coupling of a column system with icp-ms for the characterisation of colloid-mediated metal(loid)...

Acta hydrochim. hydrobiol. 33 (2005) 4, 337−345 337

George Metrevelia, Coupling of a Column System with ICP-MS forEva-Maria Kaulischa,Fritz H. Frimmela the Characterisation of Colloid-mediateda Universität Karlsruhe, Metal(loid) Transport in Porous Media

Engler-Bunte-Institut,Lehrstuhl für Wasserchemie, In this work a coupling method for the characterisation of colloid-mediated transport of theEngler-Bunte-Ring 1, metal(loid) species in porous media was developed. For this transport experiments quartz76131 Karlsruhe, Germany

sand was used as column packing material and the synthetic three-layer clay minerallaponite as model colloid. The determination of colloids was conducted by means of UVdetection. The quantification of the metal(loid) ions was carried out in two different ways:(1) The fractions collected at the column outlet were analysed with an inductively coupledplasma mass spectrometer (ICP-MS) (offline measurements); (2) the column system wasdirectly coupled with ICP-MS (online measurements). In the column experiments the influ-ence of laponite colloids on the transport of Cu, Pb, Zn, Pt and As species was investi-gated. In the offline experiments as a consequence of dilution during sample preparationno metal(loid) species at the column outlet could be found. Unlike this the breakthroughof all metal(loid)s could be detected under the same experimental column conditions inthe coupling experiments. This coupling technique offers the online detection of the metalspecies and colloidal particles with high resolution even at low concentrations and withoutany time-consuming preparation. The coupling experiments have shown that the laponiteparticles accelerate the transport of the cationic metals. For anionic metal(loid) species noinfluence of laponite on their transport behaviour was found.

Kopplung einer Säulenanlage mit ICP-MS für die Charakterisierung des kolloid-getragenen Metall(oid)-Transports in porösen Medien

In dieser Arbeit wurde für die Charakterisierung des kolloidalen Transports von Metal-l(oid)en in porösen Medien eine Kopplungstechnik entwickelt. In den Transportexperimen-ten diente Quarzsand als Füllmaterial für die Säule und das synthetisch hergestellte Drei-schicht-Tonmineral Laponit als Modellkolloid. Die Aufnahme der Durchbruchskurven vonKolloiden erfolgte mit Hilfe eines UV-Detektors. Die Quantifizierung von Metall(oid)-Ionenwurde auf zwei unterschiedliche Weisen durchgeführt: (1) Die am Säulenausgang gesam-melten Fraktionen wurden mit einem Massenspektrometer mit induktiv gekoppeltemPlasma (ICP-MS) analysiert; (2) die Säulenanlage wurde direkt mit dem ICP-MS-Systemgekoppelt. In den Säulenexperimenten wurde der Einfluss des Laponits auf den Transportder Cu-, Pb-, Zn-, Pt- und As-Spezies untersucht. Als Folge der Verdünnung bei der Pro-benvorbereitung konnten in den Offline-Experimenten am Säulenausgang keine Metal-l(oid)-Spezies gefunden werden. Im Gegensatz dazu konnten in den Kopplungsexperi-menten bei den gleichen Säulenbetriebsbedingungen die Durchbruchskurven für alleMetall(oid)e aufgenommen werden. Die Kopplungstechnik ermöglicht die Online-Detektionder Metallspezies und der Kolloide bei niedrigen Konzentrationen mit hoher Auflösung undohne zeitaufwändige Probenvorbereitung. Die Kopplungsexperimente haben gezeigt, dassdie Laponitkolloide den Transport kationischer Metalle beschleunigen. Ein Einfluss vonLaponit auf den Transport der anionischen Metall(oid)e wurde nicht festgestellt.

