Nano-hematite prepared by activation of natural siderite and its performance on immobilization of Eu(III)

Applied Geochemistry 84 (2017) 154e161

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Nano-hematite prepared by activation of natural siderite and its performance on immobilization of Eu(III) Mengxue Li, Haibo Liu*, Tianhu Chen, Wei Lin School of Resources and Environmental Engineering, Hefei University of Technology, Hefei 230009, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 31 March 2017 Received in revised form 5 June 2017 Accepted 20 June 2017 Available online 21 June 2017

Nano-hematite was synthesized by calcining siderite (FeCO3) and characterized using TEM, XRD, FT-IR, XPS, specific surface area and potentiometric titration. The characterization results showed that nanohematite was synthesized by calcining FeCO3 under atmospheric conditions. The batch experiments indicated the adsorption of Eu(III) on nano-hematite significantly increased with increasing pH 2.0e6.0 while the Eu(III) adsorption was independent of ionic strength. The presence of CO2
3 promoted the adsorption of Eu(III) on nano-hematite over a wide range of pH conditions. The adsorption kinetics and isotherms of Eu(III) on nano-hematite were fitted well by pseudo-second kinetic model and Langmuir model, respectively. The maximum adsorption capacity of Eu(III) on nano-hematite calculated by Langmuir equations was 13.02 mg/g at pH 5.5 and T ¼ 293 K. The desorption experiments revealed that the adsorption of Eu(III) on nano-hematite was an irreversible process. According to the XPS analysis, the oxygen-containing functional groups of nano-hematite (i.e., Fe-OH) played a crucial role in the Eu(III) adsorption. These findings demonstrated that the nano-hematite could be used as a valuable adsorbent for preconcentration and immobilization of Eu(III) from aqueous solutions in the environmental cleanup. © 2017 Elsevier Ltd. All rights reserved.

Editorial handling by Huaming Guo. Keywords: Nano-hematite Eu(III) Adsorption mechanism XPS analysis

1. Introduction Treatment of underground water containing radionuclides has become a momentous tasking virtue of its long term threat to humans and animals (Chen et al., 2013; Sun et al., 2012b; Wang et al., 2015a). The removal of radionuclides from aqueous solutions (e.g., Eu(III), U(VI)) has been universally studied using a wide variety of adsorbents in the late years (Ding et al., 2014, 2015; Shao et al., 2015; Sun et al., 2015). As a trivalent lanthanides, Eu(III) has been extensively investigated on various adsorbents such as clay minerals (Fan et al., 2009; Hu et al., 2010; Tertre et al., 2006; Wang et al., 2015b), metal oxides (Chen et al., 2009a; Tan et al., 2009), and carbon materials (Chen et al., 2008; Sheng et al., 2010; Sun et al., 2013; Tan et al., 2008). Wang et al. (2006b). reported the desorption of Eu(III) from humic acideAl2O3 colloid surfaces. Sun et al. (Sun et al., 2012a) found that the adsorption capacity of Eu(III) on mesoporous Al2O3/expanded graphite composites at pH 6.0 and T ¼ 293 K was 5.14 mg/g. However, the so low adsorption capacity of the natural adsorbents limited its actual application in

* Corresponding author. E-mail address: [email protected] (H. Liu). http://dx.doi.org/10.1016/j.apgeochem.2017.06.010 0883-2927/© 2017 Elsevier Ltd. All rights reserved.

environmental cleanup. To improve the adsorption capacity, numerous studies on the adsorption of Eu(III) on nano-materials were carried out in recent years such as graphene oxides (Ding et al., 2014; Ren et al., 2014; Sun et al., 2012b), nano-alumina oxides (Montavon et al., 2006, 2007; Wang et al., 2006a) and nanoiron oxides (Chen et al., 2009b; Yang et al., 2012). Shanna L. Estes et al. (2013). studied the thermodynamics of Eu(III) adsorption onto hematite. Adsorption behaviors of Eu(III) on micro-meter size hematite have already been published previously by different authors (E.M. El Afifi et al., 2016; Thomas et al., 1998). Furthermore, to reduce the cost, various natural materials were also utilized to remove Eu(III) from aqueous solution. El Afifi et al (E.M. El Afifi et al., 2016). reported the adsorption capacity of Eu(III) on natural hematite(0.25e0.425 mm) at pH 4.7 and T ¼ 298 K was 12.30 mg/g. As the particle size decreased to lower than 0.25 mm, the adsorption capacity increased to 20.81 mg/g. The capacity was relatively high and meanwhile the material was interesting. To the best of our knowledge, siderite can be transformed into hematite by calcination. Meanwhile, report on that hematite from annealing of siderite was utilized as adsorbent was rarely observed (Guo et al.,
2þ 2008), although siderite was utilized to remove AsO3
4 , F , Cu ,
et al., 2015; Erdem and Pb2þ from aqueous solution (Dankova € Ozverdi, 2005; Guo et al., 2010, 2011).

