Journal of Colloid and Interface Science 347 (2010) 282–289

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Journal of Colloid and Interface Science

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Mechanisms of uranyl and phosphate (co)sorption: Complexationand precipitation at a-Al2O3 surfaces

Catherine Galindo a,*, Mirella Del Nero a, Remi Barillon a, Eric Halter b, Benoit Made b

a Institut Pluridisciplinaire Hubert Curien, UMR 7178 CNRS/UdS, 23 rue du Loess, BP 28, 67037 Strasbourg Cedex 2, Franceb Ecole Nationale Supérieure des Mines, Mines ParisTech, Centre de Géosciences, 35 rue Saint Honoré, 77305 Fontainebleau Cedex, France

a r t i c l e i n f o a b s t r a c t

Article history:Received 1 February 2010Accepted 19 March 2010Available online 23 March 2010

Keywords:Alumina–solution interfaceUranylPhosphateSurface precipitationIn situ ATR-FTIR spectroscopyZeta potential

0021-9797/$ - see front matter � 2010 Elsevier Inc. Adoi:10.1016/j.jcis.2010.03.045

* Corresponding author. Fax: +33 3 88 10 64 31.E-mail address: catherine.galindo@ires.in2p3.fr (C

This study presents new in situ electrophoretic and ATR-FTIR data on the surface species controlling thecosorption of uranyl and phosphate ions in a-Al2O3 suspensions at acidic pH (3.3). It was shown that theuranyl sorption (i) was promoted in the presence of phosphate, (ii) induced significant changes in zetapotential of P-loaded alumina, and (iii) was governed by two mechanisms, surface complexation and sur-face precipitation, with the predominant species being mainly dependent on phosphate surface coverage.Formation of surface precipitates of uranyl phosphate at high phosphate surface coverage was inferredfrom the high negative charges imparted to the surface by uranyl and phosphate (co)sorption, and fromassignments of IR bands at 1107, 1024, and 971 cm�1 to P–O-stretching vibrations for phosphate coordi-nated to uranyl, at the alumina surface. The ATR-FTIR study showed that the precipitates of uranyl phos-phate formed at the surface of a-Al2O3 for aqueous concentrations of uranyl at trace levels. It alsoevidenced that formation of surface precipitates of U(VI)–phosphate was occurring along with the trans-formation of alumina into secondary surface precipitates of Al-phosphate, at very high phosphate con-centrations. These findings are relevant to the mechanisms of adsorption of trace uranyl on naturallyoccurring oxide surfaces, in soils with low pH where cosorption of phosphate and uranyl ions is knownto play a crucial role in the long-term retention of U.

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1. Introduction

Phosphate ions (noted here as P) affect the retention behavior ofuranium in oxic geochemical systems where U occurs in its hexa-valent state, UO2þ

2 . Uranyl phosphates are important phases forcontrolling U mobility under certain conditions, i.e., when uraniumconcentrations are high enough, or under circum-neutral pH con-ditions where the solubility of U(VI)–phosphates is low [1,2]. Theoccurrence of secondary uranyl phosphates is reported in the vicin-ity of uranium deposits [2–4] and in sediments contaminated byactinides [5]. The immobilization of U and P in low pH soils andin surface sediments involves also the (co)sorption of phosphateions with dissolved uranyl. Murakami et al. [3] identified micro-crystals of copper uranyl phosphate in Fe-rich soils overlying theU ore deposit at Koongarra (Australia). The authors suggested aretention process with an initial stage of U adsorption onto ironoxides, followed by surface precipitation of copper uranyl phos-phates. Payne et al. [6] showed that the addition of phosphate ionssubstantially increases the uptake of uranyl on the ferrihydrite, atacidic pH. They indicated that the formation of a ternary surfacecomplex involving uranium and phosphate, and/or the surface pre-

ll rights reserved.

. Galindo).

cipitation of Fe3+-phosphates having a stronger affinity for uranylthan the original ferrihydrite, provides possible explanations fortheir experimental results. Del Nero et al. [7,8] also reported thatthe formation of U(VI)-bearing surface precipitates of Fe3+-phos-phate onto iron oxides resulted in strong accumulations of U inweathering layers overlying the natural reactor zone of Bangombé(Oklo, Gabon). Jerden and Sinha [2] observed that uranyl is associ-ated with phosphorus in the unsaturated zone of soils developedover the Core Hill deposit (Virginia), where solution pH is low(4–6) and uranyl phosphate minerals are not stable. The authorsfound that uranyl: (i) is sorbed with phosphorus (as ternary surfacecomplexes or nanoprecipitates) onto Fe oxides that coated the sur-face of other minerals, and (ii) is incorporated into secondary Alphosphates of the crandallite group. Thus, the natural site studiesand the experiments have suggested different types of phosphatephases for controlling U mobility in soils and in weathering pro-files, e.g., surface complexes/precipitates of uranyl phosphate ontooxides, or (surface) precipitates of Fe3+-phosphate or Al-phosphateincorporating U in lattice, or adsorbing U at the surface.

A detailed understanding of the mechanisms and phases in-volved in the (co)sorption of uranyl and phosphate ions requiresinvestigations at the molecular level, i.e., by spectroscopic tech-niques. To our knowledge, no surface speciation study has yet beenmade in (hydrous) oxide/uranyl phosphate solution systems. Tang

C. Galindo et al. / Journal of Colloid and Interface Science 347 (2010) 282–289 283

and Reeder [9] investigated by extended X-ray absorption finestructure (EXAFS) spectroscopy the sorption of uranyl on c-alu-mina in the presence of arsenate, which was used as a chemicalanalogue of phosphate. The authors showed the existence of sur-face precipitates of uranyl arsenate (with a structure close toUO2�HAsO4, 4H2O) on c-alumina, at high concentrations of uranyland arsenate. Recent studies of the interface between the solutionsand the (hydrous) oxides of iron or aluminum have evidenced theslow transformation during phosphate sorption (in the absence ofuranyl) of the original oxide surface into Fe3+-phosphate [10] andAl-phosphate [11], respectively. Of major interest is thus the iden-tity of the uranyl phosphate species that form at the oxide/solutioninterface in the course of surface precipitation of Fe- or Al-phos-phate. Full identification requires in situ investigations, duringthe (co)sorption process, of the surface speciation, i.e., of the coor-dination environments of both phosphate and uranyl.