Keywords: Online Measurement, Mass Spectrometry, Heavy Metal, Colloidal Interaction,Tracer Experiment

Schlagwörter: Online-Messung, Massenspektrometer, Schwermetall, Kolloidale Wechsel-wirkung, Tracerexperiment

Correspondence: F. H. Frimmel, E-mail: [email protected]

DOI 10.1002/aheh.200400582 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

338 G. Metreveli et al. Acta hydrochim. hydrobiol. 33 (2005) 4, 337−345

1 Introduction

The emission of pollutants in urban areas represents an in-creasing risk for the environment. After the passage of rain-water through the soil the different contaminants like heavymetals and metalloids or organic micropollutants can reachthe groundwater and finally the drinking water supply plant.Potential emission sources of those pollutants are roadtraffic, roofs and walls of buildings, contaminated sites orwaste disposal areas. For example the emission of Pt fromautomobile traffic amounts to 270 ng per driven kilometreand car. This was measured at an “Autobahn” of the southwest of Germany [1].

In former times, the aquifer was regarded as system of twophases: the immobile soil matrix as solid phase and thegroundwater with the dissolved substances as liquid phase.The transport of pollutants was characterised by their ad-sorption and desorption at the soil matrix. Groundwater con-tains both dissolved materials and particular substances; thelatter ones mainly in colloidal form. These particles do notsediment, are particularly mobile and can transport ad-sorbed pollutants [2�5]. The most occurring colloids in natu-ral aquatic systems are for example clay minerals and NOM(natural organic matter). The transport through soils ismainly influenced by the hydraulic conditions and by interfa-cial processes.

The transport and deposition of aquatic colloids has beenstudied in laboratory column experiments or in pilot scaleexperiments by various authors. They stated that as aconsequence of the compression of diffuse double layer anda reduction of the repulsive electrostatic forces the mobilityof colloidal particles decreases with increasing ionic strength[3, 6�11]. The colloid deposition rate is depending also onthe valence of ions and the electrolyte type (symmetrical orasymmetrical stoichiometry) [6]. The transport experimentshave shown that with increasing pH value the repulsive for-ces between colloids and the grains of the solid matrix arehigher. This causes the increasing mobilisation of colloidsfrom the grain surface [3]. Furthermore the natural organicmatter plays an important role for the transport behaviour ofaquatic colloids. The adsorption of humic acids to the colloidsurface influences increasing colloid mobilisation [9, 12].The transport experiments using quartz grains as columnpacking material showed that decreasing the molecular sizeof the humic acids adsorbed to hematite colloids led to anincreased attachment efficiency [13].

The qualitative and quantitative detection of the metal spe-cies adsorbed onto or bound to the colloids has been doneby different analytical systems like atomic absorption spec-trometry (AAS), inductively coupled plasma mass spec-trometry (ICP-MS) or inductively coupled plasma optical

2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

emission spectrometry (ICP-OES). Mostly the detection ofmetal ions was carried out offline after fractionation andsample collection [4, 14�16]. This procedure comprisessome disadvantages: In many cases the preparation of thesamples is very time-consuming. For example stabilisationwith acid solutions [4, 16], dilution [16], labelling and storagein the refrigerator (before the measurements) has to bedone. For metal analysis a minimum amount of sample vol-ume is needed. Therefore the collected samples in somecases had to be diluted. As a consequence of dilution thequantification of low concentrations was not possible [16].

The direct coupling of the column system with a metal ana-lyser for example ICP-MS offers the online detection of themetal species with high resolution even at low concen-trations and without any time-consuming sample preparationor even contamination. This is very important especially fornatural samples with low metal ions concentrations like rainwater, water run-off from roads and roofs. The coupling ofICP-MS or ICP-OES with different chromatographic systems(SEC � size-exclusion chromatography, IC � ion chroma-tography) and fractionation processes (FlFFF � flow fieldflow fractionation, SdFFF � sedimentation field flow fraction-ation) has itself proven to be an efficient multi-elementmethod [15, 17�19]. However investigations of colloidaltransport processes in porous media columns with online de-tection of metals in the effluent by ICP-MS or ICP-OES havenot been reported.

