Journal of Colloid and Interface Science 303 (2006) 337–345www.elsevier.com/locate/jcis

Adsorption mechanisms of selenium oxyanions at the aluminumoxide/water interface

Derek Peak ∗

Department of Soil Science, University of Saskatchewan, 51 Campus Drive Saskatoon, SK S7N 5A8, Canada

Received 22 March 2006; accepted 2 August 2006

Available online 17 August 2006

Abstract

Sorption processes at the mineral/water interface typically control the mobility and bioaccessibility of many inorganic contaminants such asoxyanions. Selenium is an important micronutrient for human and animal health, but at elevated concentrations selenium toxicity is a concern.The objective of this study was to determine the bonding mechanisms of selenate (SeO2−

4 ) and selenite (SeO2−3 ) on hydrous aluminum oxide

(HAO) over a wide range of reaction pH using extended X-ray absorption fine structure (EXAFS) spectroscopy. Additionally, selenate adsorptionon corundum (α-Al2O3) was studied to determine if adsorption mechanisms change as the aluminum oxide surface structure changes. The overallfindings were that selenite forms a mixture of outer-sphere and inner-sphere bidentate-binuclear (corner-sharing) surface complexes on HAO,selenate forms primarily outer-sphere surface complexes on HAO, and on corundum selenate forms outer-sphere surface complexes at pH 3.5but inner-sphere monodentate surface complexes at pH 4.5 and above. It is possible that the lack of inner-sphere complex formation at pH 3.5is caused by changes in the corundum surface at low pH or secondary precipitate formation. The results are consistent with a structure-basedreactivity for metal oxides, wherein hydrous metal oxides form outer-sphere complexes with sulfate and selenate, but inner-sphere monodentatesurface complexes are formed between sulfate and selenate and α-Me2O3.© 2006 Published by Elsevier Inc.

Keywords: Adsorption; Selenate; Selenite; Aluminum oxides; Se K-edge EXAFS; Surface complexation

1. Introduction

Selenium is an important micronutrient for human and ani-mal health, but at elevated concentrations selenium toxicity is aconcern. Toxic effects of selenium are commonly encounteredin aquatic ecosystems, where the range between deficiency andtoxicity is extremely narrow [1]. In soils and sediments, sele-nium undergoes a variety of redox reactions, and can be foundin oxidation states ranging from −2 (selenide) to +6 (sele-nate) [2] with the form present in the environment being depen-dent upon soil redox status [3]. The fully oxidized (+6) form,selenate, exists as a tetrahedral oxyanion in solution as bisele-nate (HSeO−

4 ) or selenate (SeO2−4 ) with a pKa of 1.7 for this

acid dissociation. The fully protonated selenic acid is a strongacid and does not occur in water. The aqueous chemistry of se-lenate is quite similar to sulfate, and researchers have observed

* Fax: +1 306 966 6806.E-mail address: derek.peak@usask.ca.

0021-9797/$ – see front matter © 2006 Published by Elsevier Inc.doi:10.1016/j.jcis.2006.08.014

that similar surface complexes form with both oxyanions [4,5].Another important and common form of selenium in soils andsurface waters is Se(IV) which exists as the pyramidal oxyan-ion selenite (SeO2−

3 ). Selenite is a weak diprotic acid that canexist as H2SeO3, HSeO−

3 ), or SeO2−3 ) depending upon solution

pH (pKa1 is 2.64 and pKa2 is 8.4) [2].Sorption processes at the mineral/water interface typically

control the mobility and bioaccessibility of many inorganiccontaminants [6]. Adsorption complexes are responsible forthe slow reduction of oxidized selenium forms in the pres-ence of soils [7] and are an important reaction step in the re-duction of selenite to elemental selenium catalyzed by greenrusts [8]. Researchers have previously compared selenate andselenite adsorption on a variety of mineral surfaces and gen-erally found that SeO2−

4 ) is relatively weakly-bound, whileSeO2−

3 ) forms very strong complexes. It is well establishedthat iron and aluminum oxide minerals and coatings are themost common geosorbents for selenium oxyanions in soils dueto their surface chemical properties. Both iron and aluminum

338 D. Peak / Journal of Colloid and Interface Science 303 (2006) 337–345

oxides have high points of zero charge (8–9.5) [6] makingthem positively charged over most typical soil pH ranges. Bothmetal oxides also have relatively high surface areas and a highdensity of surface functional groups for ligand exchange re-actions. However, far more spectroscopic studies of oxyanionadsorption on iron oxides and hydroxides (e.g., ferrihydrite,hematite, and goethite) have been conducted compared to alu-minum hydroxide surfaces. Spectroscopic studies provide theonly means of conclusively determining how oxyanions reactwith surfaces [6], and can therefore improve both our theoret-ical and practical understanding of surface complexation reac-tions.

