James L. A.
Reed
a,
Andrew
James
b,
Thomas
Carey
c,
Neelam
Fitzgerald
c,
Simon
Kellet
d,
Antony
Nearchou
a,
Adele L.
Farrelly
a,
Harrison A. H.
Fell
a,
Phoebe K.
Allan
*a and
Joseph A.
Hriljac
*ab
aSchool of Chemistry, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK. E-mail: p.allan@bham.ac.uk; j.a.hriljac@bham.ac.uk
bDiamond Light Source Ltd, Harwell Science and Innovation Campus, Didcot, Oxfordshire OX11 0DE, UK
cNational Nuclear Laboratory, Springfields, Salwick, Preston, PR4 0XJ, UK
dSellafield Ltd, Sellafield, Seascale, Cumbria CA20 1PG, UK
First published on 1st August 2024
Controllable sorption selectivity in zeolites is crucial for their application in catalysis, gas separation and ion-exchange. Whilst existing approaches to achieving sorption selectivity with natural zeolites typically rely on screening for specific geological deposits, here we develop partial interzeolite transformation as a straightforward and highly tuneable method to achieve sorption selectivity via forming dual-phase composites with simultaneous control of both phase-ratio and morphology. The dual-cation (strontium and caesium) exchange properties of a series of granular mordenite/zeolite P composites formed from a parent natural mordenite material are demonstrated in complex, industrially relevant multi-ion environments pertinent to nuclear waste management. The relative uptake of caesium and strontium is controlled via the extent of transformation: composites exhibit significantly increased ion-exchange affinity for strontium compared to both the parent mordenite and physical mixtures of mordenite/zeolite P phases with similar phase ratios. The composite with a 40:60 mordenite:zeolite P ratio composite achieves higher uptake rates than the natural clinoptilolite material currently used to decontaminate nuclear waste streams at the Sellafield site, UK. In situ X-ray image-guided diffraction experiments during caesium exchange demonstrate that the mordenite core retains rapid caesium uptake likely responsible for the unique ion-exchange chemistry achievable through the partial inter-zeolite transformation. These results offer a straightforward and controllable route to optimised zeolite functionality and a strategy to engineer composites from low-grade natural sources at low cost and with formulation advantages for industrial deployment.
Ion-exchange, where extra-framework cations within the structure exchange with others in solution, is integral to applications including the removal of ammonia,5 heavy metals5 and radionuclides from effluent streams.6 Radionuclide uptake from aqueous waste streams is a vital part of routine global nuclear industry operations, where porous materials including zeolites,6 hexacyanoferrates,7,8 silicotitanates9,10 and sodium titanates8,11 have been utilised. Abatement of radionuclides is also essential for remediating accidental environmental releases at tragedies such as Three Mile Island (1979),12 Chernobyl (1986)13 and Fukushima (2011).14,15 Cs-137 and Sr-90 are commonly targeted species: they are highly soluble and, combined, account for 99% of medium-lived radioactivity in spent U-235 nuclear fuel.14,16 Concentrations of these radionuclides are often very low with comparatively vast quantities of competitive cations (i.e. non-radioactive cations which can sit in the structure in place of the cation of interest) present in solution.14,17 For example, a waste stream at Sellafield, UK, contained 267000 Na ions for every Cs-137 nuclide and 59000 divalent species (Ca and Mg) were in solution for each Sr-90 cation.17 Removal via ion-exchange must, therefore, be incredibly selective – this is determined by aspects of the zeolite structural chemistry including Si/Al ratio, channel size and shape, in addition to the number and nature of extra-framework cations. When multiple ions need to be removed, the competing chemistry of ions of interest can make achieving this through a single dual-uptake material extremely challenging. This problem has limited the deployment of dual-uptake systems; the majority of materials target a single species, resulting in the requirement to source two materials and adding engineering complexity.14 In zeolitic systems, the presence of eight membered rings (8 MR) are the primary selection criteria for high caesium uptake,18 but the selectivity of caesium versus other univalent competing cations (Na and K) is also important and is promoted by higher Si/Al ratios.18 In contrast, for strontium, lower Si/Al ratios are critical in order to have Al–Al pairs present to bind the divalent strontium species.18
Ion-exchange must also take place at a suitable rate for column immobilisation. In addition to aspects of the zeolite framework itself, the morphology of particles and monoliths are factors which impact the kinetics of the ion-exchange, as well as the ease of deployment in industrial column systems. Ideally, both the zeolite structural chemistry and morphology would be simultaneously closely controlled and optimised for the composition of the effluent and the deployment system.