The aims of this work were to (1) develop a new couplingmethod for the online characterisation of the colloid-me-diated transport of metal(loid)s in porous media, (2) applythe system to determine the transport behaviour of aquaticcolloids and (3) conclude on the generalization of some met-al(loid) species dependent effects.

2 Materials and methods

The column system shown in Figure 1 was constructed asfollows: A glass column (length 171 mm, inner diameter 20mm) was filled with quartz sand (F34, Quarzwerke Frechen,mean particle size 200 µm). Table 1 shows the chemicalcomposition and specific surface area of the quartz sand.Deionized water (Milli-Q Plus, Millipore) was used as eluent.The mobile phase in the column was transported upwardsfrom the bottom by a high-pressure liquid chromatographypump (HPLC pump 420, Kontron Instruments). The flow ratewas 1 mL/min. The injection of the samples was carried outby an injection valve with an injection loop of 500 µL. Thequartz sand was dispersed in deionized water and wet-packed in the column. For equilibration, the column wasrinsed with eluent over one week. For the determination ofcolloids at the column outlet a flow-through UV-VIS detector(Spectra System UV 1000, Spectra Physics Analytical) was

Acta hydrochim. hydrobiol. 33 (2005) 4, 337−345 Coupling of a Column System with ICP-MS 339

Fig. 1: Instrumental set-up of thecolumn/ICP-MS system.

Instrumenteller Aufbau des Sys-tems Säulenanlage/ICP-MS.

used. The UV signal was measured at λ = 254 nm. Datarecording of the UV-VIS detector was done by a computer.The detection of the metals and metalloids was carried outin two different ways: offline measurements and onlinemeasurements.

2.1 Offline measurements

At the column outlet after UV detection the fractions werecollected with a rate of 1/min by an autosampler (Frac-100,Pharmacia). 1 mL of the collected fraction was filled up withdeionized water to a volume of 5 mL, stabilised with 1% nitricacid (suprapur, Merck), and then analysed with an induc-tively coupled plasma mass spectrometer (ICP-MS, ELAN6000, Perkin Elmer). Before the ICP-MS analyses thestabilised samples were kept at +8 °C in the refrigerator. Thedilution of the collected fractions was necessary to getenough sample volume for the ICP-MS measurement.

2.2 Online measurements

The column system was coupled directly with the ICP-MSsystem and the metals and metalloids were continuously de-tected. For controlling the detector signal drift an internalstandard containing 20 µg/L Rh, In and Ir each was mixedto the eluent before introduction to the ICP-MS. Recoverycalculation was done by determining the difference betweenbypass and column measurements.

The hydraulic conditions within the column were determinedby tracer injections. As tracer a solution of NaNO3

(c(NaNO3) = 2 mmol/L, Riedel-de Haen) was used. The


nitrate signal was recorded with an UV detector and the so-dium signal by mean of ICP-MS. The elution time of thetracer was defined as the time where half of the tracer hadpassed the column (F1/2). It was calculated by integratingthe breakthrough curves for both detection methods. Thedead time between the UV detector and the mass spec-trometer was derived from the difference of the elution timefor both detection systems. The calculated dead time wastaken into account in all experiments. Figure 2 shows theresults of the tracer experiments.