The structure of iron oxide minerals can strongly affect theirsurface chemistry and reactivity with oxyanions. For exam-ple, researchers have determined that sulfate forms inner-spheremonodentate complexes on hematite [9,10], both inner- andouter-sphere complexes on goethite [5,11], and mostly outer-sphere complexes on HFO [10]. Some inner-sphere complex-ation of sulfate is observed on all sorbents at pH 3.5, whichis consistent with EXAFS results for selenate adsorption ongoethite, HFO, and hematite [4]. Manceau and Charlet reportedthat the changes in surface structure of HFO, goethite, and aka-ganeite led to different ratios of edge and corner sharing surfacecomplexes for arsenate, selenate, and selenite at acidic pH [12].The overall conclusions that can be drawn from previous studiesare that it is important to study oxyanions adsorption mecha-nisms over a wide range of pH and that, at least for iron oxides,it is important to directly determine reaction mechanisms on arange of mineral structures rather than making generalizationsabout metal oxides as a whole.

While iron oxides are often considered more reactive withoxyanions than aluminum oxide phases in aerobic systems,reductive dissolution of iron oxides is substantial in reducedenvironments. Under such conditions, the presence of otherredox-stable phases such as aluminum hydroxides can signif-icantly reduce the transport of contaminants [13]. Several re-searchers have compared the adsorption mechanisms of differ-ent oxyanions on iron oxides versus aluminum oxides usingspectroscopy. Wijnja and Schulthess used ATR-FTIR and Ra-man spectroscopy to study sulfate and selenate adsorption ongoethite and aged γ -Al2O3 surfaces. They found that outer-sphere complexation was the only complexation mechanismon their aluminum oxide, while inner-sphere complexes wouldform in acidic solutions on goethite. Goldberg and Johnston(2001) compared arsenate and arsenite adsorption on amor-phous Fe and Al oxides using FTIR and Raman spectroscopy.They found that arsenate forms inner-sphere surface complexeson both amorphous oxides whereas arsenite forms both inner-and outer-sphere surface complexes on amorphous Fe oxide butonly outer-sphere surface complexes on amorphous Al oxide.More recently, Peak and co-workers studied selenite adsorp-tion on a variety of aluminum-bearing mineral phases usingSe K-edge XANES and EXAFS at pH 4.5 [14]. The mineralphases included a hydroxyaluminosilicate (HAS) polymer, ahydroxyaluminum (HYA) polymer, montmorillonite, and bothHYA and HAS-coated montmorillonite. Only bidentate binu-clear inner-sphere complexation was observed on montmoril-

lonite, but for the hydroxyaluminum and hydroxyaluminosil-icate polymers, a mixture of outer-sphere and bidentate bin-uclear inner-sphere was observed. When montmorillonite wascoated with either HYA or HAS polymers then adsorption be-havior was intermediate between that of the mineral and thepure polymer. These researchers found that XANES was use-ful in detecting outer-sphere complexes, whereas EXAFS wasmore suited for identification of the types of inner-sphere bond-ing environment that was present.

It is not clear whether the tendency for oxyanions to formouter-sphere complexes on aluminum oxides is a general trendor whether it is an observation that is limited to the Al(OH)3mineral structures previously studied [5,15]. It has been sug-gested that different aluminum oxide minerals will form abayerite-type surface layer upon hydration [16] and thereforethe effect of mineral structures upon surface chemistry may notbe as significant with aluminum oxides. Additionally, the vastmajority of aluminum oxides in soils are of the Al(OH)3 form,as either gibbsite or amorphous coatings on other soil surfaces.Gibbsite is only commonly observed as a discrete mineral phasein extremely weathered acidic/tropical soils due to prolongedintensive leaching. In more temperate and calcareous soils, alu-minum hydroxide typically exists as a poorly-crystalline phaseassociated with organic matter and coating other clay miner-als [17]. Hydrous aluminum oxide (HAO) is an amorphous alu-minum hydroxide phase commonly studied as a proxy for thesepoorly crystalline aluminum oxide coatings and aggregates.Since these phases are known to be extremely reactive [17] andare expected to play an increasingly important role in sorptionprocesses as reductive dissolution of iron oxides occurs, under-standing adsorption of oxyanions on HAO is quite relevant tonatural systems.

