solid phase extraction of metal ions using immobilised chelating calixarene tetrahydroxamates

Analytica Chimica Acta 291(1994) 269-275 ELSEVIER

Solid phase extraction of metal ions using immobilised chelating calixarene tetrahydroxamates

Sharon Hutchinson a, Gary A. Kearney a, Elizabeth Horne a, Bernard Lynch a, Jeremy D. Glennon *9a, M. Anthony McKervey b, Stephen J. Harris ’

a Department of Chemists, Universiity Coke Cork Cork Ireland b School of Chemistry, Queen’s Uniuersily, B#ast, UK

’ School of Chemical Sciences, Lhbkn Ciry University, Dubliq Ireland

(Received 8th September 1993; revised manuscript received 7th December 1993)

Abstract

The synthetic macrocycles, known as calixarenes, have been shown to possess high ionophoric selectivity and form inclusion complexes with many ions. A tetrameric calixarene bearing hydroxanu ‘c acid functional groups was synthesised and supported on octadecylsilica and XAD-4 resin. When loaded with Fe(III), V(V) and C&I), the materials display the characteristic visible absorption spectra associated with hydroxamate coordination. The chemical bonding of the cahxarene tetrahydroxamic acid to silica particles was also carried out using the triethoxysilane derivative of the p-allylcaM4]arene hydroxamic acid. When packed into small cartridges, these materials are used for the solid phase extraction of transition metal ions from aqueous solution. Metal uptake profiles as a function of pH are presented for Fe(III), Co(H), Pb(II), Cd(H), Mn(II), Ni(II), Z&I) and Cu(I1). The quantitative trace enrichment of Cu(II), Zn(I1) and Mn(I1) from water samples prior to ion chromatographic analysis is demonstrated.

Key words: Calixarene; Hydroxamic acid; Solid phase extraction; Metal compiexation

1. introduction

The separation and sensitive determination of trace metal ions very often necessitates the use of preconcentration or trace enrichment [1,2]. The solid-phase extraction approach has gained rapid acceptance because of its amenability to automa- tion and the availability of a wide variety of sorbent phases.

*Corresponding author.

Chelating solid phases can be tailor-made for selective trace metal analysis, with applications in both solid-phase extraction and liquid chromatography. Appropriate chelating reagents can be chemically bonded to or otherwise immobilised onto support matrices, thus providing complexing or chelating solid phases. For example, silica immobilised 2-[(2-(triethoxysilyl)ethyl)thio]anihne has been prepared and used as a selective sorbent for the preconcentration of palladium [3]. Polymeric resins incorporating a chelating ligand have been extensively used, in particular poly-

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270 S. Hutchinson et al. /Adytica Chin&a Acta 291 (1994) 269-275

styrene-divinylbenzene matrices. One such resin containing iminodiacetic acid has been used for the pre~ncentration of heavy metals from natu- ral waters [4]. More recently, a polystyrene resin ~ntain~g ~be~lphosphon~c acid has been utilised for the separation of PMII), UWI), Cu(II), Mn(II), Z&I) and Fe(II1) [5].

Hydroxamic acids are well known as effective chelating agents for a broad range of transition metal ions [6-81. Much attention has centred on the effect of N-substitution on metal chelate stability and selectivity, and on the immobilisation of the chelating agents to the chosen solid supports, e.g., as in the coupling of hydroxamic groups to epoxy activated Sepharose [9]. Other solid support materials used to produce polymeric hydroxamic acids include cross-linked poly(acrylonitrile) [lO,ll], polyacrylic acid resin 1123, XAD-4 1131 and also silica bonded hydroxamate phases as used in the solid phase extraction of many transition metals [14].

