Carrying Gold in Supercritical C02

Jeremy D Glennon, Stephen J Harris, Andrew rtalker, Conor C McSweeney and Mark O'Connell

Analytical Division, Department ofChemistry, University College Cork, Ireland,Email.j.glennon@ucc.ie

Received: 23 March 1999

A series of fluorinated calixarene derivatives, with lower rim sulfur-donor atoms, were synthesized andstudied for the sequestration of gold using supercritical fluid extraction. Following temperature and pressureoptimization and using extracts collected in methyl iso-butyl ketone, analysis by flame atomic absorptionspectrometry reveals that Au(III) can be efficiently complexed and extracted by a novel calix[4]arene thioureareagent in supercritical CO2,

~ Pc 73P::

SupercriticalFluid

In reacting systems, supercritical fluids can makepossible homogeneous reactions by dissolvingreactants, influence reaction equilibria and rates, andopen up the way for new syntheses. In materialsscience, solid materials can be processed into veryuniform fine particles and thin film production,material coating, impregnation and dyeing haverecently been demonstrated (2).

One compound, CO2, has so far been the mostwidely used for supercritical fluid extraction (SFE)because of its convenient critical temperature, cheapness,non-explosive character and non-toxicity. Whilegenerally applicable to the extraction of non-polarorganic compounds, the addition of small amounts ofmodifiers such as the lower alcohols, extends the use ofsupercritical CO2 (SF-C02) to many polar compounds.Highly valuable extracts from medicinal plants havebeen obtained using SF-C02 for extraction and it is thesolvent of choice in many applications including

Tc{°C) Pc{atm)

CO2 31.3 72.931

N20 36.5 72.5 TcT/oC

H2O 374.1 217.7Figure 1 Phase diagram for carbon dioxide

Table 1 Critical Parameters for SelectedSupercriticalFluids

Supercritical fluids, especially those based on inertsubstances, are considered as 'clean' solvents, free from theenvironmental concerns of disposal, handling and toxicityassociated with organic solvents. A pure supercritical fluidis a substance above its critical temperature and pressure(1). Above its critical temperature, it does not condense orevaporate to form a liquid or a gas but is a fluid withproperties changing continuously from gas-like to liquid­like as the pressure increases. Above the criticaltemperature T c and pressure Pc, increasing the pressureincreases the density and thus the solvating power of thefluid. It is this solvating power that makes superaitical fluidsuseful in such processes as the decatieinstion of cotiee and inthe extraction of many important industrial chemicalsincluding medicinal compounds, natural oils and flavoursand even, organicpollutants.

The critical parameters for just three of the manyimportant substances useful as supercritical fluids aregiven in Table 1. Supercritical fluids can replace liquidsolvents in many processes such as extractions fromsolids, counter-current multistage separations,chromatographic separations and chemical reactions.

INTRODUCTION TOSUPERCRITICAL FLUIDS

52 COO' Cold Bulletin 1999,32(2)

supercritical fluid chromatography. In addition, it hasseveral advantages as an enzymatic reaction mediumover conventional organic solvents, which include gas­like diffusivities and liquid-like densities. There is also atpresent, considerable interest in the fascinatingproperties of subcritical and supercritical water assolvent and reaction media.

METAL COMPLEXES INSUPERCRITICAL C02

Surprisingly, the first demonstration of the solvatingpower of supercritical fluids involved metal species iecobalt and iron chlorides in supercritical ethanol in1879. However, since then the emphasis has been onthe extraction of organic compounds, with recently arevival in interest in metal extraction. As in organicsolvents, free metal ions are insoluble in supercriticalCO2 because of the charge neutralization requirementand the weak solute-solvent interactions. However,when metal ions are bound to well chosen organiccomplexing reagents or ligands, they can become quitesoluble in supercritical CO2. Such reagents, dependingon chemical structure, can complex metal ions in apolydentate or macrocyclic manner.

