Ni(II) sorption on natural chlorite

Applied Geochemistry 27 (2012) 1189–1193

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Ni(II) sorption on natural chlorite Åsa Zazzi a,⇑, Anna-Maria Jakobsson b, Susanna Wold c a

School of Chemical Science and Engineering, Inorganic Chemistry, Royal Institute of Technology (KTH), SE-100 44 Stockholm, Sweden Department of Chemical and Biological Engineering, Nuclear Chemistry, Chalmers Technical University, SE-412 96 Göteborg, Sweden c School of Chemical Science and Engineering, Nuclear Chemistry, Royal Institute of Technology (KTH), SE-100 44 Stockholm, Sweden b

a r t i c l e

i n f o

Article history: Received 11 February 2009 Accepted 5 March 2012 Available online 13 March 2012 Editorial handling by R.N.J. Comans

a b s t r a c t Sorption of Ni(II) onto chlorite surfaces was studied as a function of pH (5–10), ionic strength (0.01– 0.5 M) and Ni concentration (108–106 M) in an Ar atmosphere using batch sorption with radioactive 63 Ni as tracer. Such studies are important since Ni(II) is one of the major activation products in spent nuclear fuel and sorption data on minerals such as chlorite are lacking. The sorption of Ni(II) onto chlorite was dependent on pH but not ionic strength, which indicates that the process primarily comprises sorption by surface complexation. The maximum sorption was at pH  8 (Kd = 103 cm3/g). Desorption studies over a period of 1–2 weeks involving replacement of the aqueous solution indicated a low degree of desorption. The acid–base properties of the chlorite mineral were determined by titration and described using a non-electrostatic surface complexation model in FITEQL. A 2-pK NEM model and three surface complexes, Chl_OHNi2+, Chl_OHNi(OH)+ and Chl_OHNi(OH)2, gave the best fit to the sorption results using FITEQL. The high Kd values and low degree of desorption observed indicate that under expected groundwater conditions, a large fraction of Ni(II) that is potentially leachable from spent nuclear fuel may be prevented from migrating by sorption onto chlorite surfaces. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Canister failure in a deep bedrock nuclear waste repository may lead to release of radionuclides from the fuel. The multi-barrier system including the fuel itself, the Cu canister and the bentonite clay surrounding the canister is intended to prevent radionuclides from migrating to the bedrock. Furthermore, the surrounding bedrock is itself a potential barrier due to radionuclide retention by sorption. Due to the importance of the bedrock, a number of sorption studies have been carried out on different radionuclides for a variety of minerals (Lützenkirchen, 2002, 2006). The abundance of the mineral chlorite as a fracture filling material in granite justifies sorption studies to quantify the importance of sorption and further retention of radionuclides on this mineral. Forsmark is the proposed site for future disposal of nuclear waste in Sweden. Only 0.2% of Forsmark granite consists of chlorite, but chlorite is one of the dominant minerals in fractures, comprising 30–70% (Drake et al., 2006). The activation product 63Ni is an important component in spent nuclear fuel, accounting for a large proportion of its high activity levels. Nickel-63 is estimated to be present in the vicinity of the repository 300–1000 a after closure (Hedin, 2002), with a maximum calculated activity released into the near field of approximately 1000 Bq/a, peaking at 600 a after closure of the repository. ⇑ Corresponding author. Tel.: +46 (0)8 7908156; fax: +46 (0)8 212626. E-mail address: [email protected] (Å. Zazzi). 0883-2927/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apgeochem.2012.03.001

Sorption affinity onto chlorite has been reported for the cations U(VI), Cs and Yb (Bond et al., 2001; Li et al., 2002), but chlorite titrations and Ni sorption on chlorite have scarcely been studied (Zorn, 2000). The aim of this study was to determine sorption coefficients for Ni on chlorite and to provide more information on the sorption characteristics of chlorite. Experimental data obtained over the short term were extrapolated to Ni sorption onto chlorite in the long-term perspective using a modelling approach. In the surface complexation model used, reactions at the mineral/water interface, i.e. reactions of the surface hydroxyl groups with cations in solution, were included. 2. Materials and methods 2.1. Mineral The chlorite used here originated from Karlsborg in Sweden and was provided by the Swedish Museum of Natural History (chlorite catalogue number 630491). Chlorite is a secondary 2:1 phyllosilicate, characterised by a sheet-like structure. Using the nomenclature and way of describing the chlorite structure described in the literature (Bailey et al., 1971; Newman and Brown, 1987; Bailey, 1988), the chemical formula is:

ðMg5:92 FeII2:04 Al3:12 Ca0:05 K0:5 ÞðSi6:12 Al1:49 FeIII 0:39 ÞO20 ðOHÞ16

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Every chlorite has its own mineralogical history, with different degrees of isomorphic substitution generating specific formulae. The chlorite sample used in this study was first mechanically crushed to pieces of approximately 1 cm2 in area and a couple of millimetres thick, then repeatedly cooled with liquid N2 and heated to room temperature to obtain a powder. The powder was sieved, generating particles in the size fractions 120–200 lm, which were used in subsequent analyses. The specific surface area of the crushed chlorite measured by the BET method (Brunauer et al., 1938) using a Micrometrics Flow Sorb II with N2 as adsorbing gas, was found to be 0.5 ± 0.1 m2/g. 2.2. Solutions Throughout all experiments, Milli-Q deionised water (Millipore) and chemicals of analytical grade were used. Sodium perchlorate stock solutions used as the background electrolyte were prepared from NaClO4H2O (Merck p.a.) and the salt content was analysed by weighing samples dried at 393 K. 2.3. Titrations In order to study the acid–base properties of the chlorite surface, conventional continuous titrations were performed on the mineral in solution. All titrations were performed in a glove box in N2 atmosphere at room temperature (22 ± 2 °C) using 0.01 M NaOH and 0.01 M HCl as titrants. The pH was measured using a Mettler-Toledo InLAB423 combination electrode, saturated with NaCl, calibrated by Gran titrations (Gran, 1950, 1952). For the titrations, 0.1 g of chlorite was added to 10 mL of 0.1 M NaClO4/NaCl. The contact time prior to titrations varied from a few hours to a few days. Titrations were performed with either 15 min between pre-specified additions (denoted as fast titrations) or by using a stability criterion of 0.02 mV/s (denoted as rapid titrations). The time between additions was controlled by an automatic titrator, and the additions were controlled by computer software. A total volume of 1 mL was added in all fast titrations using an ABU91 Autoburette (Radiometer Copenhagen). In the rapid titrations, a Titration Manager TIM 90 Titralab (Radiometer Copenhagen) was used and the total time for a titration was approximately 20 min. 2.4. Sorption experiments The sorption experiments were performed in centrifuge tubes using a batch technique at room temperature (22 ± 2 °C). In order to minimise reactions with O2, the experiments were performed inside a glove box with an Ar atmosphere. Labelled Ni solutions were prepared from 63Ni stock as Ni(II)Cl2 in aqueous solution with an activity of 740 MBq/mL (PerkinElmer LifeScience, Inc.). Each tube contained 0.05 g or 0.1 g of crushed solid chlorite and 9.9 mL NaClO4 solution. The pH was adjusted within the range 5–10 using either 0.1 M NaOH or 0.1 M HClO4, and the sample was left to equilibrate while shaking. After 24 h, 100 lL of spiked Ni2+ solution (74 kBq/mL with [Ni(NO3)2] = 104, 105 or 106 M) was added, resulting in a total Ni concentration in the solution of 106 M, 107 M and 108 M. These low concentrations were used in order to avoid surface precipitation. The Ni sorption experiments were performed in 0.01 M, 0.1 M and 0.5 M NaClO4. The pH of each sample was measured with a combined glass/reference electrode immediately after pH adjustments and after 7 days. A drift in pH of ±0.3 pH-units was observed. The final pH measurements were used when presenting the sorption results. After 7 days of equilibration, the samples were centrifuged (Hermle Z 252MK centrifuge) at 6000 rpm (3900g) for 10 min. The b-activity of 63Ni in the aqueous phase was measured using

