Remoción y encapsulación de uranio presente en agua empleando nanopartículas de hierro cerovalente y pasivación superficial con cromo
2024
| Tesista | Ailén Salemme |
| Directores | Dra. Ing. Natalia Quici. CNEA - Argentina |
| Lugar de realización | Departamento Química Ambiental, Gerencia Química,Gerencia de Área Seguridad Nuclear y Ambiente - CNEA - Argentina |
| Fecha Defensa | 24/04/2024 |
| Jurado | Dra. Verónica Lombardo. CNEA, CONICET - Argentina |
| Código | IT/TM: 239/24 |
Título completo
Remoción y encapsulación de uranio presente en agua empleando nanopartículas de hierro cerovalente y pasivación superficial con cromo
Resumen
El uso de nanopartículas de hierro cerovalente (nZVI) para la remoción de contaminantes se destaca por su versatilidad, pero requiere de estudios detallados para su optimización. La caracterización de las nZVI es imprescindible para comprender su estructura, composición, y las transformaciones que sufren al oxidarse, lo que ayudaría a entender su evolución en el ambiente. En particular, la remoción de U(VI) es relevante en Argentina por la importancia del elemento en el ciclo del combustible nuclear.
Se estudiaron las transformaciones de nZVI comerciales por la oxidación en contacto con agua y aire, en un proceso denominado envejecimiento. Se analizaron tres etapas: (1) polvo de nZVI, (2) suspensión de nZVI sin activar y (3) suspensión de nZVI activada por 48 h. Se caracterizaron la estructura y composición de cada una mediante la determinación del contenido de Fe(0) por desplazamiento de H2, difracción de rayos X, microscopía electrónica de barrido y de transmisión, y espectroscopías Mössbauer y Raman. Se observó la disminución del contenido de Fe(0) desde la muestra (1) hasta la (3), encontrando en esta última compuestos de Fe con el mayor estado de oxidación: lepidocrocita, magnetita y hematita. Sobre la estructura, la muestra (3) se destacó por la presencia de formaciones aciculares atribuibles a la lepidocrocita.
Para el estudio de la remoción de U(VI) con nZVI, se realizaron experimentos exploratorios para optimizar las condiciones de anoxicidad y la relación molar (RM) Fe:U, probando RMs entre 10 y 400. Para RMs Fe:U mayores a 100, hubo dificultades en la separación de las nZVI e interferencia en la medición de U(VI). Luego, se comparó la eficiencia de remoción entre dos arreglos: tubos de vidrio dispuestos horizontalmente sobre un agitador orbital y un tanque con agitación vertical, para el cual se obtuvo la mayor eficiencia de remoción.
En base a resultados obtenidos previamente en el grupo, se evaluó la redisolución a largo plazo (14 días) del U(VI) con RM Fe:U 25 y 100. Se ensayó la remoción de U(VI) con y sin adición de Cr(VI), en dos etapas de reacción, la primera anóxica y la segunda óxica (con agitación continua de las suspensiones en recipientes abiertos). Se observó redisolución de U(VI) en todos los casos, excepto para RM Fe:U = 100 con adición de Cr(VI). La redisolución fue mayor con RM Fe:U = 25, y hubo una mejora con presencia de Cr(VI). Estos resultados respaldan la hipótesis de que el agregado de Cr(VI) previene parcialmente la redisolución de U(VI) al pasivar la superficie de las nZVI y sugieren que el exceso de nZVI tiene un efecto similar.
Palabras clave: nanopartículas de hierro cerovalente (nZVI), remediación ambiental, U(VI), Cr(VI)
Complete Title
Removal and encapsulation of uranium from water using zero-valent iron nanoparticles and surface passivation with chromium
Abstract
In this Thesis, the characterization and use of nanoscale zerovalent iron (nZVI) for the removal of U(VI) in aqueous systems was studied. The use of nZVI for the removal of pollutants has been studied for more than 20 years, with successful results in the removal of metals and organic pollutants, both at pilot and industrial scale. Although this technology stands out for its versatility, it still requires detailed studies to optimize the processes and expand its application. The characterization of nZVI is essential to understand the structure and composition of the material and the transformations that occur when exposed to water and air, including their effect on reactivity. This could help to understand their evolution in the environment, considering the effects to minimize potential toxicological risks. In particular, the removal of U(VI) is relevant in our country given the importance of this element in the nuclear fuel cycle, especially with a view to an expansion in the production of nuclear energy.
In first place, transformations of commercial nZVI due to oxidation by contact with water and air were studied. In practice, this process is called ageing and occurs during the activation of nZVI prior to its use in remediation. Three samples were analyzed: (1) nZVI powder, (2) unactivated nZVI suspension and (3) 48 h activated nZVI suspension. The structure and composition of each stage were characterized using a set of complementary techniques: Fe(0) determination by H2 displacement, X-ray diffraction (XRD), Mössbauer spectroscopy, Raman spectroscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). From the results it was observed that the Fe(0) content decreased from (1) to (3), the sample in which the iron compounds with the highest oxidation state were found: lepidocrocite, magnetite, hematite and possibly ferrihydrite and goethite. As for the changes in structure, sample (3) was distinguished by the presence of acicular structures, attributable to lepidocrocite. These results are in agreement with the ageing process explained in the literature.
Regarding the study of the nZVI for uranium removal, exploratory experiments were first performed to optimize the anoxicity conditions and the Fe:U molar ratio (MR) to be used, testing MRs in the range between 10 and 400. For Fe:U MRs higher than 100, difficulties were detected in the separation of the nZVI and interference in the measurement of U(VI), due to the high concentrations of Fe(total) in the solutions. Then, the removal efficiency was compared between two experimental set-ups. The first one consisted of glass tubes arranged horizontally on an orbital shaker, while the second one was a vertically agitated tank of ad hoc design. The highest removal efficiency was obtained with the tank, which can be attributed to the fact that the vertical agitation helps to keep the nZVI suspended and dispersed. Optimization of this design is important for the scale-up of this technology.
In previous work by the group, the structure of nZVI after Cr(VI) removal was analyzed, identifying the formation of a surface layer of iron hydroxychromites. Subsequently, exploratory experiments were carried out for the removal of U(VI) with Cr(VI) addition, revealing a low uranium redissolution in oxic conditions, attributed to the presence of the Cr(VI) passivating layer. Based on these results, in this Thesis the long-term re-dissolution of U(VI) was evaluated over a period of 14 days, using Fe:U MRs of 25 and 100. U(VI) removal experiments were carried out with and without the addition of Cr(VI), divided into two reaction stages: the first in anoxic medium and the second in oxic medium, with continuous stirring of the suspensions contained in vessels open to the atmosphere. Redissolution of U(VI) was observed in all systems, except in the case with MR Fe:U = 100 and addition of Cr(VI), the redissolution was more pronounced in the cases with MR Fe:U = 25, and an improvement was noted in the experiments with the presence of Cr(VI). These results support the hypothesis that the addition of Cr(VI) partially prevents the redissolution of U(VI) by passivating the nZVI surface. Furthermore, they suggest the existence of an effect of excess nZVI that contributes to slowing down this process.
Keywords: zero-valent iron nanoparticles, (nZVI), environmental remediation, U(VI), Cr(VI).
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