Sorption of U(VI) on goethite: Effects of pH, ionic strength, phosphate, carbonate and fulvic acid

ARTICLE IN PRESS Applied Radiation and Isotopes 67 (2009) 996–1000

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Sorption of U(VI) on goethite: Effects of pH, ionic strength, phosphate, carbonate and fulvic acid Zhijun Guo , Yan Li, Wangsuo Wu Radiochemistry Lab, School of Nuclear Science and Technology, Lanzhou University, Lanzhou 730000, China

a r t i c l e in f o

a b s t r a c t

Article history: Received 28 September 2008 Received in revised form 11 January 2009 Accepted 5 February 2009

U(VI) sorption on goethite was investigated as functions of pH, solid-to-liquid ratio (m/V), ionic strength and U(VI) concentration by a batch experimental method. Effects of phosphate, carbonate and fulvic acid (FA) on U(VI) sorption were examined. It was found that the sorption of U(VI) increases from 0% to 100% over the pH range of 2.5–4.5 and keeps constant in the high pH range. The sorption of U(VI) on goethite is insensitive to ionic strength. Different surface complexes in the framework of double-layer model were examined for fitting the sorption of U(VI) on goethite. A model with two mononuclear inner-sphere surface complexes, SOUO+2 and SOUO2OH, was found capable of reproducing the pH sorption edges, the sorption isotherms and the sorption data with variable m/V in this study. The proposed model can also interpret the pH sorption edge collected at P CO2 ¼ 103:58 atm without considering any ternary surface complexes of carbonate. Moreover, it was found that the presence of phosphate at relatively high concentration (6  104 mol/L) promotes U(VI) sorption. The presence of FA of 20 mg/L has little effect on the sorption of U(VI) on goethite. & 2009 Elsevier Ltd. All rights reserved.

Keywords: Uranium(VI) Sorption Goethite Surface complexation Modeling Phosphate Carbonate Fulvic acid

1. Introduction The transport of a contaminant in subsurface environment is mainly controlled by interface processes, such as sorption/ desorption, precipitation/dissolution etc. Uranium is an important contaminant concerning the sites of uranium mining and milling, and nuclear waste disposal. Thus, the sorption of U(VI) on various minerals has been extensively studied (Davis et al., 2004; Drot et al., 2007). Under oxic conditions, uranium is highly soluble due to the formation of U(VI) complexes in natural aqueous solutions. Phosphate, carbonate and humic substances are among the most common ligands existing in natural waters. The effects of these ligands on the sorption of cations (Lenhart and Honeyman, 1999; Wang and Xing, 2004) and the interactions between these ligands and various minerals (Zhong et al., 2007; Mustafa et al., 2006; Evanko and Dzombak, 1999; Rahnemaie et al., 2007) have received a lot of attention. Goethite is the most common iron (hydr)oxides in natural environment. The effects of organic and inorganic ligands on the sorption of U(VI) on goethite have also been reported (Cheng et al., 2004; Ronero-Gonza´lez et al. 2007; Villalobos et al., 2001). Surface complexation models (SCMs) have been commonly used to describe quantitatively ion sorption in geochemistry. However, many SCMs were constructed by considering just a few pH sorption edges which were collected under very limited  Corresponding author.

E-mail address: [email protected] (Z. Guo). 0969-8043/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2009.02.001

experimental conditions. As a result, some of these SCMs can not interpret the sorption data collected under other conditions. In this study, the sorption of uranium(VI) on goethite was investigated as functions of pH, solid-to-liquid ratio (m/V), ionic strength and U(VI) concentration. The effects of phosphate, carbonate and fulvic acid on the sorption of U(VI) were also studied. The sorption of U(VI) on goethite will be interpreted in the framework of surface complexation model. In addition, we will try to demonstrate the importance of other sorption data besides the pH sorption edges for setting up a proper SCM. 2. Experimental 2.1. Materials Goethite was prepared according to the procedures of Atkinson et al. (1968). The goethite was identified with powder X-ray diffraction. The nitrogen B.E.T. surface area of the goethite was determined from N2 adsorption/desorption isotherms to be 86.4 m2/g. Uranium(VI) and phosphate stock solutions were prepared from UO2(NO3)  6H2O (A.R. grade) and NaH2PO4(A.R. grade), respectively. Fulvic acid (FA; Tao et al., 1995) from Gong Xian weathered coal (Henan Province, PR China) was used. The characterization of the fulvic acid are as follows: the elemental composition (%) is C 55.95, H 2.01, N 1.11, S 1.03 and O 39.90, the number average molecular weight is 1300 Da, the capacities of carboxyl and phenolic groups are 8.16 and 1.76 meq H+/g, respectively. All other chemicals used were of analytical grade.

