Chemical Engineering Journal 245 (2014) 10–16

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Determination of colloidal pyrolusite, Eu(III) and humic substanceinteraction: A combined batch and EXAFS approach

http://dx.doi.org/10.1016/j.cej.2014.02.0211385-8947/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author at: College of Chemistry and Chemical Engineering, Shaoxing University, Huancheng West Road 508, Shaoxing, Zhejiang 312000, PR China.575 88341526; fax. +86 575 88341521.

E-mail address: gdsheng@usx.edu.cn (G. Sheng).

Guodong Sheng a,c,⇑, Qi Yang a, Feng Peng a, Hui Li b, Xing Gao d, Yuying Huang d

a College of Chemistry and Chemical Engineering, Shaoxing University, Huancheng West Road 508, Shaoxing, Zhejiang 312000, PR Chinab College of Medical Science, Shaoxing University, Huancheng West Road 508, Shaoxing, Zhejiang 312000, PR Chinac Institute of Plasma Physics, Chinese Academy of Sciences, P.O. Box 1126, Hefei 230031, PR Chinad Shanghai Synchrotron Radiation Facility (SSRF), Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201204, PR China

h i g h l i g h t s

� HA or FA promoted Eu(III) interactionat low pH values.� HA or FA reduced Eu(III) interaction at

high pH values.� Binary surface complexes and ternary

surface complexes of Eu(III) can besimultaneously formed by EXAFSstudy.� The findings are important to

understand Eu(III) physicochemicalbehavior.

g r a p h i c a l a b s t r a c t

a r t i c l e i n f o

Article history:Received 15 December 2013Received in revised form 4 February 2014Accepted 6 February 2014Available online 15 February 2014

Keywords:Eu(III)Interaction mechanismMicrostructureb-MnO2

a b s t r a c t

This work determined the role of humic acid (HA) and fulvic acid (FA), which was extracted from thenatural soil, in the interaction mechanism and microstructure of Eu(III) with pyrolusite (b-MnO2) byusing batch experiments and extended X-ray absorption fine structure (EXAFS) techniques. We combinedmacroscopic and spectroscopic approaches to see the evolution of the interaction mechanism andmicrostructure of Eu(III) with b-MnO2 in the presence of HA or FA in comparation with that in theabsence of HA or FA. The results suggested that Eu(III) interaction with b-MnO2 was obviously affectedby the addition of HA or FA. The interaction of Eu(III) with b-MnO2 was enhanced at pH < 6.5 in the pres-ence of HA or FA, while Eu(III) interaction with b-MnO2 was reduced at pH > 6.5 in the presence of HA orFA. The EXAFS fitting results provided a molecular evidence for the findings from the batch experiments.Adsorption of HA or FA onto b-MnO2 greatly modified the microstructure of Eu(III) onto b-MnO2. Onlybinary surface complexes of Eu(III) can be formed onto b-MnO2 in the absence of HA or FA, while bothbinary surface complexes and ternary surface complexes of Eu(III) can be simultaneously formed ontob-MnO2 in the presence of HA or FA, which was mainly responsible for the enhanced Eu(III) uptake atlow pH values. The results observed in this work are important for the evaluation of physicochemicalbehavior of long-lived radionuclides (lanthanides and actinides) in the natural soil and waterenvironment.

� 2014 Elsevier B.V. All rights reserved.

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G. Sheng et al. / Chemical Engineering Journal 245 (2014) 10–16 11

1. Introduction

Interfacial behavior of long-lived radionuclides (lanthanidesand actinides) at the natural particles (metal oxides and clay min-erals)/water interface is quite important for the long-term perfor-mance assessment of nuclear waste repositories [1,2]. It is wellknown that Eu(III) is a trivalent lanthanide and can be regardedas a chemical homologue of trivalent actinides because the interfa-cial behavior of trivalent lanthanides and actinides is quite similar.Therefore, the interaction and microstructure of trivalent lantha-nides and actinides at the natural particles/water interface can besimulated by using Eu(III) [3–7]. In this respect, a variety of studieshave been conducted to investigate the interfacial interaction andmicrostructure of Eu(III) at the oxide/water and clay mineral/waterinterface mainly by a combination of macroscopic and spectro-scopic techniques [1–11]. It was reported that extended X-rayabsorption fine structure (EXAFS) technique is very useful to studythe interfacial interaction and microstructure of Eu(III) with the so-lid particles at a molecule level. Nevertheless, the energy resolu-tion of conventional EXAFS technique with solid-state detector(SSD) is no less than 120 eV, making it impossible to discriminatethe La1 fluorescence peak (�5849 eV) of Eu from the Ka fluores-cence peak (�5900 eV) of Mn [12–14]. Thus, it is hardly to investi-gate the interaction and microstructure of Eu(III) with Mn-bearingmineral.