Zeolites are typically sourced one of two ways: hydrothermal synthesis from a basic colloidal gel or mining from natural deposits.19 Synthetic zeolites have high levels of phase-purity, but are expensive to synthesise and, as they crystallise as micron-scaled particles, often require complex post-synthetic pelleting prior to deployment in industrial systems.20 Natural analogues have some advantages which make them attractive for industrial ion-exchange – they are cheaper to source21 and easier to obtain as larger, industrially-relevant granules. However, of the over 240 currently known zeolite frameworks,22 only around 40 have been found naturally, with only a fraction of these available in significant quantities, making the range of achievable chemistry relatively limited.23 Natural deposits also commonly require activation by chemical treatment and often show geological variance, which can impede characterisation and result in disparities in performance which are difficult to rationalise.6 A striking example of this is the clinoptilolite (HEU topology) sourced from Mud Hills, California, which has been used as a dual ion-exchanger to remediate 100s m3 of Cs-137- and Sr-90-contaminated wastewater per day at the Site Ion-Exchange Effluent Plant (SIXEP) at Sellafield, UK.6,17 Like other clinoptilolites, Mud Hills is highly effective for removing Cs-137, but this analogue is unusual in that it outperforms other, seemingly isostructural materials, for Sr-90 uptake. While this poorly understood behaviour has been of great benefit to the nuclear industry,6,17 the supply of Mud Hills clinoptilolite is limited, with current stocks forecast to deplete in the 2030s. Further, its performance cannot be tuned to alternative or future feed-stream compositions.
A less common sourcing method is through transformation of another source of silicon and aluminium, such as such as kaolinitic rock,24 coal fly ash25 or another zeolite. These inter-zeolite transformations take place hydrothermally in alkaline conditions in the presence of a structure directing agent (SDA), commonly a tetra-substituted ammonium species26–28 or a metal cation. Sodium is most widely reported to fulfil this role,29–36 although transformations utilising other alkali,28,30,32 and alkaline earth,32,37,38 metals are possible. Metal hydroxides are often used as the sole reagent by providing both an SDA and alkaline conditions. A growing body of work has probed the atomic-scale mechanism of transformation in synthetic (powdered) zeolites.26,39,40 The first step is dissolution, whereby the parent zeolite is depolymerised via hydrolysis.41 Both the solution pH and temperature can control the rate of dissolution, which is routinely considered the rate-determining step. Once the solution concentration of dissolved species reaches the supersaturation threshold, nucleation ensues. Currently, there is no true consensus on the size of the species participating in this nucleation process, although recent work,42,43 combined with oligomer size and geometry general reactivity considerations,44,45 suggest that smaller ones (small rings and acyclic units) are more likely to be involved. Finally, autocatalytic growth of the structure proceeds until the precursors are sufficiently consumed and there is no supersaturation.39 The majority of previous work has focused on complete transformation of the zeolite to a new crystalline or non-crystalline phase.39 Whilst the atomic-scale premises of interzeolite transformation should be applicable when applied to the transformation of natural zeolites, natural zeolites are generally used as granules, meaning that both chemical (atomic-scale) and morphological (microscale) processes contribute to the functionality of the material. However, the formation and properties of dual-phase materials from partial interzeolite transformation and the impact upon particle morphology have remained unexplored. Here, we demonstrate that the partial interzeolite transformation of a natural zeolite (mordenite) is a highly controllable method to engineer zeolite composites with tuneable ratios of two phases with complementary ion-exchange chemistry. Concurrent morphological control is obtained via preservation of the parent granule morphology.
