FTIR characterization of amorphous uranyl-silicates.

FTIR characterization of amorphous uranyl-silicates ☆

D.Gorman-Lewis a ,S.Skanthakumar a ,M.P.Jensen a ,S.Mekki b ,K.L.Nagy b ,L.Soderholm a ,?

a Chemical Sciences and Engineering Division,Argonne National Laboratory,Argonne,IL 60439,USA

Department of Earth and Environmental Sciences,845W.Taylor St.,MC-186,University of Illinois at Chicago,Chicago,IL 60607-7059,USA

A B S T R A C T

A R T I C L E I N F O Article history:

Received 20November 2007

Received in revised form 8May 2008Accepted 8May 2008Editor:J.Fein Keywords:Uranium Soddyite Amorphous FTIR

Precipitation Uranyl-silicate

Ambient-temperature environments in which dissolved silica and U(VI)are present may lack the conditions necessary to readily form crystalline uranyl-silicate phases;however amorphous phases,as de ?ned by the absence of well-de ?ned Bragg re ?ections in powder X-ray diffraction patterns,are kinetically favored when solution saturation levels are appropriate.Such amorphous uranyl-silicates may be related to the crystalline phases predicted to be thermodynamically stable and in ?uence the mobility of U in the environment.

To investigate amorphous uranyl-silicates and their relation to crystalline phases we precipitated solids from solutions containing 0.05M UO 2(ClO 4)2and 0.1M Na 2SiO 3adjusted to pH values from 2.2to 9and allowed the precipitates to age in their mother liquors for approximately 6weeks at 22°C.We compared the chemical composition,X-ray diffraction patterns,and Fourier transform infrared spectra of the precipitates to those of the crystalline phases predicted by thermodynamic modeling.The precipitates were amorphous with U:Si ratios of 0.8±0.1.Their FTIR spectra revealed changes in the UO 22+and SiO 44?vibrations as a function of pH that are consistent with a shift in mid-range structural linkages from those similar to soddyite to those more like Na-boltwoodite.Structural H 2O,OH,and SiO 3OH 3?vibrations do not change as a function of pH and are consistent with a mixture of soddyite-like and Na-boltwoodite-like features.Six weeks of aging at ambient temperature is enough time for the precipitate structures to rearrange and adopt mid-range structural linkages characteristic of crystalline phases predicted by thermodynamic modeling.

1.Introduction

Many studies have been focused on the thermodynamic properties and crystal structures of uranyl-silicate minerals and their importance to the mobility of U(VI)in the environment.The impetus to investigate these crystalline phases comes from their presence in or near uraninite deposits,contaminated sites,and as experimental products of simulated geologic repository conditions (Nguyen et al.,1992;Giammar and Hering,2002;Burns and Klingensmith,2006;Gorman-Lewis et al.,2007).Uranyl-silicates synthesized from aqueous solutions typically must be aged under mild hydrothermal conditions in order to overcome kinetic barriers that would otherwise prevent their transformation to highly crystalline thermodynamically stable phases (Wronkiewicz et al.,1992,1996;Finch et al.,1999;Catalano et al.,2004).While the body of knowledge surrounding crystalline uranyl-silicates continues to grow relatively little is known about

amorphous uranyl-silicates and their effects on the mobility of U(VI)in the environment.Understanding the short-range structure and transformation kinetics of amorphous uranyl-silicates is necessary for determining how U(VI)is transported and ultimately distributed in natural settings.Many environmental settings in which dissolved silica and U(VI)are present lack the conditions necessary for the formation of crystalline uranyl-silicates phases;however,amorphous uranyl-silicates may form readily when solutions are suf ?ciently supersaturated.This relationship between crystalline and amorphous phases may be particularly important for U(VI)mobility.If present even for short time frames,amorphous phases could in ?uence the mobility of U(VI)before they undergo thermodynamically-driven rearrangement.