Metal complexes have been separated and extractedusing SF-C02, Wenclawiak et a1 have described thechromatographic separation of cobalt and chromiumacetylacetone complexes using CO2 modified withmethanol (3). In other work, a mixture of ferroceneswas shown to dissolve in pure supercritical CO2 (4). Afluorinated diketone, namely 2,2,-dimethyl­6,6,7, 7,8,8,8-heptafluoro-3, 5-octanedione has alsobeen employed to remove lanthanides and actinidesfrom solid materials (5). More recently, Wai hasdemonstrated that Cu (II) and Hg (II) can be extractedfrom solid matrices by supercritical CO2 containinglithium bisftrifluoroethyl] dithiocarbamate (LiFDDC).The choice of the fluorinated ligand is based on thefact that the solubilities of the fluorinated metalcomplexes are several orders of magnitude higher thanthe non-fluorinated analogues (6).

OXIDATION STATES ANDCOMPLEXATION OF GOLD

Precious metals such as gold and silver, in their normaloxidation states (gold in I and III and silver in I), tend

(fji;} Cold Bulletin 1999,32(2)

to form their most stable complexes with ligandscontaining 'soft' donor atoms (7). Thus complexingreagents with N- and especially S-donor atoms formstrong complexes with the precious metals. Inparticular, Au (I) forms its most stable compoundswhen bonded to 'soft' ligands, containing sulfur orphosphorus and is stabilized with respect todisproportionation by such complexation (8) . Asmedicinal compounds, goldll) thiolates (AuSR)n havebeen studied in relation to chemotherapy forrheumatoid arthritis. Au (III) is invariably complexedin all solutions, usually as anionic species such as[AuC14r and can be solvent-extracted by ion pairingwith an oxonium ion from dilute HCl solutions intoethyl acetate and diethyl ether with a very highpartition coefficient (9).

Gold metal is chemically unreactive and is notattacked by oxygen or sulfur, but reacts readily withhalogens or with solutions containing or generatingchlorine such as aqua regia; it also dissolves in cyanidesolutions in the presence of air or hydrogen peroxide toform [Au(CN)2r. The reduction of solutions of AIC14­

by various reducing agents may, under suitableconditions, give highly coloured solutions containingcolloidal gold. The complexation of gold for thepurpose of extraction from ores has mainly beencarried out using cyanide as reagent (10). The mostimportant method for extracting gold is by treatingfinely crushed ore with sodium cyanide in the presenceof oxygen to give a sodium gold cyanide complex,which is typically adsorbed onto activated carbon,before being re-extracted and reduced to the metal(11). Major problems with the cyanidation process arethe relatively slow dissolution kinetics and the healthand environmental problems associated with it.Generally, alternative gold complexing reagents whileless toxic, are less robust, excessively consumed and lesseconomically viable. Complexation has also beenshown to playa role in controlling the oxidative powerof Fe(III), used in acidic solution as an oxidant forAu (0) (12). In a patent specification Kenna suggestedthat complexation of the ferric ions could be utilized inreducing their oxidative power to a level whereoxidation of gold still proceeded at an acceptable rate,while the oxidation of thiourea was greatly reduced(13). In a separate patent, Nagaraj found that whenalkyl hydroxamic acids or their salts were used alone orin conjunction with traditional sulfide collectors onsulfide ores, the kinetics of flotation and overallrecovery of the precious metals had increasedsignificantly (14).

Precious metals usually exist together with a large

53

EXPERIMENTAL

Figure 2 Chemical structures of the fluorinated calixarenesstudiedfortheSFEofgold

Cl: R = -(CHZ}sS(CHz}z(CFz};£F3;R' = -CHz(C=O)NHOH

C2: R = -(CHz}3S(CHz}z(CFz}7CF3;R' = -CHz(C=S}N(CzHsJz

C3: R = -(CHZ}sS(CHz}z(CFz}7CF3;R' = -CHz(C=O)N(CHzCH;:SH)(CHCH3CHzCH)

C4: R = tert-butyl-;R' = CHz(C=O)O(CHz}zNH ­(C=S)NH(CHz}3S(CHz}z(CFz}7CF3

reported the desorption of gold from activated carbonusing supercritical carbon dioxide (21). Ion pairsolvation of sodium dicyanoaurate by tributylphosphate facilitates the charge neutralizationnecessary for the elution of the ionic Au (CN)z- by thenon-polar supercritical carbon dioxide.

