Speciation of arsenic in Greek travertines: Co-precipitation of arsenate with calcite

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Geochimica et Cosmochimica Acta 106 (2013) 99–110

Speciation of arsenic in Greek travertines: Co-precipitationof arsenate with calcite

Lenny H.E. Winkel a,b,⇑, Barbara Casentini a,c,d, Fabrizio Bardelli d,Andreas Voegelin a, Nikolaos P. Nikolaidis d, Laurent Charlet e

a Eawag: Swiss Federal Institute of Aquatic Science and Technology, Ueberlandstrasse 133, P.O. Box 611, 8600 Duebendorf, Switzerlandb Institute of Biogeochemistry and Pollutant Dynamics, Department of Environmental Sciences, Swiss Federal Institute of Technology

(ETH) Zurich, 8092 Zurich, Switzerlandc Water Research Institute – National Research Council, Via Salaria km 29,300, 00015 Monterotondo, RM, Italy

d Department of Environmental Engineering, Technical University of Crete, Polytechnioupolis, 73100 Chania, Greecee Institut des Sciences de la Terre, Universite Joseph Fourier, Grenoble, Maison des Geosciences, 1381 rue de la Piscine,

38400 Saint-Martin d’Heres, France

Received 22 June 2012; accepted in revised form 20 November 2012; Available online 25 December 2012

Abstract

The western part of the Chalkidiki peninsula in Northern Greece is a geothermally active area that contains high levels ofnaturally derived arsenic in its alkaline groundwaters (up to 3760 lg/L). Near wells, equilibration of these groundwaters withatmospheric carbon dioxide leads to the precipitation of travertines that contain very high levels of arsenic (up to 913 mg/kg).To determine the mechanism of arsenic uptake in these travertines, we analyzed two different types of travertine from thisregion using both bulk and micro-focused X-ray absorption spectroscopy (XAS and l-XAS) and micro-focused X-ray fluo-rescence spectroscopy (l-XRF). Bulk XAS showed that in all of the studied samples arsenic is present in the pentavalent oxi-dation state (arsenate). l-XRF analyses indicated that arsenic is closely associated with the calcite matrix and that it generallydoes not correlate well with iron. The arsenic K-edge XAS spectra of all samples closely matched each other and closelyresembled a reference spectrum for arsenate coprecipitated with calcite (rather than adsorbed or pure calcium arsenate). Ironon the other hand was found to be mainly present as a constituent of clay minerals, of presumably detrital origin, suggestingthat iron-(hydr)oxides were not sufficiently abundant to act as major scavengers for arsenic in the Chalkidiki travertines. Weestimated that calcite in these travertines could sequester at least 25% of aqueous arsenic in the form of As(V) and thus immo-bilize a substantial part of arsenic present in the geothermal groundwaters. These results may also be relevant for other areaswhere geothermal groundwaters carry arsenic to the surface and possibly as well for arsenic geochemistry in other environ-ments with CO2-enriched water.� 2012 Elsevier Ltd. All rights reserved.

1. INTRODUCTION

1.1. Background

Geogenic arsenic (As) contamination of groundwater is aserious environmental health threat. On a global scale,

0016-7037/$ - see front matter � 2012 Elsevier Ltd. All rights reserved.

http://dx.doi.org/10.1016/j.gca.2012.11.049

⇑ Corresponding author at: Eawag: Swiss Federal Institute of Aquat8600 Duebendorf, Switzerland.

E-mail address: [email protected] (L.H.E. Winkel).

South East Asia is the region that is most severely affectedby natural As contamination in groundwaters (Nicksonet al., 1998; Chowdhury et al., 2000; Berg et al., 2001; Polyaet al., 2005; Charlet and Polya, 2006; Buschmann et al.,2008; Winkel et al., 2008a,b) but also in Europe areas ofhighly enriched As groundwater exist, albeit at a more

ic Science and Technology, Ueberlandstrasse 133, P.O. Box 611,


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100 L.H.E. Winkel et al. / Geochimica et Cosmochimica Acta 106 (2013) 99–110

regional scale (Aiuppa et al., 2003; Cavar et al., 2005; Go-mez et al., 2006; Lindberg et al., 2006; Kouras et al., 2007;Angelone et al., 2009; Van Halem et al., 2009). The westernpart of the Chalkidiki peninsula in Northern Greece is suchan area that is affected by high As concentrations in thegroundwater, with reported As levels exceeding 1000 lg/L(Kouras et al., 2007) and even reaching 3760 lg/L (Casen-tini et al., 2011). In contrast to the As tainted deltas of SouthEast Asia, in the groundwaters of Chalkidiki the more oxi-dized species, arsenate (As[V]) is more common (>90%) thanarsenite (As[III]) (Casentini et al., 2011) but in both cases thesource of arsenic is geogenic. In Chalkidiki, As is trans-ported to the surface in geothermal waters, which originatefrom flows of endogenous carbon dioxide (CO2) along someof the deeper faults that cross-cut a Tertiary sediment cover(Shterev and Meladiotis, 1993). In this geothermally activearea, close to the municipality of Eleachoria, an immenseartesian flow of up to 600 m3/h started in 1983 in a pre-exist-ing borehole and stopped again after about 10 years (Loehn-ert and Papakonstantinou, 1988). In a preliminary study wefound that travertine deposits, which directly formed fromthis artesian flow, and nearby located calcareous sinterdeposits formed from chemically similar geothermalgroundwaters, contained very high levels of As (bulk Asconcentrations of up to 913 mg/kg). This exceptionally highlevel of natural As warranted further studies to improve theunderstanding of the mechanism of As uptake in these cal-cite-rich natural materials.

