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Journal of Inorganic and Organometallic Polymers and Materials https://doi.org/10.1007/s10904-020-01447-3

A Solvent Free Approach for the Preparation of Silver Modified Mesoporous Silica for Iodine Entrapment

Mícheál P. Moloney1,2 · Nicolas Massoni2 · Shani Egodawatte4 · Hans‑Conrad zur Loye3,4 · Agnès Grandjean1,3

Received: 7 November 2019 / Accepted: 9 January 2020 © Springer Science+Business Media, LLC, part of Springer Nature 2020

AbstractA green solvent-free approach, using a heating/grinding method, was employed to produce grey colored Ag functionalized mesoporous silica (here SBA-15). This material consisted of Ag0 nanoparticles with diameters of around 1.5 ± 0.45 nm inside the pores of SBA-15, and aggregated silver nanoparticles on the outside of the SiO2. Indeed due to a lack of a strong stabilizer the degree of particle aggregation is high. This easy to handle powder was then used to extract molecular Iodine. The resulting yellow powder was examined by Ultraviolet–Visible (UV–Vis) and Fourier-transform Infrared spectroscopies, X-rays diffraction, thermogravimetric analysis (TGA) and scanning and transmission electron microscopy (SEM/TEM). It was found that the aggregates of individual Ag nanoparticles disaggregate as AgI quantum dots are formed, explaining the high I2 capacity exhibited by these materials, almost 1:1 Ag to I. Despite numerous washing steps with a range of solvents (ranging from cyclohexane to water) no leaching was observed. Finally, TGA demonstrated an increased melting point for the AgI dots indicating a degree of protection from the silica support.

Keywords Solvent-free · Silver-iodide · Silver impregnation · SBA-15

1 Introduction

Although produced in relatively small quantities by the nuclear industry, the extraction and storage of radioactive iodine is important due to its biological sensitivity and long half-life (107 years) [1, 2]. Separately, the upswing in shale gas production has added new interest to this topic as shale gas produced water can have high iodide concentrations (54 mg/l) [3–6]. Although not radioactive or inherently toxic these iodides can interact with other products during the drinking water treatment processes to create harmful

side products [7]. The idea of silver functionalized supports for iodine extraction is hardly new [8–10], Ag-zeolites are typically used in nuclear industry [1, 11–14], whereas, as radiation is not a concern, in drinking water management Ag functionalized polymers can be used [3, 15, 16]. Here we expand on this work and simplify it by removing the need for solvents or stabilizing ligands making this approach greener. Taking a process reported by Tang et al. porous SBA-15 and AgNO3 are mixed, crushed, and finally heated together [17]. This allows the AgNO3 to decompose, and for nanosized metallic Ag0 particles to be formed in and on the silica structure. SBA-15 is the chosen support here as mesoporous silica; however, this simple mixing procedure should be easily transposed to other mesoporous supports, and for example commercial mesoporous silica support. This silver modified SBA-15 material was then added directly to an I2 solution where it extracted I2 at a ratio of 0.42 mol of I2 for every mole of Ag over the course of 30 min. This is almost 1:1 (Ag to I) making the sorbant quite efficient vis-a-vis silver use. Additionally, the silica support offers a certain degree of thermal protection to the AgI, raising its decomposition temperature; this is important in the nuclear industry when considering this and other materials for long term storage [18].

* Agnès Grandjean agnes.grandjean@cea.fr

1 DEN, DE2D, SEAD, Laboratory of Supercritical and Decontamination Processes, University of Montpellier, CEA, 30207 Bagnols-sur-Cèze, France

2 DEN, DE2D, SEVT, Research Laboratory for the Development of Conditioning Matrices, University of Montpellier, CEA, 30207 Bagnols-sur-Cèze, France

3 Center for Hierarchical Waste Form Materials, Columbia, SC 29208, USA

4 Department of Chemistry and Biochemistry, University of South Carolina, Columbia, SC 29208, USA

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2 Experimental

2.1 SBA‑15

SBA-15 was prepared according to the literature [19]. Briefly, 4.0 g of P123 was dissolved in 150 ml HCl solu-tion (20 ml of 36% HCl + 130 ml of Millipore water). Once a clear solution was obtained 8.5 g of TEOS (d = 0.933 g/ml) was added over 30 min. The resulting solution was then heated to 40 °C and stirred overnight. After 24 h the stirrer was removed and the solution was aged for 48 h at 100 °C. The resulting white powder (SBA-15) was filtered, washed with water. It was then placed in an oven and heated to 100 °C for 5 h before being calcinated at 550 °C for 5 h (1.5 °C/min).

