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Bioscience Horizons 2008 1(1):38-43; doi:10.1093/biohorizons/hzn003
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© Oxford University Press 2008

Dissolution kinetics of clinoptilolite and heulandite in alkaline conditions

Alissa Cotton

School of Biosciences, University of Nottingham, University Park, Nottingham, NG7 2RD, UK

*Corresponding author: Tel: +44 (0) 115 951 6255. Email: liz.bailey{at}nottingham.ac.uk

Supervisor: Dr. Elizabeth Bailey*, School of Biosciences, University of Nottingham, University Park, Nottingham NG7 2RD, UK.


   Abstract
Top
 Abstract
Introduction
 Materials and methods
 Results
 Discussion
 Conclusions
 Funding
 References
 
Temperature and pH-dependent dissolution kinetics of natural clinoptilolite and a ‘pure’ heulandite phase have been determined and compared. Si dissolution rates increased with increasing temperature and pH. Heulandite has a slightly higher dissolution rate than clinoptilolite in the pH range 8–10. Measured reaction order was 0.23 and 0.14 for clinoptilolite and heulandite, respectively. This is slightly lower than for other aluminosilicate phases at comparable temperatures. The measured dissolution rates indicate a 1 mm spherical crystal would dissolve in 38 848 years and 17 301 years for clinoptilolite and heulandite, respectively. If solution composition is maintained, and remains unsaturated with respect to Si, Al, etc., the presence of zeolites in the vicinity of an underground nuclear waste repository would be beneficial as a sorption barrier.

Key words: dissolution kinetics, clinoptilolite, heulandite


   Introduction
Top
 Abstract
 Introduction
Materials and methods
 Results
 Discussion
 Conclusions
 Funding
 References
 
Deep geological disposal (DGD) is the main viable option for disposal of high level nuclear waste as proposed by the UK government.1 DGD repository designs propose the use of a bentonite backfill with concrete, to act as a chemical barrier. Bentonite swells on contact with groundwater, and therefore act to prevent radionuclides leaching away from the repository, should steel waste canisters fail. Reaction of the low-solubility cement-based backfill with slow-flowing groundwater is likely to produce a plume of long-lasting highly alkaline conditions (approximately pH 13) as a result of the formation of calcium silicate hydrate and calcium hydroxide;2, 3


Formula

Alkaline conditions are thought to be beneficial in the repository environment, not only because radionuclides are likely to have reduced mobility at high pH, but also because research has shown that alkaline conditions, along with high temperatures caused by decaying radioactive waste, are thought to encourage formation of zeolites.4 Zeolites are expected to form within crystalline bedrock surrounding a repository, as bedrock will be the principal source of Si and Al for their formation. As the alkaline plume migrates throughout the repository, Si and Al will be dissolved. Edges of the plume are likely to contain the greatest amounts of Si and Al as there the plume would have reacted with the most rock, and also have the lowest pH (Fig. 1).


Figure 1
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  		Figure 1. Diagram of the likely pH and Si/Al concentrations with time/distance from a repository. The mineral phase likely to form is shown under the corresponding conditions.5

 
It is under these conditions that zeolites, along with other mineral phases, are likely to form. The potential for zeolites to sorb selected radionuclides arises due to the mechanisms of ion-exchange and co-precipitation, and their structure.6 Zeolites are formed from SiO4 tetrahedra, with isomorphic substitution of Al, giving rise to a negative charge within the structure which is balanced by alkali or alkaline earth cations present within the framework channels.

Radionuclides which pose a long-term threat to the safety of human health and the environment include plutonium, uranium, neptunium, caesium and strontium. Oxidation state, pH and speciation of these radionuclides are important factors in determining their mobility and potential for zeolitic sorption.7, 8

There is also evidence to suggest that zeolites have the potential to be destabilized by the flow of the alkaline plume and in turn dissolve within short timescales of just tens of months.4, 6 In terms of retardation of radionuclides in a repository, this dissolution process may prove detrimental, and may re-release any sorbed radionuclides much faster then they would have been originally released from a partially disintegrated waste canister.

