A molecular dynamics study of uranyl-carbonate complexes adsorbed on basal surfaces of clay minerals

Lei Li·Xiandong Liu·Xiancai Lu

A molecular dynamics study of uranyl-carbonate complexes adsorbed on basal surfaces of clay minerals

Lei Li·Xiandong Liu·Xiancai Lu

We use molecular dynamics simulation to study the mechanisms involved in the adsorption of aqueous uranyl species）to the basal surfaces of clay minerals，including kaolinite，pyrophyllite and montmorillonite.Uranyl ion can form various complexes with carbonates，namely，［UO2（H2O）5］2+，［UO2（H2O）3（CO3）］，［UO2（H2O）2（CO3）2］2-，［UO2（CO3）3］4-.The simulations show that at aqueous clay interfaces，both uranyl species and surface type control the adsorption pattern.The noncarbonato and monocarbonato uranyl species can form outer-sphere complexes on siloxane surfaces through electrostatic interaction，but the dicarbonato and tricarbonato uranyl complexes rarely adsorb on the siloxane surfaces.Strong outer-sphere adsorptions of the uranylcarbonate complexes on gibbsite surfaces are observed，which are fixed by hydrogen bonds between the ligands（carbonate and/or H2O）and surface hydroxyls.The sorption behaviors derived in this study provide new insights into understanding the migration and enrichment of uranium and other radionuclides.

Clay mineral·Uranyl·Carbonate· Microscopic structure

1　Introduction

The fate of chemical and radioactive wastes in the environment is related to the ability of subsurface minerals to immobilize these contaminants by processes like adsorption and precipitation.Clay minerals are considered as potential host rocks for radioactive waste repositories，protons，organic molecules（Greenwell et al.2006；Teppen et al.1998）. Thanks to unique physicochemical properties of their relatively large surface areas and substrate surface reactivity（Kalinichev et al.2007；Teppen et al.1997；Tunega et al.2002；Wang et al.2006），clays are able to control the migration and adsorption behavior of radionuclides in soils and groundwater systems（Vasconcelos and Bunker 2007）.

Uranium is a major contaminant in soil and groundwater as a result of burning coal，uranium-ore mining and its subsequent utilization to manufacture reactor fuel and nuclear weapons（Riley and Zachara 1992）.The transport of uraniumandotherradioactivesubstancestothebiosphereas dissolved constituents in the groundwater has drawn widespread concern，which is of critical importance in the environmental remediation of contaminated sites.It is wellknown that uranium has various oxidation states（i.e.，III—VI）and the transport behavior of uranium in groundwater strongly depends on its oxidation states.Trivalent and tetravalent uranium carbonato complexes are regarded as insoluble substances in groundwater（Ciavatta et al.1983；Grenthe and Lagerman 1991），and pentavalent uranium is unstable in an aqueous solution without a high carbonate content（Privalov et al.2003；Wander et al.2006）.In contrast，hexavalenturanium，existingastheuranylionforms soluble carbonate species in groundwater.

Carbonate ions are one of the most important natural complexing ligands due to their significant concentrationsin many natural waters and their strong actinide complexing ability.Some actinide nuclides have extremely longlived radiotoxicity and can form various types of complexes with carbonate ions，which exist plentifully in groundwater during their long-term transport in the geosphere.Therefore，carbonate complexes of actinides are most significant species to understand（Clark et al.1995）.

L.Li·X.Liu（?）·X.Lu

State Key Laboratory for Mineral Deposits Research，School of Earth Sciences and Engineering，Nanjing University，