The objective of this study was to directly determine thebonding mechanisms of selenate and selenite on HAO over awide range of reaction pH. Two research questions will be ad-dressed in this study. First of all, a prior study has shown thatinner- and outer-sphere complexes form between selenite andHAO at pH 4.5 [14], but the pH dependence of the reactionis unknown. Second, we are interested in comparing selenateadsorption on iron oxides and adsorption onto aluminum ox-ides using phases with the same structure. To this end, selenateadsorption on both HAO and corundum (α-Al2O3) was studiedvia XAS. Our goal was to determine whether similar differencesin adsorption mechanisms occur for these aluminum oxides ashave been observed on the iron oxides HFO and hematite. X-rayabsorption spectroscopy (XAS) was chosen as the analyticaltechnique because it is capable of determining bonding mech-anisms of adsorbed species in the presence of normal amountsof water without any sample modification. Extended X-ray ab-sorption fine structure (EXAFS) spectroscopy is particularlyrobust in the determination of inter-atomic distances in oxyan-ion surface complexes, while X-ray absorption near edge struc-ture (XANES) spectroscopy has been utilized to identify thepresence of outer-sphere complexes in systems where mixturesoccur [14]. Both techniques together can provide a reasonablyclear estimate of the local bonding environment of seleniumoxyanions in our samples.

D. Peak / Journal of Colloid and Interface Science 303 (2006) 337–345 339

2. Materials and methods

2.1. Mineral preparation

The corundum used in this experiment was purchased fromAlpha Aesar and used without further modification. It has astated purity of 99.99% (metals basis) and a stated particle sizeof 0.9–2.0 µm. Hydrous aluminum oxide (HAO) precipitate wasprepared using a method based upon that of Sims and Bing-ham for the synthesis of hydroxy-Fe precipitate [18]. A 200 mlaliquot of 1.5 M AlCl3 was placed in a 1 L beaker and neutral-ized slowly to an OH/Al ratio of 2.7 with 2.0 M NaOH undercontinuously stirred condition. The resulting precipitates wereaged for 1 h, resuspended in 1 L of deionized water, centrifugedat 1000g, and the supernatant was discarded. This process wasrepeated 5 times to remove residual Al3+ and Cl− from the sus-pension. The washed precipitates were kept as a 20 g/L stocksuspension in deionized water and used immediately for ad-sorption experiments. CO2 was excluded in the synthesis viapreparation inside a glove box under N2 atmosphere. BarnsteadDiamond NanoPure 18.2 M� water was used for the washingprocess, but CO2 was not otherwise excluded. The final pH ofthe washed suspension was ∼5.5, so carbonate levels were as-sumed to be minimal in the final product.

The surface properties of HAO and corundum were alsocharacterized prior to their use. Specific surface area wasdetermined via determined using both single and multipointBrunauer–Emmett–Teller (BET) N2 adsorption and desorptionisotherms acquired with a Quantachrome Autosorb 1 Gas Sorp-tion System (Florida, USA). The Zetasizer Nano ZS was used

Table 1Surface properties of sorbents used in this study

IEPa Surface area (m2 g−1)b

Corundum 7.9 9.88±0.5Hydrous aluminum oxide (HAO) 8.5 267±11

a Isoelectric point as determined via zeta potential.b Total external surface area as determined by 3 point N2 BET.

to determine the point of isoelectric point (IEP) through themeasurement of zeta potential as a function of pH. Mineralswere also analyzed via X-ray diffraction (XRD) on a RigakuRotaflex 200SU (Tokyo, Japan) to verify their identity. Table 1provides a summary of specific surface area and PZC of bothHAO and corundum.