Dissecting the relationship of amorphous phases,as de ?ned by the absence of well-de ?ned Bragg re ?ections in powder X-ray diffraction (XRD)patterns (Egami and Billinge,2003),to their crystalline counter-parts requires an understanding of precipitation mechanisms.When the saturation state of a solution is suf ?ciently high kinetics dominates precipitation and a nucleation-controlled process results in rapid production of an amorphous phase (Steefel and Vancappellen,1990).This drastically reduces the saturation state of the solution.In classical crystallization theory,once the saturation state of the solution has decreased the solution enters a regime in which the thermodynami-cally preferred crystalline phase begins to form.Ultimately,the forma-tion of amorphous phases may be a necessary ?rst step to forming the

Chemical Geology 253(2008)136–140

☆The submitted manuscript has been created by UChicago Argonne,LLC,Operator of Argonne National Laboratory (“Argonne ”).Argonne,a U.S.Department of Energy Of ?ce of Science laboratory,is operated under Contract No.DE-AC02-06CH11357.The https://www.360docs.net/doc/cb17557960.html,ernment retains for itself,and other acting on its behalf,a paid-up nonexclusive,irrevocable worldwide license in said article to reproduce,prepare derivative works,distribute copies to the public,and perform publicly and display publicly,by or on behalf of the Government.

?Corresponding author.

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Chemical Geology

j o u r n a l h o me pa g e :w w w.e l s ev i e r.c o m/l o c a t e /c h e mg e o

thermodynamically stable crystalline phases although the details of the process are not understood.

Recently,Soderholm et al.(2008)investigated amorphous uranyl-silicates and crystalline phases formed in aqueous solution–solid phase experiments with high energy X-ray scattering(HEXS).They described a prearranged structural unit,or synthon,that was present in all their samples(solution and solid phase)above pH4.Longer-range structural motifs of the amorphous solids,which in some cases could be related to the motifs observed in uranyl-silicate minerals, depended on how the synthons were linked https://www.360docs.net/doc/cb17557960.html,ing thermo-dynamic modeling,they proposed competitive bonding paths for linking the synthons that agreed with the short-range structures found in the HEXS pair distribution functions(PDF).

To further investigate the role of kinetics in the formation and transformation of amorphous uranyl-silicates we precipitated amor-phous phases from solutions similar to those of Soderholm et al. (2008)except that our precipitates aged in their mother liquors at ambient temperature for46days instead of24h.We analyzed the precipitates with chemical analysis,X-ray diffraction(XRD),and Fourier transform infrared spectroscopy(FTIR).The analytical results in combination with thermodynamic modeling of the experimental systems allowed us to describe the structural character of the amor-phous precipitates and relate the precipitates to crystalline phases expected with aging.

2.Experimental methods

2.1.Sample preparation and chemical analysis

All samples were prepared from stock UO2(ClO4)2and Na2SiO3 solutions.The Na2SiO3solution was prepared by adding anhydrous Na2SiO3to0.17M HClO4to achieve a concentration of0.105M.The UO2(ClO4)2solution was prepared by adding UO3(H2O)0.8to2.2M HClO4and heating gently for approximately60min to achieve a?nal concentration of0.990M U in ca.2.4M HClO4.Samples were prepared by adding50μL of the stock UO2(ClO4)2solution and950μL of the stock Na2SiO3solution to polypropylene centrifuge cones.The initial pH of the resulting solution was3.1.Small amounts of NaOH prepared from50%aqueous NaOH and stored under N2or HClO4were added to adjust pH from2.2to9.0.Substantial precipitates formed in samples from pH5.1to9.0.The resulting solid/solution mixtures were aged for 46days at22±2°C in sealed reaction vessels with minimal headspace. U and Si ratios in the precipitates were measured by dissolving2–5mg of sample in0.1M HClO4and using spectrophotometric methods to analyze U and dissolved silica(Iler,1979;Kramer-Schnabel et al., 1992).

2.2.X-ray diffraction measurements

Powder XRD analyses were obtained on solids from samples at pH5.1to9.0with a Scintag theta–theta diffractometer equipped with a Peltier detector and operating with a Cu tube.Approximately5mg of powder was placed on a zero-background quartz slide with data collected over a2θrange of15to40°with a0.01°step size and6s dwell time.

