Arsenic-Bearing Calcite in Natural Travertines: Evidence from Sequential Extraction, μXAS, and μXRF

Arsenic-Bearing Calcite in Natural Travertines: Evidence fromSequential Extraction, μXAS, and μXRFPilario Costagliola,† Fabrizio Bardelli,‡ Marco Benvenuti,† Francesco Di Benedetto,†,§ Pierfranco Lattanzi,∥

Maurizio Romanelli,†,§ Mario Paolieri,† Valentina Rimondi,†,* and Gloria Vaggelli⊥

†Dipartimento di Scienze della Terra, Universita di Firenze, Via G. La Pira 4, 50121 Firenze, Italy‡Institut des Sciences de la Terre, Universite Joseph Fourier, Maison des Geosciences, 1381 rue de la Piscine, 38400 Grenoble, France§INSTM unit of Firenze, Via della Lastruccia 3, 50010, Sesto Fiorentino, Italy∥Dipartimento di Scienze Chimiche e Geologiche, Universita di Cagliari, Via Trentino 51, 09127 Cagliari, Italy⊥Istituto di Geoscienze e Georisorse, CNR, Via Valperga Caluso 35, I-10125 Torino, Italy

*S Supporting Information

ABSTRACT: Recent studies demonstrated that syntheticcalcite may host considerable amounts of arsenic (As). Inthis paper, the concentration of As in natural calcite wasdetermined using two novel, specifically designed, sequentialextraction procedures. In addition, the oxidation state of Asand its distribution between calcite and coexisting Fe-oxyhydroxides was unravelled by μXRF elemental mappingand As K-edge μXAS spectroscopy. Our results conclusivelydemonstrate that arsenic can be found in natural calcite up to 2orders of magnitude over the normal crustal As abundances.Because of the large diffusion of calcite in the environment,this phase may exert an important control on As geochemistry,mobility, and bioavailability.

1. INTRODUCTION

Several scientific contributions were devoted to the uptake ofarsenic (As) by calcite.1−9 Most of these studies deal withsynthetic calcite, reporting As contents up to 2250 mg/kg.10

On the other hand, the efficiency of natural calcite to act as amineralogical trap for As has not been extensively studied,possibly because of the inherent difficulties of dealing withcomplex, inhomogeneous matrixes. The ability of calcite to hostand retain As has important technical (see, for example, the USpatent 8227378) and environmental implications. In fact, manyAs-contaminated groundwaters are at equilibrium or slightlysupersaturated with respect to calcite,8 suggesting a possiblecausal relationship between As in solution and the presence ofcarbonate ions (which are generally abundant in calcitesaturated waters and favor mobilization of As species adsorbedonto Fe-oxyhydroxides).11,12 Therefore, under favorableconditions, natural carbonate-rich waters may precipitatetravertine rocks with significant amounts of As.3,4,13,14

In previous studies,3,4 we presented strong spectroscopic(ESEEM and XAS) evidence, supported by density functionaltheory (DFT) structural simulations, for the incorporation ofAs in the calcite lattice in natural travertine rocks fromSouthern Tuscany (Italy). Nevertheless, since the spatialresolution of the applied techniques is millimetric at best, itwas not possible to conclusively demonstrate how much As wasactually bound to calcite. In this study, two different sequential

extraction procedures were used to determine the amount of Asin calcites from the same travertine sequence. To support theresults of sequential extractions, microscale XRF elementalmapping and As K-edge XANES spectroscopy were used tounravel the oxidation state of As, and its distribution betweencalcite and coexisting Fe(Mn)-oxyhydroxides. The informationhere acquired will be useful to establish the potentialenvironmental relevance of calcite as mineralogical trap for As.