1.2. Arsenic sequestration by calcite

The sequestration of As by calcite has been subject of anumber of studies (Goldberg and Glaubig, 1988; Sadiq,

Fig. 1. (a) Geological map of the study area in Chalkidiki, Northern Greeof the geological cross section given in panel (b). (b) N–S trending geologicthe Katsikas mountains and the lower lying Tertiary sediments that host1993). Sampling locations for this study are denoted by B (Byzantine watewell), respectively. The rectangular inset is an enlargement of this traver

1997; Cheng et al., 1999; Le Guern et al., 2003; Di Bened-etto et al., 2006; Fernandez-Martınez et al., 2006; Roman-Ross et al., 2006; Alexandratos et al., 2007; Sø et al.,2008; Yokoyama et al., 2009, 2012; Bardelli et al., 2011)motivated by the far-reaching potential for As immobiliza-tion by this common mineral. Roman-Ross et al. (2006)demonstrated that As(III) oxyanions can replace CO3

2�

groups in the calcite lattice and that As(III) can thus beincorporated in the calcite structure. Nevertheless, Søet al. (2008) showed that this substitution can only occurat comparatively high pH values and arsenite concentra-tions. Alexandratos et al. (2007) were the first to show thatalso arsenate As(V) can replace carbonate in the calcite lat-tice in batch sorption experiments carried out at pH 8.3,without changing the tetrahedral geometry or oxidationstate of As(V). Recently, Yokoyama et al. (2012) confirmedthis result over a wide range of pH (7–12) and furthermoreshowed that incorporation of As(V) can also occur fromAs(III) in solution following an oxidation step and the viaformation of calcium arsenate complexes.

1.3. Arsenic in travertine

Only a small number of studies have focused at theincorporation of As in natural calcite materials, i.g., theAs-rich lacustrine travertines in the Pecora River valley(PRV) (Italy) (Di Benedetto et al., 2006; Costagliolaet al., 2007; Bardelli et al., 2011), in western Anatolia (Tur-key) (Dogan and Dogan, 2007) and the hydrothermal trav-ertines in the Cezallier area (Massif Central, France) (LeGuern et al., 2003). Di Benedetto et al. (2006) provided evi-dence that in the PRV travertines, As(III) is incorporated inthe calcite lattice by the arsenite–carbonate substitution

ce (after Casentini et al., 2011). The line A–A0 indicates the positional transect across the Jurassic massive limestone formation forming

the Neogene and modern travertines (after Shterev and Meladiotis,r mill), and E (modern travertines formed from Eleachoria artesian

tine section.

L.H.E. Winkel et al. / Geochimica et Cosmochimica Acta 106 (2013) 99–110 101

mechanism suggested by Roman-Ross et al. (2006). RecentX-ray absorption spectroscopy (XAS) studies confirmedthis substitution but only for a small fraction of the totalamount of As present in these travertines (Bardelli et al.,2011). In both the PRV and Cezallier travertines it appearsthat adsorption on iron-oxyhydroxide surfaces is the mainprocess involved in trapping of As and that natural calcitescan thus only host a small amount of As (Le Guern et al.,2003; Bardelli et al., 2011). In contrast to the PRV andCezallier travertines, the modern travertine deposits in theChalkidiki peninsula have exceptionally high As contentsbut yet relatively low iron (Fe) contents, which seems tochallenge this finding. To understand the mechanisms thatcontrol the trapping of As in these calcite-rich materials,we sampled and analyzed two morphologically differenttravertines that have been formed from geothermal watersin the vicinity of the municipality of Eleachoria on theChalkidiki peninsula in Northern Greece (see Fig. 1). Inthis work, we combined X-ray absorption spectroscopy(XAS) and micro-X-ray fluorescence (l-XRF) studies toinvestigate the speciation of As and its chemical associationwith other elements. We show here that the Chalkidiki trav-ertine could sequester and thus immobilize a substantialpart of the As present in the CO2 rich geothermal ground-waters. These results are relevant for geothermal areaswhere groundwaters carry As to the (sub-)surface and pos-sibly also for other CO2-enriched aquifers.

2. MATERIALS AND METHODS

2.1. Fieldsite

A modern As-rich travertine deposit close to the munic-ipality of Eleachoria was sampled (see location E on themap in Fig. 1). This deposit was formed from a boreholewith artesian flow that was active from 1983 to 1999. Thewater from this borehole was repeatedly sampled and ana-lyzed in 1987 by Loehnert and Papakonstantinou (1988)and was found to be of meteoric origin (based on oxygen-18 and deuterium analyses) but labeled by components ofdeep-seated origin such as CO2 and the trace elements fluo-ride and boron. In 2007, this groundwater was sampledagain by Kouras et al. (2007) (the results of these measure-ments are given in Table EA1 in the Electronic annex). Inthe 1990’s artesian groundwater flow had largely stoppedin the western Chalkidiki area. The thickness (around 10–15 m) of modern travertine deposits is largest close to theoutflow of the well and thins out towards the main directionof flow (towards south). The deposit has a so-called accre-tionary cascade shape, which typically occurs when the rateof deposition exceeds the rate of erosion (Pentecost, 2005).

