selenite adsorption mechanisms on pure and coated ...soilchem.usask.ca/pdf/sssa06.pdf · selenite...

Selenite Adsorption Mechanisms on Pure and Coated Montmorillonite:An EXAFS and XANES Spectroscopic Study

Derek Peak,* U. K. Saha, and P. M. Huang

ABSTRACTSelenite (SeO3

22) is an oxyanion of environmental importance dueto its toxicity to animals at higher concentrations, notably waterfowland grazing animals. Sorption of SeO3

22 with mineral phases typicallycontrols the movement and bioaccessibility of SeO3

22 in soils andsediments. Previous studies have successfully utilized synchrotron-based Extended X-ray Absorption Fine Structure (EXAFS) and X-rayAbsorption Near Edge Structure (XANES) spectroscopy to determineSeO3

22 bonding mechanisms on Fe and Mn oxides, but the directevidence of SeO3

22 surface complexation mechanisms on importantmineral phases such as Al hydroxide and aluminosilicate minerals isstill lacking. In this study both EXAFS and XANES spectroscopy wasconducted on aqueous SeO3

22 solutions and on a variety of Al-bearingsorption samples at pH 4.5. The sorbents chosen were a hydroxyalu-minosilicate (HAS) polymer, a hydroxyaluminum (HYA) polymer,montmorillonite, and bothHYAandHAS coatedmontmorillonite. ForSeO3

22 sorption on montmorillonite, only bidentate binuclear inner-sphere complexation was observed. For the hydroxyaluminum andhydroxyaluminosilicate polymers, amixture of outer-sphere and biden-tate binuclear inner-sphere was observed. When montmorillonite wascoated with either HYA or HAS polymers then adsorption behaviorwas intermediate between that of the mineral and the pure polymer.Since temperate soils often contain aluminum-hydroxy and alumino-silicate coated minerals rather than discrete Al hydroxide minerals andpristine clay surfaces, the adsorption mechanisms observed on thesecoated surfaces are more realistic of the natural environment thansorption on pure minerals.

SELENIUM is a common metalloid in the soil environ-ment due to dissolution of Se-bearing minerals and

to anthropogenic sources such as wastewater from coalburning power plants. Selenium is also a micronutrientrequired for human and animal health. Selenocysteine isimportant because it is a component of glutathione per-oxidase, an enzyme that catalyzes the removal of toxicperoxides that are commonly formed during aerobicmetabolic processes (Lehninger et al., 1993). ThereforeSe uptake by forages is linked to proper health in grazinganimals. However, if soil Se levels are too high, then ittends to accumulate in forages and toxicity effects areoften observed in grazing animals (Rosenfeld and Beath,1964). Selenium toxicity is also commonly encounteredin aquatic ecosystems, as Se has a smaller range betweendeficiency and toxicity than any other element in aquaticsystems (Chapman, 1999). Bioaccumulation is oftenresponsible for Se toxicity to waterfowl, as Se levels in-crease from low concentrations in the water itself to ex-

tremely high levels in plants and fish. In waterfowl, Se isboth embryotoxic and teratogenic in high concentrations(Hoffman, 2002).

Soil Se undergoes a variety of redox reactions, and canbe found in oxidation states ranging from -2 (selenide) to16 (selenate) (Shriver et al., 1994) with the form foundin the environment being dependent on soil redox status(Huang andFuji, 1996).One important intermediate formof selenium in soils and surface waters is SeO3

22. SeleniuminSeO3

22 has anoxidation state of14, and SeO322 is aweak

diprotic acid that can exist as H2SeO3, HSeO32, or SeO3

22

depending on solution pH (pKa1 is 2.64 and pKa2 is 8.4)(Shriver et al., 1994). Selenite is considered to be one ofthemore toxic forms of Se, and as such its chemistry in thesoil environment has been researched quite extensively.Sorption processes are extremely important for SeO3

22

in soils, as adsorption between SeO322 and soil mineral

phases is often quite strong, and sorption complexes canalso explain the slow reduction of oxidized Se forms in thepresence of soils (Neal and Sposito, 1991). Sorption ofSeO3

22 is also an important reaction step in the reductionof SeO3

22 to elemental Se in the presence of green rusts(Myneni et al., 1997).

Several researchers have studied the sorption ofSeO3

22 on inorganic soil components. Hingston and co-workers (Hingston et al., 1971, 1974) found that SeO3

22

adsorption on Fe and Al oxides is pH dependent butdoes not vary with ionic strength. Additionally, theyobserved that the majority of SeO3

22 was irreversiblybound to the oxide surface. They concluded that SeO3

22

reacts to form covalent chemical bonds with metal oxidesurfaces. Zhang and Sparks (Zhang and Sparks, 1990)utilized a pressure jump chemical relaxation techniqueand observed that the reaction between SeO3

22 andgoethite consisted of two reaction steps. The fast step wasattributed to outer-sphere complex formation, and thesubsequent slower step was assigned to formation of aninner-sphere surface complex.

Several scientists have used spectroscopic techniquesto determine precisely how SeO3

22 bonds on mineralsurfaces. Hayes and coworkers (Hayes et al., 1987) usedEXAFS spectroscopy to determine bonding mechan-isms of SeO4

22 and SeO322 on goethite. They determined

that SeO322 forms inner-sphere surface complexes with

goethite, and that these complexes are bidentate bi-nuclear (bridging two adjacent Fe octahedral). Manceau

Dep. of Soil Science,Univ. of Saskatchewan, 51CampusDr., SaskatoonSK S7N 5A8 Canada. Received 21 Feb. 2005. *Corresponding author([email protected]).

Published in Soil Sci. Soc. Am. J. 70:192–203 (2006).Soil Chemistrydoi:10.2136/sssaj2005.0054ª Soil Science Society of America677 S. Segoe Rd., Madison, WI 53711 USA

Abbreviations: EXAFS, Extended x-ray absorption fine structurespectroscopy; HAS, hydroxyaluminosilicate; HAS-Mt, hydroxyalumi-nosilicate-coated montmorillonite; HYA, hydroxyaluminum; HYA-Mt, hydroxyaluminum-coated montmorillonite; pKa, acid dissociationconstant; Mt, montmorillonite; PZC, point of zero charge; PZSE,point of zero salt effect; RSF, radial structure function; XANES, X-ray Absorption Near Edge Structure spectroscopy; XAS, X-rayAbsorption Spectroscopy.