In the present study, we aimed at identifying by means of in situtechniques the mechanisms and species involved in the (co)sorp-tion of phosphate and uranyl ions in a-alumina suspensions underacidic conditions. Sorption was examined under a wide range ofinitial concentrations of aqueous phosphate ([P]I,AQ 6 400 lM),and at a low concentration of U ([U]I,AQ 6 11 lM). We investigatedthe charges and structures, i.e., the nature, of the surface speciesthat form by (co)sorption at the a-Al2O3/solution interface, andtheir dependence on key parameters such as reaction time and ura-nyl and phosphate loadings, by performing zeta potential (f) mea-surements and in situ attenuated total reflectance Fouriertransform infrared (ATR-FTIR) analyses. ATR-FTIR spectroscopyhas proven to be a powerful tool for studying the phosphate sur-face speciation, because (i) it is an in situ surface technique and(ii) it has high sensitivity for phosphate sorbed (trace concentra-tion level) at the interface. In a previous work using this technique,we have shown that, in the absence of uranyl, phosphate ions aresorbed at low pH (3.3) on a-Al2O3 via a combination of surfacereactions of complexation and precipitation of Al-phosphate,whose relative contributions depend on time and phosphate sur-face coverage [11]. The authors evidenced the formation of the sur-face precipitates of Al-phosphate by recording in situ the P–O-stretching vibration mode of phosphate sorbed at the a-Al2O3/solution interface as a function of time, using ATR-FTIR. In the pres-ent study, the technique is used—and is shown to be well suited—to identify the phosphate species of uranyl that form by (secondaryor simultaneous) addition of aqueous uranyl to the a-Al2O3/phos-phate solution systems. Studies by time-resolved laser-inducedfluorescence spectroscopy of the coordination environment(s) ofU in the uranyl phosphate species detected here by ATR-FTIR arereported in a forthcoming paper.

We present here new in situ spectroscopic data on the nature ofthe uranyl phosphate species forming on Al-oxides along the(co)sorption process of phosphate and uranyl. Although investiga-tions were conducted at low pH (3.3) to prevent precipitation ofuranyl phosphates, the mechanisms and species identified hereinare highly relevant to low pH soils and sediments, where (co)sorp-tion of phosphate and uranyl ions is known to play a crucial role inthe long-term retention of U.

2. Materials and methods

2.1. Materials

We used Alfa Aesar’s a-Al2O3 crystallites (chemical purity,99.95%; surface area, 7.6 m2 g�1) having XRD lines characteristicsof corundum. The crystallites in 0.01 M NaCl suspensions showeda particle size at pH 3.3 of 380 ± 20 nm (as determined by applyingthe Contin algorithm to photon correlation spectroscopy

measurements), and a high value of isoelectric point of �8.5 [11].Solutions were prepared by using reagent grade chemicals (HCl,NaCl, NaH2PO4, NaOH) and ultrapure water. A stock solution ofuranyl was prepared by dissolving uranyl nitrate hexahydrate(purchased from Fluka) in 0.1 M HCl.

2.2. Batch adsorption experiments

Batch experiments were performed to measure at 298 K thepartitioning of phosphate and uranyl between the colloidal andthe aqueous phases of a-Al2O3/(0.1 or 0.01 M) NaCl suspensionsat pH 3.3 (solid to solution ratio, r, of 2.5 g L�1). Batch suspensions(r = 2.5, 0.7, or 0.1 g L�1) were also used for measurement of theelectrophoretic mobility (EM) of alumina (zeta potential measure-ments). The main parameters under investigation were solid tosolution ratio, initial concentrations of aqueous phosphate([P]I,AQ 6 780 lM) and aqueous uranyl (0.55 6 [U]I,AQ 6 11 lM),and time of reaction among U, P, and a-Al2O3 (tR = 0.7, 3, or14 days). The batch experiments were conducted as follows. Sus-pensions of alumina were prepared in individual HDPE tubes at de-sired r and [P]I,AQ values. The tubes were then gently shaken at298 K. After a period of preequilibration of a-Al2O3 in the phos-phated solution (tp = 3 h), aliquots of the uranyl stock solutionwere added to the suspensions. After shaking of the tubes duringthe desired period tR, defined volumes of the suspensions were col-lected for measurements of final pH and of the EM of P-loaded and/or U-loaded alumina. The remaining suspensions were centrifugedat 8000 rpm for 2.5 h for solution–colloid separation (cutoff:15 nm). The centrifuged solutions were analyzed by ICP-MS todetermine the final aqueous concentrations of U (analytical error,5%; detection limit, 0.1 ppb) and of Al released in solution by alu-mina dissolution. Phosphate remaining in the supernatants wasdetermined by UV–Vis spectrophotometry (using a phosphate–molybdate complex at 700 nm). The corresponding experimentalerror was equal to 0.14 lM, which leads to uncertainties in theamount of phosphate sorbed ([P]S in mol g�1), for a solid to solu-tion ratio of 2.5 g L�1, of 14 and �3% for [P]I,AQ values equal to10 lM and higher than 100 lM, respectively. At low solid/solutionratios (r = 0.7 or 0.1 g L�1), the experimental errors for aqueousconcentrations did not allow accurate determination of theamount (in mol g�1) of uranyl and phosphate sorbed on alumina([U]S and [P]S, respectively). Results of EM measurements per-formed by using suspensions at low r values were thus reportedas a function of [U]I,AQ or [P]I,AQ.