We obtain a range of two-phase zeolite composite dual-ion-exchangers from a single low-grade natural mordenite source. Mordenite (MOR topology), a zeolite of high geological abundance, displays excellent affinity for caesium18 but suffers from poor strontium-uptake, meaning it is excluded as a potential dual-ion-exchanger in the nuclear industry. The inter-zeolite transformation via a hydrothermal NaOH treatment35,46,47 is known to generate a more aluminous zeolite P (GIS topology) phase, which shows increased cation capacity and enhanced Sr2+ uptake.35,46,47 This originates from the additional charge-balance required by the additional framework aluminium, stronger interactions between the more negatively charged framework and charge-dense, divalent Sr2+ cations and the presence of more aluminium pairs within the framework. Previous studies also highlight a small decrease in Cs+ uptake, although rate of uptake for both species increased.46 While mordenite structures are known to transform to zeolite P, previous studies have focused on using high concentration NaOH solutions (>2 M)35,47 to ensure complete transformation into the GIS structure.46 Here we show that partial interzeolite transformation allows the relative uptake of strontium and caesium ions of granular materials to be closely and predictably controlled. This offers a straightforward strategy to engineer optimised properties from low-grade natural sources at low cost, and with formulation advantages for industrial deployment.
(1) |
y = y0 + A1e1−x/t + A2e2−x/t | (2) |
This method allows for dynamic flow testing in a very short timeframe, requiring much smaller quantities of both ion-exchange material and solution in comparison to traditional column studies, resulting in reduced waste and costs.
Fig. 2 Analysis of zeolite composites. (a) Selected PXRD patterns of zeolite composite materials derived from mordenite (λ = 1.5406 Å). NaOH treatment concentrations are labelled. Key MOR and GIS reflections are labelled on the mordenite and 0.65 M patterns respectively, in addition to the reflections for quartz (Q). All PXRD patterns are available in the ESI 2.† (b) Weight fraction (Wf) of MOR and GIS phases present in the zeolite composites as a function of NaOH concentration (CNaOH), as determined from Rietveld refinements (error bars present but smaller than points, refinements assumed presence of MOR, GIS and quartz). (c) Si/Al ratio of materials as a function of NaOH concentration (CNaOH), as determined by XRF measurements. (d) Batch uptake data for caesium and strontium for the series of composite zeolites, in addition to activated Mud Hills clinoptilolite (MH) and mordenite (MOR). The mordenite weight fraction is also shown for reference. |
Uptake data from these experiments is summarised in Fig. 2d; the parent mordenite achieves exceptional (>96%) caesium uptake, which can be attributed to it containing both twelve- and eight-membered ring channels in its structure and its comparatively silicious framework.18 The high uptake is retained in composites containing approximately more than 60% of the MOR phase (converted in 0.45 M NaOH). At higher conversion levels, Cs-uptake rapidly decreased, with the fully converted (GIS) material showing Cs uptake of around 10%.
For strontium, the Mud Hills clinoptilolite exhibits excellent affinity (90% removal), consistent with previous literature which demonstrates an unusually high affinity compared to other clinoptilolites.6,54 For the partially and fully transformed materials, strontium uptake followed the opposite trend to caesium: the parent mordenite shows poor (∼20%) uptake, due to the high Si/Al ratio meaning there are relatively few Al-pairs suitable for binding divalent strontium. The strontium uptake increases rapidly in materials converted in up to 0.5 M NaOH (approximately 70% conversion, when Cs affinity also remains high) and continues to rise to maximum of 90% when fully-converted to GIS in 0.7 M NaOH. GIS-type zeolites have more aluminous frameworks, favouring the uptake of the more charge dense Sr2+, which requires two nearby aluminium tetrahedra to bind.18,46 Interestingly, strontium removal improved significantly in 0.20 and 0.25 M treatments when compared to parent mordenite, when no growth of a crystalline GIS-type phase or change to Si/Al ratio was detected. This is possibly due to initial desilication of the mordenite material at the surface (prior to significant crystallisation of zeolite P), or dissolution of amorphous content within the parent material. N2 porosimetry (ESI 5†) shows a reduction in surface area in a sample treated for 24 hours with 0.2 M NaOH with the loss of pores between 2 and 5 nm and the growth of pores between 5 and 30 nm, similar to other mild treatments previously reported55 and consistent with desilication leading to larger porous areas. Full-width half maximum of reflections in XRD data show insignificant changes (ESI 6†), indicating that the zeolite structure remains unchanged. The ratio of mordenite/zeolite P within the powder composites was shown to tune the relative uptake of the two ions (Fig. 2d). The material formed using a 0.5 M treatment, containing an approximately 2:3 weight ratio MOR:GIS in the composite, displays optimal dual-uptake from within these cation matrices (86% Cs uptake, 89% Sr uptake). This performance is comparable to that of Mud Hills clinoptilolite in the same conditions (93% Cs uptake, 90% Sr uptake). A 50:50 physical mixture of the starting mordenite (Na-exchanged) and ‘fully converted’ zeolite P (i.e. the material transformed in 0.7 M NaOH where no mordenite was observed in the XRD) were also tested in equivalent conditions; 94 and 67% of caesium and strontium were removed, respectively. Based on our data, a 50:50 composite material formed through hydrothermal conversion would be expected to remove approximately 90% of caesium and 85% of strontium; this increased strontium (and slight decrease in caesium) uptake is attributed to the before-discussed partial desilication of the parent mordenite which occurs concurrently to the partial inter-zeolite transformation (ESI 4†).
