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Ionic liquids for extraction of metals and metal containingcompounds from communal and industrial waste water

Lisa Fischer a, Thomas Falta a, Gunda Koellensperger a, Anja Stojanovic b, Daniel Kogelnig b,Markus Galanski b, Regina Krachler b, Bernhard K. Keppler b, Stephan Hann a,*aDepartment of Chemistry, Division of Analytical Chemistry, University of Natural Resources and Life Sciences, Muthgasse 18, A-1190

Vienna, AustriabUniversity of Vienna, Department of Inorganic Chemistry, Waehringer Strasse 42, A-1090 Vienna, Austria

a r t i c l e i n f o

Article history:

Received 17 January 2011

Received in revised form

9 June 2011

Accepted 13 June 2011

Available online 21 June 2011

Keywords:

Water treatment

Ionic liquids

Liquid phase micro-extraction

Water framework directive

* Corresponding author. Tel.: þ43 1 47654 60E-mail address: stephan.hann@boku.ac.a

0043-1354/$ e see front matter ª 2011 Elsevdoi:10.1016/j.watres.2011.06.011

a b s t r a c t

In a fundamental study the potential of ionic liquids based on quaternary ammonium- and

phosphonium cations and thiol-, thioether-, hydroxyl-, carboxylate- and thiocyanate-

functionalized anions has been assessed for future application in advanced sewage

treatment. The elimination of the metal(oid)s Ag, As, Cd, Cr, Cu, Hg, Ni, Pb, Pt, Sn, Zn and

the cancerostatic platinum compounds cisplatin and carboplatin was screened using

a liquid phase micro-extraction set-up. The analytical tool-set consisted of ICP-SFMS and

LC-ICP-MS for quantification of metal(oid)s and cancerostatic platinum compounds,

respectively. The purity of the ILs was assessed for the investigated metal(oid)s on the base

of present EU environmental quality standards and was found to be sufficient for the

intended use. In model solutions at environmental relevant concentrations extraction

efficiencies �95% could be obtained for Ag, Cu, Hg and Pt with both phosphonium- and

ammonium-based ILs bearing sulphur functionality in the form of thiosalicylate and

2-(methylthiobenzoate) anions, as well as with tricaprylmethylammonium thiocyanate

within an extraction time of 120 min. All other metals were extracted to a

lower extent (7e79%). In the case of cancerostatic platinum compounds a phosphonium-

based IL bearing thiosalicylate functionality showed high extraction efficiency for

monoaquacisplatin.

For the first time, liquid phase micro extraction with ionic liquids was applied to

industrial and communal waste water samples. The concentration of all investigated

metal(oid)s could be significantly reduced. The degree of elimination varied with the initial

concentration of metals, pH and the amount of suspended particulate matter.

ª 2011 Elsevier Ltd. All rights reserved.

1. Introduction water (Directive 2000/60/EC). The list of priority substances

The recent EuropeanUnionWater FrameworkDirective (WFD)

(2000/60/EC) sets high Environmental Quality Standards (EQS)

for priority substances in surface water regulating the annual

averages and maximum allowable concentrations in surface

86; fax: þ43 1 47654 6059.t (S. Hann).ier Ltd. All rights reserved

includes 33 organic and inorganic compounds, which have

become a serious problem in the aquatic environment due to

their toxicity, bioaccumulation and persistence. The metals

Ni, Cd, Hg, und Pb and their compounds belong to this list of

priority substances (Directive 2000/60/EC, Annex I). Their

.

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 4 6 0 1e4 6 1 44602

emission causes pollution of the water cycle leading to poor

water quality, insufficient supply of drinking water and

complicate drinking water pre-treatment (Deutsches

Umweltbundesamt, 2003). Additional negative effects are the

generation of a high toxicological risk for the aquatic organ-

isms and the bioaccumulation of the toxic compounds in the

food chain.

Several other metal compounds have been recently iden-

tified as hazardous contaminants of the aquatic environ-

ment. Among them, the cancerostatic platinum compounds

cisplatin (cis-diamminedichloroplatinum(II), carboplatin

(cisdiammine(1-1-cyclobutanedicarboxylato)platinum(II) and

oxaliplatin ([(1R,2R)-cyclohexanediamine-N,N0]oxalate(2-)-O,O0-platinum)), which are successfully and widely used in

chemotherapy (Hann et al., 2005; Kelland, 2007) pose an

environmental problem. After administration, considerable

amounts of the drugs are excreted via the patients’ urine, thus

reaching the waste water system (Kummerer and Helmers,

1997; Lenz et al., 2005). Since cisplatin is classified as carcino-

genic to animals and all other planar platinum complexes are

supposed to be carcinogenic as well (IARC, 1987; Hann et al.,

2005), efforts have been made to eliminate CPC from hospital

wastewater. However, it has been recognized that a significant

fraction of these compounds is not removed via conventional

sewage treatment (Lenz et al., 2007).

As a matter of fact, technologies for elimination of heavy

metals fromwastewater andprevention of their emission into

surface water is of particular importance. The employment of

ionic liquids (ILs) as extracting agents for elimination of those

compounds via waste water treatment could be an important

alternative to advanced sewage procedures based on adsorp-

tion and irradiation. ILs are generally defined as salts that are

liquid below 100 �C and consist entirely of ions. They show

extraordinary properties such as an extremely low vapour

pressure, high thermal stability and their physico-chemical

properties can be tuned by modifying their chemical struc-

ture (Marsh et al., 2004; Zhao et al., 2005; Pandey, 2006; Han

and Armstrong, 2007). Several studies have investigated the

use of room temperature ILs based on imidazolium-, pyr-

idinium-, pyrrolidinium- or phosphonium cations for the

extraction and separation of organic as well as inorganic

substances from aqueous media (Wei et al., 2003; Liu et al.,

2003; Papaiconomou et al., 2008; Regel-Rosocka, 2009; Ler-

tlapwasin et al., 2010; Rios et al., 2010). Further, efficient

extraction procedures formetals bound to complexing ligands

(e.g. crown-ethers, calixarenes, dithizone) into imidazolium-

based ILs were depicted (Dietz and Dzielawa, 2001; Shimojo

and Goto, 2004; Luo et al., 2004; Domanska and Rekawek,

2009). Modifying their ionic composition by appending

metal-ion ligating functional groups, selective extraction of

solutemetals by ILs can be adjusted. Rogers et al. (Visser et al.,

2001, 2002) first investigated the potential of “task specific”

ionic liquids as extractants for Hg and Cd from water using

imidazolium cations with thioether-, urea-, thiourea-

derivatized side chains (TSILs). However, the main drawback

of TSILs is that their hydrophobicity is achieved by incorpo-

ration the harmful fluorine containing anion hexa-

fluorophosphate. Therefore, they pose a severe environmental

risk due to hydrolysis and formation of hydrofluoric acid in the

presence of water or air moisture (Swatloski et al., 2003). On

the other hand, some research groups achieved the task-

specificity of ILs via anchoring a functional group onto the

anion (Kalb et al., 2006; Egorov et al., 2010; Stojanovic et al.,

2010), gaining TSILs suitable for the extraction of different

metals from aqueous solutions. For example, an ionic liquid

based on the trioctylmethylammonium cation with thio-

salicylate as anion, prepared via a halide free synthesis route,

is commercially available and has been evaluated as extract-

ing agent for heavy metals (Kalb, 2005; Kalb et al., 2006).