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and Charlet (Manceau and Charlet, 1994) publishedresults from EXAFS studies of SeO3

22 adsorbed on hy-drous ferric oxide. They also found bonding mechanismsconsistent with a bidentate binuclear bonding mecha-nism, although a bidentate mononuclear SeO3

22 surfacecomplex was also observed. This second, shorter Se-Fedistance was not observed on more crystalline Fe oxidessuch as goethite. More recently, Foster and coworkersstudied SeO3

22 sorption on Mn oxides. (Foster et al.,2003) They observed that SeO3

22 forms both bidentatemononuclear (Se-Mn 3.07 A) and monodentate surfacecomplexes (Se-Mn 3.49 A) on hydrous Mn oxides.MnSeO3�H2O precipitation could not completely beruled out in this study, but if it does occur then theprecipitate is extremely disordered and amorphous.Only one researcher has reported Se EXAFS data forSeO3

22 adsorbed on an Al oxide (Boyle-Wight et al.,2002b). Their study was conducted on g-Al2O3 at pH 7.5in the presence or absence of Co21. The researchersconcluded that SeO3

22 formed inner-sphere complexesbased on macroscopic data (Boyle-Wight et al., 2002a),but theywere not able to conclusively observe these com-plexes with EXAFS. They attributed this to Al beingan extremely weak backscatterer. However, there do ap-pear to be some second shell contributions present in theradial structure functions (RSFs) of their SeO3

22 samples(Fig. 4b of Boyle-Wight et al., 2002b). This highlightsthe need for additional studies of SeO3

22 reactivity withaluminum-bearing mineral phases.

While previous EXAFS studies have concentratedon Fe and Mn oxides, many different mineral phasescomposed of Al (e.g., aluminosilicates and aluminumoxides) are potentially important sorbents for SeO3

22 insoils. To further complicate matters, it is well known thatsoil minerals are often associated with both inorganicand organic coatings in natural systems. These conglom-erate sorbent phases may have reactivity much differentfrom their individual components. Previous research(Saha et al., 2004) used HYA and HAS coated mont-morillonite as a model system to investigate the impor-tance of coatings on SeO3

22 adsorption. It was found thatthese coatings greatly enhanced the sorption capacity ofmontmorillonite and affect adsorption rates as well.

The goal of this study was to utilize synchrotron-basedXAS (both EXAFS and XANES spectroscopy) to elu-cidate the adsorption mechanisms of SeO3

22 on HYAand HAS polymers, HYA and HAS-coated montmo-rillonite, and pure montmorillonite sorbent phases atpH 4.5. This pH was chosen for XAS studies becauseprevious researchers (Saha et al., 2004) have extensivelystudied the reaction kinetics and sorption behavior ofSeO3

22 with these surfaces and inferred that differentadsorption mechanisms are responsible for differencesin macroscopic reactivity. The spectroscopic experi-ments conducted in this research are therefore useful tovalidate the kinetic models and thermodynamic resultsfrom previous laboratory experiments (Saha et al., 2004).EXAFS spectroscopy was chosen because it is a well-established method for probing oxyanion-bonding mech-anisms onmineral surfaces, and it is expected that possibleinner-sphere complexes will be easily differentiated in our

samples based on Se-Al distance. A large amount ofspecific chemical information is also contained in theXANES region, and we believe that XANES analysis willalso be useful in probing SeO3

22 adsorption mechanismson our sorbents. For example, researchers have found thatXANES spectroscopy can distinguish outer-sphere andinner-sphere complexation of As(III) on aluminumoxides (Arai et al., 2001), outer-sphere and inner-spherebonding mechanisms of lead on montmorillonite (Strawnand Sparks, 1999), and can distinguish the protonationstates of As(III) andAs(V) in aqueous solutions (Myneniet al., 1999). TheXANES technique can be used to obtaindetailed information about precipitate geometry (includ-ing cluster size) in surface precipitates (Waychunas et al.,2003), and can determine the coordination of sulfate inFe minerals (Myneni et al., 1997). Based on the abovestudies, if outer-sphere complexation is occurring simul-taneously with inner-sphere complexation, then it shouldbe visible in the XANES spectra.

MATERIALS AND METHODS

Mineral Synthesis

Hydroxyaluminum precipitate was prepared using a methodbased on that of Sims and Bingham (Sims and Bingham, 1968)for the synthesis of hydroxy-Fe precipitate. A 200-mL aliquotof 1.5 M AlCl3 was placed in a 1-L beaker and neutralizedslowly at a rate of 0.5 mL min21 with 2.0M NaOH up to a OH/Al ratio of 2.7 under continuously stirred condition. A HASprecipitate was prepared following a method based on Su andSuarez (Su and Suarez, 1997) for the synthesis of allophone. A100 mL of 3.0MAlCl3 solution was mixed well with 100 mL of1.5 M Na2SiO3 and placed in a 1-L beaker. The resulting solu-tion, containing a Si/Al molar ratio of 0.5, was then neutralizedat the rate of 0.5 mL min21 with 2.0 M NaOH up to a OH/Alratio of 2.7 under continuously stirred condition. The resultingprecipitates were aged for 1 h, resuspended in 1 L of distilledwater, centrifuged at 1000 3 g, and the supernatant was dis-carded. The precipitates were then washed three times with600 mL of 95% ethanol followed by washing with distilledwater until the leachate did not form a precipitate with AgNO3.The washed precipitates were kept as suspension in distilledwater with a density of solid as 20 g L21 and used immediatelyfor SeO3

22 adsorption experiments. Unused precipitate wasfreeze-dried and analyzed for physical and chemical properties.

