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Adsorption of cesium on silica gel containing embedded phosphotungstic acid

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Abstract

Mesoporous silica gel containing embedded phosphotungstic acid (PTA) was synthesized by sol-gel co-condensation of tetraethoxysilane with PTA in acidic media. The obtained material had high BET surface area and pore volume. A characteristic band of the Keggin structure of PTA was present in its FT-IR spectrum while its XRD patterns were absent. This proved the embedding of PTA on a sub-molecular level. The material demonstrated high adsorption capacity of Cs. Unexpectedly, porosity of the adsorbent increased after substitution of most protons by cesium cations. Cation exchange also favored agglomeration of the material particles. Kinetic studies showed that the adsorption data correlates strongly with the pseudo-second order model. The adsorbent had two types of adsorption sites: heteropolyacid anions and silanol groups. However, adsorption on silanol groups was very sensitive to the temperature. At the increased temperature, the nature of adsorption fit the Langmuir model extremely well. The obtained results can be used in the development of an effective adsorbent for clean-up of water contaminated by radioactive cesium-137.

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... Pure PTA is very soluble in water, thus it should be immobilized in a porous matrix by sol-gel synthesis (Adetola et al. 2017). The obtained materials with PTA embedded in the silica structure on a sub-molecular level demonstrated a high adsorption capacity towards cesium (Seaton et al. 2017). However, they have a fine powder morphology with particle sizes ranging from 0.1 to 1 μm, which is unsuitable for practical applications in separation columns. ...
... The adsorbent 1 was synthesized by the sol-gel method using the procedure published earlier (Seaton et al. 2017). The obtained material contained 104 mg/g of tungsten (based on AAS analysis) or 13.6 wt% of PTA. ...
... As shown in previous works, the rate-controlled adsorption mechanism of PTA/SiO 2 is most adequately described by the pseudo-second order model (Seaton et al. 2017). With this in mind, the linear form of the pseudo-second order equation: was used to examine the rate-controlled process of adsorption, where q t and q e are the amounts of Cs + adsorbed at time t and at equilibrium (mg/g), respectively, and k 2 is the rate constant for pseudo-second-order adsorption [g/(mg min)]. ...
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The adsorption of cesium on mesoporous silica materials containing embedded phosphotungstic acid (PTA) was studied. The materials contained active adsorbent and binders: γ-Al2O3, kaolin, or charcoal. The presence of Keggin units of PTA in the bound materials was confirmed by FT-IR spectroscopy. Among all materials, the formulation with γ-Al2O3 demonstrated the highest porosity and effectiveness in adsorption. Pure PTA/SiO2 contained a significant fraction of small particles between 100 and 300 nm. However, in the alumina-bound material, they were not detected. SEM imaging showed that these particles occupied interparticle space between larger γ-Al2O3 particles. The material was stable up to 540 °C. In most materials, the adsorption of cesium decreased with increase of the binder contents but not proportionally. The adsorption capacity of all materials depended on the concentration of cesium in the solutions. Maximum adsorption was achieved after 1 h. The adsorption of cesium is controlled by intraparticle diffusion while its rate can be described by the pseudo-second-order model.
... As SSNMR can probe the chemical environment of solid samples in a non-destructive and non-invasive manner, it has been used to investigate the sequestration and binding of heavy metals and radionuclides by different adsorbents. The general approach is to characterize the adsorbents and probe their structural changes by 13 C, 29 Si, 31 P, 27 Al, and 15 N SSNMR, whereas the NMR spectra of heavy metals/radionuclides themselves which should provide more direct information of binding mechanisms, are seldomly measured due to their unfavorable NMR properties [64][65][66]. For example, heavy metal nuclei are associated with large numbers of electrons, often resulting in large chemical shift anisotropies, long relaxation time, and wide chemical shift range (e.g., ca.-5500-6000 ppm for 207 Pb in diamagnetic compounds) [67]. ...