Recently a new class of ionophoric compound, calixarenes 1151, which are the macro&c prod- ucts of phenol-formaldehyde condensations, have been shown to act as efficient extractants of alkali metals when modified at the phenolic oxygen atom [16-181. Some chemically modified calti4] arenes have been reported to be effective ionophores in ion selective electrodes for the analysis of sodium in blood plasma [19]. Chro- matographic selectivity for sodium ions on a calix(4larene silica-bonded phase has also been demonstrated [20]. Hexameric and tetrameric calixarenes containing hydroxamate functional groups have recently been reported by Nagasaki and Shinkai [21] to selectively solvent extract Fe(III), Cu(II) and Pd(I1) from aqueous solution. The alternative approach is to prepare a cheiat- ing sorbent incorporating the calixarene hydroxamate for the solid phase extraction of trace metal ions. In this work, both the covalent and non-covalent immobilisation of chelating calixarene hydroxamates onto silica particles and hydrophobic supports is described, Metal uptake profiles by solid phase extraction, using cartridges packed with these materials, were determined and their use in trace metal preconcentration prior to ion chromatographic analysis is demonstrated.

2. Experimental

The calixarene hydr~ic acid (shown in Fii. la), 25,26,27,28-tetrakis(N-hydroxycarbamoyl- metho~)-~-~~~~.-bu~lcal~4]arene was synthesised [22] as follows. To a cooled solution of 1.8 g (0.0018 mol) of the tetraethylacetate of p-tert.- butylcal$4]arene previously prepared [ 171 and 2.02 g (0.029 mol) hydroxylamine hydrochloride in a mixture of 80 ml methanol and 40 ml THF, was added a solution of 2.04 g (0.036 mol) KOH in a mixture of 24 ml methanol and I2 ml THF dropwise at -5°C following a method of Liguori [23]. After stirring for 5 h at -5°C and then 5 days at room temperature, all volatiles were removed under vacuum and dilute acetic acid was added to the residue to give 1.5 g product, which was recrystallised from dichloromethane to give colourless crystalline shiny platelets: m.p. 198- 200°C; v,,,,(KBr)/cm-’ 3230 (NH) and 1655 (GO). Found: C, 64.59; H, 7.57; 0, 20.35. Calc. for CS,H,O,,N,: C, 66.36; H, 7.28; 0, 20.40. The ‘H NMR spectrum of the product in DMSO- d, shows a broad band at 10.8 ppm attributed to the NHOH protons of the hydroxamic acid functional groups.

The p-allylcalti4larene tetraacetate was prepared from the p-ally1 derivative of calixfrllarene by treatment with ethylbromoacetate and was converted to its hydroxamic acid derivative by treatment with hydroxylamine hydrochloride in

w <b)

Fig. 1. Structure of (a) ~-te~.-bu~i~i~4~rene tetrahydroxamic acid and (b) the triethoxysilane derivative.

S. Hutchinson et aL /Analytica Chimica Acta 291(1994) 269-275 271

the presence of KOH in THF-methanol as previously described for the p-tert.-butyl derivative [ 17,221.

Treatment of this product with mercapto- propyltriethoxysilane at 70°C for 1 h in the presence of cumene hydroperoxide (free radical source for thiolene addition) gave the triethoxy derivative shown in Fig. lb.

For the chemical immobilization of calix[4] arene tetrahydroxamate on silica, 0.15 g triethoxysilane derivative of calix[4]arene tetrahydroxamate (Fig. lb) was refluxed with activated silica (1.35 g), i.e., in a 1: 9 (w/w) ratio in 15 ml chlorobenzene for 24 h in an inert atmosphere of N,(g). The chelating material was filtered and washed with chlorobenxene and suction dried. On testing with Pb(II1, a breakthrough capacity of 0.3 pm01 g-l was determined. For the activa- tion of silica, 5 pm silica (Nucleosil) was refluxed with 1: 1 cont. HCl-H,O for 8 h. The silica was filtered and washed with H,O until pH 4 was reached and then dried in a vacuum oven at 110°C for 8 h. Elemental analysis provided evi- dence for carbon coverage on the silica particles: C = 4.14%, while activated silica alone gave value of C = 0.69%.

2.2. Materials

Unless otherwise stated, all chemicals were analytical reagent grade quality. Ethylenedi- aminetetraacetic acid disodium salt (EDTA) was purchased from BDH (Poole), LiChroprep RP-18 (25-40 pm) was obtained from Merck (Darm- stadt) while Amberlite XAD4 and Tris[hydroxy- methyllaminomethane (Tris) were obtained from Sigma (Poole).