Montmorillonite (Mt), hydroxyaluminum-coated montmo-rillonite (HYA-Mt), and hydroxyaluminosilicate-coated mont-morillonite (HAS-Mt) were prepared and characterized in aprevious study (Saha et al., 2004). Surface area and point ofzero salt effect (PZSE) measurements for all sorbents used inthis study were previously reported by Saha and coworkers(Saha et al., 2004) and are compiled in Table 1. PZSE for themontmorillonite were not experimentally determined, butis typically below 3. It is important to note that PZSE is notnecessarily equal to the point of zero charge (PZC) of min-eral surfaces, and instead is the point at which titrationsconducted at different ionic strength cross one another.Researchers have shown that the PZC may differ from theexperimentally determined PZSE, but both PZC and PZSEfollow the same trends. Literature PZC values for pure alumi-num hydroxide minerals range from 8.5 to 9.5 (Sparks 1995),which is consistent with the experimentally determined PZSEresults of Saha and coworkers (2004). Based on experimentallydetermined PZSE values, at pH 4.5 one sorbent used in this

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193PEAK ET AL.: SELENITE ADSORPTION ON PURE AND COATED MONTMORILLONITE

study (Mt) will be negatively charged, two sorbents (HYA-Mtand HAS-Mt) are close to neutral/have a small surface charge,and the remaining sorbents (HYA and HAS) will be stronglypositively charged. Previous researchers have shown that outer-sphere complexation of other oxyanions such as arsenite (Araiet al., 2001; Goldberg and Johnston, 2001) and SeO4

22 (Peak andSparks, 2002; Wijnja and Schulthess, 2000) is common whensolution pH is below the PZC of metal oxides as is the case forthe pure HYA and HAS samples. It is also known that inner-sphere complexation of oxyanions can result in a surface com-plex with an overall negative charge. This leads to a decreasedPZC, and may be important in the HAS and HYA-coatedsamples which have PZSE near the experimental pH of 4.5.

Aqueous Samples

Solutions containing 50 mM total SeO3 (as Na2SeO3) at pH1.10, 3.32, 5.52, 8.50, and 11.02 were prepared in a gloveboxunder a nitrogen atmosphere to exclude oxygen from thesamples. An oxygen-free environment was necessary to obtainspectra free of SeO3

22 oxidation at alkaline pH. Initially, 5 mLof 100 mM SeO3

22 solution pH was adjusted to the desired pHwith either 1 M HCl or 1 M NaOH then enough deionizedwater (18.2MV Barnstead Diamond NanoPure) was added toproduce a 10-mL final volume. These samples were placed in acentrifuge tube, sealed with parafilm, placed into a nitrogen-filled Ziploc bag, and shipped to the Advanced Photon Source(Chicago, IL). Samples were analyzed in transmission mode byplacing the sealed tube in the path of the beam.

Adsorption Sample Preparation

A1gL21 suspension of sorbent was placed in 0.01MNaNO3,adjusted to pH 4.5, equilibrated overnight, and then Na2SeO3

was added from a freshly prepared 1 mM stock solution toreach a final concentration of 25 mM SeO3

22. Sample pH wasreadjusted to pH 4.5 after SeO3

22 addition and again if nec-essary as the reaction proceeded. After a 48-h reaction time,the suspensions were centrifuged at 1000 3 g and the super-natants were analyzed with flame Atomic Absorption Spec-trophotometry (AAS). HyA-Mt, HAS-Mt, Mt, and pure HYAand HAS adsorption studies were done open to the atmo-sphere at pH 4.5 as part of a separate research project (Sahaet al., 2004). However, dissolved carbon dioxide concentrationsin solutions at pH 4.5 are extremely low, and no oxidation ofSeO3

22 was observed in the XANES spectrum of any samples.Selenite sorption amounts (expressed as mmol m22) for allsamples are included along with reaction conditions in Table 2.One complication with this surface coverage/surface loadingcalculation arises from uncertainty in what portion of themineral surface is actually available for reaction with SeO3

22.The mmol m22 loading numbers in Table 2 are derived from thetotal (internal plus external) surface area measurements onthe sorbents. However, it is not known whether the interlayerspace of Mt remains accessible to SeO3

22 after coating withHYA or HAS. If only external surface area is considered, thesurface loadings are fairly similar for Mt, HYA-Mt, and HAS-Mt: 0.08, 0.16, and 0.09 mmol m22, respectively. Higher surfaceloadings were not attempted with these samples because wewere concerned that unreacted SeO3

22 in solution at equi-librium would be detectable in our EXAFS experiments andcould make detection of outer-sphere complexes impossible.With all the samples as prepared more than 95% of the signalwas calculated as arising from the adsorbed SeO3

22 and notfrom entrained solution. The sorption capacity of the HYAand HAS sorbents was much larger so more SeO3

22 couldbe adsorbed without issues from entrained solution. This al-lowed us to collect higher quality EXAFS data than with thecoated samples. There is some evidence that surface loadinghas an effect on sorption mechanism of oxyanions, especiallywhen both outer-sphere and inner-sphere complexes formsimultaneously. The major effect when pH is held constantis that slightly more inner-sphere complexes form at highercoverages rather than a dramatic shift in the configuration ofinner-sphere complexes.

X-Ray Absorption Spectroscopy

EXAFS spectra were collected at the SeK-edge (12.658 keV)at the PNC-CAT Bending Magnet beamline (20-BM) at the

Table 1. Characteristics of sorbents used in this study.

Surface Area, m2 g21

PZSE† External Internal Total

Montmorillonte ND‡ 72§ 529§ 601§Hydroxyaluminum (HYA) 9.38§ 122.6§ ND 122.6§Hydroxyaluminumsilicate (HAS) 8.52§ 193.7§ ND 193.7§HYA-Mt 4.7§ 128§ 322§ 450§HAS-Mt 4.4§ 108§ 379§ 487§

†Point of zero salt effect.‡Not determined.§ Cited from Saha et al. (2004).

Table 2. Structural parameters of selenite in solution and sorbed on mineral surfaces.