b-liquid scintillation counting (LSC) on a Beckman LS1801 instrument and a PACKARD Tri-Carb Liquid Scintillation Analyser Model 1500. 2.5. Desorption batch experiments After sampling for the sorption batch experiments, the rest of the ionic medium was decanted and replaced with fresh ionic solution, buffer solutions as well as the non-buffered NaClO4 of different concentrations. Samples were left to equilibrate for 7 or 14 days and treated as described in the sorption experiments. 2.6. Evaluation and analysis of data The results were interpreted with surface complexation, a model further described in the modelling section below. The data combined with existing data (Gustafsson et al., 2004) were used as input for the surface complexation simulation where sorption was calculated as the distribution coefficient Kd (presented as cm3 g1) based on initial and final activity in solution. To correct for Ni sorption to the walls of the reaction vessel, reference samples without solid phase were prepared for each series and measured at the same pH value as the samples. The activity of the samples was measured directly after addition of Ni solution and treated in parallel with the samples containing solid phase. The difference in activity after 7 days corresponded to the Ni sorbed to the walls and this value was used in the calculations to correct the distribution coefficients and the amount of Ni sorbed to the chlorite surface. Each sorption experiment was performed in duplicate or triplicate and the data points shown are the mean value of these replicates. 3. Results and discussion 3.1. Titrations Continuous titration of a suspension assumes that protons are only consumed and released by surface sites with amphoteric behaviour, i.e. sites that are not permanently charged. However, proton uptake and release can arise from other reactions, as well as from dissolution of the mineral (Shulthess and Sparks, 1986). The results of the present titrations were dependent on the time of contact prior to titration and the time between consecutive additions, which varied from minutes up to an hour for fast titrations. Titrations can also be performed on a longer time scale where the waiting time may vary from 1 day to 1 month (Lützenkirchen, 2002). The longer the time of contact, the larger the apparent surface charge observed in experiments, probably due to an unknown amount of dissolved OH being released from the chlorite, which would also explain the relatively high surface charge calculated from the titrations. The titrations were dependent on the time of contact (Fig. 1), which was assumed to be due to dissolution. To minimise the influence of dissolution the fastest titrations performed on samples with the shortest time of contact were used for determination of the acid–base reactions. However, it should be noted that there may still be some dissolution of the chlorite which cannot be separately quantified. The chlorite dissolution is strongly pH dependent. The highest dissolution rates are found at pH 2 and pH 12 with a minimum in near neutral pH (Gustafsson and Puigdomenech, 2002; Brandt et al., 2003; Lowson et al., 2005; Zazzi, 2009). The computer software FITEQL (Herbelin and Westall, 1999) was used in inverse modelling to calculate constants and site density for chlorite (Table 1). The surface site density was determined

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Å. Zazzi et al. / Applied Geochemistry 27 (2012) 1189–1193 ~2h of contact prior to titration w base ~2 h of contact prior to titration w acid ~1 week of contact prior to titration w base ~1 week of contact time prior tot titration w acid simulation

4 3 2

sigma

1 0 -1

4

4.5

5

5.5

6

6.5

7

7.5

8

8.5

9

pH

-2 -3

Table 2 Experimentally determined log Kd at pH 8.3. [Ni] M 6

10 107 108 106 107 108 106 107 108

[NaClO4] M

Log Kd 5 g/L

Log Kd 10 g/L

0.01 0.01 0.01 0.1 0.1 0.1 0.5 0.5 0.5

2.88 3.17 2.91 3.22 3.16 3.00 3.00 3.26 3.00

2.93 3.08 2.85 2.95 2.93 3.06 3.01 2.77 2.97

-4

Table 1 Results from inverse modelling of titration results using NEM in FITEQL. Reaction

