the Science of the Total Environment

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ELSEVIER The Science of the Total Environment 173/174 (1995) 301-311

Speciation observed by cation exchange

A. Dyer*, Tahira Shaheen

Department of Chemistry & Applied Chemistry, Cockcroft Building, University of Salford, Salford M5 4WT, UK

Abstract

The mediation of uptake of radioisotopes of Co, Ni & Zn by the presence of differing anion concentrations has been examined in the range 10_2 -» 10~5M with respect to metal ion concentration. The cation exchange substrates chosen were a range of zeolite (aluminosilicate) exchangers. Selection was based upon their regular structures, variation in Si/Al isomorphous replacement and microporosity. Very clear effects were observed whereby changes in cation uptake can be explained by the phenomenon of cation speciation. Observation of cation exchange capacity changes affords a method of 'sensing' speciation.

Keywords: Radioisotopes; Uptake; Cation exchange

1. Introduction

One of the major problems associated with the removal of undesirable elements from aqueous environments is the lack of knowledge of the exact nature of the species needing to be re­moved. It seems clear that the transition elements for instance, do not behave as the well defined ionic species expected from their well studied solution chemistry when they occur in water envi­ronments at low concentrations. This may well be true even of other 'simple' cations such as Cs+, Sr2+ of interest to 'nuclear waste clean-up' and

* Corresponding author

certainly is a factor when dealing with uranium, thorium and transuranic elements.

Speciation is the term commonly used to de­scribe the uncertain nature of the moieties pre­sent at minimal levels in a solution phase. The current difficulties in clarifying the species pre­sent lie in lack of instrumental methods capable of producing reliable analytical information at sub ppm level. As far as radioisotope species are concerned nuclear instrumentation is well able to assess elemental concentrations at ~ pg level and selective cation exchange can be used to distinguish specific species.

These factors have been successfully combined in the use of zeolite cation exchangers in aqueous nuclear waste treatment exemplified in the SIXEP

0048-9697/95/S09.50 © 1995 Elsevier Science BV. All rights reserved. SSDI 0048-9697(95)04776-W

302 A. L$er, T Shaheen / The Science of the Total Entironment 173 / 174 (1995) 301-311

process at Sellafield [l]. This uses a natural ze- olite (clinoptilolite) to take up 20 mol Cs (1 mol Sr) in the presence of 7 X lo5 moles of Na, 6.5 X lo3 moles of Mg and 5 X lo3 moles Ca. The work described here seeks to use the selective uptake of an environmentally interesting cation by a ze- olite to define (a) the influence of common an- ions, and (b) the concentration ranges, on the presumed cation exchange process. The absence of expected cation uptake in the presence of anionic species can be construed as the absence of expected cation species in solution. The varia- tion of anion concentration can be achieved by adding a salt of a cation with known low affinity for the exchanger.

The cations studied were Con, Ni” and Zn” in the presence of the Cl-, NO, and SOi- anions. Several zeolites were chosen for examination to provide a range of structures, Si/Al isomorphous substitution and ease of availability. A further paper will describe similar work with iron with the same anions and also the effect of simple organic anions on all four transitional elements.

2. Experimental

The synthetic zeolites used were A, X, Y, ZSM- 5 and mordenite supplied in their sodium form by Laporte Inorganics, Moorfield Rd, Widnes, Cheshire UK (they are no longer available from this source but can be obtained from other sup- pliers). In addition a sample of the natural zeolite clinoptilolite was used. This was from the Mud Hills deposit in California (that used in the SIXEP plant) and contains Na, K and Ca as its most readily exchangeable ions. The compositions of the zeolites are in Table 1.

Table 1 Composition of zeolites useda (supplier’s data)

2.1. Ion exchange The following experiments were carried out

using the radioisotopes 59-Co, 63-Ni and 65-Zn to follow cation exchange processes;

(a) uptake of Co, Ni and Zn onto the zeolites chosen, in the concentration range lo-* + 10M5M from Cl-, NO; and SO:- solutions.