The objective of this research into supercriticalfluid extraction is to avoid the use of organic modifiersand to maximize the extraction selectivity for gold. Thekey here is the matching of the gold coordinationchemistry with a SF-C02 soluble ligand, such asthrough the careful choice of donor atoms in amacrocyclic cavity. In the work reported in this paper,the use of new fluorinated macrocyclic reagents (Figure2), based on the aptly named calixarene 'molecularbaskets' is described. The SFE of Au(III) from spikedcellulose-based filter paper is chosen as a convenientapproach to test the efficiencies of the new reagents.

RRRR

excess of 'base metals', such as copper and iron. Withthe growing emphasis on economically feasiblerecycling and recovery processes, there are increasingdemands for highly selective separation techniques andreagents for the separation and recovery of preciousmetals from low quality ores and secondary sources.High selectivity can be provided by macrocyclicextractants.

A vast array of experimental data availabledemonstrates that the most effective Au (III) sorbentsand extractants from HCl solutions should be foundamong the compounds containing atoms of ether andoxide oxygen as reaction centres (15). On going fromlinear polyethers to their macrocyclic analogues, crownethers have been shown to be the most promising classof extractants for selective gold recovery from chloridesolutions. The size of the macrocyclic cavity essentiallyinfluences the gold extraction. Derivatives of 18­crown-6 were used by Yakshin and coworkers (15) inthe extraction of trace quantities of gold from HC1,while Fang and Fu (16) used benzo-15-crown-5 in thepresence of potassium chloride to extract Au(III). Tromet a1 (17) studied the solvent extraction and transferthrough bulk liquid membranes of gold and silvercyanide complexes using dicyclohexano-18-crown-6.The same group investigated the solvent extraction andtransport through a supported liquid membrane ofmetal cyanide complex salts of gold (I) and silver (I) bymacrocyclic extractant carriers (18). Bradshaw et a1have developed silica gel bonded thiamacrocycleswhich have shown high selectivity for Au (III) (19).

GOLD EXTRACTION USINGSUPERCRITICAL FLUIDS

SELECTIVE MACROCYCLICEXTRACTANTS FOR GOLD

Reports of gold extraction by SFE are few and farbetween but what is in the literature is interesting. Waiet a1 have reported the extraction of Au (III) fromcellulose paper using bistriazolo-crowns in supercriticalCO2 (20). In the presence of 5% methanol as amodifier and the addition of microlitre quantities ofwater, up to 80% of the spiked gold was extracted. Inthe absence of the modifier and spiked water, theextraction efficiency was insignificant. Otu recently

Materials

A 1000 ppm Spectrosol standard solution of Au(III) asAuC14- was obtained from Merck (Germany).Hydroxylammonium chloride (NH20H.HC1) wasalso bought from Merck. All CO2 gas cylinders werefitted with dip tubes and bought from Irish Oxygen(Cork, Ireland). All extracted samples after SFE werecollected in either methyl iso-butyl ketone (MIBK) orDMSO (both purchased from Merck) as indicated.

54 <:00' ColdBulletin 1999,32(2)

Synthesis ofFluorinated Calix[4]arene ReagentsThe chemical structures of three sulfur-containingfluorinated calixarenes are given in Figure 2,alongside the tetra-hydroxamate (C 1). The synthesisof upper rim allyl calixarenes, for subsequentthiolene addition reaction to yield fluorinatedderivatives, has been reported elsewhere by membersof this laboratory (22). Synthetic methods for lowerrim functionalization, to provide donor atoms forselective metal ion complexation, such as for Fe(III)with hydroxamate (C1) and for Ag(I) with thioamidegroups (C2), are now also well established. As theemphasis in this work is on the application of thesemacrocyclic reagents for gold extraction, only thesynthesis of the novel lower rim fluorinatedderivative and the most effective reagent (C4) isgiven below.

Synthesis ofp-t-butylcalix[4]arenetetra-l­heptadecafluorodecylthio-n-propyl-3-(2-ethyJ)­2-thiourea acetate (C4)To 0.8 g (0.005 moles) HOCH2CH2NH(C=S)NHCH2CH=CH2 (Aldrich) was added 2.4 g(0.005 moles) heptadecafluorodecanethiol (22) in 6.0 g1,2-dichloroethane and 200 mg AIBN. The entiremixture was refluxed under nitrogen for 2 h followingwhich a further 200 mg of azoisobutyronitrile wasadded and a further 2 h period of reflux was carried out.After cooling, all volatiles were removed to give a buff-coloured waxy solid of HOCH2CH2NH(C=S)NH(CH2hS(CH2)2(CF2hCF3 (3.15 g, 98%yield). IH -NMR spectral analysis verified the absence ofa chemical shift resonance at 5.92m for =CH from theallyl functionality of the reactant.