First shell Second shellSe-O Se-Al

Sorbent Loading Reaction Conditions S02† E0‡ R§,†† N¶‡‡ Ds2# R§,†† N¶‡‡ Ds2#

mmol m22 A A2 A A2

HYA 0.77 pH 4.5, I50.01 0.86 10.78 1.70 2.92 0.001 3.22 2.55 0.010HAS 0.34 pH 4.5, I50.01 0.86 10.34 1.70 2.78 0.001 3.24 2.80 0.007Mt 0.01 pH 4.5, I50.01 0.86 10.93 1.71 3.15 0.005 3.16 1.65 0.010HYA-Mt 0.05 pH 4.5, I50.01 0.86 11.21 1.70 2.5 0.002 3.16 1.78 0.008HAS-Mt 0.002 pH 4.5, I50.01 0.86 10.37 1.70 2.67 0.003 3.18 1.80 0.014Reference§§ Conc.

mMH2SeO3(aq) 50 pH 1.1 11.127 1.71 0.86 2.45 0.002HSeO3

2(aq) 50 pH 5.5 10.29 1.69 0.86 2.4 0.002

SeO322

(aq) 50 pH 11 11.653 1.70 0.86 2.61 0.002

†Amplitude reduction factor.‡Energy shift; § Interatomic distance.¶Coordination number.#Debye-Waller factor fit quality estimated accuracy.††60.02A.‡‡620%.§§EXAFS collected in tranmission mode.

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194 SOIL SCI. SOC. AM. J., VOL. 70, JANUARY–FEBRUARY 2006

Advanced Photon Source at Argonne National Laboratory.The electron storage ring was operating at 4.5 GeVand in topup mode (100 mA) for all experiments. The beamline wascalibrated using a Pt foil placed in the path of the beamdownfield from the sample chamber between I and Iref. Solu-tion samples were analyzed in transmission mode to avoid selfabsorption effects while sorption samples were analyzed influorescence mode using a solid-state 13-element Ge detector(Canberra). Solution samples were analyzed at room temper-ature while a cryostat was utilized to lower sorption samples(analyzed as moist pastes) to 25 K to avoid any beam-inducedoxidation over long scan times (up to 4 h for the most dilutesamples). An aqueous SeO3

22 sample at pH 9.8 was analyzedat both 25 K and 298 K to compare with the room temperaturedata and no significant differences were observed.

XAS Data Analysis

WinXAS version 2.3 (T. Ressler, 1997) was used for all datareduction and analysis. All scans were first checked for edgeshifts, which were corrected if observed. Multiple scans (from5 for solutions to 25 scans for the Mt sorption sample) wereaveraged and then background subtracted using a linear equa-tion for the pre-edge region and a second-order polynomial forthe post-edge region. After baseline correction, then everyspectrumwas normalized to an edge jump of 1.0. EXAFS fittingwas done by comparing experimental spectra to theoreticalsingle scattering paths for model compounds. ATOMS 3.0(Ravel, 2001) was used to construct mineral phases fromcrystallographic data, and FEFF7 (Zabinsky et al., 1995) wasthen used to calculate theoretical single scattering paths. Fitswere conducted in R space with the amplitude reduction factor(S0

2) fixed at 0.86 while coordination number, bond distance,Debye-Waller, and e0 shift were all allowed to vary. Data wasthen also fit in k space to verify that optimized parameters weresimilar in both cases. Linear combination XANES (LC-XANES) fitting was conducted on samples using the methodsrecommended by WinXAS and aqueous and adsorbed SeO3

22

reference spectra. The fitting with reference spectra wasconducted in two runs: an initial run where E0 was allowed to

vary to optimize the relative contributions of all components,and then a second run where E0 shift was fixed at 0.00.

RESULTS AND DISCUSSIONEXAFS of Selenite Aqueous Solutions

To investigate how protonation state may affectEXAFS spectra, solutions containing 50 mM Se wereanalyzed at three different pHs (1.1, 5.5, and 11.0) toproduce solutions containing only H2SeO3, HSeO3

2, or

SeO322. The k3–weighted chi data and the RSFs ob-

tained from performing a Fourier transformation on thechi data are shown in Fig. 1 (raw data denoted with solidlines). Structural parameters obtained from EXAFSfitting (fitted spectra are shown with open circles inFig. 1) are tabulated in Table 2. In all protonationstates,,2.5 oxygen atoms surrounds the central Se atomat 1.706 0.01 A. This bond distance is in good agreementwith previous studies (Hayes et al., 1987; Manceau andCharlet, 1994, etc.), and the coordination number differsfrom the true value of 3.0 by only 20% (the estimatederror for the fitting routine). Interestingly, the amplitudeof oscillations in the chi data and the intensity of the Se-Oshell in the RSF decrease as SeO3

22 becomes protonated(Fig. 1). This is most likely caused by a shift from threeequivalent Se-O distances in SeO3

22 (with bond delocal-ization) to single and double Se-O bonds in both bi-selenite and selenous acid. It was possible (data notshown) to fit the H2SeO3 data with two separate Se-Odistances (at 1.61 and 1.74 A), but the r 2 was similar tothe single shell fit that is presented in Fig. 1.

EXAFS of Selenite Adsorbed on Mineral SamplesSamples containing SeO3

22 adsorbed on HYA andHAS polymers, Mt, and HYA-Mt or HAS-Mt polymers

Fig. 1. Se K-Edge EXAFS results for aqueous selenite solutions. Solid lines denote the raw data, and open squares are the fit to theoretical standards.