Nanjing 210093，People's Republic of China

e-mail:xiandongliu@nju.edu.cn

L.Li

e-mail:lilei_earth@126.com

Uranyl adsorption onto a variety of substrates has been studied experimentally.These materials include clays（Ames et al.1983；Bachmaf and Merkel 2010；Catalano and Brown 2005；Duff and Amrhein 1996；Krepelova et al.2007，2008；Marques Fernandes et al.2012；Walter et al.2005），manganese oxides（Wang et al.2013），aluminium oxides（Catalano et al.2005），hematite（Bargar et al.1999，2000），ferrihydrite（Rossberg et al.2009），goethite（Sherman et al. 2008；Walter et al.2003），calcite（Elzinga et al.2004），granite（Baik et al.2003），silica/quartz（Drot et al.2007；Gabriel et al.2001；Lieser et al.1992；Sylwester et al.2000）and zeolites（Godelitsas et al.1996）.But batch experiments only study the macroscopic aspects of the interaction of uranium with the mineral surface such as adsorbing capacity and give little direct information on the structure and local chemical environment of the adsorbed species.In the case of uranyl adsorption onto mineral surfaces，X-ray absorption spectroscopy（XAS），which includes X-ray absorption nearedge structure（XANES）and extended X-ray absorption fine structure（EXAFS）spectroscopy，has been used to identify the number of nearest-neighbor equatorial ligand atoms and their distance to the uranium atom in surface complexes（Denecke et al.2003；Hudson et al.1999；Krepelova et al. 2008）.Outer-sphere surface complexes of various metals（including actinides）are studied experimentally with resonant anomalous X-ray reflectivity（RAXR）measurements（Fenter et al.2010；Park et al.2008；Schmidt et al.2012）. However，with these experimental measurements，it is still difficult to determine how the adsorption occurs.

Molecular simulation techniques can serve as a useful complement to sorption experiments and spectroscopic methods.The sorption behavior of specific species on different minerals can be studied directly through simulations of realisticmodelsofmolecularcomplexesandmineralsurfaces（Boek 2014；Boily and Rosso 2011；Doudou et al.2012；Glezakou and deJong 2011；Kremleva et al.2008，2012；Lectez et al.2012；Liu et al.2013b；Steele et al.2002）. Greathouse andco-workersused moleculardynamics simulation to study the structure and dynamics of the uranyl ion and its aquo，hydroxyl，and carbonate complexes in bulk water and nearthe hydratedquartz（010）surface（Greathouse et al.2002）.They found that the uranyl coordination shell exhibits pentagonal bipyramidal symmetry，with carbonate and hydroxide ions readily replacing water molecules in the first shell.They considered adsorption simulations as two kinds:outer-sphere surface complexes formed at the singly protonated surface and inner-sphere surface complexes formedatthepartiallydeprotonatedsurface.Inthepresenceof carbonate ions，an inner-sphere surface complex with monodentate surface coordination formed when only one carbonate ion was coordinated toThe dicarbonato complex never formed an inner-shere surface complex but remained anchored to the surface via hydrogen bonding between the coordinated water molecule and the surface. Greathouse and Cygan studied the adsorption of aqueous uranylspeciesontoanexternalmontmorillonitesurfaceinthe presenceofsodiumcounterionsandcarbonatoligandsin2005（Greathouse and Cygan 2005）.As concentration of uranylcarbonate complexes was increased，uranyl adsorption tendencywasfoundtodecrease.YangandZaouiinvestigatedthe adsorption of uranyl species［UO2（H2O）5］2+onto kaolinite（001）surfaces，especially onto Al（O）kaolinite surface（Yang and Zaoui 2013）.By using molecular dynamics simulations，they studied outer-sphere complexes adsorption and innerspherebidentate complexesadsorptionatdeprotonatedshortbridgesites（O—Al—O）andlong-bridgesites（AlO—AlO）.With density functional theory calculations Martorell et al.found the same possible uranyl adsorbing sites（Martorell et al. 2010）.Kerisit and Liu performed molecular dynamics（MD）simulations to investigate uranyl adsorption onto two neutral aluminosilicatesurfaces，namely，theorthoclase（001）surface and the octahedral aluminum sheet of the kaolinite（001）surface in the presence of carbonates.They found that uranyl carbonate surface complexes were unfavored on the octahedral aluminum sheet of the kaolinite surface（Kerisit and Liu 2014）.Their conclusions werederivedunder its initial setting thaturanylcarbonatecomplexesadsorbedonthesurfacewith direct bonds between uranium and surface atoms.