2.2 Silver Modification

Using the method reported by Tang et al. [17] 500 mg of the resulting silica powder and 50 mg of AgNO3 were manually ground using a pestle and mortar for several minutes. The resulting powder contains 9.1 mass% AgNO3, which corre-sponds to 5.77 mass% Ag and 4.23 mass% NO3. This results in a molar ratio before heat treatment of 0.0353:1 Ag:SiO2 (mol/mol). The resulting mixture was then placed in an oven (preheated to 130 °C) for 30 min. It was removed from the oven and ground for another minute. It was then replaced in the oven at 95 °C and left there for 5 min. This procedure was repeated 3 times before eventually heating the mixture to 350 °C for 2 h (2 °C/min). The resulting grey powder (Ag@SBA-15) was then analysed.

2.3 I2 Extraction

A known mass of I2 was dissolved in a fixed volume of cyclohexane. It was left to stir for several hours after which it was examined using UV–Vis. The intensity of the absorbance peak at 522 nm was noted and the molar extinction coefficient was calculated using the Beer Lambert law (A = ε.c.l). The iodine concentration in the isotherm sorption tests ranged from 55 to 220 mg/l and kinetics experiments were conducted with initial iodine concentration equal to 180 mg/l. A known mass of the Ag@SBA-15 was then added with a ratio mass of Ag@SBA-15 to volume of solution equal to 1. In the case of the preparation and isolation of AgI@SBA-15 for characterisation the solid was recovered by centrifugation. It was washed with cyclohexane × 2, acetone × 2, ethanol × 2 and water × 2. The powder was air dried at room temperature. With respect to the kinetic and capacity experiments a known mass of Ag@SBA-15 was added to an I2 cyclohexane solution of know concentration and volume. After the desired time had passed

the suspension was passed through a filter removing the silica solid. The remaining I2 concentration was observed using UV–Vis. The difference in absorption along with ε was then used to calculate total amount of I2 extracted, and the extrac-tion capacity of the material.

2.4 Characterizations

The silver modified Ag@SBA-15 and the I2 modified AgI@SBA-15 were examined using Ultraviolet–Visible (UV–Vis) and Fourier-transform infrared (FT-IR) spectroscopies, X-rays diffraction (XRD), thermogravimetric analysis (TGA) and scanning and transmission electronic microscopy (SEM/TEM). Changes in the surface area and porosity of the samples were examined by N2 adsorption. The I2 extraction kinetics and I2 capacity of the Ag@SBA-15 was measured indirectly using UV–Vis.

Elemental analysis (EA) of the functionalized silica’s allowed us to determine the total Ag and AgI concentrations by mass. These chemical compositions were measured by inductively coupled mass spectrometry. UV–Vis measure-ments were carried out using a Shimadzu UV-2600. FT-IR measurements were carried out using a Nicolet iS50. Pow-der XRD was performed using a Panalytical X’Pert MPD Pro instrument operated in the Bragg–Brentano geometry. A cop-per tube (λ = 1.5406 Å) with a nickel filter was used with an X’Célérator detector. Typical measurement was completed in 5 h between 10 and 90°2θ. TGA was carried out in air using a Mettler Toledo TGA/DSC 1 stare system. Sample mass was kept constant at 20 mg. Cubicles did not have lids placed on them.

The specific surface area, pore size and pore volume of the three sorbents were obtained from nitrogen adsorption–des-orption isotherms recorded at 77 K using a Micrometrics ASAP 2020 analyser.

For TEM imaging a dilute suspension of the samples dis-persed in ethanol was sonicated for 15 min. It was drop casted onto a copper grid with a thin formvar coating and dried at room temperature prior to imaging [20]. They were obtained with a Hitachi HT 7800 TEM transmission electron micro-scope and analysed using a 120 kV accelerating voltage at var-ious magnifications. SEM images and energy-dispersive X-ray (EDX) spectra were obtained with a Zeiss Supra 55 equipped with an Oxford EDX system. The samples were observed as received without polishing. P123, AgNO3 and TEOS were purchased from Sigma Aldrich.

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3 Results and Discussion

3.1 Composition of Samples and Iodine Entrapment

3.1.1 Ag@SBA‑15

Elemental Analysis (EA) of the Ag@SBA-15 showed a silver content of 68.5 ± 0.35 mg/g (6.85 ± 0.35 wt%) that corresponded well with the initial reactant concentration (5.77 wt%). This confirmed that no silver had been lost during functionalization and calcination.