We have recently shown that low cost ILs based on the

quaternary phosphonium- and ammonium ionic liquids

Cyphos�IL101 (Cytec) and Aliquat�336 (Henkel) are suitable to

eliminate platinum from aqueous solution by simple replace-

ment of the chloride anion with functionalized aromatic

anions bearing thiol- and thioether groups (Stojanovic et al.,

2010). Egorov et al. (2010) successfully applied Aliquat-based

TSIL with salicylate anion as extracting agent for iron and

copper from a model matrix, suggesting the formation of

salicylate complexes with metal species. On the other hand,

tricaprylammonium thiocyanate, [A336][SCN], is awell known

extracting agent for actinides from acid solutions via anion

exchange (Moore, 1964).We have recently evaluated this ILs as

potential extracting agent for pre-concentration of uranium

from real water matrix (Srncik et al., 2009).

In this study we have evaluated the potential of anion

functionalized ILs as extracting agents for the priority metals

Cd, Ni, Hg, Pb as well as for As, Cr, Cu, Pt, Sn, Zn. Considering

typical contact time and volume ratios during waste water

treatment a liquid phase micro-extraction set-up was imple-

mented. The analytical methods allowed rapid and accurate

high-throughput screening of the extraction efficiencies. Our

experiments included model solutions and, for the first time,

industrial waste waters from different sources considering

variable chemical and physical properties. As a further

novelty, cancerostatic platinum compounds were included in

the context of waste water treatment by ILs.

2. Experimental section

2.1. Synthesis of ionic liquids

All evaluated ILs were synthesized according to protocols

published in the literature. Ammonium and phosphonium-

based ILs were prepared via a metathesis reaction using tri-

caprylmethylammonium chloride (Aliquat�336) and trihex-

yl(tetradecyl)phosphonium chloride (Cyphos� IL 101) and

corresponding Brønsted acids or sodium salts as precursors

(Visser et al., 2002; Kogelnig et al., 2008). One- and two

dimensional NMR experiments, FTIR, elemental analysis and

electrospray ionization mass spectrometry (ESI-MS) analysis

confirmed the composition and purity of the prepared ILs. In

Table 1 the structure of ILs and their characteristic physico-

chemical parameters are displayed.

2.2. IL-extraction experiments of metals and metalcontaining compounds from aqueous solutions

Batch experiments were performed by liquid phase micro-

extraction (LPME) based on a set-up of Liu et al. (2004, 2005).

Table 1 e Physico-chemical properties of studied ionic liquids.

Ionic liquid Structure Densityg/cm3

(25 �C)

Viscosityh(cP)(25 �C)

Decompositiontemperature TD

[�C]

Chloridecontentwt%

Watercontentwt%

Watersaturation wt

%

c(Cl�)

[ppm]a

MiliQ-H2O

10 mMCaCl2

Tricaprylmethyl-

ammonium

thiosalicylate,

[A336][TS]

0.95b 3220b 270b 0.11b 1.54b 4.1b 4.0b 50b

Tricapryl-

methylammonium

2-(methylthio)

benzoate, [A336]

[MTBA]

0.94b 5242b 260b 0.39b 0.23b 15.1b 14.8b 55.7b

Tricapryl-

methylammonium

benzoate, [A336]

[BA]

0.94b 3860b 240b 0.21b 0.21b 17 18.1 456

Tricapryl-

methylammonium

hexanoate, [A336]

[Hex]

0.88c e 148c 0.59c 0.1 24.5 23.8 523

Tricapryl-

methylammonium

thiocyanate, [A336]

[SCN]

e 1017d e <0.03 0.08 2.5 4.7 182

Trihexyl(tetradecyl)

phosphonium

thiosalicylate, [PR4]

[TS]

0.93b 3875b 390b 0.56b 0.56b 11.2b 10.8b 13.1b

Trihexyl(tetradecyl)

phosphonium 2-

(methylthio)

benzoate, [PR4]

[MTBA]

0.94b 875b 350b 0.26b 0.26b 10.6b 10.5b 1

Trihexyl(tetradecyl)

phosphonium

salicylate, [PR4][Sal]

0.92b 567b 340b 0.66b 0.80b 6.5 6.2 231

a Decrease of chloride concentration in aqueous phase (10 mM CaCl2) after extraction with ILs.

b Stojanovic et al., 2010.

c Kogelnig et al., 2008.

d Kulkarni et al., 2007.

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 4 6 0 1e4 6 1 4 4603

A schematic diagram of themodified set-up is shown in Fig. 1.

A defined volume of model solution containing mg L�1

concentrations of various metals, cisplatin or carboplatin,

respectively, was transferred into a 50 mL polyethylene- (PE-)

flask. All solutions contained 10 mmol L�1 CaCl2, which had

been adjusted to pH 7.5 using suprapure o-phosphoric acid

and sodium hydroxide. For LPME, approximately 25 mL of the

investigated IL was drawn into a piece of PEEK-capillary (0.02”

ID, 1/16” OD PEEK�, 5 cm length, BESTA-Technik GmbH,

Wilhelmsfeld, Germany) utilizing a 1 mL polypropylene- (PP-)

Stir barSample solutionIL drop

PEEK-capillary(0.02” ID, 1/16” OD, 5 cm)

PP- syringe + connector

Fig. 1 e Schematic diagram of the IL-based liquid phase

micro-extraction procedure.

Table 2 e Chemical/physical properties of waste watersamples used for extraction studies (Total metalconcentration in mg LL1).

Waste water

1 2 3 4 5 6

Ag 24.3 36.2 0.23 <LOD <LOD <LOD

As 2.05 2.68 0.18 12.8 1.31 10.4

Cd <LOD 0.04 <LOD 0.053 <LOD 0.28

Cr 145 187 88.3 1.64 7.14 18.2

Cu 217 181 93.5 22.8 3.58 5.62

Hg 0.13 0.18 <LOD 0.59 <LOD <LOD

Ni 78.5 106 47.9 56.4 20.0 18.1

Pb <LOD 0.74 <LOD 412 0.70 82.6

Pt <LOD <LOD <LOD <LOD 0.06 <LOD

S 400000 460000 770000 620000 3130000 280000

Sn <LOD <LOD <LOD <LOD 0.36 0.28

Zn 3.49 6.11 8.24 40.6 11.8 93.7

pH 9.3 9.4 8.5 7.9 7.7 7.5

ECa Unknown Unknown Unknown 1600 8470 23000

xssb 137 121 68.8 75.5 411 746

<LOD: metal concentration in the reference sample below limit of

detection; the total combined uncertainty of the given concentra-

tions is 7% (coverage factor 2) (ISO Guide to the Expression of

uncertainty in measurement, 1993).

a Electrical conductivity (EC) in mS cm�1.

b xss in mg L�1.