In the above studies，some kinds of uranyl carbonate complexes，for instance，tricarbonato complex（Kubicki et al.2009），and direct contact between carbonates and surfaces have not been considered.Hence，in this study，we use molecular simulations to study the sorption of uranylcarbonate complexes on basal surfaces of three clay minerals，namely，kaolinite，pyrophyllite，and montmorillonite. These minerals are highly sorptive due to their small particle sizes，large surface areas，and chemically active surface sites（Vasconcelos and Bunker 2007）.Pyrophyllite and montmorillonite are model 2:1 clay minerals.Their two basal surfaces are both siloxane surfaces，which are common and representative surfaces in clay minerals. Additionally，the effects of electrostatic forces between surface and complexes can be studied by comparison between neutral pyrophyllite layer and charged montmorillonite layer.Kaolinite is a 1:1 layer clay composed of a repeating layer of a gibbsite surface and a siloxane surface. Due to its different surfaces，we can find the difference between them on complexes adsorption，which is of great importance.The doubly coordinated OHs on the gibbsitesurface of kaolinite have mineral chemical reactivity and the properties of gibbsite surfaces are pH-independent（Liu et al.2013a；Schoonheydt and Johnston 2006；Sposito 1984）.In our previous study，we found these OHs have an extremely high pKa（about 22.0），which indicates that these OHs do not deprotonate in common pH（Liu et al.2013a）. Therefore，we do not consider the deprotonation of gibbsite surface of kaolinite in this work.

In this study，we employ molecular dynamics simulation to simulate the process that uranyl-carbonate complexes adsorb on the clay surfaces.By analyzing the atomic density profiles and micro-structures of these complexes，we reveal the structures of adsorbed uranyl-carbonate complexes at the mineral/aqueous solution interfaces.Besides，we find that uranyl-carbonate complexes adsorb on the gibbsite and siloxane surfaces as outer-sphere adsorption due to electrostatic interaction and hydrogen bonds，respectively.These microscopic results provide a fundamental basis for investigating migration and fixation of actinides in the environment in future work.

2　Methodology

2.1Models

Box parameters of all simulations in different systems are listed in Table 1.

Pyrophyllite is a neutral 2:1-type phyllosilicate with the chemicalcompositionAl4Si8O20（OH）4.Pyrophylliteis stacked by‘‘T-O-T''layers consisting of an Al-O octahedral sheet（O）sandwiched between two Si—O tetrahedral sheets（T）.The basic structural elements of pyrophyllite and other 2:1 clay minerals like montmorillonite are similar.We follow the montmorillonite-like conventional orientation and opted for an orthogonally constrained unit cell just like earlier workers（Churakov 2006；Kremleva et al.2012）.The simulation cells contain 32 unit cells（4×4×2 units in a，b，and c dimensions）with a total of 1280 atoms in the solid. Figure 1a shows the simulation model of pyrophyllite system and the dimensions are listed in Table 1.

Ourmodelofmontmorillonitecorrespondstoalow-charge montmorillonite（0.375e/unitcell），withchargesitestotallyin the octahedral layer.The unit cell formula is Na0.375［Si8］［-Al3.625Mg0.375］O20（OH）4，where the first and second bracketedtermsrefertoionsinthetetrahedralandoctahedrallayers，respectively.Montmorillonite model contains 32 unit cells（4×4×2 units in a，b，and c dimensions）（Fig.1b）.The modelcontainstwoclaylayers，eachwith6chargesitesinthe octahedrallayer，which are causedbysubstitutions ofMg/Al. The upper interlayer compositions of montmorillonite are listed in Table 1.The bottom interlayer consists of 100 water molecules and 6 sodium atoms.

Model of kaolinite［Al2Si2O5（OH）4］consisting of 32 unit cells（4×4×2 units in a，b，and c dimensions）is shown in Fig.1c with information of interlayer compositions and box size listed in Table 1.As kaolinite has two different basal surfaces，in the aqueous regions can be placed in two ways:near the siloxane surface and gibbsite surface.So we perform simulations of these two cases，respectively.For each simulation，all aqueous ions are placed randomly within 4 A?from the surface initially.