2. SAMPLING AND ANALYTICAL METHODS2.1. Geological Outline and Location of the Sampling

Area. A detailed description of the sampling area (geology,mineralogy, and bulk chemistry) is reported elsewhere.3,4,13−15

Only a few relevant points are summarized here. Samples forthis study come from an area (Pecora Valley, PV) in SouthernTuscany, Italy (Figure 1), where an extensive outcrop of aphytoclastic travertine sequence (sensu Pentecost16) overlies asiliciclastic alluvial deposit, both belonging to a QuaternaryNeo-Autochthonous sequence.At the outcrop scale, PV travertines consist of layers of

variable thickness (up to 50−60 cm). The main phase is calcite,

Received: December 4, 2012Revised: May 9, 2013Accepted: May 21, 2013Published: May 21, 2013

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associated with variable amounts of other minerals, includingphyllosilicates (Illite, kaolinite, chlorite-vermiculite), and iron-(manganese) oxyhydroxides. Centimeter-thick layers of non-carbonate minerals document the siliciclastic feeding of thetravertine lake.The bulk As contents of travertines vary between 119 and

243 mg/kg. Arsenic is positively correlated with SiO2 (and allother oxides, notably Fe2O3), thus suggesting that the metalloidcould have been introduced in the lake together with thesiliciclastic fraction.14

2.2. Sequential Extraction Procedures. In our samples,Fe−Mn oxyhydroxides and calcite are likely the main Asresidences, although some As may adsorb onto clay minerals;17

the amount of organic C of the samples is generally low,therefore we estimate that a negligible fraction of As isassociated to organic matter. We decided then to set up asequential extraction protocol that can discriminate As boundto calcite and to Fe−Mn oxyhydroxides, minimizing thenumber of extraction steps to reduce the analytical errorstypically affecting sequential extraction procedures. Manyauthors have proposed sequential extraction protocols for Assimilar to those used for phosphorus,18−20 due to the supposedchemical affinities of these elements in soils and other naturalmatrices. However, in the available extraction protocols aspecific step dedicated to the carbonate fraction is generally notprovided.Classical extraction procedures for carbonate/Fe-oxyhydr-

oxides systems 21 include some leaching steps dedicated to Fe-oxyhydroxides, generally preceded by an acid leach carbonatedissolution step.22−26 The achieved pH (∼5) prevents anyreadsorption of cations liberated by carbonate dissolution ontothe positively charged Fe-oxyhydroxides surfaces. However, atthat pH readsorption of oxyanions (such as As, P, and Se) byFe-oxyhydroxides readily occurs,27,28 thus implying a scarceselectivity of the procedure. In contrast, an alkaline washing todesorb As from Fe-oxyhydroxides,29 followed by total attack of

the remaining phases, causes a readsorption of the leached Asby calcite, which shows its maximum adsorbing capacity for thismetalloid at pH of about 12.30 Therefore, since an extractionprotocol able to overcome the above issues does not exist, weadopted a combination of the two described methods (labeledas “A” and “B” in the following; Table 1).

Procedure A includes an alkaline leaching, adapted from thework of Jang et al.;29 the samples were kept in contact with aNaOH solution (pH ∼ 12.0) for about 20 h (step A1), toremove all As adsorbed onto Fe/Mn oxyhydroxides; after thisleaching, the only As remaining in the samples should be boundto the carbonate fraction and is released upon digestion in aquaregia (step A2).Procedure B includes two preliminary steps: samples are

washed with a nonspecific [(NH4)2SO4] and, subsequently,with a specific [(NH4)H2PO4] exchanger for arsenate (steps B1and B2, respectively), to assess the total fraction of As adsorbedonto mineral surfaces (see the work of Wenzel et al. 31). Giventhe presence in our samples of both calcite and Fe-oxyhydroxides as As-bearing phases, steps B1 and B2 areexpected to extract As bound superficially to both minerals. Thesample is then leached with the acetic buffer (step B3) todetermine the amount of As bound to calcite, and, finally, theresidue is digested in aqua regia (step B4) to measure theconcentration of As associated with Fe-oxyhydroxides.Calcimetry of the samples by using the Dietrich-Fruhling

apparatus permits the calculation of the abundance of calciteand, consequently, the absolute concentration of As in calcitefor both A and B procedures.The analytical quality of the sequential extraction was

controlled by treating the bulk sample with aqua regia (AR)and evaluating the As recovery As(R) with the followingexpression:

=∑

×RAs( )As( seq. estr. single steps)

As(AR leaching)100

The results of AR leaching are reported in Table 2.In the present approach, procedure A is designed to measure

the As content of calcite using an aqua regia leaching after analkaline washing that is intended to desorb all the As bound toFe-/Mn-oxyhydroxides. The reliability of the procedure A toestimate the As content of calcite highly depends on the

Figure 1. Schematic representation of the geological formationsoutcropping in the Pecora Valley (Southern Tuscany, Italy) andlocalization of the study area. Adapted from ref 42.

Table 1. Description of the Two Different SequentialExtraction Procedures (Labeled A and B), Employed in ThisWorka

step extractant target phase/As fraction ref

A1 NaOH, pH 12.0 As adsorbed onto Fe-ox(±aluminosilicates)

2920 h 25 °C

A2 aqua regia As bound with calcite 21, 435h 60 °C

B1 (NH4)2SO4 0.05 M easily exchangeable As 314h 25 °C

B2 (NH4)H2PO40.05M

specifically adsorbed As 31

16 h 20 °CB3 acetic buffer pH ∼5 As bound with calcite 21, 43

12 h, 25 °CB4 aqua regia As bound with Fe-ox 21, 43

5 h, 60 °CaArsenic fractions extracted during each step are reported.

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amount of As that is liberated during the step A1 from the Fe-oxyhydroxides surface by the alkaline leaching, and readsorbedonto calcite. The mechanism(s) of As adsorption on the calcitesurface are still poorly known. Sadiq32 suggests that, at pHbetween 7.5 and 9, carbonates may play an important role forAs adsorption in soils. At higher pH, since the isoelectric pointof calcite is at a pH of about 10, the adsorbing effect of Ca-carbonates should be minor. More recent studies,2 conductedin the pH range 6.0−9.5, point out that in laboratoryexperiments calcite behaves as a minor adsorbent for arsenates.However, according to Goldberg and Glaubig,30 arsenateadsorption on calcite increases from pH 6 to 10, peaksbetween pH 10 and 12, and then decreases above pH 12. Thelatter results suggest that procedure A, that includes a first step(A1) at pH ∼ 12.0, may actually overestimate the As content ofcalcite. In principle, in fact, As released from Fe-oxyhydroxidesafter alkaline washing with NaOH solution (step A1) could be(at least partly) adsorbed by calcite, and this would cause anunderestimate of the As bound to Fe-oxyhydroxides, and acorresponding overestimate of calcite-bound As measured inthe step A2. In addition, although alkaline attack (step A1) isconsidered highly efficient toward As extraction,29 it may notextract all As adsorbed onto Fe-oxyhydroxides.28 This featureimplies an additional contribution of As in the A2 step and aconsequent overestimate of the As bound to the carbonaticfraction.In the B procedure, the acetic buffer is used to selectively

dissolve the carbonate; this step is accomplished in presence ofthe Fe-oxyhydroxides that may readily adsorb the As oxyanionsliberated by the calcite dissolution. Actually, the effectiveness ofthe B procedure to exactly evaluate the As contained in calcitedepends on the side effects induced by the relatively low pH ofthe acetic buffer (pH ∼ 5.0). Under these conditions, thecapacity of Fe-/Mn-oxyhydroxides of adsorbing As-oxyanions ismaximum,33 thus leading to an underestimate of the amount ofAs bound to calcite, and a corresponding overestimate of Asbound to Fe-oxyhydroxides. This effect cannot be quantitativelyassessed and, therefore, the amount of As bound to calciterecovered by this procedure has to be regarded as a minimumestimation.2.4. Analytical and Spectroscopic Procedures. Arsenic

was determined in solution by Hydride Generation AAS using aPerkin-Elmer AAnalist100 Spectrophotometer equipped with aPerkin-Elmer FIAS 100 Hydride Generator. The analyticalquality was controlled by using international standards. The

relative difference between our results and the certified Ascontents in the standards was <7%.Both laboratory/benchtop and synchrotron radiation μXRF

were performed in this work for multielement analysis incalcite. Laboratory μXRF was performed using an Eagle III-XPL (Rontgenanalytik Messtechnik GmbH, Germany), con-trolled and run using the EDAX Vision 32 software. Thistechnique does not require any sample handling, and it allowsthe almost simultaneous measurement of both major and traceelements with accuracies <5% and ∼10%, respectively.34