Material | Structure type | Industrial deployment | Source | q max(Cs)/mg g−1 | q max(Sr)/mg g−1 |
---|---|---|---|---|---|
Mud Hills Clinoptilolite | Zeolite (HEU topology) | Cs, Sr at Sellafield, UK17 | Natural (California) | 203(24) | 57(4) |
Ionsiv® IE-911 | Crystalline silicotitanate (CST) | Cs, (Sr) at Hanford, WA, USA9 | Synthetic | 252(67)9 | — |
CsTreat® | Hexacyanoferrate | Cs at Fortum Loviisa, Finland, JAERI site (Japan), UKAEA (UK), Callaway (USA), Paks (Hungary), Fukushima Daiichi (Japan)14 | Synthetic | 46.5 (ref. 14) | — |
SrTreat® | Sodium titanate | Sr, JAERI (Japan), Fukushima Daiichi (Japan)14 | Synthetic | — | 219(13)14 |
Mordenite | Zeolite (MOR topology) | — | Natural (Java) | 167(10) | 54(16) |
MOR/GIS composite (0.50 M NaOH) | Zeolite (MOR/GIS topologies) | — | This work | 209(8) | 130(22) |
Zeolite P (0.70 M NaOH) | Zeolite (GIS topology) | — | This work | 206(24) | 146(15) |
Strontium capacity increases markedly as the transformation proceeds from 54 mg g−1 to 156 mg g−1 for the fully converted zeolite P: the more aluminous framework will contain more aluminium pairs to which strontium can adhere. This is comparable to work by Mimura,46 who reported a capacity of 161 mg g−1 for zeolite P (also synthesised from natural zeolites). The strontium capacity of the composite material (130 mg g−1) shows over double the strontium capacity of Mud Hills clinoptilolite, and although it remains lower than reported for the strontium-only ion-exchanger, SrTreat©, the simultaneous high capacity and selectivity for Cs adds appeal as a dual-uptake material.
Taken together with the batch ion-exchange experiments performed in the presence of competitive cations, the data show that the loss of Cs uptake observed in batch ion-exchanges for the transformed materials is likely due to the decrease in selectivity for Cs over K, as the Si/Al ratio reduces during the transformation, rather than a decrease in the gravimetric capacity: in batch ion-exchange, only 2–3% of the gravimetric capacity of the materials is utilised if all caesium is taken up from the solution. This is consistent with previous work by Kwon18 who found that univalent exchanges for less charge dense cations are promoted by high Si/Al ratios, and become less selective as silicon content reduces.
In the case of strontium, Mud Hills clinoptilolite and the parent mordenite have similar gravimetric uptake capacities, and yet the performance in batch ion-exchange studies is vastly different despite only a low proportion (between 3 and 13.5%) of the full capacity being required for full uptake in batch ion-exchanges (due to the enhanced selectivity of strontium over calcium). For the partially transformed materials, the increased uptake in the composites must originate, at least in part, from the large increase in strontium capacity observed as the zeolite P phase forms, although the selectivity for Sr over Ca may also be modified by the transformation.