2.3.FTIR measurements

The room temperature precipitates were aged at22°C for46days in their mother liquor prior to being decanted and left to air dry.FTIR analyses were performed using an IlluminatIR FTIR micro-spectro-meter with a diamond total attenuated re?ectance(ATR)objective in an open atmosphere.Background corrected spectra were measured over a range of400to4000cm?1with2cm?1resolution.Each spec-trum was the cumulated average of121scans.Preparing the sample mounts involved suspending approximately2–5mg of precipitate in ~200μL H2O and pipetting the suspension onto a glass slide.Once the precipitate settled on the slide the excess H2O was wicked away with a tissue and the precipitate was left to air dry.

3.Results and discussion

3.1.Chemical analysis and XRD

U:Si molar ratios in the precipitates were0.8±0.1indicating more Si in the phases than U.The ratios are dif?cult to interpret in the absence of clear mineral phase identi?cation and the presence of non-crystalline phases.The possible occurrence of unquanti?able amor-phous silica in the sample would also hinder interpretation of the molar ratios.The XRD patterns of the precipitates did not display any sharp peaks and only a broad convex feature in the baseline from ca.25to35°M consistent with non-crystalline solids(not shown).

3.2.FTIR

Infrared spectra of crystalline uranyl-silicates can be divided into six spectral regions based on previous studies:1)OH stretching from2900to3600cm?1;2)H2O bending vibrations from1590to 1700cm?1;3)uranyl stretching from800to1000cm?1;4)uranyl bending from180to350cm?1;5)SiO44?stretching from900to 1150cm?1;and6)SiO44?bending from390to570cm?1(Launer,1952; Cjeka,1999).This previous work guided assignment of peaks present in the amorphous precipitates(Table1and Fig.1)and the closest corresponding peaks in crystalline uranyl phases(see Section3.3)are provided for comparison(Table1).Peak intensity is also helpful in the assignment of bands;however,it is in?uenced by sample thickness when using an ATR objective and deviations in peak position of approximately±10cm?1are common(Hofmeister et al.,2000).Of the reference sample peaks in Table1,only those observed by Frost et al. (2006a,b)were obtained using ATR-FTIR spectroscopy.All other peaks were observed on samples typically prepared as KBr pressed pellets and measured with transmission FTIR or Diffusive Re?ectance Infrared Fourier Transform(DRIFT)spectroscopy.Frost et al.(2006a)pointed out that in some earlier reports transmittance IR data were neither placed on?at baselines nor curve-resolved,which may explain some differences among data sets.Samples from pH4.9to6.9produced identical FTIR spectra;consequently,data are shown only for the sample at pH6.9(Fig.1).

All samples display peaks associated with structurally incorpo-rated water(sharp peak at~1630cm?1)as well as surface adsorbed water(broad peak at~1530cm?1).OH stretching from surface adsorbed water,structurally incorporated water,and structurally incorporated OH?groups is apparent from the broad and more de?ned peaks present above3000cm?1(see Table1for speci?c peak position).There is only one substantial change in the spectra with regard to these peaks associated with hydration,that is the peak at ~3580cm?1in the pH6.9sample broadens at pH8.0and becomes a shoulder at pH9.0.

UO22+vibrations in the sample change substantially with an increase in pH.The pH 6.9sample exhibits one anti-symmetric UO22+vibration at~911cm?1and one symmetric UO22+vibration at ~806cm?1.These UO22+peaks are present in the pH8.0sample;how-ever,the peak at806cm?1broadens.At pH9.0there are three sub-stantial changes in the uranyl peaks:1)the peak formally at806cm?1 shifts to a lower wavenumber(773cm?1)and appears as a shoulder rather than a well-de?ned peak,2)new anti-symmetric UO22+ vibrations appear at845and884cm?1that do not appear in the samples below pH9.0,and3)the once well-de?ned anti-symmetric vibration at~911cm?1in the lower pH samples appears as a shoulder in the pH9.0sample.