To 0.55 g (0.000625 mole) p-tert-butylcalix[4]arene tetraacetic acid was added 5 ml thionylchloride and the entire mixture was refluxed, withstirring, for 2 h (calcium chloride tube attached duringreaction period). After this period, all the volatiles wereremoved under reduced pressure to give thecorresponding p- tert-butylcalix [4]arene tetra-acidchloride. To this solid was added 10 ml dry THF,followed by 0.198 g (0.0025 mole) dry pyridineand 1.6 g (0.0025 mole) HOCH2CH2NH(C=S)NH(CH2hS(CH2)2(CF 2hCF3 in 5 ml dry THFand the entire mixture was stirred at room temperatureovernight under nitrogen. Following this time, allvolatiles were removed and the residual pale yellowsolid was washed with 5% aqueous HCl and thenwater. The resulting sample was allowed to air dryovernight to afford 2.0 g of the crude product as a paleyellow solid.

(fji;} ColdBulletin 1999,32(2)

The above compound (1.58 g) was purified furtherby sequential chromatography on neutral aluminaeluting first with methylene chloride to remove solventfront and high Rf impurities and, subsequently, withmethylene chloride/methanol (14: 1) to give a paleyellow solid with R, 0.22 (0.6 g, 38% yield). CHNAnalysis: Found (%): C, 41.90; H, 4.01; N, 3.37; S,7.53. C116HI24NsSSOI2F6S' Theory (%): C, 41.33; H,3.71; N, 3.33; S, 7.61.

Instrumentation Used for Supercritical FluidExtractionExtractions were performed using an Isco SFX®supercritical fluid extraction system (Isco Inc., USA,supplied by Jones Chromatography, UK). The SFEsystem was controlled by the 260D Series Pumpcontroller and consisted of a syringe pump and heatedextractor block. A heated variable restrictor was usedand the flow rate was set at 1.0 ml min-I. Extractedsamples were collected in a liquid-trap containingMIBK or DMSO. The various working pressures wererecorded in atmospheres and the temperature of theextractor was set manually. Analysis and detection ofextracted samples were carried out using a Pye UnicamSP9 atomic absorption spectrometer and unmodifiedSF-C02 was used for all extractions.

Solubility Measurements ofLigands inSupercritical CO2

Solubility measurements of each reagent were carriedout using the Isco SFE system. A weighed amount (ca80 mg) was placed in an open-ended glass tube (3 x0.5 em Ld.). The sample tube was plugged with glasswool at both ends and inserted into the extraction cell(2.5 ml) reducing the volume of the cell to 2.2 ml. Thesample was statically extracted at 60°C under 200 or300 atm of SF-C02 for 30 min. After this time, thefluid was vented into a collection vial containing 5 mlof MIBK or DMSO. The sample tube was removedfrom the cell and weighed. The solubility wascalculated from the loss in weight of the sample tubedivided by the volume of the extraction cell and givenin terms of mmol of sample per litre of CO2.

SFE ofAu(III) Irom Solid SupportsCellulose filter paper is a convenient material forstudies of the metal extractive ability of designedreagents using supercritical fluid extraction. For theSFE of gold, 40/-11 of gold (1000 ppm gold (III)chloride standard) was spiked onto the filter paper(3x1 em). The filter paper was allowed to dry in air for30 min and was then loaded into the glass tube along

55

Sample Matrix in Extraction Cell

Figure 3 Schematic diagram ofSFE ofAu ions usingmacrocyclic extractants

techniques of liquid-liquid extraction, solid phase andmembrane extraction. Our laboratory has previouslyreported the selective supercritical fluid extraction ofFe(III) from metal mixtures using fluorinatedmolecular baskets and linear monohydroxamatereagents (22, 23).

The schematic diagram shown below illustrateshow such macrocyclic reagents, when sufficientlysoluble in supercritical CO2, can be used to permeatethrough a solid matrix, sequester the targeted metaland deposit it in a collection vessel. This paperexamines the use of new fluorinated calixarenes, aptlynamed 'molecular baskets' for the selective supercriticalfluid extraction of gold.