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at pH 4.5 were analyzed with EXAFS to determinebondingmechanisms on these sorbents. The k3–weightedchi data and the Radial Structure Functions obtainedfrom performing a Fourier transformation on the chidata are shown in Fig. 2 for all sorption samples (rawdata denoted with solid lines). Structural parametersobtained from EXAFS fitting (fitted spectra are shownwith circles in Fig. 2) are tabulated in Table 2. In allsamples there are features (denoted by arrows in Fig. 2)in both the raw chi data and the Fourier transformsthat suggest inner-sphere complexation of SeO3

22. Thereare clearly some constructive and destructive inter-ferences in the chi data that result in deviations from asingle Se-O shell (as seen in the aqueous samples), andwhen a Fourier transform is performed then a secondshell Se-Al contribution is observed between 3.16 and3.22 A in all samples. Fitting routines suggest that this Se-Al bond distance is shortest (3.16 A) when SeO3

22 issorbed on montmorillonite, longest (3.22–3.24 A) whenSeO3

22 is adsorbed on hydroxyaluminum and hydro-xyaluminosilicate polymers, and of an intermediatedistance (3.16–3.18 A) on the polymer-coated montmo-rillonite samples. This bond distance is consistent withthose calculated for bidentate binuclear (also termedbridging or corner sharing) SeO3

22 complexes. While thechange in bond distance with pure polymer sorbents islarger than the error associated with the fits (estimated at0.02 A), it is not significant enough to represent a changein bonding configuration from monodentate to biden-tate. One would predict a monodentate complex to havea Se-Al distance of,3.42 A and a bidentatemononuclear(edge-sharing) complex is expected to have a Se-Al bonddistance of,2.57 A. These numbers are based on simple

geometrical calculations based on an Al-O distance of1.72 A [as calculated for octahedral Al(H2O)631] and anSe-O distance of 1.70 A (as observed with EXAFS in thisstudy). Instead, it is more likely that there may be someslight deviation in bond lengths and coordination numberfor a bidentate binuclear bonding environment that ispossible as the sorbent structure is changed.

For the pure polymer samples, it appears that thesingle Se-Al scattering path may not fully describe thesecond shell; the coordination number for both HYA(2.55) and HAS(2.80) samples is quite large and thewidth of the second shell of the Fourier transforms isconsistent with a disordered bonding environment.This could be due to either a large static disorder in thebonding environment (meaning that a single type of sur-face complex has a large range of bond distances) ortwo distinct complexes with slightly different Se-Al dis-tances in HYA and HAS samples. It is not possible todistinguish these possible explanations with the existingEXAFS data. In the coated montmorillonite samplesthis effect is less pronounced, coordination numbers de-creases to 1.78 for HYA-Mt and 1.80 for HAS-Mt, andbond distances also shorten slightly to 3.16 A (HYA-Mt)and 3.18 A (HAS-Mt). In the pure montmorillonite ad-sorption sample, the Se-Al distance (3.16 A) is signifi-cantly shorter than the interatomic Se-Al distancesobserved in any of the pure polymer samples (3.22–3.24 A). When considered together, it appears that therea bidentate binuclear complex forms on all surfaces,but that there is a slight difference in bonding environ-ment depending on sorbent. This is reasonable giventhe nature of montmorillonite and the polymers. Onmontmorillonite, the only available Al groups for reac-

Fig. 2. Se K-Edge EXAFS results for selenite adsorbed on a range of sorbents at pH 4.5 and 0.01 M I. Solid lines denote the raw data, and opensquares are the fit to theoretical standards.Arrows denote spectral features consistent with Se-Al backscattering due to inner-sphere complexation.

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tion would occur at the fairly structured edge sites of themineral. On the HYA and HAS polymers, however,essentially all of the Al on the surface of the polymer isavailable for reaction. Additionally, these polymers areof short-range order rather than crystalline. This resultsin longer bond distances and larger coordination num-ber as SeO3

22 is essentially enveloped by the polymer.On polymer-coated montmorillonite surfaces, a portionof the polymer’s reactive surface is tied up by interactionwith the planar tetrahedral sheet and with pillaring in themontmorillonite interlayer. When reacted with SeO3

22,these polymer-coated montmorillonite sorbents haveSe-Al coordination numbers and bond distances some-what intermediate between what is observed on a puremontmorillonite surface and on a pure polymer phase.

Some of the more qualitative differences in theEXAFS spectra also deserve discussion. One interestingobservation in the raw chi data is that the amplitude ofthe montmorillonite sorption sample is much less thanthe other samples, and the amplitude of the coatedmontmorillonite samples is also somewhat decreasedrelative to the pure polymer sorption samples. This is alsovisible in the form of decreased Se-O shell height in theRSFs. This is very similar to the effects of protonation onaqueous SeO3

22. Although coordination numbers andSe-O distance was similar for all protonation states ofaqueous SeO3

22, H2SeO3 had a decrease in EXAFS am-plitude (in the chi data) and Se-O shell height (in theRSF). This may be a general effect that is caused by fulllocalization of bonds to form a formal Se 5 O doublebond and two formal Se-O single bonds. The fact that theamplitude of the chi data and the height of the Se-O shell

in the RSFs are larger in the polymer-coated and purepolymer sorption samples implies that there may besome contributions to the spectra from SeO3

22 in a dif-ferent coordination with more delocalized Se-O bond-ing. This could possibly be explained with the presenceof some additional outer-sphere complexation, whichoccurs simultaneously with inner-sphere complex for-mation. However it is difficult to conclusively assignouter-sphere complex formation with EXAFS wheninner-sphere complexes are also present. Previous re-searchers (Arai et al., 2001) have found that XANESspectroscopy is extremely useful tool in determiningouter-sphere complexation in such systems, and forthis reason XANES spectra of aqueous and adsorbedSeO3

22 were next examined in detail.

XANES of Selenite in Aqueous SolutionsBy changing solution pH to produce H2SeO3, HSeO3

2,and SeO3

22, it is possible to produce standards forXANES analysis that represent the full range of mo-lecular symmetry possible for SeO3

22. When SeO3 is pro-tonated, either partially or fully, then there are changesin molecular structure that may affect symmetry. First ofall, the proton may disrupt vertical mirror planes or itmay be placed in a position that preserves them. This willchange symmetry from CS to C1. Second, bond delocali-zation may no longer occur, which would reduce sym-metry of the structure further. Diagrams of molecularsymmetry and Lewis structures for all protonationstates of SeO3

22 are compiled in Fig. 3 to aid in the fol-lowing discussion.

Fig. 3. Relationship between protonation state, molecular symmetry, and bond delocalization for selenite species.