Three SiO44?vibrations appear in all samples and are insensitive to pH;the peak at~1080cm?1is attributed to amorphous silica,the peak

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at 621cm ?1is ascribed to SiO 44?bending,and the peak at 1382cm ?1originates from SiO 3OH 3?.Varying the pH does however appear to be responsible for the ingrowth of SiO 44?vibrations that we observe in the precipitates,namely a small broad vibration ?rst appears in the pH 8.0sample at ~966cm ?1and transforms into a well-de ?ned peak at 982cm ?1in the pH 9.0sample.

3.3.Precipitation of amorphous samples and relation to crystalline phases

Since crystalline uranyl-silicates are known to form under mild hydrothermal conditions (Nguyen et al.,1992;Moll et al.,1996;Vochten et al.,1997),the precipitates formed at ambient temperatures were not expected to display long range structure.Therefore,we choose to interpret the structures of the amorphous solids in terms of linked synthons that produce short-and mid-range order.Linkage of synthons is a framework for considering these amorphous structures that was put forth recently by Soderholm et al.(2008).

The ?rst step needed to interpret our solids within this framework is to consider the speciation predicted by thermodynamics.Thermo-dynamic predictions depend on the choice of modeled reactions and the assumptions made for the initial solution conditions.For our modeling,we considered the same reactions and equilibrium con-stants as Soderholm et al.(2008),assuming that all samples contained a ?xed amount of CO 32?.This assumption is warranted because although the samples were prepared open to the atmosphere no effort was made to allow them to equilibrate with atmospheric CO 2after they were sealed in closed reaction vessels.Consequently,we as-sumed a ?xed amount of CO 32?based on the amount of CO 2in the headspace over the solution in the reaction vessel.In addition,the only solid phases that can be considered in thermodynamic modeling are those for which published thermodynamic data are available;and in this regard,it should be noted there is a lack of solubility data for many known U(VI)phases (e.g.the sodium analogue of compreigna-cite K 2(UO 2)6O 4(OH)6·8H 2O (Kubatko et al.,2006)).The modeling is also limited by uncertainties in the amount of dissolved silica in the system.The proper input to the model for the total silica in the system (solution and solid phase),herein referred to as Si Tot ,is complicated because the initial concentration of dissolved silica,possibly present in monomeric,polymeric,and colloidal species,was well above the amorphous silica solubility limit,reported to range from 10?2.38to 10?2.71(Morey et al.,1964;Walther and Helgeson,1977).Also,the kinetics of amorphous silica precipitation are variable,and approxi-mately 300days are required to reach equilibrium from a super-saturated solution (Morey et al.,1964).Therefore,the assumption that our system is in equilibrium with amorphous silica is not

warranted.

Fig.1.FTIR spectra of samples at pH 6.9(bottom curve),pH 8.0(middle curve)and pH 9.0(top curve).Individual spectra are offset for clarity.

Table 1

FTIR peak assignments and closest correspondence to crystalline phases Sample pH Sample peak±10(cm ?1)a Corresponding peak (cm ?1)Assignment Phase c

Reference 4.9–6.9

621(s)612,615,619,625

SiO 44?(v 4)

Bolt,sodd,sodd,bolt

(d,e,d,f)806(m)

808,810

UO 22+

(v 1)

Sodd (e,d)911(s)905,909,915UO 22+(v 3)Sodd,sodd,bolt (d,e,g)1078(s)1089,1100SiO 44?(v 3)b Am Si (h,i)1382(m)1373,1384SiO 3OH 3?(δ)Na-bolt (f,k)1529(sh)1528,1578H 2O (δ)Sodd (d,e)1628(s)