Typical detection limits are a few tens of milligrams perkilogram for heavier elements (Z ≥ 19).The instrumental parameters used for determining Ca and

trace elements (Fe and As) in this study were the following:voltage, 40 kV; beam current, 1000 μA; live time, 1000 s; inaddition, a titanium primary filter (25 μm in thickness) wasinserted between anode and samples to optimize the peak tobackground ratio in the range of interest and to minimize theincidence of artifact peaks occurring in the characteristic X-rayspectra as a result of Bragg diffraction. All analyses were carriedout in vacuum conditions. The final composition was quantifiedadopting the intensity correction method proposed by Lucas-Thooth and Pyne35 and using an internal standard of calcite(M43 from Carrara Marble) for Ca and Fe. The SRM 610standard glass from the National Bureau of Standards was usedfor As quantification using a Fundamental Parameter withStandards correction routine.34 All samples were investigatedfor Ca and trace elements performing single analyses, with aspot size of 30 μm, along centimeter-size profiles. Each profileis composed by several tens of equally spaced (100−200 μm)point acquisitions.Synchrotron μXRF analyses were performed at the DiffAbs

beamline, Soleil synchrotron radiation facility (France). Anexcitation energy of 12.0 keV was selected using a Si(311)double crystal monochromator, and a beam spot size of 10 μm× 10 μm was achieved with a Kirkpatrick−Baez mirror system.The X-ray fluorescence spectra were collected with a Si(Li)detector. Samples were prepared as thin sections of about 100μm embedded in epoxy resin and glued to slides. At least three500 μm × 500 μm fluorescence maps (3 s integration time perpixel) were acquired for each sample. Obtained XRF spectraand maps were treated with PyMCA 4.4.6 software.36 On thebasis of the μXRF maps of As, Fe, and Ca, points of interestwere selected for μXAS analysis at the As K-edge. Points ofinterest were selected in As-rich spots rich either in Fe, or inCa. XAS spectra were acquired in the 11.8−12.0 keV energyrange using a 3s integration time per energy point. Two arseniccompounds (NaAsO2 and Na2AsO4·7H2O, Alfa Aesar) werealso measured as As(III) and As(V) references. Backgroundsubtraction and normalization, and linear combination fitting(LCF37) of the XAS spectra were performed using the IFEFFITpackage.38 Three to five 500 μm × 500 μm maps and one totwo μXAS spectra for each map were acquired for each selectedsample.

3. RESULTS AND DISCUSSION3.1. Arsenic Extraction. The As-contents determined for

each extraction step with the procedures A and B are shown inTable 2. The percentage difference between the total recovery(the sum of all the steps) and the expected value (the totalamount obtained by direct digestion with aqua regia: Table 2),was systematically lower than 12%. This deviation, that isconsidered a proxy for the analytical accuracy, is within the

Table 2. Concentrations of As (mg/kg) Resulting from theSteps of Sequential Extraction A and B Proceduresa

sample Refa A1 A2 ∑A B1 B2 B3 B4 ∑B

F1A 216 29 170 199 2 21 96 101 220F1B 228 27 177 204 2 27 93 90 212TR1 194 18 166 184 1 22 75 86 184TR4 243 58 191 249 3 39 81 113 236TR6 189 29 156 185 1 18 115 69 203TR7 119 8 120 128 2 13 77 31 123TR8 159 20 136 156 2 20 83 56 161

aRefa refers to the total As concentration (mg/kg) obtained from theaqua regia leaching, while ∑A and ∑B are the sum of As extractedfrom each sequential procedure. The good match of Refa and ∑A and∑B testifies the analytical accuracy of the employed sequentialextraction method.



