Surface Complexation Modeling of Yb(III) and Cs(I) Sorption on Silica

Journal of Colloid and Interface Science 212, 228 –233 (1999) Article ID jcis.1999.6086, available online at http://www.idealibrary.com on

Surface Complexation Modeling of Yb(III) and Cs(I) Sorption on Silica Nicolas Marmier, 1 Annie Delise´e, and Francine Fromage University of Reims, Faculte´ des Sciences, GRECI, BP 1039, 51687 Reims Cedex 2, France Received February 10, 1998; accepted January 13, 1999

illustrate the retention properties of the silica surface. The first one, cesium, has been widely studied. It is considered to be a mobile ion, and its affinity is greater for clays minerals than for oxide surfaces. On the other hand, the sorption behavior of rare earth elements, which present strong chemical analogies with trivalent actinides, is not very well known. Nevertheless, previous results showed good affinity between ytterbium and hematite and alumina surfaces (1). In this study, the surface complexation modeling of the cesium and ytterbium sorption on silica was performed using the experimental method tested on the previous work. The surface complexation models (SCMs) have been developed to explain sorption more precisely than empirical models. In the search to obtain a more realistic description of the observed phenomena, SCMs became more and more complicated, and the number of adjustable parameters used to account for the experiment increased (e.g., two). At the same time, nonelectrostatic surface complexation models were developed and used for natural systems (3). In such wide diversity, a uniform approach for applying SCMs has been advocated (4 – 6). This method makes the binding constants the only adjustable parameters, the others being fixed or measured, and tries to use the constants, determined on simple systems, to predict the behavior of more complex ones (7). Keeping in mind the same aim to have simple models, we have used in this work the constant capacitance model (CCM). This theoretic description makes no difference between innerand outer-sphere surface complexes and thus uses the same electrostatic description for cesium and ytterbium.

A surface complexation model is used to describe sorption of ytterbium and cesium on the silica surface. The constant capacitance model gives the description of the solid–solution interface chosen for this work. The first step in the modeling consists of extracting the surface acidity constants. The result is: SOH º SO 2 1 H 1

log Ka2 5 27.60 6 0.05.

The second step consists of the extraction of surface complexation constants for both ytterbium and cesium. The sorption of the cations is represented as follows: for the ytterbium sorption, 2H2O 1 SOH 1 Yb 31 º SOYb(OH)2 1 3H 1 log K 5 216.2 6 0.3 for the cesium sorption, SOH 1 Cs 1 º SOHCs 1

log K 5 2.05 6 0.05

SOH 1 Cs 1 º SOCs 1 H 1

log K 5 25.5 6 0.1.

In the case of cesium, the sorption of sodium is competitive and has to be considered: SOH 1 Na 1 º SOHNa 1

log K 5 2.2 6 0.1.

© 1999 Academic Press

Key Words: sorption; ytterbium; cesium; surface complexation model; lanthanide.

INTRODUCTION

MATERIALS AND METHODS

The migration of radionuclides from underground radwastes repository to the geosphere can be attenuated by sorption on natural (geological sites) or synthetic (engineered barriers) materials. Silica is one of the most common minerals in the geosphere, and silicium oxide is present in many natural and artificial compounds. In this way, its sorbing capacities have to be qualified and quantified. Two cations have been chosen to

Solid Phase

1

Silica used in this work is a commercial product (silica gel 60H Merck). The mean particle size is equal to 15 mm. The raw silica has been checked for impurities by XPS. No impurities have been detected. Its specific area, measured by the BET nitrogen adsorption method, is equal to 384 m 2 z g 21. The pH of immersion given by Merck for this silica in water is 6.5. This result has been confirmed in the lab.

To whom correspondence should be addressed. Fax: (33) 326-05-33-30. E-mail: [email protected]. 0021-9797/99 $30.00 Copyright © 1999 by Academic Press All rights of reproduction in any form reserved.

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SURFACE COMPLEXATION MODELING OF Yb AND Cs ON SILICA

Titrations are carried out under nitrogen atmosphere and suspensions are constantly stirred. Between each incremental addition, 2–5 min were allowed for the pH stabilization. Experiments have been performed after increasing stirring times, from one hour to 12 days. Experimental curves are superimposed after seven days and then the surface equilibrium is considered to be achieved. These curves are presented in Figs. 1 and 3. The remaining washed silica was washed again several times with water, and pH was recorded after each wash. The values of the recorded pH were equal to 5.67, then 6.00, and finally 6.52 after the last wash. The pH of the suspension seems to be dependent here on the number of washes and not on the value of pH zpc. Sorption of Cations

FIG. 1. Titrations experiments of 300 mg of silica in 50 cm 3 of NaNO 3 0.1 mol z L 21. Dotted lines refer to blank titration. Hollow triangles, stirring time equal to 1 h; hollow circles, stirring time equal to 1 day; hollow squares, stirring time equal to 4 days; filled triangles, stirring time equal to 6 days.