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There has not been a rigorous assignment of symme-try for all protonation states of SeO3

22 based on eithermolecular modeling or vibrational spectroscopy. How-ever, Nakamoto (Nakamoto, 1997) includes SeO3

22 inthe discussion of pyramidal XO3 molecules, and re-searchers (Wang and Zhang, 2002) have recently op-timized the structure of H2SO3, HSO3, and SO3 usingboth theoretical modeling and experimental data.The optimization of SO3 is not directly applicable toSeO3

22 because SO3 is trigonal planar unlike SO322 or

SeO322, but the effects of protonation on bond delocali-

zation are still relevant for SeO322.

Infrared and Raman spectroscopy verify that SeO322

has C3v symmetry, meaning that all three oxygens areinterchangeable (Nakamoto, 1997). In other words, thebonding described by a Se 5 O double bond in Lewisstructures is actually fully delocalized (hence the threeresonance structures drawn in Fig. 3). This is also thecase in the molecular modeling of SO3, which is reportedas having D3H symmetry (Wang and Zhang, 2002).

In contrast, H2SeO3 represents another extreme insymmetry. When the structure of H2SO3 (analogous toH2SeO3) was optimized (Wang and Zhang, 2002), it wasfound to possess CS symmetry, meaning that there isone Se5O localized double bond and two Se-OH bondswith protons arranged so that there is a reflection planein the molecule.

Biselenite (HSeO32) is somewhat more difficult to

assess, since it is possible that it may or may not havebond delocalization among the two bare oxygen atoms,and the single proton may or may not lie in the mirrorplane. Wang and Zhang (Wang and Zhang, 2002) re-ported an optimized C1 symmetry for the HOSO2

molecule, but they do not mention whether this is dueto lack of delocalization or if it is caused by proton posi-tion in the molecule. One would expect that since SO3

and SeO322 exhibit bond delocalization, HSeO3

2 wouldas well.

Given the above information about symmetry,XANES spectra of aqueous SeO3

22 species may nowbe more rigorously interpreted. Transmission modeXANES spectra of 50 mM SeO3

22 solutions adjustedwith acid or base were collected and presented in Fig. 4a.An enlargement of the 12.66- to 12.69-keV range of thesesamples is also shown in Fig. 4b to highlight differencesin the XANES region. The two most important obser-vations that can be made from the XANES spectra ofaqueous SeO3

22 solutions are: (1) H2SeO3 has featuresthat are different in position and shape from the othersamples and (2) The position of peaks in HSeO3

2 andSeO3

22 is fairly similar but the intensity of the featuresis different. These observations are completely con-sistent with the symmetry discussion above if there isbond delocalization in the unprotonated oxygens ofHSeO3

2. In this case, both HSeO32 and SeO3

22 wouldhave shared pi electron density and intermediate Se-Obond lengths. Such delocalization cannot occur withH2SeO3, and distinct long (single bond) and short(double bond) Se-O distances must occur in thismolecule. One important consequence of this interpre-tation of the data is that molecules with different sym-metry (SeO3

22 is C3V while HSeO32 is CS or C1) have

similar spectra and not molecules with the same sym-metry (both H2SeO3 and HSeO3

2 are either C1 or CS

depending on the position of protons). This suggests thatbond delocalization is the dominant force in affecting

Figure 4. (a) XANES spectra of H2SeO3, HSeO32, and SeO3

22 (b) 12.665 to 12.69 keV energy range expanded to more clearly show differences inthe near edge spectra.

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XANES spectra of SeO322. This is not too surprising

since the Se-O bond lengths in the first coordination shellwill certainly affect the geometry of the molecule, andtherefore the intensity and position ofmultiple scatteringeffects in the XANES spectra.

LC-XANES of Aqueous SolutionsAqueous samples were also analyzed that contained

two SeO322 protonation states (pH 3.3 and 8.5). These

spectra were fit with a linear combination approachusing WinXAS 2.3 and the H2SeO3, HSeO3

2, and SeO322

solutions as standards. Error in LC analysis was esti-mated at 3% by comparing the fit of all individual scansto the fit results for the overall averaged spectrum. Re-sults are shown in Fig. 5. The pH 3.3 spectrum could befit with a mixture of 82% HSeO3

2 and 18% H2SeO3, andthe pH 8.5 spectrum could be fit with a mixture of 44%HSeO3

2 and 56% SeO322. These distributions are in

agreement (within 0.5%) with the expected speciationof SeO3

22 using published pKa values of 2.63 and 8.4.

Using those pKas, the Henderson–Hasselbalch equationpredicts 82.4% HSeO3

2 and 17.6% H2SeO3 for the pH3.3 sample and 55.7% SeO3

22 and 44.3% HSeO32 for the

pH 8.5 sample. This excellent agreement between solu-tion speciation models and LC-XANES fitting suggeststhat changes in the XANES spectra as a result of Se-Obond delocalization are fairly quantitative in nature.

XANES of Selenite Adsorbed on Mineral SamplesXANES spectra of SeO3

22 adsorbed on a variety ofmineral soil components are shown in Fig. 6. The spectraof HSeO32 (aq) and H2SeO3(aq) are also included for ref-erence. HSeO3

2 is the dominant solution species at pH4.5. XANES spectra of all sorption samples containfeatures that are clearly different from HSeO3

2, whichsuggests that some inner-sphere complexation is oc-curring. This is in good agreement with the EXAFSstudies. However, while the EXAFS spectra all had gen-erally similar features, the XANES spectra for differentsorbents show some important differences. In particular,

Fig. 5. Linear combination XANES fits of aqueous samples where more than one selenite protonation state is present. LC-XANES predicteddistributions are within 0.5% of actual speciation values, and the estimated error of the LC-XANES fitting was 3%.