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 4 6 0 1e4 6 1 44604

syringe (Terumo Europe, Leuven, Belgium). The plunger was

depressed to expose a 10 mL IL drop. Extraction started by

immersion of the IL drop into the stirred sample solution. To

study the effect of contact time on metal uptake by ILs, 1 mL

sample aliquots were taken after 30, 60, 90 and 120min. Three

repetitive extraction experiments were performed for each IL.

Extraction experiments of metals from model solutions were

performed by LPME of a 20 mL volume of model solution

containing a metal concentration of 7.5 mg L�1 Ag, As, Cd, Hg,

Pb, Pt, Sn and 75.0 mg L�1 Cr, Cu, Ni and Zn (approx.). To

investigate the leaching of metals from the different ILs into

the water phase LPME was applied employing blank model

solutions with a contact time of 120 min.

After extraction, the collected aliquots were immediately

transferred into acid-cleaned 10mL PP-vials, acidified with 1%

concentrated nitric acid for stabilization (final pH < 1.5), cap-

ped and stored at 4 �C until measurement. Quantification of

the diluted samples (factor 10) was carried out by ICP-SFMS via

external calibration and internal standardization using

indium (0.5 mg L�1 in the final solution).

Several waste water samples were collected for studying

metal extraction from real matrixes. Total metal concentra-

tions, pH, electrical conductivity (EC) and suspended solid

content (xss) of the selected samples are summarized in Table

2. Gravimetric determination of the xss in real waste water

samples was performed according to DIN 38409-2 (1987).The

extraction experiments of unfiltered waste waters were per-

formed with the LPME set-up as described above. Dilution of

waste water samples prior to measurement was adjusted to

match the working range of the ICP-SFMS method. The certi-

fied reference material TM 28.3 (Low level fortified standard

prepared from Lake Ontario water, National Water Research

Institute) was used for controlling the trueness of results. All

measured values agreedwith the certified valueswithin the 2s

confidence limits.

To investigate the extraction efficiency of ILs for intact

cisplatin and carboplatin, model solutions were freshly

preparedby spiking cisplatin stock solutions (10mgL�1) into the

10 mmol L�1 CaCl2 solution (pH 7.5). 30 mL of model solution

containing 50 mg L�1 cisplatin or carboplatin were used for the

extractionexperiments,whichwereperformedaccording to the

method described above. Extraction studies concerning the

cisplatindegradationproductsmonoaqua-anddiaquacisplatin/

monohydroxo- and dihydroxocisplatin were performed by

using aged model solutions (incubation time of 48 h at 20 �C).During all extraction experiments aliquots of the model solu-

tionswere sampled after 30, 60, 90 and 120min into HPLC-vials,

placed ina cooledmetal freeautosampler (5 �C)andanalyzedby

HPLC-ICP-MS as described below. Quantification of CPC was

performed by external calibration in a working range of

0.01e50 mg L�1.

Extraction efficiencies (E ) were calculated by:

Eð%Þ ¼ �c0aq � c1aq

��c0aq � 100

where c0aq and c1aq are the total metal concentrations in the

aqueous phase before and after the respective extraction time.

2.3. Chemicals and standards

All reagents used throughout the study were of ultra-pure

grade. Ultrapure HNO3 was prepared by double sub-boiling

distillation of 65% nitric acid of p.a. grade (Merck) using

a duoPUR quartz sub-boiling unit (MLS Lab Systems GmbH,

Leutkirch, Germany). Ultra-pure water was used for the

preparation of standards and model solutions by sub-boiling

distillation of purified water (18.2 MU cm) obtained from an

ultra clear system (SG water GmbH, Barsbuttel, Germany).

Quantitative element standards (Ag, As, Cd, Cr, Cu, Hg, In, Ni,

Pb, Pt, S, Sn and Zn) were certified single element ICP stan-

dards for trace analysis, purchased from Merck KGaA (Darm-

stadt, Germany). For quantification of the metals in the

samples obtained from LPME an aqueousmulti-element stock

solution of 10.0mg L�1 (Cr, Cu, Ni, S, Zn) and 1.0mg L�1 (Ag, As,

Hg, Pb, Pt, Sn) was prepared by diluting the 1000 mg L�1 single

element ICP-MS-standard solutions in 5% nitric acid solution

and stored in PFA-bottles at 4 �C. Working solutions were

prepared immediately before use in 1% HNO3.

Table 3 e ICP-MS operating parameters used for multi-element analysis and LC-ICP-MS.

Tuningparameters

ICP-SFMS LC-ICP-MS

Plasma power 1350e1400 W 1300e1400 W

Sample gas

flow

16 L min�1 16 L min�1

Auxiliary gas

flow

0.9e1.1 L min�1 0.9e1.1 L min�1

Plasma gas

flow

0.9e1.3 L min�1 0.9e1.2 L min�1

Sample uptake

rate

PFA micro-

flow

nebulizer

0.35 ml min�1

V-groove

nebulizer

1.0 ml min�1

Isotopes

measured

R ¼ 300 195Pt

R ¼ 4500 107Ag, 111Cd, 52Cr, 65Cu, 202Hg, 60Ni,208Pb, 195Pt, 32S, 118Sn, 66Zn, 115In

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 4 6 0 1e4 6 1 4 4605

For HPLC formic acid (Suprapur� quality) and Methanol

(HPLC gradient grade) were obtained from Merck (Darmstadt,

Germany) and sub-boiled ammoniumhydroxidewas obtained

from Aldrich (Miliwaukee, WI, USA).

Solid cisplatin (purity 100%) and carboplatin were

purchased from Sigma. Stock solutions (10 mg L�1) for the

time-dependent elimination studies were prepared in case of

cisplatin in 150 mmol L�1 NaCl, while for carboplatin 5%

glucose-solution was used as solvent.

Polypropylene (PP) and Polyethylen (PE) materials (bottles,

vials, pipette tips) used for sample preparation were acid-

leached according to a routine cleaning protocol involving

incubation in nitric acid baths (10 and 1% HNO3) and rinsing

with ultra-pure water. Bottles for preparation and storage of

elemental standards weremade of perfluorated polymer (PFA)

andpre-cleanedwith anacid steamsystem (MLSLabSystems).

To avoid contamination, all sample preparation steps and

measurements were carried out under clean room conditions

(class 100000 and class 10000 with clean benches class 100,

respectively) with temperature control (20 �C) and over-

pressure (þ5 Pa).