2.2Force field and MD simulation

The CLAYFF force field（Cygan et al.2004）is used to model kaolinite，pyrophyllite，and montmorillonite，while SPC water model（Berendsen et al.1981；Teleman et al. 1987）for water molecules.CLAYFF and SPC water model have been used successfully to obtain structural and dynamical properties of hydrated mineral systems and their interfaces with aqueous solutions（Greathouse and Cygan 2005；Jinhong et al.2012；Liu and Lu 2006；Liu et al.2008，2009；Perry et al.2007）.Parameters for the aqueous uranyl ion are taken from Guilbaud and Wipff（1993a，b，1996），which have been used to model a variety of aqueous uranyl complexes including carbonate，hydroxide（Greathouse et al.2002），phosplioryl（Hutschka et al.1998），and tri-nbutlyphosphate（Baaden et al.2002）.We employ the carbonate parameters in the study（Greathouse et al.2002），which was modified to obtain satisfactory configurations of uranyl carbonate complexes.

Table 1 Initial parameters of all simulation boxes

Fig.1 Initial configurations of different clay/aqueous systems，namely，a pyrophyllite，b montmorillonite，c kaolinite.Atoms are colored as follows:O（red），H（white），Na（purple），Cl（palegreen），Si（yellow），Al（solferino），Mg（green），U（blue）

All simulations are carried out with the DL_POLY 2.20（Smith and Forester 1996）program on a parallel Linux cluster.We impose periodic boundary conditions on the three dimensions in all simulations.We use the Ewald summation method（Frenkel and Smit 2002）to calculate long-range electrostatic interactions.Also a cutoff of 9 A?is used for short-range interactions.The equations of motion are integrated by using leapfrog's algorithm with time step of 1.0 fs.In each simulation we first run NPT（number of atoms，pressure 1 atm，and temperature 298 K）simulations for at least 500 ps to equilibrate the solid/liquid interfaces. Then，1 ns NVT（number of atoms，volume，and temperature 298 K）simulations are performed to record the trajectories for statistical analysis.

3　Results and discussion

3.1In bulk water

Uranyl forms fivefold complex with 5 water molecules in bulk water without carbonate.It is also the most common coordination number found in experiments（Sylwester et al. 2000）and other simulations（Greathouse et al.2002）.［UO2（H2O）5］2+has a pentagonal bipyramidal geometry，with uranyl oxygen atoms in the axial positions（Fig.2a）. The average U-Ow（Ow stands for water oxygen）distance of 2.48 A?（Fig.2a）agrees well with the experimental value of 2.41（2）A?，obtained from EXAFS spectroscopy experiments（Hennig et al.2005）and agrees with the MD simulation value of 2.49 A?（Greathouse et al.2002；Yang and Zaoui 2013）.

For simulations of carbonate complexes，carbonates are initially placed at about 8 A?away from the uranyl ion and we find that they quickly entered the first coordination sphere upon equilibration（Fig.2）.Carbonate ions form bidentate complexes with uranium（Fig.2b）.The presence of carbonate in the uranyl complex results in a split equatorialshellabouttheuraniumatom，withoxygenatomsfrom carbonate occupying positions closer than water oxygen atoms（Fig.2b）.Whentwocarbonateionsarecoordinatedto，thefirstcoordinationshellofuranium turnstosixfold coordination shell with four carbonate oxygen atoms and two water oxygen atoms（Fig.2c）.In the presence of 3 carbonates，the first coordination shell is occupied by carbonates only（Fig.2d）.The equatorial U-Oc（carbonate oxygen）distances are 2.36—2.40 A?（Fig.2b，c，d），agreeing well with previous simulations for［UO2（H2O）3（CO3）］and［UO2（H2O）（CO3）2］2-（2.37-2.39 A?for［UO2（H2O）3（CO3）］and［UO2（H2O）（CO3）2］2-）（Greathouse et al.2002）and EXAFS experiments（2.44 A?for［UO2（CO3）3］4-）（Ikeda et al.2007）.