2.3. Zeta potential measurements

Surface charges imparted by uranyl and phosphate cosorptionwere determined from variations in zeta potential (f) of the a-Al2O3 surface with key parameters: solid to solution ratio, aqueousconcentrations ([U]I,AQ or [P]I,AQ), and/or surface loadings ([P]S or[U]S). Zeta potential variations of a-Al2O3 colloids in U-freephosphate suspensions at pH 3.3 are reported elsewhere [11]. Inthe present study, the zeta potential values were determined asfollows. Alumina suspensions were prepared at pH 3.3 and at dif-ferent values of r, [U]I,AQ, [P]I,AQ as described above. Each batchsample was taken for measurement, with a Malvern NanoZS appa-ratus, of the EM of dispersed alumina in the suspension. The EMvalue was then converted into f potential value by using theSmolukowski equation. Reported results are the average of tripli-cate measurements. The standard deviation of the f potential valueof our alumina samples did not exceed 1.5 mV. Experiments werealso undertaken to determine the isoelectric points of Al-phos-phate (purchased from Acros Organics) and U(VI)–phosphate (syn-thesized by using the procedure of Zheng et al. [12], which weretaken here as reference solids. Such information is useful due to

284 C. Galindo et al. / Journal of Colloid and Interface Science 347 (2010) 282–289

surface precipitation of Al-phosphate on a-Al2O3 during experi-ments of phosphate sorption in the absence of U [11] and to possi-ble formation of uranyl phosphate (surface) precipitates in some ofour sorption experiments.

2.4. In situ ATR-FTIR spectroscopic measurements

In situ ATR-FTIR experiments were conducted by using a BrukerEquinox IFS 55 equipped with a MCT detector, and a horizontal Gecrystal (80 mm long, 10 mm wide) manufactured by Pike (10 inter-nal reflections; angle of incidence: 45�). Before the experiment, thecrystal surface was coated with a stable layer of a-Al2O3 colloids (i)by allowing particles from CO2-free alumina suspensions (25 g L�1,pH 3.3) to settle on crystal overnight, (ii) by rinsing the coatingwith electrolyte and drying it under Ar gas flow, and (iii) by repeat-ing the procedure three times. This sample deposition method hasadvantages over methods based on deposition of wet pastes orconcentrated suspensions, such as increased sensitivity and repro-ducibility of the measurements and recordings of IR spectra as afunction of reaction time. The value of the solid to solution ratio,r, was set to �0.1 g L�1 in the ATR-FTIR experiments, as estimatedfrom the average thickness of the alumina deposits [11]. Afterdeposition, the alumina-coated crystal was brought in contact with0.01 M NaCl (pH 3.3) in the ATR cell and an FTIR spectrum was ta-ken as reference baseline. Aqueous phosphate ([P]I,AQ = 17 or50 lM) was added after a period of colloid–electrolyte equilibra-tion of 3 h (thyd = 3 h). Aqueous uranyl ([U]I,AQ = 3.5 or 7 lM) wasadded simultaneously to aqueous phosphate, or after a desired per-iod of equilibration (tp) of the alumina/phosphate solution system.FTIR spectra (2000 scans/spectrum, resolution of 4 cm�1) were col-lected every 20 min during the experiment. Spectra were con-verted into ASCII files and resolved into Gaussian lines, by theMicrosoft Origin Pro 8 software. The Levenberg–Marquardt algo-rithm, which is a form of nonlinear least-squares fitting, was usedfor the determination of each band parameter.

3. Results and discussion

3.1. Speciation and equilibrium calculations

The database of thermodynamic constants of dissolved uranylspecies and of uranyl phosphate minerals used in calculationswas taken from the critical review of Guillaumont et al. [13].Fig. EA-1.1 in Supplementary material EA-1 gives the calculatedspeciation of 1–10 lM aqueous uranyl, as a function of aqueousphosphate concentration, under conditions used in the experi-ments of P and U (co)sorption (i.e., pH 3.3, [P]I,AQ 6 780 lM,0.01 M NaCl electrolyte). Under these conditions, dissolved phos-phate is mainly in the form of a nonuranyl species, as H2PO�4 . Ura-nyl mainly exists as the free cation, UO2þ

2 , and in the form of the

0 10 20 30 40 50

Sorbed phosphate, [P]s (µmol.g -1)

Sorb

ed u

rany

l, [U

] s (µm

ol.g

-1)

0.6

0.4

0.2

0

tR=0.7o tR=3dΔ tR=14d

tR=3d;

[U]I,AQ=1µM

a

Fig. 1. Amount of uranyl sorbed ([U]S) at pH 3.3 on alumina vs. phosphate surface coveraaqueous uranyl to 2.5 g L�1 a-Al2O3/NaCl suspensions preequilibrated at varying phosph

complex ions, UO2H2POþ4 and UO2HPO4, with the percentage of Uas complexes increasing from 2% to 45% when P increases from10 to 400 lM. Results of equilibrium calculations indicate thatthe solutions are undersaturated with respect to uranyl phos-phates, under the conditions used in the ATR-FTIR experiments([P]I,AQ 6 50 lM and [U]I,AQ 6 7 lM). The solutions are slightlyoversaturated with respect to (UO2)3�(PO4)2�4H2O(s) under a lim-ited range of conditions (i.e., at [U]I,AQ P 8 lM for [P]I,AQ = 400 lM,and at [U]I,AQ P 7.5 lM for [P]I,AQ = 780 lM). These conditionswere used only in the batch experiments performed at a high alu-mina/solution ratio (2.5 g L�1) where the sorption of U is high(equal to �95%). The precipitation of the uranyl phosphate (if it oc-curred in the above-noted experiments) was expected to contrib-ute less than 7% to the loss of U from solution. Fig. EA-1.2 showsthat final experimental solutions are highly undersaturated withrespect to uranyl phosphates.