pKa

Chl OH þ Hþ ¼ Chl OHþ 2 Chl_OH = Chl_O + H+

5.6 8.2

to be 26.9 sites/nm2 ([Chl_OH] = 2.23  104 M), which is high in comparison with values reported in the literature. For example, Davis and Kent (1990) reported a site density of 2.31 sites/nm2 for all minerals where the binding to the surface was strong. One contributing parameter explaining the high number of sitedensity is the use of the total surface-area. It has been shown that different chlorites show similar degrees of sorption despite differences in surface area, within an order of magnitude (Dubois et al., 2009; Zazzi and Wold, 2009). Ideally the reactive surface area should be used in the model. However, this parameter is difficult to determine experimentally for this type of mineral (SKB, 2009) and in addition it may not be information that is available for natural minerals that are to be included in predictive modelling. The diverging results could also be related to the difficulties in achieving enough and adequate information on the proton uptake and release from the surface due to the high concentration of OHunits (16 OH per structural unit) combined with the dissolution that could not be avoided in the experiments. (Even if the fastest titrations were used only a small degree of dissolution may alter the mass balance that is used for calculating the proton uptake.) Furthermore, the site density was simulated together with the constants. Gu and Evans (2008) found that kaolinite has a ratio of 6:1 between edge sites and basal surface sites, but this type of ratio was not applied for chlorite in this study, where an even distribution of sites was assumed. 3.2. Sorption measurements The sorption of Ni(II) to chlorite revealed a strong pH dependency and the degree of sorption increased over a narrow pH range. This is typical for cation sorption and has been reported previously for Ni(II) sorption (Gustafsson et al., 2004; Bradbury and Baeyens, 2005). The highest degree of sorption was found at pH 8.3 and the corresponding log Kd values are listed in Table 2. This behaviour of Ni(II) sorption confirms that such sorption occurs through surface complexation rather than cation exchange, as was assumed by Stumm and Morgan (1996). Moreover, a comparison of sorption data for the three ionic strengths studied revealed no dependence on ionic strength, which suggests that the complexes are bound to the surface quite strongly (Hayes and Leckie, 1987; Hayes et al., 1991; Lützenkirchen, 1997).

For the solid concentrations of 5 g/L and 10 g/L studied, no observable difference in degree of sorption was detected. This can be explained by the very low surface area of chlorite, since the number of surface sites available did not increase sufficiently for any detectable effect (Fig. 2). Kd values have not been reported for Ni(II) sorption onto chlorite and therefore no direct comparison with the present study is possible. When comparing the experimental data with data on Ni(II) sorption onto other minerals (Scheidegger et al., 1996; Henning et al., 2002; Dähn et al., 2003), surface complexation of Ni(II) would have been expected to have occurred at the step edges on the chlorite surface in the present system. When comparing the sorption data with the data on Ni(II) sorption on Na-montmorillonite from Baeyens and Bradbury (1995), it was observed that at pH values of 7 and above, both chlorite and Na-montmorillonite sorbed the Ni(II) through surface complexation with log Kd values in the same order of magnitude when expressed relative to mass. On the other hand, when converting Kd to Ka, expressed per unit surface area, it was observed that Ni(II) was much more strongly sorbed to chlorite than to Na-montmorillonite due to the difference in the amount of surface sites present.

3.3. Desorption experiments After 7 or 14 days of reaction time, Ni(II) concentration did not differ significantly from the background concentrations obtained by liquid scintillation counting. As the pH decreased to approximately 4.5, 7 ± 2% of the Ni(II) sorbed was desorbed. Scheidegger and Sparks (1996) observed that at pH 4, approximately 8% of the sorbed Ni(II) was desorbed and at pH 6 approximately 3%;

4

3

log Kd

Fig. 1. Summary of continuous titrations. Effect of different contact time, with 3 min between additions and NaCl as electrolyte.

2

1

0 4

5

6

7

8

9

10

11

pH -6

0.05 g chlorite 10 M

-7

0.05 g chlorite 10 M

-8

0.05 g chlorite 10 M

0.1 g chlorite 10 M 0.1 g chlorite 10 M 0.1 g chlorite 10 M

-6 -7 -8

Fig. 2. Sorption as a function of solid concentration performed in 0.01 M NaClO4.