Titrations The titrations have been performed first on the raw silica in suspension, second on silica washed with HNO 3 10 22 mol z L 21, and then two times with deionized and boiled water. The pH of the suspension of the washed silica was equal to 4.47 after the first wash and to 5.07 after the second one. Experiments consist of alkalimetric (with NaOH 10 22 mol z 21 L ) and acidimetric (with HNO 3 10 22 mol z L 21) titrations of (a) the neat solution (background electrolyte NaNO 3 0.100 mol z l 21) (b) a solid suspension (300 mg of silica in 50 cm 3 of the NaNO 3 solution) The glass electrode used was calibrated between each experiment via titration of a dilute, standardized acid in 0.1 M ionic medium, which implies that given pH values correspond to 2log[H 1]. The concentration of the dissolved silicates has been measured after equilibration and has been taken into account in the balances (with H 4SiO 4 5 H 3SiO 42 1 H 1, log K 5 29.86 and H 4SiO 4 5 H 2SiO 422 1 2H 1, log K 5 222.96 from the Mineql database).

Cs 1 and Yb 31 (according to the constants given by Wood (8)) are the dominant aqueous species in the conditions used. Each sorption curve is the result of some 20 batch experiments with mixes of solid, cation, and solution. Different volumes of acid (HNO 3 10 22 mol z L 21) or base (NaOH 10 22 mol z L 21) are added in each suspension to make the pH edge go from 4 to 8. The total volume of solution is equal to 50 cm 3. According to the CCM hypothesis, sorption experiments have been performed for both cesium and ytterbium for an ionic strength (I) constant and equal to 0.1. Nevertheless, other experiments using different concentrations of NaNO 3 have been made to measure the competitive action of Na 1 ions. Also experiments with different ratio amounts of solid–volume of solution have been performed. The experimental edge has been enlarged in this way to improve the model in under limit conditions. Other experimental conditions are adjusted to optimize the quality of measurements. The experimental edges used are different for ytterbium and for cesium as shown in Table 1. After stirring during seven days, the pH is recorded. Then solutions are filtered through a millipore cell (0.45 mm) and the total concentration of cation in solution is determined by absorption spectrometry. The reproductivity of the cation (M n1) sorption is tested by measuring the amount of sorbed M n1 in 22 similar batches. Each batch contains the same amount of solid and the same volume of acid, cation solution, and electrolyte. The volume of the added acid is made to make the amount of sorbed cation TABLE 1 Compared Experimental Conditions Yb 31

Cs 1

NaNO 3 concentrations

0.10 and 0.05 mol z L 21

Total concentration of cations Amounts of silica

2.10 25 mol z L 21

0.1, 0.01, and 0.00 mol z L 21 4.10 25 mol z L 21

100 and 10 mg

200 and 400 mg

MARMIER, DELISE´E, AND FROMAGE

230

FIG. 2. Saturation of silica surface by H 1 and OH 2. Solid straight lines refer to the titrations which have been performed with NaNO 3 0.1 mol z L 21 alone, and solid circles refer to titrations with NaNO 3 and silica.

equal to about 50%. The standard deviations on pH values and on percentage of sorbed cations are respectively equal to s 5 0.06 pH units and s 5 3% of sorbed cation. Modeling The model used in this work is the surface complexation model. As its description is available in a large number of papers (e.g., 9), only fundamentals will be rewritten here. A finite number of reactive sites is supposed to be present on solids surfaces. These surface sites (SOH) are involved in protonation– deprotonation reactions and may fix or release ions, represented by M n1. In the studied systems, M n1 can be Yb 31 or Cs 1. Equations of the surface reactions concerning H 1, M n1, and SOH can be symbolized by the following: sH 1 1 qSOH 1 rM n1 º Hs ~SOH! q M ~s1nr!1 , r

[1]

where H s (SOH) q M r(s1nr)1 symbolizes surface complex and s, q, and r are stoichiometric coefficients.