M. Li et al. / Applied Geochemistry 84 (2017) 154e161

Therefore, in this study, the investigation of Eu(III) adsorption onto nano-hematite obtained from calcining the nature siderite was carried out. The aims of this article were to (1) synthesize nano-hematite and characterize it using TEM, XRD, FT-IR, XPS, potentiometric titration and specific surface area; (2) investigate the effect of environmental factors (reaction time, pH, ionic strength, temperature and initial concentration) on the adsorption capacity of Eu(III) on nano-hematite by batch techniques; (3) determine the adsorption mechanism between nano-hematite and Eu(III) by using XPS analysis. The highlight of this study is to utilize nano-particles in the remediation of radioactive pollution. 2. Experimental 2.1. Preparation of nano-hematite The nano-hematite was synthesized by calcining natural siderite (66.3% of siderite, 22.4% of clay minerals and quartz, 11.3 wt% of goethite) under atmospheric conditions. Briefly, the high purity of natural siderite was obtained by crushing, grinding and screening to less than 200 mesh(<0.10 mm). Afterwards, the screened powder of natural siderite was heated up to 600  C for 30 min under atmospheric conditions. Subsequently, the powder was cooled to room temperature naturally for further used. Eu(III) stock solution of 0.1 mol/L was prepared from Eu2O3 (purity 99.99%) after dissolution and dilution within 0.01 mo/L HClO4 solution. All chemicals used in this study were analytical-grade, and all the solutions were prepared with Milli-Q water.

155

experiments were carried out under the same conditions. The concentration of Eu(III) was determined by a kinetic phosphorescence analyzer(KPA-11, Richland, USA) (Sun et al., 2015). The removal percentage of Eu(III) (adsorption (%)) and adsorption capacity (Qs, mg.g
1) can be formulated as Eq. (1) (Cheng et al., 2015a) and (2) (Cheng et al., 2015b), respectively:

Adsorption ð%Þ ¼




C0
Ceq




 Q s ¼ V  C0
Ceq m

C0  100%

(1) (2)

where C0 (mg,L
1) and Ceq (mg,L
1) are initial concentration and concentration after adsorption, respectively. The m (g) and V (mL) are the mass of the nano-hematite and the volume of the suspension, respectively. All the experimental data were the average of triplicate determinations and the relative errors were within ±5%. 2.4. Kinetic models The pseudo-first-order and pseudo-second-order kinetic models were used for fitting the adsorption kinetics of Eu(III) on nano-hematite in order to confirm the underlying mechanisms
et al., 2015). The during the entire adsorption process(Dankova linear forms were given in Eqns. (3) and (4), respectively:

lnðQ e
Q t Þ ¼ lnðQ e Þ
K1  t t=qt ¼ 1

 . K2  Q e 2 þ t=Q e

(3) (4)

2.2. Characterization As-prepared nano-hematite was characterized by TEM, XRD, FTIR, XPS, specific surface area and potentiometric titration. The TEM images were provided with a Philips JSM-6490LV transmission electron microscope. The XRD patterns were conducted on a Dandonghaoyuan 2700 diffractometer using Cu Ka radiation. FT-IR measurements were conducted by using a VERTEX-70 Fourier trans-form infrared spectrometer. The XPS values were conducted with a Thermo Escalab 250 electron spectrometer. The BETnitrogen isotherms were applied in a Novawin 3000e Surface Area and Pore size Analyzer to determine the specific surface area of catalysts (SSA, m2g
1). The potentiometric acid-base titration was performed by a computer-controlled titration system (DL50 automatic titrator, Mettler Toledo). 2.3. Batch adsorption-desorption experiments The batch adsorption experiments were conducted with 2 g/L nano-hematite and 10 mg/L Eu(III) solutions in the presence of 0.01 mol/L NaClO4. The pH of the suspension was adjusted from 2.0 to 11.0 by adding negligible volume of 0.001e1.0 mol/L HNO3 and/ or NaOH solution. The adsorption isotherms were examined at pH ¼ 5.5 ± 0.1 with the initial concentration of Eu(III) ranging from 1 to 30 mg/L. The suspensions were shaken for 24 h to achieve reaction equilibrium. The desorption kinetics of Eu(III) on nano-hematite were examined after adsorption equilibrium by using 0.1 mol/L NaCl solution. In short, 3.0 mL of supernatant (the initial volume was 6.0 mL) after adsorption equilibrium was displayed by 3.0 mL of 0.1 mol/L of NaCl solution with the pH ¼ 5.5 ± 0.1 adjusted with 0.001e1.0 mol/L HNO3 solution(Ding et al., 2014; Ma et al., 2015; Sun et al., 2015). Afterwards, the aforementioned suspension was reacted for different time ranging from 5min to 24 h with continuous stirring conditions. The solid phase was separated from liquid phase by centrifugating at 9000 rpm for 15 min. The blank

