- Open Access
Characterization of silver–kaolinite (AgK): an adsorbent for long-lived 129I species
© Sadasivam and Rao. 2016
- Received: 29 December 2014
- Accepted: 15 February 2016
- Published: 23 February 2016
Bentonite is a preferred buffer and backfill material for deep geological disposal of high-level nuclear waste (HLW). Bentonite does not retain anions by virtue of its negatively charged basal surface. Imparting anion retention ability to bentonite is important to enable the expansive clay to retain long-lived 129I (iodine-129; half-life = 16 million years) species that may escape from the HLW geological repository. Silver–kaolinite (AgK) material is prepared as an additive to improve the iodide retention capacity of bentonite. The AgK is prepared by heating kaolinite–silver nitrate mix at 400 °C to study the kaolinite influence on the transition metal ion when reacting at its dehydroxylation temperature. Thermo gravimetric-Evolved Gas Detection analysis, X-ray diffraction analysis, X-ray photo electron spectroscopy and electron probe micro analysis indicated that silver occurs as AgO/Ag2O surface coating on thermally reacting kaolinite with silver nitrate at 400 °C.
- Thermal reaction
- Radioactive waste
Bentonite is identified as potential buffer material in deep geological repositories for disposal of high level radioactive wastes (HLW) owing to its very low hydraulic conductivity, large swelling ability and high adsorptive capacity to retain cations (Pusch 2008). However, owing to negative surface charge, bentonite repels anions (van Olphen 1963). 129I (Iodide-129) is a fission product encountered in nuclear power plant wastes and is generated from ion-exchange resins, filter sludge, evaporator bottoms, off-gas cartridge filter, trash, and decommissioning wastes (Zhang et al. 2002). The ability to retain iodide ions by bentonite buffer employed in deep geological repositories is important as high-level radioactive wastes contain long-lived radioactive iodide species (Oscarson et al. 1986; Krumhansl et al. 2002).
It appears that on set of dehydroxylation (Temperature range from 400 to 550 °C; above dehydration but below dehydroxylation) the clay becomes reactive and concurrently, the liberated water dissolves adjacent salt particles and catalyses the reaction (Kallai 1978). This property of kaolinite could be exploited to incorporate silver compounds on the particle surface as they (example silver oxide) have strong affinity for formation of insoluble halides (Cotton et al. 1995). So in the present work, the silver treated kaolinite material has been prepared as an additive to bentonite to improve iodide retention capacity.
Patakfalvi and Dékány (2004) reported intercalation of silver ions by disaggregating the lamellae of kaolinite using dimethyl sulfoxide (DMSO). Daniels and Rao (1983) observed that 35 meq/100 g, 63 meq/100 g, 83 meq/100 g and 106 meq/100 g of silver ions are sorbed by metakaolinite at temperatures of 25, 255, 275 and 290 °C respectively. Guided by the increased amounts of silver retention by kaolinite at elevated temperatures, the present study focuses on the kaolinite’s influence on the transition element and also discuss the possible reaction mechanism of silver nitrate and kaolinite at 400 °C (Dehydration of kaolinite starts around 400 °C. Kaolinite undergoes complete dehydroxylation at the temperature range of 450–600 °C).
Kaolinite supplied by Alminrock, Bangalore, was used in the study. Chemical dissolution of kaolinite specimen revealed that it contains, 51 % SiO2, 33 % Al2O3, 1.0 % Fe2O3, 0.15 % CaO, 0.13 % MgO, 0.22 % Na2O and 0.2 % K2O. The kaolinite specimen experienced 13.5 % weight loss on ignition. The clay has cation exchange capacity (CEC) of 2.1 meq/100 g. Analytical reagent grade silver nitrate (AgNO3; molecular weight = 169.87 g/mol) was used to prepare the silver–kaolinite specimen.