The motivation for this work stems from the recent CoWRM report9 indicating that DGD may be the most suitable disposal method in the UK, and the potential impacts on human health and the environment that may occur should radionuclides leach beyond a repository. This paper focuses on the dissolution kinetics of natural clinoptilolite and a ‘pure’ heulandite phase under conditions similar to those in/around a repository in an effort to determine how long dissolution of such zeolites would take in a repository environment, and in turn determine if their use would be viable within a repository in terms of retarding radionuclide mobility. Heulandite and clinoptilolite are both members of the Group 7 zeolites and there is debate over whether clinoptilolite is a silica-rich version of heulandite, or whether it is in fact a different mineral.10 In any case, both have a ‘10-member ring channel pore system with 8-member ring cross channels’.11 Clinoptilolite has a Si/Al ratio greater than 4, while heulandite is defined as Si/Al ratio below 4. Heulandite has low bond strength in one direction, and as a result the key difference between the two zeolites is their thermal stability.10 Clinoptilolite is stable to temperatures in excess of 500°C, while heulandite undergoes structural collapse above 350°C.11

Mineral dissolution rates are typically fastest at higher temperatures, due to an increase in energy available for reactions to take place, and at higher pH due to an increased concentration in OH– ions, which react to destabilize the structure. Natural clinoptilolite is likely to have faster dissolution rates than heulandite, as the presence of impurities causing structural instability is likely.


   Materials and methods
Top
 Abstract
 Introduction
 Materials and methods
Results
 Discussion
 Conclusions
 Funding
 References
 
A natural clinoptilolite ((Na4.5 K5.8 Ca0.4 Mg1.4) [Al16.2 Si27 O72].H2O) and a ‘pure’ heulandite ((Na2.0 K1.7 Ca3.6) [Al17.1 Si27.3 O72].H2O), both from Poona, India, were used in this study to investigate dissolution rates, and stability under varying pH and temperature conditions. Experiments were performed in solutions with pH ranging from 7.90 to 12.97. The compositions of the pH buffers used are listed in Table 1.


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  		Table 1. Composition of pH buffer reagents, showing nominal and actual pH of solutions

 
Minerals were broken up using a geological hammer and then a ball mill, before sieving to a size fraction of 212–300 µm and 125–250 µm for clinoptilolite and heulandite, respectively. Samples were rinsed with deionized water to remove any fines, and dried overnight in an oven at 30°C. Surface areas of the samples were calculated assuming a spherical particle of average diameter, giving values of 109 cm2 and 145 cm2 for clinoptilolite and heulandite, respectively. Acid digestion of the clinoptilolite phase was undertaken to establish the levels of impurities present.

Approximately 1 g of clinoptilolite and 0.5 g of heulandite were added to labelled 100 ml polyethylene reactors. A 100 ml aliquot of an appropriate buffer solution was added to each reactor, sealed to reduce evaporation and the sample shaken to ensure mixing. Reactors were then placed in a shaking water bath and gently agitated at 30°C or 60°C. Temperature was monitored using an alcohol thermometer which was checked daily. Samples were taken at intervals, ranging from a minimum of 2 h, to a maximum of 48-h intervals as the experiment progressed and steady-state dissolution was approached. Each sample of 4–5 ml of solution was removed using a pipette. It has been previously demonstrated that filtration was not necessary. Samples were stored in polypropylene sample tubes and acidified prior to analysis. Dissolution rate calculations were corrected for evaporative and sampling losses.

Solution samples were analysed using an Inductively Coupled Plasma Mass Spectrometer (ICP-MS), for elements including Si, Al, Mg, Ca and Sr. pH was measured on Day 9 of the experiment after solutions had cooled to room temperature, to see if any changes had occurred.


   Results
Top
 Abstract
 Introduction
 Materials and methods
 Results
Discussion
 Conclusions
 Funding
 References
 
Rates of dissolution were calculated from linear regions of concentration vs. time plots, after a volume correction had been applied (Fig. 2). The stoichiometries of heulandite and clinoptilolite were taken into account in order to allow a direct comparison of the Si release rate of the phases.


Figure 2
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  		Figure 2. Si dissolution rate as a function of volume corrected time for clinoptilolite at 60°C, pH 12 showing an initial increase in dissolution with time up until approximately 120 h of dissolution, after which steady state was approached.

