Cesium uptake from aqueous solutions by bentonite: a comparison of multicomponent sorption with ion-exchange models

Multi-cation exchanges involved in cesium and potassium sorption mechanisms on vermiculite and micaceous structures

Vermiculite and micaceous minerals are relevant Cs⁺ sorbents in soils and sediments. To understand the bioavailability of Cs⁺ in soils resulting from multi-cation exchanges, sorption of Cs⁺ onto clay minerals was performed in batch experiments with solutions containing Ca²⁺, Mg²⁺, and K⁺. A sequence between a vermiculite and various micaceous structures has been carried out by conditioning a vermiculite at various amounts of K. Competing cation exchanges were investigated as function of Cs⁺ concentration. The contribution of K⁺ on trace Cs⁺ desorption is probed by applying different concentrations of K⁺ on Cs-doped vermiculite and micaceous structures. Cs sorption isotherms at chemical equilibrium were combined with elemental mass balances in solution and structural analyses. Cs⁺ replaces easily Mg²⁺  > Ca²⁺ and competes scarcely with K⁺. Cs⁺ is strongly adsorbed on the various matrix, and a K/Cs ratio of about a thousand is required to remobilize Cs⁺. Cs⁺ is exchangeable as long as the clay interlayer space remains open to Ca²⁺. However, an excess of K⁺, as well as Cs⁺, in solution leads to the collapse of the interlayer spaces that locks the Cs into the structure. Once K⁺ and/or Cs⁺ collapse the interlayer space, the external sorption sites are then particularly involved in Cs sorption. Subsequently, Cs⁺ preferentially exchanges with Ca²⁺ rather than Mg²⁺. Mg²⁺ is extruded from the interlayer space by Cs⁺ and K⁺ adsorption, excluded from short interlayer space and replaced by Ca²⁺ as Cs⁺ desorbs.

Research on the remediation of cesium pollution by adsorption: Insights from bibliometric analysis

While nuclear energy with zero carbon emissions will continue to occupy an indispensable position in future scenarios for power generation, the proper disposal of nuclear waste is still highly challenging in many countries. Adsorption is currently one of the primary methods used for removal of cesium from wastewater. However, no available literature has systematically summarized advances and outlooks on the adsorptive removal of cesium, and research issues such as relevant adsorption mechanisms remain largely unexplored. In this study, a bibliometric analysis was used to quantitatively analyze 10141 publications in the Web of Science Core Collection that were published from 1900 to 2022. Current publication trends and active countries, most influential authors and institutions, journal distribution, and research hotspots and trends were reviewed and summarized. The results for the conceptual structure and evolution of investigations in this field showed three distinct periods of rapid development in recent decades. The first period concerned the scope, degree, and influences of pollution by cesium and the development of natural adsorbents. The second period included the exploration and verification of adsorption mechanisms, the fabrication and optimization of new materials, and the application of density functional theory for chemical calculations. The third period involved the development of more advanced biodegradable, nanoscale and synthetic materials with great potential for use as adsorbents as well as advances in engineering applications. Notably, the study showed that it is necessary to further enhance application-driven laboratory investigations. Future directions for research were proposed, such as the investigation of complex adsorption mechanisms, development of new materials, and engineering applications of materials developed in the laboratory. The findings will provide valuable insights and serve as a reference for researchers and policymakers as they address the adsorptive remediation of cases of pollution by cesium.