3.2. The macroscopic uranyl uptake

Fig. 1 reports the amount of U(VI) adsorbed at pH 3.3 as a func-tion of the amount of phosphate sorbed ([P]S) on a-Al2O3 colloids,for 0.01 M NaCl electrolyte solutions at different initial concentra-tions of phosphate ([P]I,AQ). The U sorption data points were ob-tained from batch experiments performed at defined values of[U]I,AQ, [P]I,AQ, and U/P/a-Al2O3 reaction time (tR), by using aluminasuspensions that were preequilibrated for 3 h with phosphate solu-tions (tP: 3 h) before the addition of aqueous uranyl. Previous ki-netic studies had indeed shown that the uptake of P in uranyl-free a-Al2O3 suspensions was rapid during the initial sorption stepand reached within the first hour �90% of P removed over a sorp-tion period of 14 days [11]. Fig. 2 reports the phosphate sorptionisotherms (which conform to the Freundlich isotherm) obtainedin the present study for different [U]I,AQ and tR values, as well assome data of Del Nero et al. [11] on the kinetics of phosphate sorp-tion in uranyl-free suspensions.

The major findings are as follows. The time dependency of thesorption of uranyl on the P-loaded colloids followed a two-steptrend that was also observable for phosphate uptake; i.e., �85%of U removed over a 14-day period was sorbed within the firsthours. The U adsorption envelopes ([U]S vs. [P]S) showed an S-typeshape, and the amount of sorbed uranyl increased nonlinearly withan increase of [U]I,AQ. The percentage of uranyl uptake was low(615%) in the absence of P and in the low-[P]I,AQ suspensions (upto [P]I,AQ = 10 lM) in which phosphate was quantitatively sorbedduring the preequilibration step, i.e., in which phosphate existedmainly as sorption species at the time of the addition of U. Theamount of uranyl uptake increased sharply with [P]S, under condi-tions that favored U–P aqueous complexation and that led to thepartitioning of P between solution and alumina and to a high P sur-face coverage (10 < [P]I,AQ < 400 lM). The removal of 1 lM aqueous

0

1

2

3

4

0 10 20 30 40 50 60Sorbed phosphate, [P]s (µmol.g -1)

d; I=0.01M

I=0.1M

[U]I,AQ=11µM

b

ge ([P]S), for different reaction times (tR) after addition of (a) 1 lM and (b) 11 lM ofate concentrations ([P]I,AQ) for 3 h (I: ionic strength).

0

10

20

30

40

50

0 200 400 600 800[P] I,AQ (µM)

[P] S

(µm

ol.g

-1)

0

10

20

30

40

50

0 200 400 600 800[P]I,AQ (µM)

oΔ: [U]I,AQ=1µM

O, tR=3dΔ, tR=14d

, [U]I,AQ=0

oΔ: [U]I,AQ=11µMba

Fig. 2. Isotherms of phosphate sorption at pH 3.3 on alumina, obtained for different reaction times after addition of (a) 1 lM and (b) 11 lM of aqueous uranyl to 2.5 g L�1 a-Al2O3/NaCl suspensions preequilibrated at varying phosphate concentrations for 3 h (see Fig. 1). Filled symbols are data obtained by Del Nero et al. [11], in the absence of U.

35

55

75

ζ (m

V)

22

32

42

52

62

72

0 10 20 30 40 50

[P]S (µmol.g-1)

zeta

pot

entia

l ζ (m

V) 0.55-1.1µM

2-5.5µM11µM

[U]I,AQ

[P]I,AQ = 10µM

[P]I,AQ = 97µM

[P]I,AQ = 388µM

[P]I,AQ = 780µM

r = 2.5g.L-1

r = 2.5g.L-1

a

b

C. Galindo et al. / Journal of Colloid and Interface Science 347 (2010) 282–289 285

uranyl was complete at a [P]I,AQ value of 400 lM. At the higher[U]I,AQ value investigated (11 lM), the amount of U sorbed at high[P]I,AQ values was almost quantitative and showed, within ourexperimental uncertainty, no dependence or a slight dependenceon [P]s. This suggested that the adsorption of U was limited bythe saturation of highly reactive surface sites and/or that mecha-nisms of U sorption showing less dependence on [P]s than surfacecomplexation were occurring predominantly from a threshold va-lue of P surface coverage (�25 lmol g�1). Applying the well-knownLangmuir and Freundlich equations indicated that sorption of ura-nyl on a-Al2O3, at a given value of [P]I,AQ and at near equilibrium(tR: 3 days), was best described (R2 > 0.99) by the Langmuir iso-therm. The value of the Langmuir equilibrium constant was foundto decrease with increasing [P]I,AQ. Thus, the main experimentalfeatures supported that multiple P-modified surface sites on alu-mina and/or several sorption mechanisms were involved in the Uuptake. Uranyl sorption was independent of ionic strength(Fig. 1), which suggested chemical sorption, while both chemisorp-tion and electrostatic attraction at the alumina surface were shownto participate in the removal of aqueous phosphate in U-free sus-pensions [11]. The contribution of the coadsorption of the uranyland phosphate ions at the alumina surface sites was possible be-cause the addition of uranyl to a-Al2O3 suspensions might increaseslightly the amount of phosphate sorbed, within our experimentaluncertainties (Fig. 2).

150 1 2 3 4

[U]s (µmol.g-1)

15

30

45

60

75

0 200 400 600 800[P]I,AQ (µM)

ζ (m

V)

r =2.5g.L-1

r =0.7g.L-1

r =0.1g.L-1

[U]I,AQ = 0 ( ), 1 ( ), 11 µM ( )

c

Fig. 3. Variation in f potential of alumina surface with: (a) phosphate surfaceloading, (b) uranyl surface loading, and (c) aqueous P concentration at different solid/solution ratios (r), for sorption experiments of phosphate and uranyl at varyingconcentrations in a-Al2O3–0.01 M NaCl suspensions at pH 3.3. Dotted lines representdata obtained by Del Nero et al. [22], in the absence of U. See Fig. 1 for definition ofparameters. Error bars are smaller than the symbols used for data points.