Extracted MetalIons as MR complexes

Targeted Au(III) Ions 0Diverse Metal Ions ••

in SupercriticalCO2

MR Ligand

o

Initially, the solubilities of the reagents weredetermined in unmodified SF-C02 using the methodof weight loss described in the experimental section, at60°C and 200 atm and also at 60°C and 300 atm. Thepresence of a fluorinated side chain greatly improvesthe solubility of the calixarenes. For C2 - C4,measured solubilities (rnmol/L) were 0.12, 0.13, 0.74at 60°C, 200 atm increasing to 2.31, 0.29 and 1.96 at60°C, 300 atm, respectively.

The selective extraction of Fe(III) in unmodified SF­CO2 from a mixture with other metal ions spiked oncellulose paper, has been demonstrated previously usingC1. Here, using Au(III) spiked onto paper, with thepressure set at 400 atm, extractions were carried outwith the temperature varied between 60 and 120°C.The extract using C1 was collected in DMSO and theextracts involving the other reagents C2, C3 and C4were collected in MIBK. Collected extracts wereanalysed at the ppm level for Au (III) by FAAS (Figure4). No gold extraction was recorded for C 1 under theabove conditions. This result was expected for C1. Apartfrom the calixarene thiol C3, the synthesized calixarenesare expected to form charged complexes with Au (III).The extraction of gold especially at the levels obtainedfor C4 is thus surprising and as suggested by Wai, theinvolvement of counter ions is suspected.

with 20 or 30 mg of the ligand as indicated. The glasstube was plugged with glass wool at both ends and alsobetween the ligand and the filter paper. The glass tubewas mounted inside a stainless steel extraction vessel,tightened into the heating block and staticallyextracted using unmodified SF-C02 for 30 min at 400atm, with temperature varied from 60 to 120°C. Theextraction cell was then vented into a collection vesselcontaining 5 ml of DMSO or MIBK for 15 min(dynamic extraction). Calixarene reagents C2, C3 andC4 are soluble in MIBK while C1 is soluble inDMSO. Having established the optimum temperaturefor each ligand, the pressure was then varied between200 and 400 atm. Collected extracts were analysed byflame atomic absorption spectrometry (FAAS). Thepercentage gold extracted was determined by directcomparison with collecting solutions spiked withstandard Au (III) solution.

The extraction was also studied as a function of theamount of water added to the spiked Au(III) on thefilter papers. The filter papers were allowed 30 min toair dry after the metal spike and were subsequentlyeach spiked with different amounts of water (0 - 40ml): Extractions were then carried out under optimumconditions, as described above. Finally extractions werecarried out on mg samples of solid Au(O) placed in theextraction cell instead of spiked filter paper. Au (0) canbe obtained in mg quantities from the standard Au(III)solution by reduction and heating or by the addition ofhydroxylamine hydrochloride. The effect of addingmicrolitre volumes of standard Fe(III) in HCl solutionas an oxidant onto the reduced gold solid was followedby FAAS analysis of collected extracts.

RESULTS AND DISCUSSION

Macrocyclic ligands have received particular attention,as host molecules, since the pioneering work ofPedersen on the synthesis and metal extractionproperties of crown ethers. His work began arevolution in macrocyclic ligand and receptor designwhich continues today as newer host molecules, suchas the calixarenes, with unique selectivities of bindingand mechanisms of release are produced. The conceptsthat the cavity size and shape in these macrocyclescould be tailor-made and fine-tuned to suit theselected metal cation diameter and that donor atomchoice determines the cation selectivity, have caughtthe imagination of many chemists over the last thirtyyears. In particular separation scientists have beenquick to demonstrate selective metal extractions, using

56 <:00' Cold Bulletin 1999,32(2)

90

80

70

600:

:8 50u

~ 40~

cJ(30

20

10

o I

60 80

~2

...... C3

...... C4

100

Temperature / °C

120

The effect of spiking water on the filter paperalongside the Au(III) was also studied. Several authorshave studied the beneficial effects of water on theextraction of metal ions in SF-C02 (6). The filter paperwas spiked with different amounts of water (0 - 40 1-11)prior to extraction with each reagent. Again C 1 extractednone of the gold. The addition of water had a muchsmaller effect than pressure variation and for C2 and C4actually decreased the amount of gold extracted by morethan 16%. However, the opposite trend was observed forthe ligand C3 where, the percentage extraction was seento increase from ca 9% to ca 10% when 40 1-11 of waterwas spiked on to the filter paper prior to extraction.