Cs selectivity and adsorption reversibility on Ca-illite and Ca-vermiculite

For understanding and predicting the Cs behavior in soils and groundwater, Cs adsorption properties of illite and vermiculite were investigated under various pH conditions and Cs concentrations. Cs adsorption and desorption experiments have been conducted with Ca-homoionic illite and Ca-vermiculite in CaCl₂ solution with an ionic strength of 0.03 and of 0.06 mol.L^-1 respectively, by focusing on cation exchanges between Cs, proton and calcium at thermodynamic equilibrium. Ca-illite displayed more affinity for Cs than Ca-vermiculite. Cs adsorption was non-linear for both clay minerals and a multi-site exchange model approach was adopted to interpret and model adsorption isotherms. Each mineral reactivity was described by their sorption site properties expressed by their exchange capacity and ionic selectivity. High-selective and low-capacity sites were shown to control Cs uptake at concentrations below 10^-8-10^-7 mol.L^-1 for both Ca-illite and Ca-vermiculite. Three high-capacity sites dominated Cs adsorption at higher concentrations. Cs adsorption reversibility was demonstrated for illite at Ca concentrations ranging from 5 10^-2 to 10^-9 mol.L^-1. The partial irreversibility of Cs adsorption onto vermiculite at Cs concentrations greater than 10^-5 mol.L^-1 was related to interlayer collapse. Reversible adsorption may occur as long as the interlayer space stays open. The irreversible adsorbed fraction was quantified and taken into account in the modeling approach to calculate the selectivity coefficient of each site.

Thermal Decomposition Behavior of Prussian Blue in Various Conditions

Prussian blue analogs (PBA) are widely studied for radioactive cesium decontamination. However, there are fewer works related to their post use storage. Considering the oxidative stabilization of the material after the selective uptake of Cs, the thermogravimetric properties in powder and bead form, with various Cs and other alkali metal ions adsorbed, and various heating rates were studied. TG-DTA taken in dry air condition shows an exothermic decomposition at ~270 °C. This temperature varied with the heating rate, mass, and the proportion of adsorbed ions. The best condition for complete oxidation of Prussian blue (PB) is found to be a gradual oxidative decomposition by heating in the temperature range of 200–220 °C until the total mass is decreased by >35%. After this, the temperature could be safely increased to >300 °C for the complete oxidative decomposition of PB that formed iron oxide and salt of the adsorbed Cs. A pilot scale test conducted using the radioactive Cs adsorbed Prussian blue microbeads (PB-b) confirmed that no Cs was released in the effluent air during the process.

Separation and recovery of ammonium from industrial wastewater containing methanol using copper hexacyanoferrate (CuHCF) electrodes

Ammonium is typically removed from wastewater by converting it to nitrogen gas using microorganisms, precluding its recovery. Copper hexacyanoferrate (CuHCF) is known to reversibly intercalate alkali cations in aqueous electrolytes due to the Prussian Blue crystal structure. We used this property to create a carbon-based intercalation electrode within an electrochemical cell. Depending on the electrode potential, it can recover NH₄⁺ from wastewater via insertion/regeneration while leaving organics. In the first phase, different binders were evaluated towards creating a stable electrode matrix, with sodium carboxymethyl cellulose giving the best performance. Subsequently, based on voltammetry, we determined an intercalation potential for NH₄⁺ removal of + 0.3 V vs. Ag/AgCl, while the regeneration potential of the electrode was + 1.1 V (vs. Ag/AgCl). Using the CuHCF electrodes 95% of the NH₄⁺ in a synthetic wastewater containing 56 mM NH₄⁺ and 68 mM methanol was removed with an energy input of 0.34 ± 0.01 Wh g^-1 NH₄⁺. A similar removal of 93% was obtained using an actual industrial wastewater (56 mM NH₄⁺, 68 mM methanol, 0.02 mM NO₂^-, 0.05 mM NO₃^-, 0.04 mM SO₄^2- and 0.34 mM ethanol), with an energy input of 0.40 ± 0.01 Wh g^-1 NH₄⁺. In both cases, there was negligible removal of organics. The stability of CuHCF electrodes was evaluated either by open circuit potential monitoring (61 h) or by cyclic voltammetry (50 h, 116 cycles). The stability during cycling of the electrode was determined in both synthetic and real streams for 25 h (125 cycles). The charge density (C cm^-1) of the CuHCF electrodes declined by 17 % and 19% after 125 cycles in the synthetic stream and the actual wastewater, respectively. This study highlights the possibility of low-cost CuHCF coated electrodes for achieving separation of NH₄⁺ from streams containing methanol. The stability of electrodes has been improved but needs to be further enhanced for large-scale applications and long-term operation.