2.2. Field sampling

In total five rock samples (E1–E5; in this order distanceto borehole decreases) were taken along the cascade out-crop (sampling location E see Fig. 1). In the vicinity ofthe Eleachoria travertine deposit, an archaeological site islocated that hosts the ruined remains of 40 water mills fromthe late Byzantine period (10th–12th century, Greek

Archaeological service, Thessaloniki). These mills were his-torically fed by one or more artesian groundwater wellsoriginating from the same karstic aquifer as the Eleachoriaborehole (Loehnert and Papakonstantinou, 1988). A num-ber of these ruined structures are partially covered by trav-ertine that precipitated from the flowing water. From oneof the former water mills (sampling location B in Fig. 1)a 39 cm wide travertine deposit was sampled in 7 pieces(samples B1–B7) (see Fig. EA2 in the Electronic annex).In contrast to the travertine deposit at Eleachoria, the trav-ertine of the Byzantine water mill shows regular lamina-tions oriented parallel to the substrate (i.e., the wall). Toobtain information on As concentrations in backgroundmaterials two additional rock samples were taken. SampleBkg1 originates from the Jurassic massive limestone forma-tion that forms the Katsikas mountains. Sample Bkg2 wastaken from the Neogene travertine limestones that line thevalley where the modern travertine was deposited, down-stream of the Eleachoria well (see Fig. 1).

2.3. Sample preparation

All travertine samples were cut in 1–1.5 cm thick slices.From each rock sample, 2–3 slices were embedded in epoxyresin and polished for benchtop l-XRF analyses. Selectedsubsamples were prepared as free-standing thin sections(�100 lm thick) for synchrotron micro-XRF and micro-XAS analyses. For these analyses, the thin sections wereplaced on Kapton tape. The remaining slices were pow-dered and homogenized for bulk quantitative and qualita-tive analyses. For bulk XAS analyses reference andtravertine samples were pressed into pellets for analysis.

2.4. Bulk quantitative analyses

2.4.1. Elemental and mineralogical analyses

Aliquots (500 mg) of the samples were digested in Teflonvessels with 2 ml nitric acid (HNO3, >69.0%, TraceSelect,Fluka Analytical, Buchs, Switzerland), 6 ml hydrochloricacid (HCl, >37%, TraceSelect, Fluka Analytical, Buchs,Switzerland) and 3 ml hydrofluoric acid (HF, 40%, Supra-pur, Merck KGaA, Darmstadt, Germany) in an AntonPaar Multiwave 3000 digestion system (Anton Paar GmbH,Graz, Austria). In a second digestion step, 18 ml of satu-rated boric acid (4% w/v) was added to the digestions ves-sels to complex free fluorides. For dilution, high-puritydeionized Milli-Q water (>18.2 MX cm, ELGA water sys-tems, UK) was used. After centrifugation, the elementalconcentrations in the supernatant were measured withICP-MS equipped with an Octapole Reaction System(ORS) (Agilent 7500). Five samples were analysed by bulkXRF analysis (Spectro Xepos, SPECTRO AnalyticalInstruments GmbH, Germany) on pressed pellets consistingof 2.5 g sample and 0.6 g licowax C (APC Solutions SA,Lonay, Switzerland). The mineralogy of powdered sampleswas analyzed using X-ray diffraction (XRD; Bruker D8with Cu lamp; Bruker AXS GmbH, Karlsruhe, Germany).For two samples of the Byzantine watermill (B5, B7), twosamples of Eleachoria travertine (E2, E4) and a backgroundsample of Neogene travertine (Bkg2), calcite lattice


parameters were calculated from bulk XRD data (spacegroup R-3c) by Rietveld refinement using FULLPROF(Rodriguez-Carvajal, 1993).

2.5. X-ray spectroscopic analyses

2.5.1. Bulk X-ray absorption spectroscopy

Bulk X-ray absorption spectra (XAS) at the As K-edgewere performed at the beamline GILDA (BM-08) (Pascar-elli et al., 1996) at the European Synchrotron RadiationFacility (ESRF) in Grenoble (France). A fixed exit mono-chromator with a pair of Si(311) crystals and a pair of Pdcoated mirrors for efficient harmonic rejection and (verti-cal) focusing of the X-ray beam were used. Dynamic sagit-tal (horizontal) focusing of the X-ray beam resulted in abeam size of 200 lm, horizontal and 100 lm vertical anda photon flux of about 109 ph � s�1 at 12 keV. Energy res-olution was estimated to be about 0.5 eV in the scanned en-ergy range (11.7–12.7 keV). The energy sampling interval inthe near edge region (11,840–11,920 eV) was 0.2 eV. Allsamples were measured in vacuum (10�6 mbar) at 77 K toreduce thermal disorder and to prevent possible beam-in-duced redox reactions. A pellet of As(III)-oxide was mea-sured as a reference with each scan for precise energyalignment. The measurements were performed in fluores-cence mode using a 13-element solid state detector (OR-TEC) suitable for the detection of dilute elements.Depending on the quality of the spectra, a number of con-secutive scans were collected, interpolated and averaged toincrease the signal-to-noise ratio.