 
An initial period of rapid Si release was observed during the first 72 h, and then dissolution slowed as steady state was approached between the sample and the solution. This pattern was observed for all ions analysed and at all solution pHs. These two different periods have been described12 as short-term dissolution and long-term steady-state dissolution, respectively.

The initial linear gradient of each graph of concentration vs. time was divided by the calculated surface area of the reacted zeolite and converted into a dissolution rate in mols cm–2s–1. The effect of pH on dissolution rate is shown in Fig. 3.


Figure 3
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  		Figure 3. Si release rate as a function of pH for Clinoptilolite at 60°C and 30°C showing an increase in dissolution rate as pH increases.

 
Si dissolution rates increased with both pH and temperature for both phases (Figs 3 and 4). Si dissolution was faster for heulandite at both temperatures which given the structural similarity of the phases, is strange as impurities present in clinoptilolite would cause structural instability resulting in faster breakdown. The reaction order determined at 30°C from the slope of a plot of Log Si release rate vs. pH is 0.23, increasing to 0.47 as temperature increases to 60°C (Fig. 5).


Figure 4
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  		Figure 4. Si release rate as a function of pH and temperature for Heulandite.

 


Figure 5
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  		Figure 5. Graph of log rate of silica release against pH for natural clinoptilolite at 30°C showing a gradient of 0.23, meaning that the reaction order of OH– ion in alkaline solution is 0.23, similar to the value of 0.3 suggested by Ragnarsdottir14 and Brady and Walther.19 The gradient increases to 0.47 at 60°C.

 
Effect of pH on dissolution
Si dissolution approached steady state after approximately 120 h (as shown in Fig. 1), whereas release of Al and Ca did not achieve steady state during the experiment.

Zeolite lifetimes
Near neutral pH (8) and 30°C dissolution rates for clinoptilolite and heulandite were applied to assess the time taken to dissolve a hypothetical 1 mm spherical crystal using the equation:12


Formula

where r{theta} is effective radius, r{theta}0 is initial radius, k{theta} is dissolution rate, V{theta} is molar volume and t is time. Results suggest that clinoptilolite and heulandite would dissolve completely in 38 848 years and 17 301 years, respectively. These values are within the large range of those given in the literature, which show values ranging from several hundred years to several millions of years. If a N2-BET measured value of heulandite surface area of 1.63 x 10–1 m2g–1 is used instead of the geometric value used in this study to calculate dissolution rates at pH 8 and 303 K, the equation given by Lasaga13 gives a lifetime value of 245 137 years for a hypothetical 1 mm spherical crystal of heulandite. Forsterite and heulandite have lifetime values of 600 000 and 300 000 years, respectively, as determined by Lasaga (1984)13 and Ragnarsdottir.14 The difference between rates calculated based upon geometric and measured surface areas is problematic. A true lifetime is difficult to ascertain as measured zeolite surface areas do not include the surface areas of internal channels.

Effect of temperature on dissolution
The effect of temperature on dissolution rate was found by estimating the activation energy. Activation energy (Ea) was calculated from the Arrhenius equation, k= Ae – Ea/RT, and a graph of ln k against 1/T used to find the gradient of the graph, from which Ea and the pre-exponential factor, A, were calculated (Fig. 6).


Figure 6
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  		Figure 6. Graph of ln k against 1/T, from which Ea and A can be calculated from the gradient. The graph is a straight line due to the use of only two temperatures in the study and thus subject to error.

 
It was assumed that Si release was first order as not enough data was available to calculate k directly. Graphs of Ea against pH were then plotted for both heulandite and clinoptilolite to see the effect of temperature and pH on dissolution rate (Fig. 7). Ea values for both zeolites increased as pH increased from pH 8 to pH 13.


Figure 7
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  		Figure 7. Relationship between activation energy and pH for clinoptilolite and heulandite.