Nuclear wastewater decontamination by 3D-Printed hierarchical zeolite monoliths

The selective removal of radioactive cationic species, specifically ¹³⁷Cs⁺ and ⁹⁰Sr²⁺, from contaminated water is critical for nuclear waste remediation processes and environmental cleanup after accidents, such as the Fukushima Daiichi Nuclear Power Plant disaster in 2011. Nanoporous silicates, such as zeolites, are most commonly used for this process but in addition to acting as selective ion exchange media must also be deployable in a correct physical form for flow columns. Herein, Digital Light Processing (DLP) three-dimensional (3D) printing was utilized to form monoliths from zeolite ion exchange powders that are known to be good for nuclear wastewater treatment. The monoliths comprise 3D porous structures that will selectively remove radionuclides in an engineered form that can be tailored to various sizes and shapes as required for any column system and can even be made with fine-grained powders unsuitable for normal gravity flow column use. 3D-printed monoliths of zeolites chabazite and 4A were made, characterized, and evaluated for their ion exchange capacities for cesium and strontium under static conditions. The 3D-printed monoliths with 50 wt% zeolite loadings exhibit Cs and Sr uptake with an equivalent ion-capacity as their pristine powders. These monoliths retain their porosity, shape and mechanical integrity in aqueous media, providing a great potential for use to not only remove radionuclides from nuclear wastewater, but more widely in other aqueous separation-based applications and processes.

3D-printed monoliths of zeolites chabazite and 4A were made, characterized, and shown effective for removing strontium and caesium from water.

Organic contaminant removal efficiency of sodium bentonite/clay (BC) mixtures in high permeability regions utilizing reclaimed wastewater: A meso-scale study

Wastewater reclamation now represents an effective measure for sustainable water resource management in arid regions, however wastewater components (organic micropollutants) may potentially impact local ecological and/or human health. Previous studies have shown that sodium bentonite/natural clay (BC) mixes may be used to effectively reduce riverbed infiltration in regions characterized by excessively high hydraulic conductivity. Accordingly, the current study sought to investigate the contaminant removal efficiency (Re) of several BC mass ratios in simulated dry riverbeds. Results indicate that the measured Re of NH₄⁺-N, COD_cr and BOD₅ increased in concurrence with an increasing sodium bentonite ratio, up to a maximum Re of 97.4% (NH₄⁺-N), 55.2% (COD_cr), and 51.5% (BOD₅). The primary contaminant removal site was shown to be the infiltration-reducing (BC) layer, accounting for approximately 40%, 60%, and 70% of NH₄⁺-N, COD_cr and BOD₅ removal, respectively. Conversely, the removal efficiency of NO₃-N was found to be low (<15%), while total phosphorous (TP) was found to actively leach from the infiltration-reduction layer, resulting in measured TP discharges 2.4-4.8 times those of initial infiltration values. The current study provides a technical baseline for the efficacy of sodium bentonite as an effective bi-functional material in areas utilizing reclaimed water i.e. concurrent reduction of infiltration rates (Function 1) and decontamination of reclaimed wastewater infiltration/recharge (Function 2). Findings indicate that sodium bentonite-clay mixes may represent a feasible alternative for managing recharge of non-potable aquifers with reclaimed wastewater.

Hybrid micro-particles as a magnetically-guidable decontaminant for cesium-eluted ash slurry

Decontamination of the radioactive cesium that is widely dispersed owing to a nuclear power station accident and concentrated in fly ash requires an effective elimination system. Radioactive fly ash contains large amounts of water-soluble cesium that can cause severe secondary contamination and represents a serious health risk, yet its complete removal is complicated and difficult. Here it is shown that a new fine-powder formulation can be magnetically guided to eliminate cesium after being mixed with the ash slurry. This formulation, termed MagCE, consists of a ferromagnetic porous structure and alkaline- and salt-resistant nickel ferrocyanide. It has potent cesium-adsorption- and magnetic-separation-properties. Because of its resistance against physical and chemical attack such as with ash particles, as well as with the high pH and salt concentration of the ash slurry, MagCE simplifies the decontamination process without the need of the continued presence of the hazardous water-soluble cesium in the treated ash.