(b) repeat of the above experiments with the addition of 0.1 M solutions of NaCl, NaNO, and Na,SO, as appropriate.

(c) as (b) with sampling over a 200-h period.

The isotopes were supplied in chloride form by Amersham International UK Conversions to the appropriate NO; or SO:- form was by use of a strong base anion exchange resin (Bayer Lewatit M500 KROH).

The exchanges used ANALAR reagents in most cases and deionized water throughout. They were performed in plastic containers in which 0.1 g tared aliquots of zeolite was in contact with 20 ml solution and equilibration times of upto 1 week. All exchangers were continuously stirred along their horizontal axis.

2.2. Radioisotope measurements The extent of exchange was monitored by loss

of radioisotope from the solution phase. 63-Ni and 65-Zn were conveniently measured by liquid scintillation counting with the most suitable con- ditions being 1:5:4 proportions for 63-Ni and 1:6:3 for 65-Zn with the proportions being respectively the ml of aqueous salt solution, scintillator solu- tion and Triton X-100. The scintillator used was PPO/POPOP in scintillation grade toluene and

Zeolite SiO, AI,O, Na,O K,O CaO MgO Fe@3 MnO H,O Total

A 41.80 36.50 21.20 0.09 0.08 0.01 0.02 - 99.80 x 46.70 33.90 19.60 0.01 0.06 0.01 0.03 - - 100.3 Y 61.90 23.70 12.50 0.20 0.37 0.11 0.51 - - 99.30 ZSM-5 97.90 0.62 0.67 0.01 0.02 0.01 0.04 - 99.27 Clinoptilolite 66.03 10.47 3.25 1.81 1.55 0.58 1.21 0.03 12.00 97.00 Mordenite 80.20 15.90 2.00 0.90 0.20 0.10 0.30 - - 99.60

“All anhydrous except clinoptilolite.

A. Dyer, T. Shaheen / The Science of the Total En ~.ironment I73 / 174 (I 995) 301-311 303

the appropriate quench corrections were made. shown by this high silica zeolite for divalent Samples containing S&Co were estimated via the cations which Rees et al. [4] explained by the Cerenkov effect using 10 ml of solution in poly- difficulty experienced by the divalent ions in spac- thene vials. Measurements were in the tritium ing themselves between the very few specific channel of a liquid scintillation spectrometer. cations sites available in the ZSM-5 structure.

3. Results and discussion

3.1. Exchanges without excess anions present Figs. l-6 illustrate the results from the equilib-

rium exchanges carried out without excess anion additions. In most of the zeolites used the ion present initially in the structure is Na+. This is present ‘as-synthesized’ and is well-known to be a cation of low selectivity to the zeolite phase. This means that the transitional element cations M” (M = Co, Ni, Zn) would be expected to be selectively scavenged into the zeolite phase [2]. This should be true at all the concentration ranges studied with the possible exception of ZSM5. This zeolite has a very high Si/Al ratio and little information is available on its ion-exchange properties in the presence of transition elements.

The zeolites which have Si/Al N 1 in their frameworks (A, X) show close to 100% uptake which may suggest that the variances shown in Y and ,mordenite reflect their higher Si/Al content (see Table 1). Note that the Si/Al content of these zeolites @i/Al N 3-5) is much less than that of ZSM5 (Si/Al m 2130) and the explana- tion invoked by Rees et al. [4], cited above, is unlikely to be valid, A more acceptable reason could be to link these low values to unfavourable access to ‘hidden sites’ under these conditions which is well-documented in earlier work [5].

A final conclusion to the assessment of the results in Table 1 is to note the instances where, at the highest concentration used (lo-’ M), there are some indications that uptake may be anion dependent (e.g. Ni and Zn in clinoptilolite, Co in Y).