ARTICLE IN PRESS Z. Guo et al. / Applied Radiation and Isotopes 67 (2009) 996–1000

where V (L) is the volume of aqueous solution and m (g) the mass of goethite. The distribution coefficient of uranium(VI) (Kd, L/g) was calculated by

2.2. Sorption A stock suspension of goethite (m/V ¼ 5 g/L) was prepared in NaCl solution (0.01, 0.1 and 0.5 mol/L NaCl). Aliquots of the goethite suspension were sampled into polyethylene tubes. The pH was adjusted with small amounts of HCl or NaOH solutions. U(VI) was added and the suspension was shaken at 2572 1C. For the experiments involving FA and phosphate, the ligand was added after addition of U(VI). When sorption steady state was reached, the solid and aqueous phases were separated by centrifugation at 18 000g for 30 min. The concentrations of uranium(VI) and phosphate in the supernatant were analyzed by using the uranyl Arsenazo(III) complex at 652 nm (Tao et al., 2000) and the blue phosphate–molybdate complex at 700 nm, respectively. The concentration of FA in the supernatant was determined at 200 nm on a PerkinElmer Lambda 35 UV/VIS spectrometer. For the U(VI) sorption system at P CO2 ¼ 103:58 atm, the pH of the suspension was adjusted with HCl for sorption at pHo7. For pH 47, calculated amounts of NaHCO3 and/or Na2CO3 were added. Then the suspensions (pH 3–10) were bubbled with air which had been conditioned with 0.1 mol/L NaCl solution for water vapor saturation. When the pH variations of the suspensions were less than 0.08 pH units per day, U(VI) was added. The suspensions were shaken at 2572 1C and the tubes were opened regularly until the pH drifts were less than 0.08 pH units a day. Phases separation and U(VI) measurements were carried out by the same methods as above. The amounts of uranium(VI)/ phosphate/ FA sorbed (q, mol/g) were calculated by the difference of the initial and final concentrations in the aqueous phase (C0 and Ceq, respectively, mol/L): V m

Kd ¼

3. Results and discussion 3.1. Effect of aqueous pH Fig. 1 shows the pH sorption edges of U(VI) at different U(VI) concentrations (5.16  105, 6.18  105 and 8.21 105 mol/L). The pH sorption edges emerge on the same line, because the U(VI) concentrations are very close. It should be helpful to interpret the sorption of U(VI) on goethite if one has an idea of U(VI) speciation. Besides pH, the speciation of U(VI) depends on U(VI) concentration because several polynuclear hydrolysis products of U(VI) exist in the high pH range. In addition, many aqueous anions also affect the U(VI) speciation. Fig. 2 shows the U(VI) speciation

80

80

Sorption (%)

100

= SOUO2+

40 20 [U(VI)] = 5.16e-5 M; [U(VI)] = 6.18e-5 M; [U(VI)] = 8.21e-5 M;

0 2

3

4

5

6

7

8

(2)

Surface complexation models similar to that proposed by Dzombak and Morel (1990) were used for modeling calculations. The site density and the intrinsic hydrolysis constants of goethite were from literatures (Missana et al., 2003, 2005). A geochemical code Phreeqc (version 2.15; Parkhurst and Appelo, 1999) was used for modeling. The Davies equation was used for activity correction of the aqueous species only. Thermodynamic data of uranium used in modeling were consistent with the NEA database (Grenthe et al., 1992). A stepwise approach to modeling the sorption data was adopted to get the best fit (Bradbury and Baeyens, 2002).

100

60

ðC 0  C eq Þ V q ¼ C eq C eq m

2.3. Modeling

(1)

Sorption (%)

q ¼ ðC 0  C eq Þ

997

= SOUO2OH [U(VI)] = 5.16e-5 M; [U(VI)] = 6.18e-5 M; [U(VI)] = 8.21e-5 M;

60

Model 2; Model 2; Model 2.

40 20

Model 1; Model 1; Model 1.