accuracy commonly accepted in sequential extraction meth-ods.39

The As contents in calcite determined by the A and Bprocedures are presented in Table 3. Here, the As-contents

determined in steps A2 and B3 were normalized to the totalamount of CaCO3 measured by calcimetry to express theresults as the concentration of As in calcite. The B3 stepsystematically yielded lower contents of As bound to thecarbonate fraction with respect to step A2. Conversely, the Asextracted from the Fe-oxyhydroxides was systematically higherin the B procedure (step B4) compared to A (step A1). Theadopted extraction procedures allowed us to establish a lowerand upper limit of As concentration in calcite from thetravertine quaternary sequence of the PV comprised between∼90 and ∼270 mg/kg As.The amount of As bound to mineral surfaces was estimated

by steps B1 and B2 because the addition of sulfate (step B1) andphosphate (step B2) should promote the release, from thesurfaces of calcite and Fe-oxyhydroxides, of the nonspecific andspecific adsorbed fractions of As, respectively. Therefore, theamount of As extracted in the step B3 should not include the Asbound to the calcite surface.According to Alexandratos et al.,5 the mechanisms and the

geometrical arrangement of arsenate adsorption on the calcitesurfacevia tridentate corner-sharing coordination of AsO4

3−

tetrahedraand of its incorporation in the calcite lattice arevery similar (i.e., adsorption onto the calcite surface can beregarded as a precursor of incorporation in the crystal lattice).In fact, in both cases, AsO4

3− tetrahedra substitute for CO32−

groups; therefore, it may be difficult to distinguish betweenadsorbed and incorporated As(V) complexes at the calcitesurface. In our experiments, As seems to be mainly specificallyadsorbed, given that the desorbed amount by using annonspecific exchanger (step B1) is practically negligible(Table 2). Following Alexandratos et al.’s5 interpretation, it islikely that at least part of the specifically adsorbed As-oxyanionextracted in step B2 might derive from AsO4

3− incorporated atthe mineral surface. As previously noted, in earlier studies3,4 wepresented strong evidence for the incorporation of As in thecalcite lattice.On the basis of the present data it was not possible to

ascertain which of the two protocols, A or B, is more accurate inmeasuring the As content of calcite in the presence of Fe-oxyhydroxides. The greatest differences in extracted As betweenthe two methods were recorded for samples with the highestFe2O3 contents.3 Given that Fe2O3 may be a proxy for thequantity of Fe-oxyhydroxides contained in the samples, theefficiency of extraction methods appears to be related to theabundance of these phases, in agreement with their competingbehavior with calcite in adsorbing As oxyanions.

3.2. Benchtop μXRF and μXAS. Two samples (F1A andTR6), characterized by similar bulk arsenic contents butdifferent Ca concentrations,3 were investigated by benchtopμXRF. The results, shown in Table S1 (Supporting Information(SI)), indicate that the chemical composition of the samples israther variable. However, CaO is almost systematically the mostabundant oxide, ranging from few units weight percent up toabout 54 wt %; high values of CaO indicate calcite-rich areas,where CaO may reach the theoretical maximum value of about56 wt %. Low CaO areas are mainly constituted by silicates.Iron content ranges from some hundreds of milligrams per

kilogram to more than 60 000 mg/kg. In general, CaO-richareas are also Fe-poor, and As and Fe are positively correlated(SI Table S1). Although Fe may be hosted in accessory silicates(e.g., chlorite) that were found in the PV travertine, Fe appearsmainly associated with Fe-oxyhydroxides; therefore, Fe contentmay be considered as a proxy to estimate the abundance of Fe-oxyhydroxides in the samples. The observed As−Fe positivecorrelation suggests that As is, in general, accommodated bythese minerals, probably bound to their surface.As expected by the bulk chemistry, calcite-rich/Fe-oxy-

hydroxides-poor (Fe < 1000 mg/kg) areas are more frequent insample TR6 rather than in sample F1A. In Figure 2, only As−