1624,1625,1630,1637

H 2O (δ)Meta,sodd

and bolt,

Na-bolt,sodd and Na-bolt

(d,e and g,d,d g and f)3228(b)3190,3265,3279,3225OH (v )Bolt,sodd,

bolt,Na-bolt

(j,e,g,f)3395(b)3390,3403,3410

OH (v )Bolt,bolt,

Na-bolt

(d,g,k)3537(m)3511,3520,3527,3565OH (v )Na-bolt,sodd,

bolt,Sodd

(f,d,g,e)3580(m)3565,3580OH (v )Sodd,Na-bolt (e,d)8.0

620(s)612,615,619,625

SiO 44?(v 4)Bolt,sodd,

sodd,bolt (d,e,d,f)806(sh)808,810UO 22+(v 1)

Sodd (e,d)

900(s)900,905

UO 22+

(v 3)Sodd (e,d and e)966(sh)963,965,966SiO 44?(v 3)Sodd (l,k,e)1085(s)1089,1100SiO 44?(v 3)b Am Si (h,i)1382(m)1373,1384SiO 3OH 3?(δ)Na-bolt (f,j)1525(sh)1528,1578H 2O (δ)Sodd (d,e)1628

(s)

1624,1625,1630,1637H 2O (δ)Meta,sodd and

bolt,Na-bolt,sodd and Na-bolt

(d,e and g,d,d and f)3324(b)3190,3330OH (v )Bolt (j,g)3395(b)3390,3403,3410

OH (v )Bolt,bolt,

Na-bolt

(d,g,k)3532(m)3520,3527,3565

OH (v )Sodd,bolt,

Sodd

(d,g,e)3584(m)3565,3580OH (v )Sodd,Na-bolt (e,d)9.0

620(s)612,615,619,625

SiO 44?(v 4)Bolt,Sodd (d,e,d,f)773(sh)780,783UO 22+(v 1)Na-bolt,bolt,sodd,bolt

(m,g)845(s)830,851,853UO 22+(v 3)Bolt,Na-bolt,bolt (g,f,d)884(m)873,880,900UO 22+(v 3)Bolt,Na-bolt,Sodd (g,m,e)903(sh)900,904,905UO 22+(v 3)Sodd

(e,d,k and e)982(s)

971,978,987,995SiO 44?(v 3)

Sodd,bolt and Na-bolt,bolt,Na-bolt

(e,g and f,d,m)1078(s)1089,1100SiO 44?(v 3)b Am Si (h,i)1382(m)1373,1384SiO 3OH 3?(δ)Na-bolt (f,j)1533(sh)1528,1578H 2O (δ)Sodd (d,e)1627(s)

1624,1625,1630,1637H 2O (δ)Meta,sodd and

bolt,Na-bolt,sodd and Na-bolt

(d,e and g,d,d and g)3324(b)3190,3330OH (v )Bolt (k,e)3395(b)3390,3403,3410

OH (v )Bolt,Na-bolt (m,g,k)3532(m)3516,3520,3527

OH (v )Sodd,sodd,

bolt

(e,d,g)3581(sh)

3565,3580

OH (v )

Sodd,Na-bolt

(e,m)

s =Strong;m =medium;w =weak;sh =shoulder;b =broad;v =stretching vibration;δ=bending vibration.b

Speci ?c vibration assignments in this work.c

Sodd =soddyite;bolt =boltwoodite;Na-bolt =sodium boltwoodite;meta =metaschoepite;am Si =amorphous silica.d

Cjeka (1999).e

Frost et al.(2006b).f

Chernorukov and Kortikov (2001).g

Frost et al.(2006a).h

Liu et al.(2007).i

Nguyen et al.(1992).j

Vochten et al.(1997).k

Plesko et al.(1992).l

Moll et al.(1995).m

Gevorkyan et al.(1979).

138 D.Gorman-Lewis et al./Chemical Geology 253(2008)136–140

Although the initial Si Tot in these systems was 0.1M,it was not possible to determine how much of the Si Tot precipitated out as amorphous silica versus uranyl-silicate.Analysis of dissolved silica concentrations in the mother liquor (0.03±0.01M)clearly indicates the solutions remained well above the amorphous silica solubility limit.Although amorphous silica is present in the systems (see above),it is clearly not in equilibrium;therefore,we chose to model our systems with the dissolved silica concentration in the mother liquor of 0.03M.