Two of the travertine samples (E2 and B7) were alsomeasured at the SUL-X beamline at the AngstromquelleKarlsruhe (ANKA, Karlsruhe, Germany) at the As andFe K-edges. Measurement points on the layered travertinesample (B7) were selected in different lamination bands(for locations see Fig. 2b). Measurements were conductedin vacuum at room temperature, using a collimated beamof �1 � 1 mm2 spot size. The energy was selected using aSi(111) double crystal monochromator, which was cali-brated by setting the first maximum of the first derivativeof the absorption edges of metallic Au and Fe foils to11,919 and 7112 eV, respectively. The Fe K-edge spectrumof the sample E4 was measured at the XAS beamline atANKA in transmission using a setup described in previouswork (Voegelin et al., 2010).

The reference spectrum of As(V) adsorbed to ferrihy-drite was available from own earlier work (Voegelinet al., 2007). Published As K-edge XAS spectra for As(V)adsorbed to calcite and coprecipitated with calcite as wellas for johnbaumite (Ca5(AsO4)3OH) (Alexandratos et al.,2007) were kindly provided by Vasso Alexandratos andEvert Elzinga. Reference spectra for Fe were available fromprevious studies and included a series of crystalline andamorphous Fe(III)-oxides and phosphates, as well as sev-eral reference spectra for Fe in different clay minerals(Voegelin et al., 2010; Frommer et al., 2011), includingthe illite IMt1, the smectite SWy-2 and the nontroniteNAu-1 purchased from the Source Clay Repository (WestLafayette, USA). All Fe and As K-edge XAS spectra were

calibrated and normalized using the software code Athena(Ravel and Newville, 2005).

2.6. Micro-focused X-ray fluorescence spectrometry and

XAS

High-resolution data on chemical composition was ob-tained by measuring polished 1 cm thick slices of travertineon a benchtop ORBIS l-XRF system (Edax, Germany).Elemental distribution maps were collected in vacuum,applying white X-ray irradiation produced by an Rh tube(50 kV and 300 lA). The X-ray primary beam was focusedto a spotsize of 30 lm diameter and a primary-beam Ti fil-ter (25 lm thickness) was applied to improve peak to back-ground ratios. After initial screening for elemental contentand interfering peak overlaps, peak intensities were mea-sured from regions of interest (ROI) for Al, As, Ba, Ca,Fe, K, Mn, Sr.

Further l-XRF analyses were performed using theID18F beamline, at the European Synchrotron RadiationFacility in Grenoble (ESRF, France). The beam was fo-cused to 5.5 lm horizontal to 1.9 lm vertical and the X-ray fluorescence signal was recorded with a Si(Li) detector.The XRF spectra were treated with PyMCA 4.4.1 software(Sole et al., 2007). Quantification of selected XRF spectrawas done with the PyMCA software based on the elementalyields derived from the NIST SRM 1577b Bovine Liver cal-ibration standard.

A thin section of sample B7 was analyzed by combinedl-XRF mapping and l-XAS at the SUL-X beamline at theAngstromquelle Karlsruhe (ANKA, Karlsruhe, Germany).Mapping and spectroscopy was conducted in vacuum atroom temperature. Beam energy was selected using aSi(111) double crystal monochromator. Focusing to a beamsize of 60 � 60 lm2 was achieved with a Kirkpatrick–Baezmirror system. An area of 3.2 by 2.3 mm2 was mapped witha step size of 40 lm and an integration time of 1 s per pixelat incident photon energies of 12.5 keV (for As, Fe, Mn,Ca) and 3.95 keV (below Ca K-edge; for K and Si). Basedon l-XRF results, selected points of interest (POI) wereanalyzed by l-XAS at the As and Fe K-edges. For analysesat the Fe K-edge, a Rh-coated Si mirror was used to rejecthigher harmonics.

3. RESULTS

3.1. Bulk chemical and mineralogical composition of the

travertines

The travertine studied here can be classified into twomain groups based on sampling location and macroscopicfeatures. Travertine samples collected from the Eleachoriadeposit (samples indicated by E) are made up of microcrys-talline calcites. Samples E1, E2 and E5 are highly porous,brittle and homogeneous in color (E1: beige and E2 andE5: orange-brown). Sample E3 is also porous and brittlebut contains thin (up to 1 mm) discrete brown layers in awhite porous microcrystalline matrix, which suggests a dif-ference in mineralogical and or elemental composition.

Fig. 2. (a) Light microscope image and elemental maps for Ca, Fe, and As of sample B7, collected by benchtop l-xrf analyses on a 1 cm thicktravertine slab. (b) Five times enlarged microscope image and elemental maps for Ca, Fe, and As collected on a thin section of the samesample (measured by l-XRF analyses at the SULX beamline at ANKA, Germany). White level of elemental maps has been adjusted to 100%(Ca), 60% (Fe) and 80% (As) of the maximum pixel intensity of the respective element. Selected positions of l-XAS measurement are markedwith circles.


Also sample E4 shows irregular patches of different shadeof reddish-brown and is much denser than the other Eleach-oria samples. At some locations in the travertine outcropincorporated plant remains, such as leafs and stems arevisible.

The 39-cm wide travertine sequence sampled from theruined Byzantine well (samples B1–B7) is composed of

alternating layers of very fine crystalline to extremely coarsecrystalline calcite fibers (up to 3 mm long) that are orientedperpendicular to the depositional surfaces and form denseaggregates. The coarser laminations are typically dark greyor pinkish grey and the smaller laminations have variablecolors that range from white through beige to brownish or-ange. These deposits can be classified as calcareous sinter,

Table 2Lattice parameters for calcite (CaCO3) obtained by Rietveldrefinements on bulk XRD data, and molar As/Ca ratios (basedon data from Table 1) for the same samples.