Fe data about these Fe-oxyhydroxides-poor areas have beenreported; as it is clear no positive correlation between As andFe is present (Figure 2). In fact, μXRF spot analyses where Feis lower than 1000 mg/kg show rather constant Asconcentration, highlighted by an angular coefficient, obtainedby a linear fit of the data, lower than ∼0.04. This indicates thatthe As distribution is no longer controlled by the presence ofFe-oxyhydroxides (Figure 2). We argue that for these calcite-rich/Fe-poor areas the mass of As adsorbed onto Feoxyhydroxides is minor whereas the amount of the metalloidassociated with calcite becomes prevalent. The Fe-poor areas ofsamples TR6 and F1A show an average As content of ∼160(±20 = 1σ) and ∼200 (±30 = 1σ) mg/kg, respectively, veryclose to the range indicated by the sequential extractions in thesame samples (Table 3). This analogy confirms that calcite maytake up As in its lattice in quantities that are at least 2 orders ofmagnitude higher than natural crustal abundance.40,41

Table 3. Absolute Concentration of As in Calcite (mg/kg)a

sample CaCO3 (wt %) step B3 step A2

F1A 71.5 134 238F1B 70.0 133 253TR1 64.5 116 257TR4 70.5 115 271TR6 82.0 140 190TR7 87.5 88 137TR8 82.0 101 166

aCalcimetry is expressed as CaCO3 (wt %).

Figure 2. Correlation of As vs Fe in Fe poor (<1000 mg/kg) areas forsamples F1A and TR6. Subgroup of the data from benchtop μXRF.



Selected elemental maps acquired at the Soleil synchrotronradiation source with 10 μm resolution on samples F1A andTR6 are shown in Figure 3 (the rest of the maps are shown inFigures S1−S6 in the SI). As already pointed out by benchtopμXRF analyses, the total (all pixels) fluorescence spectraindicate that the Ca/Fe ratio is much higher in sample TR6than F1A (Figure S7 in the SI). In addition areas withconsiderable amounts of Mn were also found in sample F1A.As can be seen from Figure 3, Ca intensities are high, and

relatively homogeneously distributed over most of the samples,and Fe has generally a negative correlation with Ca. On theother hand, confirming benchtop μXRF, As is mainly associated

with Fe as can be seen from the yellow areas in the lowermostpanels of Figure 3. Nevertheless, lower concentrations of As canalso be found in Fe-poor/Ca-rich spots (sky blue areas in thelowermost panels of Figure 3) confirming that minor amountsof As are associated with calcite where Fe-oxyhydroxides arepresent in small quantities. It should be noted that, due to thelack of a suitable standard, only the relative amounts of As, Ca,and Fe can be estimated from synchrotron μXRF.The white numbered points in Figure 3 correspond to point

μXAS acquisitions (beam spot size equal to the pixel size, i.e.,10 μm). The X-ray absorption near edge structure (XANES)region of the XAS spectra, corresponding to the points

Figure 3. Synchrotron μXRF maps of samples F1A (a) and TR6 (b). The pixel size is 10 × 10 μm. The upper panels show the distribution of Ca, Fe,and As, while the lowest panels highlight the colocalization of these element making use of RGB color combination (where R = Fe, G = As, and B =Ca). Areas lacking all the three elements under consideration (black color) correspond to holes arising from the vacuolar nature of the samples, andare due to the low thickness of the rock sections (∼100 μm).



highlighted by circles in Figure 3, is reported in Figure 4,together with reference spectra of As(III) and As(V), obtainedon synthetic calcites with adsorbed As(III) or As(V), kindlygranted by Dr. L. Winkel (see the SI for details). The energyposition of the main absorption peak of all spectra clearlyindicates that As occurs predominantly in the V oxidation state.However, the small shoulder occurring at the left of the mainabsorption peak, clearly visible in spectra 3 and 4 of Figure 4,indicates a minor contribution from As(III). Linear combina-tion fitting (LCF) using the normalized spectra of As-adsorbedsynthetic calcites suggests that the contribution of As(III),relative to the total amount of As, is around 15% in spectra 3and 4, and between 13% and 19% in eight of the fourteenspectra acquired (Table S2; Figure S8 in the SI). It is worthnoting that the estimation of the relative As(III) and As(V)amount is only indicative and that the error commonlyassociated with this technique is in the range 10−15%.Nevertheless the LCF analysis clearly indicates the presenceof As(III) in our sample, probably limited to small discretedomains, which is consistent with our previous studies.3,4