3.3. Dependency of zeta potential of a-Al2O3 on phosphate and uranylsorption at pH 3.3

Changes in f potential of a-Al2O3 colloids with key parameterswere investigated to determine charges imparted to the surfaceby the (co)sorption of phosphate and uranyl at pH 3.3. Such infor-mation is useful in distinguishing between different sorption spe-cies [10,11]. The f measurements were performed usingsuspensions obtained from batch experiments conducted at differ-ent concentrations of alumina (0.1 g L�1

6 r 6 2.5 g L�1), of aque-ous phosphate ([P]I,AQ 6 400 lM), and of aqueous uranyl addedafter 3 h (0.2 lM 6 [U]I,AQ 6 11 lM) to a-Al2O3/phosphate solutionsystems, for a tR value of 3 days. Fig. 3a and 3b show f potentialvariation with increasing sorption of phosphate and uranyl, respec-tively, for suspensions at high ratios of alumina/solution(2.5 g L�1). Also reported are data of Del Nero et al. [11] showingthat, in the absence of U, the f value of a-Al2O3: (i) decreased withP surface coverage, and (ii) exhibited a two-step and linear trenddependence on [P]S which was due to the formation of phosphatesurface complexes (cf. steep line in Fig. 3a) followed by surface pre-cipitation of Al-phosphate. We observed that phosphate surfacecoverage is the main parameter determining the f potential of P-loaded samples after uranyl sorption, with a trend of decreasing

286 C. Galindo et al. / Journal of Colloid and Interface Science 347 (2010) 282–289

f value with increasing [P]S due in part to formation, before addi-tion of uranyl, of the phosphate surface species described by DelNero et al. [11] (Fig. 3a). An important finding of the current studyis that uranyl sorption had nonetheless a significant effect on the fvalue of the P-loaded colloids. The samples at the lowest phos-phate coverage investigated (i.e., [P]S � 5 lmol g�1,[P]I,AQ � 10 lM) showed a surface less negatively charged in thepresence than in the absence of uranyl. By contrast, the reversewas observed for the samples at the highest phosphate coverageinvestigated ([P]S � 45 lmol g�1, [P]I,AQ � 780 lM). These featuresindicated that (at least) two uranyl species were involved in thesorption of U in suspensions at high alumina to solution ratios,with identity and charge of predominant species varying withphosphate concentration. By going into further detail, it appearsthat the effect of uranyl sorption on the f potential of colloids atintermediate phosphate loadings varied with the uranyl concentra-tion (cf. data at [P]I,AQ of 100–400 lM, Fig. 3b). Addition to suspen-sions of uranyl at trace concentrations ([U]I,AQ < 0.55 lM) increasedsignificantly the value of f potential of P-loaded alumina samples,while increasing the concentration of U added (up to 11 lM)—andhence the amount of U sorbed—led to decreasing values of f(Fig. 3b). The results obtained for suspensions at high alumina/solution ratios show that: (i) different uranyl surface species formin addition to the U-free phosphate species identified by Del Neroet al. [11] and (ii) the nature of these U species varies with cover-age of phosphate and uranyl, with high negatively charged speciesprevailing at high loadings.

Table 1 lists possible atomic arrangements of uranyl and phos-phate ions at the alumina surface, as well as reactions of surface li-gand exchange [11]. The uranyl surface species forming at lowconcentrations of P and U were likely uranyl surface complexes,and/or ternary complexes in which one phosphate group is directlycoordinated to the surface (reactions (1)–(3)). Only such complexescan impart to the surface less negative charges than the U-freephosphate complexes (reactions (8)–(12)). Inner-sphere complexesin which U bridge surface aluminol groups and phosphate moieties(reactions (4)–(7)) might form at higher U solution contents. Com-plexes in which uranyl is bound to the surface through a bridgingligand and lies between two ligand molecules are also possible, andsimilar species were reported to form in the ternary systems alu-mina/citrate/uranyl [14] and goethite/citrate/uranyl [15]. Mostlikely, the high negatively charged, uranyl surface species prevail-ing at high U loadings, and/or under conditions of high phosphatecoverage where Al-precipitates form [11], were surface precipi-tates of uranyl phosphate. Indeed, uranyl phosphates show verylow isoelectric point values (IEP < 2), so that their formation onalumina would impart to the surface much higher negative charges

Table 1Possible reactions of surface complexation involved in uranyl and phosphate sorptionon a-alumina.

Reactions of complexation of uranyl and phosphate at alumina surface

AlOHþ2 þH2PO�4 þ UO2þ2 $ AlOPO3HUOþ2 þH2OþHþ (1)

AlOHþ2 þH2PO�4 þ UO2þ2 $ AlOPO3UO2 þ H2Oþ 2Hþ (2)

2AlOHþ2 þ H2PO�4 þ UO2þ2 $ Al2O2PO2UOþ2 þ 2H2Oþ 2Hþ (3)

AlOHþ2 þH2PO�4 þ UO2þ2 $ AlOUO2H2PO4 þ 2Hþ (4)

AlOHþ2 þH2PO�4 þ UO2þ2 $ AlOUO2HPO�4 þ 3Hþ (5)

2AlOHþ2 þ H2PO�4 þ UO2þ2 $ Al2O2UO2HPO2�

4 þ 5Hþ (6)

2AlOHþ2 þ H2PO�4 þ UO2þ2 $ Al2O2UO2H2PO�4 þ 4Hþ (7)

Reactions of complexation of phosphate at alumina surface

AlOHþ2 þH2PO�4 $ AlOHþ2 � � �H2PO�4 (8)

AlOHþ2 þH2PO�4 $ AlOPO2�3 þ H2Oþ 2Hþ (9)

2AlOHþ2 þ H2PO�4 $ Al2O2PO�2 þ 2H2Oþ 2Hþ (10)

AlOHþ2 þH2PO�4 $ AlOPO3H� þ H2Oþ Hþ (11)

2AlOHþ2 þ H2PO�4 $ Al2O2PO2Hþ 2H2OþHþ (12)

than those imparted by (uranyl) phosphate complexes or Al-phos-phate surface precipitates (cf. Supplementary material EA-2,Fig. EA-2). The existence of surface precipitates of uranyl arsenatewas reported on c-alumina, at high concentrations of uranyl andarsenate [9].