where Qe (mg/g) and Qt(mg/g) are the adsorption concentration of Eu(III) at equilibrium and time t, respectively. K1 and K2 are the pseudo-first order and pseudo-second order kinetic rate constants, respectively. In order to have a further understanding of Eu(III) adsorption on nano-hematite, the kinetic behavior of the adsorption process was analyzed by using the intra-particle diffusion model. It was used to realize the steps involved in adsorption process and the transport of adsorbate from the exterior surface to the pores of adsorbent(Zeng et al., 2014). The model was described as Eqn. (5).

Q t ¼ ki t1=2 þ C

(5)

where ki is the intra-particle diffusion rate constant and C is a constant. The intra-particle-diffusion plot gave multi-linearity, indicating that the whole adsorption process was composed of several stages. 2.5. Isotherms models The adsorption isotherms of Eu(III) on nano-hematite can be fitted by Langmuir or Freundlich models, the linear forms are given in Eqn. (6)(Chen et al., 2009a) and(7) (Guo et al., 2011), respectively:

Ce =Q e ¼ 1=ðKL $Q m Þ þ Ce =Q m

(6)

lgQ e ¼ ð1=nÞlgCe þlgKF

(7)

where Qm (mg$g
1) is the maximum adsorption capacity of adsorbent at complete monolayer coverage. KL (L$mg
1) is a Langmuir constant which is related to the free energy of sorption. 1/ n is the heterogeneity of the adsorption sites. KF represents equilibrium coefficient which represented the partitioning of the adsorbate between the solid and liquid phases over the concentration range studied.

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2.6. Thermodynamic models In order to understand the thermodynamic properties of adsorption reaction at equilibrium state, the thermodynamic parameters (Gibbs free energy change(DG ), enthalpy change (DH ) and entropy change (DS )) could be calculated by Eqns. (8)e(10)(Yang et al., 2009; Zhao et al., 2012), respectively:

DG ¼ DH
TDS

(8)

ln K ¼ DS =R
DH =RT

(9)

Kd ¼ Q e =Ce

(10)

where R and T are the universal constant (8.314 J/mol$K) and the temperature of Kelvin, respectively. K is the adsorption equilibrium constant which could be reckoned by plotting lnKd versus Ce and extrapolating Ce to zero.

Table 1 The selective parameters of nano-hematite.

3. Results and discussion 3.1. Characterization Fig. 1 showed the TEM image of nano-hematite from the decomposition of siderite. As shown in Fig. 1A, relatively loose particle structure was observed, and the particle size can be up to nanometer level (Zhao and Guo, 2014; Zhao et al., 2014). Accordingly, the calcinations resulted in the changes of the siderite morphology compared to sheet structure (Fig. 1B) of natural siderite and the increase of SSA. Xing et al(Xing et al., 2016). reported the SSA of natural siderite was 4.17 m2$g1. Fig. 2 showed the XRD patterns of nano-hematite before and after adsorption of Eu(III). On the basis of XRD analysis, almost all of siderite was transformed into hematite after calcining at 600  C for 30min ~ iz et al., 2012). Basically no other observed in Fig. 2 (Ramirez-Mun impurities appeared, indicating that nano-hematite was successfully prepared. The transformation can be expressed as Eq. (11) (Zhao and Guo, 2014).

4FeCO3 þ O2 /2Fe2 O3 þ 4CO2

Fig. 2. XRD patterns of nano-hematite before and after adsorption of Eu(III) (H: Hematite).