Manganese (Mn) is one of the most abundant elements in thenatural environment, which can be easily oxidized, leading to avariety of Mn (hydr)oxides (a-MnO2, a-MnOOH, d-MnO2,d-MnOOH, b-MnO2, c-MnOOH). These Mn (hydr)oxides occur asfine-grained aggregates, marine, veins and fresh-water nodulesand coatings on other solid particles and rock surfaces. Besides,these Mn (hydr)oxides are chemically active with large surfacearea, and negative over a wide range of pH. So, Mn (hydr)oxidesare strong sequesters of heavy metal ions and radionuclides inthe natural water and soil environment [15–17]. As a result, greatattention has been paid to the interaction and microstructure ofheavy metal ions and radionuclides, such as Np(V), Pu(VI), Zn, Cdand UO2þ

2 with Mn (hydr)oxide [16–20]. For example, Nitscheand co-workers studied the interaction and microstructure ofNp(V)/Pu(VI) at manganite/water and hausmannite/waterinterface, and it was reported that inner-sphere complexation werethe main mechanism responsible for the interaction [16,17].The interaction and microstructure of Cd(II) and Zn(II) atthe c-MnOOH/water interface studied by EXAFS showed that Cd(II)interacted with c-MnOOH surface by the formation of mononu-clear inner-sphere complexes [19], while Zn(II) interacted withc-MnOOH surface by the formation of multinuclear inner-spherecomplexes [20]. However, because of the overlap of Mn Ka fluores-cence line and Eu La1 fluorescence line, the interactions and micro-structure of Eu(III) with Mn (hydr)oxide can be hardly studied byEXAFS.

In order to study the interaction mechanism and microstructureof Eu(III) at the Mn (hydr)oxide/water interface, it is quite neces-sary to develop new EXAFS technique with much higher energyresolution. In this respect, a bent crystal spectrometer based onthe Rowland circle geometry, the energy resolution of which isgreatly improved, can be utilized to selectively extract the fluores-cence signal from Mn Ka line to Eu La1 line [12–14]. Using this newdetector, Rakovan et al. [12] successfully collected the Eu L3 X-rayabsorption near edge structure (XANES) spectrum from a Mn- andREE-rich apatite sample, and studied the valence state of Eu in thissample. As the energy resolution of this bent crystal spectrometeris enhanced, the Eu La1 fluorescence line and the Mn Ka fluores-cence line can be easily resolved [21]. So, we can obtain the EuXAFS spectrum in a Mn-bearing samples to study the interaction

and microstructure of Eu(III) at the Mn-bearing mineral/waterinterface. In our previous reports [22,23], this high resolution EX-AFS technique with a bent crystal analyzer was used to studiedthe interaction mechanism and microstructure of Eu(III) at the b-MnO2/water and c-MnOOH/water interface as a function of solu-tion pH, contact time, and reaction temperature.

In addition, Humic Substances (HSs), such as fulvic acid (FA)and humic acid (HA), which are widely present in natural waterand soil environment, contain a large number of O-containingfunctional groups [24,25]. Because of these O-containing functionalgroups, FA and HA show strong interaction with lanthanides andactinides in natural environment, greatly affecting the fate andtransport of lanthanides and actinides. So, the role of FA and HAin the interaction and microstructure of Eu(III) at the natural min-erals/water interface is of particular interest [3–6,9–11]. It wasgenerally regarded that FA and HA enhanced Eu(III) interactionwith minerals at low pH values, whereas Eu(III) interaction de-creased at high pH values in the presence of FA or HA [3–6,9–11]. However, to the best of our knowledge, no efforts have beenfocused on illustrating the role of FA and HA in the interactionand microstructure of Eu(III) at the Mn (hydr)oxide/water interfaceat molecular level.