Fig. 3c reports f potential data obtained for batch experimentsconducted at low r values (0.7 and 0.1 g L�1) and at varying con-centrations of aqueous phosphate and uranyl. Due to the highuncertainties of the values of [P]S and [U]S obtained at such low rratios (cf. experimental section), the f data were reported inFig. 3c as a function of aqueous phosphate concentration ([P]I,AQ),for different [U]I,AQ values. The figure shows that, in the absenceof U, a decrease of the alumina concentration of a suspension ata given [P]I,AQ value led to a decrease of f potential, i.e., to an in-crease of phosphate coverage. This is consistent with the resultsof the ATR-FTIR experiments (where r is equal to �0.1 g L�1) ofDel Nero et al. [11] evidencing the formation of surface precipitatesof Al-phosphate on a-Al2O3 colloids with the low concentrations ofaqueous phosphate (�50 lM). Fig. 3c also shows that the additionof U at trace levels to the low r suspensions decreased significantlythe f potential value of the P-loaded alumina samples. Moreover,increasing the uranyl content led to decreasing f values. These re-sults confirmed that high phosphate surface loadings promoted theformation of the high negatively charged species on the aluminacolloids.

The current f potential study showed that (i) surface coveragesof U and P are main parameters controlling the nature of the uranylsurface species forming on a-Al2O3, and (ii) high negativelycharged uranyl species exist on the alumina colloids, at high phos-phate loadings. ATR-FTIR spectroscopy is a powerful technique toidentify these highly charged surface species because it allowsin situ analysis of the solid/solution interface as function of time,when using a method of Al2O3 layer deposition on the ATR crystalsetting the solid/solution ratio in experiment to a low value(r � 0.1 g L�1).

3.4. ATR-FTIR analysis

Structures at the a-Al2O3/uranyl phosphate solution interfacewere studied by performing in situ ATR-FTIR spectroscopy investi-gations of the P–O-stretching vibration modes as a function ofreaction time and of phosphate and uranyl concentrations. Thewavelength region, which is characteristic of the P–O-stretchingvibration modes for phosphate, lies between 850 and 1250 cm�1

[16,17]. The sorbed uranyl should be characterized by an antisym-metric stretching vibration m3 in the 890–930 cm�1 region[14,18,19], but it could not be observed due to the high absorbanceof a-Al2O3 below 1000 cm�1, to the high transmission thresholds ofthe crystal material, and to the cutoff of the MCT detector(�900 cm�1).

3.4.1. Analyses of solutionsThe ATR-FTIR spectrum of a solution at high phosphate concen-

trations ([P]AQ: 2.5 � 10�2 M) and at pH 3.3 was taken as a refer-ence of aqueous phosphate species (data not shown). In theregion 1000–1200 cm�1, the IR spectrum showed m3 bands at1159 and 1075 cm�1. These positions were in excellent agreementwith those reported for absorption bands of the m3 vibration (1159,1075, and 940 cm�1) of the complex H2PO�4 , which has reducedsymmetry and belongs to the point group C2v [17,20]. It was foundthat aqueous phosphate complexes were at concentrations wellbelow the detection limit of the technique (corresponding to a[P]AQ value of 800 lM at pH 3.3), under the conditions used inthe ATR-FTIR sorption experiments reported hereafter([P]I,AQ 6 200 lM; pH 3.3). Spectra were also recorded for uranylphosphate solutions at pH 3.3 having concentrations of uranyl (3

C. Galindo et al. / Journal of Colloid and Interface Science 347 (2010) 282–289 287

and 7 lM) and phosphate (0, 17, 50, and 200 lM) similar to thoseused in the ATR-FTIR sorption experiments. For all samples, the IRsignal did not depart from baseline in the region 1000–1200 cm�1.Hence, aqueous species unlikely contributed to the ATR-FTIR spec-tra of the Al2O3/(uranyl) phosphate solution interface reported inthe present study.

3.4.2. Analyses of the alumina/solution interface at low aqueousphosphate concentrations

Fig. 4a reports the time dependency of the ATR-FTIR spectra ofthe alumina/solution interface during the sorption of phosphate ata low aqueous concentration ([P]I,AQ = 17 lM) and in the absence ofU (blank experiment). The alumina colloidal layers were depositedon the ATR crystal and were contacted for 3 h with 0.01 M NaCl atpH 3.3. There was observed a poorly intense absorbance that stabi-lized within 3 h (spectrum 1) and that was likely due to hydrationof the surface and to structural reorganizations of surface hydroxylgroups. The hydrated Al2O3 layers were then equilibrated withaqueous phosphate for 6 h. The signals recorded did not departfrom baseline (spectrum 2). This feature was in agreement with aprevious study showing that sorption on a-Al2O3 of phosphate re-sulted in very low absorbance intensity up to an aqueous concen-tration ([P]I,AQ) of �50 lM, irrespective of the solid/solutioncontact time [11]. Fig. 4b reports the time dependency of theATR-FTIR spectra of the alumina/solution interface in the experi-ment of sorption of phosphate and uranyl ions, at low concentra-tions of aqueous phosphate ([P]I,AQ = 17 lM). The experiment wasconducted as follows. After hydration (thyd: 3 h) of the Al2O3 layers

a

b

Fig. 4. In situ ATR-FTIR spectra recorded as function of time during sorption ofphosphate (17 lM) and uranyl at pH 3.3 on a-Al2O3: (a) blank experiment whereonly aqueous P was added to the alumina/0.01 M NaCl system preequilibrated for3 h, and (b) experiment where aqueous P and U (twice 3.5 lM) were addedsuccessively to the alumina/0.01 M NaCl system previously aged for 3 h. (tp and tR:time of equilibration between alumina and phosphate solution before and afteraddition of U, respectively.)