CONCLUSIONS AND FUTURE WORKFigure 4 Effect oftemperature on the SFE ofgold from filter

paper at a pressure of400 atm using synthesizedfluorinated tetrameric calixarenes (20 mg reagent,40pl Au(Ill) standard, 30 min static, 15 mindynamic)

With the optimum temperature of 60°C, thepressure was then varied between 200 and 400 atm(Figure 5). No extraction of gold was again recordedwith C 1; however, some gold was extracted with C2(ca 17%) and C3 (ca 9%) at the optimum conditions,60°C, 200 atm and 60°C and 400 atm, respectively.Much higher extractions were recorded with thethiourea calix[4]arene, C4. All of the C4 reagent wasobserved to be removed from the extraction cell duringSFE carried out at 400 atm.

Extraction of gold(III) by the fluorinated calix[4]arenethiourea derivative is demonstrated on an analytical scaleusing supercritical CO2. No organic solvent modificationis needed. High extraction percentages were obtainedand as found by other workers in the field of metalextraction by SFE, it is expected that quantitativeextraction is achievable through the incorporation ofadditional washing steps to recover gold after each SFErun. Other fluorinated calix[4]arenes, functionalized onthe lower rim with thiol and thioamide groups, weremuch less efficient. No extraction of gold is possiblewhen Au(O) is used, unless an oxidant is added to theextraction cell. Addition of Fe(III) in acidic solution ontoreduced gold in the extraction cell, does however revivethe extraction of gold. Further work is in progress onselectivity, reagent recovery and metal release, and on theuse of these promising fluorinated reagents for theextraction of gold from geological and material sources.

ACKNOWLEDGEMENTS

ABOUT THE AUTHORS

Research grant support from Enterprise Irelandthrough the Basic and Strategic Research Programmesis gratefully acknowledged, as is the support in-kind ofindustrial partners.

450400350300250

~2

...... C3

...... C4

90

80

70

0: 60sg 50Jj 40cJ( 30

20

10

o'--==,*~::=IIt::::::=--+-_--+-_---I200

Pressure / atm

Figure 5 Effect ofpressure on the SFE ofgold using synthesizedfluorinated tetrameric calixarenes at their optimumtemperatures (40 pI Au(Ill) standard, 20mg ligand,30 min static, 15 min dynamic)

Jeremy D Glennon is a graduate of the NationalUniversity of Ireland, having obtained BSc and PhDdegrees in chemistry from University College Dublin.He is director of the Advanced Separations Group inAnalytical Chemistry at University College Cork

(fji;} Cold Bulletin 1999,32(2) 57

M Malte-Brun, 'UnIversal Geography', Wells and Lilly, 1824

E.E. Lungwltz, Min] Lond,1900,318

(UCC), which is dedicated to the study anddevelopment of separation and sensing methods usingmolecular recognition chemistries, clean technologyand miniaturisation. Innovative chemistries are beingdeveloped which will replace environmentallyunfriendly processes involving solvents or hazardouschemicals. In particular toxic metals are targeted forextraction and analysis, alongside recoverable preciousmetals (gold, silver) from environmental samples,industrial waste and products.

Stephen John Harris is a senior research associate atUCC, receiving a BSc(Hons) in chemistry from theUniversity of California (Berkeley) and his PhD degreefrom the University of Sussex in the field of organometallicsynthesis. He has substantial research and developmentexpertise, gained in industry at Loctite (Ireland) and atDublin City University and University College Cork. Hewas visiting professor at the Weizmann Institute of Sciencein 1991 and has a specialist knowledge of calixarene designand synthesis, recognised in 1987 with Loctites 'VernonKreble Award'. Conor C. McSweeney and Mark P.O'Connell are PhD postgraduate researchers, whileAndrew Walker is a graduate of Queens University Belfastand contributed in organic synthesis through apostdoctoral fellowship at UCc.

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