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hence the current desorption results are in good agreement with their observations, even though they investigated pyrophyllite. The observed desorption took place in the region where chlorite dissolution is dominant. The uncertainty in the degree of desorption was mainly due to the unknown volume left after the old NaClO4 solution was decanted, since the chlorite powder was not rinsed or washed prior to the start of the desorption studies. Due to the high Kd values found in the present study in combination with very fast sorption rates, it is concluded that desorption was slow, with the time of 14 days being too short to reach equilibrium. Comans (1987) showed that a reaction time of 7–8 weeks was needed in order to quantify the desorption of Cd from illite. As expected, desorption of Ni(II) from chlorite proved to be pHdependent. The low desorption of Ni(II) from chlorite coupled with high Kd values gives chlorite the ability to act as a potential barrier for retardation of migrating cations. 3.4. Surface complexation The Ni(II) sorption data were fitted using a 2-pK non-electrostatic model together with the software PHREEQC and the database wateq4f.dat (Parkhurst and Appelo, 1999). One component Chl_OH represented the entire surface, since a higher complexity could not be obtained due to lack of information on the different surface groups and their affinity for Ni. Three different reactions for Ni(II) with the chlorite surface were introduced into the NEM model whereby 0, 1 and 2 protons were released (Table 3). Inverse modelling using all the experimental data was performed to obtain the constants for the three reactions of interest. The simulations for all experimental conditions are presented together with the experimental data, in the form of% sorbed Ni as a function of pH, in Fig. 3. The surface model NEM accurately described the sorption edge, but in the area with a constant degree of Table 3 Kd Surface complexation constants obtained from simulation of experimental data using a 2-pK model in FITEQL. Formed complex 2þ

Chl OH þ Ni

pK

Chl OHNi

3.63

2þ

Chl OH þ H2 O þ Ni2þ Chl OHNiðOHÞþ þ Hþ

2.65

Chl OH þ 2H2 O þ Ni2þ Chl OHNiðOHÞ2 þ 2Hþ

11.94

Ni(II) sorption, the model overestimated the degree of sorption. Simulated Kd was 0.6 log units higher than the Kd from the experiments at the sorption maximum at pH 8.3. At the highest pH, the model predicted a decrease in sorption that was larger than that observed, and, therefore, the model was most representative in the pH area 4–8. The main interest was in sorption at pH 7–8, since hydrogeochemical calculations show that flowing water within the bedrock will be in this range (Smellie et al., 1995). Since analytical techniques designed for water chemistry were used to determine the degree of sorption to the chlorite surface, the possibility of surface precipitation at pH values higher than 9, which is also the region where the surface complexation model deviated from experimental data, cannot be excluded. Even though Ni(OH)2 formation was introduced as a parameter when establishing the NEM model, Ni(OH)2 precipitation could not be observed in the PHREEQC simulation. From the model set-up it was concluded that the most sensitive input parameter was the site density. One of the aims of this work was to use parameters experimentally determined for chlorite as far as possible in the development of the model. The surface-site density used is linked to the acid– base constants; both are derived with forward modelling of the chlorite titration data. It should be recognised that there are uncertainties in the experimentally determined parameters and the largest contributions originate from the behaviour of the chlorite i.e. the structure of chlorite and the uncertainties in determining the contribution from the surface-groups. Another parameter that has an effect on the surface site density is the assumption that the surface-sites are evenly distributed over the entire surface basal plane as well as edges. From SEM images of chlorite (Zazzi, 2009) it can be seen that the basal plane is quite smooth and that the sites are, therefore, more likely to be concentrated on the edges. This argument is in accordance with the results of Dähn et al. (2003), where it was found that Ni sorbed to the edges of montmorillonite. In work by Gu and Evans (2008) a relationship of 1:6 was for the concentration of surface sites on the basal plane relative to the edges. This type of determination has not been applied to the chlorite and the divergence between experimental results and modelling is an artefact where the modelling overestimates the number of available sites on the surface of chlorite. In the work by Gustafsson et al. (2004) the Ni sorption onto chlorite was modelled using literature values for both acid–base constants as well as surface-site density, however this model is a DLM instead of the NEM.

100 90 80

% sorbed Ni

70 60 50 40 30

Experimental

20

Simulation

10 0 4

5

6

7

8

9

10

11

12

pH Fig. 3. Experimental data together with simulation. Data points from all sorption experiments as a function of pH.