2.2. Adsorption experiments

All adsorption experiments were performed under a nitrogenatmosphere inside an anaerobic chamber to exclude CO2 andto avoid selenite oxidation. For the HAO experiments, 200 mlof 0.5 g/L HAO suspension was produced by adding 2 mlof the HAO stock suspension to 198 ml of 0.01 M NaCl indeionized water. These suspensions were adjusted to the de-sired pH for the experiments, equilibrated for 8 h, and theneither Na2SeO4 or Na2SeO3 was then added from a freshlyprepared 10 mM stock solution to reach a final concentrationof 100 µM. Sample pH was readjusted twice per day withstandardized 0.1 M HCl or NaOH as the reaction proceeded.Corundum samples were prepared by placing 0.25 g corundumpowder (Alpha Aesar) into a 200 ml solution of 0.01 M NaCland 5 µM Na2SeO4. This suspension was sonified for 2 minand then pH was adjusted as with HAO over a 24 h period. Thefresh powder and shorter reaction time was utilized in these ex-periments to attempt to minimize conversion of the aluminumoxide surface from Al2O3 to Al(OH)3. These experiments weredesigned such that greater than 95% adsorption would occur inall samples and that approximately 0.7–0.8 µmol m−2 surfaceloading would be achieved on all samples with both the HAOand corundum surfaces. This was important for two reasons.First of all, it is impossible to exclude outer-sphere complex-ation in XANES spectra of samples if substantial entrainedaqueous selenate or selenite exists. Second, it was important toachieve a similar surface loading so that comparisons could bemade as a function of pH and between the different mineralsurfaces. Surface loadings for all samples are present in Ta-ble 2.

Table 2Structural parameters of selenium oxyanions sorbed on mineral surfaces

Sorbent Seleniumoxyanion

Loading

(µmol/m2)

Reactionconditions

S0a E0

b First shell (Se–O) Second shell (Se–Al)

R (Å)c,f Nd,g �σ 2 (Å2)e R (Å)c,f Nd,g �σ 2 (Å2)e

Corundum Selenate 0.81 pH 3.5, I = 0.01 0.86 7.41 1.70 2.92 0.001 n/a n/a n/aCorundum Selenate 0.79 pH 4.5, I = 0.01 0.86 7.76 1.64 4.32 0.001 3.61 1.47 0.006Corundum Selenate 0.75 pH 6.0, I = 0.01 4.6 8.39 13.66 4.61 0.001 3.66 1.88 0.010HAO Selenate 0.72 pH 3.5, I = 0.01 0.86 8.68 1.65 4.39 0.001 n/a n/a n/aHAO Selenate 0.72 pH 4.5, I = 0.01 0.86 7.2 1.64 4.65 0.001 n/a n/a n/aHAO Selenate 0.70 pH 6.0, I = 0.01 0.86 7.63 1.65 4.41 0.001 n/a n/a n/aHAO Selenite 0.68 pH 4.5, I = 0.01 0.86 12.97 1.7 3.1 0.001 3.2 1.2 0.005HAO Selenite 0.70 pH 6.0, I = 0.01 0.86 12.98 1.7 2.53 0.002 3.21 1.5 0.008HAO Selenite 0.71 pH 8.0, I = 0.01 0.86 12.93 1.7 3.38 0.002 3.2 1.9 0.005

a Amplitude reduction factor.b Energy shift.c Interatomic distance.d Coordination number.e Debye–Waller factor.f Fit quality estimated accuracy ±0.03 Å.g Fit quality estimated accuracy ±30%.

340 D. Peak / Journal of Colloid and Interface Science 303 (2006) 337–345

After 48 h of reaction, these suspensions were shipped tothe Advanced Photon Source at Argonne National Laboratoryfor XAS analysis. Immediately prior to analysis, samples werecentrifuged at 1000g for 15 min, and the paste was placed intosample holders. No oxidation of selenite was observed in theXANES spectra of any Se(IV) samples.

2.3. X-ray absorption spectroscopy

XANES and EXAFS spectra were collected at the SeK-edge (12.658 keV) at the PNC-CAT Bending Magnet beam-line (20-BM) at the Advanced Photon Source at Argonne Na-tional Laboratory. The electron storage ring was operating at4.5 GeV and in top up mode (100 mA) for all experiments. Thebeamline was calibrated using a Pt foil placed in the path of thebeam past the sample. Edge energy was also monitored witha diode placed near I0, but out of the beam’s direct path. Thisdiode was then covered with sodium selenate powder placedon kapton tape. Solution samples were analyzed in transmis-sion mode to avoid self absorption effects and sorption sampleswere scanned in fluorescence mode using a solid-state 13-element detector (Canberra). Solution samples were analyzedat room temperature while a cryostat was utilized to lower thetemperature of sorption samples (analyzed as moist pastes) to25 K. This was done to avoid any beam-induced oxidation overlong scan times (up to four hours for the most dilute samples).WinXAS version 2.3 (written by Ressler) was used for all datareduction and analysis. All scans were first checked for edgeshifts, which were corrected if observed. Multiple scans 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, every spectrumwas normalized to an edge jump of 1.0. EXAFS fitting was per-

formed by comparing experimental spectra to theoretical scat-tering paths for model compounds using ATOMS to constructmineral phases from crystallographic data and FEFF7 [19] tocalculate theoretical scattering paths.