Metal ion stock solutions of 100 parts per million (ppml, were made up from their nitrate salts (Spectrosol or Merck 1000 ppm standards). Nitric acid (cont.), potassium hydroxide (5 M) and a 35% ammonia solution were used for pH adjustment. Deionised water from an Elgastat spectrum water purification unit, with a resistivity of 15 Ma cm or better was used for this work. All glassware was washed by soaking overnight in concentrated nitric acid.

2.3. Instrumentation

A cartridge (plastic container, 1.25 X 0.9 cm i.d.1 containing the appropriate sorbent was used in association with a masterflex peristaltic pump equipped with two channel head pumps (Cole- Parmer Instruments, Chicago, IL). An Orion Re- search Model 231 pH meter was employed for the pH measurement of all metal ion solutions. Ultraviolet and visible absorption spectra were recorded on a Shimadxu UV 260 spectropho- tometer. Atomic absorption analyses were carried out using either a Pye Unicam SP 9 spectropho- tometer or a Perkin-Elmer 2380 spectrophotome- ter. Ion chromatographic analysis was carried out using a Dionex 4500i Ion chromatographic system with a CS5 cation separator column. The mobile phase consisted of 0.05 M oxalic acid and 0.095 M lithium hydroxide with a flow rate of 1 ml min-’ at a pH of 4.8. The injection volume was 25 ~1 with post column derivatisation involving the use of 0.3 mm01 4-(2-pyridylazo)resorcinol in 1.7 M acetic acid-7.4 M ammonium hydroxide (60 : 40).

2.4. Chelating solid phase extraction cartridges containing caliwrene hydroxamates

In total three solid phase extraction cartridges of equal dimensions were constructed using either the chemically bonded calix[4]arenetetrahy- droxamate silica, ODS or XAD particles. The g-tert.-butylcal~rl]arene tetrahydroxamic acid was supported on the latter two materials as described below.

A set amount of LiChroprep RP-18 W-40 pm) (i.e., 0.35 g) was added to a plastic cartridge (1.25 X 0.9 cm i.d.) fitted with a frit at its base and methanol was pumped through to wet the octadecylsilica. Thereafter, a suspension consist- ing of 0.094 g (0.1 mm011 calixarene hydroxamate in 10 ml methanol was slowly passed through the octadecylsilica using a pressurised glass syringe. Another frit was placed on top of this silica bed and pressure was applied to ensure that the bed was tightly packed.

To immobilise the calixarene hydroxamate onto XAD-4 resin, the calixarene (0.094 g, 0.1 mm011

272 S. Hutchinron et af. /AnaIytica Chimica Acta 291@?941269-275

and 1,4-dioxan (50 ml) were stirred together in a covered 100 ml beaker until the reagent had dissolved. XAD-4 (0.2613 g) was introduced into the solution and the solution was stirred for two days, allowed to settle and the top 25 ml of the supematant solution was removed by pipette and replaced with 15 ml water. The solution was stirred for 30 min and the former step repeated three more times. The resultant solution was added as a slurry to the plastic cartridge fitted with a frit at its base. A frit was placed on top of the XAD material. The cartridges were con- nected to a peristaltic pump and water was pumped through at a rate of 1 ml min-‘.

2.5. ZrS& absorption spectrophotometry

On passage of selected metal ions through the chelating cartridges, all three solid phases gave &ours that are indicative of hydroxamate binding, red-brown for Fe(III), brown for V(V) and green for Cu(I1).

In order to further characterise complex for- mation by the immobilised calixarene hydroxamates, solutions of Fe(II1) (25 ppm, pH 2.5, 50 ml), V(V) (25 ppm, pH 4.0, 50 ml) and Cu(II) (25 ppm, pH 6.0, 100 ml> were passed separately through cartridges prepared as described above. Quantities of the chelating solid phases (0.1 g) were finely ground and dispersed in nujol and subsequent s~ctrophotometric analysis using 1 mm cells gave the visible absorption spectra of the complexes formed.