Sample ID Calcite lattice parameters (A)

a(b) c As/Ca§

Structural analysesB5 4.98564(6) 17.0548(3) 0.00071B7 4.98558(5) 17.0507(3) 0.00067E2 4.98531 (4) 17.0497(2) 0.00134E4 4.98440(7) 17.0493(3) 0.00101Bkg2 4.98206(3) 17.0353(1) 0.00046

§ Molar ratio.


which is typically characterized by well-developed lamina-tion and low porosity (Flugel, 2004).

Results of ICP-MS analyses are summarized in Table 1and show that Ca is by far the major cation present in alltravertine samples, accompanied by small amounts of Si,Al, K, Mg and Fe. The bulk As concentrations in the trav-ertines are high (between 327 and 887 mg/kg) and in threeof the travertine samples (in the recent travertine samplesE2 and E3 and Neogene travertine Bkg2, all from Eleacho-ria) As concentrations even exceeds Fe concentrations (seeTable 1). Arsenic concentrations are more variable in theEleachoria samples (327–887 mg/kg) than in the samplesfrom the Byzantine water mill (450–498 mg/kg; Table 1).Bulk XRD-analyses identified calcite as the only mineralphase (data not shown here). Calculated lattice parametersare given in Table 2.

3.2. Local elemental distribution

Elemental maps of sample B7 (from Byzantine well), ob-tained by benchtop l-XRF mapping and synchrotron l-XRF mapping (SUL-X beamline, ANKA) are given inFig. 2, in panels a and b, respectively. The elemental mapsclearly show that the thicker laminations consist of elon-gated crystals (see Fig. 2a) that are oriented perpendicularto the laminations. The thicker laminations are separatedby thinner layers with microcrystalline textures. Calciumintensities are high and relatively homogeneously distrib-uted over most of the sample. In the lower half of the Camap (Fig. 2a) a vertically oriented patch is recognizedwhere Ca and As intensities are lower, and Fe intensitiesare higher. Furthermore, both Fe and As maps in Fig. 2ashow patchy intensity patterns that seem to coincide withthe shape of the coarse crystalline calcite fibers but do notcorrelate with each other. Also on the elemental maps ob-tained by synchrotron l-XRF mapping (Fig. 2b) it is seenthat Fe occurs in localized zones characterized by lowerCa and As counts.

Table 1Concentrations of most abundant major and minor elements in selected

Sample ID Main elements (g/kg) M

Alb Cab Ka Mga Sib A

Elemental analysesB1 n.a. n.a. 0.2 2.6 n.a. 4B5 13.1 341 2.4 1.5 29.2 4B7 0.1 378 0.2 1.1 3.4 4E1 n.a. n.a. 3.3 4.3 n.a. 3E2 8.9 353 0.1 1.6 19.7 8E3 n.a. n.a. 0.1 1.9 n.a. 8E4 9.4 347 1.6 2.5 22.3 6E5 n.a. n.a. 1.2 2.2 n.a. 4Bkgl n.a. n.a. <0.3* 0.6 n.a. 2Bkg2 0.8 376 0.2 2.2 1.9 3

n.a. not analyzed.a Were measured by ICP-MS following a digestion procedure.b Measured by bulk XRF analyses.

* mg/kg.

Elemental maps of Ca, Fe and As distributions at ahigher spatial resolution (measured at beamline ID18F,ESRF) for samples B5 (Byzantine well) and E3 (Eleachoria)are shown in Fig. 3. In sample B5 (Fig. 3a), two laminationsare recognized that have different compositions than thecalcite-dominated matrix. Both laminations are depletedin Ca and enriched in Fe but lamination A (see Fe mapin Fig. 3a) is enriched in As whereas lamination B is de-pleted in As. Compared to B5, sample E3 from the Eleach-oria deposit contains about 5 times more Ca, about 12times less Fe but almost 2 times more As (Table 1). Calciumand As maps collected on sample E3 show similar elementdistributions to sample B5, with Ca and As rather evenlypresent throughout the sample (Fig. 3b). One Fe-enrichedhotspot can be identified that does not correlate with Asand Ca, which is probably an inclusion of an allochtonousFe-rich mineral grain.

Fig. 4 graphically presents the ratios between bulk con-centration of As and Fe (panel a) and strengths of correla-tions between As, Ca and Fe intensity counts (obtained bybenchtop micro-XRF measurements) (panel b) in Eleacho-ria (E) samples. Samples E4 and E5 that were collected clos-est to the outflow of the former artesian well are richer in Fe

travertine samples.

inor elements (mg/kg)

sa Bab Cra Fea Mna Nia Srb

98 n.a. 13.8 633 17.2 5.2 n.a.50 199 43.1 4852 234 26.4 27473 144 16.5 600 45.0 4.5 21968 n.a. 37.7 7640 174 21.7 n.a.87 136 4.2 336 155 10.1 33637 n.a. 11.5 395 157 7.2 n.a.58 166 30.7 4456 260 18.4 38665 n.a. 24.7 3516 146 9.9 n.a..3 n.a. 16.0 <0.2* 5.3 1.3 n.a.27 123 15.8 228 21.1 24.7 212

Fig. 3. Elemental maps obtained by micro-XRF analyses (mea-sured at ID18F, ESRF, France) for Ca, Fe and As, and RGB (As–Fe–Ca) composite maps: (a) sample B5 and (b) sample E3. Bardiagrams give Fe and As concentrations for selected points.