(internal standard)

R ¼ 10000 75As, 115In (internal standard)

2.4. Determination of total metal(oid) concentrations byinductively coupled plasma sector field mass spectrometry(ICP-SFMS)

The concentrations of the investigated metal(oid)s were

measured using an Element 2 HR-ICP-SFMS (Thermo Fisher,

Bremen, Germany). The instrument is specified for three fixed

resolution settings (R ¼ m/Dm at 10% peak valley): low reso-

lution (LR, R¼ 300), medium resolution (MR, R¼ 4500) and high

resolution (HR, R ¼ 10000). For the introduction of particle free

samples from model experiments a self-aspirating set-up

consisting of a PFA-ST micro-flow nebulizer (Elemental

Scientific Inc., Cuming, Omaha, USA) with a sample uptake of

100 mL min�1 was used. The nebulizer was combined with

a PC3 cyclonic quartz chamber (ESI) operated at 4 �C, a quartz

injector pipe and torch as well as aluminium sampler and

skimmer cones (Thermo Fisher). A slurry-type nebulization

set-up employing a V-groove nebulizer (Glas Expansion, Mel-

bourne, Australia) with an i. d. of 145 mm was used for intro-

duction of unfiltered waste water samples. Sample transport

to the nebulizer was enabled by a peristaltic pump with

a sample uptake rate of 1.0 mL min�1. All instrumental oper-

ating conditions and the selected isotopes for interference-

free ICP-SFMS measurements are listed in Table 3.

2.5. Speciation of cancerostatic platinum compoundsand degradation products by liquid chromatography (LC) -ICP-MS

Chromatographic separation of carboplatin, cisplatin and the

major degradation compounds monoaqua- and dia-

quacisplatin was performed on a Discovery HS F5 column

(3 mmparticle diameter, 150� 2.1mm, Supelco, Bellefonte, PA,

USA), applying the conditions described in detail elsewhere

(Hann et al., 2005). ICP-MS instrument settings and measure-

ment parameters are listed in Table 3. RF-power and gas flows

were daily optimized by a tuning procedure.

2.6. Data evaluation

Generation and export of transient signals (chromatograms

from LC-ICP-MS) was performed using Chromlink (Version

2.1, Perkin Elmer SCIEX) in combination with Totalchrom

(Version 6.2, Perkin Elmer SCIEX). Chromeleon software

(Version 6.70, Dionex, Sunnyvale, CA, USA) was used for

integration and evaluation of all chromatographic data.

3. Results and discussion

In this work two classes of ILs based on quaternary ammo-

nium- or phosphonium cations with functionalized anions

were comprehensively studied with regard to their potential

as novel extractants for waste water treatment. In funda-

mental experiments, the extraction efficiency for Ag, As, Cu,

Cr, Hg, Ni, Pb, Pt, Sn, Zn and cancerostatic platinum

compounds (CPC) from model solutions was assessed at

environmentally relevant concentration levels. The chemical

form of chromium, platinum and arsenic was Cr(III), [Pt(IV)

Cl6]2- and As(III), respectively. For the first time, elimination of

metals and metal compounds via ILs from real waste water

matrices was addressed. Table 1 shows the structures and

properties of the ILs, which have been included in this study.

As a pre-requisite, all prepared ILs exhibited sufficient purity

with residual chloride content of max. 0.7% wt. (a fact proving

quantitative anion exchange during preparation). Further-

more several physico-chemical properties are summarized.

As can be readily observed all ILs of the selected panel

exhibited comparable density values at room temperature

(0.88e0.95 g cm�3). However, the enhanced thermal stability

of phosphonium-based ILs compared to their ammonium

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 4 6 0 1e4 6 1 44606

analoga enabled their application in a broader temperature

range.

The experimental set-up for testing the extraction capacity

was based on liquid phase micro-extraction (LPME) shown in

Fig. 1. This set-up has been introduced by Liu et al. (2005),

studying the elimination of organometallic compounds and

other environmental pollutants from aqueous samples at high

concentrations (0.01e1 mg L�1). The method, which used an

IL/water volume ratio of 1:2000 (5 mL þ 10 mL sample) was

adopted for this work for studying time depended elimination

of metal(oid)s and metal compounds from model solutions

and waste water.

3.1. Extraction efficiencies of ILs for metal(oid)extraction

3.1.1. Leaching of metal impurities from ILs into the waterphaseIn a first step the ILs were studied regarding the leaching of

metal impurities employing LPME of blank model solutions.

Table 4 lists the concentration of metals present in the water

phase after a contact time of 120 min. The given uncertainty

represents the standard deviation (SD) of the results of three

independently prepared samples. The limits of detection

(LOD) are expressed as the threefold SD of the noise deter-

mined in the blankmodel solution (n ¼ 6). None of the studied

Table 4 e Purity of ionic liquids.

[A336][TS] [A336][BA] [A33

Conc. � Conc. � Conc.

Ag <LOD <LOD 0.06

As <LOD 0.01 0.002 0.56

Cd 0.03 0.01 0.11 0.01 <LOD

Cr <LOD 0.03 0.003 <LOD

Cu 0.49 0.25 <LOD <LOD

Hg <LOD <LOD <LOD

Ni 0.78 0.08 0.103 0.045 0.05

Pb 0.12 0.01 0.07 0.005 0.04

Pt 0.007 0.003 0.004 0.0002 <LOD

Sn <LOD <LOD <LOD

Zn 1.38 0.23 1.58 0.04 0.67

[PR4][TS] [PR4][Sal]

Conc. � Conc. � C

Ag <LOD <LOD <

As <LOD <LOD <

Cd 0.065 0.01 0.005 0.001 0.

Cr <LOD <LOD 0.

Cu <LOD <LOD 0.

Hg <LOD <LOD <

Ni <LOD 0.11 0.03 <

Pb 0.032 0.04 <LOD <

Pt <LOD <LOD <

Sn <LOD <LOD <

Zn <LOD 39.3 5.90 0.

Concentrations and standard deviations (2s) are in mg L�1; Instrumental li

deviation of the noise determined in 1% HNO3.

a Environmental Quality Standards (EQS) for priority substances in su

allowable concentration (MAC) and annual average (AA) in mg L�1 (n. a. n

ILs showed significant leaching of the metals Cd, Hg, Ni and

Pb, belonging to the priority substances listed in the Water

Framework Directive (2000/60/EC) except [A336][BA] and

[A336][Hex]. Cd concentrations determined in the water phase

were 0.11 and 0.23 mg L�1, respectively and exceeded the

maximum allowable concentration, which is 0.08e1.15 mg L�1,

depending on thewater hardness class. Significant leaching of

Hg could be excluded for all of the tested ILs. Maximum

concentrations of Ni and Pb found in the aqueous phase were

0.78 and 0.12 mg L�1 respectively, being significantly lower as

the annual average EU-quality standards of surface water.

