Speciation observed by cation exchange

Speciation observed by cation exchange

the Science of the Total Environment •I a*d H* RttatiwMp afth Man The Science of the Total Environment 173/174 (1995) 301-311 ELSEVIER Speciation o...

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the Science of the Total Environment •I a*d H* RttatiwMp afth Man

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

ELSEVIER

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 + , Sr 2 + 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

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process at Sellafield [l]. This uses a natural zeolite (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 zeolite to define (a) the influence of common anions, 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 variation 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, ZSM5 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 suppliers). 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.

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 conditions 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 solution and Triton X-100. The scintillator used was PPO/POPOP in scintillation grade toluene and

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

SiO,

AI,O,

Na,O

K,O

CaO

MgO

[email protected]

MnO

A

41.80 46.70 61.90 97.90 66.03 80.20

36.50 33.90 23.70 0.62 10.47 15.90

21.20 19.60 12.50 0.67 3.25 2.00

0.09 0.01 0.20 0.01 1.81 0.90

0.08 0.06 0.37 0.02 1.55 0.20

0.01 0.01 0.11 0.01 0.58 0.10

0.02 0.03 0.51 0.04 1.21 0.30

0.03 -

x Y ZSM-5 Clinoptilolite Mordenite

“All anhydrous except clinoptilolite.

H,O 12.00 -

Total 99.80 100.3 99.30 99.27 97.00 99.60

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the appropriate quench corrections were made. Samples containing S&Co were estimated via the Cerenkov effect using 10 ml of solution in polythene vials. Measurements were in the tritium channel of a liquid scintillation spectrometer. 3. Results and discussion 3.1. Exchanges without excess anions present

Figs. l-6 illustrate the results from the equilibrium 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 only exception to this generalized statement is clinoptilolite which, being a natural mineral, 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 exchange 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 Cland 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

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shown by this high silica zeolite for divalent cations which Rees et al. [4] explained by the difficulty experienced by the divalent ions in spacing themselves between the very few specific cations sites available in the ZSM-5 structure. 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 explanation 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 Ni and Zn in dependent (e.g. clinoptilolite, Co in Y). 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-selective 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 clinoptilolite, 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 thermodynamic activity to vary. At the lower transition element concentrations the effect of competition from Na+ and thermo-

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1 00 80 60 40 20 0

R,adioi:

100 80 60 40 20 0

Fig. 2. Radioisotope uptake on Clinoptilolite

as a function of cation and anion present (after 1 week equilibration),

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100 80 60 40 20 0

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

100 80 60 40 20 0

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

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100 80 60 40 20

ride

0

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

100 80 60 40 20 0

Radi

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307

100

a 8o g

60

= 8

40 20

Fig. 7. Effect of

xide

uilibrai tied.

100

a, 8o

8

60

= 8

40 20

Fig. 8. Effect of e

31

ide

:q1 lilibl

.ation).

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

308

100 80

3 3E

60 40 20

le

0

Fig. 9 . Effbet of e

00 80 60 40 20 0

Fig. l( I. El rft:ct of

xatic

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I73 / I74 (1995) 301-311

1 00

80 60 40 20 0

.ation).

Fig. 11. Ef feet c

100

80 60 40

[de

20 0

Anion Concentration Fig. 12.

of excess anion concentration

(M)



(0.1 M of Na salt) on cation

uptake

on ZSM-5

(after

1 week

eq ,uil ibration).

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dynamic activity changes due to anion variation do not seem to be major factors. The very wide differences seen in specific cases are clearly dependent on the nature of the cation and the anion present.The following generalised comments can be made: (9

(ii)

(iii)

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 concentration often observed with ZSM-5 can be explained 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

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 complexes then it would be expected that the charge on the zeolite framework would overcome the ‘complex’ formation. This certainly applies to ease of removal of aquo ligands from cations in these zeolites [6]. the zinc cation is least ,affected in comparison to Co” and Ni” in line with its known lower propensity to form complex species in solution. the zeolite with the widest pore opening (Y) shows low sensitivity to anion presence im-

100

80

60

1

0’

-6

I

-5

I

I

-4

1

+ X

Mordenite NaX

cl

NaY

0

ZSM-5

I

-3

log concentration Fig. 13. Effect

of 0.1 M NaNO,

on uptake

of nickel nitrate

.

I

I

-2

I

I

-1

-

after

1 week.

NiNO, by different

zeolites

0

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has been observed in 3 other high silica zeolites (Dyer, A. and Emms, T.I., unpublished) 4. Conclusions Although the inferences listed above are generalized 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 concentrations - 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 dimension of its slit shaped pores is the significant dimension which is entirely reasonable. All the other zeolites have pores which can be approximated to circular apertures.

173 / 174 (1995) 301-311

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Acknowledgements One of us (Shaheen) gratefully thanks the Ministry 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 appreciated. Mrs L. Cunliffe is thanked for the production of the figures. References ill

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. 121 A. Dyer, Modern theories of ion exchange and ion exchange selectivity, in M. Quereshi and KG. Varshney (Eds), Inorganic Ion Exchangers in Chemical Analysis, CRC Press, Boca Raton, 1991, pp 33-56. [31 A. Araya and A. Dyer, Studies on natural clinoptilolites-II cation mobilities in near homoionic clinoptilolites. J. Inorg. Nucl. Chem., 43 (1981) 595-598. L.V.C. Rees and A.K. Novak, Ion ex[41 A.M. M&leer, change and aluminium distributions in ZSM-5 zeolites. Zeolites, 11 (1991) 329-336. [51 H.S. Sherry, Cation exchange on zeolites, in E.M. Flanigen and L.B. Sand, Molecular Sieve Zeolites-I, Advances in Chemistry Series 101, Am. Chem. Sot., Washington, 1971, pp 350-379. b1 A. Dyer and R.B. Gettins, The self-diffusion of cations in synthetic zeolite ZK4, J. Inorg. Nucl. Chem., 32 (19701 319-327.