Fig. 4. Elemental concentrations and intensity counts for As, Caand Fe in travertine samples from Eleachoria. (a) Molar ratiosbetween bulk As and Fe contents (obtained by ICP-MS analyses)and (b) strengths of correlations between As, Ca and Fe intensitycounts (obtained by benchtop l-XRF analyses).


than the other Eleachoria samples and show a strong anti-correlation between Ca and Fe (R2 = 0.99 and 0.81, respec-tively). In two other samples that contain less Fe (E2 andE3) this anti-correlation is not observed and instead posi-tive correlations between Ca and As are observed(R2 = 0.48 and 0.54, in samples E3 and E2, respectively).Correlations between As–Fe do not seem to be related toboth As–Ca and Fe–Ca correlations.

Fig. 5 shows a 3 dimensional plot of the distribution ofl-XRF intensity counts (obtained by synchrotron measure-ments) for Ca, As and Fe (panel a) and the frequencies ofdistribution of l-XRF intensity counts for As–Ca (panelb), and As–Fe (panel c) in samples E3 and B5–B7. InFig. 5a two clusters of data can be recognized that aremarked with “A” and “B,” respectively. Points that makeup cluster A mainly belong to sample E3 and cluster Bmainly consists of points from samples B5–7. Even though

Fig. 4 shows that the positive correlation between As andCa in sample E3 is not very strong (R2 = 0.48), a clear trendbetween As and Ca can be observed in both groups. Fur-thermore, Fig. 5c shows that the concentration of As is gen-erally independent of Fe concentration. These resultsindicate that As could be associated with calcite rather thanwith Fe-phases. Still, since particle size and surface area ofminerals could affect obtained findings, l-XRF results ontheir own are not conclusive with respect to elemental cor-relations. Additional findings from spectroscopic studiesare presented below.

3.3. Spectroscopic analyses

Fe K-edge XANES and EXAFS spectra for Fe referencematerials and travertine samples are shown in Fig. 6. Anal-ysis of three points with different Fe contents on a thin-sec-tion of sample B7 returned very similar l-XAS spectra (notshown), suggesting a relatively homogeneous Fe species dis-tribution. The average of these three l-XAS spectra (spec-trum micro B7 in Fig. 6) closely resembled the illite IMt-1reference, suggesting that Fe was mainly contained in phyl-losilicates. The bulk spectra B5, E4 and E2 resembled thespectrum micro B7, although minor differences in theXANES and EXAFS regions indicated some variations inthe redox state of Fe in clays and minor fraction of otherFe-bearing phases. More detailed sample analysis by linearcombination fitting (LCF) indicated that all travertine

Fig. 5. Frequency plots of l-XRF intensity counts measured in samples E3 and B5–B7 (obtained at ID18F, ESRF, France) (a) 3D frequencyplot showing the distribution of intensity counts for Ca, As and Fe. The two point clouds are indicated by “A” and “B”. (b) 2D projection onthe Y–Z axis giving Ca–As intensities and (c) on the X–Z axis giving Fe–As intensities.


XANES and EXAFS spectra could be well described by acombination of reference spectra for (dominantly) Fe(III)in clays with different Fe contents (illite IMt-1, smectiteSWy-2 and nontronite NAu-1) and two Fe(III)-oxides (2-line ferrihydrite, hematite) (Fig. 6; details on LCF analysisin Electronic annex). Interestingly, goethite and lepidocro-cite were clearly rejected by LCF, whereas minor fractionsof hematite consistently improved the fits.

The bulk XANES and EXAFS As spectra of the ana-lyzed Byzantine well samples (B2, B5, B7), Eleachoria sam-ples (E2 and E4) and the Neogene background travertineBkg2 were all nearly identical (Fig. EA2, Electronic annex),indicating only minor variation in As speciation among thedifferent travertine samples. The average XANES and EX-AFS spectra obtained from these samples are shown inFig. 7AB (“As – travertine”), together with reference spec-tra for As(V) adsorbed to ferrihydrite (Voegelin et al., 2010)as well as As(V) adsorbed and coprecipitated with calcite(from Alexandratos et al.,2007). Considering the referencespectra first, it can be noted that adsorbed and co-precipi-tated As(V) have distinct spectral signatures, both in theXANES and EXAFS region. In particular, the XANESspectrum of As(V) coprecipitated with calcite exhibits twopronounced resonances at energies of �15 and �20 eVabove the white line maximum. These resonances are not

present in the reference spectrum of As(V) adsorbed on cal-cite. Inspection of back-transformed EXAFS spectra overvariable r-ranges showed that the marked splitting in theEXAFS of the As(V) coprecipitate spectrum at �4.8 A�1

in k-space originates from shells between 4.5 and 7 A in r-space, suggesting that this feature results from more distantcoordination shells or from a strong multiple scattering sig-nal. Also the comparison with the XANES and EXAFSspectra of other As(V) species (e.g., scorodite; johnbaumite;pharmacosiderite; not shown) suggested that the spectrumof As(V) coprecipitated with calcite exhibits fairly specificspectral features. The averaged travertine XANES spec-trum closely matches the spectrum of As(V) incorporatedinto the structure of calcite and exhibits the same character-istic resonances at �15 and �20 eV above the white line(see Fig. 7A). The energy position of the edge crest andthe close agreement with the spectrum of the As(V) incor-porated in calcite suggest that As in the studied travertineis dominantly As(V) and exhibits a local coordinationresembling As(V) coprecipitated with calcite. In the EX-AFS in k-space, the spectra shared the splitting at�4.8 A�1 as well as the shoulder at �6.3 A�1, but alsoexhibited marked differences. From the inspection of thecorresponding Fourier-transformed spectra (Fig. 7C), itwas found that those discrepancies resulted mainly from