Surface constants are given by K5

@Hs ~SOH! q M ~s1nr!1 # @FC/RT~s1nr!# r 1 s q n1 r e @H # @SOH# @M #

[2]

with K the surface constant, C the electrostatic potential, F the Faraday constant, R the ideal gas constant, and T the temperature (in K). In the constant capacitance model, C and surface charge density (s) are related by:

s 5 C z C,

[3]

with C the surface capacitance. In this work, C is an empirical constant chosen as being equal to 1.2 F z m 22. RESULTS

The first step of the modeling consists of extracting the surface site concentration from saturation experiments. The

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SURFACE COMPLEXATION MODELING OF Yb AND Cs ON SILICA

provide available results when the titration starting point is equal to the pH of zero point charge (pH zpc when [SOH 21] 5 [SO 2]), a correction has to be made while modeling. This is because the experimental titration starting point may be different than pH zpc (mostly considered to be close to 2 for the silica surface (e.g., 12)), as shown in Fig. 3. In the case of silica, supposing the titration starting point is equal to pH zpc may lead to modeling mistakes. When a neutral suspension is introduced into a neutral solution, a transfer of charge occurs between the two phases. An excess of charge may then be present on the surface. This excess of charge on the surface is neutralized in solution by the ions in the diffuse layer, and the entire system is neutral. The correction used here consists of subtracting the amount of H 1 or OH 2 present on the surface before titration from the total quantity of added H 1 and OH 2 during titration. This is made by testing increasing values of a correction factor for proton consuming (13). This factor (v in cm 3) is defined here as Corrected total concentration of protons ~T HC! 5 ~V 2 v!0.01/V T

[4]

with V the added volume of solutions of acid or base and V T the final volume of solution. FIG. 3. Titrations experiments of 300 mg of silica in 50 cm 3 of NaNO 3 0.1 mol z L 21. Dotted lines refer to blank titration. Hollow upside down triangles, stirring time equal to 6 days; hollow circles, stirring time equal to 10 days; hollow squares, stirring time equal to 12 days; hollow triangles, stirring time equal to 6 days, lower pH of immersion. Lines, calculations.

methodology used compares the titration curve of the solid suspension to the neat solution one (Fig. 2). In the case of silica, because of the dissolution, the neat solution corresponds to the supernatant after one week of stirring. When saturation is achieved, the amount of sorbed H 1 is supposed to be equal to the total concentration of available surface sites (T SOH). T SOH is obtained from the curves in Fig. 2 by the difference between H 1 (OH 2) present in the blank curve and H 1 (OH 2) present in the suspension for the same amount of H 1 (OH 2) added. Then T SOH is equal to 3 3 10 23 mol z L 21 for 300 mg of silica. Using such an experimental determination of the concentration of surface sites, mistakes due to correlation of adjustable parameters can be avoided while modeling. The second step consists of fitting out the surface acidity constants Ka 1 and Ka 2 using the least-square minimizing code FITEQL 1.2. (10). Details on this program are widely available in many papers (e.g., 11) and are not discussed here. Nevertheless, if many details on experimental conditions for acid– base titrations of solid surfaces and mathematical development of FITEQL are available in literature, only a few number of FITEQL users indicate the way the experimental results are implemented in the code. Indeed, as FITEQL is made to

For acid– base surface reactions: 1 SOH 1 2 º SOH 1 H

SOH º SO 2 1 H 1

Ka1

[5]

Ka2.

[6]

The value of adjusted Ka 2 is kept when the indicator of the goodness of fit (F, defined in FITEQL 1.2 User’s Guide, (10)) is minimum (Table 2). The retained value for the corrected factor v is equal to 2.0 cm 3 for the titration starting from neutral pH (curve 1) and equal to 0.5 cm 3 for the titration starting from acidic pH (curve 2). The comparison of calculated and experimental titration curves is presented in Fig. 3. The retained values are log Ka 2 5 27.60 6 TABLE 2 Comparing F and log Ka 2 with Increasing v for Titrations Starting from a Neutral pH (curve 1) and for Titrations Starting from an Acidic pH (curve 2) Curve 1

Curve 2

v (cm 3)

F

log Ka 2

v (cm 3)

F

log Ka 2

1.7 1.8 1.9 2.0 2.1

20.5 11.3 2.13 1.27 1.37

27.78 27.72 27.67 27.60 27.56

0.1 0.2 0.3 0.4 0.5 0.6

50 24.7 9.48 2.25 1.11 3.67

27.37 27.44 27.51 27.58 27.65 27.72

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MARMIER, DELISE´E, AND FROMAGE

DISCUSSION

The titration modeling results are the same for both acidic and neutral start pH. This indicates that the surface of the silica seems to be the same in the two tested experiment conditions and that both experiments can be used to extract acidic surface constants if some precautions are taken while implementing the data in the code FITEQL. The experiment results show that ytterbium binds on the silica surface more strongly than does cesium, which is not surprising considering the nature of these two cations. Indeed, if lanthanides produce complexes in solution, this is not the case for cesium. This chemical property of cesium explains its lack of affinity for oxide surfaces, which are generally considered to provide only complex surface sites. Nevertheless, the amount of cesium bound to the silica surface is not insignificant, when it is on iron oxide surfaces for example (19). The explanation of this different behavior may be in the difference observed in the pH zpc. If iron oxide surfaces are positive under pH 7 or 8, the silica surface is negative for pH over 2–3. This negative charge allows an electrostatic binding of cesium on the silica surface. The strength of this binding is weaker than

FIG. 4. Experimental results of Yb (2 3 10 25 mol z L 21) sorption on silica versus pH for different concentrations of NaNO 3 and different amounts of silica. Filled squares, NaNO 3 0.05 mol z L 21, 100 mg of silica in 50 cm 3; hollow circles, NaNO 3 0.1 mol z L 21, 100 mg of silica in 50 cm 3; hollow squares, NaNO 3 0.1 mol z L 21, 10 mg of silica in 50 cm 3; Lines, calculations.