The only exception to this generalized state- ment is clinoptilolite which, being a natural min- eral, contains exchangeable Na+, K+, Ca2+ (and perhaps Mg2+ cations) as mentioned earlier. This point will be considered later. Returning to assess the Na containing zeolites from Figs. 1 to 6 which show that, at 10e3 and lop4 M concentrations, the anticipated uptake, close to lOO%, has been observed in most cases. Exceptions arise for Co uptake into clinoptilolite, mordenite and ZSM-5 especially in the presence of the SO, = anion. The presence of this ion also diminishes the ex- change of Ni into zeolite A, mordenite, ZSM-5 and clinoptilolite. When the transition element concentration is lo-’ M increased reductions in uptake can be seen in some cases even with Cl- and NO; anions. It should be noted that the uptake of Zn2+ remains close to 100% in all zeolites at concentrations 10m3 - 1O-5 M. The results at the highest concentration (10e2 M) were unexpectedly variable. The low uptakes observed by clinoptilolite can be explained by the lack of ability of the Co, Ni & Zn ions to replace the K+ (and Ca2+ ?) ions from the zeolite [3]. Those of ZSM-5 reflect the lack of preference

3.2. Effect of additions of O.lM sodium salts with common anion

The results are shown in Figs 7-12fig 12 Detailed interpretation will not be attempted

and broad conclusions will suffice to highlight the effect of added anion.

At lo-’ M zeolite A results remain largely unaffected, but otherwise the uptake of transition element is reduced. This can be assigned to the competition of Na+ ions for cation sites i.e. the assumption that the zeolite phases are non-selec- tive to Na is not valid at this concentration. There are some instances where isotope uptake has been increased (Co SO, in clinoptilolite and ZSM-5, Ni SO, in 2SM-5) and some where the anion dependence is retained (Co, Zn in clinop- tilolite, Co, Ni in ZSM-5). Clearly these variations may be related to cation speciation but could still be due to selective changes as the presence of different anions causes the solution thermody- namic activity to vary.

At the lower transition element concentrations the effect of competition from Na+ and thermo-

304 A. Dyer, T. Shaheen / The Science of the Total Environment I73 / 174 (1995) 301-311

1

R, adioi:

00

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Fig. 2. Radioisotope uptake on Clinoptilolite as a function of cation and anion present (after 1 week equilibration),

A. Dyer, T Shaheen / The Science uf the Totnl En oironment I73 / 174 (1995) 302 -31 I 305

100

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Fig. 3. Radioisotope uptake on Mordenite as a function of cation and anion present (after 1 week equilibration).

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Fig. 4. Radioisotope uptake on Zeolite X as a function of cation and anion present (after 1 week equilibration)

306 A. Dyer, T. Shaheen / The Science of the Total Environment 173 / I74 (1995) 301-311

100

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ride

Fig. 5. Radioisotope uptake on Zeolite Y as a function of cation and anion present (after 1 week equilibration).

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A. Dyer, T. Shaheen / The Science of the Total Entironment 173 / 174 (1995) 301-311 307

xide

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308 A. LIyeq T Shaheen / The Science of the Total Environment 173 / I74 (1995) 301-311

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A. Dyer, T. Shaheen / The Science of the Total Entironment I73 / I74 (1995) 301-311 309

.ation).

[de

Anion Concentration (M) ’

of excess anion concentration (0.1 M of Na salt) on cation uptake on ZSM-5 (after 1 week eq ,uil ibration).

310 A. L$ec T Shuheen / The Science of the Total Endronment 173 / I74 (lW5) 301-311

dynamic activity changes due to anion variation do not seem to be major factors. The very wide differences seen in specific cases are clearly de- pendent on the nature of the cation and the anion present.The following generalised com- ments can be made:

(9 the zeolite frameworks carrying the highest unit charge, i.e. A and X where Si/Al + 1, often show the least sensitivity to anion species. If it were that the changes noted were a function of loose cation/anion com- plexes then it would be expected that the charge on the zeolite framework would overcome the ‘complex’ formation. This cer- tainly applies to ease of removal of aquo ligands from cations in these zeolites [6].

(ii) the zinc cation is least ,affected in compar- ison to Co” and Ni” in line with its known lower propensity to form complex species in solution.