 

   Discussion
Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
Conclusions
 Funding
 References
 
pH dependence of dissolution
Si dissolution rate pH dependence of clinoptilolite and heulandite appears to be related to the density of >Si-O–.14 As pH increases, OH– ion concentration in solution increases, thereby increasing the number of OH– ions that can be adsorbed into the 8/10-member ring channels and also onto surface functional species, leading to the formation of Si-O–, 15 via OH– attacking neutral silanol groups.16 This is ‘surface controlled’ dissolution, where detachment of metal cations from the surface is the rate-limiting step, as suggested by Carroll-Webb and Walther,12 Lasaga,13 Wieland, Wehrli and Stumm17 and Ragnarsdottir.14 The equation for Si-O– formation is:


Formula

Hydrolysis by OH– weakens the Si-O-Si and Al-O-Si bonds in the surface lattice and causes the detachment of Si-OH and Al-OH by formation of Si-O– and Al-O–. When a certain number of bonds are hydrolysed, that portion of the framework becomes unstable and detaches into solution.

Al initial dissolution rates are typically faster than Si for zeolites having Si/Al >1,18 suggesting formation of a Si-rich layer and incongruent dissolution taking place.14 Faster Al dissolution goes against the theory that under alkaline conditions, dissolution rate of aluminosilicate minerals is dependent upon surface charge at Si sites.2 Tetrahedral Al has a smaller charge than Si, so it would be expected that Al would be less likely to be polarized, so would have a lower detachment and hence dissolution rate, compared to Si. However, Si detachment would expose more Al sites and lower the activation energy by ‘decreasing oxygen coordination of the Al site’.19 Although this suggests that Al detachment would be dictated by the rate at which Si-O– bonds were polarized, it may be that the exposed Al was able to detach more easily than a Si atom with several bonds, and therefore have a faster dissolution rate than Si. It may also be the case that a negatively charged Al-complex exists under alkaline conditions, allowing preferential release of Al.2 This too has been proposed by Wilkin and Barnes (1998)20 whose research has shown that in alkaline solutions, soluble Al can exist as Al(OH)4–, and in NaOH solutions at high temperatures, the sodium aluminate ion, NaAl(OH)4 may also be present. Unfortunately the Al concentrations determined in this study were unreliable. Typically Al concentrations decreased with time to values below detection limits. An explanation for this decrease in Al concentration may be the formation of an aluminium oxyhydroxide precipitate, removing Al from solution, as suggested by Savage et al.2 Acid digestion of the natural clinoptilolite phase indicated that sufficient levels of Mn, Cu, Fe and Al were present that formation of an Al–Mn/Cu precipitate or oxyhydroxide was highly feasible.2, 21 Without reliable Al results it is difficult to ascertain whether dissolution was congruent or incongruent.

pH dependency
pH dependencies of log rate on pH appear to be 0.14 for heulandite and 0.23 for clinoptilolite, which can otherwise be described as


Formula

for heulandite and clinoptilolite, respectively, at 30°C. Where R = mol cm–2s–2, k and k' = appropriate rate constants, { > Si-O–} = concentration of metal surface species.19

A pH dependency of 0.14 for heulandite is slightly lower than that measured by Brady and Walther,19 Ragnarsdottir,14 Blum and Lasaga (1988)22 and Chou and Wollast23 for alkaline dissolution rates on a range of aluminosilicates. Although slightly higher than heulandite, a value of 0.23 for clinoptilolite at 30°C is broadly in line with values for other aluminosilicates in alkaline pH conditions.12

Effect of pHpzc
The point of zero charge (pHpzc) is the pH at which there are equal numbers of cations and anions present so that the surface has effectively no charge. The dissolution rate should be lowest at pHpzc because few H+ and OH– are absorbed, and hence fewer bonds are polarized and weakened.14 pHpzc for SiO2 is 2.4, a much lower value than that of MgO (pH 12). Typical values for AlIV2O3 are pH 5–7) and AlVI2O3 pH 8 (where IV and VI represent tetrahedrally and octahedrally co-ordinated Al, respectively) thereby suggesting that silica is most likely to be a source of negative charge at higher pH.19 For clinoptilolite, the pHpzc is ~3,24 so higher Si dissolution rates would be expected with increasing pH, as shown in the results. Heulandite is likely to have a similar pHpzc to clinoptilolite and exhibit similar behaviour. Titrations may have proved useful in determining the pHpzc of the phases in this experiment, but were not carried out due to time limitations.