(20 mg L�1 for Ni and 7.2 mg L�1 for Pb). As there are no uniform

environmental quality standards for other pollutants (As, Cr,

Cu, Ag, Zn), the ILs were evaluated regarding the maximum

permissible addition (MPA) published by the Commissie

Integraal Waterbeheer (2000). In this context only [PR4][Sal]

showed a significant release of Zn (39.3� 5.9 mg L�1) exceeding

the MPA of 7.8e52.0 mg L�1. These results indicate that [A336]

[TS], [A336][MTBA], [A336][SCN], [PR4][TS] and [PR4][MTBA] are

of sufficient purity for the intended use. However, scaling up

of IL synthesis for technical use should be carefully evaluated

concerning possible sources of contamination i.e. the

precursors of IL synthesis as well as technical equipment and

materials.

Additionally, sulphur leaching from ILs bearing S-con-

taining anions was determined. The obtained sulphur

6][MTBA] [A336][Hex] [A336][SCN]

� Conc. � Conc. �0.0006 <LOD 0.03 0.01

0.01 6.42 0.10 <LOD

0.23 0.002 0.07 0.01

0.12 0.003 0.01 0.004

<LOD <LOD

<LOD <LOD

0.004 <LOD <LOD

0.0008 0.03 0.01 0.08 0.008

0.14 0.004 <LOD

0.23 0.0009 <LOD

0.01 5.43 0.07 0.62 0.24

[PR4][MTBA] LOD MAC-EQS/AA-EQSa

onc. �LOD 0.005

LOD 0.005

05 0.01 0.0006 �0.08e1.5*/�0.08e0.25*

14 0.02 0.002

20 0.06 0.017

LOD 0.015 0.07/0.05

LOD 0.022 n. a./20

LOD 0.022 n. a./7.2

LOD 0.0004

LOD 0.045

76 0.34 0.027

mits of detection (LOD) [mg L�1] expressed as three times the standard

rface water (Directive 2006/60/EC - COM 2006 397 final), maximum

ot applicable; *depending on water hardness classes).

Table 5 e Extraction efficiency [%] (n [ 3) of evaluated ILfor the extraction of metals frommodel solutions with anextraction time of 120 mina.

[A336][TS]

[A336][BA]

[A336][MTBA]

[A336][Hex]

[A336][SCN]

[PR4][TS]

[PR4][Sal]

[PR4][MTBA]

Ag 94 56 87 66 97 82 47 100

As e e e e 7 e 5 8

Cd e 5 8 e 15 14 e 38

Cr e e 12 e 16 e 11 e

Cu 95 e 11 e 17 81 19 37

Hg 83 84 95 80 93 93 100 91

Ni e e 10 e 7 12 e e

Pb e 5 7 e 14 41 8 e

Pt 85 69 40 54 95 78 64 97

Sn 79 e 5 e 17 64 48 64

Zn e e 10 e 43 24 e e

e: Extraction efficiency <5%; the relative total combined uncer-

tainty of the given results is 7% (coverage factor 2) (ISO Guide to the

Expression of uncertainty in measurement, 1993).

a Aqueous phase 0.01 M CaCl2 (pH 7.5) with metals Ag, As, Cd, Hg,

Pb, Pt, Sn c0 7.5 and Cr, Cu, Ni, Zn 75.0 mg L�1 (approx.).

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 4 6 0 1e4 6 1 4 4607

concentrations in the range of 0.08e3.57 mg L�1 can be

attributed to sulphur containing impurities and intermediate

products (maximum of 1% sulphur impurity according to

elemental analysis) on the one hand and to the partial solu-

bility of ILs in the aqueous phase under neutral conditions on

the other hand. In the latter case, obtained sulphur concen-

trations of 2.6.10�6 to 1.1.10�4 mol L�1 would correspond to

a leaching of 0.5e13% wt. As a consequence, we conclude that

prior to their industrial use ILs have to be critically assessed

regarding solubility and toxicity. In order to eliminate this

possible disadvantage of ILs we are currently developing

procedures for immobilization of ILs onto different surfaces

e.g. in membranes (e.g. with PSF, PP, PE) or encapsulated in

biomaterials as “backbones” and on supportingmaterials. The

use of such materials as extracting agents can prevent the

transfer of ILs into the aqueous phase. However, before such

steps liquideliquid extraction studies are necessary in order

to evaluate the potential of ILs regarding metal extraction.

The biological effect of ILs similar to those investigated

within this work are provided by http://www.il-eco.uft.uni-

bremen.de/. However, it is known from the literature (Pham

et al., 2010) that the toxicity of ILs is strongly dependent on

the structure of the utilized anions and cations and a general

conclusions cannot be made without experimental data.

Nevertheless, within this study, we have implemented long-

chain quaternary ammonium cations with thiosalicylate

anion in order to avoid toxic fluorine containing anions, such

as PF6.

3.1.2. Screening of IL extraction efficiencyFor rapid assessment of the potential of the investigated ILs,

a LPME-screening was performed employing model solutions

and an extraction time of 120 min. Table 5 lists the extraction

efficiencies of the different quaternary ammonium- and

phosphonium-based ILs. It can be clearly seen that all inves-

tigated ILs revealed high extraction efficiencies for Ag, Hg and

Pt, whereas only a limited fraction of As, Cr and Ni could be

eliminated from the CaCl2 solutions after 120 min.

Extraction efficiencies for Hg ranged from 80 to 100% for all

tested ILs without obvious trends regarding IL-functionality.

This corresponds to distribution ratios >1500 and is in

accordance with a generally high solubility of Hg in both

functionalized and unfunctionalized ILs, observed by several

research groups e.g. Visser et al. (2002); Papaiconomou et al.

(2008) and Germani et al. (2007). The similar tendency can be

observed for Ag, which is highly soluble in all studied ILs,

independent of the functional group. Obviously, sulphur

containing ILs with thiosalicylate [TS], methylthiobenzoate

[MTBA] and thiocyanate [SCN] anions, respectively, showed

higher efficiencies, ranging from 82 to 100%, while ILs with

benzoate [BA], hexanoate [HEX] or salicylate [Sal] anions

showed a lower efficiency with a maximum of 66%. Corre-

sponding distribution ratios >1000 are significantly higher

compared to distribution ratios (approx. 40) observed from

Papaiconomou et al. (2008) for ILs bearing nitrile functionality.

For Pt [A336][SCN] and [PR4][MTBA] revealed excellent

extraction efficiencies (95 and 97%), whereas the extraction

rate of the corresponding quaternary ammonium IL [A336]

[MTBA] was only 40% after 120 min. Again, the efficiencies

obtained for the thiosalicylate anion containing ILs [A336][TS]

and [PR4][TS] were significantly higher as those of [BA], [HEX]

or [Sal] containing anions.