3.2Interfaces

3.2.1Atomic density profiles

An atomic density profile（Boek et al.1995；Greathouse and Cygan 2005；Perry et al.2007；Skipper et al.1995）ρi（z）is a measure of the probability of finding an atom of type i along the c axis，at distance z from a fixed origin. Atomic density profiles were obtained by time-averaging the z distribution of all atoms during the 1 ns simulation. Compared with density profiles of simple ions（i.e.，Li+，Na+，K+）reported in previous studies（Boek et al.1995；Skipper et al.1995），we need to focus on the whole complexes.Thus we can find out the spatial relationship between the complex and surface，which can help us deduce the possible form of adsorption.So we choose four kinds of atoms（uranium，uranyl oxygen，carbon，and carbonateoxygen）into consideration.Corresponding atomic density profiles are shown in Fig.3.

Fig.2 Equilibrium snapshots of uranyl complexes resulting from the simulations ofand 320 water molecules withcarbonate molecules.The result complexes are a Complexes are shown as large balls，and free water molecules as sticks.Atomic designations are U（blue），O（red），C（gray）and H（white）

Figure 3a shows atomic density profiles of the four ionic groups in pyrophyllite system.Both surfaces represent the tetrahedral SiO4surface with the oxygen atoms relaxing outwardslightlyandthesurfacehasaverysmallnetnegative charge，whichattractsionstowardit（Vasconcelosand Bunker 2007）.In the study of Vasconcelos and Bunker，they foundthe similarbehaviorofCs+ontheSiO4surface.In the absence of carbonates，the high peaks of uranium and uranyl oxygen curves suggest that the uranyl adsorbs on the layer. Two split peaks of uranyl oxygen around the high uranium peakrevealthattheuranylisnotparallelwiththesurfaceand one uranyl oxygen is closer to the surface.The curves spreading all over the z direction illustrates that the ions are stillactiveandtheadsorptionisnotstrong.Inthepresenceof carbonates，themobilityofthecomplexesislimitedasshown by smaller curve ranges.With 1 or 2 carbonates，the uranyl adsorption found above still exists.Curves of carbon and carbonate oxygen are found a little farther from the surface compared with those of uranium and first uranyl oxygen，which means carbonate is not close to the siloxane layer.In case of dicarbonato uranyl complex，the uranium peak is significantly wider，which means the adsorption isnotstable and the complex is easy to diffuse into the solution.The tricarbonato complex wanders in the middle aqueous layer，and no adsorption exists.The carbonate molecules drive the uranyl ions away from the siloxane surface，caused by the repulsion between the negative carbonates and the outward negative Si-bridging oxygen.

Fig.3 Atomic density profiles of interlayer space.The four figures stand for different systems shown as follow:a pyropyhllite，b montmorillonite，c kaolinite-siloxane，d kaolinite-gibbsite.Color key:red（uranyl oxygen），blue（uranium），black（carbon），cyan（carbonate oxygen）

In Fig.3b of montmorillonite，both surfaces are siloxane surfaces just like in the pyrophyllite.But due to the replacement of Al3+by Mg2+，the surfaces are negatively charged and free counter ions are present in the aqueous layer，which play an important role in cations adsorption.When no carbonate is in the system，the uranyl adsorption is found stable near the siloxane layer.With one carbonate，the curves of uranium and uranyl oxygen extend to a larger region.It indicates that the complex is not stable on the siloxane layer and likely to diffuse away.In cases of dicarbonato and tricarbonato complexes，the whole complexes tend to move far from the negatively charged surface due to the electrostatic repulsion.Withmoreandmoreelectronegativecarbonate，the electronegativity of complexes increases and the repulsion between complexes and the negatively charged surface increases.So the complexes with more coordinated carbonates are harder to get close to the surface.

As kaolinite has two different basal surfaces，the adsorption can be divided into two cases:near the siloxane surface and near the gibbsite surface.

In the kaolinite-siloxane system（Fig.3c），the adsorptions of noncarbonato and monocarbonato complexes are still found.In cases of dicarbonato and tricarbonato complexes，the complexes wander in the diffuse region and tend to approach to the gibbsite surface.Because of electrostatic interaction mentioned above，the carbonate complexes tend to keep away from the siloxane surface.