The calculated distribution of UO 22+species as a function of pH for the experimental composition (0.05M UO 22+,0.03M H 4SiO 4,0.0003M CO 32?,0.2M Na +)shows soddyite ((UO 2)2SiO 4·2H 2O),sodium boltwoodite (Na(UO 2)(SiO 3OH)·1.5H 2O),and metaschoepite (UO 3(H 2O)2)as the stable phases (Fig.2).Sodium boltwoodite is the sodium analogue of boltwoodite (K(UO 2)(SiO 3OH)·1.5H 2O)and there likely exists an entire solid-solution series between the two phases (Burns,1998).The dominant U(VI)solid phase is also a function of the Si Tot in the system (Fig.3).Above pH 5the uranyl-silicates are only dominant when Si Tot is relatively high.At lower Si Tot ,metaschoepite becomes the dominant UO 22+solid.

Based on possible competitive bonding paths outlined by the thermodynamic calculations it is possible to interpret structural motifs in the amorphous precipitates and relate them to the stable crystalline phases (Table 1).The ?rst observation is that UO 22+speciation is a strong function of pH.The FTIR spectra clearly indicate a progressive change in the precipitates with the ingrowth and disappearance of peaks as a function of increasing pH of the initial experimental solution as discussed above.The correspondence of the peaks in our samples with those of crystalline phases indicates comparable structural environments.

Samples at pH ≤8have two UO 22+vibrations that correspond to a soddyite-like UO 22+structural environment (Table 1).The pH 9sample has four UO 22+vibrations that correspond to peaks found in boltwoodite,Na-boltwoodite,and soddyite.The increase in the number of UO 22+vibrations indicates that as the pH increases the structural environment of UO 22+changes;however,the solid does not acquire a singular structural motif.The SiO 44?environment also undergoes considerable transformation with an increase in pH.The appearance of a SiO 44?peak that ?rst appears in the pH 8sample at 966cm ?1and by pH 9is a strong peak at 982cm ?1corresponds to a peak in soddyite at pH 8and a peak in Na-boltwoodite at pH 9.Changes in both the UO 22+and SiO 44?mid-range linkages are consis-tent with those predicted by the thermodynamic modeling,which is a shift in structural characteristics moving from a dominantly soddyite-

like environment at lower pH to a more boltwoodite/Na-boltwoodite-like environment as pH increases.

The peaks (SiO 3OH 3?,H 2O,and OH)that are persistent as a function of pH may be indicative of structural environments with energetic barriers that cannot be overcome by a 6-week aging period.The sensitivity of FTIR to SiO 3OH 3?allowed us to identify this persistent structural motif in all our samples.SiO 3OH 3?is commonly found in uranyl-silicate phases that possess an interlayer cation such as Na-boltwoodite,uranophane (Ca[(UO 2)(SiO 3OH)]2(H 2O)5),and sklodows-kite (Mg[(UO 2)(SiO 3OH)]2(H 2O)6)(Burns,1999).However,in our samples it is not possible to de ?nitively assign the presence of SiO 3OH 3?groups to the uranyl-silicates due to the presence of amor-phous silica.

Structurally bound H 2O and OH peaks do not change with pH.These groups may require mild heating and/or more time to overcome the kinetic barriers preventing rearrangement from the mixture of mid-range linkages found in the samples that correspond to both Na-boltwoodite and soddyite and to a singular structural environment found in a pure phase.The peaks from surface bound H 2O are also persistent;however,we would not expect those to change unless the precipitates were kept in a moisture free environment or treated with a dehydrating agent.

It is dif ?cult to glean any information from the persistent SiO 44?bending vibration at ~620cm ?1because it is consistent with both boltwoodite and soddyite.In addition,it is also dif ?cult to make any interpretations from the vibration associated with amorphous silica since amorphous silica has been shown to form in hydrothermal syntheses that induce arti ?cial aging of uranyl-silicates to overcome kinetic barriers (Nguyen et al.,1992).The amorphous silica persistence in our system is primarily a result of the high concentration of Si Tot in the system and not in and of itself a kinetic barrier to the restructuring of uranyl-silicates.