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the XANES spectrum of SeO322 sorbed on montmoril-

lonite has a well-defined 12.67 keV peak in the XANESregion (noted by the arrow in Fig. 6a) that is similar inboth energy and shape to H2SeO3(aq). As discussedpreviously, H2SeO3 has a fully localized double bondand two Se-O-H bonds. The fact that adsorbed SeO3

22

has features in similar positions therefore suggests that ithas similar coordination chemistry and is most likelybound as a bidentate complex. This is also consistentwith the EXAFS fit results for the montmorillonitesample. It has also been observed that SeO3

22 adsorbedon FeOOH via a bidentate binuclear inner-sphere com-plexation mechanism has a similar XANES spectrumover all pH range (data not shown). One other type ofsurface complex would result in a fully localized Se 5 Obond: monodentate adsorption of HSeO3

2 would pro-duce a monodentate biselenite complex with one ofselenite’s oxygens coordinated to Al, one to a proton,and the third present as the double-bonded O. As men-tioned in the EXAFS analysis section, this complexwould produce a Se-Al bond distance of ,3.4 A, whichwas not observed in any of the sorption samples. In-stead, the EXAFS data showed evidence for thebidentate binuclear SeO3

22 species. Based on that, themonodentate biselenite species can be ruled out.

It could also be possible that the XANES spectrum ofMt resembles aqueous H2SeO3 because H2SeO3 adsorp-tion into the interlayer of the mineral occurs. This hasbeen proposed by previous researchers (Saha et al.,2004), who predicted that H2SeO3 will be preferentiallyadsorbed in the interlayer of montmorillonite becauseHSeO3

2 and SeO322 are repelled by the negatively

charged interlayer. The mechanism of this reaction isunknown, but conceivably could involve hydrogen bond-ing between H2SeO3 and O from silica tetrahedral layersof the interlayer space. Once H2SeO3 diffuses to theinterlayer space, a ligand exchange reaction would alsobe possible. While the relative importance of interlayerH2SeO3 to the overall XANES spectrum is difficult toassess, the XANES do seem to suggest that such com-plexation cannot be disregarded.

The important changes in XANES features for SeO322

adsorbed on HYA, HYA-Mt, and Mt are enlarged inFig. 6b for clarity; HAS, HAS-Mt, and Mt spectra showthe same trends. The broadening and weakening of the12.67 keV XANES feature to a varying extent in theHYA, HAS, HYA-Mt, and HAS-Mt samples suggeststhat a fraction of the SeO3

22 is in a surface complex thatretains the molecular structure of SeO3

22 in solution(note the similarity to the HSeO3

2 spectrum). The two

Fig. 6. (a):Se K-Edge XANES spectra of selenite sorbed onto Mt, HYA,HAS, HYA-Mt, and HAS-Mt samples. HSeO3 and H2SeO3 solutions areincluded for comparison. (b) 12.665 to 12.69 keV energy range expanded to more clearly show differences in the near edge spectra for HAS,HYA, HYA-Mt, and Mt samples.

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possible sources of such a bonding environment are thepresence of unbound (entrained) or weakly bound bi-selenite in these samples. Since the entrained solutionSeO3

22 levels were ,3% of the total Se signal in allsamples, weakly bound SeO3

22 is the most likely sourceof these spectral features. This also makes sense giventhe nature of HYA and HAS polymers. Aluminum hy-droxide is strongly positively charged at pH 4.5 and soouter-sphere complexation is possible. It has previouslybeen shown that arsenite forms both outer-sphere andinner-sphere complexes on aluminum hydroxide belowthe mineral’s PZC (Arai et al., 2001) (Goldberg andJohnston, 2001), so this type of bonding mechanism iscertainly not unprecedented for HYA samples. Sincealuminosilicates have a lower PZC than aluminum hy-droxides, one would expect relatively more inner-spherecomplexes and relatively less outer-sphere complexes onHAS. When the XANES data are examined then it canindeed be observed that the features of HAS are some-what sharper and better defined than those of HYA.Montmorillonite, on the other hand, has a permanentnegative charge from isomorphic substitution. This neg-ative surface charge would make outer-sphere complex-ation of negatively charged oxyanions quite unlikely.When HYA or HAS polymers are used to pillar themontmorillonite interlayer and coat the planar sites ofthe smectite then the resulting colloid has intermediatesurface behavior. Some of the positive surface charge ofthe polymers is neutralized by the association with thenegatively charged mineral surface, which results in anoverall PZSE of pH 4.4 to 4.7 for the polymer-coatedmontmorillonite sorbents (Saha et al., 2004). Based onthat fact alone, one would expect (and can observe) arelatively small amount of outer-sphere complexationin the polymer-coated mineral samples compared to thepure polymers (Fig. 6).

When the EXAFS results and XANES results areconsidered together, a fairly complete picture of SeO3

22

adsorption on HYA, HAS, and montmorillonite be-comes clear. The proposed bonding mechanisms (con-sistent with both techniques) are summarized in Fig. 7.These bonding mechanisms are also completely consis-tent with the kinetics experiments previously conductedby Saha and coworkers (Saha et al., 2004). In those ex-periments, they used a fast and a slow reaction to modelthe kinetics of SeO3

22 adsorption on pure and coatedmontmorillonite. They determined that activation energyfor SeO3

22 adsorption on montmorillonite was muchgreater than the energy of activation for SeO3

22 adsorp-

tion on either HYA or HAS coated montmorillonite.This was the case on Mt, HYA-Mt, and HAS-Mt for boththe fast reaction (39, 11, and 19 kJmol21, respectively) andthe slow reaction (53, 32, and 27 kJ mol21, respectively)(Saha et al., 2004). This decrease in activation energy wasattributed to the presence of positively charged Al-OH21functional groups on both HYA and HAS polymers atpH 4.5. These minimize repulsion between the negativelychargedbiselenite andmontmorillonite (Saha et al., 2004).The observation (fromXAS) of outer-sphere complex for-mation on the polymers and the polymer-coated clay isalso consistent with the observed decrease in activationenergy. Saha and coworkers (Saha et al, 2004) also ob-served that the rates of reaction were significantly in-creased on theHYAandHAScoated clay sorbents. This isalso consistent with the presence of outer-sphere com-plexes, which are known (via many pressure-jump chemi-cal relaxation experiments) to have rates of formationthat are more rapid than those of inner-sphere complexformation on mineral surfaces.