Regarding Cu and Sn, the thiosalicylate containing ammo-

nium IL [A336][TS] showed high elimination rates for Cu (95%)

and Sn (82%) The distribution ratios for Cu (>1900) are compa-

rablewith theresults of Papaiconomouetal. (2008) for thioether

egroup containing ILs, where significantly lower distribution

ratios could be achieved with trioctylmethylammonium salic-

ylate IL by Egorov et al. (2010) (approx. 30). While both [TS]

containing ILs showed a remarkable extraction for Cu and Sn,

[A336][BA] did not show any affinity for these two metals. It is

interesting tomention that theviscosityof these threearomatic

ILs is in the same range (see Table 1).

Interestingly, Cd, Pb and Zn could be eliminated partly

from the model solution by [PR4][TS], but there was no effect

on the corresponding ammonium cationwith the same anion.

[A336][MTBA] showed maximal 10% extraction efficiency for

those metals and no effect was obtained with the analogue

[PR4][MTBA] for Pb and Zn while 38% of Cd could be elimi-

nated. In average, 40% of those metals could be eliminated

from the model matrix as [PR4][MTBA] shows the highest

potential for Cd, [PR4][TS] for Pb and the highest extraction

efficiency for Zn could be obtained by the thiocyanate con-

taining IL [A336][SCN]. The results indicate that not only the

functionality appended to the anion, but also the physico-

chemical properties of the investigated ILs exhibit a remark-

able impact on extraction efficiency. Our results are in

accordance with Papaiconomou et al. (2008), who have eval-

uated the elimination of numerous metals (e.g. Ag, Hg, Pd, Cu)

from aqueous solutions with nitrile- and thioether function-

alized ILs appended to pyridinium- as well as piperidinium

cations and fluor-containing anions. Although it was shown

that the extraction efficiency was predominantly governed by

the functional group, the cation ring as well as the anion was

strongly influencing the metal uptake of the investigated ILs.

In general, extraction properties of the ILs based both on the

quaternary ammonium- and phosphonium cation with [TS],

[MTBA] or [SCN] functionality were significantly higher than

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 4 6 0 1e4 6 1 44608

those of the ILs containing [BA], [Hex] or [Sal], indicating that

the sulphur containing functional groups strongly affect the

extraction efficiency. However, the presence of the sulphur

functionality cannot be regarded as the sole explanation for

the extraction efficiency. Indeed, the cationeanion combina-

tion, and therefore the IL as a whole (and resulting physico-

chemical properties), seems to have a great impact on the

extraction behaviour.

3.1.3. Time dependent extraction efficiencyTo investigate the influence of contact time on IL extraction,

time-resolved LPME experiments were performed. Fig. 2

shows the effect of contact time (0e120 min, measurement

increments of 30 min, three independent replicates) of the

tested ILs on elimination of Ag, Cd, Cu, Hg, Pb, Pt, Sn and Zn

from model solutions. The results demonstrate that the

concentration of Ag, Hg and Pt in the model solutions

decreased with increasing contact time in the case of most

ILs. An extrapolation of the observed elimination curves for

the last named metals results in an estimated elimination

efficiency of >90% for all investigated ILs except [A336]

[MTBA] and in case of Pt for [A336][Hex] as well as they

reached a steady state at approximately 40 and 50% respec-

tively. Interestingly, both the quaternary ammonium and

phosphonium ILs bearing [MTBA] functionality showed a fast

uptake of metals within 30 min as >80% to 95% of Ag, Hg and

>90% of Pt could be extracted with [PR4][MTBA] and no

significantly decrease was observed within the remaining

extraction time.

In case of Cu and Sn the cations bearing [TS] functionality

showed successful elimination potential within 120 min. It is

observable that the extraction efficiency of [A336][TS] is higher

than that of [PR4][TS] with the same contact time. These

results may be explained with the significantly lower water

content of [A336][TS] after equilibration (approx. 4.1% wt.)

compared to the phosphonium analogue (approx. 10.6% wt.),

which is in accordance with the observations of Visser et al.

(Visser et al., 2002). Their results regarding Cd and Hg

extractionwith different TSILs dissolved in the hydrophobic IL

1-butyl-3-methylimidazolium hexafluorophosphate, indi-

cated a strong impact of the decreased water content on

increasing metal-ion distribution ratios. On the other hand,

regarding [PR4][Sal], an unexpected low extraction efficiency

for Cuwas achieved, whereas Egorov et al. (2010) presented an

efficient Cu extraction (distribution ratio approx. 30) in the

bulk liquid/liquid extraction experiments with the Aliquat-

based analogue [A336][Sal]. Extraction efficiencies of the

investigated ILs for Zn, Cd and Pb are low. This may be

attributed to the pH dependence of the extraction efficiencies

leading to higher extraction ranges at higher pH (see results

for communal and industrial waste waters below) and could

also explain the higher extraction efficiencies for Cd reported

in experiments using surface water (Kogelnig et al., 2008). In

contrast to the results observed for Cu and Sn, a significant

elimination potential could be observed with the more

hydrophobic [PR4] based ILs. [PR4][MTBA] was the only IL

which showed a reasonable extraction efficiency for Cd while

moderate extraction of Zn and Pb was investigated with [PR4]

[TS]. In fact, high extraction efficiencies for those metals can

only be obtained at elevated contact times.

3.1.4. Waste water extraction experimentsThe experiments with waste water samples aimed at the

evaluation of the suitability of ILs for (i) advanced treatment of

the effluent of communal sewage treatment plants and (ii) the

first cleaning step of industrial waste waters containing high

amounts of different metal(oid)s. For this purpose the waste

waters were sampled from six different sources, i.e. two

effluents from communal sewage treatment plants (waste

water 1 and 2) and four untreated industrial waste waters

(waste water 3e6). The sampling sites were chosen consid-

ering pH value, concentration of metals and the content of

suspended solids. Physical and chemical properties describing

the waste water samples at the time of the metal extraction

experiments are summarized in Table 2. To simulate the

procedure in a waste water treatment plant, the waste water

samples were subjected to LPME without filtration or

centrifugation.

[A336][TS] and [A336][SCN] have been chosen for the

elimination studies of waste water 1 - 3 because of the

excellent extraction potential for Ag, Hg and Cu. Waste

water 4 - 6 showed a different composition regarding metal

contamination and pH as the concentrations of Cu and Ag

were low, but the samples contained high concentrations of

Pb, Cd and Zn (see Table 2). As [PR4][TS] was the only IL with

an extraction potential for Pb (approx. 40% from model

solutions) and satisfactory extraction efficiencies for Cd and

Zn, this substance was included in the waste water extrac-

tion experiments. Results of the extraction studies are shown

in Table 6.No general conclusion can be made for evaluation of the

extraction potential of tested ILs for metal(oid)s from model

matrixes and real samples as various factors are influencing

the mechanism of metal extraction. Several authors (Kalb

et al., 2006; Egorov et al., 2010; Stojanovic et al., 2010) have

evaluated a possible mechanism for task specific ILs based on

quaternary ammonium and phosphonium cations and thiol-

as well as hydroxy group containing anions. Kalb et al. (2006)

have proposed that trioctylmethylammonium thiosalicylate,

[TOMATS], can extract heavy metals by forming metal-

thiolates and additionally a complex-bond with the carbox-

ylate group of the thiosalicylate anion. Egorov et al. (2010) have

successfully demonstrated that tricaprylylmethylammonium

salicylate could extract Fe(III) and Cu(II) from aqueous solu-

tions in form of salicylate complexes. Furthermore, Preston

(1983) has shown that Co(II) and Ni(II) could be eliminated

with tricaprylylmethylammonium thiocyanate. In this case,

theextentof extractionwasshowntodependuponthe identity

of the counter anionof themetal present in theaqueousphase.