Herein, the main objective of this paper was to determine therole of HA and FA in Eu(III) nteraction and microstructure in aque-ous pyrolusite (b-MnO2) suspensions by batch and extended X-rayabsorption fine structure (EXAFS) investigations. This paper high-lights that the interaction of HA and FA with natural mineralscan greatly change the interaction and microstructure of Eu(III),thus controlling the fate and transport of trace radionuclides (lan-thanides and actinides) in heterogeneous aquatic environments.

2. Materials and methods

2.1. Chemicals and materials

The radiotracer 152+154Eu(III) with a radionuclidic and radio-chemical purity of 99.0% was purchased as the form of Eu2O3 fromthe China Institute of Atomic Energy (Beijing, China) [26]. Eu(III)stock solution was prepared from Eu2O3 after dissolution, evapora-tion and redissolution in 10�3 mol/L HClO4. The detailed informa-tion of the extraction and characterization for fulvic acid (FA)and humic acid (HA) has been provided in previous investigations[3–6]. The preparation and characterization of the adsorbent pyro-lusite (b-MnO2) was described in our previous reports [22,24]. Allother chemicals and reagents used were purchased in analyticalpurity and without further purification. The solutions in all exper-iments were prepared by Milli-Q water.

2.2. Batch interaction experiments

Batch interaction experiments were conducted under N2 condi-tions in NaClO4 solutions. The stock solutions of 0.5 g/L b-MnO2

and 0.01 mol/L NaClO4 in 10 mL polyethylene centrifuge tubeswere pre-equilibrated for 5 h before addition of Eu(III) stock solu-tion at T = 20 ± 1 �C. Then, Eu(III) stock solution (including tracequantities of the radiotracer 152+154Eu(III)) and HA or FA stock solu-tion were added the polyethylene centrifuge tubes to obtain thedesired concentrations of different components. The suspensionpH was adjusted by using 0.01 mol/L HClO4 or NaOH solutions.The suspensions were gently shaken for 2 days to obtain adsorp-tion equilibrium, and the solid was separated from the solutionby centrifugation at 18000 rpm for 30 min. The concentration of152+154Eu(III) in supernatant was determined by liquid scintillationcounting (Packard 3100 TR/AB Liquid Scintillation analyzer,PerkinElmer) with an ULTIMA GOLD ABTM (Packard) Scintillation

Fig. 1. (A) Experimental set up of crystal analyzer system in Bragg geometry forfluorescence EXAFS constructed in this study. (B) The Eu La1 fluorescence line andMn Ka fluorescence line that is discriminated by the bent crystal analyzer [22].

12 G. Sheng et al. / Chemical Engineering Journal 245 (2014) 10–16

cocktail. Under the assumptions that the added radiotracer152+154Eu(III) can disperse evenly in the non-radioactive Eu(III)stock solution and that both 152+154Eu(III) and the non-radioactiveEu(III) have similar interfacial behaviors, the adsorption percent-age of Eu(III) on b-MnO2 was calculated from the radioactivity ofthe 152+154Eu(III) (Atot) in initial solution and that of 152+154Eu(III)in supernatant (AL) (i.e., Adsorption(%) = (1 � AL/Atot)) � 100% [26].

2.3. Sample preparation for EXAFS analysis

According to a previous report [22], sample preparation forEXAFS analysis was conducted using a 1 L vessel with 0.5 g/L ofb-MnO2, 10 mg/L of HA or FA, 0.01 mol/L of NaClO4 and4.0 � 10�5 mol/L of Eu(III). Eu(III) solution was introduced as fol-lows: 10 lL increments of Eu(III) stock solution were introducedinto the suspension under constant stirring to disperse small ali-quot of Eu(III) solution, simultaneously, pH was monitored andkept to desired value during Eu(III) addition. Periods of a few min-utes between the increments were chosen to avoid Eu(III)(aq)exceeding the solubility limit of Eu(OH)3(s), while allowing com-pletion of Eu(III) addition in a reasonable amount of time. The sam-ples were shaken on a rotating oscillator and the residual wetpastes were obtained after centrifugation of the suspensions. Afterdesired time, aliquots of suspension were collected and centrifugedat 18,000 rpm. The wet pastes was then packed in a Teflon sampleholder and covered with Mylar tape. The sample was wrapped in amoist paper towel and sealed in a Ziplock bag to minimize furtherinteractions before being transported to the synchrotron beam-line. The EXAFS data collections were performed less than 2 d aftercentrifugation [26].