in 0.01 M NaCl and preequilibration (tp: 3 h) of alumina with aque-ous phosphate—both steps leading to no observable absorbance(spectrum 2)—two successive additions of aqueous uranyl(3.5 lM) were made to the system (at tR = 0 and 3 h). We observedan increase of absorbance in the region 950–1200 cm�1 on the firstaddition of uranyl to the phosphate solution (spectrum 3). Theabsorbance envelope showed a hypsochromic shift with an in-crease of reaction time among U, P, and Al2O3. The signal stabilizedafter 3 h (spectrum 6, tR: 3 h). The absorbance showed again a sud-den increase in intensity following the second addition of 3.5 lMaqueous uranyl (spectra 7–8). Resolving spectra of the Al2O3/ura-nyl phosphate solution interface showed main contributions at�1107 and �1024 cm�1, and a minor band at 971 cm�1. Theseabsorption bands were strongly correlated (Fig. 5), their relativeintensities being constant whatever the reaction time and the ura-nyl content. This supported that all three vibration bands werecharacteristics of a single surface species. Moreover, the experi-ment showed unequivocally that the single surface species wasformed by bonding of uranyl to the phosphated alumina surface,as the strong increase in absorbance observed in the region 950–1200 cm�1 coincided with an increase of [U]I,AQ added. The region900–1200 cm�1 is the spectral region containing absorption bandsattributed to stretching vibrations of the (PO4)3� and (UO2)2+ unitsin uranyl–phosphate compounds [16]. Bands at 1110 and 968 cm�1

have been reported for stretching vibrations of the phosphategroup in U(VI)–phenylphosphonate complexes [21]. A vibrationalstudy of uranyl micas [16] has given IR absorption bands at1104, 1022, and 978 cm�1 for meta-torbernite, at 1118,1074–1048, and 985–983 cm�1 for autunite and meta-autunite,and at 1117, 1054, and 985 cm�1 for saléeite; with all of thesebands being assigned to the (m3) antisymmetric stretching vibra-tion of the (PO4)3� unit. The uranyl micas also displayed bands at

950 1000 1050 1100 1150wavenumber (cm -1)

0.008

0.006

0.004

0.002

0

0.004

0.003

0.002

0.001

0

a

b

971

1024

1107

971

1024

1107

Spectrum 10 at tR: 10h

Spectrum 4 at tR: 1.5h

Abs

orba

nce

Fig. 5. Curve fittings of ATR-FTIR spectra recorded at reaction times (tR) of (a) 1.5 hand (b) 10 h, during experiments of phosphate (17 lM) and uranyl (twice 3.5 lM)sorption at pH 3.3 on Al2O3 (see Fig. 4). (Fitting after subtraction of spectrum at tR:0.)

950 1000 1050 1100 1150 1200

970 1020 1070 1120 1170

wavenumber (cm -1 )

0.012

0.008

0.004

0

0.012

0.008

0.004

0.000

Abs

orba

nce

1008 1037

1084

1137

Addition of Pat tp=0

Addition of Uat tR=0

tR=10h

tp=3h

Before U addition

After U addition

0.012

0.008

0.004

0.000

1007 1033

1086

1143

a

b

c

Fig. 6. ATR-FTIR results on phosphate (50 lM) and uranyl sorption at pH 3.3 on a-Al2O3: (a) time dependency of spectra and effect of successive additions of aqueousP and U (7 lM) to the alumina/0.01 M NaCl system previously aged for 3 h, and (band c) curve fittings of spectra recorded before and after U addition, respectively.(Fitting after subtraction of spectrum at tp: 0.)

288 C. Galindo et al. / Journal of Colloid and Interface Science 347 (2010) 282–289

900–920 cm�1, which were attributed by Frost [16] to either the(PO4)3� symmetric stretching mode or, more likely, to the antisym-metric stretching vibration of the (UO2)2+ unit. We also observed,in the ATR-FTIR spectrum of a meta-autunite sample (Supplemen-tary material EA-2, Fig. EA-2), three bands at 1113, 1008, and980 cm�1 and a low-absorbance band at �920 cm�1 that can be as-signed to stretching vibrations of the phosphate and uranyl units,respectively, according to the work of Frost [16]. Baumann et al.[22] observed spectral features similar to those of autunite andmeta-autunite for a uranyl phosphate phase forming as a second-ary mineral onto depleted uranium alloy. They assigned the bandsin the region 1000–1120 cm�1 to the antisymmetric stretchingvibrations of the phosphate groups, and the bands around915 cm�1 to the m3 mode of the (UO2)2+ unit. The absorption bandsat �1107, �1024, and 971 cm�1 observed in the current experi-ment of uranyl and phosphate (co)sorption are in good generalagreement with those reported for the m3 mode of (PO4)3� in uranylphosphate compounds. We assigned all of them to the antisym-metric stretching mode of the phosphate moiety in the single U–PO4 sorption species forming on alumina.

3.4.3. Analyses of the alumina/solution interface at higher aqueousphosphate concentrations

Similar ATR-FTIR experiments to those described above wereperformed using a higher concentration of aqueous phosphate([P]I,AQ = 50 lM). Unlike in the low-[P]I,AQ blank experiment, theabsorbance showed a significant increase when contacting the hy-drated Al2O3 layers with the U-free solution at a phosphate con-centration of 50 lM (blank experiment at [P]I,AQ = 50 lM). Therewas observed moreover an increase with time of the absorbancerelated to phosphate sorption. The spectra were similar to those re-ported by Del Nero et al. [11] for ATR-FTIR experiments of phos-phate sorption on alumina layers at [P]I,AQ values of 100 and200 lM, with resolving spectra showing positions of the absorp-tion bands at �1133, �1085, �1036, and �1008 cm�1. The authorsattributed the latter bands to the P–O-stretching vibration modesof phosphate surface species, and/or to OH-bending vibrationmodes. Bands related to OH-bending vibration modes were possi-bly due to the decrease of alumina surface charge during phos-phate sorption, that might induce transfer of proton to Al2Osurface sites, leading to Al(OH)2 surface species and favoring sur-face hydration [11]. Del Nero et al. [11] ascribed unequivocallythe absorption band at�1133 cm�1, whose position was character-istic of Al-phosphates [23–25] to the P–O-stretching vibrationmodes of phosphate sorbed on a-Al2O3 as Al-phosphate surfaceprecipitates. The formation at high P concentrations of Al-phos-phate surface precipitates on a-Al2O3 is observable in the spectra(absorption band growing at 1137-1143 cm�1) recorded duringthe experiment of phosphate and uranyl (co)sorption at a [P]I,AQ

value of 50 lM, since the time of preequilibration of alumina withthe U-free phosphate solution (Fig. 6a and b). The spectra recordedin this experiment after the addition of 7 lM aqueous uranyl weresimilar to those obtained during the preequilibration step (Fig. 6aand c). This showed that the sorption of uranyl at high phosphatesurface coverage occurred during the transformation of aluminaphase into secondary Al-phosphate surface precipitates. Likely,the strong absorbance of Al-phosphates masked the IR signal re-lated to the uranyl species. It also makes impossible the accurateand proper identification of the absorption bands of the U speciesby spectra analysis.