3. Results and discussion

3.1. Selenite XAS samples

Fig. 1 shows the results of Se K-edge EXAFS conducted onsamples of selenite adsorbed on HAO at pH 8, 6, and 4.5. Thek3-weighted chi data and fits are shown in Fig. 1a, while the ra-dial structure functions (RSFs) of the Fourier-transformed dataare shown in Fig. 1b. Fit results are also tabulated in Table 2.In all three samples there is clear evidence of two shells in theRSF: A Se–O shell (fit with ∼3 O2 at 1.70 Å) and a secondshell Se–Al (fit with ∼2 Al at 3.20 Å). The Se–O distance is ingood agreement with previous EXAFS experiments conductedon selenite, which have shown Se–O bond distances of 1.68–1.72 Å for aqueous and adsorbed selenite [12,14,20]. Basedupon simple geometrical constraints, one would predict Se–Alinteratomic distances of ∼3.55 Å for a monodentate complex,∼3.2 Å for a bidentate-binuclear (corner-sharing) complex, and∼2.6 Å for a bidentate-mononuclear (edge-sharing) complex.Therefore, the best assignment of the bonding environment forselenite on HAO over the entire pH regime is a bidentate-binuclear (corner-sharing) surface complex. This is in goodagreement with previous studies [14] of selenite adsorption onother aluminum-bearing mineral surfaces, which also reportedbidentate-binuclear surface complexation. Fig. 2 compares theFourier-filtered and back-transformed Se–O and Se–Al shellsin the pH 6.0 data from Fig. 1 to a simulation of a single shellSe–O and Se–Al fit. These simulations were performed using

Fig. 1. Se K-edge EXAFS results for selenite adsorbed on HAO as a function of pH. The left graph shows the k3-weighted chi data, while radial structure functionsare shown on the right. Raw data is denoted with a straight line, while best fits results are shown with circles.

D. Peak / Journal of Colloid and Interface Science 303 (2006) 337–345 341

Fig. 2. Comparison of Fourier-filtered backtransformed single shells to the best fit results for selenite adsorbed on HAO at pH 4.5.

the fit parameters from Table 2 with no further optimization.It is clear that excellent agreement between the simulation andthe data exists between 4 and 13k. These results provide strongevidence that an inner-sphere surface complex forms betweenselenite and HAO, and that it is of a corner-sharing geometry.This is similar to reported results for selenite bonding mecha-nisms on iron oxide minerals.

Although EXAFS results clearly demonstrated inner-spherecomplexation between selenite and HAO, they could not con-clusively eliminate the possibility of outer-sphere complexesco-occurring. To investigate this, the XANES region of oursamples was compared to XANES spectra of aqueous HSeO−

3 ),SeO2−

3 ), and selenite adsorbed on montmorillonite at pH 4.5from a previous study [14] in Fig. 3. The montmorillonite sam-ple was chosen as an inner-sphere reference for selenite-Alinteractions because the surface is negatively charged and outer-sphere complexation is unfavorable. From the XANES data itis clear that no shifts in oxidation state occurred in our ex-periments; the white line energy is unchanged in all samples.The region between 12.67 and 12.69 keV has spectral fea-tures that change substantially as the coordination of selenitechanges in our references, as previously reported [14]. Fromour samples, we can determine that selenite adsorbed on HAOhas a similar spectral signature over all pH studied, and that itdoes not totally match with either our outer-sphere (aqueous)standards or our inner-sphere (montmorillonite) standard. In-stead, the region past the main peak from 12.67 to 12.68 keVappears to be broad and featureless in our sorption samples.Previous research [14] has shown that this spectral signaturecan be reproduced by linear combination fitting using a combi-nation of the outer- and inner-sphere standards. This providesfairly strong evidence that some outer-sphere selenite adsorp-tion occurs alongside the inner-sphere complexation observedwith EXAFS. Another piece of information that supports some

Fig. 3. Se K-edge XANES spectra of selenite adsorbed on HAO as a functionof pH compared to a pure inner-sphere standard (selenite on montmorillonite)and a purely outer-sphere standard (aqueous HSeO−