... There are also a few SSNMR studies of radionuclides, which provide more straightforward information of interactions between radionuclides and solid surfaces. Sequestration of cesium (Cs) by a mesoporous silica-gel as nine-coordinated Cs þ was suggested due to the presence of a resonance at δ iso ( 133 Cs) ¼ À29.6 ppm (CsCl solution in D 2 O ¼ 0 ppm) [66]. Moreover, the 133 Cs MAS NMR Fig. 6. 13 C SSNMR studies of magnesium carbonates phase formed in CO 2 mineralization with (a) SP (Adapted from Refs. ...
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Environmental science is an interdisciplinary field, which integrates chemical, physical, and biological sciences to study environmental problems and human impact on the environment. This article highlights the use of solid-state NMR spectroscopy (SSNMR) in studies of environmental processes and remediation with examples from both laboratory studies and samples collected in the field. The contemporary topics presented include soil chemistry, environmental remediation (e.g., heavy metals and radionuclides removal, carbon dioxide mineralization), and phosphorus recovery. SSNMR is a powerful technique, which provides atomic-level information about speciation in complex environmental samples as well as the interactions between pollutants and minerals/organic matter on different environmental interfaces. The challenges in the application of SSNMR in environmental science (e.g., measurement of paramagnetic nuclei and low-gamma nuclei) are also discussed, and perspectives are provided for the future research efforts.
... The synthesized material possessed high surface area and the FT-IR spectrum confirms the embedding of PTA in a sub-molecular level. The adsorbent contain two types of adsorption sites, i.e., heteropolyacid anions and silanol groups; however, a silanol group was very sensitive to the change in temperature [81]. ...
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Radioactive contamination has become an important environmental concern after the accident occurred in Fukushima Daiichi Nuclear Power Plants.¹³⁴Cs and¹³⁷Cs are the major fission products and they are main problems in radioactive contamination. Huge amounts of Cs were released during the Fukushima Daiichi Nuclear Power Plants accident and as a result of this incident, many researchers focused on the development of adsorbents for decontamination of radiotoxic cesium. This review will critically evaluate recent advances in the preparation of Prussian blue and its analogue compounds, which are promising materials for cesium removal. Furthermore, this review will discuss recent studies on the cesium adsorption using different types of clay and clay based adsorbents and summarize various types of newly developed Cs adsorbents reported in recent years. © 2018 The Korean Society of Industrial and Engineering Chemistry. All rights reserved.
... The product was calcined at 500 °C on air for 4 h to remove the surfactant from pores. This temperature was sufficient for total removal of all organics but below decomposition temperature of immobilized PTA, which was determined to be 523 °C [21]. ...
... Co-condensation of silica gel with PTA by the sol-gel method resulted in the covalent embedding of PTA clusters in the silica network [23]. The presense of covalent bonds was earlier confirmed by solid state 29 Si NMR spectroscopy [21]. A peak at -75 ppm was attributed to the Si-O-W bonds. ...
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... The porosity in these types of materials are enhancing the surface area and simplifying its way to interact with surroundings [18,19]. Particularly, the use of inorganic mesoporous adsorbents is a promising way to formularize simple and cost-effective protocols with high sensitivity and selective uptake of target species [20][21][22][23][24][25]. Compared with the organic exchangers, the inorganic exchangers have advantages of high thermal stability and durability for many reuse cycles. ...
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... Heteropoly acid was studied extensively because its chemical properties can be varied considerably due to the structural character- istics [22,23]. Heteropoly acid could be converted into heterogeneous catalyst when it was supported on certain supporter [24]. ...
... Adsorption on organic resins or inorganic oxides is one of the most efficient methods. Inorganic adsorbents are characterized by good selectivity, low price, rapid kinetics, mechanical stability, and no swelling [10][11][12][13][14]. ...