2.4. EXAFS measurements and analysis

Eu LIII-edge EXAFS spectra at 6976.9 eV were recorded at roomtemperature on BL14 W at Shanghai Synchrotron Radiation Facility(SSRF, China). The electron beam energy was 3.5 GeV and the meanstored current was 200 mA. The source of BL14 W is a 38-pole wig-gler device with maximum magnetic field of 1.2T and the magnetperiod 80 mm inserted in the straight section of the storage ringwas used. The fluorescence signal of Eu was collected using thehigh-resolution X-ray fluorescence spectrometer that was com-posed by a spherically curved silicon crystal, detector and sampleholder [21–23]. The bent crystal, the sample and the detector justbeside the sample were located on the Rowland circle (Fig. 1A).This spectrometer can discriminate the fluorescence peak of tar-geted element from that of matrix constituents, whose fluores-cence peaks are too close in energy to be discriminated by theconventional EXAFS technique. The crystal was bent to a radiusof 18.2 cm in a glass with 4 in. diameter. The incident beam irradi-ated the sample along a 45-degree angle. The fluorescence wasgathered and received by the detector. At entrance angle close tobackscattering, the energy resolution could reach the best resolu-tion. The Eu La1 fluorescence line could be obviously resolved fromthe Mn Ka fluorescence line of the sorption samples by using thisbent crystal analyzer (Fig. 1B). So, Eu LIII-edge EXAFS spectra of theadsorption samples could be successfully collected using this high-resolution EXAFS technique.

Energy calibration, fluorescence deadtime correction and theEXAFS analysis were performed by using Athena and Artemisinterfaces to the IFFEFIT software [21–23]. The EXAFS datareduction and analyses were performed by using the followingprocedures. First, the averaged spectra were normalized withrespect to E0 determined from the second derivative of the rawspectra, and then the total atomic cross-sectional absorption wasset to unity. A low-order polynomial function was fit to thepre-edge region and the post-edge region. Next, the data were

converted from E-space to k-space and weighted by k3 tocompensate for dampening of the EXAFS amplitude with increasingk space. Fourier transformation was then performed over the krange of 2.0–10.0 Å�1 using the Kaiser-Bessel window function toobtain the radial structural functions (RSFs). Final fitting of thespectra was done on Fourier transformed k2-weighted spectra inR-space. The theoretical scattering phases and amplitudes used indata analysis were calculated with the scattering code FEFF7 [27]using the crystal structure of Eu(OH)3, Eu2O3 and Eu2MnO5. Theamplitude reduction factor (S2

0) was fixed to 1.0 to correctly repro-duce the number of neighboring atoms in the structural references.It is necessary to note that the value of S2

0 herein agrees with earlierstudies of lanthanide coordination in water [28–30]. During fitting,the Debye–Waller factors of the first shell were allowed to vary, butthose of the second shells (i.e., Eu–Mn) were fixed equal to that ofEu(OH)3(s) (i.e., 0.006 Å2). When allowed to vary, the Debye–Wallerfactors of the second shells showed no trends for different samples,and therefore the same value of 0.006 Å2 was used in the finalfitting procedure to reduce the number of free parameters. Thisconstraint was necessary due to strong correlation of Eu and Mnbackscattering contributions for the second shells. On the basis ofthis fitting mode, we can improve the comparability of fittingresults and therefore get more logical results that are helpful tointerpret the interaction mechanism at molecular level. Accuraciesfor interatomic bond distance (R) and coordination number (CN) are

Fig. 2. Role of HA/FA in Eu(III) interaction with b-MnO2 as a function of pH.

G. Sheng et al. / Chemical Engineering Journal 245 (2014) 10–16 13

0.02 Å and 20%, respectively, for the first shell, and 0.03 Å and 40%,respectively, for the second shell.