Preparation and characterization of AgK specimen
Silver retention capacity of kaolinite
% of silver nitrate in silver nitrate–kaolinite mix
mg of silver retained/g of kaolinite
After desired heating of the 20 % silver nitrate–80 % kaolinite mix, the specimen was repeatedly washed with deionised water, until it is free of unreacted silver. Thermo gravimetric analysis (TGA) and differential scanning calorimetric (DSC) analysis were performed using NETZSCH STA 409 thermal analyzer with a heating rate of 10 °C/min. Thermo gravimetric-evolved gas detection (TG-EGD) analysis was performed on 80 % kaolinite + 20 % AgNO3 mix and kaolinite specimens using a Metler-Teledo thermal analyzer model TGA/SDTA851e with Balzers ThermoStar Mass Spectrometer. The TG-EGD analysis was performed to determine the weight loss and the gases evolved during the silver nitrate–kaolinite reaction. The X-ray diffraction patterns of the materials were obtained using Cu Kα line (λ = 0.154 nm) in a Phillips Xpert diffractometer. The chemical state of silver in the AgK specimen was examined using ESCA Thermo Fischer Scientific Multi lab 2000 X-ray photoelectron spectrometer with a monochromatic Al Kα (1486.6 eV) X-ray source. JEOL JXA-8530F Electron Probe Micro analyzer (EPMA) was used to obtain the Wavelength Dispersion Spectrometry (WDS) map of silver retained in AgK specimen.
Weight loss measurements of kaolinite, silver nitrate and silver nitrate–kaolinite mixes at 400 °C for 30 min
Initial weight (g)
Weight after heating at 400 °C for 30 min (g)
Weight loss (g)
0.1 g AgNO3 + 9.9 g kaolinite (1 % AgNO3)
1 g AgNO3 + 9 g kaolinite (10 % AgNO3)
2 g AgNO3 + 8 g kaolinite (20 % AgNO3)
2.5 g AgNO3 + 7.5 g kaolinite (25 % AgNO3)
2 g AgNO3
8 g kaolinite
As per the reaction 2, the weight loss should be around 36.5 %. The observed excess weight loss could be the possible influence of atmosphere environment (Otto et al. 2014). The endothermic peak at 212 °C in the DSC curve represents the melting point of silver nitrate, while, the endotherm at 450 °C represents formation of silver metal from reduction of silver nitrate according to reaction 2.
X-ray diffraction (XRD) analysis
X-ray photo-electron spectroscopy (XPS) analysis
Electron probe micro analysis (EPMA)
Thermal decomposition reaction of AgNO3
The weight balance and thermal analysis showed that the silver retention on kaolinite surface is driven by salt-catalyzed dehydroxylation phenomena. The XRD pattern of AgK specimen indicated that the silver present in AgK specimen does not occur as Ag metal or AgNO3 molecule. The relatively high FWHM (Full Width Half Maximum) observed in the X-ray photon emission survey spectrum of AgK specimen suggested the existence of more than one silver oxide–AgO, and Ag2O in the specimen. As the photoemission is a surface phenomenon (approximate depth of penetration is 1–5 nm), it is inferred that the silver oxides in AgK apparently occur on kaolinite surface as silver oxide coatings. The electron probe micro analysis (EPMA) showed uniform distribution of silver on the surface of AgK specimen. The mass-balance calculations, XRD analysis, X-ray photon emission survey spectrum and EPMA tests with kaolinite–silver nitrate mix/AgK/Kaolinite specimen aided the formulation of chemical reaction for occurrence of uniform coatings of AgO/Ag2O on kaolinite surface of the AgK specimen. And also the AgK specimen would function as an additive to bentonite to improve its iodide retention capacity.
SS Carried out all the experimental work, interpretation of results and drafted the paper. SMR designed the work and participated in drafting the paper. Both authors read and approved the final manuscript.
The authors are grateful to Prof. Michael Plötze of Clay mineralogy group, IGT-ETH, Zurich, for providing the thermal analysis with evolved gas analysis facility.
The authors declare that they have no competing interests.
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