... Variety of porous metal phosphates have been synthesized earlier and used as excellent catalysts as well as adsorbents [5][6][7][8][9][10][11][12]. Incorporation of metal ions reveals a net negative charge to the neutral framework, which makes the material interesting for different applications [28][29][30][31][32][33][34][35][36][37]. Particularly, in the synthesis of mesoporous metal phosphates, many strategies have been followed using different inorganic structure and synthetic conditions to enhance the surface activity of the materials. ...
... Seaton et al. (44) observed that adsorption capacity of Cs (mg/g) using silica gel embedded phosphotungstic acid gradually increased with increasing Cs initial concentrations. Ding et al. (43) found that Cs + adsorption amount has increased proportionally with augmenting initial Cs concentrations. ...
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A novel isotope dilution-precipitation method has been developed to remove cesium-137 from radioactive wastewater. The process involves adding stable cesium chloride to wastewater in order to raise the total cesium concentration, which then allows both the stable and radioactive cesium ions to be precipitated together using sodium tetraphenylborate. This process was investigated utilizing laboratory solutions to determine stable cesium dose rates, mixing times, effects of pH, and filtration requirements. Once optimized, the process was then tested on synthetic wastewater and aqueous low-level waste. Experiments showed the reaction to be very quick and stable in the pH range tested, 2.5-11.5. The wastewater may need to be filtered using a 0.45-μm filter, though ferric sulfate has been shown to promote coagulation and settling, thereby eliminating the necessity for filtration. This investigation showed that this isotope dilution-precipitation process can remove Cs-37 levels below the U.S. Department of Energy's (DOE) Derived Concentration Standard (DCS) of 3.0×10(-6)μCi/mL using a single dosage, potentially allowing the wastewater to be discharged directly to sanitary sewers.
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In this study, copper hexacyanoferrate–polyacrylonitrile composite (CHCF–PAN) was prepared and used as an ion exchanger for the separation of cesium from aqueous solution. Various characterization methods including XRD, FT-IR, TG–DSC, SEM, BET and XRF were utilized for the evaluation of the synthesized ion exchanger. In order to obtain the optimum conditions for the adsorption, the influence of pH of initial aqueous solution, contact time, solution temperature and presence of the interfering cations on the distribution coefficient of cesium onto CHCF–PAN sorbent were studied. Furthermore, adsorption thermodynamic parameters namely the standard enthalpy, entropy, and Gibbs free energy were calculated and it was found that the ion exchange reaction is an endothermic and spontaneous process. Langmuir, Freundlich, Dubinin–Radushkevich (D–R) and Temkin isotherm models were fitted to the obtained experimental sorption data and it was observed that the sorption of cesium on the synthesized sorbent was better represented by the Freundlich model. Finally, in order to calculate the dynamic adsorption capacity of the synthesized ion exchanger, the column experiments were also investigated.Highlights• CHCF–PAN sorbent synthesized with porous crystalline, stable up to 200 °C. • Kd of Cs on CHCF–PAN increase with higher pH, contact time, solution temperature. • Cs adsorption on CHCF–PAN is an endothermic, spontaneous ion exchange reaction. • Sorption of Cs on CHCF–PAN was better represented by the Freundlich model. • Efficiency of adsorption column was 63.78%.
Article
Remediation of soil contaminated with 137Cs remains one of the most challenging tasks after the Chernobyl 1986 accident. The objectives of this research were to (1) identify extractants that may be used to solubilize 137Cs in soil solution, (2) study the effect of soil amendments on 137Cs accumulation by plants, and (3) evaluate the applicability of phytoextraction for environmental restoration of soil contaminated with 137Cs. The availability of 137Cs to the plants in Chernobyl soil was limited, because this radionuclide was tightly bound to exchange sites of soil particles or incorporated into the crystalline structure of primary and secondary minerals. Out of 20 soil amendments tested to increase 137Cs desorption/solubility in the soil, ammonium salts were found to be the most practical soil amendment that can potentially increase 137Cs bioavailability. Among the screened plants, Amaranth cultivars had the highest 137Cs accumulation. Three sequential crops of Indian mustard grown in one vegetation season at the experimental plot resulted in a small decrease of 137Cs specific activity within the top 15 cm of soil. Further improvements are necessary to make phytoremediation technology a feasible option for restoration of 137Cs-contaminated territories.