= SOUO2+

0

9 10 11 12

2

3

4

5

pH

6

7

8

9 10 11 12

pH

Sorption (%)

100 80 60

(= SO)2UO2

40 20

[U(VI)] = 5.16e-5 M; [U(VI)] = 6.18e-5 M; [U(VI)] = 8.21e-5 M;

0 2

3

4

5

6

7 pH

8

Model 3; Model 3; Model 3.

9 10 11 12

Fig. 1. pH sorption edges of U(VI) on goethite at m/V ¼ 5 g/L and [NaCl] ¼ 0.1 mol/L; symbols represent the experimental data; lines are the results calculated by (a) model 1, (b) model 2 and (c) model 3.

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100

K int 3 ¼

(UO2)3(OH)7(UO2)3(OH)5+

[U (VI)]/[U(VI)]TOT (%)

80

UO2(OH)3

UO22+

-

60

3 ½ SOH½UO2þ 2 

g3UO2þ

expð2F c=RTÞ

40

(UO2)4(OH)7+ UO2OH+

UO2(OH)42-

(UO2)2(OH)22+

UO2(OH)20

UO2Cl+ 0 2

4

6

8

10

12

pH

To interpret the sorption of U(VI) on goethite, other surface complexation reactions have been proposed in literatures. Reactions (3) and (10) (model 2) have been used by Missana et al. (2003):

K int 4 ¼

at [U(VI)]TOT ¼ 6  10 mol/L and [NaCl] ¼ 0.1 mol/L. As seen in Fig. 2, free UO2+ 2 is predominant over the pH range up to 5. In the high pH range, (UO2)3(OH)+5 and (UO2)3(OH) 7 successively become + the dominant U(VI) species. Thus UO2þ 2 , (UO2)3(OH)5 and  (UO2)3(OH)7 should be considered as reasonable sorbates. The surface complexation reaction of UO2þ 2 can be written as (3)

The intrinsic equilibrium constant of reaction (3) can be defined as K int 1 ¼

þ ½ SOUOþ 2 ½H  2þ ½ SOH½UO2 

gHþ expðF c=RTÞ gUO2þ

(4)

2

where F is Faraday’s constant, R the universal gas constant, T the absolute temperature, c the surface potential and g1 the activity coefficient of the aqueous species i. The surface potential can be calculated by (Dzombak and Morel, 1990):

s ¼ ð8RT 0 ce  103 Þ1=2 sinhðZ e cF=2RTÞ

(5)

where s is the surface charge density, e the dielectric constant of water (dimensionless, 78.5 at 25 1C), e0 the vacuum permittivity (8.854  1012 C/V m), ce the concentration of the background electrolyte, Ze the valence of the symmetrical background electrolyte. As seen in Fig. 1(a), reaction (3) (model 1) could reproduce the sorption of U(VI) at pHo8, but could not interpret that at higher pH. Therefore, reactions (6) and (8) concerning (UO2)3(OH)+5 and (UO2)3(OH) 7 should be considered. It was found that the pH sorption edges could be fitted with either reaction (6) or (8) in addition to reaction (3). However, the corresponding equilibrium constants of reactions (6) and (8) are not sensitive to adjustments in both cases, which implies that reactions (3) and (6), or reactions (3) and (8) may not be proper to describe the sorption of U(VI) on goethite: þ  SOH þ 3UO2þ 2 þ 5H2 O2  SOðUO2 Þ3 ðOHÞ5 þ 6H

½ SOðUO2 Þ3 ðOHÞ5 ½H  g6Hþ ¼ 3 g3UO2þ ½ SOH½UO2þ 2 

(6)

þ 6

(11)

gUO2þ 2

K int 5 ¼

(12)

½ð SOÞ2 UO2 ½Hþ 2 g2Hþ

(13)

gUO2þ ½ SOH2 ½UO2þ 2  2

The validity of models 2 and 3 was examined by the sorption data in this study. As seen in Fig. 1(b), model 2 can reproduce the pH sorption edges. The equilibrium constants of reactions (3) and (10) are identical to those obtained by Missana et al. (2003) (see Table 1). As shown in Fig. 1(c), model 3 could also reasonably fit the pH sorption edges. Moreover, it should be noted that the modeling results for three U(VI) concentrations are very close, which are consistent with the experimental results. 3.2. Effects of ionic strength and solid-to-liquid ratio Fig. 3(a) shows the sorption isotherms of U(VI) on goethite at three ionic strengths (0.01, 0.1 and 0.5 mol/L NaCl) and pH 3.8870.05. The sorption isotherms are very close, which indicates that the sorption of U(VI) is insensitive to ionic strength. Fig. 3(b) shows the dependence of U(VI) sorption on m/V at pH 3.7470.05 and [U(VI)]TOT ¼ 5.16  105 mol/L. The increasing sorption of U(VI) with m/V must be related to the increasing site capacity. As discussed in Section 3.1, three SCMs could fit to some extent the pH sorption edges of U(VI). However, which model is the best? Table 1 Modeling parameters. Goethite description Specific surface area Site density