TGA of the Ag@SBA-15 (Fig. 1) shows a mass loss step between 30 and 100 °C, this probably corresponds to dehydration as surface water is lost. This is followed by a mass increase from 100 to 205 °C which is attributed to silver oxidation and Ag2O formation [17]. This is then fol-lowed by Ag2O’s gradual decomposition. A final mass loss step beginning at around 350 °C and finishing at 600 °C (1.5%), is attributed to nitrate loss. This is, we believe, the removal of residual NO3 anions from the surface of the unmodified silver nanoparticle. These nitrates probably act as weak surface stabilizers due to the absence of a clas-sic stabilizing molecule. It therefore appears that any free nitrate (liberated from the reduced silver) is also retained, possibly sorbed by the SBA-15 or used to passivate the newly formed silver surfaces.

XRD pattern (Fig. 2) confirms the presence of cubic silver (PDF00-001-0503 Fm3m), as well as silver nitrate (PDF04-011-0004) in the Ag@SBA-15.

3.1.2 AgI@SBA15

After addition of this grey colored Ag@SBA-15 powder to a purple I2 cyclohexane solution yellow AgI@SBA-15 was formed. The rate of I2 extraction was measured indirectly using UV–Vis the results of which are seen in Fig. 3. [As a control the behavior of unfunctionalized SBA-15 in the same I2 solution was also observed. It was found to sorb a very small amount of I2 which resulted in the silica tak-ing on an orange-yellow hue. This is probably due to the silanol groups on the SiO2 hydrolyzing the molecular I2. The unfunctionalised silica would later release the iodine yellowing its container walls.]

Initial I2 extraction is quite fast with 50% of the total iodine being sorbed after 1 min (Fig. 3).

At this stage I2 uptake slows considerably with equi-librium been reached after 30 min. A Langmuir plot was used to fit isotherm sorption and then calculate the maxi-mum sorption capacity; which was found to be 68 mg/g I2 (0.535 mmol/g of I−) of Ag@SBA-15 (Fig. 3). Elemen-tal Analysis of Ag@SBA-15 showed silver content to be 0.635 ± 0.003 mmol/g Ag, accordingly this corresponds to Ag:I ratio of 1:0.84.

Elemental analysis (EA) was also performed on the washed AgI@SBA-15 solid at saturation (plateau of iso-therm). Numerous washing steps using cyclohexane, fol-lowed by ethanol, then acetone and finally water were carried

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Fig. 1 TGA of Ag@SBA-15 (black) and AgI@SBA-15 (red). The mass increase in both samples at 200 °C is ascribed to Ag2O forma-tion. However, the Ag2O starts to decompose almost immediately after formation [17, 21]. Mass loss beginning at 350 °C in the Ag@SBA-15 samples is believed to be AgNO3 decomposition. Mass loss beginning at 600  °C in the AgI@SBA-15 samples is believed to be AgI decomposition

Fig. 2 XRD of Ag@SBA-15 (black) and AgI@SBA-15 (red). Note the presence of cubic silver (PDF00-001-0503 Fm3m) as well as the continued presence of silver nitrate (PDF04-011-0004) in in the Ag@SBA-15 sample. A mixture of β-AgI (PDF04-017-0944) and γ-AgI (PDF00-001-0503) is detected after I2 extraction

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out. UV–Vis confirmed that no leaching had occurred. EA showed that no Ag loss had taken place during the I2 extrac-tion process, or the following washing steps. Secondly, the washed AgI@SBA-15 agreed with the iodine content as recorded by the Langmuir Isotherm, i.e. 68 mg/g. Literature reports sorption capacities from 4 mg/g (in case of modified zeolite) and 1000 mg/g (in case of Metal Organic Frame-work) [22]. Here this result of a ratio Ag:I agree with the results from silver loaded based materials and a mechanism of chemisorption [8, 10, 16]. Even if the sorption capacity is bit lower than many of these published value, the benefit of using this kind of material is the simplicity of synthesis route and a high extraction yield (ratio 1:1 Ag to I).

After I2 extraction XRD pattern (Fig. 2) shows the disap-pearance of the Ag and AgNO3 phases and the presence AgI phase. As expected at ambient temperature and pressure, AgI is a mixture of two phases, hexagonal phase β-AgI (a hexagonal wurtzite structure) and cubic phase γ-AgI (a cubic zinc-blende structure). [23, 24].