Article
In the geological disposal system with stable colloidal particles, the influence of the colloidal particles on the migration of radionuclides has to be evaluated. Not only the distribution coefficient, which is a measure for adsorption of radionuclides on colloidal particles, but also the reversibility of adsorption should be determined. In this study, adsorption and desorption of Cs+ ions on colloidal montmorillonite particles were investigated in the wide range of the initial Cs+ concentrations. A rapid, almost linear and reversible adsorption of Cs+ on montmorillonite was observed at basic condition, indicating that the adsorption of Cs+ on montmorillonite was mostly dominated by ion exchange. The size of the montmorillonite particles slightly affected the distribution coefficients at lower Cs+ concentrations, suggesting the contribution of surface complexation. The ion exchange and surface complexation model reproduced the adsorption and desorption behavior of Cs+. To explain the fixation of Cs+, montmorillonite was conditioned with the solutions of various Cs+ concentrations and the basal spacing was measured by X-ray diffraction (XRD). Conditioning montmorillonite at higher Cs+ concentrations > 5 × 10− 3 M decreased the basal spacing which may result in the fixation of Cs+ in the interlayer space. Since the adsorption experiments were carried out at lower Cs+ concentration < 10− 4 M, Cs+ adsorption was reversible.
Article
In Goiânia, Brazil, the accidental opening of a 137Cs teletherapy source led to the contamination of an urban area of approximately 1 km2. The restricted, local contamination patterns without any significant influence from previous contaminations (Chernobyl reactor accident and atmospheric atomic bomb test fall-out) provided a unique opportunity to study the resuspension and redeposition mechanisms in an urban area under tropical climate characteristics. Air, total deposition, rainwater, surface soil and street dust were sampled over 2 years at a garden of a house and the surroundings and analysed for 137Cs. The local meteorological conditions were recorded. In addition, some size fractionation measurements were performed. The data show a significant seasonality and a very slow long-term decrease with time for the activity concentration in air and deposition rate. Resuspension factors in the order of 10−8 to 10−9 m−1 (based on the air activity data) were derived. A wide range of values for nominal deposition velocities were found, averaging at 5–6 cm s−1. Impactor measurements indicate that about 30–60% of the total particulate matter in air is due to aerosol particles above 15 μm diameter. The data as a whole suggest that in Goiânia resuspension and deposition are mainly local phenomena and no evidence was found for a significant spreading of 137Cs from the place of primary contamination.
Article
The uptake properties of Cs + for ammonium molybdophosphate (AMP, (NH 4) 3 PMo 12 O 40 ·3H 2 O) and its composite with alginate gel polymer have been studied by the batch and column methods. The free energy for the ion exchange ([NH + 4 ] ad +Cs + NH + 4 +[Cs + ] ad) was found to have a relatively low value of −9.7 kJ/mol compared to other inorganic ion exchangers, indicating high selectivity of AMP for Cs + ions. The fine crystals of AMP exchanger were granulated with calcium alginate (CaALG) gel polymer as an immobilization matrices. The uptake rate of Cs + for AMP-CaALG composite was fairly fast and the uptake attained equilibrium within 3 h; the uptake was above 96% even in the presence of 5 M (=mol/dm 3) NaNO 3 . The distribution coefficient of Cs + , K d,Cs , decreased in the order of coexisting ions, H + >Na + >K + >NH + 4 . In a wide HNO 3 concentration region of 10 −2 –5 M, the K d,Cs value for the composite was around 10 4 cm 3 /g, while those for other elements, Na + , Sr 2+ , Co 2+ , Eu 3+ and Am 3+ , were less than 10 2 cm 3 /g. The uptake of Cs + followed a Langmuir adsorption isotherm, and the uptake capacity of Cs + increased with the content of AMP immobilized in the composite. The trace amounts of Cs + in the presence of HNO 3 were selectively adsorbed on the composite column.