3 ) and SeO2−3 ).

outer-sphere complexation is the observation that Se–Al coor-dination numbers in Table 2 are affected by pH and increasefrom 1.2 at pH 4.5 to 1.9 at pH 8. Since bond distances are notaffected enough for a conversion from monodentate to biden-tate to occur, the other possibility is that some of the adsorbedselenite is present as an additional outer-sphere complex. Theexpected effect of outer-sphere selenite on a bulk EXAFS spec-

342 D. Peak / Journal of Colloid and Interface Science 303 (2006) 337–345

Fig. 4. Se K-edge EXAFS results for selenate adsorbed on HAO as a function of pH. The left graph shows the k3-weighted chi data, while radial structure functionsare shown on the right. Raw data is denoted with a straight line, while best fits results are shown with circles.

trum is that it would decrease the relative intensity of the secondshell contributions to the overall spectrum. This would translateto a decrease in coordination number in the overall Se–Al fit,which was observed in our samples.

3.2. Selenate XAS samples

Fig. 4 shows Se K-edge EXAFS of selenate adsorptionon HAO. In all three samples, it is possible to adequately fitthe experimental data with a single Se–O shell of ∼4O2 at1.64–1.65 Å. This bond distance is consistent with previouslypublished Se–O distances of aqueous and adsorbed selenate[4,12,20]. The lack of a second shell Se–Al distance in theselenate sorption samples suggests that the coordination envi-ronment of selenate on HAO surface is outer-sphere over theentire pH range of study. This is in contrast with previousstudies [4,12] of selenate adsorption on hydrous ferric oxide(HFO). These researchers reported selenate adsorption spec-tra collected at pH 3.5 and 0.1 M ionic strength and reportedinner-sphere bidentate binuclear complexation. Unfortunately,no other spectra were reported at higher pH by either researcher.However, similar studies of the pH dependence of sulfate ad-sorption on HFO have been conducted using ATR-FTIR [10]. Itwas observed that the dominant mechanism of sulfate adsorp-tion from pH 9 to 4 was outer-sphere, and that at lower pH asecond, inner-sphere component becomes visible in the FTIRspectrum. This is more consistent with our Se EXAFS results,with the difference being that the inner-sphere complex is notapparent even at pH 3.5 on HAO.

However, one point of interest in Fig. 4a is that the fitted the-oretical path for all samples is much larger in amplitude than theactual data past 10 Å−1. This could be evidence that there is asmall amount of inner-sphere complexation, but that it is be-low the amount we can reliably fit with EXAFS. This would

also be consistent with the RSFs in Fig. 4b, as some weak fea-tures are present in all samples around 3–4 Å that could not befit in our data. When inner-sphere complexes between seleniteand HAO (Fig. 1) and selenate and corundum (Fig. 5) are fit,then the experimental data and theoretical fit are much moreclosely related. For this reason, a small amount of inner-spherecomplexation between selenate and HAO cannot be completelyruled out based upon our results.

We next investigated selenate adsorption on corundum,the aluminum analog of hematite. Because selenate and sul-fate have been shown to form inner-sphere complexes onhematite [9,10], our hypothesis was that inner-sphere complex-ation should be more likely on corundum than on HAO. Fig. 5shows results of Se K-edge EXAFS for selenate adsorption oncorundum, and the fit parameters are tabulated in Table 2. Onecan clearly see a small (10–15%) contribution from a secondshell Se–Al that is indicative of inner-sphere complexation inthe pH 4.5 and 6.0 samples. The bond distance of 3.6 Å is muchlonger than observed for selenate adsorption on iron oxides (re-ported as 2.8–3.2 Å in the literature), and is consistent with amonodentate inner-sphere surface complex. However, this Se–Al scattering was not nearly as apparent in the pH 3.5 sampleand an outer-sphere complex adequately describes the data. Aswith selenate adsorbed on HAO in Fig. 4, however, there is stillsome small feature in the 3–4 Å that could be attributed to asmall amount of inner-sphere complexation beyond our abilityto fit. One possible explanation for this is that the corundumsurface rapidly dissolves at pH 3.5 via proton-promoted disso-lution to form a bayerite layer similar in structure to our HAOphase. This would cause the adsorption mechanism to shift toan outer-sphere complex as was observed on HAO. However,at pH 3.5, Al3+ is expected to exist in aqueous solution as amonomer rather than forming a secondary aluminum hydrox-ide precipitate. This leaves two other possibilities. It is possible

D. Peak / Journal of Colloid and Interface Science 303 (2006) 337–345 343

Fig. 5. Se K-edge EXAFS results for selenate adsorbed on corundum as a function of pH. The left graph shows the k3-weighted chi data, while radial structurefunctions are shown on the right. Raw data is denoted with a straight line, while best fits results are shown with circles.