At the kaolinite-gibbsite surface，some different phenomena can be found（Fig.3d）.In the absence of carbonate，the uranyl complex goes to the siloxane surface and adsorbs on it，just like those cases observed in other systems.In the presence of carbonates，the complexes are found to adsorb on the gibbsite layer mainly，corresponding to the obvious peaks of the curves.In all three conditions with carbonates，we find that one carbonate oxygen peak is closest to the gibbsite layer.It indicates that these uranylcarbonate complexes adsorbing on the surface in these system can be attributed to some interactions between carbonate oxygen and the gibbsite layer surface.In the case of monocarbonato complex，the tails of atomic densitypeaks extend to the siloxane surface and tiny peaks can be found in the region where curves of none-carbonate condition exist.It suggests that adsorption on siloxane surface is weak.But the high and narrow peaks near the gibbsite layer illustrate that adsorption mainly happens on the gibbsite surface.For dicarbonato and tricarbonato complexes，the tails into the solution disappear，which means the complexes are fixed firmly on the gibbsite layer.

Fig.4 Equilibrium snapshots of uranyl complexes in the pyrophyllite system.The complexes are a［UO2（H2O）5］2+，b［UO2（H2O）3（CO3）］，c［UO2（H2O）2（CO3）2］2-，d［UO2（CO3）3］4-.Complexes are shown as large balls，free water molecules and mineral molecules as sticks.Atomic designations are U（blue），O（red），C（gray），Si（yellow）and H（white）

3.2.2Micro-structures of adsorbed uranyl complexes

Figure 4 shows the equilibrium snapshots of uranyl complexes in the pyrophyllite system.In Fig.4 a and b，we find the complexes are near the surface.From the simulation trajectories，we find thattheresidencetime can bemore than 500 ps.The uranyl molecule slants above the surface with oneuranyloxygenclosertothesiloxanesurface，specifically point to the di-trigonal cavity（Fig.4a，b）.For the monocarbonato complex，the adsorption site is found to be a little farther than that of［UO2（H2O）5］2+.The carbonate ligand is farther from the surface than other coordinated water molecules，which is due to the repulsion between carbonate and negatively charged surface.For the same reason，the complex of［UO2（H2O）2（CO3）2］2-with stronger electronegativity is found to adsorb on the surface at a farther distance（Fig.4c）.Due to the farther distance，the electrostaticattraction betweenuranylandsurfaceisweaker，hence the dicarbonato complex can not adsorb on the surface for a long time（at most 400 ps in our simulations）.These three complexesadsorbon the surface via electrostaticinteraction between uranyl and the surface，which makes them outersphere adsorptions.These findings are consistent with theprevious study of Greathouse and Cygan in（2006）.They also found that uranyl and oligomeric uranyl complexes can adsorb onto basal surfaces of pyrophyllite（Greathouse and Cygan 2006）.The tricarbonato complex tends to stay in the midplane of the aqueous layer shown in Fig.4d and never forms adsorption during our simulations，which is due to high negative charge of complex.

Fig.5 Equilibrium snapshots of uranyl complexes resulting fromthe simulations ofand 320 water molecules with a 0 and b 1 carbonatemolecules in the montmorillionite/aqueous system.Complexes a ］are shown as large balls，free water molecules and mineral molecules as sticks.Atomic designations are U（blue），O（red），C（gray），Si（yellow）and H（white）

Fig.6 Equilibrium snapshots of uranyl complexes resulting from the simulations ofand 320 water molecules with a 0 and b 1 carbonate molecules near the kaolinite's siloxane layer.Complexes a ］are shown as large balls，free water molecules and mineral molecules as sticks.Atomic designations are U（blue），O（red），C（gray），Si（yellow）and H（white）

In the montmorillonite system，only for noncarbonato and monocarbonato uranyl species，outer-sphere adsorptions occur.The structures of the complexes are shown in Fig.5.The complex of［UO2（H2O）5］2+is stable during the whole simulation.The monocarbonate complex gets away from the adsorbing sites occasionally.The dicarbonato and tricarbonato complexes wander in the diffuse region，and rarely get close to the surface.In the studies of Greathouse and Cygan，they also found that uranyl ions and monocarbonato species can adsorb onto the surface（Greathouse and Cygan 2005，2006）.

Figure 6 shows the equilibrium snapshots of uranyl complexes adsorbed on the siloxane surface in the kaolinitesystem.The monocarbonato complex is stable and it rarely wanders away from the adsorption sites.However，the dicarbonato and tricarbonato uranyl species cannot form adsorption complexes on the surface because of repulsion from the negatively charged surface.