TGA of AgI@SBA-15 (Fig. 1) once again shows the dehydration and Ag2O formation and decomposition steps. However, no AgNO3 decomposition step is visible. A new mass loss step slowly begins at around 450 °C before speed-ing up at 600 °C with a final steep inflection point at around 700 °C. As this step is not seen in the Ag@SBA-15 TGA curve it is assumed to be AgI decomposition. This tempera-ture range is relatively high for AgI nanoparticles as the lit-erature records their melting point at 550 °C [25]. However, according to the Langmuir plots the total iodine content of AgI@SBA-15 is 68 mg/g (6.8%). This is in good agreement with this mass loss (7%), indicating that it is in fact AgI decomposition with the accompanying loss of I2. This higher melting point may indicate that the AgI is being protected by the silica matrix. XRD of AgI@SBA-15 after a high temperature treatment (1000 °C) showed silver regeneration

with Ag0 reflections once again present (SI Fig. 2). Some AgI reflections were also visible. The continued presence of AgI after TGA analysis may be due to the liberated I2 gas remaining in the silica pores, after which it is recapture as the sample cooled.

To conclude this part, taking these results together, white colored calcinated SBA-15 mesoporous silica was function-alized with Ag0 through a grinding/heating method. The presence of nitrate and therefore AgNO3 in Ag@SBA15 could be attributed to the presence of an AgNO3 surface layer on the Ag0 nanoparticles. This could come from a small amount of free nitrate liberated during the thermal treatment possibly sorbed onto the Ag0 nanoparticle. This hypothesis was reached by a process of elimination. UV–Vis displays a well-structured Ag0 peak indicates that the sil-vers’ surface is passivated (see next section). As there are no other stabilizers present here this would suggest that it is the NO3

− anions which are passivating the silver surface. XRD showing the continued presence of AgNO3 after heat thermal treatment adds credence to this. The addition of I2 and the creation of AgI results in the loss of any AgNO3 reflections in the XRD. This further enhances the argument that AgNO3 was present as a surface layer. All the charac-terization including XRD data, elemental analysis (EA), and TGA indicate that all previously detected Ag and AgNO3 has been converted to AgI nanoparticles. These AgI seem to be closely linked to the SiO2 surface.

3.2 Microstructure and Mechanism of Iodine Sorption

Nitrogen sorption isotherms (Fig. 4 and Table 1) showed that the unfunctionalized calcinated SBA-15 had a surface area of 735 m2/g. This dropped to 582 m2/g after silver functionalization. The surface area was further reduced

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(369 m2/g) after I2 extraction. These reductions in surface area are accompanied by large decreases in the pore volume; from 0.981 cm3/g for SBA-15 to 0.784 cm3/g and finally to 0.495 cm3/g for the Ag and AgI modified samples respec-tively (Fig. 4 and Table 1). These decreases indicate that at least some of the silver and consequently the AgI are located inside, or on, the SBA-15’s pores.

TEM (Fig. 5) shows the SBA-15 pores to be around 6 nm in diameter (radius = 3 nm) in good agreement with the nitrogen sorption isotherm result.

SEM shows the presence of large 100–500 nm bright spots on the Ag@SBA-15 (Fig. 5). EDX confirmed that these are silver (SI). Therefore, silver structures are also pre-sent on the outside of the SiO2.These artefacts are quite large and at first glance may represent either a macro silver popu-lation, or large aggregates of individual silver nanoparticles.

Although the degree of aggregation made imaging dif-ficult, Silver particles are visible in the pore spaces with a

sizable population clearly visible along the pore walls. The average Ag particle size appears to be around 1.5 ± 0.45 nm (Fig. 5, SI). However, due to the lack of a stabilizer the degree of silver particle aggregation is high.

SEM of the AgI@SBA-15 shows a noteworthy change in the appearance of surface silver artefacts (Fig. 5). The large micron sized Ag aggregates have been replaced by smaller (70-300 nm) more monodisperse looking structures. EDX of these bright spots confirms the presence of both Ag and I (Fig. 4 SI). This confirms the hypothesis that the large artefacts seen on the Ag@SBA-15 were not silver micro-particles but aggregates of silver nanoparticles. This would help explain the high I2 capacity exhibited by these materi-als. As the silver is present as aggregated Ag0 nanoparticles then the addition of the strongly nucleophilic iodine to the silver surface would aid in surface charge separation, caus-ing the silver aggregates to disassemble thereby allowing the iodine to access all particles within the aggregate [26]. This hypothesis could also explain the decrease in the pore volume between Ag and AgI modified samples respectively: disaggregation of silver nanoparticles by sorbing iodine may close some pores of the silica micro and meso-structure. The AgI quantum dots are also seen as 3 nm ± 0.5 nm white spheres inside the silica pores.