(iii) the zeolite with the widest pore opening (Y) shows low sensitivity to anion presence im-

100

80

60

plying that any speciated cations can more readily gain access to the interior cation exchange sites.

(iv) if cation specification is advanced to explain these results it could be predicted that cation exchange should become negligible in the presence of salt molecules or zero charged complexes. Examples of this can be seen and are most prevalent with the divalent sulphate anion as would be expected (sulphato bidentate ligands?)

(4 it might be that the very significant increase in uptake with decrease of cation concentra- tion often observed with ZSM-5 can be ex- plained by the ingress of [MX]’ monovalent species, where X = Cl-, NO;, or l/2 SOi- which, as monovalent species, are more readily accommodated on the ZSMJ exchange sites as is the case of the general uptake of the monovalent alkali metal ions in contrast to the divalent ions as mentioned earliert4! It may also be mentioned that this phenomenon is not confined to ZSM-5 and

+ Mordenite + Mordenite X NaX cl NaY 0 ZSM-5

0’ 1 I I I 1 I . I I I I

- -6 -5 -4 -3 -2 -1 0

log concentration NiNO,

Fig. 13. Effect of 0.1 M NaNO, on uptake of nickel nitrate by different zeolites after 1 week.

A. Dyer, T. Shaheen / The Science of the Total Environment 173 / 174 (1995) 301-311 311

has been observed in 3 other high silica zeolites (Dyer, A. and Emms, T.I., unpub- lished)

4. Conclusions

Although the inferences listed above are gener- alized they are sufficiently clear as to support the general premise of this work. This is that zeolite cation exchange can be used as a ‘sensor’ to quantify the extent of speciation at low concen- trations - at least for Co and Ni. Similar effects have been observed for Fe3+ and for organic ligands. This will be reported later.

There is one final point which can be made and that is to choose the most likely circumstance for ‘speciation’ and to see if any evidence of the well known ability of zeolites to act as ‘ion sieves’. If this circumstance is acknowledged to be Ni” in the presence of the nitrato-ligand then Fig. 13 shows a reasonable correlation between extent of uptake, at lo-‘M and zeolite pore opening. The zeolite pores, of course, are f&red, regular and provide the only access to internal cation sites. The ambiguity in the low apparent uptake into mordenite can be reduced if the smallest dimen- sion of its slit shaped pores is the significant dimension which is entirely reasonable. All the other zeolites have pores which can be approxi- mated to circular apertures.

Acknowledgements

One of us (Shaheen) gratefully thanks the Min- istry of Science and Technology, Government of Pakistan, for financial support to complete this work. Gifts of zeolites from Laporte Inorganics, Widnes, Cheshire UK were also greatly appreci- ated. Mrs L. Cunliffe is thanked for the produc- tion of the figures.

References

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M. Howden and J. Pilot, The choice of ion exchanger for British Nuclear Fuels Ltd, site ion exchange effluent plant, in D. Nadin and M. Streat (Eds), Ion Exchange Technology, Ellis Horwood, Chichester, 1984, pp 66-73. A. Dyer, Modern theories of ion exchange and ion ex- change selectivity, in M. Quereshi and KG. Varshney (Eds), Inorganic Ion Exchangers in Chemical Analysis, CRC Press, Boca Raton, 1991, pp 33-56. A. Araya and A. Dyer, Studies on natural clinoptilolites-II cation mobilities in near homoionic clinoptilolites. J. In- org. Nucl. Chem., 43 (1981) 595-598. A.M. M&leer, L.V.C. Rees and A.K. Novak, Ion ex- change and aluminium distributions in ZSM-5 zeolites. Zeolites, 11 (1991) 329-336. H.S. Sherry, Cation exchange on zeolites, in E.M. Flani- gen and L.B. Sand, Molecular Sieve Zeolites-I, Advances in Chemistry Series 101, Am. Chem. Sot., Washington, 1971, pp 350-379. A. Dyer and R.B. Gettins, The self-diffusion of cations in synthetic zeolite ZK4, J. Inorg. Nucl. Chem., 32 (19701 319-327.