Furthermore the breakthrough behaviour of the metal(loid)swas investigated by injecting both colloid-free and colloid-containing metal(loid) solutions. As metal(loid)s the solutionsof Cu, Pb, Zn, Pt and As (c(metal(loid)) = 10 µmol/L) wereused. The metal(loid) solutions were prepared by dissolvingof CuCl2·2H2O, PbCl2, ZnCl2 (all: Merck), AsCl3 (Aldrich)and H2PtCl6 (ICP standard, Merck) in deionized water. Thesynthetic three-layer clay mineral laponite RD manufacturedby Laporte Industries Ltd. was used as model colloid. Thelaponite particles were dispersed in deionized water at aconcentration of 200 mg/L. The laponite primer particles areplate-like with a diameter of 25 nm and a thickness of0.92 nm (manufacturer data). The chemical composition oflaponite RD given in Table 1 corresponds to the followingformula: [Na0.7]+[(Si8Mg5.5Li0.3)O20(OH)4]0.7�. As a conse-quence of the partialsubstitution of the Mg2+ ions by Li+ ionsin the octahedral layer the face surfaces of laponite disksare negatively charged [20, 21]. The edge surfaces of lapon-ite have smaller negative or positive charges. This dependson the pH value and is caused by the ionisation and/or pro-tonation of lateral hydroxyl groups of the laponite plates [21].


Fig. 2: Dead time between UVdetector and ICP-MS system de-termined from the breakthroughcurves of the tracer.

Bestimmung der Totzeit zwischenUV-Detektor und ICP-MS-Systemaus den Durchbruchskurven desTracers.

Table 1: Chemical composition and specific surface of the used materials (manufacturer data).

Chemische Zusammensetzung und spezifische Oberfläche der verwendeten Materialien (Herstellerangaben).

Chemical composition in % SpecificSiO2 Al2O3 Fe2O3 MgO Na2O Li2O Loss on surface

ignition

Quartz sand 99.5 0.25 0.04 � � � 0.2 118 cm2/gLaponite 59.5 � � 27.5 2.8 0.8 8.2 370 m2/g

The transport experiments were carried out at pH values of5 and 7. The pH in the samples and in the eluent was ad-justed by addition of NaOH (Merck) or HCl (suprapur,Merck). Between injection series at pH 5 and 7 the columnwas rinsed with HNO3 solution (c(HNO3) = 0.1 mol/L) fol-lowed by deionized water for the release of adsorbed metal-(loid) ions and regeneration of sorption places on the quartzsand surface. For all experiments only one column wasused. The application of the “short-pulse” technique enablesbreakthrough experiments without significant blocking orfilter ripening effects due to colloid deposition [10]. For by-pass measurements the samples were diluted to a ratio of1:20 to avoid detector overflow. This dilution correspondsapproximately to the dilution effect in the column experi-ments.


3 Results and discussion

3.1 Offline measurements

In the offline experiments the breakthrough of the metal-(loid)s could not be observed. The dilution effect in the col-umn and interactions with the column packing material(quartz) in combination with the dilution step necessary forreaching the efficient sample volume to serve the ICP-MSsystem resulted in too low metal(loid) concentrations tomatch the detection limit of the ICP-MS.

3.2 Online measurements

Unlike offline measurements the breakthrough of all metal-(loid)s could be detected under the same experimental col-


umn conditions in the coupling experiments. In addition, thecoupling method enables the exact characterisation of thebreakthrough behaviour of colloids and metal(loid)s. The re-cording of the breakthrough curves for laponite was doneby UV detection. UV-VIS detection has been also used intransport experiments for colloid characterisation in thecolumn effluent by other authors [8, 9, 22, 23]. Additionally,laponite was detected by measuring the Mg trace with theICP-MS system. Mg is a component of laponite (Table 1)and can be used for its detection. The breakthrough curvesfor laponite show that the Mg trace and the UV signal corre-late well with each other (Fig. 3). At pH 5 laponite elutes inthree major fractions. The first fraction (0.85 bed volumes)represents probably the laponite agglomerates. At lower pHvalues the face and edge surfaces of laponite particles are

Fig. 3: Breakthrough curves forlaponite and lead.

Durchbruchskurven für Laponitund Blei.

Fig. 4: Breakthrough curves forlaponite and zinc.