Material | Treatment | MOR Wf | GIS Wf |
---|---|---|---|
a Activation by Na exchange. | |||
Mordenite | 1.0 M NaCla | 0.92(1) | 0.00(1) |
Composite 1 | 0.7 M NaOH, 100 °C | 0.91(2) | 0.01(1) |
Composite 2 | 0.8 M NaOH, 100 °C | 0.85(1) | 0.09(1) |
Composite 3 | 0.9 M NaOH, 100 °C | 0.62(1) | 0.28(1) |
Composite 4 | 1.0 M NaOH, 100 °C | 0.57(1) | 0.34(1) |
Composite 5 | 1.1 M NaOH, 100 °C | 0.25(1) | 0.67(1) |
Composite 6 | 1.2 M NaOH, 100 °C | 0.22(6) | 0.73(8) |
Fig. 3 Mechanism of transformation derived from imaging and local X-ray diffraction patterns. (a) Proposed macroscale-mechanism of interzeolite transformation. (b) SEM images of mordenite granule. (c) SEM images of composite (Composite 4) granule. (d) SEM images of GIS (Composite 5) granule. (e) Three-dimensionally reconstructed tomography images of a mordenite particle (left) and a dissected mordenite particle (right). (f) Three-dimensionally reconstructed tomography images of a Composite 4 particle (left) and a dissected Composite 4 particle (right). (g) Three-dimensionally reconstructed tomography images of a GIS Composite 5 particle (left) and a dissected Composite 5 particle (right). (h) Tomography cross-section of mordenite granule and diffraction beam trajectories for data presented in part (p). (i) Tomography cross-section of Composite 4 granule and diffraction beam trajectories for data presented in parts (m) and (n). (j) Zoomed-in tomography cross-section of Composite 4 granule. (k) Tomography cross-section of Composite 5 and diffraction beam trajectory for data presented in part (r). (l) Zoomed-in tomography cross-section of mordenite granule. (m) Processed diffraction data collected at point M (λ = 0.5965 Å). Key reflections form the MOR and GIS phase are labelled. Further information about impurity phases can be found in ESI 8.† (n) Processed diffraction data collected at point N (λ = 0.5965 Å). Key reflections form GIS phase are labelled. Further information about impurity phases can be found in ESI 8.† (o) Zoomed-in tomography cross-section of Composite 5 granule. (p) Processed diffraction data collected at point P (λ = 0.5965 Å). Key reflections attributed to mordenite are labelled. (q) Spatially-resolved diffraction tomography exhibiting phase distribution of MOR and GIS phases, in a granule of Composite 4. Left: distribution of MOR framework determined by integration of (620) Bragg reflection peak. Centre: distribution of GIS framework determined by integration of (200) Bragg reflection peak. Right: overlay of MOR and GIS phase maps. (r) Processed diffraction data collected at point R (λ = 0.5965 Å). Key reflections attributed to the GIS phase are labelled; further information about impurity phases can be found in ESI 11.† |
A cross-sectional tomography image for sample Composite 4 (Fig. 3i) suggests the presence of two main phases in the granule, with a lighter-contrast phase forming a ‘shell’ around the particle's interior. In line with SEM images (Fig. 3c), particles of spherical morphology are observed on the surface of the granule. Data collected by targeting the diffraction beam at the edge of the granule (point N in Fig. 3i) to collect a diffraction pattern for this ‘shell’ in isolation confirmed that GIS was the majority phase at the surface (Fig. 3n). A distinct ‘void’ region is present between the two phases, which is visible as darker contrast (Fig. 3j). The second phase is more uniform than the morphology observed in Composite 5, indicating that the transformation is incomplete. However, compared to the untransformed mordenite particle, there is increased texture to the interior including areas of porosity (darker areas in Fig. 3i), indicating that dissolution of the MOR phase is underway. Diffraction data collected from the centre of the particle (Fig. 3m) contained both the MOR and GIS phases with weight fractions of 0.75 and 0.18 respectively (ESI 11†); this is consistent with the idea of a ‘GIS shell’ encapsulating a mordenite interior. To further probe the spatial variation in phases to confirm this hypothesis, diffraction tomography data were collected on Composite 4. The intensity of key reflections for each phase (MOR (620) and GIS (200)) were integrated for each voxel, allowing for the reconstruction of the phase fractions of MOR or GIS as a 2D map. Fig. 3q shows a reconstructed slice through the centre of the particle. This confirms a higher intensity of the MOR phase at the centre of the granule with lower concentrations of GIS. Meanwhile, the shell of the granule contained large quantities of GIS at the expense of MOR, confirming the formation of a zeolite P shell during the transformation.