Fig. 6. (A) Fe K-edge XANES and (B) EXAFS spectra of Fe-oxide (2-line ferrihydrite; hematite – orange solid lines) and Fe-phyllosilicatereferences (nontronite NAu-1; smectite SWy-2; illite IMt-1 – blue solid lines) and travertine samples (micro B7: average of three micro XASspectra; B5, E4, and E2: bulk spectra on powdered pellets – black solid lines). Linear combination fits are shown as red open dots on top ofsample spectra. Numbers indicated for sample spectra indicate sum of Fe-oxide and Fe-clay references derived from LCF. Detailed LCFresults are provided in the Electronic annex. Spectrum “2L-Fh” from Voegelin et al. (2010) and spectra “IMt-1” and “SWy-2” from Frommeret al. (2011). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 7. (A) As K-edge XANES and (B) EXAFS reference spectra of As(V) adsorbed to ferrihydrite (As(V) ads Fh) as well as As(V) adsorbedto calcite (As(V) ads Cc) and coprecipitated with calcite (As(V) cpt Cc) and average from all bulk As spectra collected on travertine samples(E2, E4, B1, B5, B7, Bkg2; individual spectra shown in Fig. EA2). (C) Comparison of the magnitude (solid lines) and imaginary part (dottedlines) of the Fourier-transformed travertine spectrum (black) and the spectrum of As(V) coprecipitated with calcite (red). Spectrum “As(V)ads Fh” from Voegelin et al. (2010) and spectra “As(V) ads Cc” and “As(V) cpt Cc” from Alexandratos et al. (2007). (For interpretation of thereferences to colour in this figure legend, the reader is referred to the web version of this article.)



the marked difference at higher distances in the second shell(r-range 3–4 A). The presence of this signal at larger dis-tance in the synthetic reference material and its absence inthe averaged travertine spectrum could indicate thatAs(V) coordination in the synthetic precipitate was moreordered than in the natural material.

4. DISCUSSION

The results of this study indicate that As in travertinefrom the Chalkidiki peninsula is predominantly associatedwith the main calcite phase. These results differ from othertravertine studies such as those that were carried out on thetravertines of the Cezallier area in France (Le Guern et al.,2003) and of the Middle Pecora Valley in Italy (Di Bened-etto et al., 2006; Bardelli et al., 2011). Although the studyon Italian travertines gave evidence for the incorporationof trivalent As(III) into the calcite lattice, most As, includ-ing all As(V) was found to be adsorbed on Fe-oxides (Bard-elli et al., 2011).

Thin-sections of the travertines show that As is relativelyhomogeneously present throughout the calcite matrix. Eventhough in thin section local co-enrichment of As with Fewas found in one sample (B5), it was generally observedthat Fe-enriched structures are depleted in Ca and As. Fur-thermore, in comparison with other studied travertines, Fecontents are relatively low and in three of the studied sam-ples even lower than As contents. The combination of theseresults suggest that calcium carbonate rather than Fe-oxi-des play an important role in the trapping of As. This sug-gestion was confirmed by bulk and micro-focused XASanalyses that showed that As is exclusively present asAs(V) and coprecipitated with calcite. Substitution of thetetrahedral arsenate groups (AsO4

3�) for the planarCO3

2� group is expected to result in a significant increasein calcite unit cell parameters due to geometric incompati-bility of the arsenate tetrahedron. Indeed, compared tothe calcite lattice size of the Neogene background sampleBkg2, both a- and c-axes increase with increasing molarAs/Ca ratios in modern Eleachoria travertines (samplesE2 and E4) (see Table 2). If the arsenate groups would bein an axial position (Reeder et al., 1994; Alexandratoset al., 2007) mainly an extension of the c-axis would be ex-pected and no or minimal changes of the a-axis. Eventhough we cannot elucidate the precise reason for the unitcell expansion of the Eleachoria samples compared to thebackground sample, our results potentially indicate thatarsenate is present in a non-axial position, if we assume thatthe observed expansion is mainly related to substitution ofarsenate for carbonate.

Regarding As sequestration by other environmentallyimportant minerals, Fe-oxides and birnessite (d-MnO2)have the highest Kd values for As(V), followed by clayminerals (Smedley and Kinniburgh, 2002), as was recentlyalso pointed out by Yokoyama et al. (2012). However,Yokoyama et al. (2012) showed that As(V) can be incor-porated into calcite in a similar amount as it can besorbed on the clay minerals kaolinite and illite. Therefore,when neither Fe oxides nor clay minerals are present inlarge enough amounts to act as major scavengers for As,

calcite may serve as a host phase instead. In four samplesstudied by Fe K-edge XAS, Fe was found to be domi-nantly present as a Fe(III) in clay minerals and minorfractions of hematite. Local co-occurrence of As- andFe-enriched phases in the studied travertines from Chalk-idiki and positive correlations between Fe and As(Fig. 4b) show that As is not exclusively associated withcalcite. It is likely that As is locally adsorbed to smallamounts of Fe-oxides or Fe-containing clay minerals(Xu et al., 1988; Goldberg, 2002) e.g., in laminations offine-grained materials formed between periods of calcitedepositions (in the layered travertine deposited in the Byz-antine water mill). Presumably these clay minerals aremainly detrital (colloidally transported in the geothermalwaters) and have not formed in situ upon CO2 degassingand calcite precipitation. This is presumably also the casefor hematite, which is a common iron oxide in soils inwarmer climate zones.