Durchbruchskurven für Laponitund Zink.


oppositely charged [21, 24]. The electrostatic attraction be-tween the negatively charged faces and positively chargededges favours the formation of Laponite aggregates with a“house-of-cards” structure [24]. These aggregates elute ac-cording to size exclusion effects earlier than the tracer (bedvolume = 1.0). The size-exclusion effects were already ob-served in investigations of the colloid transport processes insoils [9�11, 14, 25]. The second fraction appears aroundthe bed volume 1.0 without retardation. The third fraction oflaponite leaves the column later than the tracer after 1.10bed volumes. At lower pH values the edge sites of laponiteare significantly protonated. The protonation processes fa-vour an increase in the positive edge charges of the par-ticles. It is attractive to assume that electrostatic attractionbetween positively charged edges of laponite and negatively


charged quartz sand surfaces results in a partial retardationof laponite particles in the column. The long tailing of the Mgand UV curves (Fig. 3) and low laponite recovery rates(Fig. 6) at pH 5 indicate the intensive interaction betweencolloids and column packing material. Without laponite, nosignificant lead transport was observed. This is due to a de-position of the metal species on the column material. In thepresence of laponite the lead cations were transported andappeared predominantly together with the first and thirdlaponite fractions. In difference to the elution at pH 5, atpH 7 the colloids and the lead elute in one peak. In thiscase the agglomeration effects and the interactions with thequartz phase are not distinguishable.

Figure 4 shows the breakthrough behaviour for laponite andzinc. At pH 5 again three laponite fractions occur, whereasone fraction is found at pH 7. For both pH values withoutlaponite no metal transport could be observed. In oppositeto the results for lead, in the presence of laponite at pH 5zinc ions elute at the same bed volume as the second lapon-ite fraction and were probably adsorbed predominantly onthis not retarded fraction. This is also reflected in the rela-tively higher zinc recovery (36%) at pH 5 (Fig. 6). The resultsfor pH 7 are similar to those of lead.

In contrast to the coupling experiments with lead and zinc,the results for copper show that the fractionation of laponitetakes place at both pH values (Fig. 5). At pH 5 the break-through behaviour of laponite is similar to the results of theexperiments for lead and zinc. At pH 7 in the presence ofcopper two laponite fractions were found. The first fractionof laponite elutes between 0.86 and 0.89 bed volumes. It isattractive to assume aggregate formation for this fraction.The aggregation processes could be caused by adsorption

Fig. 5: Breakthrough curves forlaponite and copper.

Durchbruchskurven für Laponitund Kupfer.


of copper ions on edge sites of laponite. At these bed vol-umes a small fraction of copper can be detected. Similarprocesses of copper adsorption on edge sites of three-layerclay minerals are also described in the literature. Accordingto Morton et al. [26] an increase in the pH value and in theNa+ concentration resulted in a displacement of the copperions from interlayer sites of montmorillonite particles and anincreasing adsorption of copper on the edge sites of clay.The adsorption of copper ions on the edge sites of laponiteparticles at pH 7 can cause an increased positive chargeof the edge sites, resulting in edge-face aggregation. Theinteraction between positively charged edges and negativelycharged quartz surfaces can explain the retardation of thesecond laponite fraction (bed volume = 1.07), the tailing ofbreakthrough curves (Fig. 5) and the low recovery rates oflaponite (Fig. 6) at pH 7. In the Cu-experiments, for both pHvalues there was again no metal transport in the absence oflaponite, but a breakthrough of copper for laponite contain-ing samples. At pH value of 5 the copper ions were predomi-nantly transported by the second laponite fraction.

For the experiments with cationic metal species recoverycalculations were done (Fig. 6). The recoveries for laponitewere calculated from the UV and Mg signal. The resultsshow a better immobilization of laponite particles on thequartz surfaces at the lower pH value. The obvious differ-ences between the recoveries for laponite calculated fromthe UV and Mg signal at pH 5 can be explained by increasingdissolution processes of laponite particles with decreasingpH values [27]. The recovery of the lead and copper ions atpH 7 is much lower than at pH 5 reflecting the competingsorption processes between laponite and quartz phase atdifferent pH values.