86.4 m2/g 1.3 sites/nm2 (2.210-6 mol/m2)a

Aqueous solution/solid equilibria Reactions

logKint

+

SOH+H SOH2SO +H

7.2a 10a

Model 1 + + SOH+UO2+ 2 2SOUO2+H

2.8

Model 2 + + SOH+UO2+ 2 2SOUO2+H + SOH+UO2+ 2 +H2O2SOUO2OH+2H

2.8 3.7

Model 3 + 2SOH+UO2+ 2 2(SO)2UO2+2H

1.8

2SOH+2  +

(7)

2

2 þ  SOH þ 3UO2þ 2 þ 7H2 O2  SOðUO2 Þ3 ðOHÞ7 þ8H

½ SOH½UO2þ 2 

(8)

(10)

In addition, a mononuclear bidentate surface complex, (SO)2UO2, alone (model 3) or in addition to other surface complexes, has been used (Cheng et al., 2004; Sherman et al., 2008; Ronero-Gonza´lez et al., 2007):

5

þ þ  SOH þ UO2þ 2 2  SOUO2 þH

½ SOUO2 OH½Hþ 2 g2Hþ

þ 2  SOH þ UO2þ 2 2ð SOÞ2 UO2 þ2H

Fig. 2. Speciation of U(VI) in NaCl solution; [U(VI)] ¼ 6  105 mol/L, [NaCl] ¼ 0.1 mol/L; solids are not allowed to be precipitated.

(9)

2

þ  SOH þ UO2þ 2 þH2 O2  SOUO2 OH þ 2H

20

K int 2

½ SOðUO2 Þ3 ðOHÞ7 ½Hþ 8 g8Hþ

a

Missana et al. (2003, 2005).

ARTICLE IN PRESS Z. Guo et al. / Applied Radiation and Isotopes 67 (2009) 996–1000

Sorption (%)

10-5 10-6 10-7 10-4

q (mol/g)

100

Model 1

Model 2

10-5 10-6

Sorption (%)

Model 3 q (mol/g)

60 40 20

Model 2

80 60 40 20 0 100

10-7 10-4 10

Model 1

80

0 100 Sorption (%)

q (mol/g)

10-4

999

-5

10-6

Model 3

80 60 40 20 0

10-7

10-6

10-5

10-4

10-3

0.1

1

Ceq (mol/L)

10 m/V (g/L)

100

Fig. 3. (a) Sorption isotherms of U(VI) on goethite at pH ¼ 3.8870.05, m/V ¼ 5 g/L and three ionic strengths (J 0.01 mol/L NaCl; W 0.1 mol/L NaCl; B 0.5 mol/L NaCl). (b) The dependence of U(VI) sorption on m/V at pH ¼ 3.7470.05, [NaCl] ¼ 0.1 mol/L and [U(VI)]TOT ¼ 5.16e–5 mol/L; symbols represent the experimental data; lines are the results calculated by the models.

100

80 60 40 20

[P(V)] = 0 mol/L [P(V)] = 1E-4 mol/L; [P(V)] = 6E-4 mol/L.

0

U (VI) sorption (%)

U(VI) sorption (%)

100

60 40 20 [FA] = 0 mg/L; [FA] = 20 mg/L.

0

100

100

80

= SOUO2OH

60 40

PCO = 0 atm; 2 PCO = 10-3.58 atm.

20

2

FA sorption (%)

U(VI) sorption (%)

80

80 60 40 20

= SOUO2+

[U(VI)] = 0, [U(VI)] = 5.16E-5 mol/L.

0 0 2

3

4

5

6

7 pH

8

9

10 11

2

3

4

5

6

7

8

9 10 11 12

pH

Fig. 4. pH sorption edges of U(VI) on goethite; (a) effect of phosphate; (b) effect of carbonate; (c) effect of FA; (d) FA sorption as a function of pH in the presence and absence of U(VI). Symbols in Fig. 4(a)–(c) represent the experimental data collected at [U(VI)] ¼ 5.16e–5 mol/L, [NaCl] ¼ 0.1 mol/l and m/V ¼ 5 g/L; symbols in Fig. 4(d) represent the experimental data collected at [FA] ¼ 20 mg/L, [NaCl] ¼ 0.1 mol/L and m/V ¼ 5 g/L; lines in Fig. 4(b) are the results calculated by model 2.