(11)

In order to understand the changes of the crystal structure of hematite in the reaction process, the blank experiment was conducted under the same experimental conditions. As shown in Fig. 2, the intensities of reflections were weakened after blank experiment. What's more, the reflections of nano-hematite-Eu was significantly weakened at 2q ¼ 33.27 compared with nanohematite and nano-hematite-blank which probably on account of the adsorption of Eu(III). Calculated by multipoints N2-BET method, the specific surface area of nano-hematite was about 42 m2$g
1. As described in Table 1, the pHpzc (point of zero charge) of nano-hematite was 5.4.

Fig. 1. The TEM images of nano-hematite and natural siderite (A: nano-hematite; B: natural siderite).

Adsorbents

SBET (m2/g)

pHpzc

nano-hematite natural siderite

42.54 4.17

5.43 6.06

3.2. Effect of pH and ionic strength As shown in Fig. 3, the effect of pH and ionic strength on Eu(III) adsorption on nano-hematite was explored by the batch technique. The effect of initial solution pH on Eu(III) adsorption on nanohematite was investigated at the pH ranging from 2 to 11, and the initial concentration of Eu(III) was 10 mg/L. One can see that lower removal rate of Eu(III) was observed at pH < 2.0. As depicted in Fig. 3A, the significant increase in Eu(III) adsorption was observed at the pH ranging from 2.0 to 6.0. Then it kept slowly increased at pH > 6.0, the removal rate was up to the high level of about 100% at pH ¼ 11.0. The adsorption trends could be explicated by the surface property of nano-hematite and ion distribution of Eu(III) in solution (Ding et al., 2014; Sheng et al., 2009). In previous studies (Chen et al., 2013; Sun et al., 2012a; Tan et al., 2009; Wang et al., 2006a), it was demonstrated that Eu3þ was the main species of Eu(III) aqueous solution at pH < 5.0 Afterwards, the hydrolyzed mononuclear and multinuclear species (Chen et al., 2007; Sheng et al., 2009; Wang et al., 2006b) were appeared with the increase of pH(i.e., Eu(OH)2þ). As shown in Table 1, the pHpzc of nanohematite were about 5.4, consequently, the positive charge of nano-hematite surface was observed at pH < 5.4 and negative charge at pH > 5.4. Furthermore, the lower removal rate was owing to electrostatic repulsion between positive charged nano-hematite and Eu3þspecies. The significant increase in Eu(III) adsorption could be ascribed to electrostatic attraction (Ding et al., 2014; Ren et al., 2014; Sun et al., 2012b). Fig. 3A also showed the effect of ionic strength on the adsorption behavior of Eu(III) on nano-hematite. One can be seen from Fig. 3A, no significant effect of increased ionic strength on the removal of Eu(III) was observed. According to previous reports (Ding et al., 2015; Fan et al., 2009; Hu et al., 2010; Wang et al., 2015b), the outer-sphere complexes are more sensitive to the ionic strength variations than inner-sphere complexes because the background electrolyte ions were placed in the same plane for outer-sphere surface complexes. Accordingly, inner-sphere surface complexation was speculated to dominate the adsorption of Eu(III) on nanohematite based on the above analysis.

M. Li et al. / Applied Geochemistry 84 (2017) 154e161

157

Fig. 3. A: Effect of pH and ionic strength on adsorption of Eu(III) on nano-hematite (nano-hematite-H2O represented the sample after desorption of Eu(III)). B: Potentiometric titration. CEu ¼ 10 mg/L, m/V ¼ 2.0 g/L, I ¼ 0.1 mol/L NaCl, T ¼ 293 K.

3.3. Adsorption and desorption kinetics Fig. 4A showed the adsorption-desorption kinetics of Eu(III) on nano-hematite at pH 5.5. Obviously, the adsorption process (nanohematite-Eu) has reached equilibrium within 5 h. As illustrated in Fig. 4A, the desorption capacity of Eu(III) on nano-hematite (nanohematite-Eu-H2O) significantly increased with increasing reaction time in first 5 h and then gradually tends to be gentle. One can see that the desorption kinetics of Eu(III) from nano-hematite significantly veered off its adsorption kinetics, resulting in the formation of adsorption-desorption hysteresis (Ding et al., 2014; Yuan et al., 2007). According to the study, adsorption-desorption hysteresis included reversible and irreversible hysteresis (Orecchio and Mannino, 2010). The reversible hysteresis was complete desorption without intervention, presenting as a closed hysteresis loop

between adsorption-desorption curves. And the irreversible hysteresis could be interpreted as that complete desorption cannot happen without intervention (Kan et al., 1998). Fig. 4A showed that the desorption of Eu(III) from nano-hematite was an irreversible process. As described in Table 2, the adsorption kinetics of Eu(III)