Stojanovic et al. (2010) have evaluated phosphonium-based ILs

with thiol- and thioether containing anions for the elimination

of Pt(IV). Obtained results clearly demontrated that physico-

chemical parameters (e.g. viscosity) have also a remarkable

influence for metal distribution. Therefore it is clear for the

results obtained within this study that not only a simple “task

specifity” of the evaluated ILs is responsible for the metal

distribution, but also their physico-chemical properties aswell

as the composition of the matrix.

The initial concentration of metal(oid)s in the samples was

observed to be an important factor influencing extraction

efficiency. Pb uptake from waste water 4 (c0 ¼ 412 mg L�1) was

Fig. 2 e Effect of contact time on metal extraction (n [ 3) from model solution by ammonium- and phosphonium-based ILs

compared to a reference sample. Aqueous phase: 0.01 M CaCl2 (pH 7.5) with c0 7.5 mg LL1 (Ag, Cd, Hg, Pb, Pt, Sn) and c075 mg LL1 (Cu, Zn) respectively, Vaq/VIL 2000:1. The total combined uncertainty of the given concentrations is 7%, scaled by

a coverage factor of 2 to give a confidence level of approx. 95% (ISO Guide to the expression of uncertainty in measurement;

ISO Guide to the Expression of uncertainty in measurement, 1993).

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 4 6 0 1e4 6 1 4 4609

Fig. 2 e (continued).

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 4 6 0 1e4 6 1 44610

significantly higher up to 90% than from model solutions

containing Pb concentrations in the low mg L�1 range. Ni

uptake from waste water 1 and 2 with the highest Ni content

was in the range of 86e92% and decreased with decreasing Ni

content of the water samples. The same effect was found for

Ag, as up to 75% could be extracted from awastewater sample

containing concentrations, while the extraction efficiencywas

less than 45% in amodel solutionwith an initial concentration

Table 6 e Extraction efficiencies [%] of metals fromwaste waters by ionic liquids with an extraction time of 120min (n[ 3).

Waste water 1 Waste water 2 Waste water 3

[A336][TS] [A336][SCN] [A336][TS] [A336][SCN] [A336][TS] [A336][SCN]

Ag 68% 86% 75% 72% 43% 25%

As e e 10% 7% 5% 9%

Cd * * * * * *

Cr e e 14% 8% 9% 6%

Cu 73% 73% 92% 61% 60% 24%

Hg 89% 52% * * * *

Ni 86% 86% 96% 90% 36% e

Pb * * 14% e * *

Pt * * * * * *

Sn * * * * * *

Zn 65% 69% 79% 73% 69% 44%

Waste water 4 Waste water 5 Waste water 6

[A336][TS] [A336][SCN] [PR4][TS] [A336][TS] [A336][SCN] [PR4][TS] [A336][TS] [A336][SCN] [PR4][TS]

Ag * * * * * * * * *

As 10% 8% e e e e e e e

Cd e e e * * * e e e

Cr 13% 20% 13% 18% 10% 12% 26% 3% 5%

Cu 45% 44% 53% 22% 15% 20% 10% e e

Hg * * * * * * * * *

Ni e 43% 48% 18% 8% 14% e e e

Pb 5% 79% 90% e 9% e 7% e e

Pt * * * 31% 33% e * * *

Sn * * * * * * * * *

Zn 57% 68% 86% 13% e 8% 9% e e

e: Extraction efficiency < 5%.

*: Concentration in reference sample < LODwithin 120min; the relative total combined uncertainty of the given results is 7% (coverage factor 2)

(ISO Guide to the Expression of uncertainty in measurement, 1993).

Fig. 3 e HPLC-ICP-MS chromatogram obtained from an

aged cisplatin solution (initial concentration of cisplatin

50 mg LL1, incubation time 48 h) showing the signals of

cisplatin and the degradation products monoaquacisplatin

and diaquacisplatin (full line). The dashed line shows the

effect of [PR4][TS] after an extraction time of 120 min

indicating a high and moderate extraction efficiency for

monoaquacisplatin and diaquacisplatin, respectively.

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 4 6 0 1e4 6 1 4 4611

<1 mg L�1. Elimination of Zn from waste water 1e4 was

insignificant dependent on the concentration which ranged

from 3.49 to 40.6 mg L�1. With the tested IL 44e70% of Zn could

be extracted from those waste waters, which gave signifi-

cantly higher extraction efficiencies compared to the model

solution. Low elimination rates of Zn fromwastewater 5 and 6

indicate the poor extraction from Zn strongly bound to sus-

pended solids as discussed below (Popp et al., 2008).

Generally, highly contaminated waste water yielded

superior extraction efficiencies as waters with low level

contaminations.

Another factor influencing extraction efficiency is the pH of

the samples. It is well known that the elimination efficiencies

of metal (complexes) are controlled by pH and ionic strength

of the system (Visser et al., 2002;Wei et al., 2003., Egorov et al.,

2010; Lertlapwasin et al., 2010). This effect might be caused by

a higher stability of the complexes in a more basic milieu and

would therefore explain the increased elimination of Ni, Zn

and Pb from waste water 1e4 as the pH of those samples was

in the range of 7.9e9.4 compared to the model matrix with

a pH of 7.5.

We found that the concentration of suspended particulate

matter represents a further important characteristic affecting

extraction efficiency. As published elsewhere (Popp et al.,

2008), metals are specifically distributed between suspended

particles and the water phase, which implicates that the

particles may act as competitive adsorbent (“extractant”) in

the water/IL system. This assumption is supported by the fact

that waste waters 5 and 6 containing high fractions of sus-

pended solids showed generally low extraction rates for

metals by ILs. Moreover, those samples showed a high elec-

trical conductivity of 8470 in waste water 5 and 23000 mS cm�1

in waste water 6, which suggests high contamination of total

Fig. 4 e Extraction efficiencies of phosphonium-based ILs for the extraction of inorganic Pt, cisplatin and its degradation

products mono- and diaquacisplatin (incubation for 48 h at 20 �C) from model solution with an extraction time of 120 min.