REMOVAL OF STRONTIUM (II) FROM AQUEOUS SOLUTION USING FUNCTIONALIZED CARBON NANOTUBES

This study was carried out to evaluate the environmental application of functionalized carbon nanotubes through the experimental removal of strontium (II) from water. The aim was to find the optimal condition for the removal of strontium from water under different conditions such as initial concentration of strontium, contact time and neutral pH. The functionalized multi wall carbon nanotubes (f-MWCNT) were characterized by FT-IR and scanning electron microscopy (SEM). The adsorption isotherms were correlated to Freundlich and Langmuir models and it was found that the adsorption data could be fitted better by Langmuir model than Freundlich one. The kinetic data shows that the adsorption describes well with the pseudo-second order kinetic model. Functionalized MWCNT can be used as good adsorbent for the removal of the strontium ions from polluted water according to results.

Comparison of adsorption equilibrium models for the study of CL-, NO3- and SO4(2-) removal from aqueous solutions by an anion exchange resin

The removal of chloride, nitrate and sulfate ions from aqueous solutions by a macroporous resin is studied through the ion exchange systems OH(-)/Cl(-), OH(-)/NO(3)(-), OH(-)/SO(4)(2-), and HCO(3)(-)/Cl(-), Cl(-)/NO(3)(-), Cl(-)/SO(4)(2-). They are investigated by means of Langmuir, Freundlich, Dubinin-Radushkevitch (D-R) and Dubinin-Astakhov (D-A) single-component adsorption isotherms. The sorption parameters and the fitting of the models are determined by nonlinear regression and discussed. The Langmuir model provides a fair estimation of the sorption capacity whatever the system under study, on the contrary to Freundlich and D-R models. The adsorption energies deduced from Dubinin and Langmuir isotherms are in good agreement, and the surface parameter of the D-A isotherm appears consistent. All models agree on the order of affinity OH(-)<HCO(3)(-)<Cl(-)<NO(3)(-)<SO(4)(2-), and distinguish high energy processes (OH(-)/Cl(-), OH(-)/NO(3)(-), OH(-)/SO(4)(2-), Cl(-)/SO(4)(2-)) from lower energy systems (HCO(3)(-)/Cl(-), Cl(-)/NO(3)(-)). The D-A and D-R models provide the best fit to the experimental points, indicating that the micropore volume filling theory is the best representation of the ion exchange processes under study among other adsorption isotherms. The nonlinear regression results are also compared with linear regressions. While the parameter values are not affected, the evaluation of the best fitting model is biased by linearization.

Thermodynamic modeling of Cl(-), NO3(-) and SO4(2-) removal by an anion exchange resin and comparison with Dubinin-Astakhov isotherms

The removal of chloride, nitrate, and sulfate ions from wastewaters by a macroporous ion-exchange resin is studied through the experimental results obtained for six ion exchange systems, OH(-)/Cl(-), OH(-)/NO3(-), OH(-)/SO4(2-), and HCO3(-)/Cl(-), Cl(-)/NO3(-), Cl(-)/SO4(2-). The results are described through thermodynamic modeling, considering either an ideal or a nonideal behavior of the ionic species in the liquid and solid phases. The nonidealities are determined by the Davies equation and Wilson equations in the liquid and solid phases, respectively. The results show that the resin has a strong affinity for all the target ions, and the order of affinity obtained is OH(-) < HCO3(-) < Cl(-) < NO3(-) < SO4(2-). The calculation of the changes in standard Gibbs free energies (ΔG(0)) shows that even though HCO3(-) has a lower affinity to the resin, it may affect the removal of Cl(-), and in the same way that Cl(-) may affect the removal of NO3(-) and SO4(2-). The application of nonidealities in the thermodynamic model leads to an improved fit of the model to the experimental data with average relative deviations below 1.5% except for the OH(-)/SO4(2-) system. On the other hand, considering ideal or nonideal behaviors has no significant impact on the determination of the selectivity coefficients. The thermodynamic modeling is also compared with the Dubinin-Astakhov adsorption isotherms obtained for the same ion exchange systems. Surprisingly, the latter performs significantly better than the ideal thermodynamic model and nearly as well as the nonideal thermodynamic model.

Selective capture of cesium and thallium from natural waters and simulated wastes with copper ferrocyanide functionalized mesoporous silica

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.