By making a number of assumptions we can estimatehow much of the As released from the artesian well atEleachoria could have been removed from the groundwaterupon calcite deposition. These assumptions are: (i) about90% of As in groundwater is present as As(V) (Casentiniet al., 2011) and, (ii) the chemical composition of thegroundwaters has not varied over time. Using the molarAs/Ca ratios for travertine samples E2, E4 Bkg2 andEleachoria groundwater (see Table 1 and Table EA1; Elec-tronic annex) we can estimate that for E2 and E4, respec-tively 45 and 34% of the As(V) present in thegroundwater has been sequestered by the travertine, whichis a very substantial part. According to this calculation eventhe older and more crystalline Neogene travertine (Bkg2)would still have retained 15% of As(V). These extraordi-nary results show that substantial As immobilization fromCO2-rich groundwaters is possible under natural condi-tions, provided that iron oxide concentrations are suffi-ciently low. Therewith, the present study confirms thehypothesis formulated by Bardelli et al. (2011) stating that“uptake of As by calcite could become environmentallyimportant wherever the adsorption of As onto iron oxhy-droxides is hindered.” Immobilization of As via coprecipi-tation with calcite can potentially also occur in other hotspring deposits since geothermal waters in different partsof the world have been found to be highly enriched in As(up to 50,000 lg L�1) (Smedley and Kinniburgh, 2002).Geothermal inputs to ground- and surface waters couldconsequently lead to health-threatening As concentrationsin drinking waters but until now relatively few studies havefocussed on As immobilization in geothermal deposits. Yet,considering the fact that many geothermal waters are satu-rated in calcium carbonate, the formation of calcites couldplay an important role in the immobilization of As in geo-thermal environments.

5. CONCLUSIONS

Recently formed travertines from the Chalkidiki penin-sula in northern Greece were found to contain exceptionallyhigh As levels as they were formed from As-rich geothermalgroundwaters. l-XRF studies of two morphologically


different travertines indicate that the presence of As is clo-sely associated with the calcite matrix. Fe is locally enrichedin fine layering or present as inclusions of Fe-enriched min-eral grains that probably originate from an external source.Both bulk and micro-focused XANES and EXAFS Fespectra revealed that Fe is mainly present in the structureof clay minerals (i.e., illite, smectite and nontronite) andin minor fractions of hematite. XANES and EXAFS Asspectra of the analysed travertine samples were all nearlyidentical and show that As in the studied travertine is exclu-sively present as As(V). In addition, comparison with thefeatures of both XANES and EXAFS regions of the sampleand reference spectra suggests that As is mainly coprecipi-tated with calcite, which implies that Fe-oxides/oxyhydrox-ides were not sufficiently abundant to act as a majorscavenger for As. In this manner, the modern travertinescould trap a large fraction (estimated amounts of 34%and 45%) of As(V) in the groundwater in its structure asAs(V)). Arsenic mobilization via coprecipitation with cal-cite could potentially also occur in other geothermal areasor CO2-enriched environments, such as in shallow uncon-fined aquifers that are in contact with the high-CO2 soilzone (Macpherson, 2009), but further studies are requiredto test this hypothesis.

ACKNOWLEDGMENTS

Funding for this work was provided by the EuropeanCommission (AquaTRAIN MRTN-CT-2006-035420). L.H.E.W.acknowledges funding from the Swiss National Science Founda-tion (SNF) (Grant No.: PP00P2_133619). We thank the ESRF(The GILDA beamline (BM8) and beamline ID18F) for provid-ing beamtime and Francesco D’Acapito and Juan Angel SansTresserras for their help during data collection at the GILDAand ID18F, respectively. A special thanks goes out to Chris Par-sons for his help during measurements at ESRF. We alsoacknowledge the Angstromquelle Karlsruhe (ANKA, Karlsruhe,Germany) for the provision of beamtime at the beamlines SUL-X and XAS and Jorg Gottlicher, Ralph Steininger and StefanMangold for their assistance during data collection at ANKA.We thank the members of the geochemistry group at UtrechtUniversity for letting us use their benchtop l-XRF system andwe especially thank Pieter Kleingeld and Tilly Bouten assistingwith l-XRF analyses and Thilo Behrends and Mariette Wolthersfor helpful discussions of l-XRF results. We further thankManouchehr Amini for statistical data analysis. Lastly, we thankManolis Kotronakis for his help during field sampling, AnnaAndroulaki and Daniel Moraetis for their assistance in the lab,and Vasso Alexandratos and Evert Elzinga for sharing As K-edgeXANES and EXAFS reference spectra.

APPENDIX A. SUPPLEMENTARY DATA

Supplementary data associated with this article can befound, in the online version, at http://dx.doi.org/10.1016/j.gca.2012.11.049.

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Speciation of arsenic in Greek travertines: Co-precipitation of arsenate with calcite