Solid UV–Vis measurements of Ag@SBA-15 and AgI@SBA-15 are shown in Fig. 6. The presence of a single silver plasmon band at 455 nm (and the absence of other signals) confirms the presence of Ag nanoparticles (≤ 10 nm) [27, 28]. After the addition of I2 the Ag shoulder is replaced by a strong well-structured AgI exciton peak at 424 nm [29–31]. The position, strength and shape of this peak indicates that

Fig. 4 Nitrogen sorption isotherm of SBA-15 (black), Ag@SBA-15 (red) and AgI@SBA-15 (blue). Inset: Pore size (radius) distribution for each aforementioned sample. Note decrease in pore volume

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While pore radius does not change appreciably there is a 20% decrease in pore volume after Ag addition indicating that the pores are been filled. The pore volume decreases by a further 37% after I2 addition

Surface area (m2/g)

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SBA-15 729 0.981 2.85Ag@SBA-15 582 0.784 2.67AgI@SBA-15 369 0.495 2.66

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Fig. 5 SEM (left) and TEM (right) images Ag@SBA-15 (top) and AgI@SBA-15 (bottom). Silver nanoparticles appear as 1.5 ± 0.45 nm black dots in/on silica pores and pore walls. AgI semiconductor QDs are seen as 3 nm ± 0.5 nm white dots lined up within the pore network

Fig. 6 UV-Vis of Ag@SBA-15 (black) and AgI@SBA-15 (red). The Ag nanoparticle plasmon band (455 nm) disappears and is replaced by an AgI exciton peak at 424 nm after I2 is added. Inset: Close up of Ag@SiO2

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monodisperse AgI Quantum Dots (QDs) have now been formed.

As UV–Vis of the Ag@SBA-15 (Fig. 6) shows only a sin-gle silver plasmon band this confirms the conclusions drawn from the SEM/TEM data. That is that the large objects are in fact aggregates of individual silver nanoparticles and not a macro-silver population. This is also true of the AgI as the position and sharpness of the AgI exciton peak seen in Fig. 6 demonstrates, i.e. no bulk AgI is present.

The position and shape of the highly structured AgI UV–Vis signal (despite the absence of a surface passivat-ing stabilizing ligand) shows that the AgI is nanosized and probably well bound to the silica, as already observed in the TGA conclusions.

The fact that XRD shows no Ag0 reflections again sug-gests that all the Ag0 particles and surface AgNO3 has been converted to AgI. While this is not surprising for the AgNO3 (as it is a classic Ag source for AgI formation), it would seem that instead of forming Ag/AgI core shell particles, all Ag particles have been almost wholly converted to AgI. This may be result of the Kirkendall effect, allowing for diffusion of iodine throughout the silver particles, instead of simply forming an AgI shell around an Ag core [32, 33]. [Though it should be noted that no hollow particles were observed. However; given the large excess of I2 present and the annealing carried out during Ag particle formation this is unsurprising.] This efficiency makes this material highly effective as an I2 extractant.

4 Conclusion

Therefore, in conclusion, silver functionalized porous silica was prepared by a green solvent-free approach. XRD showed that metallic silver was present while UV–Vis and TEM showed that the silver was present as Ag0 nanoparticles. It was found that the conversion of Ag to AgI was almost 100% indicating perhaps the presence of the Kirkendall effect. This type of effect is well recorded in the literature. Washing with a number of solvents ranging from cyclohexane to water resulted in no AgI leaching indicating that the particles are well bound to the silica. UV–Vis of the AgI also indicated a well-structured and passivated surface. SEM suggested a disaggregation effect after I2 addition. This further influ-enced us to believe that any large Ag objects seen on the silica were aggregated nanoparticles and not bulk silver. The presence of unmodified nano sized silver particles present a large “bare” silver surface area which allows for rapid I2 extraction with a half-life of 1 min, and for total Ag0 con-version, with a ratio Ag:I close to 1:1. Finally, this study shows that this way of synthesis leads to high stable AgI functionalized porous silica mainly due to the fact that AgI is being protected by the silica matrix. This is important when

considering the transformation of silica based materials into final waste forms through the closing of their porosity by thermal treatment.

Acknowledgements Elemental Analysis was carries out by the Labora-toire de métallographie et d’analyse chimique, DEN/MAR/SA2I/DIR, CEA Marcoule, France. Special thanks to Dr Matthew Manktelow for his considered opinion.

Author Contributions The manuscript was written through contribu-tions of all authors./All authors have given approval to the final version of the manuscript. ‡These authors contributed equally (match statement to author names with a symbol).

Funding Research was conducted in part by the Center for Hierar-chical Waste Form Materials (CHWM), an Energy Frontier Research Center (EFRC) supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineer-ing under Award DE‐SC0016574. We also thank the EDDEM-CEA project for funding this work.

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