0.05 for curve 1 and log Ka 2 5 27.65 6 0.04 for curve 2. Ka 1 is not to be considered. This is consistent with the literature data (14–18) and confirms the high acidity of the silica surface. In the final modeling step, stoichiometries and formation constants of the surface complexes are extracted from sorption experiments. Neither specific capacitance nor surface acidity constants or surface site concentration have been adjusted. The best fits of the data are obtained with the following reaction sets (Fig. 4 for ytterbium and Fig. 5 for cesium): for ytterbium sorption, 2H2O 1 SOH 1 Yb 31 º SOYb(OH)2 1 3H 1 log K 5 216.2 6 0.3

[7]

log K 5 2.05 6 0.05

[8]

SOH 1 Cs 1 º SOCs 1 1 H 1 log K 5 25.5 6 0.1

[9]

for cesium sorption, SOH 1 Cs 1 º SOHCs 1

SOH 1 Na 1 º SOHNa 1

log K 5 2.2 6 0.1.

[10]

These reaction sets are the minimum accounting for the experiments.

FIG. 5. Experimental results of Cs (4 3 10 25 mol z L 21) sorption on silica versus pH for different concentrations of NaNO 3 and different amounts of silica. Hollow triangles, NaNO 3 0.001 mol z L 21, 200 mg of silica in 50 cm 3; hollow upside down triangles, NaNO 3 0.01 mol z L 21, 200 mg of silica in 50 cm 3; hollow circles, NaNO 3 0.1 mol z L 21, 200 mg of silica in 50 cm 3; filled upside down triangles, NaNO 3 0.01 mol z L 21, 400 mg of silica in 50 cm 3; lines, calculations.

SURFACE COMPLEXATION MODELING OF Yb AND Cs ON SILICA

TABLE 3 Comparing F and Stoichiometries Used to Account for the Sorption of Cesium on 200 mg of Silica for I 5 0.1 Stoichiometries 8 9 819 9 1 10 8 1 9 1 10 a

log K

F

2.5 25.7 1.8 and 25.8 NC a 2.05, 25.5, and 2.2

26 1.8 1.0 NC a 0.8

NC, no convergence in numeric scheme.

that of a complexation’s, and so Na 1, the electrolyte cation, may compete. As shown in Table 3, the sorption of cesium on silica for I 5 0.1 can be reproduced without Eq. [10]. Nevertheless, the model which does not take into account the sorption of sodium is not able to account for all the sorption curves in Fig. 5. In the case of ytterbium, the competitive action of sodium is insignificant. This observation has been already made for the sorption of other trivalent cations on silica (20). This may be explained by the large difference in the binding strength, and so the sorption of sodium does not need to be considered in modeling. The ionic strength has no effect on the observed sorption curve in the experimental conditions used here. Using such a chemical description, it is not necessary to involve inner- or outer-sphere complexes to explain the differences observed between sorption of ytterbium and cesium, and so CCM may be used to reproduce the experimental results. According to Figs. 4 and 5, CCM can be used to account for sorption of cesium and ytterbium in various conditions of Na concentrations and for different amounts of silica in the same volume of solution. The same data set and the same constants are used for all calculations. This seems to indicate that the sorption of ytterbium and cesium is not affected by the change of the surface charge with ionic strength in the experimental conditions tested here. Such a description of the solid–solution interface, involving the comparison of binding strengths and no inner- or outer-sphere complexes partition, has been advocated by Charlet and Manceau (21) in conclusion of a spectroscopy study. CONCLUSION

Cesium can bind on the silica surface because of the very low pH zpc of this oxide. This sorption decreases when the concentration of sodium increases in solution. The sorption of

233

ytterbium is not affected by the presence of sodium in solution because of its greater binding strength. CCM is able to account for the sorbing behavior of both ytterbium and cesium, with a relatively low number of adjustable parameters used to account for the experiment results, constituted by almost 60 independent batch experiments in changing NaNO 3 concentrations conditions and for different amounts of silica. The validity of the chemical descriptions of the sorption of cesium and ytterbium on silica will be tested in tries of predictions of oxide mixtures behaviors in a forthcoming paper. ACKNOWLEDGMENT Financial support by the French Nuclear Waste Management National Agency (ANDRA) is gratefully acknowledged.

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