Table 2 Kinetic models of Eu(III) adsorption on nano-hematite.

pseudo-first-order pseudo-second-order intra-particle-diffusion

Qe (mg$g
1)

Ki(g$(mg $ min)
1

R2

C

2.20 5.03 5.00

0.90 2.03 1.94

0.6388 0.9998 0.9259

/ / 2.45

Fig. 4. A: Adsorption-desorption kinetics of Eu(III) on nano-hematite. B: pseudo-second-order model (nano-hematite-H2O represented the sample after desorption of Eu(III)). C-D: Intra-particle diffusion model. CEu ¼ 10 mg/L, m/V ¼ 2.0 g/L, I ¼ 0.1 mol/L NaCl, T ¼ 293 K, pH ¼ 5.5.

158

M. Li et al. / Applied Geochemistry 84 (2017) 154e161

Fig. 5. A: Effect of temperatures of Eu(III) adsorption on nano-hematite. B: Effect of initial concentrations on the adsorption of Eu(III) over nano-hematite in 0.1 mol/L Na2CO3 solution. C: Species of Eu(III) in 0.1 mol/L Na2CO3 solution. CEu ¼ 2e30 mg/L, m/V ¼ 2.0 g/L, I ¼ 0.1 mol/L NaCl, pH ¼ 5.5, t ¼ 24 h.

adsorption on nano-hematite could be well fitted by pseudosecond kinetic model (R2 ¼ 0.9998) compared to pseudo-first order kinetic model (R2 ¼ 0.6388). The adsorption kinetics suggested that the chemisorption of Eu(III) on nano-hematite was the ratelimiting step (Ding et al., 2014; Kim et al., 2014; Wang et al., 2010). As depicted in Fig. 4C, the fitting curves for nano-hematite was plotted by first 5 spots because of the following points have reached a balance which would affect the linear relationship. And the related parameters were listed in Table 2. The linear straight line segment didn't passing through the origin, which suggested that the external diffusion and internal diffusion dominated the whole process of Eu(III) adsorption.

Eu(III), the blank experiment was carried out under the same conditions with the initial concentration of 10 mg/L. After reaction for 24 h, the concentration of Eu(III) in solutions was 9.75 mg/L, demonstrating that the effect of self-precipitation on the adsorption of Eu(III) was very small and almost can be ignored. Fig. 5B displayed the effect of concentration on Eu(III) adsorption on nano-hematite in 0.1 mol/L Na2CO3 solution. Consistent with the result of above, the amount of Eu(III) adsorbed on nanohematite increased with increasing solution concentration. Unlike with the variation tendency compared to the reaction without Na2CO3 solution, the adsorption rate increased with the increasing of the initial concentration of the solution, which proved that the presence of CO2
3 contributed to the adsorption process. Ion distribution of Eu(III) in Na2CO3 solution was exhibited in Fig. 5C. þ Eu(III) was mainly existed with the species of EuHCO2þ 3 , EuCO3 , Eu(CO3)-2 at pH ¼ 5.5. Combined with the pHpzc ¼ 5.4 of nanohematite, the negatively charged nano-hematite surface was easily reacted with positively charged Eu(III) species as a result of electrostatic attraction. Calculated by two models, the relative parameters were displayed in Table 3. On the basis of these records, the adsorption of Eu(III) on nano-hematite could be well fitted by Langmuir model (R2 > 0.9896) than Freundlich model(R2 < 0.9893), which revealed

3.4. Adsorption isotherms Fig. 5A showed the effect of concentration and temperature on Eu(III) adsorption on nano-hematite. As shown in Fig. 5A, the adsorption of Eu(III) on nano-hematite significantly increased with increasing solution concentration. The amount of Eu(III) adsorbed on nano-hematite was highest at T ¼ 333 K and the lowest at T ¼ 293 K, which accounted for that a higher temperature can promote the adsorption process(Sheng et al., 2010; Sun et al., 2012b; Wang et al., 2006a). To investigate the self-precipitation of