Article
12-tungstophosphoric acid (PW12) has been supported on dealuminated zeolite Y, containing a secondary pore system with the predominant pore radii of 15 Å. The interaction between the heteropolyacid and the zeolite matrix has been studied using 29Si, 27Al and 31P MAS NMR. Distribution of two types of the Keggin anions, strongly and weakly interacting with the support, has been estimated quantitatively. Increased acidic properties of the PW12/zeolite Y system were demonstrated by a change in the selectivity pattern of m-xylene transformations in the gas phase (significantly increased dealkylation and transalkylation paths at the expense of isomerization). Redox properties were evaluated in oxidation reactions with H2O2 in the liquid phase. It has been shown that the PW12/zeolite Y catalysts can selectively oxidize a number of organic compounds at relatively low temperatures.
Article
Distribution coefficients of cesium on natural and cation-enriched (Na+, K+, NH4+ and Ca+2) forms of clinoptilolite were measured by batch, radioactive tracer technique. The measurements were carried out for an initial cesium concentration range of 10−6–10−1 mol/dm3 and at temperatures of 25, 40, 60 and 80 °C. Experimental isotherms evaluated from distribution coefficients were fit to Langmuir, Freundlich and Dubinin-Radushkevich (D-R)models. Of the models tested, D-R model was found to represent the isotherms better in a wider range of concentrations than either Langmuir or Freundlich model. Breakthrough behavior of cesium on natural and cation-enriched forms of clinoptilolite for a particular set of conditions were also determined in a small size column. Column parameters were evaluated using mass transfer zone concept.
Article
The structures and vibrational spectra of the α-Keggin heteropolyanions and [GaMo12O40]5− have been calculated using density functional theory. The calculations represent the first non-empirical study of the vibrational frequencies of this important class of polyoxometalates. The agreement between the previously reported vibrational spectra and the calculated values is, in general, good. A number of previously reported assignments have been confirmed or clarified. The calculations are extremely computationally demanding requiring precise energies and geometries and cannot presently be considered a standard task for the study of these large, heavy element cluster anions. Characteristic group frequencies for the type I polyoxometalates with isopolyanions with the Lindqvist structure and heteropolyanions with the α-Keggin structure have been identified and the effect of the heteroatom on these frequencies studied. The vibrational analyses confirm the high symmetry for these anions suggested by previous geometry calculations.
Article
The acidity of heteropolytungstic acids containing cesium is of great interest because of their unique catalytic and physical properties. Preparation and characterization of CsxH3−xPW12O40 (x = 1, 2, 2.5 and 3) have been performed using FTIR for functional groups, XRD for crystal structure, 31P MAS NMR for the environment of the phosphorous. Calorimetry and adsorption of pyridine interaction in cyclohexane slurry (Cal–ad method) was used to quantify the number of acid sites and the corresponding enthalpy of adsorption. The distribution of proton and cesium ions in the salt structures is nearly homogeneous, as confirmed by XRD, 31P MAS NMR and enthalpy measurements. The acidity, systematically obtained in this work, indicates different proton strengths, with the following order: H3PW > Cs2HPW ≅ Cs2.5H0.5PW > CsH2PW ≫ Cs3PW, with ΔH1 = −32.7, −28.3, −27.4, −20.8 and −7.8 kcal mol−1, respectively. These enthalpies are strongly influenced by an endothermic term related to interstitial expansion of the structure by entrance of the pyridine molecule, which is evidenced by XRD results. The total amount of protons titrated by pyridine in all solids is larger compared to the calculated population on surface. This supports penetration by the polar probe into all of the solids. For Cs2.5H0.5PW the strongest protons are exposed nearly exclusively on the surface of this solid, as demonstrated by Cal–ad results for site-1 calculation. The combination of FTIR and adsorption site density demonstrated the effect of substituting Cs for H, changing the Brønsted acid site density to hydrogen-bonded ones.
