, extends the use to polar compounds. Above the critical temperature Tc and Pc, increasing the pressure increases the solvating power of the fluid. It is this solvating power that makes supercritical fluids useful in the synthesis and extraction of many important industrial chemicals including natural products, oils, flavors, medicinal compounds and organic pollutants but which with innovative design, can be the medium for selective and efficient environmental processes, for cleaning, for advanced material generation, and for metal extraction and analysis. 2
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Innovative research carried out in this area of green chemistry can alleviate the environmental problems created in many anthropogenic activities, such as the remediation of contaminated soil and waste, and lead to the development of new chemistries for tomorrow's clean technologies. Clean chemistries for precious and heavy metal complexation and extraction by sc-C0 , capable of replacing environmentally unfriendly processes using solvents or hazardous chemicals, are being researched in our supercritical fluid centre at UCC. Considerable progress has been made on the design and development of new linear and macrocyclic reagents for selective metal extraction. In particular, the synthesis of fluorinated hydroxamic acids has led to demonstrations of their extractive power in supercritical fluid C 0 . In this way the biochelation ability of siderophores found in soil microorganisms has been harnessed for metal extraction from solid samples by SFE (25). 2
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Gold in Supercritical Fluids Research into elucidating how gold is solubilised and carried, reacts and is extracted under supercritical conditions is of growing scientific importance but
Gopalan et al.; Supercritical Carbon Dioxide ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
70 has great practical potential in precious metal analysis and recovery. There is an environmental imperative to explore such cleaner technology for efficient gold extraction to avoid environmental damage, such as the extensive environmental contamination that has occurred in the Brazilian Amazon from the mercury-gold amalgamation process. The published work on the SFE of gold has been limited to date. Wai et ai (26) have successfully extracted A u using bistriazolo-crowns. In the presence of 5% methanol as a modifier to the C 0 and microlitre quantities of spiked water onto the filter paper containing A u , up to 79% was extracted. Otu (27) recently reported the desorption of gold from activated carbon using supercritical carbon dioxide. Ion pair solvation of sodium dicyanoaurate (Na ... Au(CN) ") by tributyl phosphate facilitates the charge neutralisation necessary for the elution of the ionic Au(CN) " by the non-polar supercritical carbon dioxide. 3+
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In this chapter, a review of our recent SFE work is provided, covering the design and application of new fluorinated macrocyclic calixarenes and the discovery of the effectiveness of linear flourinated thiourea reagents for gold complexation and extraction in unmodified sc-C0 . 2
Experimental Materials The thiourea derivative, l-[3,5-di(trifluoromethyl)phenyl]-2-thiourea (Tl) was purchased from Fluorochem Ltd (Derbyshire, UK). A 1000 ppm spectrosol solution of A u as an AuCl " solution was obtained from BDH Chemicals Ltd. (Poole, England). A l l C 0 gas cylinders were fitted with dip tubes and bought from Irish Oxygen (Cork, Ireland). All extracted samples after SFE were collected in either methyl iso-butyl ketone (MIBK), DMSO (both purchased from BDH) or methanol (Merck, Germany) as indicated. 3+
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Synthesis of Ligands A series of novel fluorinated macrocyclic calixarene and fluorinated thiourea ligands were synthesised at UCC for used in the SFE of metal ions (Figures 1 and 5). Among die fluorinated calixarenes listed, C2-C4 were designed for gold extraction and are of particular relevance in the results presented in this paper. The synthetic methods are either published elsewhere (28,29) or in preparation for detailed publication.