We performed then an experiment in which aqueous phosphate([P]I,AQ = 50 lM) and aqueous uranyl ([U]I,AQ = 7 lM) were addedsimultaneously to the 0.01 M NaCl electrolyte in contact with thealumina colloidal layer (thyd: 3 h). Fig. 7a shows the time depen-dency of the ATR-FTIR spectra recorded after the simultaneousaddition of P and U. Resolving spectra with five absorption bands

provided accurate fits to the experimental data, and showed posi-tions of the absorption bands at 1138, 1109, 1084, 1030, and972 cm�1 (Fig. 7b). These results supported strongly that the broadsignals were due to the contribution of multiple vibration bands forAl-phosphate surface precipitates (i.e., P–O-stretching bands and/or OH-bending bands at �1133, �1085, and �1033 cm�1;Fig. 6b), and for uranyl phosphate surface species showing posi-tions of P–O-stretching bands similar to those observed in thelow-[P]I,AQ experiments (i.e., at �1107, �1024, and 971 cm�1,Fig. 5a and b). Thus, the current ATR-FTIR study demonstrated thata single type of surface species of uranyl phosphate formed at a lowratio a-Al2O3/uranyl phosphate solution, whatever the aqueousphosphate and uranyl concentrations. Because their position wasinsensitive to the [U]I,AQ and [P]I,AQ values, and to the transforma-tion rate of alumina into secondary Al-phosphate, we assigned theabsorption bands at �1107, �1024, and 971 cm�1 to the P–O-stretching vibrations of surface precipitates of uranyl phosphate.The decrease in f potential values with [U]I,AQ, observable inFig. 3b, is thus due to the increase of the surface precipitation ofuranyl phosphate, imparting higher and higher negative chargeto P-loaded alumina.

0.010

0.005

0

-0.005

0.011

0.007

0.003

-0.001

972

1030

1084

1109

1138

wavenumber (cm -1 )

Addition of Pand U at t R=0

t hyd=3h

tR=6.5h

At t R: 3h

tR=3h

Abs

orba

nce

a

b

950 1000 1050 1100 1150 1200 1250

Fig. 7. ATR-FTIR results on phosphate (50 lM) and uranyl sorption at pH 3.3 on a-Al2O3: (a) time dependency of spectra and effect of simultaneous addition ofaqueous P and U (7 lM) to the alumina–0.01 M NaCl system previously aged for 3 h,and (b and c) curve fitting of spectrum recorded 3h aftre the addition of P and U.(Fitting after subtraction of spectrum at tp: 0.)

C. Galindo et al. / Journal of Colloid and Interface Science 347 (2010) 282–289 289

4. Summary and conclusions

This paper provides spectroscopic and electrophoretic data onthe identity of species controlling the cosorption of uranyl andphosphate ions at pH 3.3 on a-Al2O3 colloids. It gives insights intothe effect of the reaction time, U loading, and P surface coverage,whose increase (in absence of U) favors the surface precipitationof Al-phosphate after the formation of phosphate surface complex[11]. The in situ surface speciation study reported here is evidencethat, in the presence of phosphate ions, uranyl sorbed on a-Al2O3

via a combination of surface reactions of complexation and precip-itation, with the predominant species being mainly dependent onphosphate surface coverage. Under conditions of low P loadingfavorable to the formation of phosphate surface complexes, thesorption of uranyl was found to be limited. Because they impartedto surface less negative charges than the U-free phosphate com-plexes, the uranyl sorption species forming at low P coverage werelikely uranyl complexes and/or ternary complexes in which onephosphate group is directly coordinated to the surface. Increasingthe extent of surface site occupancy by phosphate promoted dra-matically the uranyl uptake and favored the formation of speciesimparting high negative surface charges, such as surface precipi-tates of uranyl phosphate. Formation of uranyl phosphate surfaceprecipitates at high P surface coverage was inferred from assign-ment of ATR-FTIR bands at 1107, 1024, and 971 cm�1 (whose posi-tions were insensitive to the aqueous concentrations of U and P) toP–O-stretching vibrations for phosphate coordinated to uranyl, at

the alumina surface. The ATR-FTIR study showed that uranyl phos-phate formed at the surface of highly P-loaded a-Al2O3 whateverthe rate of transformation of alumina into surface precipitates ofAl-phosphate. It also evident that the precipitation of uranyl phos-phate is possible for low aqueous concentrations of phosphate anduranyl (including traces of uranyl), when occurring at oxide sur-faces. To our knowledge, this is the first published paper that re-ports the in situ identification of precipitates of uranyl phosphateat the oxide/solution interface and that provides spectroscopic evi-dence for the formation of such precipitates at trace levels ofuranyl.

These spectroscopic results, although obtained at a very low pHvalue (3.3), are relevant to soils and sediments at acidic pH wherethe cosorption of phosphate and uranyl ion is known to play a rolein the long-term retention of U. They stress that precipitation ofuranyl phosphate occurs at the surfaces of natural Al-oxides, i.e.,on oxide surfaces transforming into Al-phosphate, for concentra-tions of P and U in soil solution that are much below those corre-sponding to thermodynamic equilibrium. Such knowledge must beintegrated in geochemical codes in order to improve the reliabilityof predictive modeling of uranyl cycling in soils.

Acknowledgments

This work was supported by REALISE, the ‘‘Alsace Region Re-search Network in Environmental Sciences and Engineering” andthe Alsace Region. We also thank GNR Paris for financial support.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.jcis.2010.03.045.

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