2.6. pH dependency of transition metal uptake

Metal solutions (5 ppm, 25 ml) were adjusted to the required pH and pumped at a rate of 1 ml min-’ through the cartridge. This was followed by 25 ml of water to wash off any remaining uncomplexed metal ions. Thereafter the metal was eluted with 25 ml of 0.01 M EDTA or acidified water (pH 2.0). Subsequent atomic absorption analysis gave the % uptake of the various metal ions and % recovery of the eluted metals. With the silica material, the pH range studied was 2.0-7.5 and the pH was adjusted using 5 M KOH. The pH range studied for the XAD-4 resin

was extended to 8.5 and this required the addition of 2 mm01 Tris as a buffer to the metal solutions to prevent precipitation.

The dependency of the uptake of transition metals on the flow rate was also investigated for CuUI), Co@), Pb(II), Fe(III), Mn(III, NXII), Zn~fI) and Cd(H). With the pH of the aqueous metal ion solutions chosen for maximum complexation, the solution flow rate was varied from OS-12 ml min-‘.

2.7. ~eco~entration and analysis of trace metal ions in water samples using ion chromatography and atomic a~o~~n s~ctro~t~

The use of the chelating cartridges for the preconcentration and analysis of transition metal ions present in laboratory tap water was exam- ined. A 250-ml sample of fresh tap water was acidified to pH 2.0, boiled for ten minutes 1241, allowed to cool and then filtered through a 0.45- pm filter (Tris, 0.0606 g, 2 mm01 was added as a buffer after cooling, for applications involving the XAD-4 immobili~d calixarene). After adjusting the pH to 7.5 for the silica based materials and to 8.5 for the XAD based resin using a 35% ammonia solution, the sample was pumped through the cartridge at a flow rate of 1 ml min-‘. There- after, 50 ml of deionised water was washed through and any complexed metals were eluted off with 10 ml of acidified water (pH 2.0). Ion chromatographic and atomic absorption analyses of the initial sample, filtrate and eluate, were used to determine the concentration, %uptake and recovery for the metals present in the sample.

3. Results and discussion

The hydrox~~ acid functional group when present in microbial siderophores plays an impor- tant part in the sequestration of FeQII) from aqueous solution into microbial cells. The very high stability constants for complexation of Fe(II1) can be attributed to the nature of the oxygen donor groups, N-substitution on the hydroxamate and in some cases, ~nfo~ational stability. The

S. Hutchinson et al. /Adytica Chimica Acta 291 (1994) 269-275 273

calix[4]arene tetrahydroxamic acid, under study here provides a structurally rigid organic back- bone and with its anchored chelating groups is essentially a synthetic siderophore. Unlike most naturally occurring siderophores, the calX4larene tetrahydroxamic acid (Fig. la) is water insoluble. This property is useful in solid phase extraction applications, as the physically immobilised calixarene would be less easily leached from a suit- ably chosen support material. On the down side however, could be the high hydrophobicity, which might not allow satisfactory water-receptor contact for metal complexation to occur.

A characteristic feature of Fe(III) complexation by hydroxamic acids in aqueous solution and as solid complexes is the presence of an absorption band in the 420-480 nm region. In order to verify that hydroxamate complexation of metal ions, can occur, when the calX4larene tetrahydroxamic acid is immobilised onto ODS or XAD, molecular absorption spectra were recorded for the materials dispersed in nujol following expo- sure to aqueous solution of metal ions. The calx4larene tetrahydroxamic acid supported on ODS displayed broad bands with A, values at - 430 nm for Fe0111 and V(V) while Cu(II) gave a broad absorption at - 740 nm (Fig. 2). These bands are characteristic of hydroxamate complexation and represent significant wavelength shifts and increases in absorption values from those

15 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9

PH Fig. 3. Effect of pH on percentage metal extraction of metal ions by .calixarene tetrahydroxamic acid supported on XAD-4 resin.

obtained in the spectra of the uncomplexed metal ions in aqueous solutions. The results also indi- cate that sufficient water-receptor contact exists when the calixarene receptor is supported on ODS or XAD resin. Similar metal binding prop-

Fig. 2. Absorption spectra of metal calixarene tetrahydroxamate complexes supported on ODS: (a) Fe(III), (b) V(V) and (c) C&I).