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This question can be answered by the sorption isotherms and the relationship between sorption and m/V. As shown in Fig. 3(a) and (b), it is obvious that model 2 is the best because it can reproduce the sorption isotherms and the dependence of sorption on m/V in addition to the pH sorption edges of U(VI). It is popular that only one or a few pH sorption edges were used to construct SCMs. This study demonstrates that sorption isotherms and the relationship between sorption and m/V are necessary constrains for a proper SCM.

mononuclear inner-sphere surface complexes SOUO+2 and SOUO2OH, can be used to interpret the sorption of U(VI) on goethite. The proposed model can reproduce the pH sorption edge of U(VI) at P CO2 ¼ 103:58 atm without considering any ternary surface complexes; (4) the effect of phosphate on the sorption of U(VI) is strongly dependent on phosphate concentration; (5) the presence of FA (20 mg/L) has little effects on U(VI) sorption on goethite.

3.3. Effects of FA, phosphate and carbonate

Acknowledgments

Fig. 4(a) shows the pH sorption edges of U(VI) in the presence of phosphate. As seen in Fig. 4(a), the effect of phosphate on the sorption of U(VI) strongly depends on its concentration. The presence of phosphate at 1.0  104 mol/L has little influence on U(VI) sorption, whereas the presence of phosphate at 6.0  104 mol/L obviously promotes the sorption of U(VI) over the pH range from 2.3 to 4.5. It is well known that phosphate has relatively strong affinity for mineral surface. Several inner-sphere surface complexes of phosphate on goethite have been confirmed by spectroscopic techniques. FT-IR results obtained by Persson et al. (1996) indicated that inner-sphere monodentate phosphate surface species are dominant on goethite surface. Luengo et al. (2006) concluded that the bidentate surface complexes (FeO)2PO2 and (FeO)2(OH)PO are formed at pH 4.5, while at pH 7.7 and 9, (FeO)2PO2 is the dominant species. Zhong et al. (2007) concluded that surface binding changes from monodentate complexation to bidentate complexation with increasing surface phosphate coverage Thus, the effects of phosphate on U(VI) sorption on goethite must be related to the binding phosphate species on goethite. A ternary surface complex of phosphate and U(VI) has been proposed by Cheng et al. (2004). Therefore, the increase in U(VI) sorption at relatively high phosphate concentration (6.0  104 mol/L) should result from ternary surface complexation on goethite. Fig. 4(b) shows the pH sorption edge of U(VI) sorption on goethite in the presence of air at PCO2 ¼ 103:58 atm. As seen in Fig. 4(b), the presence of air has little effects on U(VI) sorption on goethite at pH up to 8.2. However, the sorption of U(VI) at PCO2 ¼ 103:58 atm decreases with pH very rapidly at pH48.2. Similar results at other pressures of CO2 have been reported by Villalobos et al. (2001). Modeling exercises demonstrated that model 2 can successfully describe the pH sorption edge of U(VI) at PCO2 ¼ 103:58 atm without considering any ternary surface complexes of carbonate Fig. 4(b). Thus, the decreasing sorption of U(VI) in the high pH range at PCO2 ¼ 103:58 atm must result from the complexation of U(VI) and carbonate in aqueous phase. Fig. 4(c) illustrates the pH sorption edge of U(VI) in the presence of FA (20 mg/L). It is obvious that FA of 20 mg/L has little effects on the sorption of U(VI) on goethite. Fig. 4(d) shows the effect of U(VI) on the sorption of FA on goethite. The sorption of FA is decreased in the presence of U(VI) especially in the low pH range. The decrease in the sorption of FA may result from the complexation of U(VI) and FA in aqueous phase.

The financial support by the National Natural Science Foundation of China (nos. 20501010 and J0630962) is gratefully appreciated.

4. Conclusions Based on the results and discussion above, a few conclusions can be drawn as follows: (1) the sorption of U(VI) on goethite is strongly pH-dependent, but insensitive to ionic strength; (2) sorption isotherms and the relationship between U(VI) sorption and m/V are important for a proper SCM; (3) a model with two

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