Table 3 Langmuir and Freundlich model of Eu(III) adsorption on nano-hematite. Temperature (K)

Langmuir Qm (mg$g
1)

KL (L$mg
1)

R2

Freundlich KF (mg$g-1)/(mg$L)
n

1/n

R2

293 313 333

5.07 9.66 13.02

1.20 0.60 0.52

0.9896 0.9908 0.9935

1.68 2.62 3.26

0.47 0.55 0.57

0.9706 0.9893 0.9879

M. Li et al. / Applied Geochemistry 84 (2017) 154e161 Table 4 Thermodynamic parameters for the adsorption of Eu(III) on nano-hematite. Temperature (K)

DG /(kJ$mol
1)

DS /(J$mol
1.K
1)

DH /(kJ$mol
1)

293 313 333


4.39
4.58
5.29

22.44 22.44 22.44

2.19 2.45 2.19

that the adsorption process was monolayer adsorption. As shown in Table 3, the maximum adsorption capacity of Eu(III) on nanohematite was 13.02 mg g
1 at pH 5.5 and T ¼ 333 K. The capacity was slightly higher than 12.30 mg g
1of Eu(III) adsorption on nature hematite(0.25e0.425 mm) at pH 4.7 and T ¼ 298 K, however, it was lower than 20.81 mg/g as the particle size lower than 0.25 mm in this report (E.M. El Afifi et al., 2016). 3.5. Thermodynamic parameters As illustrated in Table 4, the negative DG values for Eu(III) adsorption on nano-hematite was observed, which revealed that adsorption process was a spontaneous process. The DG values became more negative with the increase of temperature suggested that more efficient adsorption at high temperature (Ding et al., 2015). The positive DH values illustrated that Eu(III) adsorption on nano-hematite was an endothermic process (Hu et al., 2011; Zhao et al., 2011). Furthermore, the positive DS values indicated that the adsorption of Eu(III) on nano-hematite increased the disorder of the solid-solution system (Yang et al., 2010; Zhao et al., 2012). 3.6. Adsorption mechanism Fig. 6A showed the FT-IR spectra of the nano-hematite before

159

and after adsorption of Eu(III). The bands appeared at 544 and 448 cm
1 were due to Fe-O vibration peaks of nano-hematite (Li et al., 2006), respectively. After adsorption of Eu(III), the absorption band of Fe-O was slightly shifted and the intensity was significantly weakened. The peak presented at 1459 cm
1 was attributed to the deformation of hydroxyl vibration of water. Afterwards, the peaks observed at 3320 cm
1 should be the O-H stretching vibration of adsorbed water (Sun et al., 2016). To determine the interaction mechanism between radionuclides and nano-hematite, the XPS spectra of survey and high resolution scans for O 1s, Fe 2p, and Eu 3d on the nano-hematite before and after adsorption of Eu(III) and desorption of Eu(III) was observed in Fig. 6. As shown in Fig. 6B, Eu 3d appeared in nano-hematite-Eu and nano-hematite-Eu-H2O but no in nano-hematite, which suggested that Eu(III) was adsorbed on the nano-hematite. The weakly shift of binding energy and the decrease of peak area of the O 1s spectra was exhibited in Fig. 6B, which illustrated that the Eu(III) adsorption on nano-hematite was ascribed to the oxygen-containing functional groups (Estes et al., 2013). Afterwards, the spectral line of Fe 2p was also presented in Fig. 6B. Fig. 6C showed the high resolution scans of O 1s of nano-hematite before and after adsorption of Eu(III). Owing to the adsorption of Eu(III), the relative intensity of O 1s was weakened after adsorption. The separation of the O1speak composed of Fe-O-Fe and Fe-O-H was attributed to the different chemical environment. The peak area of Fe-O-H slightly increased after Eu(III) adsorption(39.13% for nano-hematite-Eu, 32.07% for nano-hematite-Eu-H2O and 28.04% for nanohematite), suggesting that Eu(III) adsorption onto nano-hematite by chemisorbed OH
groups (Chen et al., 2016). As depicted in Table 5, a higher binding energy of O1s of nano-hematite-Eu compared to nano-hematite was observed, which was attributed to the increased negative charge of oxygen atom in nano-hematite after adsorption (Tan et al., 2009; Wen et al., 2014). According to the

Fig. 6. A: FTIR spectra of nano-hematite. B-D: The survey and high resolution scans of XPS spectra of Eu(III) adsorption on nano-hematite (B: Total XPS spectra of survey and high resolution scans; C: O 1s; D: Eu 3d.).