274 S. Hutchinson et al. /Analytica Chin&a Acta 291 (1994) 269-275

erties were exhibited by the chemically bonded calixI4larene tetrahydroxamate silica phase.

Using flame atomic absorption spectrometry, the percentages of metal uptake were determined for Fe(III), Co(H), Pb(II), Cd(H), Mn(II), NXII), Zn(II> and C&I) as a function of pH. The uptake profile obtained for the cartridge containing calixarene supported on XAD is given in Fig. 3. A similar pattern of metal uptake was obtained using the calixarene supported on ODS. Fe(III), Pb01) and Cu(I1) are quantitatively extracted below pH 5.0 while the quantitative extraction of the other metals requires a pH as high as 8.0. With the exception of Pb(II), this behaviour is similar to the extraction behaviour of alkyl-sub- stituted hydroxamic acids such as the N-phenyl- acylhydroxamic acids reported by Hojjatie et al. [25]. Under these conditions, however, the calix[4]arene tetrahydroxamic acid displays a dif- ferent selectivity for Pb(II), with complexation occurring in the range pH 2-5 rather than pH 4-8.

The metal uptake profile for the chemically bonded calixarene tetrahydroxamate silica is shown as a function of pH in Fig. 4. The pH dependency of metal uptake for Pb(II), Cu(I1) and the other metal ions studied is similar to the supported calixarene tetrahydroxamic acid phases. The most noticeable difference is that for NXII), Zn(II), Co(H) and Mn(II), the uptake curves are shifted to a lower pH and overlap to a greater extent. This could be due to improved water-receptor contact on the silica phase or to additional complexation, possibly through the sul- phur containing anchoring side chain.

For all three chelating sorbents, elution of the complexed metal ions was readily achieved using 0.01 M EDTA or acidified water (pH 2.0). In addition the chelating phases were reusable and stable to metal loading and washing cycles with minimum reduction in metal complexation capacity on repetitive usage.

3.1. Applications

The chelating calixarene cartridges are useful for the off-line and on-line preconcentration of trace metal ions from aqueous samples, in con-

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7 m

90

80

R

a

9

46

D

a,

10

0 L5 2 25 3 35 4 45 5..55 6 65 7 ?5 9 95 9

pn Fig. 4. Effect of pH on percentage metal extraction of metal ions by chemically bonded calixarene tetrahydroxamic acid on silica.

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Fig. 5. Ion chromatographs of tap water sample (25 ~1 injec- tions), (a) original sample, (b) after passing through chelating cartridge.

S. Hutchinson et al. /Analytica Chimica Acta 291 (1994) 269-275 275

junction with atomic absorption spectrometry or ion chromatography. To demonstrate this, the preconcentration of trace Cu(II), Mn(I1) and Zn(I1) ions from a laboratory tap water sample was carried out using a cartridge containing the calixarene tetrahydroxamate on XAD resin. The pH was adjusted to 8.5 to ensure quantitative uptake and metals were removed using acid elution. A 25-fold preconcentration factor was used and the metal content of the acid wash was determined by FAAS, yielding the following con- centrations (ppm) for the original water sample: Cu(I1) (0.41), Mn(I1) (0.04) and Zn(I1) (0.29). Direct analysis of the original water sample by ion chromatography using post column derivatisation with PAR yielded metal ion levels (ppm) of Cu(I1) (0.411, Mn(I1) (0.05) and Zn(I1) (0.30). The effectiveness of the chelating cartridge is illus- trated in Fig. 5 where the ion chromatographs for the water samples .are given for, before and after cartridge treatment. Further studies of the applications of these chelating calixarene cartridges in environmental trace metal analysis, are in progress.

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solid phase extraction of metal ions using immobilised chelating calixarene tetrahydroxamates

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