3. Results and discussion

3.1. Macroscopic experimental results

The pH dependence of Eu(III) interaction with b-MnO2 in theabsence or presence of HA/FA is shown in Fig. 2. We can see thatEu(III) interaction with b-MnO2 increases at low pH values in thepresence of HA or FA, while Eu(III) interaction decreases at highpH values. Generally speaking, Eu(III) interaction at the mineral/water interface in the presence of HA or FA is explained by acompetition between adsorbed HA/FA and non-adsorbed HA/FAin solution. Surface adsorbed HA/FA affect Eu(III) interaction atthe mineral/water interfaces by changing surface charges and/oraltering available interaction sites for Eu(III) [24–26]. The zeta po-tential of HA or FA was negative at pH > 2.0 [3–5,25], thus, HA/FAwith negative charge can be easily adsorbed on b-MnO2 surfacesat low pH values because of the strong electrostatic attractioninteraction. Fig. 3 shows the results of HA/FA interactionwith b-MnO2 as a function of solution pH, and it can be seen that�90% of HA/FA can be adsorbed onto b-MnO2 surfaces at low pHvalues, and then HA/FA interaction with b-MnO2 decreases with

Fig. 3. Adsorption of HA/FA onto b-MnO2 as a function of pH.

pH increasing. So, at low pH values, the net positive surface chargeof b-MnO2 can be reduced which is caused by the adsorption ofnegatively charged HA or FA, resulting in more favorable electro-static interactions with solution Eu(III), and thus HA/FA promotedEu(III) interaction with b-MnO2. At high pH values, because of thestrong electrostatic repulsion interaction, the negatively chargedHA/FA can be hardly adsorbed on the negatively charged b-MnO2

surface. A lot of HA/FA remains in the solution, the soluble HA/FAforms strong HA–Eu or FA–Eu complexes, thereby competitivelydeceases Eu(III) interaction with HA/b-MnO2 or FA/b-MnO2 hy-brids. Recently, Janot et al. [1,31] studied colloidal a-Al2O3, Eu(III)and humic substances interactions by a combined macroscopic andspectroscopic methods, and found similar results.

We can also see from Fig. 2 that the role of FA in Eu(III) interac-tion with b-MnO2 is much stronger than that of HA. As we know,HA and FA was extracted from the same soil, both of which havesimilar O-containing functional groups, the proportions andconfigurations of these O-containing functional groups are quitedifferent [3–5]. It was reported that the surface site density of FA(2.71 � 10�2 mol/g) is higher than that of HA (6.46 � 10�3 mol/g)[3,25], this means that FA can provide more available interactionsites with Eu(III). So, the effect of FA on Eu(III) interaction withb-MnO2 is much stronger than that of HA. In previous investiga-tions, Tan et al. [3,5] studied the influence of HA/FA on Eu(III) up-take mechanism on hydrous alumina and TiO2 (anatase and rutile),Fan et al. [4] studied the sorption of Eu(III) on attapulgite in thepresence of HA/FA, Hu et al. [9] studied the effects of HA on theadsorption of Eu(III) on GMZ bentonite, Sheng et al. [11] studiedthe influence of HA/FA on the sorption of Eu(III) to titanate nano-tubes, and similar foundlings have been reported.

3.2. Microscopic investigation

In order to investigate the microstructure and interaction ofEu(III) at b-MnO2/water interface, a combination of macroscopicand microscopic data becomes quite necessary. So, the microscopicinsight into the nature of the microstructure and interaction

Fig. 4. XANES spectra for Eu of the reference samples and adsorption samples.

Fig. 5. k3-weighted EXAFS spectra (A) and radial structure functions (RSFs) (B) produced by forward Fourier transforms and imaginary parts (uncorrected for phase shift) ofthe reference samples.

14 G. Sheng et al. / Chemical Engineering Journal 245 (2014) 10–16

mechanism of Eu(III) onto b-MnO2 in the presence or absence ofHA/FA was further studied by X-ray absorption fine structure(XAFS). Fig. 4 showed the X-ray absorption near edge structure(XANES) spectra of the samples, we can see that the X-ray absorp-tion is dominated by an intense adsorption at �6990 eV, which isindicative of trivalent Eu in all reference and adsorption samples[7,21].

The EXAFS spectra of Eu reference samples (Eu(III)aq, Eu(OH)3,and Eu2O3) are shown in Fig. 5. The EXAFS spectrum of Eu(III)aq

is shown by a single sinuous oscillation from O surrounding centralEu atom in the first shell. In contrast to Eu(III)aq, Eu(OH)3, andEu2O3 show more complex structural features in the k range ofEXAFS spectra, indicating the presence of heavy atoms surroundingcentral Eu atom at higher shells. The radial structure functions(RSFs) show that Eu(III)aq has only one peak at �1.9 Å, suggestinga single shell of backscattering atoms surrounding central Eu atom,

Table 1EXAFS results of reference samples and reacted samples at Eu LIII-edge.