Temperature dependence of dissolution
Higher activation energies were expected for heulandite rather than clinoptilolite, due to the presence of impurities causing structural defects in the clinoptilolite. This was not observed and may be a result of the higher Si/Al ratio, meaning a greater number of Si-O bonds must be broken, requiring more energy. It may also be due to low bond strength in one direction within heulandite thus reducing thermal stability.10 Due to rates being measured at only two temperatures 30°C and 60°C, the graph of ln K against 1/T was a straight line, and therefore the Ea values are not as accurate as if there had been more than two values.

Zeolite lifetime values
Temperatures are likely to be approximately 30°C in a repository after just several years.25 Lasaga13 used Ea to estimate the effect of depth on reaction rate. Using a geothermal gradient of 30°C/km, rates of reaction will increase by 10 km–1, which is important to consider when determining lifetimes.

The lifetime values of clinoptilolite and heulandite at 30°C were of the same order of magnitude, with approximately 21 500 years difference, although it should be stressed that these values are subject to error mainly as a result of the surface area calculation. Heulandite had a faster dissolution rate at 30°C at lower pH thus partially explaining the longer lifetime of clinoptilolite. The presence of other ‘impure’ elements within clinoptilolite may aid the formation of other minerals such as analcime on dissolution,26 and thus prolong its lifetime. SEM analysis of the samples would have been useful to determine if secondary minerals had formed, but this was impossible due to time limitations. Assuming reaction rates increase by a factor of 10,13 and lifetime values decrease by the same factor, total dissolution times may only take 103 years, which is important to consider when determining repository depth.


   Conclusions
Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 Conclusions
Funding
 References
 
This study shows that the effect of pH on zeolite dissolution is strong. Dissolution in alkaline solution most likely occurs as a result of polarization and hence de-protonation of Si-OH, forming Si-O– which in turn detaches into solution.

The effect of temperature on zeolite dissolution is important, as increasing temperatures increased dissolution rates. There is little difference between clinoptilolite and heulandite Ea values, suggesting both would be ideal within a repository.

The reaction order for heulandite and clinoptilolite were 0.14 and 0.23, respectively, slightly lower than previously determined for a range of aluminosilicate phases at similar temperatures. Clinoptilolite showed a faster dissolution rate than heulandite at higher temperatures and pH, probably due to the presence of impurities in the natural sample. In terms of their use in a repository, the clinoptilolite sample is most similar to that that would be expected to form, as a natural sample is likely to have impurities present. Further work needs to be carried out to see the effects that impurities may have on dissolution rates. Dissolution rates obtained here, applied to a hypothetical 1 mm spherical crystal show that both zeolites would dissolve in 104 years, once surface area uncertainties had been accounted for. Clinoptilolite had a longer calculated lifetime due to slower dissolution at 30°C at which lifetimes were calculated. A lifetime in the region of 104 years that under high pH and moderate temperatures in a repository environment, indicates that zeolites may prove a useful sorption barrier.


   Funding
Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 Conclusions
 Funding
References
 
This work was funded by the University of Nottingham.


   Acknowledgements
 
Sincere thanks and gratitude to Dr Elizabeth Bailey for all of her advice, time and support. Also thanks to my family and partner for their support. Dr C Rochelle and staff at BGS Keyworth are thanked for providing materials, advice and BET surface area measurements.


   References
Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 Conclusions
 Funding
 References
 