Gopalan et al.; Supercritical Carbon Dioxide ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
71 Supercritical Fluid Extraction and Flame Atomic Absorption Analysis Extractions were performed using an Isco SFX supercritical fluid extraction system (Isco Inc., USA, supplied by Jones Chromatography, UK). The SFE system was controlled by the 260D Series Pump controller, allowing programming of modifier addition, pressure automatic refill and continuous flow and consisted of a syringe pump and heated extractor block. A heated variable restrictor was used (part no. 68 395 005) and the flow rate was set at 1.0 ml min" . Extracted samples were collected in a liquid-trap containing methanol, MIBK or DMSO. The various pressures were set in atmospheres and the temperature of the extractor was set manually. Analysis and detection of extracted samples were carried out using a Pye Unicam SP9 atomic absorption spectrophotometer (FAAS) or the CARY/1E/UV visible spectrophotometer. Unmodified sc-C0 (i.e. without the addition of an organic modifier, such as MeOH) was used for all extractions. 1
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Solubility Measurements of Calixarene Ligands in Supercritical C 0
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Solubility measurements of the series of novel fluorinated macrocyclic calixarene ligands were carried out using the Isco SFE system. A weighed amount (ca. 80 mg) was placed in an opened ended glass tube (3 χ 0.5 cm i.d.), which was plugged with glass wool at both ends and inserted into the extraction cell (2.5 ml) reducing the volume of the cell to 2.2 ml. The sample was statically extracted at 60°C under 200 or 350 atm of sc-C0 for 30 min, unless otherwise stated. After this time, the fluid was vented into a collection vial containing 5 ml of MIBK, DMSO or methanol. The sample tube was removed from the cell and weighed. The solubility was calculated from the loss in weight of the sample tube divided by the volume of the extraction cell and given in terms of mmol of sample per litre of C 0 . 2
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Optimisation of SFE of A u from cellulose paper 3+
For the A u extraction experiments using thefluorinatedligands, 40 of A u (1000 ppm gold(III) chloride standard, BDH) was spiked onto filter paper (3xlcm). The filter paper was allowed to dry in air for 30 min and was then loaded into the glass tube along 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 sc-C0 for 30 min at 60°C and at applied pressures between 200 and 400 atm. The extraction cell was then vented into a collection vessel containing 5 ml of methanol, DMSO or MIBK for 15 min (dynamic extraction). For the ligands studied, T1-T4 were soluble 3+
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Gopalan et al.; Supercritical Carbon Dioxide ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
72 in methanol, C2, C3 and C4 were soluble in MIBK and R4 was soluble in DMSO. Having established the optimum pressure for each ligand, the temperature was then varied between 60 and 120°C. Collected extracts were analysed by FAAS or UVVisible spectrophotometry. The percentage A u extracted was determined by direct comparison with collecting solutions spiked with standard A u solution. 3+
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Finally, the extraction was studied as a function of the amount of water added to the spiked A u on the cellulose filter paper. The papers (3xlcm) were spiked with 40 jiL A u as previously described. The wet filter papers were allowed 30 min to air dry and were each spiked with different amounts of water (0-80 |iL). Following the extraction procedure described above, under the optimum temperature and pressure conditions, a static extraction of 30 min followed by 15 min dynamic extraction into a collecting solution of 5 ml methanol, MIBK or DMSO was applied for each sample. The collected solutions after SFE were analysed by FAAS for their gold content 3+
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Results and Discussion Molecular Baskets in sc-C0
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Considerable progress has been made at U C C on the design and development of new linear and macrocyclic reagents for metal extraction, based on the chemistries given below. The project work to-date has focussed on organic synthesis, solubility measurements and the study of the efficiency and selectivity of extraction. The first examples of the use of calixarenes in supercritical fluids have been reported by this laboratory (28). The fluorination of calixarenes at the upper rim provides a convenient means to increase the solubility of these molecular baskets in sc-C0 . These new molecular recognition calixarene reagents have been synthesised to exhibit unique selectivity for metal ions through the use of a suitably sized cavity for complexation. 2
Solubilities of Calixarene Ligands in se-C0
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Solubility measurements were carried out for a number of the above calixarenes, using the method of weight loss, at a temperature of 60°C and at pressures of 200 and 350 atm, including those designed to be gold selective, C2C4 ( measured at 300 atm). The solubility of each ligand increased significantly at the higher pressure studied. The presence of a fluorinated side chain greatly improves the solubility of the calixarenes; this is clearly evident when the solubilities of a range of different fluorinated and non-fluorinated calixarenes are compared. Lower rim functionalisation of the calixarenes to contain chelating groups based on oxygen, nitrogen or sulphur donor atoms lowers the solubility relative to the fluorinated calix[4]arenes R l and R3 (Table I).
Gopalan et al.; Supercritical Carbon Dioxide ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
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RI*
R = i-butyl
R' = H
RI
R = (CH ) S(CH ) (CF ) CF
R' = H
R2