Sample conditions Shell R (Å) CN r2 (Å2) Rf

Eu(III)aq Eu–O 2.42(3) 8.9(2) 0.007(5) 0.019Eu(OH)3 Eu–O 2.40(5) 8.1(3) 0.008(4) 0.021

Eu–Eu 3.59(3) 2.4(2) 0.007(4) 0.047Eu2O3 Eu–O 2.36(5) 6.2(7) 0.008(7) 0.022

Eu–Eu 3.61(3) 6.4(5) 0.007(4) 0.055Eu(III)/b-MnO2 system Eu–O 2.40(5) 7.6(5) 0.005(6) 0.025

Eu–Mn 3.75(4) 2.4(3) 0.006(6) 0.039Eu(III)/b-MnO2/HA system Eu–O 2.42(4) 7.3(4) 0.005(5) 0.021

Eu–Mn 3.76(5) 1.9(3) 0.006(6) 0.046Eu–C 2.72(5) 2.2(4) 0.007(5) 0.098

Eu(III)/b-MnO2/FA system Eu–O 2.39(3) 7.1(7) 0.005(6) 0.023Eu–Mn 3.75(4) 1.9(3) 0.006(6) 0.056Eu–C 2.71(3) 2.1(5) 0.007(5) 0.087

R: Interatomic distance; CN: Coordination number; r2: Debye–Waller factor; Rf:The residual factor; Rf =

Pk(k3xexp � k3xcalc)/

Pk(k3xexp) measures the quality of the

model Fourier-filtered contribution (xcalc) with respect to the experimental con-tribution (xexp). Euaq, Eu(OH)3 and Eu2O3 are named as reference samples, whereasthe other samples with adsorbed Eu(III) are named as adsorption samples.

while for Eu(OH)3, and Eu2O3, the spectra reveal clear second peakat �3.1 Å due to the contribution of higher-shell atoms. Structuralparameters from EXAFS analysis are shown in Table 1. For Eu(III)aq,Eu consists of �8.9 O at REu–O � 2.42 Å. For Eu(OH)3, and Eu2O3, Euis coordinated with�8.1 O at REu–O � 3.59 ÅA

0

in the first shell. Whilein the second shell, Eu consists of �2.4 Eu at REu–Eu � 3.11 ÅA

0

inEu(OH)3, and �6.4 Eu at REu–Eu � 3.61 ÅA

0

in Eu(OH)3. These resultsare in good agreement with previous works [5,11,26].

In the presence of HA/FA, Eu(III) interaction with b-MnO2

enhanced at pH < 6.5, whereas Eu(III) interaction withb-MnO2 decreased at pH > 6.5 (Fig. 2). In order to further studythe microscopic insights into the role of HA or FA in the interactionmechanism of Eu(III) with b-MnO2, three uptake samples of Eu(III)in the absence or presence of HA/FA with the concentration of10 mg/L, which is indicative of natural environment, were pre-pared for EXAFS analysis. The k3-weighted EXAFS spectra and ra-dial structure functions (RSFs) for the interaction samples ofEu(III)/b-MnO2 system, Eu(III)/HA/b-MnO2 system and Eu(III)/FA/b-MnO2 system is shown in Fig. 6, giving the microscopic evidencefor the changes of interaction mechanism and microstructure ofEu(III) with b-MnO2 in the absence or presence of HA/FA. Table 1shows the microscopic coordination results from the EXAFS fitting.For the interaction sample of Eu(III)/b-MnO2 system, Eu is coordi-nated with �7.6 O at REu–O � 2.40 ÅA

0

in the first shell, while in thehigher shell, Eu consists of �2.4 Mn at REu–Mn � 3.75 ÅA

0

. The pres-ence of the higher shell from Mn backscatter indicates the forma-tion of inner-sphere surface complexes of Eu(III) onto b-MnO2,where Eu(III) directly interact with the surface hydroxyl groupsof b-MnO2. For the interaction sample of Eu(III)/HA/b-MnO2

system, Eu is coordinated with �7.3 O at REu–O � 2.42 ÅA0

in thefirst shell, while in the higher shell, Eu consists of �1.9 Mn atREu–Mn � 3.76 ÅA