Fig. 6: Recoveries for laponite and the metals.

Wiederfindungen für Laponit und die Metalle.

Fig. 7: Breakthrough curves forplatinum at pH 7.

Durchbruchskurven für Platin beipH = 7.

Breakthrough experiments were also carried out with anionicmetal(loid)s. Figure 7 shows the breakthrough behaviour forplatinate at a pH value of 7. No colloidal transport of platinumcan be seen. At the column outlet the platinum was found atthe same elution time both in the presence and absence oflaponite. Platinum eluted after 1.08 bed volumes with a smallretardation due to interaction with the quartz phase. The cal-culated recoveries for platinum prove also this interactionprocesses. At 1.6 bed volumes only 39% (in the presence


of laponite) and 40% (in the absence of laponite) of the in-jected platinum had passed the column. At pH 5 the break-through behaviour of platinum is the same as at pH 7, whichis a consequence of the same species present.

The breakthrough of arsenic was investigated by injectinglaponite-containing as well as laponite-free arsenic solu-tions. The results of the experiments at pH 5 are shown inFigure 8. An interaction between arsenic and quartz sand is


Fig. 8: Breakthrough curves forarsenic at pH 5.

Durchbruchskurven für Arsen beipH = 5.

obvious. The retardation of arsenic was even more pro-nounced than the one of platinum. A good explanation forthe behaviour of arsenic is reflected in the pKa values. Ar-senic appears predominantly undissociated as H3AsO3 atpH values below 9 [28]:

H3AsO3 + H2O v H2AsO3� + H3O+ pK1 = 9.2

H2AsO3� + H2O v HAsO3

2� + H3O+ pK2 = 12.1

At the column outlet after 12 bed volumes 70% (in the ab-sence of laponite) and 65% (in the presence of laponite) ofthe injected As were detected. At pH 7 arsenic showed astronger interaction with the quartz phase and eluted with ahigher retardation. Furthermore a small fraction of arsenic(0.8%) was transported by colloids (for breakthrough curvesat pH 7 see Frimmel et al. [29]).

4 Conclusions

For the investigation of colloid-mediated transport processesin porous media a new coupling method (column system/ICP-MS) was developed. This method enables the onlinecharacterisation of colloids as well as of metal species ad-sorbed on or bound to the colloid surfaces. The couplingof the column with the ICP-MS system offers the followingadvantages compared with the offline measurements:

� Continuous breakthrough curves for colloids and metal-(loid)s are obtained with high resolution.

� Samples with low concentration of colloids and metal-(loid)s can be detected.


� An exact calculation of recovery rates is now possible.

� Time-consuming sample preparation is avoided.

In the coupling experiments with cationic metals the influ-ence of colloids on the transport of Cu, Pb and Zn could beobserved. By injecting laponite containing metal solutionsthe breakthrough of these metals were proved at low andneutral pH values. No significant metal transport was foundfor the laponite-free metal solutions. All of these metal cat-ions adsorbed completely onto the quartz particles. In con-trast to the cationic metal solutions platinum and arsenicwere found at the column outlet in the presence as well asin the absence of laponite.

The results show that the coupling method (column system/metal analyser) can be successfully used for the characteri-sation of a wide variety of colloidal transport processes. Thisis very important especially for the investigations of the realsamples of rain water with low colloid and metal concen-tration. In addition, the colloid-mediated separation ofanionic and cationic element species in packed columns ispromising for practical application in water purification.

Acknowledgements

The authors wish to thank the German Research Foundation(Deutsche Forschungsgemeinschaft � DFG) for financialsupport within the KORESI project (Colloidal Transport ofSubstances during the Seepage of Rainwater) and theResearch Training Group GRK 366 (Interfacial Phenomenain Aquatic Systems and Aqueous Phases).


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[Received: 15 July 2004; accepted: 4 February 2005]

coupling of a column system with icp-ms for the characterisation of colloid-mediated metal(loid)...

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