Å. Zazzi et al. / Applied Geochemistry 27 (2012) 1189–1193

4. Summary The Ni(II) sorption to chlorite surfaces observed in this study showed characteristic sorption behaviour for surface complexation of cations, i.e. a strong pH dependency and an increasing degree of sorption over a narrow pH range. Chlorite will thus provide a strong barrier for any escaping radionuclides in the groundwater of deep geological repositories with neutral pH. Site investigations and calculations have shown that the pH in areas around the future repositories will be in the range 7–9 (Laaksoharju et al., 2008), if account is taken of the variations in pH due to saline water and melt water intrusions and dissolution of different mineral phases. This pH range corresponds to the stability window of chlorite dissolution (Gustafsson and Puigdomenech, 2002; Lowson et al., 2005) and the maximum sorption capacity of the chlorite surface. There is no reason to believe that the groundwater chemistry would change to an acid pH where Ni(II) starts to desorb from the chlorite surface to a large extent, as was shown experimentally in this study. As long as the pH exceeds 7, the Ni(II) will stay sorbed. Titrations with short contact time must be applied to the chlorite system if the information is to be used for inverse modelling of chlorite surface protonation. For macroscopic determination of sorption data, a typical model for that kind of experiment is required. The sorption data were fitted with a non-electrostatic model to fit titration and sorption data. These models are based on one of the aims of this work, which has been to use parameters experimentally determined for chlorite as far as possible in the development of the model. Acknowledgement We thank Zoltán Szabó for valuable comments during preparation of the manuscript, the Swedish Museum of Natural History for providing the chlorite and SKB for financial support. References Baeyens, B., Bradbury, M.H., 1995. A Quantitative Mechanistic Desorption of Ni, Zn and Ca Sorption on Na-Montmorillionite. Part II: Sorption Measurements. PSI Würenlingen nad Villigen. Bailey, S.W., 1988. Chlorites: structures and crystal chemistry. In: Bailey, S.W. (Ed.), Hydrous Phyllosilicates (exclusive of micas). Reviews in Mineralogy, vol. 19, pp. 347–404 (Chapter 10). Bailey, S.W., Brindley, G.W., Johns, W.D., Martin, R.T., Ross, M., 1971. Summary of national and international recommendations on clay mineral nomenclature by 1969–1970 Clay. Miner. Soc. Nomencl. Commun. Clays Clay Miner. 64, 129–132. Bond, K.A., Boult, K.A., Green, A., Linklater, C.M., 2001. Sorption of Uranium(VI), Plutonium and Thorium onto Aluminium Oxide, Muscovite and Chlorite: An Experimental and Modelling Study. AEA Technology Plc, Harwell, Oxfordshire. Bradbury, M.H., Baeyens, B., 2005. Experimental and Modelling Investigations on Na-illite: Acid–Base Behaviour and the Sorption of Strontium, Nickel, Europium and Uranyl. Wettingen, Paul Scherrer Institut. Brandt, F., Bosbach, D., Krawczyk-Bärsch, Arnold, T., Bernhard, G., 2003. Chlorite dissolution in the acid pH-range: a combined microscopic and macroscopic approach. Geochim. Cosmochim. Acta 67, 1451–1461. Brunauer, S., Emmett, P.H., Teller, E., 1938. Adsorption of gases in multimolecular layers. J. Am. Chem. Soc. 60, 309–319. Comans, R.N.J., 1987. Adsorption, desorption and isotopic exchange of cadmium on illite: evidence for complete reversibility. Water Res. 21, 1573–1576. Dähn, R., Scheidegger, A.M., Manceau, A., Schlegel, M.L., Baeyens, B., Bradbury, M.H., Chateigner, D., 2003. Structural evidence for the sorption of Ni(II) atoms on the edges of montmorillonite clay minerals: a polarized X-ray absorption fine structure study. Geochim. Cosmochim. Acta 67, 1–15. Davis, J.A., Kent, D.B., 1990. Surface complexation modeling in aqueous geochemistry. In: Hochella Jr. M.F., White, A.F. (Eds.), Mineral–Water Interface Geochemistry. Reviews in Mineralogy, vol. 23, pp. 177–260.

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