Significant morphological differences are observed in Composite 5 (Fig. 3g, k and o). The large, spherical sub-particles observed by SEM can be seen on the exterior of the granule in the tomography imaging, up to approximately 30 μm in diameter. Interestingly, the interior also contains similar sub-particles, albeit of only around 10 μm in diameter. Diffraction data collected at the point labelled R (Fig. 3r) confirms GIS as the dominant phase, with mordenite and quartz impurities (ESI 11†). This is in agreement with the bulk diffraction pattern (ESI 9†). We note that the diffraction beam is passing through the full particle, so the diffraction pattern (Fig. 3r) will contain phase information from all regions; thus, it is not possible from this data to determine whether the MOR phase remains concentrated in the centre from incomplete conversion of the particle or spread throughout. Based on these findings, we propose the macroscale mechanism of these transformations to be one of surface dissolution-recrystallisation, resulting in a dense ‘outer shell’ encapsulating a partially dissolved interior. Nucleation onto the parent material is likely preferred due to the elevated concentration of SDAs at the aluminous surface. The base then penetrates the outer shell, dissolving the interior, which then recrystallises into zeolite P (Fig. 3a).
The ability of the material to remove ions from the reservoir was calculated from the rate that the activity (number of decays per second) in the reservoir decreased after a given volume of active simulant (Vc) had passed through the material; the lower the activity of a given volume, the more radionuclides the material has sorbed. Because the active simulant liquor was pumped through the column and recirculated back into the liquor reservoir, the radionuclides become more dilute within the solution as the experiment progresses, which will reduce the rate of removal. This can be corrected for to give the probability of sorption at a given volume (α), which can be estimated using eqn (3) (Vr = reservoir volume, Vf = flow rate, C = concentration of reservoir for a particular species, see ESI 13† for derivation). A summary of sorption probabilities at three volume intervals (50, 300 and 1500 mL) is provided in Table 3.
(3) |
Material | α 50(Cs) | α 300(Cs) | α 1500(Cs) | α 50(Sr) | α 300(Sr) | α 1500(Sr) |
---|---|---|---|---|---|---|
Mud Hills clinoptilolite | 0.53 | 0.36 | 0.23 | 0.53 | 0.26 | 0.18 |
Mordenite | 0.63 | 0.38 | 0.21 | 0.60 | 0.23 | 0.14 |
Composite 2 (85:9 MOR:GIS ratio) | 0.70 | 0.42 | 0.20 | 0.74 | 0.29 | 0.16 |
Composite 4 (57:34 MOR:GIS ratio) | 0.83 | 0.42 | 0.19 | 0.98 | 0.34 | 0.16 |
Both Mud Hills clinoptilolite and the activated, untransformed mordenite display very similar Cs-137 uptake curves, in agreement with the good Cs sorption behaviour observed in earlier powder capacity studies, and with previous literature.18 The slightly higher initial uptake of Cs in mordenite compared to Mud Hills might reflect the enhanced ionic diffusion through larger 12 membered rings compared to 10 membered rings in the HEU structure, although small changes in particle permeability may also affect this (see Mud Hills clinoptilolite xCT, ESI 14†). Both composites show faster initial uptake of Cs compared to both Mud Hills and the parent mordenite; this is reflected in the higher initial α50 values (0.70 and 0.83 vs. 0.53 and 0.63, for Composite 2, Composite 4, Mud Hills and mordenite, respectively). This is unlikely to originate from enhanced ionic diffusion within the newly-formed GIS zeolite crystal structure, given that the GIS framework contains relatively narrow 8- and 4-membered ring units. Instead, this is likely to reflect the enhanced diffusion through the mordenite phase thanks to desilication and microscopic diffusion through the more porous composite (clearly observed in Fig. 3h–k), both features are induced by the transformation.
The initial uptake rate of Sr-90 in the mordenite material is higher than Mud Hills clinoptilolite (α50 of 0.60 and 0.53, respectively). However, after ∼150 mL throughput, the rate of uptake for mordenite significantly reduces and remains lower than for Mud Hills clinoptilolite throughout the experiment. Composites 2 and 4 showed significantly enhanced uptake of Sr-90 at low volumes compared to both naturally sourced analogues resulting in the trend: Composite 4 > Composite 2 > Mud Hills clinoptilolite > mordenite. The is likely to originate from both the enhanced strontium capacity, as revealed through adsorption isotherms, and the increased surface area and porosity, which can be observed in xCT images (Fig. 3h–k); this allows easier access of ions to surface adsorption sites, an effect which is likely to be significant for the larger hydrated strontium ion when compared to caesium.