However, in agreement with the literature,5,6 it is likely that Asuptake by calcite mostly occurs in the As(V) form.

4. ENVIRONMENTAL RELEVANCE

The fact that natural calcite from PV travertines contains As inamounts that are 2 orders of magnitude higher than normalcrustal abundances is of significant environmental importance.The occurrence of arsenic-bearing calcite may be revealed byseveral techniques, but (relatively low cost) sequentialextraction procedures may provide satisfactory results. Even if

calcite is likely to host lower As concentrations with respect toother common minerals, like sulphides, oxides, oxyhydroxides,or sulphates, this mineral phase is ubiquitous and widely stablein the near-surface environment. Moreover, given that As canbe trapped in the crystal lattice3,4 and not only adsorbed at themineral surface like in the case of Fe-oxyhydroxides, calcitebehaves as a long-term effective trap, since its dissolutionusually takes place under acidic conditions only, but it is ratherinsensitive to superficial exchange and redox reactions. All thesefeatures make calcite a suitable and, probably, underestimatedmineral trap for As.

■ ASSOCIATED CONTENT

*S Supporting InformationSynthesis of the As(III) and As(V) standards, μXRF analysis forCa, Fe, and As (Table S1), all the elemental maps acquired atSoleil synchrotron radiation (Figure S1−S6), the representativetotal fluorescence signal of samples TR6 and F1A (Figure S7),the relative amounts of As(III) and As(V) retrieved from LCFrefinements (Table S2), and the LCF curves of spectra 1−4(Figure S8). This material is available free of charge via theInternet at http://pubs.acs.org.

■ AUTHOR INFORMATION

Corresponding Author*Tel.: +39 55 2757507. Fax: +39 55 284571. E-mail: [email protected].

NotesThe authors declare no competing financial interest.

Figure 4. XANES spectra acquired on samples TR6 (1 and 2) and F1A (3 and 4) and of the As(III) and As(V) references (As-(III)-Ca and As-(V)-Ca). The position of the main absorption peak compared to the As(III) and As(V) references indicates that As is predominantly in the 5+ oxidationstate in all spectra. Linear combination fitting (LCF) using the As(III) and As(V) reference spectra revealed that the contribution of As(III) inspectra 3 and 4, indicated by the small shoulder at the left of the main absorption peak, is around 15%. The presence of As(III) in spectra 1 and 2cannot be confirmed being the amount determined by LCF less than 10%, i.e. comparable to the error usually associated to LCF procedure. Thenumbers associated to each spectrum correspond to the points of interest highlighted by circles in Figure 3.



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■ ACKNOWLEDGMENTS

Financial support was provided by local (ex-60%) researchfunds to P.C. and M.B., MIUR PRIN 2010-2011 to P.C., andRegione Toscana “Progetti finalizzati sulla Silice” to F.B. andM.R. The Ente Cassa di Risparmio di Firenze funded someinstruments used for this research. The μXRF benchtopequipment is owned by the Interdepartmental Center “G.Scansetti” for Studies on Asbestos and Other Toxic Particulates.We thank Dr. Lenny Winkel for providing the As-calcitestandards. mXRF measurements were performed at Synchro-tron Soleil (France) on DiffAbs beamline. We thank Dr.Dominique Thiaudiere and Dr. Cristian Mocuta for the helpgiven during those measurements. We also thank the fouranonymous reviewers for useful comments.

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Arsenic-Bearing Calcite in Natural Travertines: Evidence from Sequential Extraction, μXAS, and μXRF