LC-XANES of Selenite Adsorbed onPolymer-Coated Montmorillonite

To estimate the relative distribution of SeO322 on the

components of polymer-coated clays, LC-XANES fittingwas conducted on the polymer-coated samples fromFig. 6 using the pure polymers and the montmorillonitespectra as standards. The results are shown in Fig. 8.Error in LC analysis was estimated at 5% by comparingthe fit of individual scans to fit results for the overallaveraged spectrum. It was observed that,54% of the ad-sorbed SeO3

22 on HYA-Mt was bound to the montmo-rillonite surface, while on HAS-Mt approximately 42%was bound to the clay mineral phase. The most likelyexplanation for this difference in sorption is the differ-ence in surface area for the two-coated sorbents. HAS-coated Mt has 487 m2 g21 surface area while HYA hasonly 450 m2 g21; PZSE for the two coated clays in bothcases is similar: 4.7 for HYA and 4.4 for HAS (Saha et al.,2004). The surface area differences suggest that in theHAS-Mt sample there is relatively more polymer avail-able for reaction with SeO3

22 than in the case of theHYA-Mt. It was reported by Saha and coworkers thatmore Al (1.21 mol Al31kg21 Mt) for HAS versus 1.16 molAl31kg21 Mt for HYA-Mt) sorbed on Mt from HAS sol-utions than HYA solutions during the sorbent synthesis.There is an important assumption made in the choice

of standards used in the above analysis. Because there isboth outer-sphere and inner-sphere adsorption in theHYA and HAS samples, using these mixed bonding en-vironment samples as standards assumes that the sorp-tion mechanisms on the polymers is unchanged byinteraction with montmorillonite. It is also possible thatthe interactionwith theMt surface could change the ratioof outer-sphere to inner-sphere on the polymer coating.However, one would expect that if these polymer-Mtinteractions were dramatically different for the HAS andHYA samples then it would be reflected in the PZSE ofthe coated minerals. Instead, the PZSE values are quitesimilar (4.7 for HYA-Mt and 4.4 for HAS-Mt). In view of

Fig. 7. Selenite adsorption mechanisms for pH 4.5 samples consistentwith EXAFS and XANES spectroscopy.

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their PZSE values (Saha et al., 2004), HAS-Mt shouldform slightly more inner-sphere complexes and slightlyfewer outer-sphere complexes compared with HYA-Mt.This surface charge effect coupled with the increasedtotal amount of polymer on the HAS-Mt sample (viasurface area and via mol Al31 adsorbed) is consistent withthe data presented in Figs. 6 and 8.

Larger Significance of this StudyFrom an environmental standpoint, these results are

the first to clearly demonstrate the mechanism of SeO322

adsorption on aluminum-bearing mineral phases. Onmontmorillonite, only bidentate binuclear inner-spherecomplexation was observed. For the hydroxyaluminumand hydroxyaluminosilicate polymers, a mixture ofouter-sphere and bidentate binuclear inner-sphere wasobserved. When montmorillonite was coated with eitherHYA or HAS polymers then adsorption behavior wasintermediate between that of the mineral and the purepolymer. The adsorption behavior of these polymer-coated minerals is especially important to natural sys-tems. Temperate soils often contain aluminum-hydroxyand aluminosilicate coated minerals rather than discrete

aluminum hydroxide minerals and pristine clay surfaces.More XAS studies are planned for contaminant sorptionon more realistic and complicated sorbents such asorganic and inorganic polymer coated clay minerals.

The finding (from XANES) that outer-sphere com-plexes form on the HAS, HYA, HYA-Mt, and HAS-Mtsamples but not on pure Mt is consistent with the mac-roscopic kinetics experiments previously conducted bySaha and coworkers (Saha et al., 2004). They determinedthat activation energy for SeO3

22 adsorption on montmo-rillonite was much greater than the energy of activationon HYA-Mt or HAS-Mt. They also observed that reac-tion rates were more rapid on HYA-Mt and HAS-Mt,which is also consistent with the presence of outer-spherecomplexes. This effect on kinetics parameters demon-strates that shifts in complexation from outer-sphere toinner-sphere can have a large effect on macroscopic ad-sorption experiments, and therefore on larger-scale trans-port of SeO3

22 in the environment.The XAS results also show that it is possible to infer

bonding mechanisms of SeO322 on mineral phases reliably

with XANES as well as with EXAFS spectroscopy. TheEXAFS results also suggest that itmay be possible to inferthe presence of outer-sphere oxyanion complexation

Fig. 8. Linear combination XANES fits of selenite on HYA and HAS coated montmorillonite. Estimated error for the fitting is 6 5%.

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in samples where inner-sphere complexes are also pres-ent by the amplitude of theEXAFS spectra and the heightand shape of the first (Me-O) shell in RSFs. However,XANES spectra remain more conclusive and reliable forobserving outer-sphere complex formation. The inter-pretation of Se XANES data relies on changes in molecu-lar geometry and Se-O bond delocalization that occuras SeO3

22 changes protonation state in solution or formsinner-sphere complexes on surfaces.

Careful use of XANES spectral features in determi-nation of bonding configuration can extend the utility ofX-ray absorption spectroscopy in environmental samplesfor two reasons. First is the fact that concentrationsin natural samples are often too low for EXAFS spectros-copy to be conducted. Additionally, many synchrotron-based studies now utilized microprobe beamlines whereXANES spectroscopy is far more practical to conductthan is EXAFS.

ACKNOWLEDGMENTS

This research was supported by funding from NaturalSciences and Engineering Research Council of Canada Dis-covery grants #261432 and 2383 and by the SaskatchewanSynchrotron Institute. Additionally, assistance in beamline con-figuration and optimization were provided by Drs. J. Cross, R.Gordon, and S. Heald of PNC-CAT at the Advanced PhotonSource at Argonne National Laboratory. PNC-CAT facilitiesat the Advanced Photon Source, and research at these facilities,are supported by the US DOE Office of Science Grant No.DEFG03-97ER45628, the University of Washington, a majorfacilities access grant from NSERC, Simon Fraser Universityand theAdvanced Photon Source. Use of the Advanced PhotonSource is also supported by the U. S. Department of Energy,Office of Science, Office of Basic Energy Sciences, under Con-tract No. W-31-109-Eng-38. D.P. also thanks Dr. S.D. Siciliano(University of Saskatchewan) for his careful review of the workand suggestions.

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