Fig. 6. Comparison of Fourier-filtered backtransformed single shells to the best fit results for selenate adsorbed on α-Al2O3 at pH 4.5.

that some relaxation of the corundum surface structure occursat low pH. This would not require the dissolution and repre-cipitation of a different Al(OH)3 surface. The other possibilityis that a new Al–SeO4 precipitate phase of some sort might beforming under acidic conditions.

As the second shell fit with Se–Al in our pH 4.5 and 6.0samples is fairly weak, we also compared the Fourier-filteredand back-transformed first and second shells in the pH 6.0 se-lenate on corundum spectrum to a simulation of a single shellSe–O and Se–Al fit using the parameters in Table 2. The resultof this simulation is shown in Fig. 6. It is clear that excellent

agreement between simulation and the experimental data existsover a range 4–13k. However, it should be noted that the totalcontribution of this Se–Al shell is relatively small (9–11%) forboth inner-sphere samples. If the spectra from Figs. 1 and 5 arevisually compared, it is clear that the second shell peak is muchstronger in the selenite sorption samples. In previous studies ofselenate adsorption on iron oxides [21], the weak second shellcontribution was attributed to mixed inner- and outer-spherecomplex formation on the goethite surface. The same phenom-enon may occur on corundum, which would explain the lowintensity of the inner-sphere features in our samples. Alterna-

344 D. Peak / Journal of Colloid and Interface Science 303 (2006) 337–345

Fig. 7. Se K-edge XANES spectra of selenate adsorbed on HAO as a functionof pH compared to an inner-sphere standard (selenate adsorbed on FeOOH atpH 3.5) and a purely outer-sphere standard (aqueous SeO2−

4 ). Note the sim-ilarities in all spectra; this suggests that XANES is not as sensitive to localcoordination environment as selenite.

tively, the surface could be changing in water even over the timescale of our experiment. If that occurs then it is expected thatcomplexation mechanisms similar to HAO could occur.

Se K-edge XANES spectra for all of the selenate sorptionsamples on corundum and HAO were also collected and com-pared to references. Fig. 7 contains these XANES spectra ofselenate adsorbed on HAO and corundum as a function of pHcompared to an inner-sphere standard (selenate adsorbed onFeOOH at pH 3.5 and 1.0 M I) and a purely outer-sphere stan-dard (aqueous SeO2−

4 ). Note that unlike the selenite XANESdata in Fig. 3, all spectra in Fig. 7 appear virtually identical.This suggests that XANES is not as sensitive to coordinationenvironment for selenate as it is for selenite. The best expla-nation for this is the extremely rigid tetrahedron that selenateforms. Its metal center is +6, and very little change in XANESspectra are observed upon protonation, inner-sphere coordina-tion, or precipitation simply because the Se–O bond angles anddistances do not vary by a great deal.

As a follow-up study, a macroscopic pH adsorption enve-lope of selenate on HAO was performed at both 0.01 and 0.1ionic strength at 1 g/L HAO. Sodium nitrate was used as thebackground electrolyte, and an initial selenate concentration of1.25 mM was used. The results are graphed in Fig. 8. Fromthe pH envelope it is clear that there is a strong ionic strength

Fig. 8. Macroscopic pH envelope for selenate adsorption on HAO at 0.1 and0.01 M ionic strength.

Fig. 9. Bonding mechanisms consistent with Se XAS results. Note that a smallamount of inner-sphere complexation is possible between selenate and HAO,and a small amount of outer-sphere complexation is possible between selenateand corundum.

dependence on the adsorption, which implies that some outer-sphere complexation is occurring on this surface.

To summarize our results, Fig. 9 shows surface complexesfor selenite on HAO and selenate on HAO and corundum.The overall findings are that, for the pH range studied: (i) se-lenite forms inner-sphere bidentate-binuclear (corner-sharing)and some outer-sphere surface complexes on HAO, (ii) sele-nate forms primarily outer-sphere surface complexes on HAO,but a very small amount of inner-sphere complexes are poten-tially co-occurring, and (iii) selenate forms mostly outer-spheresurface complexes on corundum at pH 3.5 but mostly inner-sphere monodentate inner-sphere surface complexes at pH 4.5and above. It is possible that the lack of inner-sphere complexformation at pH 3.5 is caused by some sort of proton-promotedchanges in the corundum surface, and it is also possible basedon the weak second shell contribution that some outer-spherecomplexation also occurs at pH 4.5 and above on corundum.