0

. For the adsorption sample of Eu(III)/FA/b-MnO2,Eu is coordinated with �7.3 O at REu–O � 2.42 ÅA

0

in the first shell,while in the higher shell, Eu consists of �1.9 Mn at REu–Mn � 3.75 ÅA

0

.According to these results, we can see that all the three interactionsamples have similar O and Mn backscatter, indicating thecoordination environment of O atom is the same and the direct

Fig. 6. k3-weighted EXAFS spectra (A) and radial structure functions (RSFs) (B) produced by forward Fourier transforms and imaginary parts (uncorrected for phase shift) ofEu uptake on b-MnO2 in the absence/presence of HA/FA.

Fig. 7. A pictorial representation of (A) binary surface complexes and (B) ternary surface complexes for Eu(III) interaction at the b-MnO2/water interfaces in the presence/absence of HA/FA.

G. Sheng et al. / Chemical Engineering Journal 245 (2014) 10–16 15

interaction of Eu with the surface hydroxyl groups of b-MnO2

in theabsence or presence of HA/FA. We have reported that the obtainedREu–Mn value of �3.75 Å was indicative of a bidentate Eu(III) species(i.e., edge-sharing) formed on b-MnO2, which is invariant withaging time, pH and initial Eu concentrations [22]. However, forthe interaction samples of Eu(III)/HA/b-MnO2 and Eu(III)/FA/b-MnO2 system, the spectral intensities of 3.0–4.0 Å are greatlydecreased, despite of the increased macroscopic uptake capacityin the presence of HA/FA. This loss is replaced by C atoms fromHA/FA. From the EXAFS fitting, we can see that Eu is coordinatedwith �2.2 C at REu–C � 2.72 ÅA

0

and �2.1 C at REu–C � 2.71 ÅA0

(Table 1) for the interaction samples of Eu(III)/HA/b-MnO2 andEu(III)/FA/b-MnO2 system, respectively. This result suggests that

multiple species contribute to Eu(III) interaction with b-MnO2,some are similar to those measured in HA/FA-free b-MnO2 suspen-sions in which binary surface complexes is formed where Eu(III)interact with surface hydroxyl groups on the b-MnO2 surface,and some are attributed to the formation of ‘‘ligand-bridging’’ternary surface complexes where b-MnO2 first interact with theO-containing functional groups at HA/FA which are adsorbed ontob-MnO2 surface simultaneously. Fig. 7 shows the possible adsorp-tion sites of Eu(III) on b-MnO2. This microscopic finding is in goodconsistent with the macroscopic result. Knowledge obtained fromthe results will be useful to elucidate the interaction mechanismand microstructure of long-lived radionuclides (lanthanides andactinides) at the natural mineral/water interfaces.

16 G. Sheng et al. / Chemical Engineering Journal 245 (2014) 10–16

4. Conclusion

The role of FA/HA in the interaction and microstructure ofEu(III) with b-MnO2 was studied by a combined macroscopic andspectroscopic techniques. The presence of HA/FA enhanced theinteraction of Eu(III) with b-MnO2 at low pH values, whereas re-duced Eu(III) interaction with b-MnO2 at high pH values. The pres-ence of HA/FA can provide more interaction sites to bond Eu(III) atb-MnO2 than that on bare b-MnO2 and thus increase Eu(III) inter-action with b-MnO2. For the interaction samples of Eu(III)/HA/b-MnO2 and Eu(III)/FA/b-MnO2 system, the spectral intensities of3.0–4.0 Å are greatly decreased, which is replaced by C atoms fromHA/FA. EXAFS results showed that binary surface complexes andternary surface complexes can be simultaneously formed in thepresence of HA/FA, leading to the enhanced interaction. The resultsare important for the evaluation of interaction mechanism andmicrostructure of long-lived radionuclides (lanthanides and actin-ides) with oxide minerals, and their fate and transport in theenvironment.

Acknowledgements

Financial supports from National Natural Science Foundation ofChina (21207092, 11175244), China Postdoctoral Science Founda-tion Grant (2013M531536) and the Doctoral Scientific ResearchFoundation of Shaoxing University (20125038) are acknowledged.The authors thank beamline BL14W1 (Shanghai SynchrotronRadiation Facility) for providing the beam time.

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