Article
Copper(II) ferrocyanide on mesoporous silica (FC-Cu-EDA-SAMMS) has been evaluated against iron(III) hexacyanoferrate(II) (insoluble Prussian Blue) for removing cesium (Cs(+)) and thallium (Tl(+)) from natural waters and simulated acidic and alkaline wastes. From pH 0.1-7.3, FC-Cu-EDA-SAMMS had greater affinities for Cs and Tl and was less affected by the solution pH, competing cations, and matrices. SAMMS also outperformed Prussian Blue in terms of adsorption capacities (e.g., 21.7 versus 2.6 mg Cs/g in acidic waste stimulant (pH 1.1), 28.3 versus 5.8 mg Tl/g in seawater), and rate (e.g., over 95 wt% of Cs was removed from seawater after 2 min with SAMMS, while only 75 wt% was removed with Prussian Blue). SAMMS also had higher stability (e.g., 2.5-13-fold less Fe dissolved from 2 to 24 h of contact time). In addition to environmental applications, SAMMS has great potential to be used as orally administered drug for limiting the absorption of radioactive Cs and toxic Tl in gastrointestinal tract.
Article
The removal of cesium from an aqueous solution by an adsorption-microfiltration (AMF) process was investigated in jar tests and lab-scale tests. The adsorbent was K(2)Zn(3)[Fe(CN)(6)](2). The obtained cesium data in the jar test fit a Freundlich-type isotherm well. In the lab-scale test, the mean cesium concentration of the raw water and the effluent were 106.87 microg/L and 0.59 microg/L, respectively, the mean removal of cesium was 99.44%, and the mean decontamination factors (DF) and concentration factors (CF) were 208 and 539, respectively. The removal of cesium in the lab-scale test was better than that in the jar test because the old adsorbents remaining in the reactor still had adsorption capacity with the premise of no significant desorption being observed, and the continuous renewal of the adsorbent surface improved the adsorption capacity of the adsorbent. Some of the suspended solids were deposited on the bottom of the reactor, which would affect the mixing of adsorbents with the raw water and the renewing of the adsorbent surface. Membrane fouling was the main physical fouling mechanism, and the cake layer was the main filtration resistance. Specific flux (SF) decreased step by step during the whole period of operation due to membrane fouling and concentration polarization. The quality of the effluent was good and the turbidity remained lower than 0.1NTU, and the toxic anion, CN(-), could not be detected because of its low concentration, this indicated that the effluent was safe. The AMF process was feasible for practical application in the treatment of liquid waste containing cesium.
Article
Upon exposure to ultraviolet (UV) radiation, non-toxic hexacyanoferrate (II) (Fe(CN)6(-4)) undergoes direct photolysis, resulting in the liberation of toxic free cyanide (HCN,CN-). This experimental study employed manipulation of several environmental parameters with the objective of characterizing their effects on the photolysis rate of hexacyanoferrate (II). The photolysis rate was not affected significantly by varying (1) the initial hexacyanoferrate (II) concentration (from 10 to 400 microg/l as total CN), (2) the solution turbidity (kaolin clay concentration from 0 to 5 mg/l), and (3) pH (from pH 4 to 12). Parameters that exhibited a significant effect (significance level, alpha < 0.05) on the photolysis rate included the intensity of incident ultraviolet radiation (from 30 to 110 micromol/m2s photons) and the concentration of dissolved organic matter (color) from added humic acid (from 0 to 10 mg/l). In addition, observations made by spiking both deionized and natural waters demonstrated that the rate of hexacyanoferrate (II) photolysis (1) significantly exceeded the rate of free cyanide formation from photolysis and (2) exhibited significant retardation that directly depended on the free cyanide concentration in solution. The hexacyanoferrate (II) photolysis data were consistent with a simple, semi-empirical kinetic model that included the reversible formation of at least one cyanoferrate intermediate. The reverse reaction, in turn, behaved in a manner that was consistent with a second order rate law with respect to free cyanide concentration.