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M. Li et al. / Applied Geochemistry 84 (2017) 154e161

Table 5 The binding energy of XPS analysis of nano-hematite. sample

Eu 3d5/2

Eu 3d3/2

Fe-O-Fe

Fe-O-H

Fe 2p

nano-hematite nano-hematite-Eu nano-hematite-Eu-H2O

/ 1134.8 1134.9

/ 1164.3 1164.3

529.8 530.1 530.0

531.5 532.0 531.7

710.9 710.7 711.0

survey scans of XPS spectra, it was explicitly confirmed that the adsorption of Eu(III) on nano-hematite was bound up to oxygencontaining functional groups. As shown in Fig. 6D, Eu 3d spectra after adsorption could be characterized with two doublet-peaks such as Eu 3d5/2 and Eu 3d3/2 but no Eu 3d spectra appeared in nano-hematite which further proved that Eu(III) was successfully adsorbed on nano-hematite-Eu. Compared to nano-hematite-Eu, the intensity of Eu3dof nano-hematite-Eu-H2O was slightly weakened, indicating that Eu(III) was desorbed from nano-hematite-Eu. 4. Conclusions According to the results of characterization, nano-hematite was satisfactorily synthesized by calcination of natural siderite at 600  C. Batch adsorption results indicated that the Eu(III) adsorption was remarkably increased at the pH 2.0e6.0 and kept slowly increased at pH > 6.0. No effect of ionic strength revealed that inner-sphere surface complexation dominated the adsorption of Eu(III) on nano-hematite. In addition, the desorption of Eu(III) from nano-hematite was an irreversible process. The adsorption kinetics of Eu(III) adsorption on nano-hematite could be well fitted by pseudo-second kinetic model. The external diffusion and internal diffusion dominated the whole process of Eu(III) adsorption derived from intra-particle diffusion model. The adsorption isotherms of Eu(III) could be well fitted by Langmuir model, the maximum adsorption capacity of Eu(III) on nano-hematite was 13.02 mg g^-1 at T ¼ 333 K, pH ¼ 5.5 and the presence of CO2
3 favored the adsorption. The calculated thermodynamic parameters explained that the adsorption of Eu(III) was an endothermic and spontaneous process. Furthermore, the adsorption of Eu(III) on nano-hematite was ascribed to the oxygen-containing functional groups derived from XPS analysis. Acknowledgement Financial support from National Natural Science Foundation of China (No. 41402030, 41572029) and the Fundamental Research Funds for the Central Universities (JZ2017HGTB0196) are acknowledged. References Chen, H., Zhao, Y., Wang, A., 2007. Removal of Cu(II) from aqueous solution by adsorption onto acid-activated palygorskite. J. Hazard. Mater. 149, 346e354. Chen, C.L., Hu, J., Xu, D., Tan, X.L., Meng, Y., Wang, X.K., 2008. Surface complexation modeling of Sr(II) and Eu(III) adsorption onto oxidized multiwall carbon nanotubes. J.Colloid Interface Sci. 323, 33e41. Chen, C.L., Hu, J., Shao, D.D., Li, J.X., Wang, X.K., 2009a. Adsorption behavior of multiwall carbon nanotube/iron oxide magnetic composites for Ni(II) and Sr(II). J. Hazard.Mater 164, 923e928. Chen, C.L., Wang, X.K., Nagatsu, M., 2009b. Europium adsorption on multiwall carbon nanotube/iron oxide magnetic composite in the presence of polyacrylic acid. Environ. Sci. Technol. 43, 2362e2367. Chen, C.L., Yang, X., Wei, J., Tan, X.L., Wang, X.K., 2013. Eu(III) uptake on rectorite in the presence of humic acid: a macroscopic and spectroscopic study. J.Colloid Interface Sci. 393, 249e256. Chen, L., Zhao, D., Chen, S., Wang, X., Chen, C., 2016. One-step fabrication of amino functionalized magnetic graphene oxide composite for uranium(VI) removal. J.Colloid Interface Sci. 472, 99e107. Cheng, W.C., Ding, C.C., Sun, Y.B., Wang, X.K., 2015a. Fabrication of fungus/attapulgite composites and their removal of U(VI) from aqueous solution. Chem.

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