Fig.7 Equilibrium snapshots of uranyl complexes resulting from the simulations ofand 320 water molecules with a 0，b 1，c 2，d 3carbonate molecules near the kaolinite's gibbsite surface.The complexes are a Complexes are shown as large balls，free water molecules and mineral molecules as sticks.Atomic designations are U（blue），O（red），C（gray），Si（yellow），Al（solferino）and H（white）

In the kaolinite-gibbsite system，in the absence of carbonate，the uranyl complex is initially placed near the gibbsite surface within 4 A?.But it rapidly leaves the gibbsite surface and gets close to the siloxane surface. Then stable outer-sphere adsorption occurs（Fig.7a），which is same as the structure shown above（Fig.6a）.This tendency was also seen in earlier study（Liu et al.2011）.

In the presence of carbonate ions，carbonate-uranyl complexescanadsorbonthegibbsitesurface（Fig.7b，c，d）.These carbonate-uranyl complexes bind on the gibbsite layer via hydrogenbonds.Thesharpradialdistributionfunction（RDF）peaks around 1.5 A?in Fig.8 suggest that there is strong interaction between carbonate oxygen and hydroxyl hydrogen on the gibbsite surface（Fig.8）.In Fig.7c，one can see that hydrogen bond can also form between the hydrogen of the water ligand and the oxygen of surface hydroxyl，which contributes to the complex adsorption.Figure 7d shows that twooutwardcarbonateoxygenatomscaninteractwithsurface hydrogen simultaneously，which firmly fastens the complexes.It is found that monocarbonate complex can leave the gibbsite surface occationally，but it comes back soon.Dicarbonato and tricarbonato uranyl complexes are fixed on the surface during the whole NVT simulation period.

Fig.8 Plots of the Oc—Ho radial distribution functions from simulations with a 1，b 2 and c 3 carbonates in the system near the gibbsite surface. Oc carbonate oxygen，Ho hydroxyl hydrogen on the gibbsite surface

Table 2 Adsorption patterns in different clay systems

4　Summary

Molecular dynamics simulations were used to study the interaction between uranyl-carbonate complexes and the basal surfaces of clay minerals.Table 2 summarizes the adsorption patterns derived from the simulations.In pyrophyllite system，［UO2（H2O）5］2+，［UO2（H2O）3（CO3）］and［UO2（H2O）2（CO3）2］2-can form outer-sphere complexes on the siloxane surface through electrostatic interaction，while the tricarbonato uranyl complex tends to diffuse away from the siloxane surface.With one or two carbonate ions in the coordination shell of uranium，the uranyl-carbonate complexes adsorb on the surface at farther distances than that of［UO2（H2O）5］2-.In montmorillonite andkaolinite-siloxane system，outer-sphere adsorption of noncarbonato and monocarbonato complexes on siloxane surface is observed.Noncarbonato uranyl complex cannot stay on the kaolinite-gibbsite surface，but it adsorbs at the siloxane surface in the outer-sphere way.In the presence of carbonate ions，we find that different outer-sphere complexes occur via hydrogen bonds at the gibbsite surface of kaolinite.Both the outward carbonate oxygens and water ligands form hydrogen bonds with hydroxyls on the gibbsite surface.These hydrogen bonds fix the complexes on the surface.Overall，we have revealed the microscopic structures of uranyl-carbonate complexes adsorbed at clay interfaces，which are helpful for understanding the transport and fixation of radioactive elements.

AcknowledgmentsWe thank National Science Foundation of China（Nos.41002013，40973029，41273074 and 41222015），the Foundation for the Author of National Excellent Doctoral Dissertation of PR China（No.201228），Newton International Fellowship Program and the financial support from the State Key Laboratoryfor Mineral Deposits Research.We are grateful to the High Performance Computing Center of Nanjing University for using the IBM Blade cluster system.

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24 December 2014/Revised:29 December 2014/Accepted:29 December 2014/Published online:18 April 2015 ?Science Press，Institute of Geochemistry，CAS and Springer-Verlag Berlin Heidelberg 2015