Aqueous phase 0.01 M CaCl2 (pH 7.5) with Pt c0 7.5 mg LL1 and intact cisplatin c0 50 mg LL1, respectively. The inset shows the

extraction efficiencies of the different ILs for extraction of monoaquacisplatin (dashed lines) and diaquacisplatin (full lines).

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 4 6 0 1e4 6 1 44612

dissolved solids (TDS). It can be concluded that both, the

interaction of metal(oid)s with suspended particulate matter

and TDS, strongly influenced the mechanism of extraction

leading to decreased extraction efficiencies.

Nevertheless, the results indicate that a variety of toxic

metals could be efficiently extracted from contaminated

water by the use of small amounts of ILs. We can also assume

that several selective TSILs might be used in parallel to

separate multiple contaminants from a mixed waste water

stream. As evaluated ILs are miscible with each other it is

absolutely realistic that the efficiency for the extraction

process might also be increased, when choosing the correct

composition of the IL extracting system. As a pre-requisite,

the selection of appropriate ILs or task specific IL mixtures for

waste water treatment certainly requires a preliminary char-

acterisation of the waste water.

3.2. Extraction efficiencies of ILs for extraction ofcancerostatic platinum compounds from model solutions

The potential of ILs regarding the elimination of the cancero-

static platinum compounds (CPC) from model solutions was

tested, as recent investigations have shown that hazardous

platinum containing compounds are only partially removed by

conventional sewage treatment (Lenz et al., 2007). Since oxali-

platin is supposed to be exclusively present in waste water

treatment plants in the form of its biotransformation products

and various adducts with biomolecules, our elimination exper-

iments were restricted to cisplatin and carboplatin. In waste

water, carboplatin is mainly present as intact drug, while

cisplatin is degraded e in dependence on pH, chloride concen-

tration and age of the waste water e to the twomajor aquation

products cis-[PtCl(H2O)(NH3)2]þ (monoaquacisplatin) and cis-

[Pt(H2O)2(NH3)2]2þ (diaquacisplatin) as well as to the neutral

hydroxocomplexes cis-[PtCl(OH)(NH3)2] (monohydroxocisplatin)

and cis-[Pt(OH)2(NH3)2] (dihydroxocisplatin). In order to detect

these species in the model solutions used for the elimination

experiments, speciation analysis by HPLC-ICP-MS was per-

formed. Fig. 3 shows the chromatographic separation of

cisplatin, monoaquacisplatin and diaquacisplatin in an aged

standard solution (incubation time 48h)with an initial cisplatin

concentration of 50 mg L�1.

For investigation of the IL extraction efficiencies for intact

platinum drugs, LPME experiments were performed with

freshly prepared model solutions spiked with cisplatin and

carboplatin at concentration levels of 50 mg L�1. Compared to

the excellent results obtained for inorganic Pt employing

phosphonium-based IL bearing thiosalicylate and -2-(meth-

ylthio)benzoate functionality, neither intact cisplatin nor

carboplatin were significantly extracted by these ILs.

In order to investigate the potential of the tested IL

regarding the elimination of cisplatin degradation products,

extraction experiments on aged model solutions (48 h) were

performed. For phosphonium-based ILs increased extraction

efficiencies could be observed for the cisplatin metabolites.

The phosphonium-based IL bearing thiosalicylate function-

ality showed a significantly higher extraction efficiency for

monoaquacisplatin as the -2-(methylthio)benzoate function-

ality. In the case of diaquacisplatin no significant difference in

the extraction efficiency of these two phosphonium-based IL

could be observed (see Fig. 4). These results are of particular

relevance for the elimination of cisplatin, since it is known

from a previous study that more than 75% of this drug enters

sewage treatment as highly active monoaquacisplatin/mon-

ohydroxocisplatin (Hann et al., 2003).

4. Conclusion

This study reports the evaluation of various anion func-

tionalized ammonium- and phosphonium-based ILs

regarding purity and extraction efficiencies for metal(oid)s

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 4 6 0 1e4 6 1 4 4613

and cancerostatic platinum compounds from model

matrixes and from communal and industrial waste water

samples applying liquid-liquid extraction procedures. Con-

cerning partial solubility of ILs in water, an appropriate

immobilisation strategy is a pre-requisite to suppress the

loss of ILs into the water phase. Current investigations deal

with the development of appropriate immobilisation

procedures. According to our results, task specific ILs

represent an auspicious procedure for selective cleaning of

contaminated waste water. In particular, the application of

thiol- and thioether- functionalized ILs [A336][TS], [A336]

[MTBA], [PR4][TS] and [PR4][MTBA] for communal waste

water treatment and the use of [A336][SCN] for industrial

waste water with high level Zn contamination is recom-

mended. As a matter of fact the variation of ILs and IL

mixtures is only possible in laboratory experiments. The

application of ILs on a large industrial scale needs the

selection of a low number of low cost ILs, which have to be

proposed for authorization by REACH legislation.

Due to the fact that functionalized ILs incorporating com-

plexing thiol-groups did not indicate significant affinity

towards planar intact platinum complexes, another approach

should be investigated in the future. Since the hydrophobicity

and polarizability of ILs seem to have a remarkable impact on

the extraction processes, a design of strongly hydrophobic,

apolar ILs, and hence extraction of apolar platinum complexes

from the aqueous phase as awhole,may be a useful approach.

Future research should concern procedures for stripping

and recovery of metals andmetal compounds from ILs as well

as the recycling of ILs for a potential use on a larger scale.

Kalb et al. (2006) could strip extracted heavy metals from

the commercially available anion functionalized IL tri-

octylmethylammonium thiosalicylate [TOMATS] via oxida-

tion of the thiol-group with HNO3. This was also successfully

achieved for uranium with tricaprylylmethylammonium thi-

osalicylate [A336][TS], as demonstrated in a previous study

(Srncik et al., 2009). For analytical purposes - such as selective

pre-concentration of desired metals or facilitating sample

preparation methods e this is a prosperous result. However,

due to the oxidation the IL is destroyed, making recycling

impossible. Therefore we are currently evaluating different

back-extracting agents in order to establish continuous

application of ILs. Preliminary experiments using EDTA as

back-extracting agent turned out to be efficient for the

recovery of e.g. Pb(II) from the IL [A336][TS], making recycling

possible. Furthermore, electro deposition ofmetals would also

be an interesting field of further research, especially for noble

metals such as Pt.

In our opinion, with an appropriate immobilization

strategy to avoid water miscibility of ILs and an effective

back extraction procedure which enables the reuse of ILs

a positive life-cycle analysis may be expected.

Acknowledgements

This work was supported by grants from the Austrian Federal

Ministry of Agriculture, Forestry, Environment and Water

Management (BMLFUW, Project Title: “Elimination of priority

substances from waste water“). Maria Fuerhacker (University

of Natural Resources and Life Sciences e BOKU Vienna,

Department of Water, Atmosphere and Environment) is

gratefully acknowledged for providing the waste water

samples.

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