  1. Nirex Report N/I22. The viability of a phased geological repository concept for the long-term management of the UK's radioactive waste. (2005) Nirex.
  2. Savage D, Rochelle C, Moore Y, et al. Analcite reactions at 25–90 °C in hyperalkaline fluids. Miner Mag (2001) 65:571–687.[CrossRef]
  3. Colic L, Meike A. Thermodynamics of calcium silicate hydrates development of a database to model concrete dissolution at 25°C using the EQ3/6 geochemical modeling code. Technical Report (1997) Lawrence Livermore National Laboratory UCRL-ID-132088.
  4. Savage D. Zeolite occurrence, stability and behaviour. Maqarin natural analogue study Phase III SKB. In: Technical Report TR-98-04 (2004) 1. Sweden: Stockholm.
  5. Savage D. Review of the potential effects of alkaline plume migration from a cementitious repository for radioactive waste. Implications for performance assessment. In: R&D Technical Report P60 (1997) UK: Environment Agency.
  6. Baker AJ, Bateman K, Hyslop EK, et al. Research on the alkaline disturbed zone resulting from cement-water-rock reactions around a cementitious repository. In: Nirex Report N/054 (2002).
  7. Nitsche H, Prussin T, Muller A, et al. Measured solubilities and speciations of Neptunium, Plutonium and Americium in a typical groundwater J-13 from the Yucca Mountain region. In: Milestone Report 3010-WBS (1993) Los Alamos LA-125-62-MS.
  8. Triay IR, Cotter CR, Kraus SM, et al. Radionuclide sorption in Yucca Mountain tuffs with J-13 well water: Neptunium, Uranum and Plutonium. (1996) Technical Report. Los Alamos LA-12956-MS.
  9. Committee on Radioactive Waste Management. Managing our radioactive waste safely. In: Technical Report CoRWM Doc 700 (2006).
  10. Breck DW. Zeolite Molecular Sieves: Structure, Chemistry and Use (2006) New York: John Wiley and Sons.
  11. Zhao D, Cleare K, Oliver C, et al. Characteristics of the synthetic heulandite-clinoptilolite family of zeolite. Microporous Mesoporous Mater (1998) 21:371–379.[CrossRef]
  12. Carroll-Webb SA, Walther JV. A surface complex reaction model for the pH-dependence of corundum and kaolinite dissolution rates. Geochimica et Cosmochimica Acta (1988) 52:2609–2623.[CrossRef][ISI]
  13. Lasaga AC. Chemical kinetics of water-rock interactions. J Geophys Res (1984) 89:4009–4025.[CrossRef][ISI]
  14. Ragnarsdottir KV. Dissolution kinetics of heulandite at pH 2-12 and 25°C. Geochimica et Cosmochimica Acta (1993) 57:2439–2449.[CrossRef][ISI]
  15. Yamamoto S, Sugiyama S, Matsuoka O, et al. Dissolution of zeolite in acidic and alkaline aqueous solutions as revealed by AFM Imaging. J Phys Chem (1996) 100:18474–18482.[CrossRef][ISI]
  16. Bickmore BR, Nagy KL, Gray AK, et al. The effect of Al(OH)[4]– on the dissolution rate of quartz. Geochimica et Cosmochimica Acta (2006) 70:290–305.[CrossRef][ISI]
  17. Wieland E, Wehrli B, Stumm W. The coordination chemistry of weathering: III. A generalization on the dissolution rates of minerals. Geochimica and Cosmochimica Acta (1988) 52:1969–1981.[CrossRef]
  18. Cizmek A, Komunjer L, Subotic B, et al. Kinetics of zeolite dissolutions: Part 4. Influence of the concentration of silicon in the liquid phase on the kinetics of ZSM-5 dissolution. Zeolites (1994) 14:182–189.[CrossRef][ISI]
  19. Brady PV, Walther JV. Controls on silicate dissolution rates in neutral and basic pH solutions at 25°C. Geochimica and Cosmochimica Acta (1989) 53:2823–2829.[CrossRef]
  20. Wilkin RT, Barnes HL. Solubility and stability of zeolites in aqueous solution: I. Analcime, Na-, and K-clinoptilolite. Am Miner (1998) 83:746–761.[Abstract]
  21. Drake IJ, Zhang Y, Briggs D, et al. The local environment of Cu2+ in Cu-Y Zeolite and its relationship to the synthesis of dimethyl carbonate. J Phys Chem (2006) 110:11654–11664.
  22. Blum A, Lasaga AC. Role of surface speciation in the low-temperature dissolution of minerals. Nature (1988) 331:431–433.[CrossRef]
  23. Chou L, Wollast R. Steady-state kinetics and dissolution mechanisms of albite. Am J Sci (1985) 285:963–993.[Abstract/Free Full Text]
  24. Gainer G. Boron adsorption on hematite and clinoptilolite. In: Technical Report (1993) Los Alamos, LA SUB-93-64.
  25. Pultarova I. Heat conduction in radioactive waste repository. In: Technical Report (2006) Prague: Czech Technical University.
  26. Wilkin RT, Barnes HL. Nucleation and growth kinetics of analcime from precursor Na-clinoptilolite. Am Miner (2000) 85:1329–1341.[Abstract/Free Full Text]
Submitted on 30 September 2007; accepted on 17 December 2007


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