The composites continue to show increased uptake of strontium compared to the parent mordenite for all throughput volumes used in these experiments. These trends are also supported by batch kinetic studies conducted on powdered composites (ESI 15†). Around 1000 mL of throughput, Mud Hills clinoptilolite surpasses the uptake capacity of the composites. This perhaps marks the point at which the majority of surface adsorption sites are taken up, and diffusion through the bulk of the material becomes the limiting factor. At this point, although the GIS phase has significantly higher capacity than either HEU or MOR, the small 8 MRs will slow ion diffusion. It is noteworthy that all four Sr curves remain further from equilibrium compared to the caesium analogues at the end of the experiment, likely due to the slower movement of the large, hydrated, divalent strontium ion. This is consistent with both complimentary batch kinetic studies (ESI 15†) and literature.18
The choice of the base concentration affords an extraordinary degree of control over the phase ratio, in this case the ratio of mordenite and zeolite P phases; this is achieved over the full interconversion phase range. The impact of this is two-fold: firstly, this process serves to diversify the chemistry of natural zeolites, meaning that both the library of potential natural sources for ion-exchangers, and the potential applications for natural zeolites are vastly widened. Secondly, the fine chemical control over the phase composition delivers materials with excellent discrimination between uptake of strontium and caesium allowing the overall ion-exchange properties to be finely, and predictably, tuned. This opens up the possibility that ion-exchange properties of mineral-source-derived composites could be designed for a given waste-stream.
The ability to readily generate column-ready, morphologically-controlled granular composites in a simple, low-cost, one pot method, whilst retaining the advantages of natural zeolites over synthetic sources, adds industrial appeal to this process. The inter-zeolite transformations core–shell morphologies from a low-grade source, without the need for additional components (e.g. binders) or processing that would be required for other mixed phase systems, such as a 50:50 mix of parent mordenite and fully converted zeolite P, which also exhibits poorer affinity for strontium. The crucial retention of the size of the parent particles in the dual-phase composites is likely the result of the dense outer shell of zeolite P which initially forms on the surface of granules during the transformation. Our combined SEM/xCT/XRD characterisations show that this shell is present in the partially transformed materials and remains even when the transformation is close to complete, with more porous areas forming in the centre of the composites. In situ xCT/XRD experiments during ion-exchange demonstrate that the core remains accessible to the caesium. In addition to the deployment advantages of maintained granularity, our RIX measurements highlight that the partially transformed granular composites demonstrate a remarkable improvement in both Cs-137 and Sr-90 uptake rates compared to the initial mordenite system. In fact, this enhancement is to such an extent that the granular composites exhibited superior Cs-137 and Sr-90 uptake rates when compared to industry standard, Mud Hills clinoptilolite, either because of the altered ion-exchange properties of the phases after the partial transformation, or because of the difference in porosity results in improved ion diffusion into the particles. These results open up inter-zeolite transformation as a route to more efficient waste processing, in addition to the potential to design the adsorber's morphology for a particular column system and contact time. We note that when selecting a material to deploy in nuclear applications, it is important to consider both the mechanical properties of the material and its impact on the cost of material immobilisation, medium-long term storage in a radioactive waste store and eventual disposal in a geological repository; these factors will be explored in future work. Taken together, the chemical and morphological tunability of the process presented here offer a straightforward and cost-effective way to tune both the thermodynamics and kinetics of ion-exchange, opening up the possibility of “designer” composites tailored towards specific effluent streams and systems. Beyond nuclear waste management, species selectivity is crucial to many applications of zeolites across the chemical sciences. Granular zeolite composites with tuneable chemical functionality and morphology may find ready application in areas where there are multiple species of interest, for example water purification, multi-gas absorbers,59,60 multi-molecular separators61 or multi-process catalysts,62 or where the combination of the complementary characteristics of different frameworks (e.g. species selectivity and adsorption capacity) may prove advantageous. While our focus here has been on extending the functionality of a natural zeolite, similar control over partial transformations should be possible in synthetic zeolite monoliths allowing the rich vein of inter-zeolite transformation chemistry40 to be used for the production of tuneable composite particles.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4sc02664k |
This journal is © The Royal Society of Chemistry 2024 |