Soderholm et al.(2008)identi ?ed a uranyl-silicate synthon that was present in their all their experimental solutions and solids.The linkage of synthons dictated the mid-range correlations observed in their amorphous samples.Their hydrothermally treated solid pro-ducts (4days at 150°C)were all amorphous,with the exception of the sample at pH 5.They observed a transition from soddyite and soddyite-like precipitates at low pH to more Na-boltwoodite-like precipitates at higher pH,a transition we also observed in our ambient-temperature samples.The similarity in the preparation of our experimental samples to those of Soderholm et al.(2008)and the observation of similar structural characteristics suggests

their

Fig.2.Uranyl species distribution as a function of pH under the experimental conditions (0.05M UO 22+,0.03M H 4SiO 4,0.0003M CO 32?,0.2M Na +,ionic strength 0.2

M).

Fig.3.Plot of dominant uranyl phase as a function of total H 4SiO 4in the system under the following conditions of 0.05M UO 22+,0.0003M CO 32?,0.2M Na +,and ionic strength of 0.2M.When no solid phase is present,the most abundant solution species is shown.

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previously identi?ed synthon is also present in the samples discussed herein.

Interpreting our results in this framework allows us to relate our amorphous precipitates to the thermodynamically predicted phases on the molecular level.This level of understanding is necessary in order to determine the impact amorphous phases may have on the mobility of U(VI)in the environment.The transformation of the linkages between synthons in the amorphous precipitate to produce thermodynamically predicted phases is a central factor in determining the potential environmental impact of amorphous phases composed of synthon aggregates.In our work we observe this crucial transition toward the thermodynamically predicted phases beginning within 6weeks.This prearranged uranyl-silicate building block may suf?ciently lower the energetic barriers of the UO22+and SiO44?environments to allow the changes we observe from a dominantly soddyite-like environment at lower pH to a more boltwoodite/Na-boltwoodite-like environment as pH increases.In addition,6weeks is a relevant time frame in terms of accidental release of U into the environment.The energetics that favor crystallization ultimately serve to further stabilize the solid phases thereby limiting U(VI)mobility.

4.Conclusions

Analyzing samples aged for6weeks with FTIR provided a direct probe of H2O,OH,and SiO4?groups within the structural environment of the solid as well as a tool to investigate the UO22+environment.We determined that a6-week ambient ambient-temperature-aging period is enough time to allow the mid-range linkages involving UO22+and SiO44?groups within the precipitates to adopt structural similarities to crystalline phases predicted by thermodynamic modeling.The similarities represent a shift from structural environ-ments more like soddyite to those more like Na-boltwoodite.The persistent peaks in the FTIR spectra produced by H2O,OH,and SiO3OH3?groups may be indicative of structural environments with energetic barriers that cannot be overcome in six weeks of aging at ambient temperature.The persistent structural mid-range features in the precipitates are a mixture of those from soddyite and Na-boltwoodite indicating that more time or heat is necessary to cause complete thermodynamic structural rearrangement.Our results give a sense at the molecular level of the effect aging has on the transfor-mation of amorphous precipitates to crystalline uranyl-silicates.The results highlight the potential signi?cance of these amorphous materials and as well as the time and temperature dependence of their structural characteristics.Should they form in the environment, they are likely to have a direct short-term and long-term impact on U(VI)mobility as they transform into thermodynamically stable crystalline phases.Further characterization of these amorphous phases and the solutions from which they form as a function of time are necessary to fully understand the impact amorphous uranyl-silicates have on the fate and transport of U(VI).

Acknowledgements

This work was funded by the U.S.Department of Energy,Of?ce of Biological and Environmental Research,Environmental Remediation Sciences Program under grant number DE-FG02-06ER64193to the University of Illinois at Chicago and Contract DE-AC02-06CH11357to Argonne National Laboratory.

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