The observed complexation mechanisms are generally con-sistent with reported sulfate and selenate adsorption mecha-nisms on iron oxides at pH 4.5 and above. Sulfate and sele-nate have very similar solution and surface chemistry, similar

D. Peak / Journal of Colloid and Interface Science 303 (2006) 337–345 345

size, and have previously been determined to react with sur-faces via similar mechanisms. Sulfate is known to form inner-sphere monodentate surface complexes on α-Fe2O3 [9,10] andan inner-sphere monodentate surface complex also formed onthe fresh α-Al2O3 surface used in this study. Sulfate was shownto adsorb on HFO via an outer-sphere mechanism [10], andselenate also forms mostly outer-sphere surface complexes onHAO under these conditions. This suggests that there is a linkbetween surface structure and surface reactivity in metal oxidesthat is common between iron and aluminum oxides.

However, the pH 3.5 selenate adsorption samples on alu-minum phases do not share the above consistency with ironoxides. While sulfate and selenate both form inner-spheremonodentate complexes on α-Fe2O3 at pH 3.5 [4,9,10], onlyouter-sphere complexes are able to be modeled on α-Al2O3in this study at pH 3.5. While inner-sphere surface complexesof selenate were found on HFO at pH 3.5 by previous re-searchers [10,12], only outer-sphere complexes are formed asthe dominant phase on HAO. The best possible explanationsfor this disparity relate to the higher solubility of aluminum ox-ides at low pH and their tendency to convert to Al(OH)3 formin aqueous suspensions. First of all, there is a large differencein solubility between iron oxide and aluminum oxide phases atpH 3.5. Since the aluminum oxide surface itself is thermody-namically unstable under very acidic conditions, it is possiblethat some mixed aluminum selenate precipitate phase might beforming. It is known that an aluminum analog of schwertman-nite (an iron oxyhydroxy-sulfate mineral) forms in conditionssimilar to our pH 3.5 samples. Alternatively, it has also beenshown that γ -Al2O3 will spontaneously convert to α-Al(OH)3in water over 30 days [5]. It has been suggested that all alu-minum oxide phases can be modeled with the same parametersbecause in water their surfaces all relax to this same Al(OH)3bayerite layer. These differences in the stability of aluminumoxides could be responsible for the difference in adsorption onaluminum versus iron oxide minerals observed in our experi-ments.

4. Summary

The results of this experiment highlight the fact that adsorp-tion mechanisms of oxyanions on mineral surfaces are stronglyinfluenced by sorbent surface properties. This has been demon-strated for sulfate adsorption on iron oxides, but to our knowl-edge this has never been observed on aluminum oxides priorto this study. The results are also generally consistent with astructure-based reactivity for metal oxides, which is in con-trast to the typical thinking that most oxyanions form strongcomplexes with iron oxides but outer-sphere complexes on alu-minum oxides. Instead, it appears that the general trend is thathydrous aluminum and iron oxides (HAO and HFO) form outer-sphere complexes with sulfate and selenate, and that inner-sphere monodentate surface complexes are formed between sul-fate and selenate and the α-Me2O3 structure. It would be useful

for future research that compared more metal oxides to verifythat this trend is valid for MeOOH mineral surfaces.

Acknowledgments

This research was supported by funding from Natural Sci-ences and Engineering Research Council of Canada Discov-ery grants #261432 and by the Saskatchewan Synchrotron In-stitute. Additionally, assistance in beamline configuration andoptimization were provided by Drs. J. Cross, R. Gordon, andS. Heald of PNC-CAT at the Advanced Photon Source atArgonne National Laboratory. PNC-CAT facilities at the Ad-vanced Photon Source, and research at these facilities, are sup-ported by the US DOE Office of Science Grant No. DEFG03-97ER45628, the University of Washington, a major facilitiesaccess grant from NSERC, Simon Fraser University and theAdvanced Photon Source. Use of the Advanced Photon Sourceis also supported by the U.S. Department of Energy, Officeof Science, Office of Basic Energy Sciences, under ContractNo. W-31-109-Eng-38.

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